Aromatic amides. VIII. Z,E Isomer ratios and stereochemistry about aryl-nitrogen bonds in N-aryl amides and thioamides

Abstract

For 4?-substituted 2?,6?-diethylacetanilides, the Z/E isomer ratio depends on the nature of the 4?- substituent, the Z isomer being relatively favoured by electron-withdrawing substituents. The acyla- tion shifts of the ortho aromatic protons for 4?-substituted thiopivalanilides are greatest when the substituent is electron- withdrawing. These relationships complement the previously observed changes in isomer ratios of thioamides and acylation shifts of amides. It is proposed that the Z isomer is more nearly planar when the 4?- substituent is electron-withdrawing. As the substituent becomes more electron-releasing, the amide group is able to twist more out of the ring plane and the Z isomer is destabilized, relative to the E isomer, by n-π electron repulsions.