NMR Motional Averaging in the Isotopically Perturbed Cope Rearrangement

The effect of deuterium substitution on the positional equilibrium and rate of the Cope rearrangement in barbaralone-d1 has been investigated by 1H and 13C dynamic NMR spectroscopy. Deuterium favors the allylic position over the cyclopropyl position. Skewing of the equilibrium requires that the Cope rearrangement be considered as a nonmutual exchange process that transports 13C−1H spin pairs between unequally populated molecular configurations. The NMR fast-exchange results are critically interpreted in light of a recently proposed alternative averaging theory (Jones, D. H.; Kurur, N. D.; Weitekamp, D. P. Bull. Magn. Reson. 1992, 14, 214). Our experimental results are consistent with the traditional theory but do not agree with the prediction of the alternative theory. The use of 1H−13C inverse detection is demonstrated for the observation of very broad 13C resonances in the coalescence region via their directly attached exchange-narrowed 1H signals, and the contribution of rapid chemical exchange to the ...