REACTIONS OF COBALT(I) SUPERNUCLEOPHILES . ALKYLATION OF VITAMIN B12S COBALOXIMES(I) AND RELATED COMPOUNDS

Results of kinetic measurements are presented which indicate that the reactions of alkyl halides with vitamin B12a, cobaloximes(I), and other Co(I) chelates proceed by a classical SN2 mechanism, the Co(I) centers being some of the most powerful nucleophiles known. The evidence for this mechanism providing the dominant reaction pathway is based mainly on the analysis of substrate structural effects on the substitution rates. Although the absolute reactivities of the Co(I) nucleophiles are up to 107 times greater than those, e.g., of iodide ion, the relative reactivities with various substrates are very similar. Surprisingly, the rates of reactions of the alkyl halides studied are no more sensitive to steric effects of the corrin ligand system than to those of the cobaloxime moiety. Steric hindrance by out-of-plane corrin ligands appear in later stages of the Co-C bond formation process, as evidenced by the instability of secondary alkyl cobalamins in contrast to the corresponding c baloxime derivatives. The factors influencing the nucleophilicity of the Co(I) chelates, in particular, the effects of axial bases, ligand structure, and possible mechanistic alternatives of the alkylation reactions are discussed. The presence of the coordinated 5,6-dimethylbenzimidazole does not cause a substantial change of the Co(I) nucleophilicity of vitamin B12a. © 1969, American Chemical Society. All rights reserved.