Electrocarboxylation of arylmethyl chlorides catalyzed by cobalt 6,6′-bis(2-hydroxyphenyl)-2,2′-bipyridine

Abstract

The electrocatalytic carboxylation of 4-(trifluoromethyl)benzyl chloride, 4-methoxybenzyl chloride and diphenylmethyl chloride in N,N-dimethylformamide (DMF) by [Co(dobpy)] (dobpy=6,6′-bis(2-hydroxyphenyl)-2,2′-bipyridine dianion) has been investigated. Electrocarboxylation of the arylmethyl chlorides in CO2-saturated DMF solution in the presence of the cobalt catalyst resulted in the formation of a mixture of the corresponding arylacetic acid and substituted toluene. The ratio of carboxylic acid to hydrocarbon in the product mixture was dependent on the nature of the arylmethyl chlorides and the electrolysis potential. Voltammetric studies indicated that the electrogenerated [CoI(dobpy)]− underwent oxidative addition with RCl to give [CoIII(dobpy)R]. The unstable one-electron-reduced complex [CoII(dobpy)R]− underwent homolytic CoC bond cleavage to generate R, which in turn could undergo further reduction to give the carbanion R− either at the electrode or in the solution. On the one hand, R− was trapped by CO2 to produce carboxylic acid; on the other hand, the protonation of R− and hydrogen-atom abstraction by R would lead to the production of RH. [Co(dobpy)] is a more selective catalyst in the production of carboxylic acid than [Co(TPP)] (TPP=5,10,15,20-meso-tetraphenylporphine dianion) and a more robust catalyst than [Co(salen)] (salen=N,N′-bis(salicylidene)ethane-1,2-diamine dianion).