Ph 3 PCN 2 : A stable reagent for carbon-atom transfer

Precise modification of a chemical site in a molecule at the single-atom level is one of the most elegant yet difficult transformations in chemistry. A reagent specifically designed for chemoselective introduction of monoatomic carbon is a particularly formidable challenge. Here, we report a straightforward, azide-free synthesis of a crystalline and isolable diazophosphorus ylide, Ph 3 PCN 2 , a stable compound with a carbon atom bonded to two chemically labile groups, triphenylphosphine (PPh 3 ) and dinitrogen (N 2 ). Without any additives, the diazophosphorus ylide serves as a highly selective transfer reagent for fragments, including Ph 3 PC, to deliver phosphorus ylide–terminated heterocumulenes and CN 2 to produce multisubstituted pyrazoles. Ultimately, even exclusive carbon-atom transfer is possible. In reactions with aldehydes and acyclic and cyclic ketones (R 2 C=O), the carbon-atom substitution forms a vinylidene (R 2 C=C:) en route to alkynes or butatrienes.