NHC‐Palladium‐Catalyzed Cascade Annulation of Alkynoic Acids with Unactivated Alkenes in Ionic Liquids

A practical and green NHC-palladium-catalyzed cascade annulation of alkynoic acids with unactivated alkenes under an oxygen atmosphere in ionic liquid for the assembly of structurally diverse alkyl substituted α-methylene-γ-lactones is accomplished. In the presence of 0.5 mol% of IPr−Pd−Py−Cl₂ in [Amim]Cl, both the alkynoic acids and unactivated alkenes were well tolerated, thus giving the desired products in moderate to good yields with excellent Z/E selectivity.

Abstract

An N-heterocyclic carbene palladium (NHC-palladium)-catalyzed cascade annulation of alkynoic acids with unactivated alkenes under an oxygen atmosphere in ionic liquids is described. This protocol enables facile access for the preparation of diverse alkyl substituted α-methylene-γ-lactones in moderate to good yields with excellent Z/E selectivity. Moreover, the ionic liquid not only acts as the green solvent, but also as the eco-friendly chlorine source, thus avoiding the use of excess metal halides such as cupric chloride. Some preliminary mechanistic studies were performed, which further revealed possible pathways for this cascade annulation.

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