Passage from Concerted to Stepwise Dissociative Electron Transfer as a Function of the Molecular Structure and of the Energy of the Incoming Electron. Electrochemical Reduction of Aryldialkyl Sulfonium Cations

Abstract

The electrochemical reductive cleavage of the carbon-sulfur bond in the title compounds offers the example of a reaction where the concerted or stepwise character of the electron-transfer-bond-breaking process is a function of molecular structure. As with the reductive cleavage of the carbon-halogen bond in benzyl halides and of the nitrogen-halogen bond in aromatic N-halosultams, the two main factors governing the nature of the mechanism are the LUMO energy and the bond strength in the starting molecule: the higher the former and the weaker the latter, the greater the tendency for the concerted mechanism to prevail over the stepwise mechanism and vice versa. Consistently with the effect of these two mechanism-governing factors, two borderline cases were identified where the reaction passes from the concerted pathway to the stepwise pathway upon increasing the driving force by raising the scan rate and thus shifting the reduction potential toward negative values. The reasons for possible variations of the concerted or stepwise character of the mechanism of reductive cleavages upon changing the mode of electron injection are discussed. The electrochemical reduction of sulfonium salts has been the object of several studies in the past from which the reaction mechanism in Scheme 1 has emerged.283 The radical resulting from the initial reductive cleavage reaction may give rise tovarious products not involving further consumption of electrons or, in most cases, be further reduced to the carbanion, which is eventually converted into the corresponding hydrocarbon. The starting sulfonium cation plays the role of the acid in the latter reaction when the medium does not contain additional acids, as when the reaction is carried out in unbuffered aprotic solvents and when at least one carbon bonded to the sulfur bears at least one hydrogen. Half of the starting sulfonium cation is then converted into the corresponding ylid, and the overall reaction follows consequently a one electron per molecule stoichiometry (Scheme 2).4 Previous electrochemical studies indicate that the most stable radical is formed upon reductive cleavage as, for example, phenacyl, detected under the form of acetophenone, in phenacyldiethyl and phenylmethyl sulfonium s a l t ~ ; ~ M cyanomethyl in the dimethylcyanomethyl sulfonium cation;S* benzyl in the benzyldimethyl sulfonium cation, as detected by means of a radical t r a ~ ; ~ b and methyl, isopropyl, benzyl, p-cyanobenzyl, phenacyl, and .CH2(Ph)C=C(CN)2 in the corresponding phenylmethyl and 1 -naphthylmethyl sulfonium salts, as attested by the formation (1) (a) Universite Denis Diderot. (b) Eastman Kodak Co. (2) (a) For a review, see: ref 2b. (b) Chambers, J. Q. Organic Sulfur Compounds. In Encyclopedia ofEIectrochemistry ofrhe Elements, Organic Section; Bard, A. J., Lund, H., Eds.; Dekker: New York, 1978; Vol. XII, pp (3) (a) Saeva, F. D.; Morgan, B. P. J. Am. Chem. Soc. 1984, 106,4124. (b) Saeva, F. D. Tetrahedron 1986,42, 6123. (c) Saeva, F. D. Top. Curr. Chem. 1990, 156,61. (4) (a) The roleof ylid formationhasbeen pointedout in earlypolarographic studies of phenacyldiethyl and phenylmethyl sulfonium Ylid formation during the reduction in aprotic solvents has been taken advantage of for synthetic purposes, making them react with carbonyl compounds.”.’ (b) Savbnt, J.-M. C. R. Hebd. Seances Acad. Sci. 1963, 257, 448. (c) Savht, J.-M. C. R. Hebd. SeancesAcad.Sci. 1964,258,585. (d) Savht, J.-M. Bull. SOC. Chim. 1967, 481. (e) Shono, T.; Mitani, M. Tetrahedron Lett. 1969, 687. ( f ) Shono, T.; Akazawa, T.; Mitani, M. Tetrahedron 1969, 687. (5) (a) Wagenknecht, J. H.; Baizer, M. M. J . Electrochem. SOC. 1967, 114, 1095. (b) Baizer, M. M. J . Org. Chem. 1966, 31, 3847. 329-502. 0002-7863/94/ 1516-7864$04.50/0 Scheme 1 + R e + S 7-” + e ’ 1 ( in one or two steps ? )