In order to clarify crucial roles of an intramolecular axial base in alkylation and dealkylation reactions of vitamin B12, hydrophobic vitamin B12 derivatives bearing a proximal base capable of coordinating to the nuclear cobalt at the α - and β-sites, [Cob(II)(α -Im)6 C1ester]ClO4 and [Cob(II)(β -Im)6 C2ester]C1O4, respectively, and a hydrophobic vitamin B12 capped with a fragment involving the imidazolyl moiety, [Cob(II)(Im: cap)5 C1ester]C1O4, were prepared. Their ESR spectra indicated that the three different imidazolyl moieties were completely coordinated to the nuclear cobalt, and such intramolecular coordination acted to shift the Co(II)/Co(I) redox potential to a cathodic side. The extent of potential shift was comparable to that observed upon addition of N-methylimidazole to a hydrophobic vitamin B12 without an axial base at a 50: 1 molar ratio. The axial base did not significantly show a kinetic effect on the alkylation of hydrophobic vitamin B12s, since the univalent cobalt has no detectable tendency to interact with an axial ligand. The imidazolyl segment introduced into the β-site inhibited alkylation reactions due to steric reasons. All the intramolecular axial bases treated in this study enhanced cleavage of the cobalt-carbon bond under aerobic irradiation conditions as originated from their steric and electronic effects.
All Science Journal Classification (ASJC) codes
- Chemical Engineering(all)