The interplay of steric and electronic factors affecting geometrical isomerism of diaryl ketimine derivatives

Abstract

The evidence bearing on the mechanism of the Schmidt reaction with ketones is critically reexamined. It is shown that dehydration of an initially formed azidohydrin (I) to a ketiminodiazonium ion (II), which may or may not equilibrate between its geometrically isomeric forms (IIa or b), can account for the various reported ratios of isomeric amides produced from unsymmetrical ketones. The previously anomalous behavior of ortho-substituted diaryl ketones can be resolved by taking into account the influence of conjugative effects on the preferred rotational positions. The ratios of amides produced from a series of o-alkylbenzophenones by both the Schmidt reaction and oximation plus Beckmann rearrangement have been determined at different temperatures; there are appreciable differences in the product ratios and their temperature coefficients from the two reactions.