Convenient access to π-conjugated 1,3-azaphospholes from alkynes via [3 + 2]-cycloaddition and reductive aromatization

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When incorporated into π-conjugated systems, phosphorus-based heterocycles have gained significant attention due to their unique chemical and physical properties. However, their syntheses remain challenging. Herein, we describe a novel [3+2]-cycloaddition between 1,3-dipolar triflatophosphanes and alkynes, affording azaphospholium salts. These salts can be reduced to generate the targeted 1,3-azaphospholes and an NHC ( N -heterocyclic carbene) byproduct. The reaction is high-yielding, scalable, and can tolerate a wide array of sterically and electronically diverse alkynes, leading to a library of 1,3-azaphospholes (19 examples, up to 99% yield). As a broad range of structural modifications is covered, information on structure-property relationships is extracted from joint experimental and theoretical analysis, revealing efficient strategies for further optimizations. Their high thermal stability and tunable electronic structure render them a promising platform for future applications.