Reactions of B12r with aliphatic free radicals: a pulse-radiolysis study

Abstract

The spectra of the intermediates formed in the reactions of B/sub 12r/ with the free radicals Br/sub 2//sup -/., CO/sub 2//sup -/., .CH/sub 2/C(CH/sub 3/)/sub 2/OH, .C(CH/sub 3/)/sub 2/OH, . CH/sub 2/CHO, and .CH(OH)CH/sub 2/OH are reported. The results indicate that Br/sub 2//sup -/. oxidizes B/sub 12r/ to B/sub 12a/, via an inner-sphere mechanism, and CO/sub 2//sup -/ . reduces B/sub 12r/ to B/sub 12s/. All the aliphatic free radicals studied, .R, react with B/sub 12r/, yielding as the first product a pseudocoenzyme denoted Co/sup III/-R. Co/sup III/-CH/sub 2/C(CH/sub 3/)/sub 2/OH is stable for over a second in the pH range 3 to 10 as is Co/sup III/-CH/sub 2/CHO. The latter compound hydrolyzes in acid solutions to yield B/sub 12a/ and CH/sub 3/CHO. Co/sup III/-C(CH/sub 3/)/sub 2/OH and Co/sup III/-CH(OH)CH/sub 2/OH decompose heterolytically to yield mainly B/sub 12s/; a side reaction that probably yields Co/sup III/-H via a ..beta..-hydride shift is also observed. The kinetics of decomposition of Co/sup III/-CH(OH)CH/sub 2/OH in neutral solutions are reported. No water elimination from the latter intermediate occurs. The reasons for the latter observation are discussed. 6 figures.