Generation and Trapping of a 1-Phosphafulvene: An Illustration of the P═C/C═C Analogy.

1-Phosphafulvenes can be easily generated by dissociation of dimers resulting from the reaction of phospholes with aldimines. As electron-rich partners, they act as 4π phosphadienic systems toward alkenes and alkynes in [4 + 2] cycloaddition reactions. As electron-poor partners, they act either as 2π systems toward conjugated dienes in [2 + 4] cycloaddition reactions via their P═C double bond or as 6π systems toward phosphadienes in [6 + 4] cycloaddition reactions.