Investigations into the stereoselectivity of enamine reactions: bromination and deuteriation

Contrary to previous reports in the literature, Hickmott et al have shown that the stereoselectivity of the deuteriation of enamines of 4- t - butylcyclohexanone is low and indicative of a reactant- like transition state. Their results showed that preference for axial attack was in the order: enol ether > morpholine enamine > pyrrolidine enamine. These results were explained in terms of the relative positions on the reaction co-ordinate. The aim of this project was to determine whether the ideas proposed by Hickmott et al could be applied to the reaction of enamines with other electrophilic reagents. It was decided to investigate the stereoselectivity of bromination of different enamines of 4 -t- butylcyclohexanone. The pyrrolidine, morpholine, N- methylaniline piperidine, and diisobutylamine enamines of 4- t- butylcyclohexanone were therefore synthesised. These enamines were then brominated at 0 degrees C and at -78 degrees C. The ratios of the axial 2- bromo-4- t- butylcyclohexanone and the equatorial 2- bromo -4- t - butylcyclohexanone were obtained from the 13C, 1H n. m. r. and capillary gas chromatography of the crude reaction mixtures. Results show that the stereoselectivity of the morpholine, pyrrolidine and piperidine enamines of 4 - t - butycyclohexanone is in agreement with the theory proposed by Hickmott and Woodward. However, the stereoselectivity of bromination of the diisobutylamine and N - methylaniline enamines were suprisingly low