Phosphine/AgF-Catalyzed Ethynylative Addition of Allenoates to Enones

Abstract

While α-substituted allenoates are well established as versatile 4C-synthons in phosphine-catalyzed (4+n) annulations for the construction of cyclic frameworks, their α-carbons typically remain inert toward bond formation, implying an intrinsic limitation of classical phosphine catalysis. Herein, we report a phosphine/AgF cocatalysis that can enable the ethynylative addition of α-substituted allenoates to enones, affording acyclic products bearing an all-carbon quaternary stereocenter at the α-carbon. The reaction pathway is proposed to involve a silver enolate–vinyl fluorophosphorane intermediate. For this Ag(I)–P(V) species, the Ag(I) moiety might act as a Lewis acid to facilitate the α-addition reaction to the enone, while the P(V) moiety could be amenable to serve as a leaving group for E1cb-type elimination, eventually furnishing ethynylative addition. More importantly, the Ag(I) center provides a versatile handle to achieve an asymmetric variant with high enantioselectivity by harnessing its ability to coordinate with chiral bidentate ligands.