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 R-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.

Circular dichroism of tocopherols versus tocotrienols

“…1); the same sign has been observed for other tocol derivatives (9-15) with 2R configuration. [17][18][19][20][21][22] Assuming that the long alkyl chain at C-2 occupies preferentially an equatorial position (vide infra), the 2R configuration imparts to the dihydropyrane ring a half-chair conformation with positive (P) helicity (Scheme 2). Thus, a correlation P helicity $ negative 1 L b CD band is found, in keeping with the trend observed for chromanes, including 5-and/or 7-oxygenated derivatives;…”

Spectroscopic Characterization of Neutral and Cation Radicals of α-Tocopherol and Related Molecules: A Satisfactory Denouement

“…More recently, UV-visible spectra recorded during the electrochemical oxidation of the tocopherols (and two related model compounds, including MOH) in an organic solvent, with a supporting electrolyte and an organic acid (for suppressing the deprotonation of the cation), have become available; [14][15][16] the choice of the solvent, acetonitrile or DCM, did not exert any noticeable influence on the spectra, and the shapes of the visible bands were in fair agreement with those observed in TFA. 4,13 On the other hand, a recent visible absorption spectrum of TOAc •+ , with p ) 492 in DCM, 17 bears hardly any resemblance to the other spectra mentioned above. It will be shown later that the electrochemical spectra 15,16 imply that ε 465…”

An optically transparent thin-layer electrochemical cell for the study of vibrational circular dichroism of chiral redox-active molecules

“…7,8 Consequently, VCD has become an essential tool employed in biophysical, biochemical, and pharmaceutical research laboratories. [9][10][11][12][13][14][15][16] To the best of our knowledge, the only VCD spectra of chiral organic radicals have been reported by Mori et al 17 However, these radical cations were obtained on addition of a chemical oxidant, so that many aspects of the chiroptical activity (e.g., the reversibility of the oxidation, and optically transparent thin-layer electrochemical (OTTLE) 18 cell applicable for VCD measurements on electrochemically generated radical ions, using a commercially available VCD spectrometer.…”