Nucleophilic Migratory Cyclopropenation of Activated Alkynes: A Nonmetal Approach to Unbiased Cyclopropenes.

An unprecedented reductive [2 + 1] annulation of α-keto esters with alkynones mediated by P(NMe2)3 is described. Although this nonmetal cyclopropenation is a nucleophilic process, attributed to the ester migration via a formal [2 + 2] cycloaddition reaction of Kukhtin-Ramirez adducts and alkynones followed by a fragmentation, cyclopropenes with an unbiased alkene scaffold are formed in good to excellent yields, thus providing a promising complementarity to electrophilic metal-catalyzed cyclopropenation.