Homolytic reactions of ligated boranes. Part 16. Enantioselective hydrogen-atom abstraction by chiral amine–boryl radicals: catalytic kinetic resolution of esters and of camphor

Abstract

UV irradiation of an oxirane solution containing di-tert-butyl peroxide, an amine–alkylborane complex (in which the B-alkyl group is optically active) and a racemic ester or camphor as substrate, gives rise initially to the tert-butoxyl radical which rapidly abstracts hydrogen from the complex to form an optically active amine–boryl radical. The amine–boryl radical then abstracts hydrogen enantioselectively from a C–H group α to the carbonyl group in the substrate to regenerate the amine–alkylborane. This abstraction reaction has been used to bring about catalytic kinetic resolution of the substrate, using the bis(isopinocampheylborane) complex of N,N,N′,N′-tetramethylethylenediamine and some of its derivatives as catalysts. After partial consumption of the substrate, the amount remaining and its enantiomeric excess (ee) have been used to derive enantioselectivity constants for hydrogen-atom abstraction. Enantioselectivity varies considerably with the structure of the substrate. The highest selectivity was observed for hydrogen abstraction from dimethyl 2,2-dimethyl-1,3-dioxolane-trans-4,5-dicarboxylate, when after 75% consumption of initially-racemic ester at –90 °C, the residual substrate showed an ee of 97%. A transition-state model is proposed to account for the observed enantioselectivities.