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Formation of Ruthenium Carbenes by gem‐Hydrogen Transfer to Internal Alkynes: Implications for Alkyne trans‐Hydrogenation

Abstract

Insights into the mechanism of the unusual trans‐hydrogenation of internal alkynes catalyzed by {Cp*Ru} complexes were gained by para‐hydrogen (p‐H2) induced polarization (PHIP) transfer NMR spectroscopy. It was found that the productive trans‐reduction competes with a pathway in which both H atoms of H2 are delivered to a single alkyne C atom of the substrate while the second alkyne C atom is converted into a metal carbene. This “geminal hydrogenation” mode seems unprecedented; it was independently confirmed by the isolation and structural characterization of a ruthenium carbene complex stabilized by secondary inter‐ligand interactions. A detailed DFT study shows that the trans alkene and the carbene complex originate from a common metallacyclopropene intermediate. Furthermore, the computational analysis and the PHIP NMR data concur in that the metal carbene is the major gateway to olefin isomerization and over‐reduction, which frequently interfere with regular alkyne trans‐hydrogenation.

Let’s stay together: Certain ruthenium complexes are able to catalyze the stereochemically highly unusual trans‐hydrogenation of internal alkynes, but can also effect a geminal hydrogenation, during which both H atoms of a single H2 precursor are delivered to the very same alkyne C atom whilst the neighboring C atom is converted into a metal carbene. The significance of this finding is analyzed by PHIP NMR spectroscopy (PHIP=para‐hydrogen‐induced polarization transfer) and density functional calculations.

Authors:   Markus Leutzsch, Larry M. Wolf, Puneet Gupta, Michael Fuchs, Walter Thiel, Christophe Farès, Alois Fürstner
Journal:   Angewandte Chemie International Edition
Volume:   54
edition:   42
Year:   2015
Pages:   12431
DOI:   10.1002/anie.201506075
Publication date:   31-Aug-2015
Facts, background information, dossiers
  • ruthenium
  • NMR spectroscopy
  • hydrogenation
  • isomerization
  • Formation
  • density
  • analysis
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