Model Studies for Coenzyme B12 Dependent Enzyme-Catalyzed Rearrangements. Kinetics and Mechanism of Decomposition of Formylmethylcobalamin and Its Acetals
Silverman, R.B. and David Dolphin
J. Am. Chem. Soc.
Syntheses of formylmethylcobalamin (I) 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 Co-C bond cleavage to B12b and a vinyl ether, and normal acetal hydrolysis to I, followed by a rate-determioning fission of the Co-C bond to B12b and acetaldehyde. The rate law of decomposition of I is d[I]/dt = -K[I][H O+ ]; 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 Co-C bond.