(3+2) Annulation of 4-Acetoxy Allenoate with Aldimine Enabled by AgF-Assisted P(III)/P(V) Catalysis

A phosphine-catalyzed (3+2) annulation of 4-acetoxy allenoate and aldimine with the assistance of AgF is described. The success of this reaction hinges on the metathesis between the enolate-phosphonium zwitterion and AgF, leading to a key intermediate comprising of silver enolate and a fluorophosphorane P(V)-moiety. The former is able to undergo a Mannich reaction with aldimine, whereas the latter initiates a cascade sequence of AcO-elimination/aza-addition, thus furnishing the P(III)/P(V) catalysis. By taking advantage of the silver enolate, a preliminary attempt at an asymmetric variant was conducted with the combination of an achiral phosphine catalyst and a chiral bis(oxazolinyl)pyridine ligand (PyBox), giving moderate enantioselectivity. AgF is capable of converting the enolate-phosphonium zwitterion into a novel silver enolate-fluorophosphorane species, which enables the development of (3+2) annulation of 4-acetoxy allenoate and aldimine via P(III)/P(V) catalysis. The proof-of-concept study has demonstrated that the silver enolate moiety offers an alternative approach to realize an asymmetric variant merely by the introduction of an additional chiral PyBox ligand.image