Abstract
Cobalamins (B12) play various important roles in vivo. Most B12-dependent enzymes are divided into three main subfamilies: adenosylcobalamin-dependent isomerases, methylcobalamin-dependent methyltransferases, and dehalogenases. Mimicking these B12 enzyme functions under non-enzymatic conditions offers good understanding of their elaborate reaction mechanisms. Furthermore, bio-inspiration offers a new approach to catalytic design for green and eco-friendly molecular transformations. As part of a study based on vitamin B12 derivatives including heptamethyl cobyrinate perchlorate, we describe biomimetic and bioinspired catalytic reactions with B12 enzyme functions. The reactions are classified according to the corresponding three B12 enzyme subfamilies, with a focus on our recent development on electrochemical and photochemical catalytic systems. Other important reactions are also described, with a focus on radical-involved reactions in terms of organic synthesis.
Highlights
In B12, the cobalt center is coordinated by four equatorial pyrroles of the corrin ring and 2,3-dimethylbenzimidazole as a lower axial ligand (Figure 1a) [6,7,8]
Cob(III)alamins with upper alkyl ligands are quite interesting because of their structural relevance to methylcobalamin and adenosylcobalamin that serve as organometallic cofactors in B12-dependent enzymes
A cyclophane-type B12 artificial enzyme mediated similar carbon-skeleton rearrangements [32]. We developed another artificial enzyme composed of human serum albumin (HSA) and heptapropyl cobyrinate [34]
Summary
Mechanistic investigations revealed that the formation of the two-electron-reduced species of Co(III)-monoalkylated complex of 1 was vital for carbon-skeleton rearrangement reactions. The electrolysis of diethyl 2-bromomethyl-2-phenylmalonate at −2.0 V vs Scheme 2: Electrochemical carbon-skeleton arrangements mediated by B12 model complexes. The electrochemical carbon-skeleton rearrangement reactions were successfully mediated by simple B12 model complexes 2 (Figure 3). The reduction of the Co(III)-monoalkylated complex and the oxidation of the Co(III)-dialkylated complex proceeded at the cathode and anode, respectively [61] These processes were coupled to achieve the 1,2-migration of functional groups. Further investigations with diethyl 2-bromomethyl-2-phenylmalonate as a substrate confirmed that the carboxylic ester-migrated product was formed via not a radical, but a cationic intermediate that was generated by the fragmentation to the monoalkylated complex at the anode (Scheme 2). The B12-Ru@MOF is the best system for the functional simulation of MMCM among our B12 artificial enzymatic systems
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