J.-L Mathieu

Tschan

Jia Guo

Sumesh K Raman

Emilie Brulé

Thierry Roisnel

Marie-Noelle Rager

Rémi Legay

Guillaume Durieux

Baptiste Rigaud

Christophe M Thomas
email: christophe.thomas@chimie-paristech.fr

Zinc and cobalt complexes based on tripodal ligands: synthesis, structure and reactivity toward lactide †

Keywords:

The coordination chemistry of a series of pro-ligands ([L 1 ]-[L 6 ]) with cobalt and zinc derivatives has been studied. All complexes have been characterized by multinuclear NMR, elemental analysis, and by singlecrystal X-ray diffraction studies. Polymerization of rac-lactide takes place at 130 °C in the presence of cobalt and zinc complexes to yield polymers under solvent free conditions with controlled molecular masses and narrow polydispersities.

Introduction

Biodegradable polymers have recently gained great attention as a replacement for synthetic ( petroleum-based) materials. 1 Among the new biodegradable polymers that have been developed during the last decade, 2 polylactides (PLAs) are of particular interest. 3 The availability of monomer feedstocks from renewable resources has given PLAs an increasing prominence in the market place. 4 Methods for the synthesis of these polymers now rely on the controlled ring-opening polymerization (ROP) of lactide by well-defined metal initiators, which produce high-molecular-weight polymers with interesting mechanical properties. 5 The coordination-insertion mechanism of lactide polymerization proceeds via coordination of the monomer to the metal centre, followed by insertion into the metal-alkoxide species through the acyl-oxygen bond with retention of configuration. This results in a new metal-alkoxide species capable of further insertion. In a biomimetic spirit, [START_REF] Williams | Inspired by the cooperativity of dizinc sites in hydrolytic enzymes, Hillmyer and Tolman have demonstrated that dinuclear zinc complexes supported by a bulky monophenoxide ligand polymerize LA with good control and at a rate faster than any other Zn-containing system reported previously[END_REF] a mechanistic analogy may be drawn between this process and the hydrolysis reactions performed by a large class of zinc metalloenzymes.

The ubiquity of zinc in metalloenzyme chemistry has been related to its flexible coordination chemistry, substitutional lability, Lewis acidity, combined with intermediate polarizability. 7 Much of the importance of zinc enzymes originates from their amidase activity involving the cleavage of amide bonds. [START_REF]In particular, metallo-β-lactamases are important native zinc enzymes that catalyze the hydrolysis of β-lactam antibiotics. 9 The motivation for their use comes from the fact that important zinc enzymes such as the peptidase astacin exhibit amidase activity involving the cleavage of amide bonds. Astacin is an endopeptidase for which the active site has a unusual trigonal bipyramidal geometry, with the zinc coordinated to three histidine residues, a tyrosine residue, and a water molecule. See for instance[END_REF] For instance, the metallo β-lactamase scaffold is designed to accommodate one or two zinc ions able to activate a nucleophilic hydroxide for the hydrolysis of the β-lactam ring. Upon substitution with Co 2+ , the resulting enzyme is a more efficient catalyst than the native enzyme for the hydrolysis of some β-lactams suggesting that the role of the metal centre is principally to provide the nucleophilic hydroxide, rather than to act as a Lewis acid to polarize the carbonyl group. 7b In our search for new polymerization catalysts, we focused our efforts on investigating the catalytic activity of mononuclear zinc and cobalt complexes supported by tripod ligands. 9 The coordination chemistry of tripod ligands has been intensively studied in recent years. Of particular interest are pyrazolylborate ligands to which Vahrenkamp and Parkin have contributed synthetic, model complex, and complex reactivity studies. 10 Another important class of tripod ligands are tris( picolyl)amine derivatives which can easily be obtained with different donor arms. Depending on the nature and number of donor groups, such compounds can accommodate a broad range of coordination geometries as multidentate ligands for various metal centres. Some of the resulting complexes can be used as efficient catalysts in a variety of processes. Early reports on tripodal ligands were focused on the coordination properties of monoanionic phenoxy based ligands of the type [L 2 NO] -but there is no example of a Although efficient single site initiators have been reported for the controlled ROP of lactide, a great challenge remains in performing catalytic polymerization of lactide, thus optimizing productivity of the metal-based species and minimizing contamination of polymer with metal residues. Inoue first reported the use of metalloporphyrins in the immortal polymerization of lactones using transfer agents such as alcohols. 12 This initial discovery inspired the development of other catalytic systems. 13 As already mentioned, model studies of different zinc enzymes have been used to determine how the coordination environment around the zinc centre affects the activity of the model. To this end, researchers have constructed active zinc enzyme mimics. In particular, tripodal zinc complexes have been widely investigated as models. For instance, it has been shown that, in the presence of methanol or ethanol, a tris( pyrazolyl)borate zinc complex acts as a catalyst for the ring-opening of β-butyrolactone, resulting in the formation of the corresponding esters of β-hydroxybutyric acid. In this regard, we envisaged that our [L 2 NO]M(Cl) n and [L 3 N]M(Cl) n chloride complexes would have the potential to be labile to alcohol exchange; that is, a rapid exchange would occur between the metal species and alcohol. Since the growing species of lactide polymerization is an alkoxide, alcohol would then behave as a reversible chain transfer agent. Herein we report the synthesis, characterization, and LA polymerization reactivity of different zinc(II) and cobalt(II) complexes.

Synthesis and structural characterization of the pro-ligands

The tripodal pro-ligands described in this study were prepared in a straightforward manner, by reductive amination, following a one-pot procedure using commercially available or easily accessible starting materials (Fig. 1). 11c,14,15 The new compounds [L 2 ]H, [L 3 ]H, L 4 , and L 5 were obtained in good to excellent yields. [L 3 ]H was prepared by two successive reductive amination reactions from picolyl amine, introducing first the phenolic moiety followed by the phosphine arm. L 5 is obtained in a one-step reaction between diphenylphosphinobenzaldehyde and an ammonium salt. All compounds were obtained as colourless or yellowish powders, and their identity was confirmed by NMR and high-resolution mass spectrometry or elemental analysis. Such a ligand architecture allows for steric and electronic variations on the different donor fragments (Fig. 1).

The crystal structures of compounds [L 1 ]H, [L 2 ]H, L 4 , L 5 , and L 5 •HCl were determined by X-ray diffraction studies (Fig. 23456). A summary of the relevant crystallographic data and refinement parameters for [L 1 ]H, [L 2 ]H, L 4 , L 5 and the protonated ligand L 5 •HCl can be found in Table 1. Interestingly, the molecular structure of the ammonium cation [L 5 H] + (Fig. 6) revealed the steric properties of pro-ligand L 5 in a structural motif akin to that involving metal coordination, due to the interaction of all coordinating atoms, the nitrogen and the three phosphorus atoms with the proton H1. These interactions are evidenced when comparing with the molecular structure of L 5 (Fig. 5) in which the tripod minimizes the steric interactions between all atoms. Considering the plane containing the three methylenic carbons, the nitrogen atom is above the plane and the three phosphorus atoms located on the other side; the lone pair of nitrogen is pointing outwards from the NP 3 tetrahedra. In [L 5 H] + , the proton is inside the NP 3 tetrahedra, showing the cavity that could create the ligand L 5 around a metal centre upon coordination. 7). These compounds were isolated in good yields and are readily soluble in chlorinated solvents. These complexes were characterized by elemental analysis, NMR spectroscopy and molar conductivity, UV/Vis measurements, and X-ray diffraction studies for some of them. The results of molar conductivity measurements suggest that all zinc complexes are non-electrolytes in DMF (see ESI †). In accordance with previous studies and 1 H NMR data (vide infra), these observations are indicative of the formation of (di)chloride species, in which one of the pyridyl/ phosphino moieties may remain uncoordinated. By contrast the molar conductivity values of cobalt(II) dichlorides indicate that one of the coordinated chloride has been replaced by a DMF molecule. This assignment of cationic structures was supported by UV/Vis analyses of 7, 9 and 11.

The monochloro zinc and cobalt complexes 1-3 and 7-9 were synthesized via a salt metathesis route (Scheme 1). The salt metathesis reactions are two-step reactions. First a solution of the pro-ligand is combined with sodium hydride in order to form an anionic phenoxide. In a second step, the preformed sodium-phenoxide is added to a metal chloride and the synthesis of the metal complex is accompanied by the formation of sodium chloride. As an example, the synthetic scheme for the preparation of 1 is shown in Scheme 1. The coordination of the ligand L 1 to the zinc metal centre is confirmed by 1 H NMR analysis (Fig. 8). Firstly, the signal of the pyridine α-proton is shifted from 8.4 ppm to 9.4 ppm. Secondly, an AB pattern for benzylic CH 2 protons confirms the non-equivalence of the benzylic protons of each picolyl arm upon coordination (C-C bond rotation is now impossible). Finally, the benzylic protons of the phenolic arm remain equivalent and appear as a singlet in the 1 H NMR spectrum, suggesting the formation of a monomeric species with rigid C s ) 2 ]} 1/2 . In the 1 H NMR of complex 5 (Fig. 9), all the signals, especially the benzylic CH 2 , are broadened singlets, suggesting a fluxional structure, probably due to the weak interaction between the zinc and the phosphorus atoms; the small coordination induced shifts Δδ 31 P (complex-free ligand) are 4.2, 0 and -2.5 (δ 31 P(L 5 ) = -16.3 ppm; δ 31 P(5) = -11.1, -16.3, -18.8 ppm), and support weak interactions of two of the phosphorus atoms with the zinc metal centre. Variable-temperature NMR spectroscopy of complex 5 in CDCl 3 revealed decoalescence of the resonances upon lowering the temperature, eventually resulting in the observation of individual resonances for two NCH 2 -phosphine groups at -30 °C. These observations are also consistent with only two of the phosphine groups coordinated onto the metal centre and fast exchange of the coordinated and free phosphine groups on the NMR timescale at relatively high temperature. The enhanced steric flexibility within L 5 , originating from the presence of less donating phosphino groups, and the corresponding more opened coordination sphere at the metal centre might be responsible for the formation of a mixture of monomeric (5) and dimeric/ oligomeric species. 11 In 31 P NMR spectra, one shielded and one deshielded signals compared to the free ligand signal are observed (see the Experimental section). The same situation, that is, a coordinated pyridyl/phosphino group and a free pyridyl/phosphino group, can be envisioned for the other metal complexes. Alternatively, we cannot rule out that all σ-donor groups are coordinated to the metal centre in 1-12, either in a symmetric or a dissymmetric fashion. In the case of nitrogen containing pro-ligand homologues, observation of the exchange mechanism between two pyridine arms on the zinc atom was reported for a (N 4 )ZnCl 2 system (N 4 = N-[(2pyridyl)methyl]-2,2′-dipyridylamine). 16 In addition, as already observed by Kol in zinc complexes supported by aminophenolate ligands bearing 2-methoxyethyl arms, one arm can be tightly coordinated and the other one can be weakly coordinated onto the metal centre. 17

The crystal structure of complex 2 was determined by X-ray diffraction studies. Complex 2 is monomeric in the solid state and features a five-coordinated zinc centre in a distorted bipyramidal trigonal geometry (Fig. 10). Furthermore, the geometric parameter τ defined by Addison and Reedijk is 0.80, which implies that the coordination geometry is best described as a distorted bipyramidal trigonal. [START_REF] Addison | The τ value ranges from 0 (square pyramidal) to 1 (trigonal bipyramidal)[END_REF] Consistent with the NMR observations in solution for complex 1 (vide supra), the two pyridyl groups per molecule are coordinated, together with the N and O atoms of the ligand, forming one six-membered metallacycle and two five-membered metallacycles. Therefore, though potentially tridentate in solution, the tripodal aminophenolate is actually tetradentate. The coordination sphere is completed by the chloro ligand. The Zn-O and Zn-N bond distances fall in normal ranges for similar complexes. 11 The Zn(1)-N(11) (2.123(6) Å) and Zn(1)-N(1) (2.141(6) Å) bond distances are comparable, likely reflecting a relatively strong σ-donor effect of the pyridyl groups. The six-membered metallacycle adopts a boat conformation, as observed in other trisaminophenolate zinc complexes.

Paramagnetic NMR spectroscopy of the cobalt complexes

The NMR of paramagnetic species has been extensively studied over the past few decades. [19][START_REF] Mao | [END_REF][21][22] In diamagnetic molecules, the chemical shift arises from the orbital shift linked to the shield constant. In paramagnetic molecules, a hyperfine shift is added modifying the observed chemical shift. Even though the presence of a paramagnetic centre in the molecule modifies the NMR spectra, some information can still be extracted from NMR experiments. 23 Despite the acceleration of relaxation processes, the integration of NMR signals is still linked to the number of nuclei involved in the resonance. 24 Moreover, by setting the parameters to the correct values, recording of 2D NMR spectra is possible, allowing seeing the correlations between nuclei spin magnetic moments.

As expected, the cobalt(II) complex (L 6 )CoCl 2 12 is paramagnetic, displaying broad signals in its 1 H and 31 P NMR spectra. 25 The 1 H NMR spectrum displays ten resonances over the fourteen expected for a C S symmetrical complex (Fig. 11). The most down-fielded resonances at 131.5, 64.3 and 43 ppm can be easily attributed to protons on the pyridine cores as 2). 26 The COSY experiment with the {43.0 ppm; -5.6 ppm} correlation gives the last signal for the pyridines cycle (Fig. 1S †). The attribution of the different signals is allowed by the correlation between relaxation times and the distance to the paramagnetic centre. The broadest peak at 131.5 ppm corresponds to the nucleus in α position. The two signals at 64.3 ppm and 43.0 ppm can be attributed to the β and β′ protons, respectively. The phenyl groups of the triarylphosphine are easily assigned with the peak integration. Those aromatic cycles are the only possibility to have four or two equivalent protons. The protons in ortho position are the closest to the metal centre and thus are the most shielded and shifted to -12.5 ppm. In addition the meta protons resonate at 13.6 ppm and the two para protons at -7.5 ppm. The relative position of these signals is different from the position of the pyridine proton signals, which can be explained by the difference of the coordination mode. The direct bonding through the nitrogen donor atom increases the residual electronic spin density on the pyridine atoms yielding a stronger Fermi contact interaction. Finally, for the last aromatic ring, 4 non-equivalent protons are identified. The proton a in ortho position is assigned to the -3.3 ppm signal, the broadest and quickest relaxing. The other signals are attributed following the same pattern as the two phenyl rings. As protons on all aromatic cycles are subjected to a strong paramagnetic shift, all donor atoms are coordinated to the metal centre. In this spectrum the benzylic protons are not detected either due to their fast relaxation at the NMR timescale or due to an excessive peak width.

The description of the previous complex enables a faster interpretation of other cobalt(II) complexes with the same framework. In particular, the 1 H NMR spectrum of 7 displays the same paramagnetic pattern (Fig. 12). The pyridine proton signals are observed in the same range as in 12. The main difference concerns the two aromatic protons of the phenolate ring which resonate at 46.3 ppm and 29.3 ppm. Like previously, the most down-fielded signal is attributed to the b position. The greater paramagnetic shift of these protons is explained by the increased electronic spin density on the ring as there are fewer rings on which it can be delocalized. Even if they are in the diamagnetic region, the tBu protons are less shielded than in the free protonated ligand. The protons in ortho position appear as a broad signal at 3.45 ppm and the other ones as a sharper signal at 3.75 ppm.

As a d 7 ion, the cobalt centre can either be in a low spin configuration (S = 1/2) or a high spin configuration (S = 3/2). A change of the spin configuration in a given geometry has a strong influence on the ionic radius. 27 The magnetic moment can be directly measured by SQUID experiments or by the NMR Evan's method. 28,29 Therefore, the magnetic moment of complex 12 was measured with both techniques and gave, respectively, μ eff = 4.0μ B and μ eff = 3.9μ B . As the complex is not in the same state for these two methods, the results can differ due to different accessible configurations in the liquid state. Nevertheless, both methods indicate a cobalt in its high spin configuration. 30 The magnetic moment of 7, determined using

A C C E P T E D M A N U S C R I P T
Evans' method in solution, was found to be 4.8µ B , consistent with a d 7 metal centre having three unpaired electrons.

Reactivity towards rac-lactide

A range of catalytic species have been employed to mediate the polymerization of LA. 31 However, in spite of great efforts in this field, there is still a demand for highly active, nontoxic, and stable complexes that can be easily handled. In this regard, ROP catalysts have not been widely investigated in bulk polymerizations, although there is significant interest in using metal-based catalysts that are active in bulk monomers. Indeed, bulk polymerizations are industrially attractive and compliant with the principles of green chemistry. 32 For instance, Feijen et al. demonstrated that a chiral Schiff-base (SALEN type) aluminium complex based on (R,R)-cyclohexanediamine is an isoselective initiator for the polymerization of racemic lactide at 130 °C. 33 Interestingly, this catalytic system proceeded under solvent free conditions, and gave high-yield reactions, controlled molecular weights and narrow polydispersities. Unfortunately, this aluminium-based initiator was not sufficiently active to be industrially viable, as two days were necessary to convert 200 eq. of lactide. 33 Also, Hillmyer reported a detailed study of analogous Zn, Mg, and Co dinuclear complexes, showing that the dizinc complex is the most active of the series; in melt lactide at 140 °C, more than 4000 eq. of lactide were converted by the dinuclear zinc complex to produce atactic polylactide with low molecular weight control (M n 57 kg mol -1 ; M w /M n 2.6). 34 Only one other example of cobalt-based [Co(OAr) 3 ][Na(THF) 6 ] complexes was reported for melt lactide polymerization with moderate activity and control. 35 As noted earlier, a great challenge in the production of PLA is the catalytic polymerization of lactide, thus optimizing productivity of the metal-based species and minimizing contamination of polymer with metal residues. In this regard, we were interested in investigating the catalytic behaviour of our chloride complexes for lactide polymerization in combination with an alcohol under solvent free conditions. Representative results observed in these polymerizations using benzyl alcohol as a reversible transfer agent are collected in Table 3. All complexes [START_REF]Complexes 6 and 12 are insoluble in melt lactide and could not be tested[END_REF] were tested for ring-opening polymerization of rac-lactide at 130 °C under solvent free conditions. As control experiments the homoleptic zinc and cobalt dichloride precursors were tested in the presence or absence of alcohol for the ring-opening polymerization of rac-lactide. By contrast to previous observations, [START_REF] Börner | [END_REF] we observed activity in the case of zinc dichloride in the presence or absence of alcohol at 130 °C, even if the polymerization rate and control were low (Table 3, entry 1). However, CoCl 2 is inactive for the ROP of lactide in bulk at 130 °C (entry 2). Generally, in the absence of alcohol, the Zn complexes (Table 3, entries 4, 7, 9 and 11) and the Co complexes (Table 3, entry 17) gave also modest reactivities and controls.

Notably, the combination based on zinc complex 1 and 10 equiv. of benzyl alcohol readily polymerized 130 equiv. of rac-LA in 3 h at 130 °C, leading to PLA with an experimentally determined M n value in close agreement with the calculated one (entry 3). Interestingly, for most of the systems tested, we observed that in the presence of an excess of benzyl alcohol (10 equiv.), polymers with narrow polydispersities and controlled molecular weights, which could be predicted from the monomer-to-alcohol ratio, were obtained. 38 Depending on the ligand framework, the zinc initiator is not always more active than its Co analogue. For instance, the cobalt complex 7 (entry 13), which bears ligand L 1 with a tert-butyl group in the ortho position of the phenolate, is as active as the Zn derivative 1 (entry 3). When the ortho-substituent is a bulkier CPh 3 substituent, the corresponding zinc complex 2 is much more active and

Dalton Transactions Paper

This exhibits the highest activity observed in this study with a TOF up to 8800 h -1 (entry 6). However, introduction of a bulkier ortho-substituent in the N 3 O ligand also decreases the control of polymerization (M n shorter than the expected one and PDI broadened). Remarkably, as opposed to the chloro metal complexes that were previously evaluated under similar conditions, the zinc complex 2 is significantly more efficient. [START_REF] Börner | [END_REF] Indeed, these data lie in the same range as the highest activities obtained with unstable catalytic systems, the major difference being that the present zinc catalytic system is much easier to handle. [START_REF] Börner | [END_REF] The phosphorus atom is a soft donor and has a weak interaction with the zinc metal centre. Therefore, by replacing a pyridine arm by a phosphine arm, the metal centre is more accessible and more electrophilic. This is evidenced by the ROP of LA by an increased polymerization rate for 3 compared to 1 (entries 3 and 8). Also, the replacement of a pyridyl group in 4 by a phosphine group in 5 increases the activity of the initiators from a TOF of 218 to 770 h -1 respectively (entries 10 and 12). We assume that, since the phosphine ligand L 5 is more labile than the ligand L 4 , the environment around the zinc metal centre is less hindered, resulting in better activity. Similar to what was obtained in the case of N 3 O ligands, PDI are slightly broadened (from 1.05 to 1.4) as the activity increases. In the case of neutral N m P n ligand derivatives, the zinc is bonded to two chloride atoms, meaning that in the presence of benzyl alcohol, the propagation of two polymer chains can occur. However the molecular weights of the polymer obtained for the zinc dichloride complexes 4 and 5 suggest that only one polymeric chain is formed per metal atom.

Among the cobalt derivatives, complexes 8 and 11 exhibit good activity and excellent control for ROP of lactide at 130 °C (entries 14 and 18). As already observed for zinc complexes, the cobalt complex 8 (entry 14), which bears ligand L 2 with a trityl group in the ortho position of the phenolate, is much more active than the Co analog 7 bearing tert-butyl-substituted ligand L 1 (entry 13). The poor reactivity of complex 10 (entry 16) might arise from the low solubility of the corresponding pre-catalyst. Overall, such performances compare well with those reported for some related, efficient cobalt and zinc chloro systems. 5 Finally, a somehow disappointing feature of these systems remains the absence of stereocontrol achieved in the ROP of racemic lactide, whatever the substituents on the ligands and the nature of the donor fragments.

Conclusion

We have reported a new series of zinc and cobalt chloride complexes supported by readily available tripodal ligands. The corresponding versatile systems enable the production of large quantities of polymer with small amounts of a catalyst, optimizing productivity and thus minimizing the contamination of polymer with metal residues. These organometallic catalysts, which are easily synthesized, are thermally robust, thus allowing their use under bulk conditions. These results suggest a number of new avenues for biodegradable polymers. Indeed the discovery of efficient and productive organometallic catalysts for the synthesis of biodegradable polymers is a crucial requirement for a sustainable growth of the chemical industry.

Experimental section

General considerations

All reactions were carried out under an argon atmosphere using Schlenk techniques or a glove box (<1 ppm O 2 , <2 ppm H 2 O). Tetrahydrofuran, pentane and toluene were dried with a solvent purification system (PureSolv, Innovative Technology, Inc.) and degassed by freeze pump thaw cycles prior to use. Anhydrous dichloroethane, etheral solution of HCl (1 M), 2-picolylamine, 3,5-di-tert-butyl-4-hydroxybenzaldehyde, and sodium triacetoxyborohydride (stored in a glove box) were purchased from Aldrich and used as received. Anhydrous cobalt dichloride (CoCl 2 ) and zinc dichloride (ZnCl 2 ) were purchased from Strem Chemicals. 2-Diphenylphosphinobenzaldehyde was prepared by a previously described method. 39 Deuterated solvents from Eurisotop were dried and degassed by freeze pump thaw cycles (deuterated chloroform was distilled over phosphorus pentoxide, and deuterated benzene was distilled over sodium/benzophenone). NMR spectra were recorded on a Bruker Avance 300, Avance 400 or AM-500 MHz spectrometer (at the Ecole Nationale Supérieure de Chimie de Paris and at University of Caen). Chemical shifts are reported in ppm and coupling constants J in hertz. NMR spectra were calibrated using residual 1 H and 13 C resonances of deuterated solvents. External calibration for 31 P was made with 85% H 3 PO 4 as a standard. Assignment of signals was made from 13 C{ 1 H}{ 31 P}, 2D 1 H-1 H COSY, 1 H- 13 C HMQC and HMBC NMR experiments. The paramagnetic spectrum sequence was as follows: a π/6 pulse followed by a 550 ms acquisition and a 2 s delay for a spectrum width of 220 ppm with 64k points. After the FFT, a 0.3 Hz filter is applied. For the T 1 measurements, the inversion-recovery sequence was used: π pulse, variable delay τ, π/2 pulse and acquisition. The paramagnetic COSY 45 used an acquisition time of 32 ms and a delay of 1 s with TD1 = 512, TD2 = 2048 with an accumulation of 64 scans. For the paramagnetic complexes, numbers in brackets are the peak halfwidth in Hz and the number of protons for this signal. X-ray diffraction analyses were performed at the Laboratoire de Diffractométrie X, UMR 6226, CNRS-Université de Rennes 1 (France). Elemental analyses (C, H, N) were performed at the Elemental Analysis Service of the London Metropolitan University (UK) and are the average of two independent determinations. High-resolution mass spectra were performed at Institut Parisien de Chimie Moléculaire, UMR CNRS 7201, University Pierre et Marie Curie (France). Size exclusion chromatography (SEC) of polymers was performed in THF at 35 °C using an Agilent 1260 Infinity Series GPC (ResiPore 3 μm, 300 × 7.5 mm, 1.0 mL min -1 , UV (250 nm) and RI (PL-GPC 220) The number average molecular masses (M n ) and polydispersity index (M w /M n ) of the polymers were calculated with reference to universal calibration vs. polystyrene standards (limits M w = 200 to 400 000 g mol -1 ). SQUID measurements were recorded on a MPMS-XL7 apparatus. UV-visible spectra were recorded on a Safas-Monaco spectrometer from 250 nm to 1000 nm with 2 nm steps. Molar conductivities of freshly prepared 10 -3 mol L -1 solutions of zinc and cobalt complexes in DMF were measured on a CDM 210 conductivity meter.

General procedure: synthesis of N-(2-hydroxybenzyl)-N,N-bis-(2-pyridylmethyl)amine proligands (N 3 O)H

Into a solution of di-(2-picolyl)amine (1 g, 5 mmol) and appropriate salicylaldehyde (5.5 mmol) in 1,2-dichloroethane (12.5 mL), sodium triacetoxyborohydride (1.7 g, 8 mmol) was added. The mixture was stirred overnight at room temperature, and water (15 mL) was poured into the solution and stirred for another 30 min. The organic phase was separated from aqueous phase and dried over magnesium sulfate (MgSO 4 ).

The solvent was removed under vacuum and the final product was purified by recrystallisation in cold methanol. L 3 ]H): two steps of reductive amination (Fig. 14)

N-(3,5-Ditert-butyl-2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)- amine (N 3 O tBu )H ([L 1 ]H) (
N-(3,5-Ditert-butyl-2-hydroxybenzyl)-N-(2-(diphenylphosphino)- benzyl)-N-bis(2-pyridylmethyl)amine (N 2 PO)H ([
A solution of 3,5-ditert-butyl-4-hydroxybenzaldehyde (2.38 g, 10.1 mmol) and 2-(aminomethyl)pyridine (1 g, 9.2 mmol) in methanol (100 mL) was stirred under reflux for 2 h. The solution was cooled down to room temperature and sodium borohydride (1.74 g, 46 mmol) was added in small portions. After NaBH 4 was completely dissolved, the solvent was removed under vacuum. Dichloromethane (100 mL) and water (100 mL) were added to the crude product and the mixture was stirred for 1 h. The organic phase was washed two times more with water and dried over magnesium sulfate (MgSO 4 ). After removal of solvent, the product was further dried under vacuum for 24 h. It was then dissolved in 1,2-dichloroethane (23 mL) along with 2-diphenylphosphino-benzaldehyde (2.93 g, 10.1 mmol) and sodium triacetoxyborohydride (3.2 g, 14.7 mmol) was then added. The mixture was stirred overnight at room temperature, and water (15 mL) was poured into the solution to stir for another 30 min. The organic phase was separated from aqueous phase and dried over magnesium sulfate (MgSO 4 ). The solvent was removed under vacuum and the final product was purified by recrystallization in cold methanol to give a white powder (4.1 g, 6.9 mmol, 75%). Synthesis of N,N-bis(2-(diphenylphosphino)benzyl)-1-( pyridin-2-yl)methanamine (N 2 P 2 , L 4 ) (Fig. 15)

A solution of 2-pyridinemethylamine (0.279 g, 2.60 mmol) and 2-diphenylphosphino-benzaldehyde (1.50 g, 5.17 mmol) in degassed dichloroethane (10 mL) was stirred for a few minutes. Then, sodium triacetoxyborohydride (1.42 g, 6.72 mmol) was added. The heterogeneous mixture was stirred for 6 h at room temperature, until full conversion of the aldehyde (the disappearance of the aldehyde can be monitored by 31 P NMR). When the reaction was completed, the solvent was evaporated and the product was extracted with ethylacetate (AcOEt). The organic phase was washed with an aqueous saturated solution of NaHCO 3 and the organic phase was separated and dried over magnesium sulfate (MgSO 4 ). The solvent was evaporated to give the crude product as a white solid foam.

The solid was washed several times with pentane and dried. L 4 was obtained pure as a white powder (1.53 g, 2.33 mmol, 90%). Single crystals for X-ray diffraction structure analysis were obtained by slow evaporation of a concentrated solution of L 4 in acetone. Synthesis of tris(2-(diphenylphosphino)benzyl)amine (NP 3 , L 5 ) (Fig. 15)

A suspension of ammonium benzoate (0.154 g, 1.11 mmol) in a solution of 2-diphenylphosphinobenzaldehyde (1.00 g, 3.45 mmol) in degassed dichloroethane (10 mL) was stirred for a few minutes. Then, sodium triacetoxyborohydride (1.06 g, 5 mmol) was added. The heterogeneous mixture was stirred for 72 h at room temperature, until full conversion of the aldehyde (the disappearance of the aldehyde can be monitored by 31 P NMR). When the reaction was complete, the solvent was evaporated and the product was extracted with ethylacetate.

The organic phase was washed with an aqueous solution of NaOH (1 M) three times. The organic phase was separated and dried over magnesium sulfate (MgSO 4 ). The solvent was evaporated to give the crude mixture as a solid foam. The solid can be purified by chromatography over a silica gel column (gradient of elution from hexane to hexane-diethyl ether 2 : 1 v/v, R f ∼ 0.5) and obtained as a white foam (0.6 g, 0.71 mmol, 61%). If there is no other amine by-product, L 5 can also be purified by precipitation of the ammonium chloride salt (cf. synthesis of L 5 •HCl) followed by deprotonation (a solution of L 5 •HCl in dichloromethane is vigorously stirred with aqueous NaOH (1 M)). Single crystals for X-ray diffraction structure analysis were obtained by slow evaporation of a concentrated solution of L 5 in CH 2 Cl 2 . 1 H NMR (300 MHz, 25 °C, D6-acetone): δ = 7.77 (m, 3H, H3), 7.30 (m, 21H, H-Ar), 7.12 (m, 15H, H-Ar) 6.72 (m, 3H, H6), 3.78 (d, 4 J P-H = 2.8 Hz, 6H, H1); 13 1,2-dichloroethane (20 mL). After 5 min sodium triacetoxyborohydride (1.82 g, 12.24 mmol) was added. The reaction mixture was stirred at room temperature for 4 h before the reaction was diluted with ethylacetate and quenched by a saturated NaHCO 3 solution. The aqueous phase was then extracted with AcOEt (3 × 15 mL) and the organic layers were combined, dried over MgSO 4 and solvents were removed under reduced pressure. The resulting yellow oil was then recrystallized from cold diethyl ether to yield a pale yellow solid (2.45 g, 85%). 1 H NMR (300 MHz, CDCl 3 ) δ 8.47 (d, J = 4.1 Hz, 2H, H1), 7.79 (dd, J = 7.1, 4.4 Hz, 1H), 7.56 (td, J = 7.7, 1.8 Hz, 2H), 7.41-7.02 (m, 16H), 6.87 (dd, J = 6.7, 4.3 Hz, 1H), 3.94 (d, 4 J P-H = 2.0 Hz, 2H, H3), 3.75 (s, 4H, H2); 13 C NMR (75 MHz, CDCl 3 ) δ 159. 7, 149.0, 137.1, 136.9, 136.4, 134.1, 133.9, 129.2, 128.9, 128.7, 128.6, 127.2, 122.9, 121.9, 59.9, 57.0, 56.7; 31 P NMR (121 MHz, CDCl 3 ) δ -16.4.

General procedure: synthesis of 1-3 and 7-9

Into a solution of an appropriate pro-ligand (0.2 mmol) in THF (10 mL), sodium hydride (9.6 mg, 0.4 mmol) was added.

The mixture was stirred at room temperature for 10 min and a dichloride salt (Zn or Co, 0.2 mmol) was then added. The mixture was stirred at 60 °C for 48 h and then filtered through a metal cannula equipped with a paper filter. THF was removed and the solid obtained was washed three times with pentane. The final product was dried under vacuum for at least 24 h.

(L 1 )ZnCl (1)

A white solid was obtained in 90% yield. UV/visible (CH 2 Cl 2 ) λ (ε) 633 nm (308 M -1 cm -1 ), 575 nm (228 M -1 cm -1 ).

Synthesis of (L 4 )ZnCl 2 (4)

A solution of zinc dichloride (0.04 g, 0.29 mmol) and L 4 (0.204 g, 0.31 mmol) in tetrahydrofuran (3 mL) was stirred overnight at 50 °C. Then, the solvent was evaporated and the white solid obtained was washed three times with pentane. Complex 4 was dried thoroughly under vacuum and obtained as a white solid, containing 0.1 mole of THF (confirmed by 1 H NMR analysis) (0.150 g, 0.23 mmol, 78% Synthesis of (L 5 )ZnCl 2 (5)

A solution of zinc dichloride (0.03 g, 0.22 mmol) and L 5 (0.20 g, 0.24 mmol) in tetrahydrofuran (3 mL) was stirred overnight at 50 °C. Then, the solvent was evaporated and the white solid obtained was washed three times with pentane. The white solid 5 was dried thoroughly under vacuum (0.160 g, 0.16 mmol, 72%). -16.3, -18.8. Anal. calcd (%) for C 57 H 48 Cl 2 NP 3 Zn: C, 70. 13; H, 4.96; N, 1.43. Found: C, 70.05; H, 5.11; N, 1.35.

Synthesis of (L 6 )ZnCl 2 (6)

A solution of zinc dichloride (0.03 g, 0.22 mmol) and L 6 (0.11 g, 0.24 mmol) in tetrahydrofuran (3 mL) was stirred overnight at 50 °C. Then, the solvent was evaporated and the white solid obtained was washed three times with pentane. The white solid 6 was then dried thoroughly under vacuum (0.140 g, 0.23 mmol, 96%). 1 H NMR (300 MHz, 25 °C, CDCl 3 ) δ 9.19 (d, J = 4.8 Hz, 2H), 8.12 (s, 1H), 7.94 (s, 4H), 7.62-7.02 (m, 14H), 6.77 (t, J = 8.5 Hz, 1H), 4.57 (br, 4H), 4.32 (s, 2H). 13 C NMR (75 MHz, CDCl 3 ) δ 156. 17, 148.72, 141.17, 139.64, 139.43, 135.45, 135.33, 134.38, 133.61, 133.43, 132.46, 131.22, 129.21, 129.08, 127.25, 126.88, 126.75, 126.29, 125.93, 124.22, 58.23, 57.09, 31.36, 22.43, 13.93; 31 P NMR (121 MHz, 25 °C, CDCl 3 ) δ -18.04.

Synthesis of (L 4 )CoCl 2 (10)

A solution of cobalt dichloride (0.03 g, 0.23 mmol) and L 4 (0.167 g, 0.25 mmol) in tetrahydrofuran (3 mL) was stirred overnight at 50 °C (a purple precipitate formed instantaneously). Then, the solvent was removed with a cannula and the purple solid was washed with tetrahydrofuran and dried thoroughly under vacuum to give 10 as a pure compound (0.144 g, 0.18 mmol, 80% Synthesis of (L 5 )CoCl 2 (11)

A solution of cobalt dichloride (0.03 g, 0.23 mmol) and L 5 (0.214 g, 0.25 mmol) in tetrahydrofuran (3 mL) was stirred overnight at 50 °C. A blue solution was obtained with the formation of a brown precipitate. Then, the solvent was evaporated and the brown solid obtained was washed three times with pentane. The brown solid 11 was dried thoroughly under vacuum (0.205 g, 0.21 mmol, 92%). Anal. calcd (%) for C 57 H 48 Cl 2 CoNP 3 : C, 70.60; H, 4.99; N, 1.44. Found: C, 70.48; H, 4.89; N, 1.55; UV/visible (CH 2 Cl 2 ) λ (ε) 733 nm (91 M -1 cm -1 ), 665 nm (163 M -1 cm -1 ), 600 nm (144 M -1 cm -1 ), 497 nm (96 M -1 cm -1 ).

Synthesis of (L 6 )CoCl 2 (12) A solution of cobalt dichloride (0.03 g, 0.23 mmol) and L 6 (0.12 g, 0.25 mmol) in tetrahydrofuran (3 mL) was stirred overnight at 50 °C. A blue solution was obtained with the formation of a brown precipitate. Then, the solvent was evaporated and the brown solid obtained was washed three times with pentane. The brown solid 12 was dried thoroughly under vacuum (0.13 g, 0.22 mmol, 95%). cm -1 ), 520 nm (175 M -1 cm -1 ).

Typical polymerization procedure

All polymerizations were performed under argon in a 15 mL flame dried Schlenk tube equipped with a Teflon coated stirring bar. In a glove box, a Schlenk tube was charged with the appropriate complex (0.009 mmol), benzyl alcohol (0.01 g, 0.090 mmol) and rac-lactide (1.30 g. 9 mmol) and then outside of the glove box, the tube was immersed in a 130 °C oil bath.

The reaction mixture was vigorously stirred for the desired time and the polymerization was quenched by rapid cooling of the reaction vessel. Conversion was determined by 1 H NMR analysis of the crude mixture (integrating the methyl region in the spectrum). The crude mixture was then dissolved in 8 mL of dichloromethane and the polymer further precipitated from 400 mL of ice-cold ethanol. The resulting polymer was dried under vacuum at 50 °C until the weight was constant. In the case of a polymerization with lower conversion, the crude was precipitated from an appropriate mixture of diethyl ether and pentane.

  soft phosphorus donor atoms. 11 Therefore we decided to investigate the coordination of tripodal containing [L 2 NO] and [L 3 N] donor environments and the catalytic behavior of the corresponding zinc and cobalt chloride complexes.

Fig. 1

 1 Fig. 1 Synthesis of [L n ].

Fig. 3

 3 Fig. 3 Molecular structure of pro-ligand [L 2 ]H (hydrogen atoms and solvent molecules are omitted for clarity).

Fig. 2

 2 Fig. 2 Molecular structure of pro-ligand [L 1 ]H (hydrogen atoms and solvent molecules are omitted for clarity).

Fig. 4

 4 Fig. 4 Molecular structure of pro-ligand L 4 (hydrogen atoms are omitted for clarity).

Fig. 5

 5 Fig. 5 Molecular structure of tripodal pro-ligand L 5 (hydrogen atoms are omitted for clarity).

  Synthesis and structural characterisation of the cobalt(II) and zinc(II) complexesThe coordination chemistry of the ligands was studied towards zinc and cobalt dichloride. [L 2 NO] and [L 3 N] ligands were used to synthesize a series of twelve new complexes 1-12, such as [L 2 NO]MCl and [L 3 N]MCl 2 (M = Zn, Co). Reactions of the neutral pro-ligands L 4 , L 5 , and L 6 with one equiv. of ZnCl 2 or CoCl 2 in tetrahydrofuran at 50 °C afford cleanly the bischlorozinc and -cobalt complexes 4-6 and 10-12 (Fig.

Fig. 6

 6 Fig. 6 Molecular structure of tripodal ammonium salt L 5 •HCl (hydrogen atoms and chloride anion are omitted for clarity).

  NMR timescale. As further supported by the solid state structure of 2 (vide infra), these observations are consistent with both pyridyl groups coordinated onto the metal centre. The same features are also observed for L 2 and L 3 coordinated zinc complexes (see the Experimental section).

Fig. 7

 7 Fig. 7 Complexes 1-12 (R = Ph).

Fig. 9

 9 Fig.9Variable temperature 1 H NMR experiment for (L 5 )ZnCl 2 (5); benzylic region (400 MHz, from 40 °C to -30 °C, CDCl 3 ).

Fig. 8 1

 8 Fig. 8 1 H NMR spectra of pro-ligand [L 1 ]H (up) and 1 (down) (CDCl 3 , 25 °C, 300 MHz).

Fig. 10

 10 Fig. 10 Molecular structure of 2 (hydrogen atoms and solvent molecules are omitted for clarity).

  often attributed in similar iron(II) complexes (Table

Fig

  Fig. 11 1 H NMR spectrum of complex 12 (CDCl 3 , 25 °C, 300 MHz).

Fig. 12

 12 Fig. 12 1 H NMR spectrum of complex 7 (CDCl 3 , 25 °C, 300 MHz).

  journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 4550-4564 | 4557

Fig. 13 )

 13 White solid with 85% yield. 1 H NMR (300 MHz, 25 °C, CDCl 3 ): δ = 8.47(d, J = 4.2 Hz, 2H, H1), 7.53 (t, J = 4.2 Hz, 2H, H2), 7.29(d, J = 4.8 Hz, 2H, H4), 7.13 (d, J = 2.4 Hz, 1H, H9), 7.08 (m, 2H, H3), 6.79 (d, J = 2.4 Hz, 1H, H11), 3.80 (s, 4H, H6), 3.73 (s, 2H, H7), 1.38 (s, 9H, H17), 1.19 (s, 9H, H14);13 C NMR (75 MHz, 25 °C, CDCl 3 ): δ = 158.1 (C5), 153.8 (C13), 149.0 (C1), 140.4 (C8), 136.6 (C4), 135.6 (C12), 124.5 (C9), 123.6 (C2), 123.1 (C11), 122.2 (C3), 121.7 (C10), 56.5 (C6), 58.3 (C7), 34.7 (C16), 34.1 (C15), 31.7 (C17), 29.7 (C14). N-(3-Trityl-5-methyl-2-hydroxybenzyl)-N,N-bis(2-pyridylmethyl)amine (N 3 O CPh3 )H ([L 2 ]H) (Fig. 13) White solid with 75% yield. 1 H NMR (300 MHz, 25 °C, CDCl 3 ): δ = 8.32 (d, J = 4 Hz, 2H, H1), 7.42 (t-d, J 1 = 4 Hz, J 2 = 1.8 Hz, 2H, H2), 7.30-6.95 (m, 17H, H3, H14), 6.85 (d, J = 9 Hz, 2H, H4), 6.82 (d, J = 1.

  Fig. 14 [L 3 ]H with position numbering. Fig. 13 [L 1 ]H and [L 2 ]H with position numbering.

  1 H NMR (500 MHz, 25 °C, CDCl 3 ): δ = 8.34 (m, 1H, H5), 7.70 (m, 2H, H9), 7.53 (td, J = 7.71, J = 1.9 Hz, 1H, H3), 7.38 (brd, J = 7.71 Hz, 1H, H2), 7.29 (m, 14H, H17, H16, H10), 7.18 (m, 8H, H15), 7.08 (m, 3H, H11, H4), 6.79 (m, 2H, H12), 3.83 (d, 4 J P-H = 2.1 Hz, 4H, H7), 3.65 (s, 2H, H6); 13 C NMR (125 MHz, 25 °C, CDCl 3 ): δ = 159.9 (C1), 148.7 (C5), 143.8 (d, J P-C = 22.0 Hz, C8), 136.8 (d, J = 11.0 Hz, C14), 136.2 (C3), 136.1 (d, J P-C = 14.5 Hz, C13), 133.9 (d, J P-C = 19.6 Hz, C15), 133.3 (C12), 128.82 (C9), 128.80 (C10), 128.6 (C17), 128.5 (d, J P-C = 6.9 Hz, C16), 126.9 (C11), 122.7 (C2), 121.7 (C4), 59.9 (C6), 56.1 (d, J P-C = 23.4 Hz, C7); 31 P{ 1 H} (200 MHz, 25 °C, CDCl 3 ): δ = -16.3. Anal. calcd (%) for C 44 H 38 N 2 P 2 : C, 80.47;H, 5.83; N, 4.27. Found: C, 80.37; H, 5.72; N, 4.18.

  C{ 1 H} NMR (125 MHz, 25 °C, CDCl 3 ): δ = 144.1 (d, J P-C = 22.5 Hz, C2), 136.9 (d, J P-C = 11.2 Hz, C8), 135.9 (d, J P-C = 14.5 Hz, C7), 133.9 (d, J P-C = 19.5 Hz, C9), 133.0 (C6), 128.8 (C3), 128.7 (d, J P-C = 5.1 Hz, C4), 128.5 (C11), 128.4 (d, J P-C = 6.9 Hz, C10), 126.7 (C5), 55.7 (d, J P-C = 23.6 Hz, C1); 31 P{ 1 H} (200 MHz, 25 °C, D6-acetone): δ = -16.3. Anal. calcd (%) for C 57 H 48 NP 3 : C, 81.51; H, 5.76; N, 1.67. Found: C, 81.44; H, 5.81; N, 1.55. Synthesis of tris(2-(diphenylphosphino)benzyl)ammonium chloride (by protonation of L 5 with HCl) (L 5 •HCl) L 5 was dissolved in diethyl ether and an etheral solution of HCl (1 M) was added dropwise until no further precipitate was formed. The solid was filtered off over celite and recovered by dissolution in dichloromethane (CH 2 Cl 2 ). The solvent was evaporated to dryness and L 5 •HCl was obtained as a white solid quantitatively. Single crystals for X-ray diffraction structure analysis were obtained by slow evaporation of a concentrated solution of L 5 •HCl in CHCl 3 . 1 H NMR (300 MHz, 25 °C, CDCl 3 ): δ = 8.72 (brs, 1H, NH), 7.69 (d, J = 7.1 Hz, 4H, H-Ar), 7.45-7.00 (brm, 38H, H-Ar), 5.12 (brs, 6H, N-CH 2 ); 31 P{ 1 H} (120 MHz, 25 °C, CDCl 3 ): δ = -17.8. Anal. calcd (%) for C 57 H 49 ClNP 3 : C, 75.08; H, 5.42; N, 1.54. Found: C, 75.18; H, 5.54; N, 1.55. Synthesis of N,N-bis( pyridin-2-yl)-1-(2-(diphenylphosphino)benzyl)methanamine (N 3 P, L 6 )2-(Diphenylphosphino)benzaldehyde (1.75 g, 6.12 mmol) and di-(2-picolylamine) (1.08 mL, 6.12 mmol) were dissolved in

Fig. 15 L

 15 Fig. 15 L 4 and L 5 with position numbering.

  1 H NMR (400 MHz, 25 °C, CDCl 3 ): δ = 7.75-6.70 (m, 42H, H-Ar), 4.89 (br, CH 2 ), 4.09 (br, CH 2 ), 3.71 Dalton Transactions Paper This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 4550-4564 | 4561 CH 2 ); 31 P{ 1 H} (120 MHz, 25 °C, CDCl 3 ): δ = -11.1,

  1 H NMR (300 MHz, 25 °C, CDCl 3 ) δ 131.4 (800, 2H), 64.3 (75, 2H), 43.0 (60, 2H),13.6 (30, 4H), 13.2 (40, 2H) 3.6 (25, 1H), -3.3 (45, 1H), -5.6 (65, 2H), -7.5 (110, 2H), -12.5 (240, 4H); magnetic susceptibility (CDCl 3 , 293 K) μ eff = 4.1μ B (SQUID, 293 K) μ eff = 3.9μ B ; ESI MS m/z 567.3 ([M -Cl] + ), 287.0 ([M -2Cl + MeCN] 2+ ), 266.3 ([M -2Cl] 2+ ); UV-visible (CH 2 Cl 2 ) λ (ε) 660 nm (130 M -1

Table 1

 1 Summary of X-ray data collection and refinement

		[L 1 ]H	[L 2 ]H	L 4	L 5	L 5 •HCl	2
	Empirical formula	C 27 H 35 N 3 O	C 39 H 35 N 3 O	C 44 H 38 N 2 P 2	C 57 H 48 NP 3	C 60 H 52 Cl 10 NP 3	C 45 H 40 ClN 3 OZn
	Formula weight	417.58	561.7	656.7	839.9	1234.44	739.62
	T (K)	100(2)	100(2)	150(2)	150(2)	150(2)	150(2)
	Wavelength (Å)	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073
	Crystal system Space group	Monoclinic P21/n	Triclinic P 1 ˉP2 1 /c Monoclinic	Triclinic P 1 ˉP1 ˉP2 1 /n Triclinic Monoclinic
	a (Å)	10.052(2)	10.9042(6)	14.0718(9)	15.6653(16)	10.8501(19)	9.1363(15)
	α (°)	90	112.417(2)	90	98.534(4)	93.544(6)	90
	b (Å)	24.616(6)	13.0721(6)	13.5580(8)	18.0030(18)	13.843(2)	22.025(5)
	β (°)	110.971(12)	91.203(2)	98.664(2)	112.551(4)	95.702(6)	91.470(11)
	c (Å)	10.268(2)	14.7332(7)	19.6384(9)	18.1172(19)	20.392(3)	19.679(4)
	γ (°)	90	104.786(2)	90	92.618(4)	96.184(7)	90
	V (Å 3 )	2372.4(9)	1860.58(16)	3704.0(4)	4637.0(8)	3021.8(8)	3958.6(14)
	Z D c (g cm -3 ) µ (mm -1 )	4 1.169 0.071	2 1.003 0.060	4 1.178 0.150	2 1.203 0.167	2 1.357 0.579	4 1.241 0.725
	F(000)	904	596	1384	1768	1268	1544
	Crystal size (mm)	0.55 × 0.5 × 0.43	0.55 × 0.32 × 0.18	0.27 × 0.18 × 0.11	0.56 × 0.39 × 0.25	0.57 × 0.51 × 0.42	0.1 × 0.1 × 0.1
	Crystal color	Colourless	Colourless	Colourless	Colourless	Colourless	Colourless
	Collected reflections	17 275	34 702	30 098	65 501	44 988	42 068
	Unique reflections	5268	8390	8432	20 787	13 664	8975
	R int	0.069	0.0481	0.0504	0.0637	0.0554	0.2219
	Variable parameters	282	391	433	1099	671	407
	Goodness-of-fit	1.013	1.093	1.055	1.038	1.052	0.912
	R 1 , wR 2 (I > 2σ) a Δρ max/min (e Å -3 )	0.0672, 0.1496 0.28, -0.262	0.0524, 0.1518 0.243, -0.235	0.0451, 0.122 0.32, -0.259	0.0488, 0.1132 0.356, -0.34	0.0631, 0.1481 1.563, -1.534	0.0986, 0.2197 0.798, -1.086
	a R 1 = ∑||F o | -|F c ||/∑|F o |; wR 2 = {∑[w(F o 2 -F c 2 ) 2 ]/∑[w(F o 2

Table 2

 2 Chemical shift, spin-lattice relaxation time, half-height width and integration of the 1 H NMR spectrum of 12 in CDCl 3

		α	β	β ′	m + b	b ′	c	a	γ	p	o
	δ (ppm)	131.5	64.3	43.0	13.6	13.2	3.6	-3.3	-5.6	-7.5	-12.5
	T 1 (ms)	0.4	14.4	7.6	26.1	35.2	38.5	10.2	21.2	49	1.3
	ν 1/2 (Hz)	800	75	60	30	40	25	45	65	110	240
	Integration	2	2	2	5	1	1	1	2	2	4

| Dalton Trans., 2014, 43, 4550-4564

This journal is © The Royal Society of Chemistry 2014

Table 3

 3 Polymerization of lactide using Co and Zn complexes a

	Entry	Initiator	[LA]/[I]	[BnOH]/[I]	Time (h)	Conv. b (%)	M n,calc.	c (g mol -1 )	M n	d (g mol -1 )	M w /M n	d	TOF (h -1 )
	1	ZnCl 2	1000	10	3	11	ND e		ND		ND		37
	2	CoCl 2	1000	10	3	0	ND		ND		ND		0
	3	1	1000	10	3	13	1800		1100	1.06		43
	4	2	1000	-	0.25	28	40 300		41 000	2.02		1120
	5	2	1000	10	0.25	75	10 800		10 500	2.00		3000
	6	2	5000	10	0.25	44	31 700		17 000	1.71		8800
	7	3	1000	-	2	27	38 800		8200	1.32		135
	8	3	1000	10	2	83	11 900		11 300	1.16		415
	9	4	1000	-	3	5	ND		ND		ND		16
	10	4	1000	10	3	66	9300		6700	1.05		218
	11	5	1000	-	1	12	17 300		20 800	1.22		120
	12	5	1000	10	1	77	11 000		12 300	1.38		770
	13	7	1000	10	3	13	ND		ND		ND		43
	14	8	1000	10	3	45	6400		5000	1.21		150
	15	9	1000	10	3	26	3700		2300	1.07		86
	16	10	1000	10	3	10	ND		ND		ND		33
	17	11	1000	-	3	7	ND		ND		ND		23
	18	11	1000	10	3	60	8600		7300	1.08		200

a Polymerization was performed in melt rac-lactide at 130 °C. b Conversion was determined by 1 H NMR (integration of the methyl resonances of LA ad PLA). c M n,calc = 144.00 × ([LA]/[I]) × conv./([BnOH]/[I]). d M n and M w /M n of polymer determined by SEC-RI in THF at RT using polystyrene standards and M n corrected by the Mark-Houwink parameter (0.58). e Not determined.

  ). Anal. calcd (%) for C 44 H 38 Cl 2 ColN 2 P 2 : C, 67.19; H, 4.87; N, 3.56. Found: C, 67.08; H, 4.93; N, 3.45.

This journal is © The Royal Society of Chemistry 2014

This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 4550-4564 | 4553

This journal is © The Royal Society of Chemistry 2014 Dalton Trans., 2014, 43, 4550-4564 | 4555

Acknowledgements

CNRS, ENSCP, ANR (grant ANR-10-PDOC-010-01), Fondation Pierre-Gilles de Gennes, and the French Ministry of Research and Higher Education are thanked for financial support of this work. The authors would like to thank Purac for a generous loan of rac-lactide. C. M. T. is grateful to the Institut Universitaire de France.


Solene Guiheneuf

Ludovic Paquin

François Carreaux

Emilie Durieu

Thierry Roisnel

Laurent Meijer

Jean-Pierre Bazureau
email: jean-pierre.bazureau@univ-rennes1.fr

J.-P Bazureau

Br, MeO = No2

F, Br = Me

New 5-ylidene rhodanine derivatives based on the dispacamide A model

Keywords: Rhodanine, 5-Ylidene rhodanine, Knoevenagel condensation, Microwave condensation, 2-Thioxo-imidazoline

A practical approach for the preparation of (5Z ) 5-ylidene rhodanine derivatives bearing the (4,5dihalogeno-pyrrol-2-yl)carbamoyl fragment of dispacamide A is reported. The new compounds were obtained in good yields (19-88 %) by Knoevenagel condensation according to a solution-phase microwave dielectric heating protocol in the presence of organic bases (piperidine, TEA, and AcONa) from a set of N -substituted rhodanines 2(a-i). The ten synthetic products 3(a-j) have been synthesized with a Z -geometry about their exocyclic double bond and the structure of one of these compounds (3) was confirmed by a single X-ray diffraction analysis. The new (5Z ) 5-ylidene rhodanine derivatives 3(a-j) were tested against eight protein kinases.

Introduction

During the two last decades, the 5-arylidene-2-thioxothiazolidine-4-ones and 5-arylidene rhodanine derivatives have been the subject of intensive research by organic chemists and biologists because such compounds represent privileged scaffolds in drug discovery. A survey of recent literature showed that these compounds display a wide range of pharmaceutical properties. For example, epalrestat I (Fig. 1) was used in the treatment of diabetic peripheral neuropathy [START_REF] Hotta | Long-term clinical effects of Epalrestat, an aldose reductase inhibitor, on diabetic peripheral neuropathy[END_REF] and has been evaluated as aldose reductase inhibitor [START_REF] Ruiz | Aldose reductase structures: implications for mechanism and inhibition[END_REF]. The 5-benzylidene rhodanine core (compound II) has been shown to inhibit the pancreatic cholesterol esterase [START_REF] Heng | New cholesterol esterase inhibitors based on rhodanine and thiazolidinedione scaffolds[END_REF] (CEase). A series of dimeric analogs based on BH3I [START_REF] Degterev | Identification of small-molecule inhibitors of interaction between the BH3 domain and Bcl-x L[END_REF] have been developed as small-molecule Bcl-2 antagonists (compounds III) for apoptosis through a complete SAR study [START_REF] Wang | Development of dimeric modulators for anti-apoptotic Bcl-2 proteins[END_REF]. To discover chemical probes to further understand the function of human DNA polymerase λ in cancer by high-throughput screening (HTS) using SYBR Green-based assay [START_REF] Summerer | Enhanced fidelity in mismatch extension by DNA polymerase through directed combinatorial enzyme design[END_REF] and three 5-arylidene-2-thioxo-thiazolidine-4-ones (compounds IV) were identified as potent inhibitors. For Alzheimer's disease, compounds containing the 5-arylidene rhodanine moiety are reported to have an inhibitory effect (compounds V) of tau aggregation [START_REF] Strittmatter | Small molecule inhibitors of human DNA polymerase λ[END_REF], amyloid polypeptide fibril formation [START_REF] Bulic | Development of tau aggregation inhibitors for Alzheimer's disease[END_REF][START_REF] Bulic | Tau protein and tau aggregation inhibitors[END_REF], regulation of cathepsin-D immunoreactivity (compound VI) in the senile plaques [START_REF] Mishra | Small-molecule inhibitors of islet amyloid polypeptide fibril formation[END_REF][START_REF] Whitesitt | Synthesis and structure-activity relationships of benzophenones as inhibitors of cathepsin D[END_REF]. In addition, rhodanine-based molecules have become a popular small-molecule family of inhibitors for numerus targets in malaria [START_REF] Kumar | Discovery of a rhodanine class of compounds as inhibitors of Plasmodium falciparum enoyl-acyl carrier protein reductase[END_REF][START_REF] Soltero-Higgin | Identification of inhibitors for UDP-galactopyranose mutase[END_REF], Hepatitis C [START_REF] Powers | SAR and mode of action of novel non-nucleoside inhibitors of hepatitis C NS5b RNA polymerase[END_REF], HIV infection [START_REF] Rajamaki | Exploration of novel thiobarbituric acid-, rhodanine-and thiohydantoin-based HIV-1 integrase inhibitors[END_REF][START_REF] Dayam | β-Diketo acid pharmacophore. 1. Discovery of structurally diverse inhibitors of HIV-1 integrase inhibitors[END_REF], obstructive pulmonary disease and asthma [START_REF] Kodimuthali | Recent advances on phosphodiesterase 4 inhibitors for the treatment of asthma and chronic obstructive pulmonary disease[END_REF], anthrax and botulinum [START_REF] Sherida | Rhodanine derivatives as selective protease inhibitors against bacterial toxins[END_REF]. Our group, during the last 10 years, has investigated the chemical development of analogs of marine sponge alkaloid as inhibitors of protein kinases. Protein kinases represented a class of enzymes, which catalyze protein phosphorylation, a key cellular regulatory mechanism that is frequently deregulated in human diseases. We have recently identified the marine sponge alkaloid leucettamine B as an inhibitor of DYRKs/CLKs [19,[START_REF] Debdab | Synthesis and preliminary biological evaluation of new derivatives of the marine alkaloid leucettamine B as kinase inhibitors[END_REF]. The synthesis of analogs of leucettamine B named leucettines (leucettine L 41 VII) and the biological characterization of leucettines [START_REF] Debdab | Leucettines, a class of potent inhibitors of cdc2-like kinases and dual specificity, tyrosine phosphorylation regulated kinases derived from the marine sponge leucettamine B: modulation of alternative pre-RNA splicing[END_REF] showed that this family of kinase inhibitors deserves futher optimization as potential therapeutics against neurodegenerative diseases such as Alzheimer's diseases [START_REF] Tahtouh | Selectivity, cocrystal structures, and neuroprotective properties of leucettines, a family of protein kinase inhibitors derived from the marine sponge alkaloid leucettamine B[END_REF][START_REF] Burgy | Chemical synthesis and biological validation of immobilized protein kinase inhibitory leucettines[END_REF]. In parallel, the 2-amino imidazoline-4-one moiety present in the marine sponge alkaloid dispacamide A (Fig. 1) [START_REF] Cafieri | Dispacamides, anti-Histamine alkaloids from Caribbean Agelas Sponges[END_REF] represented also an attractive scaffold for the search of potential new inhibitor of protein kinases. Recently, we have developed an efficient approach to dispacamide A and its analogs [START_REF] Guiheneuf | An efficient approach to dispacamide A and its derivatives[END_REF] in seven steps with an overall yield ranging from 12 to 33 %. Unfortunately, the preliminary biological screening results of these new dispacamide A derivatives [START_REF] Guiheneuf | Microwave in organic synthesis[END_REF] showed moderate inhibition activities against serine/threonine kinases. In our efforts to discover new low molecular weight inhibitors of disease-relevant protein kinases, we focused now our attention on the synthesis of N -3-substituted 5-ylidene rhodanine derivatives based on dispacamide A model (Fig. 2). In this context, the 2-amino-imidazoline-4-one platform of dispacamide A was replaced by a rhodanine platform.

In this approach, the planned retrosynthesis of these 5-ylidene rhodanine derivatives is based around the key aldol condensation between the N -(4,5-dihalogeno pyrrol-2-yl) carbamoyl aldehyde building-block and various 2thioxo-thiazolidine-4-ones. This route was envisaged to be amenable to the design of new 5-ylidene rhodanine derivatives for their preliminary screening as kinase inhibitors. Our goal in this study is the development of a convenient, high yielding and robust reaction protocol for the preparation of new N -3-substituted 5-yliden-2-thioxo-thiazolidine-4-one derivatives.

Results and discussion

The overall strategy to the target 5-ylidene N -substituted rhodanine derivatives is outlined in Scheme 1. For this study, we have examined the reactivity of various 2-thioxothiazolidine-4-ones 2(a-i) with the N -(4,5-dihalogeno pyrrol-2-yl) carbamoyl aldehydes 1(a,b) in Knoevenagel condensation under microwave dielectric heating. For the synthesis of small molecules with potential biological activity, the use of microwave irradiation is growing in importance [27,[START_REF]Ultrasound and microwave: recent advances in organic chemistry[END_REF] because the major benefits of performing reaction under microwave irradiation are higher product yields and shorter reaction times as compared to reactions which run with conventional heating (i.e., in oil bath). A key advan- Scheme 1 General synthetic approach for 5-ylidene rhodanines derivatives 3(a-j) and structure of the starting rhodanines 2(a-i)

123 A C C E P T E D M A N U S C R I P T O H N O N H X X S N S O O H N N H X X N S S O 1
S N S O N 2(f-i) H R 1 O H 2 N CO 2 H
β-alanine tage of modern commercial scientific laboratory microwave apparatus is their ability to control reaction conditions precisely, by monitoring temperature/pressure, and reaction times.

The N -(4,5-dihalogeno pyrrol-2-yl) carbamoyl aldehyde partners 1(a,b) were prepared in three steps according to our previously published method [START_REF] Cafieri | Dispacamides, anti-Histamine alkaloids from Caribbean Agelas Sponges[END_REF]. Starting from commercial readily available 2-trichloroacetyl pyrrole, a regioselective halogenation was conducted by addition of bromine (X = Br) or sulfuryl chloride (X = Cl) to give the corresponding C-4, C-5 dihalogeno pyrroles (70-90 %), which were then coupled with 3,3-diethoxy-1-aminopropane (75-88 %). The corresponding N -(4,5-dihalogenopyrrol-2-yl) carbamoyl acetals were deprotected with p-TsOH at 55 • C after 6 h (98 %) and led to the aldehyde building-blocks 1(a,b) in good overall yields (1a 60 % for X = Br; 1b 66 % for X = Cl).

For the present study, we have investigated the chemical reactivity of a series of various N -substituted rhodanines 2(a-i) for Knoevenagel condensation in the presence of alde-hydes 1(a,b). Among these compounds, rhodanine 2a, 2-(4-oxo-2-thioxo-thiazolidin-3-yl)acetic acid 2b and 3-amino rhodanine 2d are commercially available. Access to 3-(4oxo-2-thioxo-thiazolidin-3-yl)propanoic acid 2c (Scheme 2) could be accomplished by the reaction of β-alanine with carbon disulfide and bromoacetic acid in aqueous potassium hydroxide [29]. After 3 h at room temperature, the reaction mixture was acidified at pH 4 and the desired insoluble compound 2c was obtained in 44 % yield (Table 1) by simple filtration. To introduce the arylsulfonamide functionality on the rhodanine moiety in compound 2e, we used the Power's approach [START_REF] Powers | SAR and mode of action of novel non-nucleoside inhibitors of hepatitis C NS5b RNA polymerase[END_REF]: firstly, the formation of the arylsulfonyl hydrazide was accomplished by treatment of sulfonyl chloride at 0 • C with hydrazine in THF followed by aqueous work-up and, secondly, the arylsulfonyl hydrazide treated with bis-(carbomethyl)trithiocarbonate [START_REF] Kamila | Microwave-assisted synthesis of novel bis(2-thioxothiazolidin-4-one) derivatives as potential GSK-3 inhibitors[END_REF][START_REF] Nitsche | Aqueous microwave-assisted one-pot synthesis of N-substituted rhodanines[END_REF] Next, the preparation of compounds 2(f-i) involved the use of commercial 3-amino rhodanine 2d as starting product. Among the conditions studied, we found that reaction of 2d with benzoyl chloride in THF at 60 • C gave good conversion to 2f (90 %) after 2 h. For compounds 2(g-i), optimal reaction conditions were obtained only in dry toluene at 50 • C. After a reaction time ranging from 8 (for 2g, h) to 23 h (for 2i) which were monitored by thin-layer chromatography on silica gel using dichloromethane/ethanol (9:1) as eluent, we obtained the crystallized compounds 2(g-i) in good to high yields (84-98 %). With the desired N -substituted rhodanines 2(a-i) and the N -(4,5-dihalogeno pyrrol-2-yl)carbamoyl aldehydes 1(a, b) in hand, we proceeded to examine the Knoevenagel condensation under microwave dielectric heating for the synthesis of new 5-ylidene rhodanines 3(a-j) based on the dispacamide A model. In literature, the condensation of an aryl aldehyde to 2-thioxo-thiazolidine-4-one required the presence of a base such as piperidine in ethanol [START_REF] Heng | New cholesterol esterase inhibitors based on rhodanine and thiazolidinedione scaffolds[END_REF][START_REF] Safonov | New benzimidazoles as thrombopoietin receptor agonists[END_REF], piperidine/AcOH in ethanol under microwave irradiation [START_REF] Tomašić | Synthesis and biological evaluation of new glutamic acid-based inhibitors of MurD ligase[END_REF][START_REF] Tomašić | Synthesis and antibacterial activity of 5-ylidenethiazolidin-4-ones and 5-benzylidene-4,6-pyrimidinediones[END_REF][START_REF] Zidar | Discovery of novel 5-benzylidenerhodanine and 5-benzylidenethiazolidine-2,4-dione inhibitors of MurD ligase[END_REF][START_REF] Zidar | New 5-benzylidenethiazolidin-4-one inhibitors of bacterial MurD ligase: design, synthesis, crystal structures, and biological evaluation[END_REF] or with a catalytic amount of piperidinium acetate in refluxing toluene [START_REF] Chen | Synthesis and biological evaluation of thiazolidine-2,4-dione and 2,4-thione derivatives as inhibitors of translation initiation[END_REF], AcONa in refluxed AcOH [START_REF] Whitesitt | Synthesis and structure-activity relationships of benzophenones as inhibitors of cathepsin D[END_REF]40] or concentrated ammonia solution in the presence of NH 4 Cl [START_REF] Opletalova | Synthesis and characterization of (Z )-5arylmethylidenerhodanines with photosynthesis-inhibiting properties[END_REF]. The use of solventless reaction conditions has also been employed with task specific ionic liquids (TSILs) [START_REF] Alizadeh | A solvent-free protocol for the green synthesis of arylalkylidene rhodanines in a task-specific ionic liquid[END_REF][START_REF] Gong | Green synthesis of 5-benzylidene rhodanine derivatives catalyzed by 1-butyl-3methyl imidazolium hydroxide in water[END_REF] or with a solid inorganic support (Al 2 O 3 or KSF) under microwave [START_REF] Ben-Alloum | Synthèse de nouvelles spiro-rhodanine-pyrazolines par addition dipolaire-1,3 de la diphenylnitrilimine sur quelques 5-arylidènerhodanines en "milieu sec" et sous irradiation micro-onde[END_REF]. In this context, we decided to perform the condensation reaction under microwave dielectric heating and we screened a range of reaction parameters in order to find optimal reaction conditions. Reaction optimization for the synthesis of compounds 3(a-j) consisted of varying the reaction temperature (70-150 • C), the irradiation power (50-200 W), the reaction time (20-40 min), the nature of the solvent (EtOH, AcOH, or AcOEt), the base (piperidine, Et 3 N or AcONa) and the ratio of rhodanine 2 with the base. The reactions were conducted in borosilicate vials of 10 mL equipped with snap caps (at the end of the irradiation reaction time, cooling was realized automatically by compressed air). As shown in Table 2, the use of 0.1 equivalent of piperidine/AcOH in ethanol has been employed for the synthesis of N -3-substituted 5-ylidene rhodanine derivatives 3(a-c) and3f in poor (3f 19 %), moderate (3c 46 %) to good yields (3a 88 % and 3b 82 %). In contrast, any effort to obtain Knoevenagel condensation adducts from the reaction of the other N -substituted rhodanines 2 with aldehydes 1(a, b) was unsuccessful using these reaction conditions. This implies that new specific synthetic protocols has to be devised for the others products 3. Finally, treatment of 1 and 2 with a stoichiometric mixture of AcONa/AcOH (1 equivalent) during 20 min at 120-140 • C afforded the compounds 3d and 3(g-j) in yields ranging from 46 to 74 %. It is noteworthy that access to compound 3e required the use of triethylamine (TEA) in glacial acetic acid instead of piperidine or AcONa and the condensation reaction was conducted in ethyl acetate (AcOEt). Compound 3e was synthesized in 71 % yield after a reaction time of 30 min using moderate reaction temperature (70 • C) and irradiation power (50 W) in a glass vial without snap caps (or in open vessel at atmospheric pressure).

Structures of the desired compounds 3(a-j) were substantiated by 1 H, 13 C NMR and HRMS analyses. In theory, E and Z geometrical isomers around the exocyclic double (CH=C) are possible for the N -3-substituted 5-ylidene rhodanine derivatives 3(a-j). 1 H NMR spectra of these compounds 3 show only one signal for the methylene proton (CH=) in the range of 6.82-7.20 ppm, at lower field val-

Compound CDK1 CDK2 CDK5/p25 GSK3α/β CK1 DYRK1A CLK1 Dispacamide A - - - >10 - >10 - 3a >10 >10 >10 - - 5.7 - 3b >10 >10 >10 >10 >10 >10 >10 3(c-j) - - - - - - -
a Compounds were tested at various concentrations on each kinase as described in "Experimental" section. IC 50 values are reported in μM,inactive at the highest concentration tested (10 μM); >10 inhibitory but IC 50 >10 μM tem [START_REF] Sing | Arylalkylidene rhodanine with bulky and hydrophobic functional group as selective HCV NS3 protease inhibitor[END_REF]. Finally, the Z -configuration and also the chemical structure of 3 were confirmed by the single X-ray diffraction analysis of 4,5-dibromo-1H -pyrrol-2-carboxylic acid [3-(4-oxo-2-thioxo-thiazolidin-5-ylidene)-propyl]-amide 3a (Fig. 3).

As an initial effort to investigate their in vitro bioactivity, the new 5-ylidene rhodanine derivatives 3(a-j) were tested against three protein kinases relevant to Alzheimer's disease, CK1α/β (casein kinase 1α/β), CDK5 (cyclin-dependent kinase 5)/p25 and GSK-3α/β glycogen synthase kinase 3α/β. All assays were run in the presence of 15 μM ATP and appropriate protein substrates (RRKHAAIGpSAYSITA peptide for CK1α/β, histone H1 for CDK5/p25, GS-1 (YRRAAVPPSPSLSRHSSPHQSpEDEEE) peptide for GSK-3α/β). IC 50 values were determined from doseresponse curves and are provided in Table 3.

These results are summarized in Table 3. The unsubstituted compound 3a showed promising inhibitory activity against DYRK1A with IC 50 value in the single-digit micro-molar range (3a: IC 50 5.7 μM). Yet the other new 5-ylidene rhodanine derivatives 3(c-j) substituted in position N -3 were considered inactive on CDKs, GSK3α/β,CK1, CLK1 and DYRK1A since they displayed IC 50 values above 10 μM, these results suggested that these structures cannot be accommodated in the ATP binding site of these protein kinases by hydrogen bond interactions with specific amino-acids of the ATP-binding pocket of each protein kinase.

Conclusion

In summary, the present study described a practical approach to new 5-ylidene rhodanine derivatives 3(a-j) bearing the (4,5-dihalogeno-pyrrol-2-yl)carbamoyl fragment of dispacamide A as potential inhibitors of protein kinases for therapeutics against neurodegenerative diseases. The key step of this solution phase organic synthesis involved a Knoevenagel condensation under microwave dielectric heat- This microwave-assisted condensation afforded new (5Z ) 5ylidene rhodanines derivatives in a stereo-controlled fashion with yields ranging from 19 to 88 % and in high purity after purification by preparative chromatography on silica gel. This work should enable further biological evaluations, new analogs though the simple synthetic process described here, and further studies towards a complete structure-activity relationship (SAR). These studies are on going in our laboratory.

Chemistry experimental part

Melting points were determined on a Kofler melting point apparatus and were uncorrected. Thin-layer chromatography (TLC) was accomplished on 0.2-mm precoated plates of silica gel 60 F-254 (Merck). Visualization was made with ultraviolet light (254 and 365 nm) or with a fluorescence indicator. 1 H NMR spectra were recorded on BRUKER AC 300 P (300 MHz) spectrometer, 13 C NMR spectra on BRUKER AC 300 P (75 MHz) spectrometer. Chemical shifts are expressed in parts per million downfield from tetramethylsilane as an internal standard. Data are given in the following order: δ value, multiplicity (s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad), number of protons, coupling constants J is given in Hz. The mass spectra (HRMS) were taken respectively on a MS/MS ZAB-Spec Tof Micromass (EBE TOF geometry) at an ionizing potential of 8 eV and on a VARIAN MAT 311 at an ionizing potential of 70 eV in the "Centre Régional de Mesures Physiques de l'Ouest" (CRMPO, Rennes). Reactions under microwave irradiations were realized in the Explorer®24 CEM microwave reactor (CEM France) and also in the Anton Paar Monowave 300®microwave reactor (Anton Paar France) using borosilicate glass vials of 10 mL equipped with snap caps (at the end of the irradiation, cooling reaction was realized by compressed air). The microwave instrument consists of a continuous focused microwave power output from 0 to 300 W for the Explorer®24 CEM apparatus and from 0 to 800 W for the Anton Paar Monowave 300®apparatus. All the experiments were performed using stirring option. The target temperature was reached with a ramp of 2 min and the chosen microwave power stay constant to hold the mixture at this temperature. The reaction temperature is monitored using calibrated infrared sensor and the reaction time included the ramp period. The microwave irradiation parameters (power and temperature) were monitored by the ChemDriver software package for the Explorer®24 CEM apparatus and by the Monowave software package for the Anton Paar Monowave 300®reactor. Preparative chromatographies were realized on a Combi Flash R f 200 psi (Serlabo Technologies France) using pre-packed column of silica gel 60 F 254 Merck equipped with a DAD UV/Vis 200-360 nm detector. Elemental analyses were performed on a Flash Microanalyzer EA1112 CHNS/O Thermo Electron in the "Centre Régional de Mesures Physiques de l'Ouest" (CRMPO, Rennes). Solvents were evaporated with a BUCHI rotary evaporator. All reagents and solvents were purchased from Acros, Aldrich Chimie, and Fluka France and were used without further purification. The starting aldehydes 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3-oxo-propyl)amide 1a and 4,5-dichloro-1H -pyrrole-2-carboxylic acid (3oxo-propyl)-amide 1b were synthesized according to our previous methods described in literature [19].

3-(4-Oxo-2-thioxo-thiazolidin-3-yl)-propionic acid (2c)

In a 25 mL two-necked round-bottomed flask provided with a magnetic stirrer and condenser, β-alanine (500 mg, 5.61 mmol) was solubilized in a solution of 22 % potassium hydroxide at room temperature. To this homogeneous solution was added carbon disulfide (0.37 mL, 6.12 mmol) dropwise during 10 min at 25 • C. After stirring over a period of 3 h, commercial bromoacetic acid (781 mg, 5.61 mmol) was added in small portions to the reaction mixture. The yellowish solution was vigorously stirred at 25 • C during 3 h. The resulting orange solution was acidified at pH 4 with conc sulfuric acid then, stirred over a period of 16 h and orange needles appeared in the suspension. The insoluble product 2c was collected by filtration and dried under high vacuum (10 -2 Torr) at 25

N-(4-Oxo-2-thioxo-thiazolidin-3-yl)-benzenesulfonamide (2e)

In a 25 mL two-necked round-bottomed flask provided with a magnetic stirrer and condenser, a suspension of commercial phenylsulfonyl hydrazine (500 mg, 2.9 mmol) in 3 mL of deionized water was vigorously stirred at 95 • C during 2 h and produced an homogeneous yellow solution. To this solution was added commercial bis-(carboxymethyl)trithiocarbonate (657 mg, 2.9 mmol). The reaction mixture was stirred at 95 • C over a period of 22 h. Water was evaporated in vacuo during 7 h and the desired insoluble compound 2e was collected by filtration then was submitted to purification by recrystallization in 3 mL of absolute ethanol. The product 2e was obtained as white needles in 41 % yield (344 mg). Mp = 152-154 • C. 1

N-(4-Oxo-2-thioxo-thiazolidin-3-yl)-benzamide (2f)

In a 25 mL round-bottomed flask provided with a magnetic stirrer and condenser, a mixture of commercial 3-amino rhodanine 2d (500 mg, 3.37 mmol) and benzoyl chloride (0.39 mL, 3.37 mmol) was stirred in 5 mL of dry tetrahydrofuran at 60 • C during 2 h. The solvent of the reaction mixture was eliminated in a rotary evaporator under reduced pressure and the yellowish crude residue crystallized after cooling down to room temperature. The crude solid was washed with toluene (2×5 mL) and was collected by filtration. The desired product 2f was obtained as a yellowish powder in 90 % yield (767 mg) and was further used without purification. Mp = 196-198

4-Methoxy-N-(4-oxo-2-thioxo-thiazolidin-3-yl)-benzamide (2g)

In a 25 mL round-bottomed flask provided with a magnetic stirrer and condenser, a mixture of commercial 3-amino rhodanine 2d (300 mg, 2.02 mmol) and p-methoxybenzoyl chloride (345 mg, 2.02 mmol) was dispersed in 3 mL of dry toluene. The reaction mixture was heated at 50 • C under vigorous magnetic stirring and the reaction was monitored by TLC on silica plates using dichloromethane/ethanol (9:1) as eluent (2g R f 0.8). After 8 h, the solvent of the reaction mixture was eliminated in a rotary evaporator under reduced pressure and the crude oil was dried under high vacuum (10 -2 Torr) at 25 • C for 3 h. The crude oil crystallized after standing at room temperature. The desired product 2g was obtained as yellowish powder in 98 % yield (570 mg) and was further used without purification. Mp = 190-192 • C. 1 H NMR (DMSO-d 6 ) δ: 3.84 (s, 3H, CH 3 O); 4.51 (s, 2H, H-5 , CH 2 S); 7.08 (d, 2H, J = 8.9 Hz, H-3, Ar); 7.92 (d, 2H, J = 8.9 Hz, H-2, Ar); 11.39 (br s, 1H, NH). 13

N-(4-Oxo-2-thioxo-thiazolidin-3-yl)-2-phenyl-acetamide (2h)

In a 25 mL round-bottomed flask provided with a magnetic stirrer and condenser, a mixture of commercial 3-amino rhodanine 2d (300 mg, 2.02 mmol) and phenylacetyl chloride (0.27 mL, 2.02 mmol) was dispersed in 3 mL of dry toluene. The reaction mixture was heated at 50 • C under vigorous magnetic stirring and the reaction was monitored by TLC on silica plates using dichloromethane/ethanol (9:1) as eluent (2h R f 0.75). After 8 h, the solvent of the reaction mixture was eliminated in a rotary evaporator under reduced pressure and the crude product was dried under high vacuum (10 -2 Torr) at 25

N-(4-Oxo-2-thioxo-thiazolidin-3-yl)-3-phenylpropionamide (2i)

In a 25 mL round-bottomed flask provided with a magnetic stirrer and condenser, a mixture of commercial 3-amino rhodanine 2d (50 mg, 0.34 mmol) and 2-phenylpropionyl chloride (50 μL, 0.34 mmol) was dispersed in 1 mL of dry toluene. The reaction mixture was heated at 50 • C under vigorous magnetic stirring and the reaction was monitored by TLC on silica plates using dichloromethane/ethanol (9:1) as eluent (2i R f 0.75). After 23 h, the solvent of the reaction mixture was eliminated in a rotary evaporator under reduced pressure. The crude residue was dissolved in dichloromethane (15 mL) then, this organic layer was washed with 4 mL of saturated hydrogen carbonate NaHCO 3 and was dried over MgSO 4 . After filtration, the solvent of the filtrate was eliminated in vacuo and the crude residue was dried under high vacuum (10 -2 Torr) at 25

Standard procedure for the preparation of 4,5-dihalogeno-1H-pyrrol-2-carboxylic acid [3-(4-oxo-2-thioxothiazolidin-5-ylidene) -propyl]-amide 3(a, b)

In a 10 mL glass tube were placed successively 4,5dihalogeno-1H -pyrrole-2-carboxylic acid (3-oxo-propyl)amide 1(a, b) (3.09 mmol), commercial rhodanine 2a (3.09 mmol, 1 equiv.), piperidine (0.31 mmol, 0.1 equiv.) and glacial acetic acid (0.31 mmol, 0.1 equiv.) in 12 mL of absolute ethanol. The glass tube was sealed with a snap cap and placed in the Explorer®24 CEM microwave cavity (P = 300 W). The mixture was irradiated at 150 • C (with a power of 200 W) for 20 min under vigorous magnetic stirring. After microwave dielectric heating, the crude reaction mixture was allowed to cool down at room temperature and the volatile compounds of the reaction mixture were eliminated in a rotary evaporator under reduced pressure. The crude residue was submitted to purification by preparative chromatography (Combi Flash R f 200 psi apparatus, detector UV 254 nm) on pre-packed column of silica gel 60F 254 Merck using dichloromethane/ethanol (9:1) as eluent. Pooling and evaporation of the solvents in vacuo gave the expected compounds 3(a, b) which, were dried under high vacuum (10 -2 Torr) at 25 • C for 1 h.

4,5-Dibromo-1H-pyrrol-2-carboxylic acid [3-(4-oxo-2thioxo-thiazolidin-5-ylidene)-propyl]-amide (3a)

Orange powder. R f = 0.83. Yield = 88 %. Mp = 156 • C (decomposition). 1

4,5-Dichloro-1H-pyrrol-2-carboxylic acid [3-(4-oxo-2thioxo-thiazolidin-5-ylidene)-propyl]-amide (3b)

Orange powder. R f = 0.79. Yield = 82 %. Mp = 180 • C (decomposition). 1 H NMR (DMSO-d 6 ) δ: 2.40 (q, 2H, J = 6.5 Hz, CH 2 CH=); 3.43 (q, 2H, J = 6.5 Hz, NHCH 2 ); 6.82 (m, 1H, =CH); 6.84 (s, 1H, H-3, =CH, Ar); 8.34 (t, 1H, J = 5.8 Hz, NHCO); 12.74 (br s, 1H, NH, Ar); 13.58 (br s, 1H, H-3 , NH). 13 (

5-{3-[(4,5-Dibromo-1H-pyrrol-2-carbonyl)-amino]-propylidene} -4-oxo-2-thioxo-thiazolidin-3-yl)-acetic acid (3c)

In a 10 mL glass tube were placed successively 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3-oxo-propyl)-amide 1a (200 mg, 0.62 mmol), commercial 2-(4-oxo-2-thioxo-thiazolidin-3-yl)-acetic acid 2b (118 mg, 0.62 mmol, 1 equiv.), piperidine (6 μL, 0.062 mmol, 0.1 equiv.), glacial acetic acid (4 μL, 0.062 mmol, 0.1 equiv.) in 2.5 mL of absolute ethanol. The glass tube was sealed with a snap cap and placed in the Monowave®200 Anton-Paar microwave cavity (P = 800 W). The mixture was irradiated at 110 • C for 40 min under vigorous magnetic stirring. After microwave dielectric heating, the crude reaction mixture was allowed to cool down at room temperature and the volatile compounds of the reaction mixture were eliminated in a rotary evaporator under reduced pressure. The crude residue was submitted to purification by preparative chromatography (Combi Flash R f 200 psi apparatus, detector UV 254 nm) on pre-packed column of silica gel 60F 254 Merck using dichloromethane/methanol (9:1) as eluent (3c: R f 0.10). Pooling and evaporation of the solvents in vacuo gave the desired compound 3c which, was dried under high vacuum (10 -2 Torr) at 25 (

5-{3-[(4,5-Dibromo-1H-pyrrol-2-carbonyl)-amino]-propylidene} -4-oxo-2-thioxo-thiazolidin-3-yl)-propionic acid (3d)

In a 10 mL glass tube were placed successively 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3-oxo-propyl)-amide 1a (79 mg, 0.24 mmol), 3-(4-oxo-2-thioxo-thiazolidin-3-yl)propionic acid 2c (50 mg, 0.24 mmol, 1 equiv.), commercial sodium acetate AcONa (20 mg, 0.24 mmol, 1 equiv.), and glacial acetic acid (90 μL, 1.6 mmol, 6.58 equiv.). The glass tube was sealed with a snap cap and placed in the Explor-er®24 CEM microwave cavity (P = 300 W). The mixture

123 A C C E P T E D M A N U S C R I P T
was irradiated at 140 • C (with a power of 60 W) for 20 min under vigorous magnetic stirring. After microwave dielectric heating, the crude reaction mixture was allowed to cool down at room temperature and to the oily crude reaction mixture was added 1 mL of deionized water. After triturating, the insoluble product 3d was collected by filtration and was submitted to purification by preparative chromatography (Combi Flash R f 200 psi apparatus, detector UV 254 nm) on pre-packed column of silica gel 60F 254 Merck using dichloromethane/ethanol (9:1) as eluent (3d R f 0.49). Pooling and evaporation of the solvents in vacuo gave the desired compound 3d which, was dried under high vacuum (10 -2 Torr) at 25 • C for 1 h. 3d was obtained as yellowish needles in 46 % yield (58 mg). Mp = 90 • C (decomposition). 1

4,5-Dibromo-1H-pyrrol-2-carboxylic acid [3-(3-amino-4oxo-2-thioxo-thiazolidin-5-ylidene)-propyl]-amide (3e)

In a 10 mL glass tube were placed successively 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3-oxo-propyl)-amide 1a (100 mg, 0.31 mmol), commercial 3-amino rhodanine 2d (46 mg, 0.31 mmol, 1 equiv.), dry triethylamine Et 3 N (4 μL, 0.031 mmol, 0.1 equiv.), glacial acetic acid AcOH (2 μL, 0.031 mmol, 0.1 equiv.) in 1 mL of ethyl acetate AcOEt. The glass tube was placed (open vessel) in the Explorer®24 CEM microwave cavity (P = 300 W) and the mixture was irradiated at 70 • C (with a power of 50 W) for 30 min under vigorous magnetic stirring. After microwave dielectric heating, the crude reaction mixture was allowed to cool down at room temperature and to the crude reaction mixture was added 5 mL of cyclohexane. This heterogeneous mixture in the tube was submitted to ultrasound in a Branson 1510 apparatus at 25 • C during 30 min. The yellowish desired compound 3e was collected by filtration, washed with Et 2 O (2 × 5 mL) and was purified by preparative chromatography (Combi Flash R f 200 psi apparatus, detector UV 254 nm) on pre-packed column of silica gel 60F 254 Merck using dichloromethane/methanol (9:1) as eluent (3e R f 0.69). Pooling and evaporation of the solvents in vacuo gave the desired compound 3e which, was dried under high vacuum (10 -2 Torr) at 25

4,5-Dibromo-1H-pyrrol-2-carboxylic acid [3-(3-benzenesulfonylamino-4-oxo-2-thioxo-thiazolidin-5-ylidene)propyl]-amide (3f)

In a 10 mL glass tube were placed successively 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3-oxo-propyl)-amide 1a (100 mg, 0.31 mmol), N-(4-oxo-2-thioxo-thiazolidin-3-yl)benzenesulfonamide 2e (89 mg, 0.31 mmol, 1 equiv.), piperidine (3 μL, 0.031 mmol, 0.1 equiv.), glacial acetic acid (2 μL, 0.031 mmol, 0.1 equiv.) in 1.2 mL of absolute ethanol. The glass tube was sealed with a snap cap and placed in the Monowave®200 Anton-Paar microwave cavity (P = 800 W). The mixture was irradiated at 150 • C for 20 min under vigorous magnetic stirring. After microwave dielectric heating, the crude reaction mixture was allowed to cool down at room temperature and the volatile compounds of the reaction mixture were eliminated in a rotary evaporator under reduced pressure. To the black crude residue was added Et 2 0 (5 mL) and after triturating, the insoluble compound 3f was collected by filtration. Then, 3f was submitted to purification by preparative chromatography (Combi Flash R f 200 psi apparatus, detector UV 254 nm) on pre-packed column of silica gel 60F 254 Merck using dichloromethane/methanol (9:1) as eluent (3f R f 0.69). Pooling and evaporation of the solvents in vacuo gave the desired compound 3f which, was dried under high vacuum (10 -2 Torr) at 25

Standard procedure for the preparation of 4,5-dibromo-1Hpyrrol-2-carboxylic acid [3-(3-substituted amino-4-oxo-2thioxo-thiazolidin-5-ylidene)-propyl]-amide (3g) and 3(i, j).

In a 10 mL glass tube were placed successively 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3-oxo-propyl)-amide 1a (

4,5-Dibromo-1H-pyrrol-2-carboxylic acid {3-[3-(4methoxy-benzoylamino)-4-oxo-2-thioxo-thiazolidin-5ylidene]-propyl}-amide (3h)

Compound 3h was prepared in 74 % yield (154 mg) from 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3-oxo-propyl)amide 1a (115 mg, 0.35 mmol), 4-methoxy-N -(4-oxo-2thioxo-thiazolidin-3-yl)-benzamide 2g (100 mg, 0.

4,5-Dibromo-1H-pyrrol-2-carboxylic acid [3-(4-oxo-3phenylacetylamino-2-thioxo-thiazolidin-5-ylidene)-propyl]amide (3i)

Compound 3i (R f 0.71) was prepared in 58 % yield (126 mg) from 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3oxo-propyl)-amide 1a (122 mg, 0.38 mmol), N -(4-oxo-2-thioxo-thiazolidin-3-yl)-2-phenyl-acetamide 2h (100 mg, 0.38 mmol), sodium acetate AcONa (31 mg, 0.38 mmol) in glacial acetic acid (140 μL, 2.47 mmol, 6.58 equiv.) according to the standard procedure. Mp = 100 • C (decomposition). 1

-(3-phenyl-propionylamino)-2-thioxo-thiazolidin-5-ylidene]-propyl}-amide (3j)

Compound 3j (R f 0.77) was prepared in 49 % yield (46 mg) from 4,5-dibromo-1H -pyrrole-2-carboxylic acid (3oxo-propyl)-amide 1a (52 mg, 0.16 mmol), N -(4-oxo-2thioxo-thiazolidin-3-yl)-3-phenyl-propionamide 2i (45 mg, 0.16 mmol), sodium acetate AcONa (14 mg, 0.16 mmol) in glacial acetic acid (61 μL, 1.06 mmol, 6.58 equiv.) according to the standard procedure. Mp = 100 • C (decomposition). 1 .647 g cm -3 , μ = 4.815 mm -1 . The structure was solved by direct methods using the SIR97 program [START_REF] Altomare | SIR97: a new tool for crystal structure determination and refinement[END_REF], and then refined with full-matrix least-square methods based on F 2 (SHELXL-97) [START_REF] Sheldrick | A short history of SHELX[END_REF] with the aid of the WINGX [START_REF] Farrugia | WinGX suite for small-molecule singlecrystal crystallography[END_REF]program. The contribution of the disordered solvents to the calculated structure factors was estimated following the BYPASS algorithm [START_REF] Van Der Sluis | BYPASS: an effective method for the refinement of crystal structures containing disordered solvent regions[END_REF], implemented as the SQUEEZE option in PLATON [START_REF] Spek | Single-crystal structure validation with the program PLATON[END_REF]. A new data set, free of solvent contribution, was then used in the final refinement. All non-hydrogen atoms were refined with anisotropic atomic displacement parameters. H atoms were finally included in their calculated positions. A final refinement on F 2 with 8064 unique intensities and 361 parameters converged at ωR(F 2 ) = 0.2134(R(F) = 0.0795) for 2888 observed reflections with I > 2σ (I). Crystallographic data for the structure of 3a in this paper have been deposited in the Cambridge Crystallographic Data Centre as supplementary publication number CCDC 938832. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK [fax: +44-0-1223-336033 or e-mail:deposit@ccdc.cam.ac.uk].

X-ray crystallographic data for 4,5-dibromo-1Hpyrrol-2-carboxylic acid [3-(4-oxo-2-thioxothiazolidin-5-ylidene)-propyl]-amide (3a)

Biochemistry part

Protein kinase assay buffers

Buffer A 10 mM MgCl 2 , 1 mM EGTA, 1 mM DTT, 25 mM Tris-HCl pH 7.5, 50 μg heparin/mL. Buffer B 50 mM MgCl 2 , 90 mM NaCl, 30 mM Tris-HCl pH 7.4.

Kinase preparations and assays

Kinase activities for each enzyme were assayed in buffer A or B, with their corresponding substrates, in the presence of 15 μM ATP in a final volume of 30 μL. After 30-min incubation at 30 • C, the reaction was stopped by harvesting, using a FilterMate harvester (Packard), onto P81 phosphocellulose papers (GE Healthcare) which were washed in 1 % phosphoric acid. Scintillation fluid was added and the radioactivity measured in a Packard counter. Blank values were subtracted and activities calculated as pmoles of phosphate incorporated during the 30-min incubation. The activities were expressed in % of the maximal activity, i.e., in the absence of inhibitors. Controls were performed with appropriate dilutions of DMSO.

CDK1/cyclin B (M phase starfish oocytes, native), CDK2/ cyclin E (human, recombinant, from A. Echalier), and CDK5/p25 (human, recombinant, expressed in E. coli) [START_REF] Leclerc | Indirubins inhibit glycogen synthase kinase-3β and CDK5/P25, two protein kinases involved in abnormal tau phosphorylation in Alzheimer's disease: a property common to most cyclin-dependant kinase inhibitors?[END_REF] were assayed in Buffer A (supplemented extemporaneously with 0.15 mg BSA/mL, except for CDK2) with 25 μg of histone H1.

DYRK1A (rat, recombinant, expressed in E. coli as GST fusion protein, provided by Dr. W. Becker), DYRK1A (human, recombinant, expressed in E. coli as GST fusion proteins), DYRK4 (human, recombinant, expressed in insect cells), CLK1, 2, 3, and 4 (mouse, recombinant, expressed in E. coli as GST fusion proteins) was assayed as described for CDK1/cyclin B with 1 μg of RS peptide (GRSRSRSRSRSR) as a substrate. Native DYRK1A was purified from rat brain, taking advantage of the natural poly-histidine sequence located in the C-terminal domain of DYRK1A, by affinity chromatography on cobalt-sepharose beads (Clontech). Briefly, after a 30-min preclearing incubation at 4 • C with sepharose beads, rat brain lysates were incubated with cobaltsepharose beads (400 μg total proteins/20 μL beads). Kinase activity of native DYRK1A was directly assessed on the beads in buffer A (+0.5 mg BSA/mL) using the Woodtide substrate (KKISGRLSPIMTEQ).

GSK-3α/β (porcine brain, native, affinity purified on axin-sepharose beads), GSK-3α and GSK3β (human, recombinant, expressed in insect cells) and PfGSK-3 (Plasmodium falciparum, recombinant, expressed in E. coli) were assayed as described for CDK1/cyclin B, but using a GSK-3 specific substrate (GS-1: YRRAAVPPSPSLSRHSSPHQpSEDEEE, where pS stands for phosphorylated serine) [START_REF] Primot | Purification of GSK-3 by affinity chromatography on immobilized axin[END_REF].

Casein kinase 1 (CK1δ/ε) (porcine brain, native) was assayed with 0.67 μg of CKS peptide (RRKHAAIGp-SAYSITA), a CK1 specific substrate [START_REF] Reinhardt | Purification of CK1 by affinity chromatography on immobilised axin[END_REF].

CLK1 (mouse, recombinant, expressed in E. coli as GST fusion proteins) was assayed in buffer A (+0.15 mg BSA/mL) with RS peptide (GRSRSRSRSRSR) (1 μg/assay).

Fig. 1 Fig. 2 N

 12 Fig. 1 Structures of some bioactive 5-arylidene rhodanine derivatives (I-VI) and leucettine L 41 (VII) Fig. 2 N -3-substituted 5-ylidene rhodanine derivatives based on dispacamide A model

  in water at 95 • C for 22 h produced 3-(arylsulfonylamino)rhodanine 2e in 41 % yield after purification by crystallization from ethanol.

Table 2 aFig. 3 Table 3

 233 Fig. 3 Ortep diagram of 4,5-dibromo-1H -pyrrol-2carboxylic acid [3-(4-oxo-2-thioxo-thiazolidin-5-ylidene)-propyl]-amide 3a obtained by X-ray diffraction

  ing from N -(4,5-dihalogeno-pyrrol-2-yl) carbamoyl aldehydes 1(a, b) and N -substituted rhodanines 2(a-i) partners.

  H NMR (DMSO-d 6 ) δ: 4.75 (s, 2H, H-5 , CH 2 ); (m, 2H, H-3, Ar); 8.13 (m, 1H, H-4, Ar); 8.37 (m, 2H, H-3, Ar); 11.31 (br s, 1H, NH). 13 C NMR (DMSO-d 6 ) δ: 32.95 (C-5 , CH 2 S); 127.23 (C-2, Ar); 128.97 (C-3, Ar); 133.34 (C-4, Ar); 140.78 (C-1, Ar); 170.30 (C-4 , C=O); 199.40 (C-2 , C=S). HRMS, m/z: 310.9594 found (calculated for C 9 H 8 N 2 O 3 NaS 3 [M+Na] + requires 310.9594).

5 -

 5 Dibromo-1H-pyrrol-2-carboxylic acid {3-[4-oxo-3

( 2 (

 2 C 11 H 9 Br 2 N 3 O 2 S 2 )); M = 878.3. APEXII, Bruker-AXS diffractometer, Mo-Kα radiation (λ = 0.71073 Å), T = 150(2) K; triclinic P-1 (I.T.#2), a = 7.0648(6), b = 14.9877(13), c = 17.6708(13)Å, α = 75.216(3), β = 87.475(3), γ = 78.233(3) • , V = 1771.1(3) Å 3 , Z = 2, d = 1

Table 1

 1 Results for the preparation of N -substituted rhodanines 2c

, 2(e-i)

	Compound	Structure	Yield a (%)	Compound	Structure	Yield a (%)
	2c		S	44	2g		S	O	98
		S	N	CO 2 H		S	N	N	O
								H
			O				O
	2e			41	2h			98

  • C for 2 h. The product 2c (503 mg, 44 % yield) was further used without purification. Mp = 162-164 • C. 1 H NMR (DMSO-d 6 ) δ: 4.05 (m, 4H, H-3, CH 2 ); 4.22 (s, 2H, H-5 , CH 2 ). 13 C NMR (DMSOd 6 ) δ: 30.57 (C-2, CH 2 CO); 35.90 (C-3, NCH 2 ); 39.57 (C-5 , CH 2 S); 171.04 (C-1, C=O); 174.04 (C-4 , C=O); 202.96 (C-2 , C=S). HRMS, m/z: 249.9585 found (calculated for C 6 H 6 NO 3 Na 2 S 2 [M-H+2Na] + requires 249.9585).

  H NMR (DMSO-d 6 ) δ: 4.53 (s, 2H, H-5 , CH 2 ); 7.56 (m, 2H, H-3, Ar); 7.66 (m, 1H, H-4, Ar); 7.93 (m, 2H, H-2, Ar); 11.56 (br s, 1H, NH). 13 C NMR (DMSO-d 6 ) δ: 33.43 (C-5 , CH 2 S); 127.75 (C-2, Ar); 128.72 (C-3, Ar); 130.90 (C-1, Ar); 132.76 (C-4, Ar); 164.44 (NHC=O); 170.42 (C-4 , C=O); 200.14 (C-2 , C=S). HRMS, m/z: 274.9927 found (calculated for C 10 H 8 N 2 O 2 NaS 2 [M+Na] + requires 274.9925).

• C.

1

  C=S). HRMS, m/z: 305.0031 found (calculated for C 11 H 10 N 2 O 3 NaS 2 [M+Na] + requires 305.0030).

	C
	NMR (DMSO-d 6 ) δ: 33.34 (C-5 , CH 2 S); 55.48 (CH 3 O);
	113.95 (C-3, Ar); 122.96 (C-1, Ar); 129.79 (C-2, Ar); 162.67
	(C-4, Ar); 163.79 (NHC=O); 170.51 (C-4 , C=O); 200.29
	(C-2 ,

  C=S). HRMS, m/z: 289.0082 found (calculated for C 11 H 10 N 2 O 2 NaS 2 [M+Na] + requires 289.0081).

	• C for 3 h. The desired product 2h was
	obtained as yellowish powder in 98 % yield (538 mg) and
	was further used without purification. Mp = 172-174 • C.
	1 H NMR (DMSO-d 6 ) δ: 3.66 (s, 2H, CH 2 ); 4.40-4.41 (br
	s, 2H, H-5 , CH 2 S); 7.31-7.32 (m, 5H, H-2, H-3, H-4, Ar);
	11.41 (br s, 1H, NH). 13 C NMR (DMSO-d 6 ) δ: 33.38 (C-
	5 , CH 2 S); 126.52 (C-4, Ar); 128.19 (C-3, Ar); 129.11 (C-2,
	Ar); 134.65 (C-1, Ar); 168.39 (NHC=O); 170.18 (C-4 C=O);
	199.99 (C-2 ,

  371.9411 found (calculated for C 11 H 9 N 3 O 35 2 Cl 2 NaS 2 [M+Na] + requires 371.9411).

	C=O); 167.48 (C-4 , C=O); 195.94 (C-2 , C=S). HRMS,
	m/z:
	C NMR (DMSO-d 6 ) δ: 32.26 (CH 2 CH=); 36.80
	(NHCH 2 ); 107.92 (C-4, =CCl, Ar); 109.65 (C-3, =CH, Ar);
	114.94 (C-4 , C=O); 124.61 (C-5, =CCl, Ar); 124.74 (C-
	5 , C=CH; 130.38 (C-2, Ar); 134.96 (=CH); 159.12 (NH-

  H NMR (DMSO-d 6 ) δ: 2.45 (m, 2H, CH 2 CH=); 2.56 (m, 2H, CH 2 CO); 3.41 (m, 2H, NHCH 2 ); 4.15 (t, 2H, J = 7.8 Hz, 2H, NCH 2 ); 6.89 (s, 1H, H-3, =C-H, Ar); 7.01 (t, 1H, J = 7.5 Hz, C=CH); 8.32 (t, 1H, J = 5.5 Hz, 1H, NHCO); 12.72 (br s, 1H, H-1, NH, Ar). 13 C NMR (DMSO-d 6 ) δ: 30.70 (CH 2 CH=); 32.23 (CH 2 CO); 36.82 (NCH 2 ); 97.80 (C-4, =CBr, Ar); 104.72 (C-5, =CBr, Ar); 112.60 (C-3, =CH, Ar); 127.17 (C-5 , C=CH); 127.87 (C-2, Ar); 136.61 (CH=); 159.04 (NH-C=O); 164.86 (C-4 , C=O); 171.67 (CO 2 H); 193.65 (C-2 , C=S). HRMS, m/z: 507.8636 found (calculated for C 14 H 12 N 3 O 79 4 Br 2 S 2 [M-H] -requires 507.8636).

  DMSO-d 6 ) δ: 2.45 (m, 2H, CH 2 CH=); 3.41 (m, 2H, NCH 2 ); 5.99 (m, 1H, C=CH); 6.95 (s, 1H, H-3, Ar); 8.28 (m, 1H, NH-CO); 12.79 (br s, 1H, NH, Ar). HRMS, m/z: 452.8693 found (calculated for C 11 H 11 N 4 O 79 2 Br 2 S 2 [M+H] + requires 452.8690).

• C for 1 h. 3e was obtained as a yellowish powder in 71 % yield (100 mg).

1

H NMR (

Acknowledgments One of us (S.G.) wishes to thank the "Ministère de la Recherche et de l'Enseignement Supérieur" for research fellowships. Financial support of this program carried out under the French National Cancer Institute "Cancéropôle Grand Ouest" by contracts PRIR 04-8390 and ACI 04-2254, is gratefully acknowledged.