Light‐Driven gem Hydrogenation: An Orthogonal Entry into “Second‐Generation” Ruthenium Carbene Catalysts for Olefin Metathesis

Abstract The newly discovered light‐driven gem hydrogenation of alkynes opens an unconventional yet efficient entry into five‐coordinate Grubbs‐type ruthenium carbene complexes with cis‐disposed chloride ligands. Representatives of this class featuring a chelate substructure formed by an iodo‐substituted benzylidene unit react with (substituted) 2‐isopropoxystyrene to give prototypical “second‐generation” Grubbs‐Hoveyda complexes for olefin metathesis. The new approach to this venerable catalyst family is safe and versatile as it uses a triple bond rather than phenyldiazomethane as the ultimate carbene source and does not require any sacrificial phosphines.

The structure was solved by SHELXT and refined by full-matrix least-squares (SHELXL) against

S4
Photolysis experiments were performed in the PhotoRedOxBox TC from HepatoChem ( Figure S3) equipped with an EvoluChem™ LED (365 nm, 18 W). When the cooling function was used, water was used as the cooling agent (23°C).
Most reactions were carried out in quartz Schlenk tubes, but control experiments showed that standard borosilicate glassware is equally suitable and can be used without any noticeable effect on yield and purity.

STARTING MATERIALS
The compounds 2b, 2 2e, 3 2g, 4 S1, 5 S2, 6 S3, 7 S4, 8 S5, 9 S6, 10 S7, 11 1b, 12 1a, 12 1e, 13 and 1h 14 were prepared according to literature procedures. iodobenzaldehyde (1.00 g, 4.31 mmol) in THF (10 mL) was introduced and stirring continued for 30 min before diethyl chlorophosphate (747 μL, 5.17 mmol) was added dropwise and the mixture was warmed to room temperature. After stirring for 3 h, the mixture was cooled to -78°C and KO t Bu (2.42 g, 21.6 mmol) was added in one portion. After stirring for 15 min, the mixture was warmed to room temperature and stirring was continued overnight. Sat. aq. NH4Cl (15 mL) was added, the organic layer was separated and the aqueous phase was extracted with EtOAc S6 (3 × 100 mL). The combined organic layers were dried over Na2SO4 and the solvent was removed under reduced pressure. The residue was purified by flash chromatography (silica, pentane) to provide the title compound as a colorless oil (914 mg, 88%  was added dropwise to a solution of ethyl phenyl sulfone (650 mg, 3.82 mmol) in THF (40 mL) at -78°C. After stirring for 30 min, a solution of quinoline-8carbaldehyde (500 mg, 3.18 mmol) in THF (10 mL) was added. After stirring for another 30 min, diethyl chlorophosphate (552 μL, 3.82 mmol) was added dropwise and the mixture was warmed to room temperature. After stirring for 3 h, the mixture was cooled to -78°C and KO t Bu (1.78 g, 15.9 mmol) was added in one portion. The mixture was stirred for 15 min at this temperature and at room temperature overnight. Sat. aq. NH4Cl (10 mL) was added, the organic layer was separated and the aqueous layer was extracted with EtOAc (3 × 50 mL). The combined organic phases were dried over Na2SO4 and the solvent was removed under reduced pressure. The residue was purified by flash chromatography (silica, pentane/tert-butyl methyl ether, 1:1 → 1:2) to provide the title compound as an orange oil (310 mg, 58%). 1 H NMR (400 MHz, CDCl3) δ 9.04 (dd, J = 4.2, 1. 8

RUTHENIUM η 6 -CYMENE PRECATALYSTS
Representative Procedure. Preparation of the Ruthenium Complex 1d. KHMDS (568 mg, 2.85 mmol) was added in one portion to a suspension of imidazolium salt S4 (1.28 g, 2.19 mmol) in Et2O (35 mL) at room temperature. The mixture was vigorously stirred for 10 min before it was filtered through a pad of Celite and the filter cake was carefully rinsed with Et2O (20 mL). The combined filtrates were evaporated in high vacuum to provide the corresponding N-heterocyclic carbene (NHC) as a pale yellow solid (1.05 g, 88%) which was directly used in the next step.
After stirring for 1 h in the dark, the solvent was removed in high vacuum. The red-orange residue was dissolved in Et2O  13   Hz, 3H). 13   Hz, 3H). 13  Ruthenium Complex 5f. Prepared analogously from the ruthenium cymene complex 1e (101 mg, 165 μmol) and alkyne 2b (61.6 mg, 247 μmol). The crude material residue was triturated with pentane and the supernatant was filtered off. The residual green solid was washed with pentane (2 × 20 mL) before it was re-dissolved in CH2Cl2

S14
Table S1 NMR analysis of the cis-configured Ru carbene cis-5g; this dataset was recorded on a 600 MHz spectrometer in CD2Cl2.

gem-HYDROGENATION UNDER H2/D2-ATMOSPHERE
A 1:1 mixture of H2 and D2 was prepared by using a gas-tight 10 mL Hamilton syringe: a gas volume of 5 mL was taken out of a balloon filled with H2 and of a second balloon filled with D2.
Alkyne 2b (40.3 mg, 166 μmol) was added to a solution of the ruthenium cymene complex 1b (78.9 mg, 114 μmol) in CD2Cl2 (2 mL) in a flame-dried quartz Schlenk tube under Ar and the tube was closed with a rubber septum. The total volume of 10 mL H2/D2 was then flushed through the quartz Schlenk tube using an outlet cannula; this procedure was repeated nine times.
The mixture was then stirred under this H2/D2-atmosphere at room temperature overnight with constant irradiation at λ = 365 nm. The mixture was loaded on a silica pad, eluting with CH2Cl2/ Et2O (1:1, 30 mL). A brown-yellow band was collected and a dark residue stayed on top of the silica pad. The product-containing fractions were concentrated and the dark brown residue was dissolved in CH2Cl2 (2 mL). A brown solid was precipitated upon addition of pentane (30 mL) at room temperature and the supernatant was filtered off. Drying of the residue in high vacuum S20 afforded a beige-brown solid (56.6 mg, 62%

S21
A 1:1 mixture of H2 and D2 was prepared by using a gas-tight 10 mL Hamilton syringe: a gas volume of 5 mL was taken out of a balloon filled with H2 and of a second balloon filled with D2.
Alkyne 2b (74.2 mg, 307 μmol) was added to a solution of the ruthenium cymene complex 1b (142 mg, 204 μmol) in CD2Cl2 (4 mL) in a flame-dried quartz Schlenk tube under Ar and the tube was closed with a rubber septum. The total volume of 10 mL H2/D2 was then flushed through the quartz Schlenk tube using an outlet cannula; this procedure was repeated nine times.
The mixture was then stirred under H2/D2-atmosphere at room temperature for 30 min with constant irradiation at λ = 365 nm. At this point, a gas-tight 1 mL Hamilton syringe was used to take a 1 mL aliquot of the gas in the headspace of the Schlenk tube, which was analyzed by MS (Finnigan MAT SSQ 7000). The sample was directly injected via the GC-inlet of the spectrometer and no column was used. No adequate mass calibration is possible in this mass range: therefore the masses were determined by comparison to an authentic HD sample and helium as references within the measurement. The sample does contain H2, HD and D2, see Figure S5. in a flame-dried Schlenk tube under argon. The mixture was stirred overnight at 80° C (bath temperature). After cooling to room temperature, the mixture was concentrated in high vacuum. The dark brown residue was triturated with pentane at -78° C, the supernatant was filtered off and the residue was washed with pentane (2 × 10 mL). The residue was then purified by flash chromatography (silica, CH2Cl2) to provide the title compound as a yellow-brown solid (625 mg,