Reduction with metal hydrides—XIX : Competitive reduction of alkylcyclohexanones: Regarding the concepts of steric approach control and product development control

Abstract

Competitive rate studies have been carried out for the reduction of 4-t-butylcyclohexanone, 3,5-dimethylcyclohexanone, 3,3,5-trimethylcyclohexanone, 3,3,5,5-tetramethylcyclohexanone and 2,2-dimethyl-4-t-butylclohexanone by LAH in ether, lithium aluminium tri-t-butoxyhydride in THF, lithium aluminium trimethoxyhydride in THF and NaBH 4 in isopropyl alcohol and sodium triisopropoxyborohydride in isopropyl alcohol. Relative rate constants for attack of the various ketones from the axial and from the equatorial side were computed by reducing the ketones in pairs and analyzing the alcohol mixtures thus obtained. The relative rate constants support the concept of “steric approach control” but suggest that “product development control” plays at best a minor role especially in the reductions with aluminohydrides. Alternative explanations for the 10:1 ratio of equatorial to axial alcohol observed in the lithium aluminium hydride reduction of 4-t-butylcyclohexanone in ether are suggested.