OPTICALLY-ACTIVE AMINES .36. APPLICATION OF THE BENZENE CHIRALITY RULE TO RING-SUBSTITUTED MANDELIC-ACIDS

The positive sign of the 1L(b) Cotton effects (CEs) from about 250 to 270 nm in the circular dichroism (CD) spectrum of (R)-mandelic acid and its sodium salt is determined by vibronic borrowing from allowed transitions at shorter wavelength. On ring substitution, transition moments are induced in the benzene ring bonds adjacent to the attachment bond of the chiral group, resulting in enhanced coupling of the 1L(b) transition with the chiral group. The sign of the 1L(b) CEs for a ring-substituted (R)-mandelic acid may be the same or opposite to that of (R)-mandelic acid, but the sign of its 1L(b) CEs can often be correlated with its absolute configuration provided the ring position of the substituent and its spectroscopic moment is taken into account.