CONFORMATION AND STEREODYNAMICS OF A HIGHLY HINDERED ETHANE, 1,2-BIS(2,4,6-TRI-TERT-BUTYLPHENYL)ETHANE, AND OF THE CORRESPONDING ETHYL RADICAL

The variable-temperature dynamic NMR spectra of the title compound (1, ArCH2CH2Ar, where Ar = 2,4,6-tri-tert-butylphenyl) allowed the free energy of activation (ΔG⋆ = 12.9 kcal mol-1) to be determined for the internal molecular motions. Molecular mechanics calculations indicate that 1 adopts the same eclipsed conformation as found in an X-ray diffraction study. These calculations also show that the internal motion is due to CH2-CH2 rotation highly correlated with Ar-CH2 rotation. The computed pathway for this process predicts that CH2-CH2 rotation is accompanied by rotation about one of the Ar-CH2 bonds. This conclusion has been confirmed by the experimental observation that the rate constant determined, at a given temperature, for the CH2-CH2 rotation is twice as large as that determined for the Ar-CH2 rotation. In the analogous disulfide 3 (ArCH2SSCH2Ar) no such correlated motion exists and the barrier (ΔG⋆ = 8.15 kcal mol-1) is that of an isolated S-S rotation. The rate constants determined by monitoring the CH2 signal of 3 are in fact equal to those obtained by monitoring the tert-butyl signal at the same temperature, as opposed to the case of 1. In the corresponding ethyl radical, line-width alternation of the ESR spectra yields the activation parameters for the CH-CH2 rotation (ΔG⋆ = 6.3 kcal mol-1, ΔH⋆ = 6.3 kcal mol-1, ΔS⋆ =-0.2 eu). Analysis of the hyperfine splittings suggests that the conformation of the radical is staggered, with the Ar-CH2 bond making an angle of 56° with the pz orbital of the sp2 carbon bearing the unpaired electron. There is also an indication that, despite the crowded situation, there is still conjugation between the Ar group and the radical center. © 1990, American Chemical Society. All rights reserved.