NMR STUDIES OF HINDERED ROTATION. THE DIELS-ALDER ADDUCT OF N-N-BUTYLMALEIMIDE WITH PHENCYCLONE : RESTRICTED MOTION OF BRIDGEHEAD PHENYLS

Abstract The Diels-Alder adduct of phencyclone and N-n-butylmaleimide has been prepared, and NMR studies have been carried out in CDCl3 solution at ambient temperatures by one-and two-dimensional 1H NMR (300 MHz) and 13C NMR (75 MHz) techniques. The resulting spectra appear to be consistent with slow rotation about the hindered C(sp2)-C(sp3) bonds to the bridgehead unsubstituted phenyls, i.e., slow exchange limit (SEL) spectra. Full rigorous 1H spectral assignments have been made via high-resolution COSY experiments. The number of signals in the 13C NMR aryl region were also consistent with hindered phenyl rotations; preliminary 13C assignments are given. Striking evidence for magnetic anisotropic effects due to the phenanthrene moiety, bridging ketone carbonyl, and bridgehead phenyls are discussed, supporting endo stereochemical assignment of the adduct.