Electrochemical reduction of unsaturated nitriles in acetonitrile and nitromethane

The electrochemical reduction of five conjugated and non-conjugated nitriles, CH₃CHCHCN (1a, trans/cis = 62/38%); CH₃CH₂CHCHCN (1b, cis); CH₃CHCHCH₂CN (1c, trans), CH₃CHC(CH₃)CN (1d) and CH₂CHCH(CH₃)CN (1e) have been investigated in acetonitrile, and of two derivatives, 1a and CH₂C(CH₃)CN (1f) in nitromethane. The results show that unlike activated nitriles which undergo the known electrohydrodimerization (EHD) process in protic media, forming "tail-to-tail" dimeric products, the dimerization in acetonitrile affords mainly "head-to-tail" dimers and other products involving addition of "H" and "CH₂CN" moieties to the carbon-carbon double bond. Moreover, it is noteworthy that the electrochemical dimerization in acetonitrile affords branched dinitriles with an odd number of carbons between the two nitrile groups, rather than the common dimers with an even number of carbons. This type of dimer could serve as a precursor for branched diamines (and diacids) in the polyamide industry. In nitromethane, no dimeric but "condensation" products were formed, involving one molecule of solvent and one or two molecules of nitrilic substrate.