Comparison of rotational barriers of related diphenylmethyl anions and cations studied by C NMR spectroscopy: Mechanistic and structural considerations.
The rotational barriers of diphenylmethyl anions and cations were measured through their temperature-dependent (13)C NMR spectra. The ground state structures were found to possess a symmetrical propeller nature. The mechanism for phenyl rotation involves a nonsynchronous process, in which in the transition state one ring is coplanar with, and the other is perpendicular to, the plane formed by the central carbon and its bonds. The barriers of rotation in the carbanions vary with the strength of chelation of the potassium gegenion, whereas the (13)C NMR shifts remain unaffected. Changing substituents in the para position of the aromatic rings allowed both the elucidation of structure and mechanism and the observation of effects of changes in electron distribution on the height of the barrier and the (13)C NMR shifts.
Olah
G A
GA
Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, California 90007.
Watkins
M I
MI