Nickel(I) Salen-Catalyzed Reduction of 1-Haloalkyl-2-oxocycloalkanecarboxylates: Three- and Four-Carbon Ring Expansions

Abstract

Cyclic voltammetry and controlled-potential electrolysis have been employed to study the catalytic reduction of methyl 1-(4-halobutyl)-2-oxocyclopentanecarboxylate and ethyl 1-(3-halopropyl)-2-oxocyclohexanecarboxylate by nickel(I) salen electrogenerated at carbon cathodes in dimethylformamide containing tetramethylammonium tetrafluoroborate. Electrocatalytic reduction of the aforementioned compounds, in either their bromo or iodo forms, affords the corresponding ring-expanded products (methyl cyclononanone-5-carboxylate and ethyl cyclononanone-5-carboxylate) in up to 26% yield, along with the dehalogenated species (methyl 1-butyl-2-oxocyclopentanecarboxylate and ethyl 1-propyl-2-oxocyclohexanecarboxylate) and their unsaturated counterparts [methyl 1-(but-3-enyl)-2-oxocyclopentanecarboxylate and ethyl 1-allyl-2-oxocyclohexanecarboxylate]. However, the principal products are dimers—dimethyl -(octane-1,8-diyl)bis(2-oxocyclopentanecarboxylate) derived from methyl 1-(4-halobutyl)-2-oxocyclopentanecarboxylate, and diethyl -(hexane-1,6-diyl)bis(2-oxocyclohexanecarboxylate) arising from ethyl 1-(3-halopropyl)-2-oxocyclohexanecarboxylate—obtained in yields ranging from 52% to 83%. A mechanistic scheme, involving alkyl radicals formed via nickel(I) salen-catalyzed cleavage of the carbon-halogen bond of each substrate, is proposed to account for the formation of the various products.