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Stereomutations of Two-Bladed Propeller Derivatives:  Ortho-Substituted Diaryl Ethylene and Diaryl Ketone

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journal contribution
posted on 21.01.2005, 00:00 by Lodovico Lunazzi, Andrea Mazzanti, Mirko Minzoni
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).