Lewis acid catalysis of the ene addition of chloral and bromal to olefins; stereochemical and mechanistic studies

Abstract

The Lewis acid catalysed ene additions of chloral and bromal to alkenes are completely regiospecific, moderately regioselective, and often highly stereoselective. Diastereoselectivity in the addition to (–)-β-pinene was a function of the Lewis acid, and with TiCl₄ essentially quantitative asymmetric induction was observed. The stereochemical phenomena are explained satisfactorily by assuming the active enophiles possess transoid structures such as (1), that a concerted or rapid stepwise mechanism operates, and that product formation occurs predominantly through the least hindered encounter complex of the olefin and (1). In the case of 2-methylbut-2-ene, however, there is some evidence for an additional stereo-electronic contribution, the ‘cis-effect’. Stereochemical assignments are supported by X-ray structural data. Ketones, hydrohalogenated ene adducts, or rearrangement products are formed (mainly in the additions to olefins of moderate reactivity) indicating the participation of Friedel-Crafts type dipolar intermediates. The ene adducts themselves could be formed via dipolar intermediates or in competing ‘concerted’ reactions; the stepwise mechanism must operate in some reactions because of the observation of Wagner-Meerwein rearrangements. Olefin reactivity over the series, measured by the competitive technique, towards chloral–AlCl₃ showed a ca. 900-fold variation in rate; ‘ene’ reactivity decreases more steeply.