ORGANIC FREE-RADICAL-PROMOTED ISOMERIZATION OF ALPHA-ALKYLCOBINAMIDESAND BETA-ALKYLCOBINAMIDES

Anaerobic reaction of alpha- or beta-alkylcobinamides (alpha- or beta- RCbi(+)'s) with the corresponding alkyl free radical, R(.) (where R = CH3, CH3CH2, or CH3CH2OCH2CH2), generated by the Fenton reaction using Fe2+ and an alkyl hydroperoxide, RC(CH3)(2)OOH, causes isomerization and leads to mixtures of alpha- and beta-RCbi(+)'s. The reaction does not occur, however, under aerobic conditions or under anaerobic condit ions in the presence of an excess of the free radical scavenger H-Temp o. In addition, alpha-CH(3)CH(2)Cbi(+) reacts with 50 molar equiv of t ert-butyl hydroperoxide and Fe2+ to give a mixture of alkylcobinamides that contains 6% alpha-CH(3)Cbi(+) and 94% beta-CH(3)Cbi(+), showing that multiple transalkylations occur. A Co-II-induced isomerization an d the S(H)2 mechanism are ruled out on the basis of the known reactivi ty of RCbi(+) and product analysis. A mechanism is proposed which invo lves a direct oxidative free radical displacement by an R(.) to the me tal of RCbi(+) via a dialkylcobalt(IV) corrinoid species. Since the re action leads to equilibration of the two diastereomers under mild cond itions, it can be used to study the equilibria between diastereomeric RCbi(+)'s. Thus, the equilibrium for the diastereomeric ethyl-13-epico binamides, in which the e propionamide side chain of the corrin ring h as been epimerized from the alpha to the beta face of the corrinoid, l ies significantly more toward the a diastereomer than that for the nor mal ethylcobinamides. This represents the most direct experimental evi dence obtained to date that the corrin ring side chains control the re lative steric accessibility of the two faces of the cobalt corrinoids.