Electrochemistry and catalytic properties of amphiphilic vitamin B12 derivatives in nonaqueous media.

D Gryko,Y Hisaeda,K Goliszewska,M Giedyk,H Shimakoshi

doi:10.1039/c6dt00355a

Abstract

The reduction pathway of cobalester (CN)Cble, an amphiphilic vitamin B12 derivative, was investigated in organic solvents under electrochemical conditions and compared with mono- and dicyanocobyrinates. The redox characteristics were determined using cyclic voltammetry and spectroelectrochemical methods. The presence of a nucleotide moiety in B12-derivative impedes the in situ formation of dicyano-species thus facilitating the (CN)Co(iii) to Co(i) reduction. The (CN)Cble shows stepwise reduction to Co(i) via (CN)Co(ii). The reduction of (CN)Co(ii)/Co(i) was found to depend on cyanide-solvent exchange equilibrium with weakly coordinating solvents and bulky peripheral chains promoting intact (CN)Co(ii) species existence. The studied complexes were also utilized as catalysts in bulk electrolysis of benzyl bromide affording bibenzyl in very good yield.

Highlights

In order to understand the influence of the nucleotide fragment on the redox behaviour of cobalesters (2) in organic solvents, a series of cyclic voltammograms was recorded
Such redox behaviour is in agreement with that observed by Lexa and Savéant for cyanocobalamin in water and DMSO,[22,23] but is distinctively different from heptamethyl cobyrinates 3 and 4 for which Co(III)/Co(II) and Co(II)/Co(I) redox couples are well separated (Fig. 2C and D)
To direct reduction at −1.4 V vs. Ag–AgCl, they could undergo axial ligand exchange to yieldCo(II) complexes 6 and 10, which are readily reduced at more positive potentials. This stays in contrast to what was proposed for cyanocobalamin (1) in protic solvents, where no (CN)Co(II) intermediate was considered in the absence of purposely added cyanide.[23]

Summary

Introduction

Vitamin B12 (cyanocobalamin 1) is a highly functionalised organometalic complex comprising a corrin ring with the central cobalt cation and peripheral amide chains as shown in Fig. 1.1,2 As a cofactor for B12-dependent enzymes it plays an important role in the normal functioning of mammalian organisms.[3,4,5] Inspired by nature, scientists successfully employed cyanocobalamin (1) and its derivatives as catalysts in organic synthesis, due to their ability to form and selectively cleave Co–C bonds.[6,7,8,9] The scope of B12-catalyzed reactions is continuously expanding, including such processes as dehalogenation, addition to double bonds, ring expansion, C–H functionalization etc.The catalytic properties of vitamin B12 derivatives rely on redox chemistry of the central cobalt cation with the reduction of Co(III) to active Co(II) or Co(I) forms being critical.[10]. We present the redox characteristics of cobalester (2) with regard to factors influencing the reduction of Co(III) to catalytically active Co(II) and Co(I) species and demonstrate its application as a catalyst in potential-controlled electrolysis of benzyl bromide. This indicates an efficient formation of Co(I) species of cobalester (2) at less negative potentials compared to cobyrinate derivatives having the cyanide ligand.

Results

Conclusion