Electrochemical Reduction of Ketones Mediated by (Dimethylpyrrolidinio)mercury. Reductive Cyclization of Unsaturated Ketones and Redox Catalysis Studies

Article abstract of DOI:10.1021/ja00219a042

The mechanism of dimethylpyrrolidinium (DMP(+1)) mediation of the reductive cyclization of 6-hepten-2-one at a mercury electrode was studied.The initial step of the reduction involves formation of a solid composite, (DMP)Hg5, which then transfers an electron to the ketone, regenerating DMP(1+) and Hg^o.The ketyl radical anion cyclizes to form a primary alkyl radical, which can either abstract a hydrogen atom from the solvent, dimethylformamide, or be reduced to the corresponding carbanion.Protonation gives 1,2-dimethylcyclopentanol.Both direct and DMP(1+)-mediated reductions of 7-octen-2-one, 5-phenyl-2-pentanone, and 6-heptyn-2-one were studied.All three compounds give cyclic alcohols when the reduction is mediated by DMP(1+).The former two compounds give the corresponding straight-chain alcohols when reduced directly.The latter gives the same cyclized product upon both direct or DMP(1+)-mediated reduction.Redox catalysis studies show that DMP(1+) is a remarkably effective catalysts for the reduction of ketones.The effectiveness of the catalysis correlates with the expected rates of cyclization of the unsaturated ketones and is consistent with a mechanism involving mediation by solid (DMP)Hg5.Redox catalysis studies of the reduction of cyclohexanones indicate that DMP(1+)-mediated reductions are accelerated by increasing , and the results are discussed in terms of a mechanism leading to pinacol formation.