Absolute Asymmetric Photocyclization of Triisopropylbenzophenone Derivatives in Crystals and Their Morphological Changes

Although 4-(2,4,6-triisopropylbenzoyl)­benzyl­benzamide is an achiral molecule, chiral crystals can form through spontaneous crystallization in a methanol solution. In the M crystal, twofold helical hydrogen-bond chains form in a counterclockwise direction among the molecules along the a axis to generate crystal chirality. The solid-state circular dichroism spectra of the two enantiomorphous crystals as Nujol mulls show a good mirror-image relationship. UV irradiation of the M crystal at >290 nm caused highly enantioselective Norrish type II photocyclization to yield the (R)-cyclobutenol with 94% ee in 100% yield as the sole product, resulting in successful absolute asymmetric synthesis. In contrast, the (S)-cyclobutenol was obtained from the P crystal with 95% ee in 100% yield. The high enantiodifferentiation in the crystalline-state photocyclization is attributed to the shorter distance between the carbonyl oxygen atom and one of the methine γ-hydrogen atoms of the two o-isopropyl groups as well as the smooth transformation with minimum molecular motion because of the similar shapes of the reactant and product molecules. UV irradiation of the platelike crystals resulted in a crack in the direction perpendicular to the long axis (the a axis of the unit cell), likely because the hydrogen-bond chains were broken during the photocyclization.