Catalytic Hydrogenolysis of Substituted Diaryl Ethers by Using Ruthenium Nanoparticles on an Acidic Supported Ionic Liquid Phase (Ru@SILP-SO₃H)

Published as part of the 30 Years SYNLETT – Pearl Anniversary Issue

Abstract

Catalytic hydrogenolysis of diaryl ethers is achieved by using ruthenium nanoparticles immobilized on an acidic supported ionic liquid phase (Ru@SILP-SO₃H) as a multifunctional catalyst. The catalyst components are assembled through a molecular approach ensuring synergistic action of the metal and acid functions. The resulting catalyst is highly active for the hydrogenolysis of various diaryl ethers. For symmetric substrates such as diphenyl ether, hydrogenolysis is followed by full hydrodeoxygenation producing the corresponding cycloalkanes as the main products. For unsymmetric substrates, the cleavage of the C–O bond is regioselective and occurs adjacent to the unsubstituted phenyl ring. As hydrogenation of benzene is faster than hydrodeoxygenation over the Ru@SILP-SO₃H catalyst, controlled mixtures of cyclohexane and substituted phenols are accessible with good selectivity. Application of Ru@SILP-SO₃H catalyst in continuous-flow hydrogenolysis of 2-methoxy-4-methylphenoxybenzene is demonstrated with use of commercial equipment.

Key words

ruthenium nanoparticles - acidic SILP - multifunctional catalysis - diaryl ether cleavage - hydrogenolysis - deoxygenation - lignin

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• 13 Experimental Procedures Safety warning: High-pressure experiments with compressed H₂ must be carried out only with appropriate equipment and under rigorous safety precautions.General: If not otherwise stated, the syntheses of the ionic liquids (ILs), the supported ionic liquid phases (SILPs), and the nanoparticles immobilized on SILPs (Ru@SILP and Ru@SILP-SO₃H) were carried out under an inert atmosphere by using standard Schlenk techniques or inside a glovebox. After synthesis, ILs, SILPs, Ru@SILP, and Ru@SILP-SO₃H were stored under an inert atmosphere. If not otherwise stated solvents were used after distillation without any further purification. For reactions under an inert atmosphere, solvents were additionally dried with molecular sieves (4 Å) and degassed by flushing solvents with argon. For catalysis decalin (from ACROS, 99% anhydrous) was used without purification. The precursor [Ru(2-methylallyl)₂(cod)] was commercially available from Umicore. All other chemicals and solvents were purchased from commercial suppliers and used without purification.Synthesis of catalysts: The Ru@SILP, Ru@SILP-SO₃H, and Ru@IL-SO₃H were synthesized as previously reported and Ru@SiO₂ was synthesized accordingly.^12a Synthesis of substrates: The synthesis of diarylethers was carried out as previously reported.^1cA round-bottom Schlenk-flask (250 mL) was equipped with a magnetic stir bar, copper iodide (456 mg, 2.40 mmol, 10.0 mol%), picolinic acid (590 mg, 4.80 mmol, 20.0 mol%), and potassium phosphate (10.2 g, 48.0 mmol, 2.00 equiv). A second Schlenk flask (100 mL) was charged with the aryl iodide (24.0 mmol, 1 equiv), the phenol (28.8 mmol, 1.2 equiv), and anhydrous DMSO (50 mL). The solution of flask 2 was transferred into flask 1 under flowing Argon and the reaction mixture was stirred for 20 h at 100 °C, subsequently. After cooling down, the reaction mixture was diluted with a 1:1 mixture of a saturated aqueous solution of NH₄Cl (200 mL) and H₂O (200 mL). After extraction with CH₂Cl₂ (3 × 200 mL) the combined organic phases were washed with a 5% aqueous solution of KOH (300 mL) and brine (300 mL). After drying the mixture with Na₂SO₄ the solvent was removed and the crude reaction mixture was preadsorbed on silica gel. The purification of the crude mixture was achieved with use of silica gel. Remaining iodine compounds caused the deactivation of the catalyst in subsequent catalytic reactions and therefore had to be separated carefully. This afforded in some cases purification by two subsequent chromatography columns. Full characterization of all substrates can be found in the Supporting Information.2,6-Dimethoxy-4-methylphenoxybenzene: Prepared according to the general procedure by using iodobenzene (5.00 g, 24.0 mmol) and 2,6-dimethoxy-4-methylphenol (4.8 g, 28.8 mmol). The crude product was purified by flash column chromatography (eluent: ethyl acetate/n-pentane 1:8). 2,6-Dimethoxy-4-methylphenoxybenzene was obtained as a slightly off-white solid (4.46 g, 18.3 mmol) in 76% yield. ¹H NMR (500 MHz, CDCl₃): δ = 7.29–7.26 (m, 2 H), 7.02–6.98 (m, 1 H), 6.93–6.90 (m, 2 H), 6.51 (s, 2 H), 3.79 (s, 6 H), 2.42 (s, 3 H) ppm. ¹³C NMR (125.7 MHz, CDCl₃): δ = 158.77 (C), 153.23 (C), 135.65 (C), 129.84 (C), 129.39 (CH), 121.52 (CH), 114.87 (CH), 106.28 (CH), 56.33 (CH₃), 22.24 (CH₃) ppm. HRMS (EI): m/z 244.11.Batch catalysis: In a typical experiment, Ru@SILP (37.5 mg, 0.0012 mmol Ru), substrate (0.6 mmol, 500 equiv), and decalin (0.5 mL) were combined in a glass insert and placed in a high-pressure autoclave. After purging the autoclave with H₂, the reaction mixture was stirred at 170 °C in an aluminium heating cone under 120 bar H₂ (pressurized at 100 bar H₂ at rt). After the reaction, the autoclave was cooled in an ice bath, carefully vented, and the reaction mixture filtered before GC analysis with use of hexadecane as an internal standard. In some cases a gap in mass balance was observed because of the loss of cyclohexane in the headspace of the autoclave (scale-up decreased this error).Isolated yield (5a): The catalysis was scaled up by a factor of 5. After reaction the decalin phase was isolated and the catalyst washed with decalin (1 × 5 mL). The combined organic phases were extracted with aqueous KOH (3 × 10 mL, 0.3 M). The KOH phase was washed with n-pentane (3 × 10 mL) and neutralized with aqueous HCl, subsequently. After neutralization, the aqueous phase was extracted with CH₂Cl₂ (4 × 20 mL). After drying the CH₂Cl₂ phase with MgSO₄ and removal of the solvent a yellow-brownish oil (186 mg, 1.35 mmol) was obtained in 45% yield (See Supporting Information, Figure S11 for characterization).Continuous flow catalysis: A 70 mm CatCart^® (V_{packed reactor} = 0.5126 mL) was filled with Ru@SILP-SO₃H (439 mg, 0,014 mmol Ru) and installed into the H-Cube Pro^TM. Prior to catalysis the catalyst was flushed with decalin (1 mL min^–1 for 30 min). Then, the reactor was pressurized to 40 bar H₂ (90 NmL min^–1) and heated to 170 °C. When reaching stable reaction conditions, the substrate solution (0.025 M in decalin) was introduced into the system (0.3 mL min^–1). The reaction was allowed to equilibrate for 30 min before the first samples were collected. The reaction mixture was analysed by GC analysis with use of hexadecane as an internal standard.
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