Ugo Authesserre
email: ugo.authesserre@univ-tlse3.fr

V S V S N Swamy

Nathalie Saffon-Merceron
email: nathalie.saffon@univ-tlse3.fr

Antoine Baceiredo
email: antoine.baceiredo@univ-tlse3.fr

Tsuyoshi Kato
email: tsuyoshi.kato@univ-tlse3.fr

Eddy Maerten
email: eddy.maerten@univ-tlse3.fr

New insight into the reactivity of S,S-bis-ylide

Keywords: bis-ylides, ylide, carbone, phosphenium, carbene

The present work focuses on the reactivity of S,S-bis-ylide 2, which presents a strong nucleophilic character as evidenced by reactions with methyl iodide and CO2 affording C-methylated salts 3 and betaine 4, respectively. Derivatization of betaine 4 affords the corresponding ester derivative 6 which was fully characterized by NMR spectroscopy and X-ray diffraction analysis. Furthermore, an original reaction with phosphenium ions leads to the formation of a transient push-pull phosphino(sulfonio)carbene 8 which rearranges to give stabilized sulfonium ylide derivative 7.

Introduction

Since the discovery of ylides in the 1920s [1,2], their chemistry has experienced an important grow [START_REF] Clark | Nitrogen, Oxygen and Sulfur Ylide Chemistry[END_REF][START_REF] Wittig | [END_REF][5] and they have become very important synthetic tools [START_REF] Abell | Organophosphorus Reagents[END_REF][START_REF] Schobert | Organophosphorus Reagents[END_REF][START_REF] Edmonds | Modern Carbonyl Olefination[END_REF][START_REF] Kolodiazhny | Phosphorus Ylides: Chemistry and Application in Organic Synthesis[END_REF]. Bisylides, which are the combination of two ylide functions on a single carbon atom, have attracted significant attention from many research groups [START_REF]Transition Metal Complexes of Neutral η1-Carbon Ligands[END_REF][11][12][13][14][15][16] Indeed, since the first synthesis of carbodiphosphoranes I by Ramirez in the 60s [17], many models have been developed (Figure 1). Among them, we can mention carbodicarbenes II prepared by Bertrand's group [18] after initial theoretical predictions by Frenking [19], the mixed carbophosphinocarbene III initially synthetized by Fürstner and Alcarazo and later by Ong et al. [20,21,22] and mixed phosphonio/sulfonio IV and phosphonio/sulfoxonio V (X = O) bis-ylides prepared by our group [23][24][25]. Finally, major contributions were made by Fujii's group with various combinations of selenium/sulfur or sulfur/sulfur stabilized carbones V-VIII [26][27][28][START_REF] Morosaki | Phosphorus Sulfur Silicon[END_REF][30]. Naturally, this diversity of structures has brought considerable knowledge on these species that exhibit a very diverse reactivity ranging from their use as ligands in organometallic chemistry [15] to activation of small molecules [21] and even application as atomic carbon sources [23]. Bis-ylides main application remains their use as ligands because of the peculiar environment of the central carbon atom. In particular, the presence of two lone pairs located on a single carbon atom [START_REF]Transition Metal Complexes of Neutral η1-Carbon Ligands[END_REF][11][12][13][14][15][16] offers unique coordination modes for transition metals. Indeed, Stephen took advantage of the presence of the two lone pairs to stabilize certain transition metals, achieving high efficiency in hydrogenation reactions [31]. Ong's group has evidenced that the unexpected accepting character of carbodicarbenes at the N-heterocyclic carbene fragments was benefic for an ambiphilic-type reactivity allowing the activation of small molecules [32,33]. It is obvious that the reactivity and applications of bis-ylides are directly linked to the combination of the two ylide functions, but overall remain rare. In 2010, we have reported the first stable and isolable mixed P,S-bis-ylide IV [23] which can be used as atomic carbon reagent to introduce one carbon atom into various organic molecules [34]. In particular, the labile character of sulfide ligand in IV facilitates substitution reactions at the central carbon atom and thus atomic carbon transfer reactions. Therefore, S,S-bis-ylide VI constituted of two labile sulfide moieties should be an excellent atomic carbon reagent. However, probably due to the thermal instability of VI, only a few reactions with Au(I), demonstrating its carbone character were described [30] and no reaction with organic reagents has been reported to date. In this article, we report original reactions of S,S-bis-ylide VI with CO2 and phosphenium ions (Figure 1).

Results and Discussion

Monoprotonated precursor 1 (Scheme 1) has been prepared according to the method previously described by Fujii [30]. The spectroscopic data are in good agreement with the reported values. In particular, the central proton and carbon resonances of 1 appear at  = 3.88 ppm and  = 19.4 ppm, respectively in 1 H and 13 C NMR spectra. Because of the thermal instability of S,S-bis-ylide 2, the deprotonation was then realized in THF-d8 using potassium hexamethyldisilazane at -80 °C and was analyzed at low temperature, in situ, in the presence of conjugated acid. With the appearance of a bright yellow color, in the 1 H NMR spectrum the C-H signal in disappeared while the central carbon in the 13 C NMR spectrum was lower field shifted at  = 39.0 ppm compared to 1. Bis-ylide 2 starts to decompose at -30 °C to afford black carbon precipitate and diphenyl sulfide. The formation of S,S-bis-ylide 2 was unambiguously confirmed by its methylation reaction (Scheme 1). Indeed, upon addition of one equivalent of methyl iodide to a THF solution of 2 at -80 °C, the yellow color immediately disappeared concomitantly with the precipitation of a white solid. After treatment, the desired C-methylated salt 3 was isolated in 85% yield. The methylation was confirmed by the presence of two characteristic signals for the methyl group at  = 1.67 ppm and  = 10.2 ppm in the 1 H and 13 C NMR spectra, respectively. C-methylated salt 3 was isolated in the crystalline form layering diethyl ether onto a saturated dichloromethane solution and its structure was confirmed by X-ray diffraction analysis (Figure 2). As expected, the S-C bond lengths in 3 (1.682 and 1.687 Å) are very similar to those of the protonated bis-ylide precursor 1 (1.685 Å and 1.676 Å) [30] and much shorter than a S-C single bond length (1.81 Å), showing the delocalization of the negative charge from the central carbon atom to the SPh2 fragments, and justifying the planar environment around the C1 atom (∑°= 359.7°). The S1-C1-S2 angle is also almost identical to that of 1. It is well known that carbodiphosphoranes (P,P-bis-ylides) react with CO2 to form zwiterrionic donor-acceptor complexes that can coordinate transition metals or evolve upon heating towards phosphine oxyde and phosphoranylideneketene [35][36][37][38][39]. The sulfur analogue 2 also reacts with CO2 (3 bars) at -70 °C affording the corresponding CO2 adduct 4 (Scheme 2). 13 C NMR analysis of the resulting solution at -20 °C shows two broad signals appearing at  = 167.2 ppm and  = 40.16 ppm corresponding to the coordinated CO2 fragment and the central carbon atom, respectively, in good agreement with the formation of betaine 4. However, at room temperature, zwiterionic CO2 adduct 4 reacts slowly hexamethyldisilazane byproduct to afford the O-silylated ester 5 [START_REF]THF/pentane solution and its structure was established by X-ray diffraction analysis (see supporting information). Unfortunately, treatment to isolate 5 leads to decomposition[END_REF]. Alternatively, CO2 adduct 4 can be trapped by adding one equivalent of MeI to generate the O-methylated salt 6, which is stable and was easily isolated in 80% yield. Compound 6 has been fully characterized by NMR spectroscopy (see experimental section and supporting information). Single crystals of 6 were grown layering pentane onto a saturated DCM at room temperature [START_REF](6-OTf) salts co-crystalized. Both structures are very similar and the complete data can be found in the supporting information[END_REF] and then analyzed by X-ray diffraction confirming the expected structure (Figure 3). The S1-C2 and S2-C2 bond lengths (1.720(2) Å and 1.713(3) Å, respectively) are slightly longer than in those of 3 (1.682 Å and 1.687 Å). The central carbon C1 shows a planar environment and the C1-C2 bond length is short 1.437(4) Å. This bonding pattern can be explained by a major contribution of the ester fragment to stabilize the negative charge [START_REF]The same tendency was observed in the O-silylated salt 5 (see supporting information)[END_REF]. In order to demonstrate the leaving group ability of sulfide group, we have considered the reaction of S,S-bis-ylide 2 with an electrophile bearing a lone pair such as a phosphenium ion (Scheme 3) [START_REF] Dellus | [END_REF]. The reaction of 2 with bis(diisopropylamino)phosphenium ion at -80 °C affords ylide 7 with a good selectivity when the reaction mixture is rapidly brought to room temperature [44]. The structure of 7 was established by X-ray diffraction analysis (Figure 4). Variable temperature NMR analysis indicate the formation of adduct 8 ( = 48.3 ppm in 31 P NMR) at -70°C that starts to evolve towards sulfonium ylide 7 ( = 25.7 ppm in 31 P NMR) above -40°C (carbene intermediate 9 was not detected). The process can be explained by the initial attack of nucleophilic bis-ylide 2 on the positively charged phosphenium salt generating a phosphino cationic ylide intermediate 8 (Scheme 4). The -donation of phosphine lone pair induces sulfide elimination and formation of phosphino(sulfonio)carbene 9. This push-pull carbene, substituted by a -donating phosphino-and a -accepting sulfonio-group, exhibits a strong electrophilic character on the phosphorus center [45], and therefore reacts with triflate anion to afford 10. Finally, rearrangement, starting with the nucleophilic attack of central carbon to the sulfoxide moiety, occurs to afford sulfonium ylide 7 which is stabilized by two -accepting substituents such as trifluoromethanesulfonyl and phosphonyl groups, as indicated by the short C1-S2 and C1-P1 bond lengths [1.694(2) and 1.807(2) Å, respectively]. Thanks to the high leaving group ability of sulfide ligand, such a strongly electrophilic phosphino(sulfonio)carbene 9, which reacts even with the generally non-coordinating triflate anion, can be generated from 8 by elimination of diphenyl sulfide. It should be noted that, in the case of carbodiphosphoranes, such an addition-elimination reaction does not take place. Instead, they react with a chlorophosphine to form a stable adduct similar to 8 [46,47].

Materials and Methods

General Comments

All manipulations were performed under an inert atmosphere of argon by using standard Schlenk techniques or high-pressure NMR tube techniques. Dry and oxygen-free solvents were used. 1 H, 13 C, 19 F and 31 P NMR spectra were recorded on Brucker Avance II 300 MHz, Avance III HD 400 MHz and Avance I and II 500 MHz spectrometers (Brucker, Karlsruhe, Germany). Chemical shifts are expressed in parts per million with residual solvent signals as internal reference ( 1 H and 13 C{ 1 H}). 19 F and 31 P NMR chemical shifts were reported in ppm relative to CFCl3 and 85% H3PO4, respectively. The following abbreviations and their combinations are used: br, broad; s, singlet; d, doublet; t, triplet; q, quartet; hept, heptuplet; m, multiplet. 1 H and 13 C resonance signals were attributed by means of 2D COSY, HSQC and HMBC experiments. Mass spectra were recorded on a Hewlett Packard 5989A spectrometer (Hewlett-Packard, Palo Alto, CA, United States). HRMS was realized on a Xevo G2 QTof apparatus (Waters, Milford, United States). Melting points were measured in a sealed glass tube on a Stuart SMP-30 melting point apparatus. All commercially available reagents were used without further purification otherwise noted. 1 was prepared following previously reported procedure [30].

Synthesis

S,S-Bis-ylide 2: In a J. Young NMR tube, protonated precursor of bis-ylide 1 (50 mg, 0.094 mmol, 1 eq.) was dissolved in THF-d8 (0.2 mL). At -80 °C, a solution of KHMDS (19 mg, 0.095 mmol, 1 eq.) in 0.2 mL of THF-d8 was added. The NMR tube was then closed and carefully shaken the mixture while remaining at low temperature. The yellow colored solution was obtained. Because of thermal instability of 2, the product was used or characterized at low temperature in presence of hexamethyldisilazane without any purification. 1 H NMR (400 MHz, 213 K, THF-d8): δ = 7.41-7.26 (m, 12H, CHar), 8.02-7.90 (m, 8H, CHar). 13 C-methylated salt 3: In a Schlenk flask containing bis-ylide precursor 1 (50 mg, 0.094 mmol, 1 eq.) and KHMDS (19 mg, 0.095 mmol, 1 eq.) was added 0.5 mL of THF at -80 °C. After the yellow coloration of the solution, 1 equivalent of iodomethane (5.9 µL, 0.094 mmol, 1eq) was added at the same temperature. The solution immediately became colorless and a white precipitate appeared. After 15 min, the solution was allowed to warm up to RT, then the volatiles were removed under reduced pressure. The crude was extracted with CH2Cl2 (3 x 2 mL). The resulting solution was dried using MgSO4 then evaporated to afford 3 as white solid (42 mg, 0.080 mmol, 85% yield). Crystals were grown layering diethyl ether onto a saturated CH2Cl2 solution. 1 H NMR (500 MHz, 298 K, CD3CN): δ = 1.67 (s, 3H, CH3), 7.54-7.57 (m, 8H, CHar), 7.59-7.69 (m, 12H, CHar). 13

CO2-adduct 4:

In a high-pressure NMR tube, to a solution of bis-ylide 2, freshly prepared from the protonated precursor 1 (50 mg, 0.094 mmol, 1 eq.) and KHMDS (19 mg, 0.094 mmol, 1 eq.) in THF-d8 (0.4 mL) at -70 °C, 3 bars of CO2 were applied. The tube was shaken carefully leading to a colorless solution. Because of thermal instability of 4, the product was characterized at 0 °C in presence of hexamethyldisilazane and CO2 without any purification. 1 H NMR (400 MHz, 273 K, CD3CN): δ = 7.30-7.42 (br s, 12H, CHar), 7.65-7.88 (br s, 8H, CHar). 13 O-methylated CO2 adduct 6: In a pressure NMR tube, to a solution containing freshly generated CO2 adduct 4 by above mentioned method, at -70 °C then warmed up to room temperature and CO2 pressure released. Subsequently, MeI (1.0 eq, 6 µL) was added then precipitation occurs overnight. After removal of volatiles under reduced pressure, the crude was washed with pentane (3 x 2 mL). Then 6 was extracted from the crude using DCM. After concentration, the solution was layered with pentane affording 6 as colorless crystals at room temperature (42 mg, 0.075 mmol, 80% yield). 1 H NMR (300 MHz, 298 K, CD2Cl2): δ = 3.67 (s, 3H, CH3), 7.55-7.65 (m, 16H, CHar), 7.66-7.75 (m, 4H, CHar). 13 Sulfonium ylide 7: In a Schlenk flask containing bis-ylide precursor 1 (50 mg, 0.094 mmol, 1 eq.) and KHMDS (19 mg, 0.094 mmol, 1 eq.) was added 0.5 mL of THF at -80 °C. After the yellow coloration of the solution, 1 equivalent of phosphenium triflate (35.7 mg, 0.094 mmol, 1eq) was added at the same temperature. The solution was rapidly brought to room temperature then the volatiles were removed under reduced pressure. The crude was extracted with CH2Cl2 (3 x 2 mL). The resulting solution was dried then evaporated. The crude was purified by flash silica gel chromatography using pentane and ethyl acetate as solvents. 3 was obtained as white solid (32 mg, 0.056 mmol, 60% yield). Crystals can be grown from a saturated CHCl3 solution. 1 H NMR (400 MHz, 298 K, C6D6): δ = 1.04-1.24 (bs, 12H, CH3), 1.24-1.34 (d, JPC = 7.0 Hz, 12H, CH3), ), 3.80-4.10 (bs, 4H, CH), 6.80-7.00 (bs, 6H, CHar), 7.90-8.20 (bs, 4H, CHar). 13

X-ray Data

The data of the structures for 3, 5, 6-I, 6-OTf and 7 were collected at 193 K on a Bruker-AXS APEX II CCD Quazar diffractometer [START_REF] Schobert | Organophosphorus Reagents[END_REF] equipped with a 30 W air-cooled microfocus source and on a Brucker-AXS D8-Venture diffractometer (3, 5, 6-I and 6-OTf) equipped with a Photon III-C14 detector with MoKα radiation (wavelength = 0.71073 Å) by using phi-and omega-scans. The data were integrated with SAINT, and an empirical absorption correction with SADABS was applied [START_REF] Sadabs | Program for data correction[END_REF]. The structures were solved using an intrinsic phasing method (ShelXT) [START_REF] Sheldrick | Integrated space-group and crystal-structure determination[END_REF] and refined using the least-squares method on F 2 (ShelXL-2014) [START_REF] Sheldrick | Crystal structure refinement with SHELXL[END_REF]. All non-H atoms were treated anisotropically. All H atoms attached to C atoms were fixed geometrically and treated as riding on their parent atoms with C-H = 0.95 Å (aromatic), 0.98 Å (CH3), 0.99 Å (CH2) or 1.0 Å (CH) with Uiso(H) = 1.2Ueq(CH, CH2) or Uiso(H) = 1.5Ueq(CH3). The structures 3 and 6-I were found to be disordered. Several restraints (SAME, SADI, SIMU, DELU) were applied to refine these disorders and to avoid the collapse of the structure during the least-squares refinement by the large anisotropic displacement parameters.

Conclusions

In summary, S,S-bis-ylide 2 readily reacts with MeI to give the C-methylated salt 3. Moreover, 2 also reacts with CO2 to give betaine 4 which was characterized in solution by NMR spectroscopy at low temperature. CO2 adduct 4 was still reactive and undergoes Osilylation and O-alkylation reactions, in the presence of HMDS and MeI respectively, to afford the corresponding salts 5 and 6. Finally, the high leaving group ability of sulfide ligand in 2 was demonstrated by an original reaction with phosphenium ions, yielding a stabilized ylide 7 probably through the transient formation of a strongly electrophilic phosphino(sulfonio)carbene 9. Efforts are currently underway to extend the potential of this type of ylides.

Figure 1 .

 1 Figure 1. Carbodiphosphoranes I, carbodicarbenes II, carbophosphinocarbenes III, phosphonio/sulfonio bis-ylides IV, phosphonio/sulfoxonio or phosphonio/iminosulfonio bis-ylides V, sulfonio/sulfonio bis-ylides VI, selenio-or sulfonio-/iminosulfonio VII and bis iminosulfanebisylides VIII.

Scheme 1 .

 1 Scheme 1. Synthesis of S,S-bis-ylide 2 and alkylation reaction with MeI.

Figure 2 .

 2 Figure 2. Molecular structure of C-methylated salt 3. Ellipsoids are drawn at the 30% probability level; hydrogen and disordered atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): S1-C1 1.682(9), C1-S2 1.687(9), C1-C2 1.525(12), S1-C3 1.794(4), S1-C9 1.759(3), S2-C21 1.772(4), S2-C15 1.776(3). S1-C1-S2 111.6(6), S2-C1-C2 123.8(6), C2-C1-S1 124.3(6), C9-S1-C1 107.4(3), C1-S1-C3 111.6(4), C3-S1-C9 100.2(2), C1-S2-C15 107.6(3), C15-S2-C21 101.5(2), C21-S2-C1 111.9(4).

Scheme 2 .

 2 Scheme 2. Reactivity of S,S-bis-ylide 2 with CO2 and corresponding esters 5 and 6.

Figure 3 .

 3 Figure 3. Cationic part of 6. Ellipsoids are drawn at the 30% probability level; hydrogen and disordered atoms and counter ion ( -I) were omitted for clarity. Selected bond lengths (Å) and angles (°): S1-C2 1.720(2), C2-S2 1.713(3), C1-C2 1.437(4), C1-O1 1.214(3), C1-O2 1.351(3), O2-C3 1.438(4), S1-C4 1.783(3), S1-C10 1.803(5), S2-C22 1.793(3), S2-C16 1.794(3). S1-C2-S2 117.5(1), S2-C2-C1 127.6(2), C1-C2-S1 114.1(2), C2-C1-O2 110.8(2), O2-C1-O1 122.6(3), O1-C1-C2 126.6(2).

Scheme 3 .

 3 Scheme 3. Reactivity of S,S-bis-ylide 2 with a phosphenium ion.

Figure 4 Scheme 4 .

 44 Figure 4 Molecular structure of 7. Ellipsoids are drawn at the 30% probability level; hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (°): S1-C1 1.724(2), C1-S2 1.694(2), C1-P1 1.807(2), S1-C14 1.803(2), S1-C20 1.788(2), P1-O1 1.487(1), P1-N1 1.669(2), P1-N2 1.656(2), S2-O2 1.434(2), S2-O3 1.437(1), S2-C26 1.841(2), C26-F1 1.327(2), C26-F2 1.336(2), C26-F3 1.333(2). S1-C1-P1 120.80(10), S1-C1-S2 109.97(10), P1-C1-S2 127.57(11).

  C{ 1 H} NMR (101 MHz, 213 K, THF-d8): δ = 39.0 (s, SCS), 127.1 (s, CHar), 129.3 (s, CHar), 129.6 (s, CHpara), 148.2 (s, Cipso).

  C{ 1 H} NMR (125 MHz, 298 K, CD3CN): δ = 10.2 (s, CH3), 23.4 (s, SCS), 129.7 (s, CHar), 131.4 (s, CHar), 132.7 (s, Cipso), 133.3 (s, CHpara). Mp = 116 °C (decomposition). HRMS (ESI+): m/z [M] + calculated for C26H23S2 = 399.1241 found = 399.1248.

  C{ 1 H} NMR (100 MHz, 273 K, CD3CN): δ = 40.2 (br s, SCS), 130.7 (s, CHar), 130.8 (s, CHar), 132.6 (s, CHpara), 134.4 (s, Cipso), 167.2 (br s, C-CO2).

  C{ 1 H} NMR (75 MHz, 298 K, CD2Cl2): δ = 44.0 (s, SCS), 52.6 (s, CH3), 129.2 (s, Cipso), 129.7 (s, CHar), 131.1 (s, CHar), 133.7 (s, CHpara), 164.2 (s, CO2CH3). Mp = 160 °C (decomposition). HRMS (ESI+): m/z [M] + calculated for C27H23O2S2 = 443.1139 found = 443.1140.

Intermediate 8 :

 8 C{ 1 H} NMR (126 MHz, 203 K, THF-d8): δ = 24.4 (bs, CH), 48.1 (d, JPC = 6.3 Hz, CH), 55.0 (d, JPC = 158.0 Hz, SCS), 122.5 (q, JFC = 321.9 Hz, CF3), 128.2 (s, CHar), 130.4 (s, CHar), 131.4 (s, CHar), 138.3 (s, Cipso). 31 P NMR (121 MHz, 298 K, C6D6): δ = 25.7. 19 F NMR (282 MHz, 298 K, C6D6): δ = -76.4. HRMS (ESI+): m/z [M+H] + calculated for C26H38F3N2O3PS2 = 579.2092 found = 579.2104. Variable temperature analysis was performed at -70 °C in THF-d8. Data attributed to intermediate 8 extracted from the crude analysis. 1 H NMR (400 MHz, 203 K, THF-d8): δ = 0.70-0.90 (bs, 8H, CH3), 1.10-1.40 (bs, 16H, CH3), 3.50-4.00 (bs, 4H, CH), 7.20-8.00 (m, 20H, CHar). 13 C{ 1 H} NMR (126 MHz, 203 K, THF-d8): δ = 24.4 (bs, CH3), 24.7 (bs, CH3), 34.5 (d, JPC = 96.8 Hz, SCS), 47.1 (d, JPC = 17.5 Hz CH), 49.6 (bs, CH), 122.4 (q, JFC = 322.6 Hz, CF3), 128.7 (s, CHar), 131.3 (s, CHar), 133.7 (s, CHar), 139.1 (s, Cipso). 31 P NMR (121 MHz, 203 K, THF-d8): δ = 46.1.

  Supplementary crystallographic data for CCDC-2250100 (3), CCDC-2250101 (5), CCDC-2250102 (6-I), CCDC-2250103 (6-OTf), CCDC-2250104 (7) can be obtained free of charge from The Cambridge Crystallographic Data Centre via https://www.ccdc.cam.ac.uk/structures/.

Funding: This research was funded by The Agence Nationale de la Recherche, ANR-19-CE07-0013.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement: Not applicable.

Acknowledgments:

The Agence Nationale de la Recherche (ANR-19-CE07-0013) and the Ministère de l'Enseignement Supérieur et de la Recherche are gratefully acknowledged for Ph.D. grant to U.A. and post-doctoral fellowship for V.S.V.S.N.S. The authors would like to thank the CNRS and the Université de Toulouse, UPS for financial support. NMR, mass and x-ray diffraction services from the Institut de Chimie de Toulouse (ICT -FR2599) are acknowledged for support.

Supplementary Materials: The following supporting information are available online at: www.mdpi.com/xxx/s1. NMR spectra and crystallographic data Author Contributions: Conceptualization, E.M., T.K., A.B.; investigation, U.A., V.S.V.S.N.S.; X-ray structural studies: N.S.-M.; writing-original draft preparation, E.M.; writing-review and editing, all authors; supervision, E.M., T.K.; project administration, E.M.; funding acquisition, E.M., T.K., A.B. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest:

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Sample Availability: Samples of the compounds are not available from the authors.