Electroanalytical investigation of the cobalt-catalyzed electrochemical dimerization of allylic acetates. Role played by iron(II) ions

Cyclic voltammetry and preparative-scale electrolyses have been used to investigate the electrochemical behavior of CoBr2 in the presence of allyl acetate, CH2CHCH2OAc, or cinnamyl acetate, PhCHCHCH2OAc, in an acetonitrile+pyridine (v/v=9:1) mixture. A complexation reaction between the electrogenerated cobalt(I) and allyl acetate leading to the corresponding (η2-allyl-OAc)cobalt(I) has been detected. This intermediate undergoes a slow oxidative addition affording a (η3-allyl)cobalt(III) complex which reduces at the same potential as the starting compound CoBr2. The rate constant for the complexation reaction has been estimated. The slow oxidative addition of (η2-allyl-OAc)cobalt(I) allowed us to observe its electrochemical reduction into the corresponding (η2-allyl-OAc)cobalt(0). Preparative-scale electrolyses of cinnamyl acetate in the presence of CoBr2 demonstrate the formation of the dimer 1,6-diphenyl-1,5-hexadiene from (η3-allyl)cobalt(II) or (η2-allyl-OAc)cobalt(0). However, the reduction of allylic acetate derivatives is not strictly catalytic since CoBr2 is poisoned by acetate anions leading to cobalt species of the type Co(OAc)x which are reducible at more negative potentials. However, the presence of iron(II) ions allows the regeneration of the cobalt catalyst precursor owing to its complexation with acetate anions.