Tunable photochemical 6 heterocyclization reactions mediated by a boron Lewis acid

Lewis acid catalysts are of great interest for regulating the excitation energy, reactivity, and enantioselectivity of photochemical transformations. However, the understanding of their catalytic performance toward 6 pi photocyclization is still quite sparse. We provide here a mechanistic insight into the regulation effect of a boron Lewis acid (BF3) on the photochemical 6 pi heterocyclization of a beta-enaminone investigated using multi-configurational ab initio calculations. Our state-of-the-art computational results reveal that the 6 pi heterocyclization of a free enaminone substrate tends to take place in the triplet (3)pi pi* state, which relies on a (1)n pi* electronic excitation and an El-Sayed type singlet-triplet crossing. The excitation energies and properties of the enaminone can be photochemically tuned in the presence of a BF3 Lewis acid catalyst. As an important consequence, the catalyzed 6 pi heterocyclization can proceed smoothly on the singlet (1)pi pi* hypersurface, and it may also occur in triplet manifold when choosing a suitable photosensitizer enabling the direct photochemical activation of the enaminone to the triplet excited state.