Analysis of Interconversion between Atropisomers of Chiral Substituted 9,9’‐Bicarbazole
Toshifumi Kobayashi, Fumitaka Ishiwari, Takanori Fukushima, Kengo Hanaya, Takeshi Sugai, Shuhei Higashibayashi
Abstract
Abstract Interconversion of atropisomers of chiral 3,3’‐di‐ tert ‐butyl‐9,9’‐bicarbazole linked by a single N−N bond was analyzed by DFT calculations and experiments. The calculations revealed that the trans transition state has a lower energy for the interconversion than the cis transition state. The lowest transition state of interconversion between natural dixiamycins A and B containing a 9,9’‐bicarbazole structure was found to be the trans transition state. The calculations also indicated that degradation through N−N bond cleavage was faster than racemization. Atropisomeric enantiomers of 3,3’‐di‐ tert ‐butyl‐9,9’‐bicarbazole were resolved by chiral HPLC for the first time, and then racemization and N−N bond cleavage were investigated by heating experiments, showing that racemization did not take place below the degradation temperature.