Chiroptical Properties of Organic Radical Cations. The Electronic and Vibrational Circular Dichroism Spectra of α-Tocopherol Derivatives and Sterically Hindered Chiral Hydroquinone Ethers

Abstract

Qualitatively and quantitatively reliable electronic and vibrational circular dichroism (ECD and VCD) spectra of chiral organic radical cations were obtained for the first time with α-tocopherol derivatives and sterically hindered chiral hydroquinone ethers. The isolation and spectral measurements of chiral radical cation salts were made possible by using nitrosonium or antimony derivatives as electron-transfer oxidants, which can cleanly oxidize the substrate donors without giving any byproducts in the sample solution. Such reliable ECD spectra enabled us to fully examine the chiroptical properties of organic radical cations and also compare them with those of the corresponding neutral compounds. The observed VCD spectra of neutral and radical cationic species of chiral hydroquinone ether were nicely simulated by density functional theory (DFT) calculations, from which the relative contribution of each radical cation conformer in solution was evaluated. Thus, the combined synthetic, spectroscopic, and theoretical protocol, composed of chiral modification, clean oxidation to form stable radical cations, ECD/VCD spectral analyses, and DFT calculations, was demonstrated to be a powerful, indispensable tool for elucidating a comprehensive picture of radical cationic species in solution