Reactions of B_12r with Aliphatic Free Radicals: A Pulse-Radiolysis Study

The spectra of the intermediates formed in the reactions of B_12r with the free radicals Br₂⁻·, CO₂⁻·, CH₂C(CH₃)₂OH, ·C(CH₃)₂OH, ·CH₂CHO, and ·CH(OH)CH₂OH are reported. The results indicate that Br₂⁻·oxidizes B_12r to B_12a, via an inner-sphere mechanism, and CO₂⁻ reduces B_12r to B_12s. All the aliphatic free radicals studied, R, react with B_12r, yielding as the first product a pseudocoenzyme denoted C^III-R. Co^III-CH₂C(CH₃)₂OH is stable for over a second in the pH range 3–10 as is Co^III-CH₂CHO. The latter compound hydrolyzes in acid solutions to yield B_12a and CH₃CHO. Co^III-C(CH₃)₂OH and Co^III-CH(OH)CH₂OH decompose heterolytically to yield mainly B_12s; a side reaction that probably yields Co^III-H via a β-hydride shift is also observed. The kinetics of decomposition of Co^III-CH(OH)CH₂OH in neutral solutions are reported. No water elimination from the latter intermediate occurs. The reasons for the latter observation are discussed.