A radical-polar crossover approach to complex nitrogen heterocycles via the triplet state.

The transition from radical to ionic reactivity is a key design feature of many photochemical reactions, enabling complex transformations not possible under either mechanistic regime alone. Ground-state alkenes are common substrates in existing methods of this type, serving as radical acceptors to generate open-shell intermediates from which the radical-polar crossover (RPC) event is oxidatively or reductively triggered by a photocatalyst. Here, we describe an alternative RPC mechanism proceeding via an alkene triplet diradical. In this transformation, an iodine radical liberated during a homolytic aromatic substitution step functions as a single-electron oxidant to generate an iminium electrophile that can be intercepted en route to complex natural-product-like amines. An enantioselective variant of the reaction, enabled by an oxidatively installed sulfinyl leaving group, points to the generality of this underdeveloped pattern of diradical reactivity, paving the way for other triplet-state reactions that incorporate both one- and two-electron bond-forming processes.