Hamed Emad M, Doai Hanh, McLaughlin Christopher K, Houmam Abdelaziz
Electrochemical Technology Center, Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N1G 2W1.
J Am Chem Soc. 2006 May 24;128(20):6595-604. doi: 10.1021/ja056730u.
Important aspects of the electrochemical reduction of a series of substituted benzyl thiocyanates were investigated. A striking change in the reductive cleavage mechanism as a function of the substituent on the aryl ring of the benzyl thiocyanate was observed, and more importantly, a regioselective bond cleavage was encountered. A reductive alpha-cleavage (CH(2)-S bond) was seen for cyano and nitro-substituted benzyl thiocyanates leading to the formation of the corresponding nitro-substituted dibenzyls. With other substituents (CH(3)O, CH(3), H, Cl, and F), both the alpha (CH(2)-S) and the beta (S-CN) bonds could be cleaved as a result of an electrochemical reduction leading to the formation of the corresponding substituted monosulfides, disulfides, and toluenes. These final products are generated through either a protonation or a nucleophilic reaction of the two-electron reduction-produced anion on the parent molecule. The dissociative electron transfer theory and its extension to the formation/dissociation of radical anions, as well as its extension to the case of strong in-cage interactions between the produced fragments ("sticky" dissociative electron transfer (ET)), along with the theoretical calculation results helped rationalize (i) the observed change in the ET mechanism, (ii) the dissociation of the radical anion intermediates formed during the electrochemical reduction of the nitro-substituted benzyl thiocyanates, and more importantly (iii) the regioselective reductive bond cleavage.
研究了一系列取代苄基硫氰酸酯的电化学还原的重要方面。观察到还原裂解机理随苄基硫氰酸酯芳环上取代基的变化而发生显著改变，更重要的是，遇到了区域选择性键裂解。氰基和硝基取代的苄基硫氰酸酯发生还原α-裂解（CH(2)-S键），导致相应的硝基取代二苄基的形成。对于其他取代基（CH(3)O、CH(3)、H、Cl和F），电化学还原会导致α（CH(2)-S）键和β（S-CN）键均发生裂解，从而形成相应的取代单硫化物、二硫化物和甲苯。这些最终产物是通过双电子还原产生的阴离子在母体分子上的质子化或亲核反应生成的。离解电子转移理论及其对自由基阴离子形成/离解的扩展，以及其对所产生片段之间强笼内相互作用情况（“粘性”离解电子转移（ET））的扩展，连同理论计算结果有助于解释：（i）观察到的ET机理变化；（ii）硝基取代苄基硫氰酸酯电化学还原过程中形成的自由基阴离子中间体的离解；更重要的是（iii）区域选择性还原键裂解。
