Analysis of Interconversion between Atropisomers of Chiral Substituted 9,9’‐Bicarbazole

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.