Experimental and Theoretical Study of the CD Spectra and Conformational Properties of Axially Chiral 2,2‘-, 3,3‘-, and 4,4‘-Biphenol Ethers

Abstract

New chiral biphenol ethers (Biph22, Biph33, and Biph44), carrying (R)-methylpropyloxy groups at 2,2'-, 3,3'-, and 4,4'-positions of biphenyl, were prepared. The introduced peripheral chiral groups in these biphenol ethers induce an (averaged) axial chirality to give predominant aR- or aS-rotamers. The chiroptical properties of these axially chiral biphenol ethers in polar and nonpolar solvents were determined experimentally and were compared with the corresponding theoretical values to determine their conformational behavior in solution. Geometry optimization at the DFT-D/TZV2P level and subsequent time-dependent density functional theory (TD-DFT) at the BH-LYP/TZV2P level treatments to obtain rotatory strengths revealed that 6 out of 18 conformers (aR-Tg-, aR-Tg+, aR-G+t, aS-Tg-, aS-Tg+, and aS-G+t) are crucial to reproduce the experimental circular dichroism (CD) spectra and optical rotations. Although biphenyl molecules are in conformational equilibrium with varying interplanar angles in solution, our static approach to the prediction of the experimental CD spectra is simply based on pairs of thermally populated, local-minimum structures, that is, the dynamic behavior of the systems or the vibrational wave functions are not considered. The relative energies computed at the SCS-MP2/TZVPP level in the gas phase or in acetonitrile solution using the conductor-like screening model (COSMO) were found to be accurate enough to calculate the thermal population of the relevant conformers. Although most of the CD signals mutually cancel out each other between a pair of aR- and aS-rotamers, the remaining Cotton effects due to a small preference for a single rotamer produce characteristic CD spectra. In general (and somewhat unexpectedly), the delicate cancellation effects in the CD spectra are accurately described by the theoretical approach, and the simulated CD spectra are in excellent agreement with the experimental ones though observed rotatory strengths being always smaller (by 5-20 times) than the theoretical data. Accordingly, slight preferences for the P (or aR)-configuration for Biph22 and M-configuration for Biph33 and Biph44 are determined. The preference for the opposite isomer in the case of Biph22 is due to larger attractive intramolecular interactions between the chiral alkyl groups. This is also consistent with the lower oxidation potential found for Biph22 (DeltaE approximately 0.1 V), as compared with those for Biph33 and Biph44. The CT complex formation of Biph22-44 with various acceptors was also studied by UV-vis and CD spectroscopic methods.