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 (DG¹ = 6.45, 4.65 and £ 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 the o,o’-diisopropylbenzophenone 2 is predicted by calculations to exist essentially as a C2-type conformer, a result which was confirmed by the low temperature NMR spectra. The interconversion barrier for the enantiomeric forms of this conformer was also measured (DG¹ = 6.35 kcal mol-1).