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 (ΔG⧧ = 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, 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 (ΔG⧧ = 6.35 kcal mol-1).