CARBANIONS .3. NUCLEAR MAGNETIC-RESONANCE SPECTROSCOPIC AND THEORETICAL-STUDY OF HOMOAROMATICITY IN CYCLOHEXADIENYL ANIONS

By analogy with the homocyclopropenyl (1) and homotropylium cations (2), cyclohexadienyl anions (3) might also be expected to be nonplanar, homoaromatic species. Several such anions, prepared by proton abstraction from the corresponding 1,3- and 1,4-cyclohexadienes, were studied by 1H and 13CNMR spectroscopy in ND3 solution. Interpretation of the chemical shifts, coupling constants, and the results of low-temperature studies at 300 (1H NMR) and 67.88 MHz (13C NMR) indicated these cyclohexadienyl anions rather to be planar nonhomoaromatic species. MINDO/3 calculations on the parent cyclohexadienyl anion agreed; the planar form was found to be a shallow energy minimum. The large spatial separation between the π-system termini and the stabilizing interaction of π* (CH2) with the HOMO of the pentadienyl fragment are responsible for the absence of the expected homoaromaticity. © 1978, American Chemical Society. All rights reserved.