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=Me2IMe, (Me2IPr)-Pr-i). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)(n); n=3, 4, 5, 6] and Ph2C=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.