Enantioselective Synthesis of Cyclopentenes by (3+2) Annulation via a 2-Carbon Phosphonium.

Herein we report a catalytic enantioselective (3+2) annulation, in which a vinyl phosphonium intermediate serves as the 2-carbon component. The reaction involves an α-umpolung β-umpolung coupling sequence, enabled by β-haloacrylates and chiral enantioenriched phosphepine catalysts. The reaction shows good generality, providing access to an array of cyclopentenes, with mechanistic studies supporting stereospecific formation of the vinyl phosphonium intermediate which, then undergoes annulation with turn over limiting catalyst elimination. Beyond defining a new approach to cyclopentenes, these studies demonstrate that β-haloacrylates can replace ynoates in reaction designs that require exclusive umpolung coupling at the α- and β-positions.