Protonation Behavior of 1,1'-Bi-2-naphthol and Insights into Its Acid-Catalyzed Atropisomerization.

The behavior of 1,1'-bi-2-naphthol (BINOL) in variety of (super)acid media has been studied by NMR. The results are combined with the theoretical (DFT) study of the role of mono- and diprotonated forms of BINOL in the acid-catalyzed atropisomerization of this compound. It is demonstrated that the process of enantiomeric configuration exchange proceeds mainly via internal rotation around the C1(sp3)-C1'(sp3) bond in intermediates such as C1-monoprotonated keto or C1,C1'-diprotonated forms of BINOL, depending on the acidity level.