Grubbs Metathesis Enabled by a Light‐Driven gem‐Hydrogenation of Internal Alkynes

Abstract [(NHC)(cymene)RuCl2] (NHC=N‐heterocyclic carbene) complexes instigate a light‐driven gem‐hydrogenation of internal alkynes with concomitant formation of discrete Grubbs‐type ruthenium carbene species. This unorthodox reactivity mode is harnessed in the form of a “hydrogenative metathesis” reaction, which converts an enyne substrate into a cyclic alkene. The intervention of ruthenium carbenes formed in the actual gem‐hydrogenation step was proven by the isolation and crystallographic characterization of a rather unusual representative of this series carrying an unconfined alkyl group on a disubstituted carbene center.


General Remarks
All reactions were carried out in flame-dried glassware under argon, ensuring rigorously inert conditions. The solvents were purified by distillation over the indicated drying agents and were stored and handled under argon: CH2Cl2 (CaH2), MeCN (CaH2), pentane (Na/K alloy), THF (Na/K alloy).
NMR spectra were recorded on Bruker AV400 or AV500 spectrometers at 298 K unless otherwise indicated, with the chemical shift (δ) given in ppm relative to TMS and the coupling constants (J) in Hz. The solvent signals were used as references 1  Unless stated otherwise, all commercially available compounds (abcr, Acros, TCI, Aldrich, Alfa Aesar) were used without further purification.
Both hydrogen and deuterium were handled with standard balloon techniques.
The Schlenk tube was flushed with hydrogen for 2 min through an outlet cannula (the cannula did not reach into the solution to ensure that only the head space of the tube was flushed). After the first 10 seconds of flushing with hydrogen, the light source was switched on and the reaction mixture was stirred for 30 min under hydrogen atmosphere. After cooling to room temperature, all volatile components were removed under high vacuum. The remaining crude material was suspended in pentane (10 mL) and the suspension was vigorously stirred for

Kinetic Profiling of Complex 20
In order to characterize complex 20 further, we made use of a standard method to assess the catalytic performance of olefin metathesis catalyst developed by Grubbs et al. 3 To this end, the ring closing metathesis of diallylmalonate (22) was carried out with complex 20 and the commercially available Grubbs II and Hoveyda-Grubbs II catalysts for comparison.
In an argon-filled glovebox, an NMR tube with a screw cap septum top was charged with a stock solution of the corresponding complex in CD2Cl2 (1 mol%, 0.5 mL). The sample was brought out of the glovebox and equilibrated at 30°C in the NMR machine before diallylmalonate 22 (    Supporting Information S18 Supporting Information S19 Interestingly, significant amounts of C3-and C5-products are detected as well: their formation is attributed to an isomerization/carbene formation/cross dimerization sequence (see Figure S- 4 Cross-dimerization of these "tertiary" ruthenium carbenes with the "secondary" ruthenium carbene explains the observation of C3-and C5-products.

S21
The model reaction was also run with 5 mol% and 20 mol% catalyst loading. Headspace GC analysis showed that the proportion of C4 and C3 compounds is essentially independent of the loading. However, the ratio between unsaturated and saturated compounds does change in that the proportion of alkanes is significantly increased at higher loading; this finding is again in line with a secondary hydrogenation process.
It is of note that Fogg, Jensen and coworkers have recently studied a pyridine-stabilized ruthenium ethylidene complex, which decomposes via bimolecular coupling to give a mixture of butene,

Supporting Information
S22 pentene and propene in a 2:1:1 ratio. 5 Whilst this observation is qualitatively similar to our observations, we note that pentenes and pentane are underrepresented (relative to propane/propene) in our study. The difference might be explained by the fact that different analytical tools were used (GC/MS versus NMR); since pentenes/pentane are less volatile than propene/propane, they might be underrepresented in the gas phase. Efforts to quantify the pentenes by NMR spectroscopy in the liquid phase were unsuccessful due to severe signal overlap with product 2. Moreover, one might conceive that dimerization of a propylidene complex with an ethylidene complex is (somewhat) slower than the dimerization of a propylidene complex with a methylidene complex.
Hydrogenative metathesis with a 1:1 mixture of enynes 1a 6 and 1b confirms the bimolecular decomposition of the secondary carbene complex. The increased amount of C3-products and concomitant decrease of C4-and C2-products strongly supports the notion that bimolecular alkylidene coupling is operative. The fact that the C2:C3:C4 ratio is not 1:2:1, as statistically S29

VT NMR Studies of [(NHC)(p-cymene)RuCl2] Complexes
A flame-dried pressure NMR tube was charged with a solution of the corresponding complex -benzene (500 µL). The NMR tube was inserted into the NMR probe head and spectra were acquired in a temperature range from 25 °C to 100 °C (10 °C increments).   (1 mL). The Schlenk tube was closed with a septum and then transferred into the photolysis apparatus. A hydrogen-filled balloon was connected to a needle which was pierced through the septum and the Schlenk tube was flushed with hydrogen for 2 min through an outlet cannula (the cannula did not reach into the solution to make sure that only the head space of the tube was flushed). The ethylidene complex 24 (42.0 mg, 20 mol%) in toluene (2 mL) was added via syringe and the resulting mixture was stirred for 60 min under H2 atmosphere. The mixture was diluted with pentane (5 mL) and then filtered through a short pad of silica. The filtrate was evaporated and the residue was analyzed by 1 H NMR spectroscopy, which showed the formation of product 2 in 64% NMR yield; the analytical data of this compound are compiled below.
After stirring for 30 min, the mixture was cooled to -78 °C and a solution of acetophenone (43.4 µL, 372 µmol) in THF (0.5 mL) was added. After stirring at -78 °C for 90 min, the mixture was stirred overnight at ambient Supporting Information S41 temperature. Water (10 mL) was introduced and the mixture was extracted with EtOAc (3 × 50 mL). The combined organic layers were washed with brine and dried over Na2SO4. The solvent was removed under reduced pressure and the residue purified by flash chromatography (silica, pentane/tert-butyl methyl ether 3:1) to provide an inseparable mixture of the desired alcohol and remaining starting material.
A solution of this crude material (42.3 mg, 185 µmol) in THF (0.5 mL) was added dropwise to a suspension of NaH (13.3 mg, 556 µmol) in THF (1 mL) at 0 °C. After warming to ambient temperature, the mixture was stirred for additional 15 min, before it was cooled to 0 °C and methyl iodide (60 µL, 964 µmol) was added. The mixture was stirred overnight at ambient temperature before the reaction was quenched with water (10 mL) and tert-butyl methyl ether (20 mL). The aqueous layer was extracted with tert-butyl methyl ether (3 × 20 mL), the combined organic phases were washed with brine and dried over Na2SO4, the solvent was removed under reduced pressure and the residue was purified by flash chromatography (silica, pentane/tert-butyl methyl

Synthesis of the Precatalysts
Since preliminary results showed that the purity of the catalyst has a significant impact on the outcome of the catalytic transformation, a modified and optimized synthesis procedure for  During this study, the authors used several batches of 3c and did not notice any significant decomposition or change in the catalytic performance when the material was stored in the dark at 20 °C.
Solutions of such complexes should therefore be handled in the dark; the light of the fume-hood should be switched off during any manipulations.

Hydrogenative Metathesis Reactions General Procedure: Intramolecular Hydrogenative Enyne Metathesis
A flame-dried quartz Schlenk tube was charged with [(IPr)(p-cymene)RuCl2] (13.8 mg, 0.02 mmol, 10 mol%), the substrate (0.2 mmol) and toluene (2 mL, 0.1 M). The Schlenk tube was closed with a septum and then transferred into the photolysis apparatus. A hydrogen-filled balloon was connected to a needle which was pierced through the septum and the Schlenk tube was flushed with hydrogen for 2 min through an outlet cannula (the cannula did not reach into the solution to make sure that only the head space of the tube was flushed). After the first 10 seconds of flushing with hydrogen, the light source was switched on and the reaction mixture was stirred for 60 min under hydrogen atmosphere. The reaction mixture was diluted with pentane (5 mL) and then filtered through a short pad of silica. The filtrate was evaporated and the residue purified by flash chromatography.
When the same reaction was performed in ordinary laboratory glassware (instead of the quartz Schlenk-tube), a yield of 75% was obtained.
The analytical data of 2 matched those previously reported in the literature. [21] The following compounds were prepared analogously: