Enantioselective Energy Transfer-Enabled Cyclization using a Privileged Al-Salen Photocatalyst

Chiral catalysts that can engage multiple substrates, via distinct activation modes, to deliver enantioenriched products with high levels of fidelity are often described as “privileged”. Antipodal to enzymatic specificity, this generality enables the reactivity - selectivity divide in ground state landscapes to be effectively reconciled. Achieving this latitude in excited state paradigms remains a frontier, and efforts to identify privileged chiral photocatalysts are currently a core area of research. Aluminum salen complexes are emergent contenders on account of their well-defined photophysical properties and competence in photo-induced single electron transfer processes: this has recently been leveraged in the deracemization of cyclopropyl ketones. To expand the activation repertoire of Al-salen photocatalysts, an unprecedented enantioselective energy transfer (EnT) catalysis-enabled photocyclization of acrylanilides has been developed. This operationally simple strategy allows reactivity and enantioselectivity to be simultaneously regulated by an inexpensive, commercial chiral Al-salen complex upon irradiation at λ = 400 nm: this allows diverse cyclic products to be forged with high levels of enantioselectivity (up to 96:4 e.r.). Establishing this dichotomy in excited state activation modes serves to consolidate the privileged status of chiral Al-salen complexes in enantioselective photocatalysts and to complement their ubiquity in ground-state regimes.