Formation of ruthenium carbenes by gem-hydrogen transfer to internal alkynes: implications for alkyne trans-hydrogenation.
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.
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
Markus
M
Leutzsch
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
Larry M
LM
Wolf
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
Puneet
P
Gupta
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
Michael
M
Fuchs
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
Walter
W
Thiel
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany).
Christophe
C
Farès
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim/Ruhr (Germany). fuerstner@kofo.mpg.de.
Alois
A
Fürstner
NMR spectroscopy; alkynes; carbenes; density functional calculations; hydrogenation; ruthenium