Chemical vs. Redox Catalysis of Electrochemical Reactions. Reduction of trans-1,2-Dibromocyclohexane by Electrogenerated Aromatic Anion Radicals and Low Oxidation State Metalloporphyrins

Article abstract of DOI:10.1021/ja00255a036

Homogenous Catalysis of the electrochemical reduction of trans-1,2-dibromocyclohexane is investigated in a series of catalysts comprising aromatic and heteroaromatic compounds and several metalloporphyrins.The aromatic anion radicals and the reduced Zn and Cu porphyrins give rise to a typical redox catalysis involving an outer-sphere electron transfer from the mediator to the substrate.The rate-determining step of the reaction as well as that of the direct electrochemical reduction at glassy carbon consists of the injection of one electron concerted with the cleavage of one carbon-bromine bond.The final olefin is obtained in successive steps after injection of a second electron and cleavage of a second bromide ion.The direct reduction and the redox-catalysed reaction follow the same quadratic activation driving force free energy relationship involving two additive reorganization factors.One, accounting for about 80percent of the standard activation energy, concerns the breaking of the C-Br bond giving a contribution approximately equal to one-fourth of the bond dissociation energy.The remainder of the activation barrier involves solvent reorganization.Nickel, iron, and cobalt porphyrins (at the formal metal "I" oxidation state) react much more rapidly than redox catalysts having the same standard potential.This points to an inner-sphere mechanism: abstraction of one Br followed by or concerted with the elimination of the second Br or S_N2 displacement of one Br(1-) followed by or concerted with the elimination of the second Br(1-) before or after the injection of a second electron.

Full text of DOI:10.1021/ja00255a036

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Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.