Reductive Co Alkylation of Heptamethyl Cobyrinate with the Methylthiomalonate ( S)-Methyl 3-Bromo-2-[(ethylthio)carbonyl]-2-methylpropanoate The methylthiomalonate(−)-( S)-Methyl 3-bromo-2-[(ethylthio)carbonyl]-2-methylpropanoate( 5a)was prepared from dimethyl methylmalonate in five steps via the stereospecific cleavage of the ( pro- S)-ester group of 1 with pig-liver esterase in an overall yield of 26.5% ( Scheme 4a). Reductive Co alkylation of heptamethyl Coβ-perchlorato cob (II)yrinate ( 8) with 5a by electrosynthesis lead to the alkylcobalt complex 9a in 40% yield ( Scheme 4b). The O2-dependent reactions of the methyhnalonyl fragment produced by photolysis of 9a and its deuterated derivative 9c are reported ( Scheme 5).