Regioselective Bond Cleavage in the Dissociative Electron Transfer to Benzyl Thiocyanates Abdelaziz Houmam,* ,² Emad M. Hamed, ² Philippe Hapiot, ‡ John M. Motto, ² and Adrian L. Schwan ² Department of Chemistry and Biochemistry, UniVersity of Guelph, Guelph, Ontario, Canada N1G 2W1, and Laboratoire d’Electrochimie Mole´ culaire, Synthe` se et Electrosynthe` se Organiques, UMR CNRS 6510, UniVersite´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France Received June 16, 2003; E-mail: houmam@chembio.uoguelph.ca Since the introduction of the dissociative electron transfer (ET) theory 1 much attention has been given to understanding whether ET and bond breaking are concerted or successive steps and to factors controlling the occurrence of either mechanisms. 2 According to this theory, the reaction activation energy depends on both thermodynamic and kinetic factors, through a quadratic activation- driving force relationship. 1 The difference in the reaction free energy between the two ET mechanisms can be expressed by the corresponding standard potentials (eq 1). The weaker the bond and the more positive E X • /X - 0 , the more favorable the thermodynamics of the concerted mechanism are. The intrinsic barrier contains solvent and inner reorganizations (ΔG 0,s q + ΔG 0,i q ). For a concerted ET, the main contribution to ΔG 0,i q is that of the bond dissociation. 1,2 In most reported studies, however, the same bond (R-X) is broken when the ET driving force is changed for a series of compounds. 2 While regioselective bond cleavage in dissociative ET reactions has been reported, concrete examples are however limited, and the factors controlling the regioselectivity remain unclear. 3 We recently reported the electro- chemistry of a series of aryl thiocyanates and showed that their reduction results in the cleavage of the S-CN bond (-cleavage) and involves a unique autocatalytic process. 4 Here we report the electrochemical reduction of benzyl (1) and p-nitrobenzyl (2) thiocyanates. Not only is a change of the ET mechanism observed, but more interestingly, a clear-cut example of a regioselective bond cleavage is also encountered. Furthermore, we show that both phenomena may be understood on the basis of the dissociative ET theory and its extension to the formation/dissociation reactions of radical ions. 1b,2 The cyclic voltammetry of benzyl thiocyanate (1) in acetonitrile (Figure 1a) displays an irreversible reduction peak at a potential E p )-2.20 V vs SCE. The peak width 4 has a value of 160 mV. The variation of the peak potential with the log(ν) is linear with a slope equal to 94 mV per unit log(ν). This first reduction peak corresponds to the consumption of one electron per molecule. A second irreversible reduction peak is observed at -2.53 V vs SCE and corresponds to the reduction of dibenzyl disulfide (Figure 1a). The coefficient transfer values determined from both the first reduction peak width 4 and from the E p - log(ν) plot 4 correspond to 0.29 and 0.31 respectively, i.e., much lower that 0.5, indicating a reaction kinetically controlled by an irreversible electron-transfer step. 1,2 The initial ET and the S-CN bond breaking (-cleavage) are concerted; the produced radical thiyl is immediately reduced at the electrode, yielding the benzyl thiolate anion. As for the aryl thiocyanates, the benzyl thiolate reacts on 1 to yield dibenzyl disulfide (Scheme 1). An important result here is the absence of autocatalysis, shown by trace crossing in the electrochemical reduction of the aryl thiocyanates. The reason being that dibenzyl disulfide is more difficult to reduce than 1. Electrolysis of 1 confirmed the stoichiometry of 1 electron per molecule and the formation of dibenzyl disulfide (3: 72%); it shows further the formation of dibenzyl sulfide (4: 26%) which results from the attack of the benzyl thiolate on the benzylic carbon of 1 with the ejection of thiocyanate anion. This has been confirmed by a control reaction where tetrabutylammonium benzyl thiolate and 1 have been mixed to yield both 3 and 4. 5 The cyclic voltammogram of p-nitrobenzyl thiocyanate (2) displays an irreversible reduction peak at a potential E p )-0.96 V vs SCE (Figure 1b). Its height corresponds to the consumption of 1 electron per molecule. The peak width has a value of 72 mV and the slope of the E p vs log(ν) plot is equal to 48 mV per unit log(ν). These peak characteristics correspond to a stepwise ET involving the intermediacy of a radical anion and with a mixed kinetic control by both the ET and the bond dissociation steps. This first irreversible peak is followed by a second reversible peak (E 0 )-1.19 V vs SCE) corresponding to the reduction of 4,4′- dinitrodibenzyl (5), by comparison with an authentic sample (Figure 1b), formed as a result of a chemical reaction following the ET where the thiocyanate anion is the leaving group (R-cleavage) and not the cyanide as seen for compound 1 in agreement with previous studies. 6 Electrolysis of 2 confirmed the stoichiometry of 1 electron ² University of Guelph. ‡ Universite´ de Rennes. E RX//RX •- 0 - E RX/R • +X •- 0 ) E RX/RX •- 0 + D R-X - E X • /X - 0 - TΔS RX/R • +X • (1) Figure 1. Cyclic voltammetry in CH3CN/TBAF (0.1 M) at a glassy carbon electrode, ν ) 0.2 V/s, temperature ) 20 °C of (a) 1: 2.35 mM ( ____ ), 3: 1.3 mM ( ___ ) and (b) 2: 2 mM ( ____ ), 5: 1.5 mM ( ___ ). Scheme 1 Published on Web 09/30/2003 12676 9 J. AM. CHEM. SOC. 2003, 125, 12676-12677 10.1021/ja036710x CCC: $25.00 © 2003 American Chemical Society