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.

H 2 activation and C-H bond formation (via TS R1-R3 /TS R1'-R3' ), which is different from the stepwise process in the neutral case ( Figure S4). C2 is more easily hydrogenated than in the neutral case by 6.0 kcal mol -1 . Other than this, we find no major differences.    Table S1. Listed are the SCF energy, zero point vibrational energy (ZPVE), enthalpy correction (H corr ), and Gibbs free energy correction (G corr ) determined on the gas-phase geometries for all stationary points calculated using the neutral Ru catalyst with the 2-butyne substrate. The single imaginary frequency ( i cm -1 ) is also listed for all transition states. Single point solvent (DCM) corrected SCF energies on the gas phase geometries are also documented. All energies are in atomic units. SCF  8. Comparison of the computed gas-phase structure and the X-ray structure of 9b Table S4. Selected bond distances (Å) and angles (°) of structure 9b (X-ray) / C2 (DFT). M06/def2-TZVP (ultrafine grid) was used for geometry optimization. The DFT computed geometry is in close agreement with the X-ray derived structure. Hydrogen atoms are removed for clarity.

Coordinates
All graphics included in this section were generated using the CYLview program. 6 XYZ coordinates (Å) for the molecules present in the hydrogenation of 2butyne using the neutral catalyst A0  CCDC 1406683 contains the supporting crystallographic data for this paper. These data can be The acquired 1 H NMR spectra were referenced to the residual solvent signal (δ CHDCl2 = 5.32 ppm) [2] .
The 13 C NMR spectra were referenced with the Ξ-scale. [3,4] For the OPSY-EXSY spectrum, a mixing time of 300 ms was used.
NMR data was processed with Bruker's Topspin 3.2. For the simulation of the NMR spectra, the DAISY module of Topspin was used. The NMR assignment of the carbenes 9 was performed with MestreNova 9.1.
para-Hydrogen Generation. The p-H 2 enrichment above the thermal equilibrium of 25% was achieved in two different ways.
Initially, the p-H 2 was enriched to 50% using the "U-shaped tube" method ( Figure S-3). [5] The tube was filled with a mixture (3:1) of activated charcoal (Norit PK1-3, Sigma Aldrich) and iron(III) oxide (99%, meshed powder, Alfa Aesar). The filled tube was evacuated and heated with a heat gun (150 °C) to remove any residual water and oxygen from the catalyst. This tube was used several times before the catalyst had to be reactivated. To enrich the p-H 2 , the tube was loaded with 20 bar of hydrogen gas (99.995%, dry) and placed in a Dewar flask filled with liquid nitrogen (77 K). After an equilibration time of 1h, the enriched hydrogen gas was transferred to an evacuated storage bottle or directly transferred to the NMR tube.

S7
Sign of the J HH Couplings. The shape of the PASADENA antiphase signal gives information about the sign of the coupling constant. In the case of a positive coupling the first signal of the doublet is positive, whereas the second one is negative. This can be seen for the vicinal coupling of the olefinic protons of 5a (Fehler! Verweisquelle konnte nicht gefunden werden., left). If the coupling is negative, the sign of the antiphase signals is inverted. This can be nicely seen in the case the hyperpolarized protons of 7a and 6a proving that the coupling is indeed negative ( Figure S1, middle & left). Negative couplings are normally observed for geminal proton-proton couplings.

S9
The 1 H-OPSY-COSY spectrum contains various structure informations about the carbene intermediates 6a and 7b. On the one hand it clearly shows the cross peaks between the geminal protons (H2αH2β, H1αH1β). On the other hand asymmetrical cross peaks to the CH 3 -groups (H3) can be seen. The asymmetrical cross peaks are explained by the different polarization of the methyl and the methylene protons. The hyperpolarized geminal protons generate this cross peak (H1/2H3), whereas the non-hyperpolarized methyl group may generate a cross peak (H3H2/1), but the intensity is lower than the noise level and so not visible. Exchange Spectroscopy. The analysis of the OPSY-EXSY spectrum of the reaction 4b to 5b finds exchange correlations from the hyperpolarized hydrogens of the carbene 6c to a number of products and by-products, namely 5b, 10, 11 and free H 2 , as shown in Figure 4 of the main text of the publication. These results are in excellent accord with the pathways 1 and 2 as proposed in Scheme 3. These observed exchange correlations are depicted by red arrows in Scheme S1. Interestingly, pathway 1 correctly predicts that only one of the olefinic hydrogens originates from the carbene 6c.