INFLUENCE OF THE AXIAL ALKYL LIGAND ON THE REDUCTION POTENTIAL OF ALKYLCOB(III)ALAMINS AND ALKYLCOB(III)YRINATES

The irreversible‐reduction potentials of 26 alkylcob(III)alamins (RCblIII 1a–z) and 26 alkylcob(III)yrinates (R‘Cby’III; 2a–z) (Ep 1a–z and Ep 2a–z, resp.) have been measured in situ by single‐scan voltammetry of hydroxocob(III)alamin hydrochloride (vitamin B12b‐ HCl; 1) or heptamethyl cob(II)yrinate perchlorate (ClO4‘Cby’II; 2) in presence of the corresponding alkyl halides (RX;3a–z) in DMF. The reduction potentials of alkylcobalt complexes exhibiting half‐life times as short as a few seconds become measurable by this technique. Thermodynamic cycles prove that the observed reduction potentials are closely related to the standard reduction potentials E°(RCoIII + e−⇆R⋅ + CoI). Electron‐withdrawing groups and/or an increased degree of substitution at the Co‐bound C‐atom in RCblIII and, R‘Cby’III shift Ep(1a–z) and Ep (2a–z) towards positive potentials. Linear correlations have been found between Ep(1a–z) (Ep(2a–z)) of RCblIII (R‘Cby’III) and the pKa of RH (or the Taft σ*‐ or the Hammett σ p−‐values of R) within each class of R, i. e. MeCblIII (Me‘Cby’III), primary RCblIII (R‘Cby’III) and secondary RCblIII (R‘Cby’III). The correlations allow to distinguish between electronic effects of the Co‐bound alkyl residues and their steric interactions with the corrin side chains. The correlations have further been used to visualize the light‐induced formal insertion of an olefin into the Co, C‐bond of an alkylcobalamin (Scheme 2, 1a→1u), a key step in the vitamin‐B12‐catalized C, C‐bond formation. Copyright © 1990 Verlag GmbH & Co. KGaA, Weinheim