Structure, Conformation, Stereodynamics, Dimer Formation and Absolute Configuration of Axially Chiral Atropisomers of Hindered Biphenyl Carbinols

NMR spectra of biphenyl derivatives bearing a single CR2OH substituent in the ortho position indicate that they exist as sp (more stable ) and ap (less stable) conformers, due to the restricted rotation about the Ar-CR2OH bond. When R=Et (compound 2) the corresponding rotation barrier was determined (7.5 kcal mol-1) by line shape simulation of the low temperature NMR spectra. Introduction of the prochiral i-Pr group in the position 3’ of a biphenyl with the CMe2OH substituent in the position 2 (4) allowed the determination of the enantiomerisation barrier (due to the Ar-Ar bond rotation) for the stereolabile axially chiral atropisomers (13.95 kcal mol-1). DFT computations of these barriers were all in agreement with the experiments. Biphenyls bearing two CR2OH groups in the 2,2’ positions were found to exist as configurationally stable atropisomers: when R=Me (7) they were separated by enantioselective HPLC and the absolute configuration assigned on the basis of the corresponding CD spectra