An electrochemical and spectroelectrochemical (IR) investigation of the reduction of RCo(II)TPP (R = benzyl or butyl; TPP = tetraphenylporphyrin): mechanistic implications in the CoTPP catalyzed electrocarboxylation of alkyl halides

The electrochemical reduction of the intermediates generated from the reaction of Co(I)TPP- (TPP = tetraphenyl porphyrin) with benzyl chloride (PhCH2Cl) or butyl bromide (BuBr) have been studied by use of electrochemistry and IR-spectroelectrochemistry. The rates of formation of PhCH2Co(III)TPP, and of decomposition of the reduction product [PhCH2Co(II)TPP](-) are much faster than those found for the corresponding BuBr system. The decomposition of [RCo(II)TPP](-) proceeds via homolytic Co-C bond cleavage, to generate the corresponding organic radical at negative potential, where RCo(III)TPP can be reduced to [RCo(II)TPP](-). A mechanism involving the reaction of organic radicals with the porphyrin ring and transfer from cobalt to a nitrogen of the pyrrole ring, leading to the formation of an N-alkyl cobalt porphyrin complex is proposed, which is consistent with the destruction of catalyst during electrocatalytic carboxylation and reduction of an organic halide. (C) 1998 Elsevier Science S.A. All rights reserved.