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

Abstract

Abstract 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.