Chiral and racemic tetramorphs of 2,6-di-t-butylditolylfuchsone

Abstract

The title molecule 4-(α,α-ditolylmethylene)-2,6-di-t-butyl-1,4- benzoquinone (abbreviated as di-t-butylditolylfuchsone and numbered 2-t-Bu) serendipitously afforded four concomitant polymorphs during routine purification by column chromatography in the same solvent elution fraction. Polymorph I crystallized in chiral space group P2 1. Polymorphs II, III, and IV crystallized in centrosymmetric space groups P2 1/n, Pbca, and C2/c, respectively. The role of bulky t-Bu groups for crystallization in the chiral space group is discussed for 2,6-ditolyl and 2,6-diphenyl fuchsones. α,α-Diphenylmethylene-2,6-di-t-butyl-1,4-benzoquinone (di-t-butyldiphenylfuchsone, 1-t-Bu) crystallized in P2 1 (one polymorph) and P2 1/c (two polymorphs) space groups. Unfavorable steric repulsions due to bulky t-Bu groups result in voids in the crystal structures of centrosymmetric polymorphs II and III. Phase transformation of racemic structure II to III and finally to chiral polymorph I was monitored by thermal microscopy and differential scanning calorimetry. X-ray diffraction confirmed the phase transformation to be a single-crystal-to-single-crystal event. The chiral polymorph I is the stable modification in the tetramorphic system. Several randomly picked single crystals of 2-t-Bu polymorph I had the same absolute chirality by circular dichroism spectroscopy. A new molecule capable of exhibiting conformational chirality via atropisomerism is identified. © 2012 American Chemical Society.