Dictating Photoreactivity through Restricted Bond Rotations: Cross-Photoaddition of Atropisomeric Acrylimide Derivatives under UV/Visible-Light Irradiation

Nonbiaryl atropisomeric acrylimides
underwent facile [2 + 2] photocycloaddition
leading to cross-cyclobutane adducts with very high stereospecificity
(enantiomeric excess (ee): 99% and diastereomeric excess (de): 99%).
The photoreactions proceeded smoothly in isotropic media for both
direct and triplet sensitized irradiations. The reactions were also
found to be very efficient in the solid state where the same cross-cyclobutane
adduct was observed. Photophysical studies enabled us to understand
the excited-state photochemistry of acrylimides. The triplet energy
was found to be ∼63 kcal/mol. The reactions proceeded predominantly
via a singlet excited state upon direct irradiation with very poor
intersystem crossing that was ascertained by quantification of the
generated singlet oxygen. The reactions progressed smoothly with triplet
sensitization with UV or visible-light irradiations. Laser flash photolysis
experiments established the triplet transient of atropisomeric acrylimides
with a triplet lifetime at room temperature of ∼40 ns.