Nucleophilic addition to linear nitrilium ions (–CN–) leading to single isomers. Isolation of relatively stable isoimides and their rearrangement to imides limited by substrate Z–E isomerisation

Abstract

The N-anilinonitrilium ions formed by solvolysis of halides (5) and (10) react with acetate or methanol to give a single isomer (20) or (23) in which the entering nucleophile and the forming lone pair on nitrogen are trans. The reaction is kinetically (rather than thermodynamically) controlled since the Z-isomer, which is formed exclusively in the initial reaction, undergoes isomerisation to the E-isomer at elevated temperatures. The observed stereospecificity is not due to selective solvation of the nitrilium ion by the departing halide ion since (5; R = Bu^t), which has the Z-configuration gives (20) or (23) with retention of configuration. The rate determining step for the O → N acyl group migration [(12)→(11)] is inversion of the configuration at nitrogen (Z–E isomerisation); in the E-isomer the nucleophilic lone pair on nitrogen and the acyl group are adjacent. Stable O-acylisoimides can be isolated by slowing the rate of Z–E isomerisation. Substituent effects on Z–E isomerisation have been measured and contrast with those observed for simple imine systems.