Indirect Electroreduction of 2-Alkyl-2-(bromomethyl)alkanones with Cobaloxime to Form 3-Alkyl-2-alkenones via 1,2-Acyl Migration

Inokuchi, T.; Tsuji, M.; Kawafuchi, H.; Torii, S. J. Org. Chem. 1991, 56, 5945.

Indirect electroreduction of 2-alkyl-2-(bromomethyl)cycloalkanones 1 in the presence of (chloropyridine)cobaloxime(III) as a recyclable mediator gives the corresponding one-carbon homologated 3-alkyl-2-cycloalkenones 2 in 51-74% yields along with their saturated isomers 3 (ca. 4% yields) and the unrearranged products 4(5-23% yields). This rearrangement reaction is feasible by use of 20 mol% of the cobaloxime for the substrate 1a and the formation of the 3-alkylcycloalkanone 3a otherwise increases to 28% yield with the lesser amount of the cobaloxime. The ring-enlargement 1 can be rationalized in terms of a 1,2-acyl migration of the primary carbon radical, formed by the action of electrogenerated cobaloxime(I) species toward the carbon-bromine bond of 1. The resulting tertiary carbon radical would undergo synchronous recombination with the cobaloxime(II) species and the subsequentb-elimination of Co(III)H species from the intermediary alkylcobaloxime(III) complexes leads to enones 2. The triphenyltin hydride reduction of bromomethyl ketones 1 induces the 1,2-acyl rearrangement predominantly under high dilution conditions to produce the corresponding saturated 3-alkylalkanones 3 as major products along with a small amount of the unrearranged 4.