Conformational studies by dynamic NMR. 78. Stereomutation of the helical enantiomers of trigonal carbon diaryl-substituted compounds: Dimesitylketone, dimesitylthioketone, and dimesitylethylene

The free energies of activation for the enantiomerization of the title comp ounds (Mes(2)C=X, Mes = 2,4,6,-trimethylphenyl) were determined by dynamic NMR to be 4.6, 6.5, and 9.2 kcal mol(-1) for X = O, S, and CH2, respectivel y. Single-crystal X-ray diffraction showed that the structure of dimesitylk etone is that of a propeller (Cz symmetry) with the mesityl rings twisted b y 50 degrees with respect to the plane of carbonyl. The same structure was predicted by molecular mechanics calculations, which also produced good agr eement between computed and experimental barriers for a dynamic process whe re a disrotatory one-ring flip pathway reverses the helicity of the conform ational enantiomers. Solid-state NMR spectra indicated that the enantiomeri zation barrier in the crystal must be much higher (at least 19 kcal mol(-1) ) than that in solution. Contrary to the case of dimesitylketone, the calcu lated barrier of dimesitylethylene agrees better with the experimental valu e if the enantiomerization process is assumed to be a conrotatory two-ring flip pathway.