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 activities are based on the formation of a pi-allyl hydride intermediate. It has been observed that the beta-hydrogen atom of coordinated olefin may migrate to the metal giving eta(3)-allylic hydrides which have been characterized by their H-1 NMR spectra. This is the first case where the rearrangement of tungsten(0) alkenes coordinated to a pi-allylic hydrides intermediate, fundamental to many catalytic mechanisms, can be observed directly. In the presence of Lewis acid the same alkene complexes are catalytically active toward olefin metathesis. It seems reasonable that the catalytically active alkylidene compounds are formed from the coordinated alkene via pi-allylic hydride and metallacyclobutane precursors.