Stereomutations of two-bladed propeller derivatives: ortho-substituted diaryl ethylene and diaryl ketone

Stereochemical analysis, supported by ab initio computations, predicts the existence of three possible stable helical conformers for o,o'-diisopropyl-1,1'-diphenylethylene (1) and o,o'-diisopropylbenzophenone (2). At low temperature the NMR spectra of 1 showed distinct sets of signals for these conformers, thus allowing the measurement of the three barriers involved in the related stereomutation processes to be obtained (DeltaG=6.45, 4.65, and < or =4.0 kcal mol(-1)). The NMR spectra also indicate that the asymmetric conformer (C1 point group) is the most stable one in solution, as anticipated by calculations. X-ray diffraction confirmed that this structure is that adopted in the crystalline state. On the other hand, o,o'-diisopropylbenzophenone (2) is predicted by calculations to exist essentially as a C2-type conformer, a result that was confirmed by the low-temperature NMR spectra. The interconversion barrier for the enantiomeric forms of this conformer was also measured (DeltaG=6.3(5) kcal mol(-1)).