Selective Hydrogenation and Hydrodeoxygenation of Aromatic Ketones to Cyclohexane Derivatives Using a Rh@SILP Catalyst

Abstract Rhodium nanoparticles immobilized on an acid‐free triphenylphosphonium‐based supported ionic liquid phase (Rh@SILP(Ph3‐P‐NTf2)) enabled the selective hydrogenation and hydrodeoxygenation of aromatic ketones. The flexible molecular approach used to assemble the individual catalyst components (SiO2, ionic liquid, nanoparticles) led to outstanding catalytic properties. In particular, intimate contact between the nanoparticles and the phosphonium ionic liquid is required for the deoxygenation reactivity. The Rh@SILP(Ph3‐P‐NTf2) catalyst was active for the hydrodeoxygenation of benzylic ketones under mild conditions, and the product distribution for non‐benzylic ketones was controlled with high selectivity between the hydrogenated (alcohol) and hydrodeoxygenated (alkane) products by adjusting the reaction temperature. The versatile Rh@SILP(Ph3‐P‐NTf2) catalyst opens the way to the production of a wide range of high‐value cyclohexane derivatives by the hydrogenation and/or hydrodeoxygenation of Friedel–Crafts acylation products and lignin‐derived aromatic ketones.


Synthesis of trioctyl(3-(triethoxysilyl)propyl)phosphonium NTf2
For the subsequent anion exchange no Schlenk techniques were used. Trioctyl (3-(triethoxysilyl)propyl)phosphonium iodide (7.0 g, 10 mmol, 1eq) was dissolved in 15 mL DCM. In a separate flask, LiNTf2 (2.87 g, 10 mmol, 1eq) was dissolved in 2.5 mL water. The solutions were mixed and vigorously stirred for 30 min at room temperature. The organic phase was washed three times with water, dried over MgSO4 and the solvent was removed under reduced pressure. The resulting pale viscous liquid was dried over night at 60 °C in vacuo. Yield = 85%. (Abbreviation: Oct3-P-NTf2)

SILP synthesis:
General procedure for the synthesis of SILP A solution of the ionic liquid (9.0 mmol) in 20 mL anhydrous DCM was added to a suspension of 10.3 g dehydroxylated silica (500 °C, high vacuum, 16 h) in 90 mL anhydrous toluene. The reaction mixture was refluxed at 130 °C for 2-5 days under inert atmosphere, before the solvent was carefully removed by decantation. The SILP was washed 4 times with dry DCM and dried in vacuo at 60 °C for 16 h. The solvent of the combined organic phases was removed under reduced pressure to determine the amount of not grafted IL. (Total IL grafted = starting amount of ILrecovered IL). [2] NPs@SILP synthesis:

Synthesis of Rh@SILP
A solution of [Rh(allyl)3] (11.3 mg, 0.05 mmol) in 2 mL dry DCM was added to a suspension of 500 mg support (SILP or SiO2) in 5 mL dry DCM. The reaction mixture was stirred for 1 h at room temperature and the support changed its color to yellow/brown. The solvent is carefully removed under reduced pressure and the impregnated support material was transferred to an autoclave within the glovebox. The metal precursor was reduced under the following conditions: 100 °C, 100 bar H2 (at r.t), 2h. A grey/black powder was obtained and stored under air.

Synthesis of Ru@SILP
A solution of [Ru(cod)(cot)] (15.7 mg, 0.05 mmol) in 2 mL dry DCM was added to a suspension of 500 mg SILP in 5 mL dry DCM. The reaction mixture was stirred for 1 h at room temperature and the SILP changed its color grey. The solvent is carefully removed under reduced pressure and the impregnated SILP was transferred to an autoclave within the glovebox. The metal precursor was reduced under the following conditions: 150 °C, 20 bar H2 (at r.t), 1h. A grey/black powder was obtained and stored under inert atmosphere.

Catalytic reactions
All high pressure reactions were carried out using in-house manufactured 10 or 20 mL stainless steel finger autoclaves. Catalytic reactions were performed in glass inlets.
The substrate (0.1 mmol, 50 eq.), the catalyst (20 mg, 0.002 mmol metal) and n-heptane (375 mg) were mixed in a glass inlet and placed in a high pressure autoclave. The reactor was purged (5 times) and pressurized with H2, before it was brought to reaction temperature using a preheated aluminum cone. Mixing was guaranteed by using a magnetic stirrer bar at 500 rpm. The reactions were stopped by quickly cooling the autoclave in a water-bath to room temperature and careful depressurization. The reaction mixture was filtered using a syringe filter and the solution was analyzed by GC-FID using tetradecane as an internal standard.
For the recycling experiments, the catalyst was separated via centrifugation from the solution. The solution was submit to GC analysis and the catalyst was washed with n-heptane (1mL) before a fresh substrate solution was added and the reaction restarted under the same conditions.

Analytics
All solution state NMR were recorded on a Bruker Ascend 400 spectrometer at room temperature. The coupling constants (J) are given in Hertz (Hz), and the chemical shifts (δ) are expressed in ppm, relative to TMS at 25 °C. Gas chromatography (GC) was performed on a Shimadzu GC-2030 equipped with a FID-detector and a CP-WAX-52CB column from Agilent. Gas chromatography coupled with a mass spectrometer (GC-MS) were performed on a Shimadzu QP2020. All TEM images were recorded on a Hitachi HF2000 operating at 200 kV. Elemental Analysis was measured externally in "Mikroanalytisches Laboratorium Kolbe, Oberhausen Germany". BET measurements were performed on a Quadrasorb SI from Quantachrom Instruments. FTIR spectra of SILPs and dehydroxylated SiO2 were obtained using a Bruker Alpha spectrometer in the DRIFT mode and 29 Si solid-state NMR spectra were obtained using a Bruker AVIII-500 spectrometer.