A Combined Experimental and Theoretical Study on the Conformation of Multiarmed Chiral Aryl Ethers

Abstract

Four series of multiarmed chiral aryl ethers carrying two, three, five, or eight side-chains on a variety of aromatic core molecules (2 − 5) were prepared. The structure and conformation of 2 and 3 (in the solid state) were determined by the X-ray crystallographic analyses. While a pair of alternated (anti) conformers (i.e, up−down and down−up) were found in the crystal of 2, three side-arms in 3 were aligned in the same direction to give a C₃-symmetric syn-conformation. Examinations by dispersion-corrected density functional (DFT-D) calculations revealed that two out of six anti- and two out of four syn-conformers of 2 are energetically most important. Two calculated structures of anti-conformers are in good agreement with those found in the solid state by X-ray analysis. Similarly, relevant conformations of syn-3, fully alternated 4, and C₅-symmetric 5 were optimized at the DFT-D-B-LYP/TZVP level. The structure and conformation of the side-arms in 2 − 5 in solution were further studied by temperature dependent ¹H NMR and UV−vis spectroscopy. In addition, comparative experimental and theoretical CD spectral studies were carried out in order to elucidate the contribution of the thermodynamically less-stable minor isomers in solution. The CD spectral changes observed for 2 and 3 at varying temperatures were quite different, while the parent chiral arene 1, as well as 4 and 5, only showed an increased intensity of the negative Cotton effect for the ¹L_b band. The latter behavior is readily accounted for in terms of the conformational freezing of the chiral groups at low temperatures. The unusual CD spectral behavior observed for 2 and 3 was rationalized by the conformational alteration of the side-arms. Because of attractive van der Waals interactions between the aromatic units of the arms in nonpolar solvents, the syn-conformations become gradually more important for 2 at low temperatures, which eventually results in a weak positive Cotton effect for the ¹L_b band. This was also supported by the SCS-MP2/TZVPP single-point energy calculations for the relevant conformers of 2. For 3, the contribution of the C₃-symmetrical conformer becomes more important than the less-symmetrical isomers at low temperatures. The conformations of 2 and 3 in their excited states as well as in the oxidized states were also examined.