Asymmetric induction in addition of N-nitrenes to alkenes: 2-(chiral)substituted benzimidazole-derived N-nitrene additions to alkenes

Abstract

Oxidation of the optically active 1-aminobenzimidazoles (5)–(7) with lead tetra-acetate in the presence of various prochiral alkenes yields mixtures of the diastereoisomeric aziridines with little asymmetric induction (Table). Oxidation of theracemic 1-amino-2-(1,2,2-trimethylpropyl)-1H-benzimidazole (12) in the presence of α-methylene-γ-butyrolactone (1) yields the stereoisomeric azirid (16) in a 5.5:1 ratio, and oxidation in the-presence of γ,γ-dimethyl-α-methylene-γ-butyrolactone (18) gives the aziridine (19) stereospecifically. The stereochemistry of (19) was determined by X-ray crystallography.

A rationalisation for the stereospecificity of addition of the intermediate N-nitrene, produced by oxidation of (12) to the lactone (18), is given in terms of competitive reaction via two different transition states both of which lead to the same sense of chiral induction (Scheme 5).

Significant asymmetric induction is also found in the addition of the nitrene derived from oxidation of (12) to (E)-but-2-ene (5.2:1 ratio-of stereoisomers) and styrene (5.6:1 ratio of stereoisomers).