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 [3]. 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 [4]. 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.

[1]: Saner Poplata and Thorsten Bach, J. Am. Chem. Soc., 2018, 140, 3228-3231.
[2]: Daniel P. Schwinger and Thorsten Bach, Acc. Chem. Res., 2020, 53, 9, 1933-1943.
[3]: Christoph Brenninger, John D. Jollife and Thorsten Bach, Angew. Chem. Int. Ed. 2018, 57, 14338-14349.
[4]: Hongjuan Wang, Xiaoyan Cao, Xuebo Chen, Weihai Fang and Michael Dolg, Angew. Chem. Int. Ed., 2015, 54, 14295-14298.