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.