Metathesis of P=C Bonds Catalyzed by N-Heterocyclic Carbenes.

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p-block elements. A high-yielding P=C bond metathesis of phosphaalkenes (ArP=CPh2 , Ar=Mes, o-Tol, Ph) has been discovered that is catalyzed by N-heterocyclic carbenes (NHC=Me2 IMe, Me2 Ii Pr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)n ; n=3, 4, 5, 6] and Ph2 C=CPh2 . Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o-Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo-(ArP)2 CPh2 and cyclo-(ArP)4 CPh2 have also been observed. This work represents a rare application of non-metal-based catalysts for transformations involving main-group elements.