Bridged (β-alkoxyalkyl)CoIII(salen) complexes by intramolecular alkoxycobaltation of unactivated alkenes: new models for coenzyme B12

Abstract

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 containing a β-alkoxy-substituted three- or four-carbon bridge between cobalt and the equatorial ligand. NMR and UV–VIS spectroscopic studies show that product formation is a three-stage process involving (1) oxidation of cobalt(II) to produce an (alkoxo)cobalt(III) complex, (2) intramolecular interaction of cobalt(III) with the alkenyl double bond to yield a carbocationic intermediate, and (3) nucleophilic attack by the alcohol. In the case of cobalt(II) complex 6e (R = 3-methylbut-3-en-1-yl), the major product is bridged β-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. Application of the alkoxycobaltation reaction to 6d and ethane-1,2-diol afforded bridged [β-(2-hydroxyethoxy)alkyl]Co(salen) complex 20, a simple model for coenzyme B₁₂ with a built-in substrate. The molecular structure of 20 has been determined by X-ray diffraction methods.