Model Studies for Coenzyme B₁₂ Dependent Enzyme-Catalyzed Rearrangements. Kinetics and Mechanism of Decomposition of Formylmethylcobalamin and Its Acetals

Syntheses of formylmethylcobalamin and the kinetics and mechanism of decomposition of the aldehyde and its acetals are reported. Two different pathways were observed for the acid-catalyzed decomposition of the cobalamin acetals: direct cobalt-carbon bond cleavage to B_12b and a vinyl ether, and normal acetal hydrolysis to formylmethylcobalamin, followed by a rate-determining fission of the cobalt-carbon bond to B_12b and acetaldehyde. The rate law of decomposition of formylmethylcobalamin (A) is d[A]/dt = -K [A][H₃O⁺]; the acid sensitivity and bimolecularity of the decomposition were rationalized as an initial protonation of the formyl carbonyl followed by a direct cleavage of the cobalt-carbon bond.