Enamine chemistry: Pt. 32

Investigations into the bromination of 4-t-butycyclohexanone enamines have shown that the ratio of axial : equatorial bromine incorporation varies with the enamine used (pyrrolidine 51:49; piperidine 66:34; morpholine 74:26; N-methylaniline 52:48; di-isobutylamine 52:48). This has been rationalized in terms of direct C-bromination of the pyrrolidine, N-methylaniline, and di-isobutylamine enamines via an early reactant-like transition state, whereas the piperidine and morpholine enamines are believed to react via initial N-bromination and rearrange to the C-bromo-iminium salt via a product-like transition state. Equilibration of all bromo-iminium slats with triethylamine prior to hydrolysis gives mainly trans-2-bromo-4-t-butylcyclohexanone on hydrolysis. 3 figs., 10 refs., 3 tabs