The photocatalytic [2+2] cycloaddition of cyclic enones has recently received renewed attention. It was discovered that it can be catalyzed by chiral Lewis acids (1,3,2-oxazaborolidines) to yield a variety of bicyclic products with high enantioselectivity [1,2]. The key to this reaction is a redshift of the bright UV absorption band of the enone caused by the Lewis acid, which allows for a selective excitation of the chiral complex . To better understand this process, the absorption spectra of basic cyclohexenone-Lewis-acid complexes were calculated using XMS-CASPT2. Both, a bright ππ∗ singlet state and a darker nπ∗ singlet state were found to contribute to the absorption significantly. Furthermore, their spectral shifts differ depending on the Lewis acid.
It is known that the [2+2] photocycloaddition proceeds via a triplet state . Hence, we also elucidated the reaction path from the Frank-Condon point to the triplet minima by calculating critical points and by non-adiabatic molecular dynamics simulations. Based on these results, we devised an experiment that should allow us to directly observe the triplet formation using UV transient absorption spectroscopy.
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