Phosphine-Catalyzed [3 + 2] and [2 + 4] Annulations of Ketenimines with Electron-Deficient Alkenes or α,β-Unsaturated Carbonyl Compounds

Phosphine-catalyzed annulation reactions of allenoates represent a powerful strategy for constructing carbo- and heterocyclic compounds. However, applications involving heteroatom surrogates of allenes remain scarce. Herein we report the first example of ketenimines serving as phosphine acceptors to enable divergent [3 + 2] and [2 + 4] annulations with thiazolone-derived alkenes, delivering fused and spirocyclic thiazole derivatives through modulation of β'-substituents on ketenimines. This protocol achieves high yields, broad substrate scope, scalability, and applicability to further transformations. This work demonstrates the potential of ketenimines as aza surrogates of allenoates in phosphine catalysis, introducing heteroatom diversity beyond carbon/oxygen while retaining high reactivity and a broad reaction scope.