Bridged (beta-alkoxyalkyl)Co-III(salen) complexes by intramolecular alkoxycobaltation of unactivated alkenes: new models for coenzyme B-12

Co-II(salen) derivatives (salen = {N,N'-ethylenebis[salicylideneaminato]}) whose ethanediyl moiety carries an alkenyl side-chain R [R = prop-2-en-1-yl (6a), 2-methylprop-2-en-1-yl (6b), but-2-en-1-yl (6c), but-3-en-1-yl (6d)] react with oxygen and alcohols to give organocobalt(III) complexes contain ing a beta-alkoxy-substituted three- or four-carbon bridge between cobalt a nd the equatorial ligand. NMR and UV-VIS spectroscopic studies show that pr oduct formation is a three-stage process involving (1) oxidation of cobalt( II) to produce an (alkoxo)cobalt(III) complex, (2) intramolecular interacti on of cobalt(III) with the alkenyl double bond to yield a carbocationic int ermediate, and (3) nucleophilic attack by the alcohol. In the case of cobal t(II) complex 6e (R = 3-methylbut-3-en-1-yl), the major product is bridged beta-methylene organocobalt(III) complex 10, demonstrating that proton loss competes with addition of alcohols when the intermediate organocobalt(III) species has a substantial degree of tertiary carbocation character. Applic ation of the alkoxycobaltation reaction to 6d and ethane-1,2-diol afforded bridged [beta-(2-hydroxyethoxy)alkyl]Co(salen) complex 20, a simple model f or coenzyme B-12 with a built-in substrate. The molecular structure of 20 h as been determined by X-ray diffraction methods.