42496-08-2Relevant academic research and scientific papers
Conformational studies by dynamic NMR.78.1 Stereonmutation of the helical enantiomers of trigonal carbon diaryl-substituted compounds: Dimesitylketone, dimesitylthioketone, and dimesitylethylene
Grilli,Lunazzi,Mazzanti,Casarini,Femoni
, p. 488 - 495 (2001)
The free energies of activation for the enantiomerization of the title compounds (Mes2C=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, respectively. Single-crystal X-ray diffraction showed that the structure of dimesitylketone is that of a propeller (C2 symmetry) with the mesityl rings twisted by 50° with respect to the plane of carbonyl. The same structure was predicted by molecular mechanics calculations, which also produced good agreement between computed and experimental barriers for a dynamic process where a disrotatory one-ring flip pathway reverses the helicity of the conformational enantiomers. Solid-state NMR spectra indicated that the enantiomerization 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 calculated barrier of dimesitylethylene agrees better with the experimental value if the enantiomerization process is assumed to be a conrotatory two-ring flip pathway.
