Intermolecular Formal [2 + 1] Cycloaddition with Sulfoxonium Ylides via Energy Transfer Photocatalysis

Among the recognized methods to build cyclopropanes via photocatalytic [2 + 1] cycloaddition, the strategy of activation of the C1 synthon is exclusively dominant, while the photocatalytic activation of C2 synthon coupled with appropriate C1 synthon remains overlooked. Herein, we disclose a visible-light photocatalytic cis-cyclopropanation of maleimides with sulfoxonium ylides under mild, metal-, and oxidant-free conditions to efficiently construct 3-azabicyclo[3.1.0]hexane derivatives. In particular, a high stereoselectivity of cis-cyclopropanation can be accomplished through solvent-bridged H-bonding interactions. A combined experimental and computational mechanistic study suggests that the activation of C2 synthon, maleimides, via photocatalytic triplet–triplet energy transfer (EnT) is achieved to conduct the [2 + 1] cycloaddition reaction with sulfoxonium ylides. The afforded bicyclic framework can undergo facile downstream transformations, delivering stereochemically defined cyclopropane derivatives. In addition, this photocatalytic [2 + 1] cycloaddition via distinct EnT activation of C2 synthon is also appropriate for coumarins to result in the cyclopropa[c]coumarin derivatives. Moreover, benzophenone imines and 2-benzoylpyridines are also suitable for the photocatalyzed [2 + 1] cycloaddition with sulfoxonium ylides via energy transfer mechanism to construct the corresponding aziridine and oxirane derivatives, respectively, demonstrating the general applicability and practicability of this approach.