Transformations of Cyclic Olefins Mediated by Tungsten Nitrosyl Complexes

This report describes investigations that have elucidated the nature, extent, and mechanism of the cyclic-olefin oligomerization effected by a series of tungsten precatalysts, with particular focus on Cp*W(NO)(CH<sub>2</sub>CMe<sub>3</sub>)<sub>2</sub> (<b>1</b>) and Cp*W(NO)(CH<sub>2</sub>SiMe<sub>3</sub>)(η<sup>2</sup>-CPhCH<sub>2</sub>) (<b>2</b>). Upon thermolysis, these precatalysts oligomerize simple cyclic olefins, from cyclopentene to cyclooctene, into ring-retaining oligomers as high as dodecamers (depending on the substrate) with remaining sites of unsaturation. Precatalyst initiation involves the coupling of one equivalent of the substrate with the reactive 16<i>e</i> intermediate thermally generated by the precatalyst (i.e., an alkylidene by <b>1</b> or an η<sup>2</sup>-alkyne complex by <b>2</b>), followed by rearrangement of the coupled ligand in the metal’s coordination sphere either to an olefin or to a diene (with concomitant loss of two hydrogen atoms). The rearranged ligand is displaced from the metal center as two equivalents of substrate coordinate to form a putative bis-olefin complex, Cp*W(NO)(cyclic olefin)<sub>2</sub>, that represents the convergent entry point to the catalytic cycle for the precatalysts. The coordinated olefins undergo metal-mediated coupling to form a metallacyclopentane complex. The metallacycle then undergoes β-hydrogen activation and reductive elimination to generate an η<sup>2</sup>-cyclic-olefin dimer. Further incorporation of substrate leads to formation of trimers and higher oligomers. Alternatively, expulsion of any coordinated oligomer from the tungsten center regenerates the reactive bis-olefin complex. Finally, decomposition of the tungsten catalyst species is consistent with a bimetallic pathway. All new organometallic complexes have been characterized by conventional spectroscopic and analytical methods, and the solid-state molecular structures of several compounds have been established by X-ray crystallographic analyses.

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