Coenzyme B12 serves as a cofactor in various enzymatic reactions in which a hydrogen atom is interchanged with a substituent on an adjacent carbon atom. Measurement of the dissociation energy of the coenzyme's cobalt-carbon bond and studies of the rearrangement of model free radicals related to those derived from methylmalonyl-coenzyme A suggest that these enzymatic reactions occur through homolytic dissociation of the coenzyme's cobalt-carbon bond, abstraction of a hydrogen atom from the substrate by the coenzyme-derived 5'-deoxyadenosyl radical, and rearrangement of the resulting substrate radical. The only role thus far identified for coenzyme B12 in these reactions--namely, that of a free radical precursor--reflects the weakness, and facile dissociation, of the cobalt-carbon bond.

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Rhodium complexes containing chiral phosphine ligands catalyze the hydrogenation of olefinic substrates such as alpha-aminoacrylic acid derivatives, producing chiral products with very high optical yields. Elucidation of the mechanisms of such reactions leads to the conclusion that the stereoselection is dictated not by the preferred initial binding of the substrate to the chiral catalyst, but rather by the much higher reactivity of the minor diastereomer of the catalyst-substrate adduct corresponding to the less favored binding mode.

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