Stable Chiral Dithia-Bridged Heterohelicene Radical Cation
A stable axially chiral radical cation of dithia-bridged heterohelicene has been synthesized and analyzed using experimental and theoretical methods.
Our recent study  is another demonstration of the fruitful interplay between theory and experiment discussed in the previous post. What makes this work remarkable is that the publications on stable helical radical ions are rather scarce, while it is important for various applications to have enantiomerically pure stable radical cations with known absolute configuration. My colleagues in Erlangen first prepared the racemic neutral heterohelicene with a dithia-bridged triphenylamine moiety, which we had already encountered in the previous post and in the study of pyridyl N-heterotriangulenes prepared for photovoltaics. Then the enantiomers were separated by HPLC by my colleagues in Max-Planck-Institut für Kohlenforschung. Finally, it was shown that the pure enantiomeric compound undergoes reversible oxidation with retention of P-helicity. The latter was done by determining absolute configurations of the neutral and radical cation compounds through extensive comparison of experimental and theoretical (calculated by my US colleagues) electronic circular dichroism (ECD) spectra — a nice example of synergy between two disciplines.
For potential chiroptical application it is important to know how stable is the chiral compound. The radical cation appears to be chemically stable, but of course it is also important that it does not change its configuration too fast. The above finding that heterohelicine does not change its P-helicity upon oxidation is quite promising in this respect. However, oxidation leads to the flattening of heterohelicine as found by both X-ray analysis and theoretical calculations, which potentially may lower the racemization barrier. Indeed, our calculations have shown that the racemization of both neutral heterohelicine and its radical cation proceeds via the transition states in which the ortho-atoms lie in one plane. Thus, the racemization barrier is lower for the radical cation by ca. 5 kcal/mol compared to the neutral compound at the B3LYP/TZVP level of theory. Nevertheless, it is still rather high for heterohelicene radical cation (ΔG≠298=28.1 kcal/mol) and comparable to the barriers known for other neutral helicines.
1. Bettina D. Gliemann, Ana G. Petrovic, Eva M. Zolnhofer, Pavlo O. Dral, Frank Hampel, Georg Breitenbruch, Schulze Philipp, Vijay Raghavan, Karsten Meyer, Prasad L. Polavarapu, Nina Berova, Milan Kivala, Configurationally Stable Chiral Dithia-Bridged Heterohelicene Radical Cation: Electronic Structure and Absolute Configuration. Chem. Asian J. 2017, 12, 31–35. DOI: 10.1002/asia.201601452.