Hydrogenation of Terminal Alkenes Catalyzed by Air‐Stable Mn(I) Complexes Bearing an N‐Heterocyclic Carbene‐Based PCP Pincer Ligand

Abstract Efficient hydrogenations of terminal alkenes with molecular hydrogen catalyzed by well‐defined bench stable Mn(I) complexes containing an N‐heterocyclic carbene‐based PCP pincer ligand are described. These reactions are environmentally benign and atom economic, implementing an inexpensive, earth abundant non‐precious metal catalyst. A range of aromatic and aliphatic alkenes were efficiently converted into alkanes in good to excellent yields. The hydrogenation proceeds at 100 °C with catalyst loadings of 0.25–0.5 mol %, 2.5–5 mol % base (KOtBu) and a hydrogen pressure of 20 bar. Mechanistic insight into the catalytic reaction is provided by means of DFT calculations.

5 was synthesized using a modified procedure from literature. [1]raformaldehyde (241 mg, 8 mmol, 1 equiv.)and diisopropylphosphine (948 mg, 8 mmol, 1 equiv.)were put in a Schlenk flask that was flushed with argon beforehand.The reaction mixture was heated to 60°C and stirred for 18 hours.The resulting oil and was then left to cool to room temperature, after which dry dichloromethane (20 mL) was added.o-Phenylenediamine (389 mg, 3.6 mmol, 0.45 equiv.) was added, and the solution was stirred for 48 hours at ambient temperature.Black selenium (695 mg, 8.8 mmol, 1.1 equiv.) was slowly added and the mixture was stirred for 1 hour.The suspension was then filtered off over a patch of celite.The residue was washed vigorously with dichloromethane after which the organic phase was taken to dryness.The product was purified via silica gel chromatography (eluent: dichloromethane), yielding 1301 mg (62%) of 5 as a yellow solid.

Poisoning with Triethylphosphine
Inside an argon flushed glovebox, a screw cap vial (8 mL) was charged with 4-chlorostyrene (26 µL, 216 µmol, 1 equiv.),KO t Bu (2.5 mg, 22 µmol, 10 mol%), 2 (2.5 mg, 4.3 µmol, 2.0 mol%), triethylphosphine (32 µL, 216 µmol, 1 equiv.)and dry toluene (1 mL) and closed under argon atmosphere.The vial was transferred out of the glovebox and was flushed multiple times with hydrogen gas in an autoclave.It was subsequently put under hydrogen gas at 20 bar while stirring for 18 hours at 100°C.Afterwards the reaction mixture was allowed to reach room temperature, depressurized and exposed to air to quench the catalyst.The sample was analyzed via GC-MS.
A conversion of only 9% could be observed.

Poisoning with Mercury
Inside an argon flushed glovebox, a screw cap vial (8 mL) was charged with 4-chlorostyrene (26 µL, 216 µmol, 1 equiv.),KO t Bu (2.5 mg, 22 µmol, 10 mol%), 2 (2.5 mg, 4.3 µmol, 2.0 mol%), a drop of mercury and dry toluene (1 mL) and closed under argon atmosphere.The vial was transferred out of the glovebox and was flushed multiple times with hydrogen gas in an autoclave.It was subsequently put under hydrogen gas at 20 bar while stirring for 18 hours at 100°C.Afterwards the reaction mixture was allowed to reach room temperature, depressurized and exposed to air to quench the catalyst.The sample was analyzed via GC-MS.Full conversion was observed.

Control Experiment without Catalyst
Inside an argon flushed glovebox, a screw cap vial (8 mL) was charged with 4-chlorostyrene (26 µL, 216 µmol, 1 equiv.),KO t Bu (2.5 mg, 22 µmol, 10 mol%) and dry toluene (1 mL) and closed under argon atmosphere.The vial was transferred out of the glovebox and was flushed multiple times with hydrogen gas in an autoclave.It was subsequently put under hydrogen gas at 20 bar while stirring for 18 hours at 100°C.Afterwards the reaction mixture was allowed to reach room temperature, depressurized and exposed to air.The sample was analyzed via GC-MS.No conversion was observed.

Control Experiment with Hydride
Inside an argon flushed glovebox, a screw cap vial (8 mL) was charged with 4-chlorostyrene (26 µL, 216 µmol, 1 equiv.), 4 (1.1 mg, 2.2 µmol, 1.0 mol%), KO t Bu (0.5 mg, 4.4 µmol, 2.0 mol%) and dry toluene (1 mL) and closed under argon atmosphere.The vial was transferred out of the glovebox and was flushed multiple times with hydrogen gas in an autoclave.It was subsequently put under hydrogen gas at 50 bar while stirring for 18 hours at 100°C.Afterwards the reaction mixture was allowed to reach room temperature, depressurized and exposed to air.The sample was analyzed via GC-MS.Full conversion was observed.

Control Experiment with Hydride without Base
Inside an argon flushed glovebox, a screw cap vial (8 mL) was charged with 4-chlorostyrene (26 µL, 216 µmol, 1 equiv.), 4 (2.1 mg, 4.3 µmol, 2.0 mol%) and dry toluene (1 mL) and closed under argon atmosphere.The vial was transferred out of the glovebox and was flushed multiple times with hydrogen gas in an autoclave.It was subsequently put under hydrogen gas at 20 bar while stirring for 18 hours at 100°C.Afterwards the reaction mixture was allowed to reach room temperature, depressurized and exposed to air.The sample was analyzed via GC-MS.No conversion was observed.This data corresponds with previously reported data. [3]Ethylanisole (3d)

Figure S2 .
Figure S2.Free-energy Profile for the Deprotonation of C and Addition of Styrene.Free energies (kcal/mol) are referred to A. The C-atom marked with a circle is formally negatively charged.