Reactivity of electrogenerated polysul–de ions towards acyl thioanhydrides and anhydrides in N,N-dimethylacetamide Julie Robert, Meriem Anouti and Jacky Paris* L aboratoire de Physicochimie des Interfaces et des Milieux UFR Sciences et Reç actionnels, T echniques, Parc de Grandmont, 37200 T ours, France The reactivity of electrogenerated polysul–de ions in N,N-dimethylacetamide has been followed S3~~ (HS62~) by spectroelectrochemistry of a series of RC(O)X species : thioanhydrides X\SC(O)R 1a, 2a) (R\CH3 C6H5 and anhydrides X\OC(O)R 3b, 4b, 5b, 6b).With thioanhydrides two steps (R\CH3 n-C3H7 t-C4H9 C6H5 were evidenced: (i) formation of RC(O)S~ in equilibrium with from both fast substitution at the RC(O)S2~ trigonal carbon and exclusion from the nucleofugic anion X~: (ii) subsequent reaction of on RC(O)S2~ substrates leading to diacyl disul–des. With anhydrides the –rst step only occurs at a slower rate.The electrolysis of sulfur in the presence of 1a or 2a allowed the preparative scale formation of as RC(O)S2C(O)R isolated products from the ìelectrochemical insertion of sulfur œ in diacyl monosul–des. Reç activiteç des ions polysulfures e ç lectrogeç neç reç s dans le dimeç thylaceç tamide vis-a` -vis des thioanhydrides et anhydrides dœacides carboxyliques La reç activiteç des ions polysulfures eç lectrogeç neç reç s dans le N,N- S3~~ (HS62~) dimeç thylaceç tamide a eç teç suivie par spectroeç lectrochimie vis-a` -vis dœune seç rie de deç riveç s RC(O)X: thioanhydrides X\SC(O)R 1a, 2a), anhydrides X\OC(O)R 3b, 4b, 5b, 6b).(R\CH3 C6H5 (R\CH3 n-C3H7 t-C4H9 C6H5 Avec les thioanhydrides, deux eç tapes sont mises en eç vidence : (i) formation des ions RC(O)S~ et en RC(O)S2~ eç quilibre du fait de la substitution sur le carbone trigonal et de lœobtention de lœanion nucleç ofuge X~; (ii) reç action ulteç rieure des ions sur les substrats conduisant aux diacyldisulfures. Avec les anhydrides seule la RC(O)S2~ premie` re eç tape sœeÜectuant plus lentement est observeç e.Lœeç lectrolyse du soufre en preç sence des espe` ces 1a ou 2a reç aliseç e au niveau preç paratif a permis dœisoler les diacyldisulfures issus de ì lœinsertion eç lectrochimique du soufreœ sur les diacylmonosulfures. As reported recently,1 acyl chlorides ìinstantaneouslyœ react with ions in N,N-dimethylacetamide, a dipolar S3~~ (HS62~) aprotic medium, to produce diacyldisul–des (62»75% yield).Two successive steps were evidenced by spectroelectrochemistry : (i) initial substitution (eqn 1) of the leaving group, with concurrent equilibria (eqns 2 and 3) as established by direct addition of sulfur to thiocarboxylate ions :2 RC(O)Cl]2 S3~~]RC(O)S~]5/2 S2]Cl~ (1) 2 RC(O)S~]3 S2H[RC(O)]2S2]2 S3~~ (2) 2 RC(O)S~]S2H2 RC(O)S2~ (3) and (ii) subsequent reaction (eqn 4) of species : RC(O)S2~ RC(O)S2~]RC(O)Cl][RC(O)]2S2]Cl~ (4) Eqns 1 and 4 are analogous to those implied in the formation of diacylperoxides from RC(O)X [X\Cl, OC(O)R] and superoxide ions in aprotic media.3 O2~~ We report here on the relative reactivities of electrogenerated ions towards acylating agents : S3~~ ìthioanhydridesœ 1a, 2a) and [RC(O)]2S (R\CH3 C6H5 anhydrides 3b, 4b, 5b, [RC(O)]2O (R\CH3 n-C3H7 t-C4H9 6b).Reactions were followed at 20 °C by UV-vis absorp- C6H5 tion spectrophotometry coupled with stationary voltammetry. Results Sulfur-polysul–de ion characteristics in DMA The partial dissociation (eqn 5) of cyclooctasulfur into S8 S2 molecules was recently proposed by our group in dimethylacetamide: 4 S8H4 S2 (5) K1(297 K)\[S2]4/[S8]\10~7 mol3 dm~9 (6) In aprotic media such as DMA, sulfur reduces in two twoelectron steps with respect to the cyclic form [waves R1, S84 V vs. reference and R2, V, experi- E1@2\[0.40 E1@2\[1.10 mental value lA mmol~1 dm3] on a rotating i(R1)/[S8]0\34 gold-disc electrode.In the presence of excess of sulfur we expect the initial single-electron transfer to be S2]e~]S2~~ followed by the reaction of with the dimeric ions, up S2 S42~ to the formation of or species.4,5 The S62~ (HS3~~) S82~ stable product of the overall electrolysis of at controlled S8 potential on R1 (eqn 10) is the blue anion-radical S3~~ nm, dm3 mol~1 cm~1) through the (jmax\617 emax\4390 disproportionation (eqn 8) of the carmine red ions S82~ nm, dm3 mol~1 cm~1; (jmax1\515 emax1\3800 jmax2\360 nm, dm3 mol~1 cm~1) : emax2\9000 S8]2 e~]S82~ (7) S82~Hb f 2S3~~]S2 (8) K2(297 K)\[S3~~]2[S2]/[S82~]\1.7]10~6 mol2 dm~6 (9) S8]8/3 e~]8/3 S3~~ (10) ions are in equilibrium with their dimer S3~~ S62~ (jmax\ 465 nm, dm3 mol~1 cm~1) : emax\3100 S62~H2 S3~~ (11) New J.Chem., 1998, Pages 53»56 53l / nm l / nm ( a) ( b) A A E / V i / mA K3\[S3~~]2/[S62~]\0.043 mol dm~3 (12) UV-vis absorption spectra mol~1 cm~1) of (ei/dm3 S8 , ions between 250 and 750 nm were pre- S82~, S62~, S3~~ viously reported.5 In dilute solutions remains low with [S62~] respect to (i.e., 16% at total concentration [S3~~] [S3 ~~]0T\ mol dm~3). and [S3~~]]2 [S6 2~]\5.0]10~3 S82~ S([1/3) ions oxidize (O1) and reduce (R2) at the same potentials V; V].[E1@2(O1)\[0.20 E1@2(R2)\[1.10 Reactivity of ions with the thioanhydrides 1a, 2a S3 ~ó As observed with acyl chlorides,1 the addition of the thioanhydrides 1a, 2a to a sulfur solution greatly enhances the limiting current of the reduction wave R1: i(R1)exp./i(R1)th.\ 2.3 (1a) and 2.0 (2a) for This homoge- [(RCO)2S]/[S8]0\2.0. neous catalytic eÜect (eqns 7 and 13) agrees with the fast regeneration (eqn 13) of sulfur in the course of the reaction of polysul–de ions with substrates RC(O)X [X\Cl1, SC(O)R] in the diÜusion layer : RC(O)X]S82~]RC(O)S~]7/8 S8]X~ (13) Here the nucleofuge X~ and the substitution product would be the same species : RC(O)S~.This was veri–ed by the addition of a concentrated solution of thioanhydride 1a or 2a in DMA (2.0»7.0]10~2 mol dm~3) to ions of total con- S3~~ centrations close to 5.0]10~3 mol dm~3.Fig. 1 and [S3~~]0 T 2 show the evolution of A\f(j) and i\f(E) as a function of the ratio for the example y\[RC(O)X]/[S3~~]0 T R\CH3 with mol dm~3. As long as y remains [S3~~]0T\5.22]10~3 below B0.15 (Fig. 1a), decreases in favor of A617 (S3~~) A515 and with an isosbestic point at 540.5 nm; there is A360 (S82~) Fig. 1 (a) Evolution of UV-vis spectra during the addition of diacetyl sul–de 1a to an S([1/3) solution, mol [S3~~]0T\5.22]10~3 dm~3. The thickness of the cell was 0.1 cm; y\ (curve 1), 0.03 (2), 0.05 (3), 0.08 (4), 0.11 (5), [(RCO)2S]/[S3~~]0T\0 0.14 (6), 0.15 (7). (b) The same as (a) with y\0.15 (7), 0.24 (8), 0.33 (9), 0.50 (10), 0.71 (11), 0.84 (12), 1.27 (13) Fig. 2 Evolution of voltammograms during the reaction of diacetyl sul–de 1a with S([1/3) ions. Same conditions as for Fig. 1. Rotating gold-disc electrode, )\1000 rev min~1, diameter\2 mm; E vs. Ag/ AgCl, KCl satd in (0.1 mol dm~3) reference DMA»N(Et)4ClO4 no sign of R1 on any of the voltammograms. The stoichi- (S8) ometry (eqn 15) is the same as with acyl chlorides : sulfur coming from the substitution (eqn 14) totally reacts with S3~~ ions in excess according to eqn 8b: [RC(O)]2S]2 S3~~]RC(O)S~]5/2 S2]RC(O)S~ (14) [RC(O)2]2S]7 S3~~]2 RC(O)S~]5/2 S82~ (15) At the same time, the oxidation wave of RC(O)S~/RC(O)S2~ ions (the electroanalytic process eqns 16]3]17 previously described,2 V) increases at the expense of the E1@2\]0.09 one V).S82~/S3~~ (E1@2\[0.20 2 RC(O)S~][RC(O)]2S2]2 e~ (16) 2 RC(O)S2~][RC(O)]2S2]S2]2 e~ (17) The subsequent consumptions of the two ions and S3~~ S82~ by a shift in the equilibrium (eqn 8f) (0.15\y\0.5) permits the detection of sulfur by the growth of its cathodic wave R1 V).For y\0.5 (stoichiometry of eqn 14), the (E1@2\[0.40 equilibria (eqns 2 and 3) bear out the remaining presence of polysul–de ions in the solution ; the and concentra- S3~~ S82~ tions calculated from and the constants2 K1, K2 , K3 K4 , K5 lead to and values close to the experimental ones A617 A515 (^10%): K4\[RC(O)S2~][S3~~]2/[RC(O)S~]2[S2]3 \(12^2) dm6 mol~2 (18) K5\[RC(O)S2~]2/[RC(O)S~]2[S2]1\(48^4) dm3 mol~1 (19) With further additions of (0.5\y\1.0, curves [RC(O)]2S 10»13), and ions continue to be consumed S3~~ S82~ [decrease in and i(O)] but these species cannot be A617 , A515 totally eliminated because of the weak oxidation (eqn 2) of the nucleofugic RC(O)S~ ions.Low concentrations of ions (eqn 3) are revealed in the spectra (Fig. 1b, CH3C(O)S2~ curves 12, 13) by their characteristic absorbances2 (jmax1\ 336 nm, dm3 mol~1 cm~1; nm, emax1\4800 jmax2\467 dm3 mol~1 cm~1). i(R1) continues to rise with emax2\800 values greater than those of generated due to a catalytic S8 , eÜect analogous to eqns 7]13, which was previously noticed when diacyldisul–des were added to sulfur.2 At y\1, the oxidation current of ions is in agreement RC(O)S~/RC(O)S2~ with that resulting from the overall eqn 20. 2 [RC(O)]2S]2 S3~~][RC(O)]2S2]2 S2]2RC(O)S~ (20) 54 New J. Chem., 1998, Pages 53»56E/ V i / mA E / V i / mA Fig. 3 Evolution of voltammograms during the electrolysis of a solution with mol dm~3 in the presence of diben- [S8]0\1.05]10~3 zoylsul–de, mol dm~3 at E\[1.0 V vs. [2a]0\1.74]10~3 reference n F mol~1 2a\0 (curve 1), 0.37 (2), 0.75 (3), 1.12 (4), 1.49 (5), 1.86 (6), 2.24 (7), 2.62 (8) With the addition of sulfur, ions are not oxi- C6H5C(O)S~ dized in accordance with eqn 22 and the residual formation of ions (eqn 3) is only detected by their electro- RC(O)S2~ catalytic and kinetic oxidation wave2 (eqns 16, 3]17, E1@2\ ]0.35 V).The evolution of i\f(E) and A\f(j) for the reaction of with are the same as with [C6H5C(O)]2S S3~~ R\alkyl (0\y\1) except that ions totally dis- S82~/S3~~ appear at y\0.5. The electrochemical reduction of sulfur (EB[1.0 V) in the presence of the thioanhydrides 1a, 2a, which con–rms the preceding results, is illustrated in Fig. 3 with the experimental conditions : mol dm~3, [(C6H5CO)2S]0\1.74]10~3 mol dm~3. For 0\n F mol~1 2a\2 [S8]0\1.05]10~3 (curves 2»6) the decrease of the catalytic current i(R1) goes with the increase of the anodic waves of the RC(O)S2~ V) and RC(O)S~ V) ions.2 Two (E1@2\]0.35 (E1@2\]0.72 steps were observed when solutions were elec- RC(O)Cl]S8 trolyzed in the same way:1 (i) initial formation (eqn 21) of diacyldisul–de (0\n\1), with only appearance of the oxidation current of Cl~ ions on the voltammograms: 2 RC(O)X]S8]2 e~][RC(O)]2S2]3/4 S8]2 X~ (21) and (ii) reduction (eqn 22) of by polysul–de ions [RC(O)]2S2 (1\n\2), with the growth of the anodic wave of RC(O)S~ Fig. 4 Evolution of voltammograms during the addition of trimethylacetic anhydride 5b to an S([1/3) solution, [S3~~]0T\5.69 ]10~3 mol dm~3. (curve 1), 0.14 (2), 0.25 y\[RCO)2S]/[S3~~]0T\0 (3), 0.47 (4), 0.79 (5), 1.22 (6) ions : [RC(O)]2S2]S8]2 e~]2 RC(O)S~](S8) (22) In our particular case X~ species are RC(O)S~ ions, which are then generated on the basis 1 RC(O)S~/1 F.The overall process looks like the noteworthy ìelectrochemical insertion œ (eqn 23) of sulfur into thioanhydrides : 2 [RC(O)]2S]S2]2 e~][RC(O)]2S2]2 RC(O)S~ (23) Beyond n\2, ions result from the reduction of S82~/S3~~ sulfur (growth of and i(O) at V, A617 , A515 E1@2\[0.20 curves 7, 8). The electrolysis of 1a, 2a with sulfur [RC(O)]2S added as a ìmediatorœ at a ratio of 8 [S8]0/[RC(O)]2SB2.5 were performed on a preparative scale Mn\1 F mol~1 was the only product isolated [RC(O)]2SN.[RC(O)]2S2 (R\ yield 48%; 74%). CH3, R\C6H5 , Reactivity of ions with anhydrides 3bñ6b S3 ~ó An analogous study was carried out with anhydrides as substrates. Whatever the nature of R (3b»6b) the enhancement of the reduction current of sulfur was only observed at R2 potentials with the addition of [RC(O)]2O: i(R1]R2)exp./i(R1 for As noticed on the ]R2)th.B1.5 [(RCO)2O]/[S8]0\2.0.–rst wave R1 with thioanhydrides, this observation agrees with the catalytic eÜect (eqns 24]25), which implies here the more reducing agents These last species were not gen- S42~.4,6 erated in the present study by quantitative electrolysis of sulfur. S8]4 e~]2 S42~ (24) [RC(O)]2O]S42~]RC(O)S~]3/8 S8]RCO2~ (25) Thiocarboxylate ions (R\3»6) proved to be practically unreactive towards anhydrides at room temperature: the maximal absorbance and the oxidation wave of a solution A262\1.90 of mol dm~3 V) [CH3C(O)S~]0\2.90]10~3 (E1@2\]0.31 only decreased by 5% in the presence of [CH3C(O)]2O\3.0 ]10~3 mol dm~3 whereas no spectroelectrochemical changes were noticed for [(t- mol dm~3 C4H9CO)2S~]\2.70]10~3 with [(t- mol dm~3.In the C4H9CO)2O]ad\3.60]10~3 presence of sulfur, the reactions were limited : for mol dm3; 8 [CH3C(O)S~]0\2.27]10~3 [S8]0\11.0 ]10~3 mol dm~3; mol dm~3, [(CH3CO)2O]ad\8.5]10~3 the anodic current of ions RC(O)S2~/RC(O)S~ (E1@2\]0.05 V) retained 60% of its initial value at equilibrium after 10 min while i(R1) increased because of the catalytic eÜect (eqns 7]13) due to the partial formation of Under [RC(O)]2S2 .the same conditions solutions were unre- C6H5C(O)S~]S8 active towards benzoic anhydride. When anhydrides 3b»6b were added to ions (R\t- Fig. 4), the evolutions S3~~ C4H9 , of the spectra and voltammograms for 0\y\0.5 were identical to those observed with thioanhydrides (i.e., Figs. 1a and 1b, curves 1»10) or acyl chlorides ;1 however, except for R\ the reactions slowed down for y greater than 0.3 : as an C6H5 , example for mol dm~3, y\0.40, equi- [S3~~]0T\5.50]10~3 libria were attained after 1 min with and 8 min R\n-C3H7 with Beyond y\0.5, the addition of alkyl sub- R\t-C4H9 .strates only partially consumed ions and RC(O)S~/RC(O)S2~ at a slow rate : e.g., from curve 6 of Fig. 4 which was recorded at y\1.22 after 15 min, 80% of anionic species remained in solution. Discussion Diacyldisul–des are usually synthesized by chemical7 or electrochemical8 oxidation of thiocarboxylate ions and reactions of acyl chlorides with or under PTC con- Li2S29 Na2Sx ditions.10 Our results establish that these species are readily New J. Chem., 1998, Pages 53»56 55obtained by the reactions of thioanhydrides with poly- S3~~ sul–de ions at room temperature, as observed with acyl chlorides. 1 In both cases, thiocarboxylate ions coming from the fast nucleophilic substitution on the carbonyl carbon react in the presence of sulfur with the organic substrates RC(O)X. The formation of species can be explained by an [RC(O)]2S2 enhanced reactivity of intermediate ions compared RC(O)S2~ to RC(O)S~.This a eÜect,11 already displayed with RS2~ ions,5 probably competes with the displacement of the equilibrium (eqn 2f) by consumption of the stronger nucleo- S3~~ philes. The lower reactivity of anhydrides in general compared to that of acyl chlorides12 only allows access to RC(O)S~ ions. With respect to thioanhydrides, the same observation agrees with ìthe relative weakness of the overlapping of the C(2p) and S(3p) orbitals in the carbon»sulfur bondœ as noted by Cronyn et al.13 Thioanhydrides can be easily prepared by acylation of thiocarboxylate ions.14,15 These more stable species appear to be as efficient acylating agents as acyl chlorides in aprotic media.Experimental Materials and equipment Diacetyl sul–de 1a and anhydrides 3b»6b were obtained from Aldrich and used as received (purity[98%).Dibenzoyl sul–de 2a (mp 45»47 °C, lit.15 47»48 °C) was previously synthesized2 by addition of benzoyl chloride to electrogenerated thiobenzoate ions from thiobenzoic acid. Spectroelectrochemical experiments were carried out in DMA (Aldrich) with added tetraethylammonium perchlorate (Fluka, 0.1 mol dm~3) at 20 °C with equipment, electrodes and the —ow-through cell previously described.4 Potential values refer to Ag/AgCl, KCl satd in (0.1 mol dm~3).DMA/N(Et)4ClO4 Analysis of diacyl disul–des was performed by GC-MS (Hewlett-Packard 5989 A) and NMR spectroscopy (Bruker AC 200 spectrometer, as solvent, J values in Hz at CDCl3 200.132 and 50.323 mHz for 1H and 13C NMR, respectively).Generation of S(‘1/3) ions S([1/3) solutions (40 cm3) were prepared at concentrations near 5]10~3 mol dm~3 before addition of concentrated RC(O)X substrates in DMA cm3) by electro- (Vmax\4 reduction of sulfur at controlled potential (R2, EB[1.4 V) on a large gold-grid electrode.4 ions were the S3~~ (HS62~) only species in solutions when reached its maximum A617 value.Preparative electrolysis and were obtained by electro- [CH3C(O)]2S2 [C6H5C(O)]2S2 lysis of sulfur ([0.7 V\E\[0.5 V) with added homologous diacyl monosul–des up to 1 F mol~1 (eqn 23). [RC(O)]2S The experimental conditions (two-compartment cell, electrodes, procedure and puri–cation) were the same as with acyl chlorides.1 The intensity remained at a high value (200»250 mA) in the course of the electroreductions because of catalytic eÜects with both substrates and products [RC(O)]2S [RC(O)]2S2 .Diacetyl disul–de. Diacetyl sul–de: 1.18 g (10 mmol); S8: 0.77 g (24 mmol S). Product: diacetyl disul–de (0.36 g, 48%); 2.54 (6 H, s) ; 28.8 (2 C) and 189.5 (2 C); m/z 150 (M`, dH dC \2%) and 43 (100). Dibenzoyl disul–de. Dibenzoyl sul–de: 0.72 g (3 mmol); S8 : 0.24 g (7.5 mmol S).Product dibenzoyl disul–de (0.41 g, 74%); mp 135»136 °C (lit. :16 136»136.5 °C); 7.53»7.75 (6 H, m) dH and 8.13 (4 H, d, 7.4 Hz); 128.1 (4 C), 129 (4 C), 133.8 3J1H dC (2 C), 134 (2 C), and 186 (2 C); direct introduction mode m/z 274 (M`, 4%), 105 (50), 77 (100) and 51 (25). References 1 J. Robert, M. Anouti, M. Abarbri and J. Paris, J. Chem. Soc., Perkin T rans. 2, 1997, 1759. 2 J. Robert, M. Anouti and J. Paris, J. Chem. Soc., Perkin T rans. 2, 1997, 473. 3 (a) R. Johnson, T etrahedron L ett., 1976, 5, 331; (b) D. T. Sawyer, J. J. Stamp and K. A. Menton, J. Org. Chem., 1983, 48, 337; (c) J. P. Stanley, J. Org. Chem., 1980, 45, 1413; (d) A. Le Berre and Y. Berguer, Bull. Soc. Chim. Fr., 1966, 7, 2368. 4 G. Bosser and J. Paris, New J. Chem., 1995, 19, 391 and references therein. 5 G. Bosser, M. Anouti and J. Paris, J. Chem. Soc., Perkin T rans. 2, 1996, 1993. 6 J. Paris and V. Plichon, Electrochim. Acta, 1982, 27, 1501. 7 R. L. Franck and J. R. Blegen, Org. Synth. Coll., 1955, 3, 116»118. 8 Y. Hirabayashi and T. Mazume, Bull. Chem. Soc., Jpn., 1966, 39, 1971. 9 J. A. Gladysz, V. K. Wong and B. S. Jick, T etrahedron, 1979, 35, 2329. 10 (a) M. Kodomari, M. Fukuda and S. Yoshitomi, Synthesis, 1981, 8, 637; (b) J. X. Wang, W. Cui, Y. Hu and K. Zhao, Synth. Commun., 1995, 25, 889. 11 J. E. Dixon and T. C. Bruice, J. Am. Chem. Soc., 1972, 94, 2052 and references therein. 12 J. March, Advanced Organic Chemistry: Reactions, Mechanisms and Structure, Wiley, New York, 1992, p. 409. 13 M. W. Cronyn, M. Chang Pin and R. A. Wall, J. Am. Chem. Soc., 1955, 77 3031. 14 (a) E. E. Reid, Organic Chemistry of Bivalent Sulfur, Chemical Publishing, New York, 1962, vol. 4, pp. 11»58; (b) H. Boé hme and H. P. Steudel, L iebigs Ann. Chem., 1969, 730, 121. 15 M. Mikolajczyk, P. Kielbasinski and H. M. Schiebel, J. Chem. Soc., Perkin T rans. 1, 1976, 564. 16 C. Christophersen and P. Carlsen, T etrahedron, 1976, 32, 745. Received 28th May 1997; Paper 7/06743G 56 New J. Chem., 1998, Pages 53»56