Experimental and Theoretical Studies on the Chiroptical Properties of Donor-Acceptor Binaphthyls. Effects of Dynamic Conformer Population on Circular Dichroism

The axial chirality of biaryls has attracted much attention as effective chiral auxiliaries and ligands in asymmetric synthesis and chirality sensing. In this study, the chiroptical properties of donor-acceptor binaphtyl DA and symmetrical DD were investigated. The amplitude of the main-band couplet was enhanced in the circular dichroism (CD) spectra of DA; due to the effective cation-pi, interactions altering the potential energy profile against the rotational angle about the binaphthyl's C1-C1' bond. A shallow unsymmetrical potential curve was obtained for DA, which is in contrast to the almost symmetrical double well potential for DD. The theoretical calculations of the CD spectra at the R1-CC2 level using the two-state model composed of the s-cis and s-trans conformations sucessfully reproduce the experimental CD. Dynamic conformer distribution over a wide range of dihedral angle in the chiral binaphthyls was shown to be critical in discussing the performance of such axially chiral molecules.