NMR Studies of Hindered Rotation. The Diels-Alder Adduct of Phencyclone with p-Benzoquinone: Restricted Motion of Bridgehead Phenyls

The Diels-Alder adduct of phencyclone with p-benzoquinone has been examined by ¹H NMR at 300 MHz in CDCl₃ at ambient temperatures for evidence of hindered rotation of the unsubstituted bridgehead phenyl groups. The nearly first-order spectrum exhibits four approximate doublets and five approximate triplets in the aromatic region, of roughly equal intensity (ca. 2H). This is consistent with a slow-exchange limit (SEL) spectrum of the hindered phenyls. For rapidly rotating phenyls, the predicted fast-exchange limit (FEL) spectrum would have shown two 2H doublets and one 4H doublet, in addition to three 2H triplets and one 4H triplet, in the aryl region. Full ¹H assignments have been made, based on the two-dimensional ¹H-¹H homonuclear chemical shift correlation spectrum (COSY) and expected magnetic anisotropy effects.