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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&gt;96 : 4) in 95% yield. Conformational analysis clarified the preference for the trans versus cis isomer.

Crystallization-Induced Diastereomeric Transformation of <i>N</i>-2′-<i>t</i>-Butyl-6′-iodobenzoyl-3-bromocarbazole