Crystallization-Induced Diastereomeric Transformation of N-2′-t-Butyl-6′-iodobenzoyl-3-bromocarbazole

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We previously reported the atropisomeric properties of 2′-t-butyl-6′-iodo-substituted N-benzoyl-3-bromocarbazole, i.e., the steric or electronic effects of the substituents restricted the rotation about the N–C7′ and C7′–C1′ bonds to separate four stereoisomers (cis/trans for the N–C7′ axis, aR/aS for the C7′–C1′ axis). Furthermore, the 2′-t-butyl-6′-iodo-substituted N-benzoyl 3-bromocarbazole was confirmed to be a gear molecule, in which the rotation about the C7′–C1′ bond was in perfect concert with that about the N–C7′ bond. Herein, we report a unique crystallization-induced diastereomeric transformation found in this molecule. In the isolation process, where the product is recrystallized from the diastereoisomeric mixture, in situ isomerization and selective crystallization could lead to a diastereomeric transformation, and a mixture of diastereomers (trans/cis=54 : 46) was converted to trans-isomer-enriched crystals (trans/cis>96 : 4) in 95% yield. Conformational analysis clarified the preference for the trans versus cis isomer.
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