A relay energy transfer paradigm for asymmetric photocatalyzed [4+2] cycloadditions

In asymmetric energy transfer photocatalysis, direct incorporation of conventional chiral catalysts has achieved satisfactory enantiocontrol in several transformations. However, the efficiency and even feasibility of this mode are still limited by the energy transfer barrier that arises from the inevitable catalyst-mediated spatial segregation. To overcome this underappreciated constraint, we designed a relay energy transfer catalytic mode in which the catalyst acts as a bridge for energy transfer between the photosensitizer and the substrate. Guided by this concept, we engineered a class of chiral energy transfer acid catalysts capable of delivering high triplet energy. These catalysts effectively circumvent the inherent stoichiometric dependence on acid activators in dearomative [4+2] cyclization between quinolines and alkenes. The tunability of side arms and the proximity of the catalytic site to the chiral source optimize regio-, diastereo-, and enantioselectivities.