Optical activity and helicity enhancement of diphenylmethyl molecular propellers

The diphenylmethyl group is a molecular propeller that can detect the chirality of a nearby stereogenic center through electronic circular dichroism spectra, within the short wavelength region of the phenyl ¹B transitions (190–200 nm). A positive exciton Cotton effect was associated with a positive (P) helicity of the diphenylmethyl group, while the opposite was true for M helicity. Higher helicity discrimination was observed in the presence of a hydroxy group (in hydroxydiphenylmethyl derivatives) which formed an intramolecular hydrogen bond, thus reducing the number of accessible conformers of the molecule. We found that the hydroxydiphenylmethyl groups in chiral tartaric acid TADDOL derivatives were preferably heterohelical (P, M).