Unveiling the impact of the light source and steric factors on [2 + 2] heterocycloaddition reactions

Information gained from in-depth mechanistic investigations can be used to control the selectivity of reactions, leading to expansion of the generality of synthetic processes and discovery of new reactivity. Here, we investigate the mechanism of light-driven [2 + 2] heterocycloadditions (Paternò–Büchi reactions) between indoles and ketones to develop insight into these processes. Using ground-state ultraviolet–visible absorption and transient absorption spectroscopy, together with density functional theory calculations, we found that the reactions can proceed via an exciplex or electron–donor–acceptor complex, which are key intermediates in determining the stereoselectivity of the reactions. We used this discovery to control the diastereoselectivity of the reactions, gaining access to previously inaccessible diastereoisomeric variants. When moving from 370 to 456 nm irradiation, the electron– donor–acceptor complex is increasingly favoured, and the diastereomeric ratio (d.r.) of the product moves from >99:<1 to 47:53. In contrast, switching from methyl to ipropyl substitution favours the exciplex intermediate, reversing the d.r. from 89:11 to 16:84. Our study shows how light and steric parameters can be rationally used to control the diastereoselectivity of photoreactions, creating mechanistic pathways to previously inaccessible stereochemical variants.