Bidirectional Elongation Strategy Using Ambiphilic Radical Linchpin for Modular Access to 1,4-Dicarbonyls via Sequential Photocatalysis

Organic molecules which connect themselves with multiple substrates by sequential C−C bond formations can be utilized as linchpin compounds in multicomponent process. While they are useful for rapidly increasing the molecular complexity, most of the reported linchpin coupling methods rely on the use of organometallic species as strong carbon nucleophiles to form C−C bonds, which narrows the functional group compatibility. Here, we describe a metal-free, radical-mediated coupling approach using a formyl-stabilized phosphonium ylide as a multifunctional linchpin under visible-light photoredox conditions. The present method demonstrates an ambiphilic character of the phosphonium ylide, which serves both as a nucleophilic and an electrophilic carbon-centered radical source. The stepwise and controllable generation of these radical intermediates allows sequential photocatalysis involving two mechanistically distinct radical additions, both of which are initiated by the same photocatalyst in one pot with a high functional-group tolerance. The methodology enables a bidirectional assembly of the linchpin with two electronically differentiated alkene fragments and thus offers rapid and modular access to 1,4-dicarbonyl compounds as versatile synthetic intermediates.