Yohan Champouret

K Cory

Cory Macleod

Kevin M Smith

Brian O Patrick

Rinaldo Poli

K Cory Macleod

Controlled Radical Polymerization of Vinyl Acetate with Cyclopentadienyl Chromium β-Diketiminate Complexes: ATRP vs OMRP

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

There has been considerable interest in recent years in the controlled polymerization of vinyl acetate (VAc), since the resulting polymers as well as statistical and block copolymers have many different applications, related to the subsequent conversion of the hydrophobic acetate functions to hydrophilic alcohol functions. This monomer can only be polymerized by the radical pathway but controlled chain growth is difficult due to the relatively high reactivity of the propagating radical species compared to more tractable substrates such as styrene or acrylates. 1 Among various methods, comprising degenerate transfer with dithiocarbamates, 2,3 xanthates, 4,5 and organotellurium and -antimony compounds, 6 the use of cobalt complexes either in a degenerative transfer mode 7,8 or in a reversible termination mode 9,10 (organometallic mediated radical polymerization, or OMRP) has proven to afford the best level of control, with polydispersity indexes below 1.1 in certain cases. 11 In the OMRP mechanism, unwanted bimolecular radical coupling reactions are effectively prevented by the reversible formation and homolytic cleavage of a metal-alkyl bond (Scheme 1). 12 L y /Mt x+1 -P n L y /Mt x + P n k t bimolecular terminations

k p + M k act k deact

Scheme 1

On the basis of the well established ability of Cr II complexes such as [Cr(H2O)6] 2+ or solvated CrCl2 to effectively trap organic radicals at close to diffusion controlled rates to form Cr III -alkyl species, 13 we have recently tested a few high spin organometallic Cr II complexes of type CpCr(nacnac Ar,Ar' ), where nacnac Ar,Ar' is a bulky aryl-substituted -diketiminato ligand, ArNC(CH3)CHC(CH3)NAr', as moderating agents in VAc polymerization and have shown that they can indeed trap the growing PVAc radical chains effectively and reversibly. 14,15 The polymerization at 30°C initiated by the thermally labile diazo compound 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile) (V-70, t1/2 = 10 h) in the presence of the more sterically hindered complex with Ar = Ar' = C6H3iPr2-2,6 or Dipp, 1, was relatively fast (70% conversion in 46 h using VAc/V-70/1=500/0.8/1), albeit much slower than the control experiment carried out without Cr complex, but poorly controlled (Mw/Mn = 1.8) consistent with a relatively slow trapping rate. The less sterically hindered system with Ar = Ar' = C6H3Me2-2,6 or Xyl, 2, on the other hand, revealed a much less reversible OMRP trapping process, since polymerization stopped at a conversion of ca. 12% at 50°C and did not progress further even at 90°C. 14 Further work using the neopentyl compound CpCr III (nacnac Xyl,Xyl )(CH2CMe3) as a single-molecule initiator, however, showed activation and sustained (albeit very slow) polymerization at room temperature with better control (Mw/Mn = 1.46 at 14% conversion after 400 h), but complete deactivation occurred at higher temperatures (> 50°C), this being traced to a decomposition process with formation of the acetate complex

CpCr III (nacnac Xyl,Xyl )(O2CCH3). 15 We are also interested in the interplay between ATRP and OMRP mechanisms for controlled chain growth. 12 When the metal complex in the reduced state is able to abstract a halogen atom from a halogen-terminated dormant chain, yielding the chain carrying radical and an oxidized metal complex, a moderating equilibrium based on this atom transfer process can also be installed. For the half-sandwich CpCr(nacnac) system, this ATRP activation/deactivation equilibrium is possible since the corresponding chloro complexes of Cr III are known, [15][16][17] notably

CpCr III (nacnac Ar,Ar' )Cl with Ar = Ar' = Dipp (3) 16 or Xyl (4). 15 The first such interplay was reported for the polymerization of styrene mediated by half-sandwich Mo complexes, CpMo III Cl2L2 (L2 = (PMe3)2, dppe, or  4 -C4H6). 18 The interplay of the two trapping processes is schematically shown in the free energy diagram of Figure 1. When the polymerization is carried out under OMRP conditions (either from the reduced complex Mt n and an external radical source such as V-70, or from an organometallic initiator Mt n+1 -R0), no interplay with ATRP can occur if the metal complex is devoid of halogen ligand. This is the case for the above mentioned VAc polymerization mediated by complexes 1 and 2. On the other hand, when the polymerization is carried out under ATRP conditions (Mt n + R0-X), or the so-called "reverse" ATRP conditions (Mt n+1 -X and an external source of R0 • such as V-70), then the active chain may be trapped both by the Mt n+1 -X complex generated during the activation (under ATRP conditions) or initially present (under reverse ATRP conditions), giving rise to the ATRP equilibrium, or by Mt n initially present (under ATRP conditions) or generated by the initial ATRP trapping (under reverse ATRP conditions), giving rise to the OMRP equilibrium. For each type of activation/deactivation equilibrium, three energetic parameters are of importance: a thermodynamic parameter G and the two activation barriers G ‡ act and G ‡ deact for the activation and deactivation process (G = G ‡ act -G ‡ deact), as shown in Figure 1a.

R • +Mt n -Mt n Mt n+1 -R -Mt n+1 -X +Mt n+1 -X Mt n + R-X (b) (a)
ATRP OMRP When ATRP trapping is thermodynamically more favorable (case of Figure 1a), the dormant chain prevalently contains halogenated chain ends. Different situations have been described, where G ATRP >>> G OMRP , as in the case of copper systems where control is essentially only insured by ATRP, 19 or GATRP > GOMRP (ATRP dormant species marginally more stable than the OMRP dormant species), as in the case of some molybdenum 18 and osmium systems. 20,21 In the latter cases, the two trapping equilibria cooperate during the ATRP process and a more or less pronounced degree of control can also be observed under pure OMRP conditions.

G ATRP G OMRP G ‡ act,ATRP G ‡ deact,ATRP G ‡ deact,OMRP G ‡ act,OMRP
It is also possible to envisage, however, a case where OMRP trapping dominates (GATRP < GOMRP), as shown in Figure 1b. This situation has not yet been highlighted, to the best of our knowledge. We would like to present here the first example of this situation. Chromium(II) complexes are known to possess a strong reducing power, thus the Cr III -Cl bond may be expected to be rather strong in a homolytic sense, compared for instance to a Fe III -Cl or Cu II -Cl bond. In turn, this is expected to reduce the GATRP value for a given R-X system, since HATRP = [BDE(R-X)-BDE(Mt n+1 -X)] 12 while SATRP should be small. Vinyl acetate is a monomer that leads to rather strong C-X bonds for X = Cl or Br. 22 In order to activate dormant chains with strong C-X bonds, metal complexes capable of yielding stronger Mt n+1 -X bonds are necessary. Under these circumstances, stronger Mt n+1 -C bonds should also be formed, likely leading to irreversible OMRP trapping, unless the Mt n+1 -C bond strength is attenuated by steric interactions and this is indeed the case for the CpCr II (nacnac) system, as demonstrated in our previous contributions. 14,15 Experimental Section Materials. All procedures, unless otherwise stated, were carried out under dry, oxygen-free argon or nitrogen, using standard Schlenk and glove box techniques.

Solvents were dried by using the method of Grubbs 23 or distilled under argon from appropriate drying agents and degassed by three freeze-vacuum-thaw cycles prior to use. [START_REF] Armarego | Purification of Laboratory Chemicals[END_REF] Methyl propionate, phenacyl chloride and KN(SiMe3)2 were purchased from Aldrich and used as received. Complexes CpCr II (nacnac Ar,Ar ) (Ar = Dipp, 1; Xyl, 2), 14,16 CpCr III (nacnac Ar,Ar )Cl (Ar = Dipp, 3; Xyl, 4), 15,16 CpCr III (nacnac Xyl,Xyl )I, 17 and CH3CH(Cl)OOCCH3 [START_REF] Zakrzewski | [END_REF] were prepared according to literature procedures.

General procedures for the radical polymerization of vinyl acetate. (a)

ATRP procedure: Cr II + methyl 2-chloropropionate. All polymerizations were conducted following the same experimental protocol. As a representative example, the Schlenk tube was charged with complex 1 (36 mg, 0.086 mmol, 1 equiv) and methyl 2-chloropropionate (5 L, 0.043 mmol, 0.5 equiv), followed by the addition of degassed vinyl acetate (4 mL, 43 mmol, 500 equiv). The Schlenk tube was degassed by three freeze-vacuum-thaw cycles and then immersed in an oil bath preheated at 50°C. At the desired time, the Schlenk flask was rapidly cooled to room temperature by immersion into iced water before sample withdrawal. The monomer conversion was determined gravimetrically after removal of the unconverted monomer under reduced pressure and the resulting residue was used for SEC characterization.

(b) Reverse ATRP procedure: Cr III Cl + V-70. All polymerizations were conducted following the same experimental protocol. A typical experiment is described here as a representative example with complex 4, VAc and 0.5 eq. of V-70 (Cr:V-70:VAc=1:0.5:500). All operations were carried out under a protective argon atmosphere. Complex 4 (39.3 mg, 0.086 mmol, 1 equiv) and V-70 (13.2 mg 0.043 mmol, 0.5 eq.) were introduced in a Schlenk tube, followed by the addition of degassed vinyl acetate (4 mL, 43 mmol, 500 equiv). The Schlenk tube was degassed by three freeze-vacuum-thaw cycles and then immersed in an oil bath preheated at 50 °C. At the desired time, the Schlenk flask was rapidly cooled to room temperature by immersion into iced water before sample withdrawal. The monomer conversion was determined gravimetrically after removal of the unconverted monomer under reduced pressure, and the resulting residue was used for SEC characterization. with CpCr III (nacnac Xyl,Xyl )I (52.9 mg, 0.0963 mmol) dissolved in toluene (4 mL) in a

Schlenk flask overnight at room temperature. The solvent was evaporated in vacuo, the residue was extracted with hexanes (2 mL), filtered through Celite and cooled to -35 ˚C for 3 days to yield crystals of 5 (5.6 mg, 11 %) suitable for x-ray crystallographic analysis. Due to the presence of co-crystallized iodide starting material, satisfactory elemental analysis for 5 was not obtained. UV/vis (hexanes; λmax, nm (ε, M -1 cm -1 )): 407 (7600), 607 (500).

X-ray characterization of compound 5.

A single crystal of compound 5 was mounted on a glass fiber and centered on the optical path of a Bruker X8 APEX II diffractometer with graphite-monochromated Mo Kα radiation. The data were collected at a temperature of -100.0 ± 0.1 ˚C in a series of  and  scans in 0.5˚ oscillations. Data were collected and integrated using the Bruker SAINT software package [START_REF] Saint | Bruker Analytical X-ray System[END_REF] and were corrected for absorption effects using the multiscan technique (TWINABS) [START_REF] Twinabs | Bruker Nonius scaling and absoption for twinned crystals -V2008/2[END_REF] and for Lorentz and polarization effects. The structure was solved by direct methods with SIR92. [START_REF] Altomare | [END_REF] Compound 5 crystallizes with 10 % CpCr III (nacnac Xyl,Xyl )I starting material in the crystal lattice. The material crystallizes as a two component split crystal with the two components related by a 33˚ rotation about the (1 0 0) real axis. Data were integrated for both twin components, including both overlapped and non-overlapped reflections. The structure was solved using non-overlapped data from the major twin component. Subsequent refinements of 5

were carried out using an HKLF 5 format data set containing complete from component 1 and all overlapped reflections from component 2. The batch scale refinement showed a roughly 82:18 ratio between the major and minor twin components. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions but were not refined. All refinements were performed using the SHELXTL crystallographic software package of Bruker-AXS. [START_REF] Sheldrick | version 5.1[END_REF] The molecular drawing was generated by the use of ORTEP-3. 31

Computational details. QM/MM calculations were carried out by use of the Gaussian03 suite of programs 32 with use of the B3LYP functional 33 CpCr II (nacnac Ar,Ar ), on the organometallic dormant complexes CpCr III (nacnac Ar,Ar )-(CHMeOAc), and on the transition states between them for (CHMeOAc) trapping (Ar = Ph, Xyl, Dipp) have previously been described at the same level of theory. 15 Frequency calculations confirmed the nature of the optimized geometries as global energy minima. The electronic energies were corrected by the ZPVE term and converted to enthalpy and to Gibbs free energies at 25°C on the basis of the ideal gas and harmonic oscillator approximations.

Results

(a) ATRP of VAc mediated by complexes 1 and 2

Polymerizations of VAc were carried out in the presence of compounds 1 or 2 as catalysts under standard ATRP conditions with CH3CH(Cl)COOCH3 as initiator.

Compound CH3CH(Cl)COOCH3 was chosen as an initiator because its C-Cl bond is weaker than that of the ATRP PVAc-Cl dormant chain, 22 thus ensuring a fast consumption of initiator. Although bromide compounds are typically preferred as initiators for ATRP, we selected a reactive chloro derivative in this case because the expected product of atom transfer, the Cr III chloride complexes 3 and 4, also exist and can been independently used to install the same atom transfer equilibrium by reverse ATRP (vide infra). The operating conditions and conversion results are given in Figure 2. Note that these reactions were carried out at 50°C. In the presence of 1 equiv of initiator per Cr II atom, monomer consumption was initially rapid but subsequently reached a plateau at 26% and 16% conversion with compounds 1 and 2, respectively. The efficiency factor is very low (ca. 0.11 in both experiments) and the polymer obtained has a relatively narrow polydispersity (Mw/Mn in the range Polymerizations were also carried out with compounds 1 and 2 under similar conditions with 0.5 equiv of initiator per Cr II but now working at room temperature.

For both compounds greater conversions were attained relative to the experiments at 50°C. As shown in Figure 3 the monomer consumption is initially rapid but slows over time, although polymerization does not stop, whereas no sustained polymerization had been observed at 50°C beyond 3.5 h (Figure 2). These results

suggest the occurrence of a thermal decomposition pathway at 50°C, which is not prevalent at room temperature. This behavior parallels that of the same polymerization carried out under OMRP conditions and hence suggests that a switch to OMRP control occurs (see Discussion). Like at 50°C, the conversion attained with 1 are greater than with 2. The Mn deviated little from the theoretical values (Mn/Mn,th up to a maximum of 2) and the Mw/Mn gradually increased throughout the duration of the reaction to a final value of 2.4 with 1 and 1.75 with 2. Cr-O bond (Scheme 2). In order to independently synthesize one of these complexes in the absence of 4, methyl propionate was deprotonated with KN(SiMe3)2, 26 and the isolated potassium enolate was reacted with CpCr(nacnac Xyl,Xyl )I. 17

Scheme 2

In contrast with the above fast reactions, compound 2 reacted much more slowly with one equiv of either 1-chloroethyl acetate or chloromethyl pivalate at room temperature (significant conversion after 24 h), and no reaction of 2 with neopentyl chloride was observed at room temp or at 50°C (UV/Vis). In these cases, the UV/Vis monitoring gave clear evidence of the formation of 4 and a Cr III alkyl product (broad band at ~550 nm). Interestingly, whereas the absorption assigned to the putative CpCr(nacnac Xyl,Xyl )[CH(OOCCH3)CH3] product decreased in intensity at extended reaction times, that assigned to the putative CpCr(nacnac Xyl,Xyl )[CH2OOCtBu] product maintained its intensity unaltered, suggesting greater stability for the alkyl compound The calculations indicate that the ATRP dormant state is less favored than the OMRP dormant state for all Ar substituents at the Eel and H levels. Only when introducing the gas-phase (-TS) term, i.e. going to the G level, the stability order is inverted (only for Xyl and Dipp systems). In solution, however, the major portion of the entropy term (translational + rotational) is partially quenched by the solvent lattice and the actual change of G is expected to differ from the gas-phase value and to be closer to the H value. The calculations also indicate that the steric effect operated by the Ar substituents is more pronounced on the OMRP activation/deactivation equilibrium (difference between the Ph and Dipp systems of 7.3, 8.6 and 10.8 kcal mol -1 at the Eel, H, and G levels) than on the ATRP equilibrium (corresponding differences of 1.3, 6.2 and 6.9 kcal mol -1 ), because the smaller size of the Cl atom relative to the CH(CH3)OAc fragment induces a less severe rearrangement of the coordination sphere on going from Cr II to Cr III . A greater steric hindrance disfavors the active state for the ATRP equilibrium and the dormant state for the OMRP equilibrium, so GATRP increases in the order Ph < Xyl < Dipp (the same holds for HATRP, whereas an inversion is predicted at the Eel,ATRP level, see Figure 6) and GOMRP increases in the opposite order and by a greater extent. Finally, the calculations indicate that the energetic barrier leading from the active radical to the ATRP dormant state is much greater than that leading to the OMRP dormant state.

Whereas the latter is almost non existent at the E ‡ el and H ‡ levels (negative values are obtained after the PV correction for the Ph and Dipp systems) and in the 10-15 kcal mol -1 range at the gas-phase G ‡ level, the former is ca. 10 kcal mol -1 at the E ‡ el and H ‡ levels and > 20 kcal mol -1 at the G ‡ level. The geometries obtained for the ATRP transition states are very similar for the three systems and little dependent on the aryl steric requirements. As the steric bulk increases, the transition state is earlier along the Cl atom transfer coordinate, with a shorter Cr-Cl and a longer Cl-C distance, because of the greater steric push exerted by the nacnac ligand while regaining its more relaxed conformation in the 2-legged piano stool configuration of the Cr II complex.

The difference between OMRP and AT RP trapping barriers at the H ‡ level can be easily rationalized since OMRP trapping involves a simple bond making process and is therefore only requiring a small coordination sphere reorganization to allow the accommodation of the new ligand, whereas ATRP trapping involves simultaneous bond breaking and making processes. The PVAc • growing chains are therefore predicted to be trapped much more rapidly by the Cr II complex than by the Cr III -Cl complex. Note that, although the (-TS) term has nearly no effect on the thermodynamics of the ATRP equilibrium (HATRP  GATRP) because the reaction conserves the number of molecules, the activation process is very much affected by the entropy term because of the associative character of the atom transfer process. In conclusion, OMRP trapping is both thermodynamically and kinetically favored over ATRP trapping.

We have also explored the activation of compound CH3CH(Cl)COOCH3 by The adduct formation between the CH3CHCOOCH3 radical and complex 2 provides a stabilization energy of -15.8 kcal mol -1 for the C bonded form and -17.6 kcal mol -1 for the O-bonded (-methoxy enolate) form. On the H scale, these bond energies become -9.4 and -14.3 kcal mol -1 , respectively. Hence, the enolate form is calculated as the more stable one. Both isomers, however, have a smaller bond dissociation energy than the OMRP dormant species of the PVAc model system (CH3CHOOCCH3 radical).

Discussion

The polymerization results show that the ATRP initiator CH3CH(Cl)COOCH3 is activated by CpCr II (nacnac) (compounds 1 and 2) to start a slow polymerization of VAc. The initiator is also found to react with 2 in the absence of monomer, to presumably yield a mixture of 3 and the enolate complex CpCr(nacnac Xyl,Xyl )[OC(OCH3)=CHCH3] (Scheme 2). Indeed, the DFT calculations show that the enolate is more stable than the C-bonded form of the CH3CHCOOCH3 radical. In the presence of VAc, the primary CH3(COOCH3)CH • radical has the choice to either react with a molecule of CpCr II (nacnac) to irreversibly yield the Cr III enolate, or add to monomer to start the PVAc chain growth. The very low initiator efficiency observed for the process is likely related to the competitive trapping by chromium and indeed complex CpCr(nacnac Xyl,Xyl )[OC(OCH3)=CHCH3] was shown to be incapable to reinitiate polymerization. This low initiator efficiency factor could not be caused by extensive radical coupling termination during initiation, because a significant build-up of Cr II , which would necessarily accompany such irreversible terminations, was not observed for these reactions.

Once the primary radical has started a PVAc chain, reversible trapping of this chain occurs as indicated by the sustained polymerization, even though the level of control is poor. Thus, a dormant species is formed. Of the two possible trapping processes, namely ATRP and OMRP, the latter must be preferentially operating, as indicated by a combination of experimental and theoretical evidence. On the experimental side, the reaction between compound 2 and CH3CH(Cl)OAc, which has a C-Cl bond strength very close to that of the putative PVAc-Cl dormant species, was shown to be relatively slow. The evolution of the polymerization rate at 25°C (Figure 3) is in fact quite similar to that of the corresponding polymerization initiated by CpCr III (nacnac Xyl,Xyl )(CH2CMe3) under the same conditions, which is necessarily controlled by the OMRP equilibrium. 15 The decrease of the polymerization rate with time and the deactivation at higher temperature have been completely rationalized in the previous OMRP study as resulting from occasional head-to-head monomer insertions followed by acetate group transfer, leading to the inactive CpCr(nacnac Xyl,Xyl )(OAc) compound. 15 Hence, the trend of polymerization rate is additional persuasive evidence that the VAc polymerization is moderated by the OMRP equilibrium also when it is initiated by the ATRP initiator. It is not surprising that similar results are also obtained under reverse ATRP conditions.

The computational results confirm the above conclusion and allow other interesting considerations. Faster activation of the more active methyl

Conclusions

This contribution has served to illustrate the general principle that interplay between atom transfer and metal-carbon bond formation for the radical trapping in controlled radical polymerization may thermodynamically favor formation of the organometallic dormant species and has shown the first reported example where this is proven to occur. The contribution also shows, by computation of the activation barriers of the two competitive radical trapping processes, that the metal-carbon bond formation is greatly preferred for the kinetic standpoint. Although this is intuitively obvious, a clear statement to this effect and the explicit calculation of the two competitive barriers had not yet been reported to the best of our knowledge.

The system investigated here has no significant value for the practical development of efficient controlled polymerization protocols for vinyl acetate, a challenging monomer, because of a variety of problems. The low initiator efficiency factors related to the competitive formation of Cr III enolates might be easily eliminated through the use of other sources of stabilized radicals devoid of conjugated O atoms, for instance PhCH(Cl)CH3. Sustained polymerization is however hampered by acetate transfer to the oxophilic metal center. This process has so far not been observed for cobalt systems, which appear so far the best ones for controlling the radical polymerization of this monomer. However, the half-sandwich Cr system reported here has been useful to better appreciate the scope of the ATRP-OMRP interplay.

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Use of a half-sandwich Cr II /Cr III system to control the radical polymerization of vinyl acetate has revealed for the first time that the interplay between two moderating mechanism, atom transfer radical polymerization (ATRP) and organometallic mediated radical polymerization (OMRP), may lie in favor of the latter, both kinetically and thermodynamically.

Figure 1 .

 1 Figure 1. Schematic energy diagrams for the interplay of ATRP and OMRP trapping. (a) Case of dominant ATRP. (b) Case of dominant OMRP.

( c )

 c From a Cr III -enolate complex. Compounds CpCr III (nacnac Xyl,Xyl )I (151 mg, 0.275 mmol) and K[OC(OCH3)=CHCH3] (42.3 mg, 0.335 mmol, 1.22 equiv) were placed in a Schlenk flask followed by the addition of toluene (15 mL). The mixture was stirred overnight at room temperature at which point the solvent was evaporated in vacuo. The residue was extracted with hexanes (10 mL) and filtered through Celite (UV/vis: λmax = 397 nm, 635 nm). The green solution was cooled to -35 ˚C to yield 25.1 mg of compound CpCr III (nacnac Xyl,Xyl )[OC(OCH3)=CHCH3] (18%). This batch was then dissolved in VAc (2 mL). Stirring at room temperature for 54 h and then warming to 50°C for 21.5 h did not result in any polymerization process.Synthesis of compound CpCr(nacnacXyl,Xyl )[OC(Ph)=CH2], 5. A toluene (1 mL) suspension of K[OC(Ph)=CH2] (16.9 mg, 0.107 mmol, 1.11 equiv) was reacted

  within the DFT methodology for the QM part and of the UFF34 for the MM part. The basis set chosen for the QM calculations comprised the 6-31G* set for the H, C, N, O and Cl atoms, and the SDD set, which includes a pseudopotential, augmented by an f polarization function with the optimized35 1.941 coefficient for the Cr atom. The cutoff between the QM and MM parts was placed at the level of the Ar-N bonds, with the NC(Me)CHC(Me)N diketiminato moiety being treated quantomechanically together with the Cr atom, the Cp ring, the Cl ligand and the CH(OAc)CH3 fragment, while the entire aryl substituents were handled at the MM level. The input geometry of compound 3 was obtained from the crystallographically characterized compound15 and those of the related systems with Ar = Ph or Dipp were adapted from it. For the Cl-atom transfer transition states, the input geometries were constructed following an initial scan of the Cr-Cl distance with the Cl-CH(OAc)CH3 molecule being initially placed next to the CpCr II (nacnac Ar,Ar ) system in a direction suitable for the eventual formation of the Cr-Cl bond, with a linear Cr•••Cl-C angle. Spin contamination was negligible, all calculations converging with <S 2 > close to the expected values(3.75 for the quartet state of Cr III , 6.00 for the quintet state of the atom transfer transition states). The maximum deviations were 3.7966 for the quartet states (Ar = Dipp) and 6.0848 for the quintet state (Ar = Dipp). The calculations on the Cr II complexes

Figure 2 .

 2 Figure 2. Conversion as a function of time for the VAc polymerization initiated by CH 3 CH(Cl)COOCH 3 (In-Cl) in the presence of compounds 1 and 2. Conditions: VAc/Cr II /CH3CH(Cl)COOCH3 = 500:1:(1 or 0.5). T = 50°C.

Figure 3 . 4

 34 Figure 3. Conversion as a function of time for the VAc polymerization initiated by CH 3 CH(Cl)COOCH 3 (In-Cl) in the presence of compound 1 (diamonds) or 2 (squares). Conditions: VAc/Cr II /CH3CH(Cl)COOCH3 = 500:1:0.5. T = room temp.

Figure 4 .

 4 Figure 4. Conversion as a function of time for the VAc polymerization initiated by V-70 in the presence of compound 3 (a) or 4 (b). Conditions: VAc/Cr/V-70

  Xyl,Xyl )[OC(OCH3)=CHCH3] could not be obtained, the corresponding reaction sequence with acetophenone provided suitable crystals of CpCr(nacnac Xyl,Xyl )[OC(=CH2)Ph], 5. Complex 5 co-crystallized with ~10% of unreacted Cr(III) iodide starting material, a problem that was also observed in the structure of CpCr(nacnac Dep,Dep )(CH3).17 However, the structure confirmed the bonding mode of the organic fragment as an enolate bound through oxygen. The large Cr-O-C bond angle of 5 (Figure5) is consistent with an O-bound enolate ligand as opposed to a C-bound ligand which would be expected to have a considerably smaller bond angle. In addition, the two hydrogen atoms bound to C27 were clearly visible during structural refinements of the crystallographic data for compound 5 and consistent with sp 2 hybridization at the carbon atom. As the UV/Vis spectra of both Xyl,Xyl )[OC(OCH3)=CHCH3] complex also has a Cr-O bond. Isolated CpCr(nacnac Xyl,Xyl )[OC(OCH3)=CHCH3] did not initiate the polymerization of vinyl acetate, at room temp or when heated to 50°C.

Figure 5 .

 5 Figure 5. Thermal ellipsoid diagram (50 %) of compound 5. Compound 5 co-crystallizes with approx. 10 % iodide starting material; the I atom of the co-crystallized starting material and all H atoms are omitted for clarity. Selected bond lengths (Å): Cr(1)-N(1), 2.007(3); Cr(1)-N(2), 2.023(3); Cr(1)-CNT, 1.925; Cr(1)-O(1), 1.878(4); O(1)-C(28), 1.367(18); C(27)-C(28), 1.37(3). Selected bond angles (deg): N(1)-Cr(1)-N(2), 89.61(13); N(1)-Cr(1)-O(1), 90.26(15); N(2)-Cr(1)-O(1), 91.07(15); CNT-Cr(1)-O(1), 126.17; Cr(1)-O(1)-C(28), 145.9(12).

Figure 6 .

 6 Figure 6. Energy diagram for the ATRP and OMRP activation/deactivation involving the CpCr II (nacnac Ar,Ar ) and CH3CH(Cl)OAc systems (Ar = Ph, red; Xyl, blue; Dipp, purple). For each localized energy minimum or transition state, the three given numbers indicate respectively the change of electronic energy (Eel), enthalpy (H) and Gibbs free energy (G) in kcal/mol. The data are plotted on the Eel scale.

Figure 7 .

 7 Figure 7. Geometries of the transition states for the ATRP activation/deactivation process involving the Ph (above), Xyl (middle) and Dipp (below) systems.

complex 2 ,

 2 as well as the relative stability of the C-bonded and O-bonded Cr III complexes formed by addition of the CH3CHCOOCH3 radical to 2, namely compounds CpCr(nacnac Xyl,Xyl )(CH(COOCH3)CH3) and CpCr(nacnac Xyl,Xyl )-(OC(OCH3)=CHCH3). We limited these calculations to the thermodynamics, without optimization of the relevant transition states. The relative energy of 2 + CH3CH(Cl)COOCH3 with respect to 4 + CH3CHCOOCH3, is +1.4, -0.8 and -1.6 kcal mol -1 at the E, H, and G levels, respectively. The more favorable activation of CH3CH(Cl)COOCH3 relative to CH3CH(Cl)OOCCH3 (Figure6) is caused by its weaker C-Cl bond (BDE of -68.4 for the former vs. -79.0 for the latter). Even though the transition state for the Cl atom transfer process was not optimized in this case, the activation energy can be easily predicted to be lower than for the methyl 2-cloroproprionate, because the C-Cl bond breaking process requires less energy while the Cr-Cl bond making process provides the same thermodynamic drive. This result is in line with the experimental observations: the stoichiometric reaction of 2 with CH3CH(Cl)COOCH3 is rapid whereas that with CH3CH(Cl)OOCCH3 is slow.

2 -of 1 or 2 ,

 22 Scheme 3

  1.44-1.56 for 1, 1.34-1.69 for 2). Lowering the amount of initiator to 0.5 equiv per Cr II yields a similar trend with lower plateau conversion values and lower initiator

efficiency (ca. 0.08 in both cases). This final conversion is again higher with compound 1 (7%) than with compound 2 (5.4%). Note that the maximum conversion is reached after approximately the same time (ca. 3.5 h) for all four experiments.

Acknowledgement. We are grateful to the Agence Nationale de la Recherche (contract ANR No. NT05-2_42140) for financial support. RP also thanks the Institut Universitaire de France for additional support and the Centre Interuniversitaire de Calcul de Toulouse (CICT, project CALMIP) and the Centre Informatique National de l'Enseignement Supérieur (CINES) for granting free computational time. KMS thanks the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation, and the University of British Columbia for financial support.

with the primary alkyl group.

(d) DFT studies

We have decided to recur to DFT calculations in order to address several issues.

A first issue is the relative stabilities of the ATRP and OMRP dormant states with respect to the active radical. The 1-chloroethyl acetate molecule was chosen as a suitable model for the PVAc-Cl dormant chain of the ATRP process, whereas the CpCr(nacnac Ar,Ar )(CHMeOAc) was considered suitable to model the CpCr(nacnac Ar,Ar )(PVAc) dormant chain of the OMRP process. The relative energy of the latter and the steric effect of the Ar group (Ph, Xyl, Dipp) on the Cr-C bond dissociation energy has already been calculated and presented in our previous contribution. 15 The previous study also included the calculation of the transition state for the OMRP trapping process. Therefore, the new calculations of the chloride complexes and of the transition states for the ATRP activation/deactivation process for the three aryl systems were conducted at the same level of theory (see Experimental Section for details). The energetic results are summarized in Figure 6 and the transition state geometries are shown in Figure 7.

Supporting Information Available. Tables of Mn and PDI vs. conversion for all polymerizations; key geometrical parameters for the optimized geometries; Cartesian coordinates for all new optimized geometries; complete crystallographic data for complex 5 (16 pages).


Vincent Escande

Laetitia Garoux

Claire Grison

Yann Thillier

Françoise Debart

Jean-Jacques Vasseur

Clothilde Boulanger

Claude Grison
email: claude.grison@cefe.cnrs.fr

Ecological catalysis and phytoextraction: symbiosis for future

Keywords: sustainable chemistry, ecological catalysis, heterogeneous catalysis, biomolecules, phytoextraction 1

Metallophyte plants derived from phytoextraction are used as starting materials to prepare novel polymetallic catalysts. Polymetallic catalyst activity is used in many Lewis acid catalyzed reactions according to the polymetallic catalyst preparation. The synergetic catalysis of these systems leads to efficient syntheses of complex biomolecules such as dihydropyrimidinone, 5'-capped DNA and RNA, and glycosyl aminoacid. These new polymetallic catalysts also bring new possibilities in Green Catalysis, that we named "Ecological Catalysis".

Introduction

Ecological Catalysis in sustainable chemistry context

In recent years, Organic Chemistry has focused on environment-friendly processes. The "Green Chemistry" concept is an interesting approach to reduce negative environmental impacts of organic synthesis. Modern chemistry requires chemical processes designed to reduce waste production; which facilitate efficient reuse and recycling; and which use starting material derived from renewable resources or from abundant wastes. [START_REF] Anastas | Green Chemistry: Theory and Practice[END_REF][START_REF] Winterton | [END_REF][3] Consequently, chemical engineering is now showing a considerable interest in catalytic processes. Amongst other approaches, chemo-and bio-catalysis represent particularly current and relevant topics in research and development of new catalytic processes. In this context, metal heterogeneous catalysis, nanocatalysis, electrocatalysis, photocatalysis or biocatalysis are key technologies to achieve green synthesis. [START_REF] Sheldon | Green Chemistry and Catalysis[END_REF][START_REF] Polshettiwar | [END_REF][START_REF] Kyriacou | Electrocatalysis for organic synthesis[END_REF][START_REF] Drauz | Enzyme catalysis in organic synthesis[END_REF] Catalytic synthetic methodologies approach now sustainability but don't solve environmental problems yet. That is why we wanted to achieve the synthesis of molecules of interest with raw material, derived from restoration processes of polluted sites, as new catalysts. The generalization of this methodology should thus encourage the development of restoration programs of polluted sites, thanks to the economic interest of the molecules produced according to this new approach in Catalysis. This issue is therefore devoted to the development of a new concept of plant-based polymetallic catalysis, that we call "Ecological Catalysis". The concept combines the catalytic green chemistry field and the important implications of a solution to metallic element persistence and bio-accumulation.

Connecting phytoextraction of trace metals, environmental opportunities and innovative plantbased chemistry

Trace metals (TM) in soils present major environmental problems worldwide. Metal-hyperaccumulating plants, or metallophytes, extract TM from contaminated soils and concentrate them in their shoots. Their discovery was a wonderful opportunity to remove TM from the environment. Exploring the utility of these plants had led to the development of phytoextraction. Today, using plants to clean up the environment is achievable on a large-scale, cost-effective and has good public acceptance. The main disadvantage is the lack of real economic opportunities. [START_REF] Glass | Phytoremediation of Toxic Metals: Using Plants to Clean up the Environment[END_REF][START_REF] Chaney | Phytoremediation of Contaminated Soil and Water[END_REF][START_REF] Li | [END_REF][11] Therefore, we demonstrated that metallophytes could be the basis of a novel, plant-inspired, metallo-catalytic platform for green synthesis of molecules of biological interest. Zinc hyperaccumulating plants are an attractive resource for new chemical perspectives. [12,13] For example, Noccaea caerulescens and Anthyllis vulneraria, are able to concentrate about 120 000 ppm of Zn II in calcined shoots. [14] They represent fascinating models for the preparation of ecological catalysts. [14][15][16]

Experimental

Preparation of catalytic extracts from metallophyte species

Leaves were harvested before flowering, air-dried and crushed. The obtained solid (150 g) was calcined at 400 °C for 5 h and the resulting powder (24 g) was added to 1 L of a solution of 5 M HCl solution. The solution was heated at 60 °C and stirred for 2 h. The reaction mixture was filtered on celite. The resulting solutions, composed of different metal chlorides, were then concentrated under vacuum. Dry residues were either used crude or partially purified in order to decrease the concentration of alkali and alkaline earth metals in the catalytic solid. With Zn hyperaccumulating metallophytes (N. caerulescens and A. vulneraria), Amberlite IRA 400 ion exchange resin was used for adsorption of Zn II on the resin, and elution of alkali and alkaline earth cations (fraction 1). Treatment with 0.5 M HCl eliminated a part of Fe III fixed on the resin before the elution of heavy metals Zn II , Cd II , Pb II (fraction 2) with 0.005 M HCl. When mentioned, a mixture of these plant-derived Lewis acids and montmorillonite K10 was prepared by co-grinding with mortar and pestle, at room temperature. 2.0 g of montmorillonite K10 were mixed with the plant-derived Lewis acids (amount corresponding to 1.1 mmol of metal of interest/g of support). This mixture was activated at 100°C for 15 min before use.

Characterization of catalytic extracts from metallophyte species

Chemical analysis of the plant extract samples after calcinations (400°C for 3 h) was performed by X-Ray Fluorescence spectrometry (XRF) using a BRUKER AXS S4 Explorer wavelength-dispersive spectrometer. The quantitative analysis of major and expected elements was performed on beaded samples for overcoming problems of particle size variation as well as mineralogy effects: the powdered sample is mixed with a Li 2 B 4 O 7 flux with a flux / sample ratio equal to 8, heated in a crucible between 400-600 °C, then cast in a platinum dish to produce a homogeneous glass-like bead. ICP-MS was used to confirm the composition of the various plant extracts obtained. ICP-MS analyses were performed using the Metal Analysis of total dissolved solutes in water. The samples were acidified with nitric acid 2.5% and stirred for 30 min. The digestates were diluted to 0.005 g.L -1 . Three blanks are recorded for each step of the digestion and dilution procedure on a HR-ICP-MS Thermo Scientific Element XR.

X-ray diffraction (XRD) data measurements on the samples dried at 110°C for 2 hours were performed by using a BRUKER diffractometer (D8 advance, with a Cu K radiation =1.54086 Å) equipped with a Lynxeyes detector.

FTIR measurements were carried out using pyridine as probe molecule. The samples were pressed into wafers (8 mg.cm -2 ) and activated in the IR cell under flowing air (1cm 3 .s -1 ) at 400°C for 10 h and then under vacuum (10 -3 Pa) for 1 h. A PerkinElmer Spectrum 100 FT-IR spectrometer was used for recording the spectra. Excess gaseous pyridine was adsorbed, then the samples were degassed for 15 minutes at 25°C (10 -3 Pa) and a first spectrum was recorded. The samples were then degassed for 15 minutes at 150°C (10 -3 Pa) to eliminate the physisorbed pyridine and a second spectrum was recorded.

Use of catalytic extracts (Green Lewis Acid Catalysts) in organic synthesis

Chemicals

Reagents and solvents were purchased from Aldrich, excepted guanosine-5'-diphosphate (GDP) salts, purchased from Jena Bioscience and 1,8-diazabicyclo[5.4.0]undec-7-ene purchased form Acros Organics. Reagents were used without further purification unless otherwise noted. Pyridine, acetonitrile, triethylamine, butylamine and trin-butylamine were distilled on calcium hydride. All reactions were performed under argon (or nitrogen) and stirring unless otherwise noted. When needed, glassware was dried overnight in an oven (T > 100 °C).

Purification and analysis of reaction products

Flash column chromatography was performed using Chromagel Carlo Erba Reactifs SDS silica 60 A CC 35-70 µm. Reactions were monitored using Merck Kieselgel 60 F254 aluminium. TLC's were visualized by UV fluorescence (254 nm) then one of the following: KMnO 4 , ninhydrine, phosphomolybdic acid solution, phosphotungstic acid solution.

Analytical and semi-preparative high performance liquid chromatographies were performed on a Dionex DX 600 HPLC system or a Dionex U 3000 HPLC system equipped with anion-exchange DNAPac PA 100 columns (4 x 250 mm for analysis or 9 x 250 mm for purification, Dionex). The following HPLC solvent systems were used: 5% CH 3 CN in 25 mM Tris-HCl buffer, pH 8 (buffer A) and 5% CH 3 CN containing 400 mM NaClO 4 in 25 mM Tris-HCl buffer, pH 8 (buffer B). Flow rates were 1.5 mL.min -1 and 5 mL.min -1 for analysis and semipreparative purposes, respectively.

NMR spectra were recorded on a Brüker Avance 300 spectrometer at room temperature, 1 H frequency is at 300 MHz, 13

Representative procedure for the synthesis of dihydropyrimidinones with Green Lewis Acid

Catalyst: synthesis of 5-ethoxycarbonyl-6-methyl-4-isobutyl-3,4-dihydropyrimidin-2(1H)-one (4)

A solution of ethyl acetoacetate 2 (781 mg, 6.0 mmol), 3-methylbutyraldehyde 3 (345 mg, 4.0 mmol) and urea 1 (360 mg, 6.0 mmol) in 95% ethanol (10 mL) was heated to reflux in the presence of montmorillonite K10supported crude Green Lewis Acid Catalyst derived from Zn hyperaccumulator plants (1650 mg, amount corresponding to 1.0 mmol of zinc following ICP-MS dosing) with 3 drops of hydrochloric acid (12 M) for 5 h (TLC). The reaction mixture was filtered in order to remove the catalyst, which can be reactivated by heating, after been washed with ethanol (3x10 mL then 150°C, 5h

Representative procedure for the selective deprotection of methyl ester: synthesis of 6,7dideoxy-1,2 :3,4-di-O-isopropyldine-7-[(9-flurenylmethoxycarbonyl)amino]-D-glycero-α-D-galactooctopyranuronic acid (9)

A solution of 0.87 mL of NaOH 1M (0.9 mmol, 1.2 eq.) is added at room temperature to 1.8 eq of Ca(II) from fraction 1 dissolved in 16 mL of a mixture of isopropanol/water 7/3 (v/v). After 5 min of stirring, 0.4 g of methyl ester 8 (7.2 mmol, 1 eq.) is slowly added. The reaction mixture is stirred for 3h30 then diluted by addition of 10 mL of diethyl ether. pH is adjusted to 4 by addition of hydrochloric acid solution (1M) then the aqueous layer is extracted with diethyl ether. Organic layers are combined, dried with MgSO 4 , filtrated and evaporated under reduced pressure. The obtained residue is purified by column chromatography on silica gel (hexane/EtOAc 1/1 v/v). A white solid is obtained with 82% yield. Rf=0.5 (EtOH/EtOAc 1/9 v/v) ; IR 3450, 1700 cm -1 ; 1 H NMR (CDCl 3 , 300 MHz) δ: 7.74 (d, 2H, 3 J=7.4Hz), 7.58 (d, 2H, 3 J=7.0Hz), 7.41-7.24 (m, 4H), 5.74 (d, 1H, NH, 3 J =7.6Hz), 5.49 (d, 1H, 3 J=5.2Hz), 4.58 (dd, 1H, 3 J=5.2Hz, 3 J=2.3Hz), 4.24-4.17 (m, 1H), 4.16-4.08 (m, 1H), 3.97-3.85 (m, 1H), 2.48-2.31 (m, 1H), 2.17-2.09 (m, 1H), 1.44 (s, 3H), 1.43 (s, 3H), 1.32 (s, 3H), 1.28 (s, 3H).

Representative procedure for the coupling of solid-supported T 6 phosphoro-imidazolidate with GDP: synthesis of 5'-guanosyl triphosphate hexa-2'-deoxythymidylate (GpppT 6 ) (12)

Capping reaction with GDP (10)

In a dry 2 mL microcentifuge tube (Costar), bis (tri-n-butylammonium) GDP 10 (103 mg, 0.14 mmol) and the correct amount of freshly dehydrated catalytic extracts ([Zn] = 0.4 µM) were mixed in anhydrous DMF (0.5 mL). The tube was closed and the mixture was vortexed for 5 minutes on a Top-Mix 1118 (Fischerbrand) and centrifuged in a tabletop centrifuge (Sigma 1-13) at 6000 min-1 for 30 seconds. This operation was repeated twice. The supernatant was taken using a glass syringe filled with 3 beads of 4Å molecular sieves. Using another syringe, the solution was applied to the column containing the solid-supported 5'-phosphoroimidazolidate oligonucleotide 11 (prepared following a method described by Thillier and coworkers [17]), and left to react for 18 h at 30 °C. The solution was removed and the support was washed with water (2 x 2 mL), then with a 0.1 M aqueous solution of EDTA (pH 7, 2 x 2 mL), and dry CH 3 CN (4 x 2 mL). Finally the column was dried by blowing argon through it during 1 min.

Deprotection and release of GpppT 6 (12)

The solid-supported GpppT 6 12 was deprotected and released from the support as follows: firstly, a 1 M solution of 1,8-diazadicyclo-[5.4.0]undec-7-ene (DBU) in anhydrous CH 3 CN was applied to the column for 3 min. Then the solution was removed and the solid-support was washed with anhydrous CH 3 CN. The support was dried by a 1 min flush with argon. Secondly, a 30 % aqueous ammonia solution was applied to the column in three batches (1.5 mL, 1mL, 0.5 mL) for 30 min each. The three ammonia fractions were collected in a 4 mL screw-capped glass vial and were left to react at room temperature for 1.5 h. The fully deprotected oligonucleotides were transferred to 50 mL round-bottomed flasks and isopropylamine (15% of total volume: 0.45 mL) was added only to the solutions of GpppT 6 12. Then the mixtures were evaporated under reduced pressure with a bath at 30°C maximum until the volumes were reduced to 0.3 mL. The mixtures were coevaporated three times with 1 mL of water following the same protocol. The residues were redissolved in water (1.5 mL divided in three portions for flask rinse: 0.8 mL, 0.4 mL, 0.3 mL) and transferred to 2 mL Eppendorf-vials then lyophilized from water. The crude GpppT 6 was analyzed on a Dionex DX 600 HPLC system monitored at 260 nm with a 0%-30% linear gradient of buffer B (5% CH 3 CN

3.

Results and discussion

Catalysts characterization by XRF and XRD analysis

The first step of the process was the thermic treatment of leaves at 400°C, followed by the addition of HCl (1M) and the concentration of the solution, which led to an unusual mixture of metallic chlorides and oxides. The potential of zinc hyperaccumulating plants for Ecological Catalysis was based on the total mineral composition of contaminated biomass. Zn II , Cd II and Pb II were due to the TM hyperaccumulation ability of metallophyte plants. Na I , K I , Ca II , Mg II , Fe III were also present as they are essential for plant growth.

The challenge was to develop ecological chemical methodologies, where each metallic fraction was useful and presented catalytic properties in organic synthesis. Hence, we investigated a partial separation of metallic derivatives through the ion exchange separation. It was found to be a solventless and efficient process. The use of Amberlite IRA 400 resulted in the preparation of three very active catalytic systems from a single batch of biomass residues: the crude mixture Zn II -Fe III -Al III -Cd II -Pb II -Mg II -Ca II ; a solid highly enriched in Zn II ; and a solid highly concentrated in Ca II . X-ray fluorescence (XRF) was used to determine the chemical composition analysis of all fractions from the plant extracts obtained (Table 1).

Table. 1. Composition of obtained plant extract for Ecological Catalysis

The XRF data were confirmed by inductively coupled plasma mass spectroscopy (ICP-MS). As indicated in Table 1, data from the crude extract confirmed the exceptional capacity of Noccaea caerulescens and Anthyllis vulneraria for Zn hyperaccumulation. The extract contained high amounts of transition metals: 6.74% of Zn II , and 2.75% of Fe III , which are two powerful Lewis acids. This is interesting since several Lewis acids were gathered in one extract, a potential ecological catalyst. A significant amount of Ca II (10.30%) should also be noted. Simple calculations revealed that solutions could not be considered to simply contain a hydrated mixture of ZnCl 2 , CaCl 2 and FeCl 3 . A liquid-liquid extraction with the trioctylamine (TOA) / toluene system had been employed to identify soluble species. A number of salts containing [ZnCl 3 ] -and [ZnCl 4 ] 2-, [ZnCl 2 (PO 4 )FeCl 4 ] - anions were isolated after extraction and analysis by LC MS.

XRD pattern was performed for crude extract (Figure 1) and for each fraction separated with Amberlite resin (Figures 2 and3). The main diffraction peaks of the predominant minerals are shown for reference purposes. Table 2 summarizes these data.

The X-ray diffraction of Ca II -enriched extract (Fraction 1) revealed a mixture of calcium-magnesium salt (CaMg 2 Cl 6 (H 2 0) 12 , CaMg(CO 3 ) 2 ) , KCaCl 3 and CaCO 3 . The Zn II -enriched extract (Fraction 2) corresponded to an amorphous, hygroscopic and even deliquescent mixture similar to ZnCl 2 .The mineral analysis clearly revealed that Zn II was the major cation (33%), which is also the major Lewis acid and therefore a potential efficient catalyst. As expected Ca II salts were eliminated. A treatment with dioxane [18] allowed the formation of crystalline species. According to obtained XRD data, two crystal structures of ZnCl 3), the potassium zinc tetrachloride detected in the crude extract was found in the fraction 2. The presence of ZnCl 2 probably indicated that K 2 ZnCl 4 was converted into more stable ZnCl 2 , KCl and KClO 3 species.

The source of chlorate (fraction 2 treated with dioxan) and perchlorates (fraction 1 and crude extract supported) was not clear. The hypothesis of K 2 ZnCl 4 decomposition and subsequent oxidation was not satisfactory. This oxidation was not explained by the redox properties of present cations. The oxyanion chlorides didn't exist in the crude fraction (not supported). Thus, they didn't derive from plants. Probably, the oxidation of chloride into chlorate and perchlorate occurred during the purification and the preparation of the supported extract.

Thus, the ecological catalysts had a complex and an original composition. It brings new perspectives such as the formation and the stabilization of [ZnCl 4 ] 2-and [ZnCl 3 ] -, which have provided conflicting results and which are rarely observed in solution. [START_REF] Burgess | Encyclopedia of Inorganic Chemistry[END_REF] According to their Kd, they constitute masked form of ZnCl 2 in solution. The association of these species with other metallic cations was also an exciting and unusual situation. This novel polymetallic composition should be investigated in catalyzed organic synthesis. The direct conversion of catalytic solid as zeolite like-materials had been investigated. According to Vanden Eynde and co-workers, [START_REF] Vanden Eynde | [END_REF] the methodology was based on a co-grinding; montmorillonite K10 was placed in a porcelain mortar and air-dried crude extract was then added and mixed with montmorillonite K10 using a pestle to obtain a homogeneous powder. The choice of conditions was also guided by the outcome of the experiment and was therefore considered realistic for an economic feasible process. The subsequent solid was characterized by XRF (Figure 4) and XRD (Figure 5). Fortunately, the mass percentage of Zn II was maintained, since the amount of chloride decreased. K 2 ZnCl 4 was the observed sole zinc chloride (Table 3).

Characterization of the Lewis acidity by pyridine adsorption

Pyridine is widely used as a probe molecule for determination of Lewis acidity on solid acids, by monitoring the bands in the range of 1400-1650 cm -1 arising from its ring vibration modes. [21,22] Infrared spectra of pyridine adsorbed on crude fraction were recorded at 25°C and 150°C in order to distinguish frequencies of physisorbed pyridine from frequencies of pyridine coordinated to Lewis sites (Figure 6).

Fig. 6. IR spectra of adsorbed pyridine on crude extract taken following brief outgassing at the indicated temperatures

Figure 6 shows that a band at 1440 cm -1 observed at 25°C disappeared after outgassing at 150°C, and could thus be attributed to physisorbed, weakly bonded, pyridine. [21] In the same range, a band at 1450 cm -1 was observed at 150°C. This band was characteristic of pyridine, which was still strongly bonded at this temperature, by coordination to Lewis acid sites. [22] It was the first indication of the extract Lewis acidity. The decrease of the band intensity at 1486-1487 cm -1 is more delicate to explain. It could be attributed to physisorbed pyridine, as the intensity was strongly reduced after heating. Looking at bands in the 1590-1640 cm -1 range, the band at 1599 cm -1 observed at 25°C disappeared at 150°C, this one being characteristic of hydrogen-bonded pyridine. [21] On the other hand, several bands were observed in the 1600-1640 cm -1 range, with small variations in frequencies, depending on outgassing temperature (1608 and 1631 cm -1 at 25°C ; 1609, 1628 and 1639 cm -1 at 150°C). The continued existence of bands in this range, in spite of heating at elevated temperature, indicated the presence of strongly bonded pyridine. In previous studies, these bands had been attributed to pyridine coordinated to Lewis acid sites. The occurrence of these bands at different frequencies in this range may account for the involvement of different types of Lewis acid sites. [22] This hypothesis was supported by the previous X-ray data (Table 2), highlighting the presence of different Lewis acids in the extract.

3.3.

Synthetic applications of ecological catalysts in the transformations of biomolecules

Dihydropyrimidinone synthesis

The first example illustrated the non-conventional catalytic activity of the crude mixture derived from N. caerulescens and A. vulneraria in supported multicomponent reactions. Biginelli reaction was an interesting example, because this reaction led to dihydropyrimidinone heterocycles, starting from aldehyde, CH-acidic carbonyl component and urea-type molecule. [23][START_REF] Kappe | Organic Reactions[END_REF][START_REF] Suresh | [END_REF] Recently dihydropyrimidinones have been the object of an increased interest, as these molecules exhibit exciting biological features. [26,27] Among the pharmacological reported properties, calcium channel modulators, α 1a adrenoreceptor-selective antagonists and compounds targeting the mitotic machinery can be cited as examples. Since the discovery of this reaction by Pietro Biginelli in 1893 [28] the yield of this reaction has been improved especially by the use of Lewis acid catalysts. [START_REF] Kappe | Organic Reactions[END_REF][START_REF] Suresh | [END_REF] Despite important progress achieved, yields are still low when aliphatic aldehydes are used, probably because of self aldol condensation, a side reaction which does not occur when aromatic aldehydes are used. [29][30][31] However, when using Green Lewis Acid Catalyst derived from Zn metallophytes in this reaction, excellent yields were obtained, even with aliphatic aldehydes as building blocks. The yield was increased in comparison with others published catalytic systems [32][33][34][35][36] as shown in Table 4, illustrating the synthesis of 5ethoxycarbonyl-4-isobutyl-6-methyl-3,4-dihydropyrimidin-2(1H)-one 4 (Scheme 1).

Scheme 1. Synthesis of dihydropyrimidinone 4 with various catalysts (see Table 4) As it was mentioned by Mitra and coworkers that montmorillonite KSF clay could catalyze Biginelli reaction under microwave irradiation, [36] blank reaction was performed with the sole support, montmorillonite K10, and hydrochloric acid, without Green Lewis Acid Catalyst (3 drops of concentrated hydrochloric acid were added in order to ensure the complete solubility of potentially formed bisureide, as previously reported [37]). As shown in Table 5, when the reaction was performed with the sole montmorillonite K10/HCl, the yield was only 67 %, which proves that the high yield obtained with supported Zn Green Lewis Acid Catalyst was due to the Lewis acids provided by the hyperaccumulating plants. Encouraged by this result, we tried the reaction with other aliphatic and aromatic aldehydes (Scheme 2 and Table 5). Again, good yields of dihydropyrimidinones were obtained and, each time, they were superior to the yields with sole montmorillonite K10/HCl. Furthermore, the Zn Green Lewis Acid Catalyst was recycled by filtration, washing with ethanol, and reactivation by heating (150°C, 5h). The yield of dihydropyrimidinone 4 remained basically unchanged (88 %, entry 4), showing that the Zn Green Lewis Acid Catalyst could be reused after recycling.

Scheme 2. Synthesis of various dihydropyrimidinones with Zn Green Lewis Acid Catalyst or with sole montmorillonite K10/HCl (see Table 5) The experimental conditions using Zn Green Lewis acid Catalyst are consistent with the principles of Green Chemistry, especially because the solvent is non-toxic, cheap, and may be biosourced.

In order to explain the high activity of Zn Green Lewis Acid Catalyst in this transformation, we propose that cooperative effects exist between the different Lewis acids of our ecological catalysts. The most commonly accepted mechanism for the Biginelli reaction, proposed by Kappe in 1997, involves the formation of a Nacyliminium ion intermediate. [38] In a similar mechanism, Hu and coworkers suggested that Lewis acid can interact with three distinct sites during the reaction. [39] Following this mechanism, Lewis acid acts by coordination to the urea oxygen and to the nitrogen atom of the acyl imine [40][41][42][43] Due to the binding of each reaction site to its "optimal" Lewis acid partner, the reaction should be better catalyzed by a polymetallic catalyst, which offers different acid sites.

Scheme 3. Co-catalysis of the Biginelli reaction with different Lewis acids

This synergetic effect could thus explain the high activity of the Zn Green Lewis acid in the Biginelli reaction with aliphatic aldehydes, giving poor yields with classical catalysts. This hypothesis is sustained by other published examples of Lewis acid co-catalysis in the Biginelli reaction, leading to excellent yields. [44,45] However, a supplementary explanation about the power of the ecological catalyst for aliphatic aldehydes can be highlighted. As shown in Figure 1, the major species of Zn II is K 2 ZnCl 4 . That supposes a progressive release of ZnCl 2 in solution, which limits the concurrent enolization of aldehyde.

Chemoselective hydrolysis in peptide chemistry

Another stimulating example of the activity of the Green Lewis Acid Catalyst concerns chemoselective hydrolysis. Peptide synthesis requires the use of orthogonal protecting groups, to allow modification of selected sites without reaction of other functions of the molecule during the synthesis. [START_REF] Sureshbabu | Amino Acids, Peptides and Proteins in Organic Chemistry[END_REF][START_REF] Sewald | Peptides: Chemistry and Biology[END_REF][START_REF] Isidro-Llobet | [END_REF] The 9fluorenylmethoxycarbonyl group (Fmoc) is a base-labile α-amino protecting group widely used in peptide synthesis. [49,50] When carboxyl-protecting groups are involved during a synthesis, the choice of using Fmoc as α-amino protecting group presupposes that these carboxyl-protecting groups should not be cleaved during Fmoc deprotection. This is generally avoided by the use of orthogonal carboxyl-protecting groups, such as tert-butyl, allyloxycarbonyl or benzyl, resisting to Fmoc cleavage conditions. [START_REF] Isidro-Llobet | [END_REF] An alternative had been proposed by Pascal and Sola, showing that the use of CaCl 2 with NaOH allowed the selective basic cleavage of carboxylic ester function in the presence of Fmoc group. [51] The authors proposed that this chemoselective deprotection was due to two effects: first, "stabilization of the negatively charged tetrahedral transition state by coordination with Ca 2+ facilitating ester hydrolysis", second, "reduction of the hydroxide ion concentration due to the formation and incomplete dissociation of calcium hydroxide", in favour of hydrolysis of the ester and limiting βelimination of Fmoc group. Due to the high content of calcium of fraction 1 of the Green Lewis Acid Catalyst, we formulated the hypothesis that this catalytic fraction would be an excellent alternative to Pascal and Sola conditions. The eluted first fraction of crude ecological catalyst, highly enriched in CaCl 2 , was concentrated. The controlled addition of NaOH (1M) until pH 4 allowed the chemoselective hydrolysis of the methyl ester of 6,7dideoxy-1,2 :3,4-di-O-isopropyldine-7-[(9-flurenylmethoxy carbonyl)amino]-D-glycero-α-D-galactooctopyranuronic methyl ester 8, without the cleavage of the base labile Fmoc. The expected galactosyl aminoacid 9 was obtained with 82% yield (Scheme 4).

Scheme 4.

Selective deprotection of carboxyl group without the cleavage of Fmoc with Ca-enriched Green Lewis Acid Catalyst (fraction 1).

In order to ensure that this methodology is adapted to substrates different of sugars, two methyl esters of amino acid-based Fmoc derivatives were tested successfully (Table 6). The mass of catalyst is estimated following ICP-MS dosing, to correspond to 2.0 equivalents of Ca(II). Reaction conducted in 15 mL of a mixture of isopropanol/water 7/3 (v/v), at room temperature.

b Isolated yield.

The good yields obtained with these substrates confirm the interest of this method in peptide chemistry, for selective deprotection of methyl esters.

Synthesis of 5'-capped oligonucleotides

Convenient access and availability of large quantities of capped DNA and RNA are of great interest for biologists for structural and mechanistic studies of complexes with RNA capping enzymes. [START_REF] Shuman | Progress in Nucleic Acid Research and Molecular Biology[END_REF][START_REF] Hodel | [END_REF][54][55] The need to develop an efficient method to functionalize RNA at their 5'-end with the cap structures guanosyl triphosphate (Gppp) attracted our attention and led us to study the performance of our ecological catalysts in the transformation. Starting our studies with a DNA homosequence as a model, we found that the ZnCl 2 enriched solid (fraction 2), was an excellent catalyst for the coupling reaction between the guanosine-5'-diphosphate (GDP) bis(tetrabutylammonium) salt 10 and the 5'-phosphorimidazolidate derived from a solid-supported hexathymidylate (T 6 -CPG) 11 (Scheme 5). The obtained yield for the synthesis of 5'-terminal capped oligonucleotides GpppT 6 12 (65%) was better than pure ZnCl 2 (55%). Therefore a synergetic effect might exist between the present Lewis acids which improved catalytic performance, when a mixture of borderline and soft Lewis acids was made (fraction 2). These observations showed again that a combination of different metal halides led to more active systems than individual components. The result is consistent with the activation of three basic Lewis centres during the coupling: phosphate groups of GDP, imidazole and phosphate moiety of phosphoroimidazolidate (Scheme 6). This adjustment of coordination by relative affinity is a real advantage of the Ecological Catalysis.

Conclusion

The potential utility of this new plant-inspired catalysis was clearly demonstrated by its application to the synthesis of complex heterocycles, glycosyl aminoacid and 5'-capped DNA and RNA. Ecological catalysts derived from metallophytes provided improved catalyst performances compared to the most known catalysts. The wide variety of reactions that are catalyzed by ecological catalysts, and the ease of preparation of such catalysts create manifold opportunities for the development of catalytic processes. The development of Znhyperaccumulating plants for organic synthesis leads to innovative and fascinating plant-based materials. They represent a more sustainable and promising system and suggest that sustainable chemistry and performance can be combined. Other novel developments of Ecological Catalysis are certain to emerge in the future.

Fig. 1 .Fig. 2 . 1 Fig. 3 .

 1213 Fig. 1. XRD pattern of crude extract

Fig. 4 .Fig. 5 .

 45 Fig. 4. Chemical composition of supported crude extract

Figure 7

 7 Figure 7 demonstrates the synthetic utility of these ecological catalytic systems through multifunctional biomolecule synthesis.

Fig. 7 .

 7 Fig. 7. Synthetic applications of ecological catalysts in the transformations of biomolecules

Scheme 5 .

 5 Scheme 5. Coupling reaction between GDP 10 and T 6 -CPG 11 catalyzed by Zn-enriched Green Lewis Acid Catalyst (fraction 2)

Scheme 6 .

 6 Scheme 6. Activation of basic Lewis centres of reactants by different Lewis acids of Zn-enriched Green Lewis Acid Catalyst

  NMR (DMSO-d 6 , 300 MHz) δ: 8.92 (s, 1H, NH), 7.39 (s, 1H, NH), 3.99-4.12 (m, 3H, H4 and OCH 2 Me), 2.16 (s, 3H, C(6)-Me), 1.70 (m, 1H, CH 2 CHMe 2 ), 1.38 (m, 1H, CH 2 CHMe 2 ), 1.19 (t, 3H, OCH 2 Me, 3 J=7.2 Hz), 1.10 (m, 1H, CH 2 CHMe 2 ), 0.86 (d, 6H, CH 2 CHMe 2 , 3 J=6.6 Hz). MS (EI + ) calcd for C 12 H 20 N 2 O 3 [M] + 240.2, found 241.2 [M+1] + .

	). The solution was poured into crushed ice (20 g) and
	stirred for 20 min. The solid separated was filtered under suction, washed with cold water (30 mL) and
	recrystallized from hot ethanol, affording pure product (870 mg, 91%), mp 179-181 °C (lit. 178-180 °C) ; IR
	3229, 3107, 2951, 1698, 1649 cm -1 ; 1 H

13 C NMR (DMSO-d 6 , 75 MHz) δ: 165.2, 152.8, 148.1, 100.3, 59.0, 48.1, 46.0, 23.6, 22.8, 21.3, 17.6, 14.1.

13 C NMR (

  DMSO-d 6 , 75 MHz) δ: 176.3, 156.3, 143.3, 141.3, 127.7, 127.1, 125.1, 120.0, 109.6, 108.8, 94.5, 72.6, 70.4, 67.2, 64.5, 51.2, 47.2, 31.6, 25.9, 25.7, 24.9, 24.6. MS (EI + ) calcd for C 29 H 33 NO 9 [M] + 539.2, found 540.2 [M+1] + .

  containing 400 mM NaClO 4 in 25 mM Tris-HCl buffer, pH 8) in buffer A (5% CH 3 CN in 25 mM Tris-HCl buffer, pH 8). MALDI-TOF characterization in negative mode: calcd for C 70 H 93 N 17 O 57 P 8 [M-H] -2267.35, found 2267.25 [M-H] -.

Table 2 .

 2 XRD pattern of crude extract and eluted fractionsAccording to Pearson's HSAB principle, the catalytic solids generated from Zn metallophyte species led to modulation of the hard/soft ratio. The obtained catalytic solids could be distinguished according to three types of Lewis acid level. The purified N. caerulescens / A. vulneraria extract, called fraction 1, led to a Pearson's "Hard Lewis Acid" mixture, because Mg II , Ca II , Al III , Fe III contributions represented 99.6 % of the cationic mixture. Purified N. caerulescens / A. vulneraria extract, called fraction 2, led to a Pearson's "Borderline Lewis Acid" composition, with a mixture of borderline and soft Lewis acids (Zn II + Cd II + Pb II = 64 %). Crude extracts were constituted by miscellaneous cations with respect to Pearson's classification system.

	Fraction	Mineral	Formula (simplified)
	Crude	Calcium magnesium	CaMg 2 Cl 6 (H 2 0) 12
		chloride hydrate
		Anhydrite, syn	CaSO 4
		Potassium zinc	K 2 ZnCl 4
		tetrachloride
		Sylvite, syn	KCl
		Magnesium	MgP 4 O 11
		ultraphosphate
	Fraction 1	Chlorocalcite, syn	KCaCl 3
		Calcium magnesium	CaMg 2 Cl 6 (H 2 O) 12
		chloride hydrate
		Calcium carbonate	CaCO 3
		Calcium perchlorate	Ca(ClO 4 ) 2
		Anhydrite, syn	CaSO 4
		Sylvite, syn	KCl
		Dolomite	CaMg(CO 3 ) 2

Table 3 .

 3 XRD pattern of supported crude extract

	Fraction	Mineral	Formula (simplified)
		Calcium magnesium	CaMg 2 Cl 6 (H 2 0) 12
		chloride hydrate
		Anhydrite, syn	CaSO 4

Potassium zinc tetrachloride K 2 ZnCl 4

	Supported
	crude on
	montmorillonite
	K 10
	Potassium perchlorate	KClO 4
	Chlorocalcite, hydrate CaCl 2 (H 2 0) 2
	Calcium magnesium	Ca 2 MgSi 2 O 7
	silicate
	Quartz	SiO 2
	Tridymite	SiO 2

Table 4 .

 4 Activity comparison of different catalysts in the Biginelli reaction involving urea 1, ethyl acetoacetate 2 and isovaleraldehyde 3.

	Catalyst	Solvent	Time	Yield (%)	Reference
	Zn Green Lewis
	Acid Catalyst supported on	EtOH 95%	5 h	91	this work
	K10/HCl
	Ionic liquid [bmim]Cl•2AlCl 3	Ionic liquid [bmim]Cl•2AlCl 3	1 h 15	87	[35]
	Montmorillonite KSF	Solvent free irradiation) (microwave	16 min	85	[36]
	Sulfated tungstate	Solvent free	1 h 05	65	[34]
	Proline-based
	organocatalyst, pentafluorobenzoic acid,	THF/dioxane (1:1)	96 h	60	[33]
	Ph 3 CNH 3 + CF 3 COO -
	ZnI 2 , 300 MPa	acetonitrile	4 h	55	[32]

Table 5 .

 5 Conditions of synthesis and isolated yieldsThe mass of montmorillonite K10 et the volume of hydrochloric acid were the same as those used in the preparation of Zn Green Lewis Acid Catalyst supported on K10/HCl.

	Entry R	Catalyst	Yield (%)
	1	(CH 3 ) 2 CHCH 2	Zn Green Lewis Acid Catalyst supported on K10/HCl	91
	2	(CH 3 ) 2 CHCH 2	K10/HCl a	67
	3	(CH 3 ) 2 CHCH 2	K10	58
	4	(CH 3 ) 2 CHCH 2	Recycled b Zn Green Lewis Acid Catalyst supported on K10/HCl	88
	5	CH 3 CH 2	Zn Green Lewis Acid Catalyst supported on K10/HCl	93
	6	CH 3 CH 2	K10/HCl	54
	7	CH 3 CH 2 CH 2	Zn Green Lewis Acid Catalyst supported on K10/HCl	90
	8	CH 3 CH 2 CH 2	K10/HCl	78
	9	Ph	Zn Green Lewis Acid Catalyst supported on K10/HCl	87
	10	Ph	K10/HCl	53
	11	3-BrC 6 H 4	Zn Green Lewis Acid Catalyst supported on K10/HCl	96
	12	3-BrC 6 H 4	K10/HCl	74

a b

Catalyst recycled one time.

  7, stabilizing the intermediate (Scheme 3). Moreover, when 1,3-dicarbonyl component is used as CH-acidic carbonyl component, chelation of the Lewis acid should stabilize the enol tautomer. Using a co-catalytic system formed of two Lewis acids, BF 3 .OEt 2 and Cu(OAc) 2 , Hu and coworkers suggested that both Lewis acids activate different sites according to their highest affinity. That is why we propose that the Zn Green Lewis Acid Catalyst should act in the same way: as this catalyst is a mixture of various transition metal salts, with different Lewis acidity, each metal salt should coordinate to one of the three sites for which it would have the best affinity. As each reaction site has a proper Lewis basicity, it should coordinate preferentially to one kind of Lewis acid, following principles of Hard-Soft Acid-Base theory.

Table 6 .

 6 Deprotection of carboxyl group without the cleavage of Fmoc of amino acid derivatives

	Entry Substrate	Catalyst a	Time Yield b (%)
	1	Fmoc-Gly-OMe	Ca-enriched Green Lewis Acid Catalyst (fraction 1)	1 h	84
	2	Fmoc-Gly-Phe-Pro-OMe	Ca-enriched Green Lewis Acid Catalyst (fraction 1)	6 h	71
	a

ACKNOWLEDGMENTS

The authors would like to thank ANR (11ECOT 011 01), ADEME, FEDER program for financial supports.