ChemInform Abstract: The Radical Anions of 1,2‐Diphenylcyclohexene and Structurally Related Compounds. Conformational ESR and ENDOR Studies

Abstract

ESR, ENDOR, and TRIPLE resonance studies have been performed on the radical anions of 1,2-diphenylcyclohex-1-ene (4), 1,2-di(perdeuteriophenyl)cyclohex-1-ene ((D10)4) the trans-configurated 3,4-diphenyl-8-oxabicyclo[4.3.0]non-3-ene (5) and its 2,2,5,5-tetradeuterio derivative (D4)5, and 2,3-diphenyl-8,9,10-trinorborn-2-ene (6). The spectra of exhibit strong temperature dependence along with a specific broadening of ESR hyperfine lines and proton ENDOR signals. The coupling constant, which bears the main responsibility for these features, is that of the β-protons in the quasi-equatorial positions of the cyclohexene ring, and the experimental findings are readily rationlized in terms of relatively modest conformational changes without invoking the inversion of the half-chair form. The hyperfine data for the β-protons in closely resemble the corresponding low-temperature values for , However, the ‘unusual’ features observed for are absent in the ESR and ENDOR spectra of , because the half-chair conformation of the cyclohexene ring in is deprived of its flexibility. Although the boat form of this ring in is also rigid, the spectra of are temperature-dependent, due to an interconversion between two propeller-like conformations of the phenyl groups. The pertinent barrier is 30 ± 5 kJ·mol−1. An analogous interconversion presumably takes place in and as well, but, unlike , it is not amenable to experimental study.