Conformational Aspects of Substituents in Enamines. X-Ray Structure Analyses and Quantum Chemical Calculations *

X-ray structure determination of the enamine (Z)-4-(6'-t-butoxycarbonyl-2',2'-dimethyl-5'-phenyl3'-hexen-3'-yl)morpholine (1) reveals that certain bulky groups may enforce a relative orientation of the molecular subunits without conjugative interaction between the nitrogen lone pair and the olefinic double bond. According to the results of quantum chemical ab initio calculations the experimentally found arrangement would be the least favourable one in the absence of such substituents. A careful analysis of the molecular structure of 1 shows that this unusual arrangement is due to the presence of both, the aand the ^-substituent. In (£)-4-(3'-t-butoxycarbonylmethyl-l '-phenyl-r-penten-r-yl)morpholine (2) rather the nitrogen lone pair than the phenyl n system is in conjugation with the olefinic double bound. The results of ab initio calculations on model compounds show that conjugation of the double bond with the nitrogen lone pair is by 2 6 kcal /mol more favourable than conjugative interaction between the phenyl group and the C = C bond. Closer examination of the molecular structure of 2, however, led to the conclusion that it is predominantly the ^-substituent which forces the phenyl ring in a position where conjugation with the enamine double bond is not possible.