REACTIVITY OF BIS(ALKENE) TETRACARBONYL COMPLEXES OF TUNGSTEN - EVIDENCE FOR ALKENE TO PI-ALLYL HYDRIDE REARRANGEMENT

The bis(alkene)tetracarbonyl complexes of tungsten were identified as olefin isomerization and metathesis catalyst precursors whose activiti es are based on the formation of a pi-allyl hydride intermediate. It h as been observed that the beta-hydrogen atom of coordinated olefin may migrate to the metal giving eta(3)-allylic hydrides which have been c haracterized by their H-1 NMR spectra. This is the first case where th e rearrangement of tungsten(0) alkenes coordinated to a pi-allylic hyd rides intermediate, fundamental to many catalytic mechanisms, can be o bserved directly. In the presence of Lewis acid the same alkene comple xes are catalytically active toward olefin metathesis. It seems reason able that the catalytically active alkylidene compounds are formed fro m the coordinated alkene via pi-allylic hydride and metallacyclobutane precursors.