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Metal Alkyls With Alkylidynic Metal-Carbon Bond Character: Key Electronic Structures In Alkane Metathesis Precatalysts.

C. Copéret, C. P. Gordon
Published 2020 · Chemistry, Medicine

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The homologation of alkanes via alkane metathesis is catalyzed at low temperatures (150 °C) by the silica-supported species  (SiO)WMe 5 and  (SiO)TaMe 4 , while (SiO)TaMe 3 Cp* is inactive. The contrasting reactivity is paralleled by differences in  13 C NMR signature; the former display significantly more deshielded isotropic chemical shifts (δ iso ) and almost axially symmetric chemical shift tensors, similar to what is observed in their molecular precursors TaMe 5 and WM 6 . Analysis of the chemical shift tensors reveals the presence of a triple-bond character in their metal-carbon (formally single) bond. This electronic structure is reflected in their propensity to generate alkylidynes and to participate in alkane metathesis, further supporting that alkylidynes are key reaction intermediates in that reaction. This study establishes chemical shift as a descriptor to identify potential alkane metathesis catalysts.
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