Regioselective S—O vs. C—O bond cleavage in sulfenate ester radical anions

Abstract

The electron transfer (ET) reduction of benzyl benzenesulfenate ester (1) and tert-butyl benzenesulfenate ester (2) was investigated using electrochemical techniques. Analysis of the cyclic voltammetry of each compound suggests that the ET reduction proceeds via a stepwise dissociative mechanism. The voltammograms of 1 are similar to those of diaryl disulfides and it was found through controlled potential electrolysis (CPE) product studies that ET reduction leads to S-O bond cleavage. The voltammograms of 2 are dramatically different with a sharper dissociative wave occurring at a more negative peak potential. CPE experiments indicate products that result from ET leading to C-O bond cleavage in this case. DFT calculations of the singly occupied molecular orbitals (SOMOs) of 1 and 2 were performed and offer a rationale for the different reactivity of the two radical anions.Résumé : On a étudié la réaction de réduction par transfert d'électron (TE) du benzènesulfénate de benzyle (1) et du benzènesulfénate de tert-butyle (2) en faisant appel à des techniques électrochimiques. L'analyse de la voltampéro-métrie cyclique de chaque composé suggère que la réduction par TE se produit par un mécanisme par dissociations successives. Le voltampérogramme du composé 1 est semblable à celui des disulfures de diaryles et, sur la base d'études des produits par électrolyse à potentiel contrôlé (EPC), on a trouvé que la réduction par TE conduit à une coupure de la liaison S-O. Le voltampérogramme du composé 2 est dramatiquement différent et il présente une vague dissociative qui se présente à un pic de potentiel beaucoup plus négatif. Les études d'EPC indiquent que, dans ce cas, les produits qui résultent d'un TE proviennent d'une coupure d'une liaison C-O. Des calculs effectués sur la base de la théorie de la densité fonctionnelle des orbitales moléculaires à occupation singulet (SOMO) des composés 1 et 2 et ils permettent de présenter une rationalisation pour les réactivités différentes des deux radicaux anions.

Electron Transfer Initiated Reactions: Bond Formation and Bond Dissociation

“…Bond cleavage following reduction of the initial reactant has been shown to take place for a wide variety of species including neutral structures, ions and radicals. Reductive DET has been shown to lead to the dissociation of a large variety of chemical bonds including C−C, – C−S, – C−O, – C−N, – O−O, – N−S, – N−O, – S−S, – S−O, C−halogen, – N−halogen, – and S−halogen, – among others. The most investigated reductive ET-initiated cleavage process is probably the reduction of organic halides (Scheme ).…”

“…Bond cleavage following reduction of the initial reactant has been shown to take place for a wide variety of species including neutral structures, ions and radicals. Reductive DET has been shown to lead to the dissociation of a large variety of chemical bonds including C-C, [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22] C-S, [23][24][25][26][27][28][29][30][31] C-O, [32][33][34][35][36][37][38][39][40][41][42] C-N, [43][44][45][46] O-O, [47][48][49][50][51] N-S, [43][44][45][46]<...…”

Reaction mechanisms : Part (i) Radical and radical ion reactions

“…71 Fragmentation of the intermediate free radical has been rationalized on the basis of a curve-crossing model. 72 ð13Þ Other studies of reductive bond cleavage which appeared in 2005 include the Ti-mediated ring opening of epoxides, 73 C-S bond cleavage in sulfides, 74 S-O and C-O bond cleavage in sulfenate esters, 75 and C-O bond cleavage in N-alkyl-4picolinium esters (a system cleverly used in the context of a photo-labile protecting group for carboxylic acids, Scheme 23). 76 One electron reduction and C-N bond cleavage were invoked to explain the chemistry of diazoparaquinone natural products in the context of DNA damage.…”