Optically Active Amines. 40. Application of the Benzene Sector Rule to the Circular Dichroism of Chiral Benzylcarbinamines and Benzylcarbinols

The sign of the 1 L b Cotton effects (CEs) from about 240 to 270 nm in the circular dichroism (CD) of enantiomers of chiral benzylcarbinamines and benzylcarbinols is correlated with their absolute configurations using the benzene sector rule and a consideration of the equilibrium between their two conformers of lowest energy and of oppositely signed rotatory powers. For chiral benzylalkylcarbinamines, carbinamine salts, and carbinols, which show a single sign for their 1 L b CEs, a shift in the conformational equilibrium can explain a sign reversal of the CEs with a change in solvent, (R)-2-amino-1-phenylpropane showing negative 1 L b CEs in methanol but positive ones in cyclohexane. l-Phenylalanine in water shows positive 1 L b CEs, but in methanol it shows both negative and positive CD maxima. In methanol, the two longest wavelength maxima comprise a double CE associated with the band origin absorption maximum at 267 nm, the conformational equilibrium shifted from the positive conformer of l-phenylalanine to its negative one and the greater amount of the latter is now detected at slightly longer wavelength in the CD spectrum.