Palladium‐Catalyzed Hydrolytic Cleavage of Aromatic C−O Bonds
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Angewandte Communications Chemie International Edition:DOI:10.1002/anie.201611076 Heterogeneous Catalysis Hot Paper German Edition:DOI:10.1002/ange.201611076 Palladium-Catalyzed Hydrolytic Cleavage of Aromatic C OBonds À Meng Wang,Hui Shi, Donald M. Camaioni, and Johannes A. Lercher* Abstract: Metallic palladium surfaces are highly selective in observed to promote hydrolysis in the presence of H2, promoting the reductive hydrolysis of aromatic ethers in although to alesser extent than hydrogenolysis.[10a] In aqueous phase at relatively mild temperatures and pressures previous work, our group hypothesized that hydrolysis of H2.Atquantitative conversions,the selectivity to hydrolysis occurs along the same reaction path as hydrogenolysis,with products of PhOR ethers was observed to range from 50% cleavage of the ether bond by Ni and subsequent addition of [10a] (R = Ph) to greater than 90%(R= n-C4H9,cyclohexyl, and HC and OHC (from water dissociation). PhCH2CH2). By analysis of the evolution of products with and Herein, we report that supported Pd catalysts are active 18 without incorporation of H2 O, the pathway was concluded to and highly selective towards ether hydrolysis (> 80%at be initiated by palladium metal catalyzedpartial hydrogena- complete conversion) at relatively mild temperatures (ca. tion of the phenyl group to an enol ether.Water then rapidly 2008C) in aqueous phase with pressurized H2 (typically adds to the enol ether to form ahemiacetal, which then 40 bar). As opposed to conventional acid-catalyzed hydrolysis undergoes elimination to cyclohexanone and phenol/alkanol (ArOR + H O ArOH + ROH), we show that hydrolytic 2 ! products.Aremarkable feature of the reaction is that the aryl ether cleavage on Pd occurs by ahitherto unconsidered stronger Ph Obond is cleaved rather than the weaker aliphatic mechanism requiring H addition followed by water attack. À 2 O Rbond. Hereafter, we define this pathway as “reductive hydrolysis” À (C H OR + nH + H O C H O + ROH). Prior work 6 5 2 2 ! 6 (6+2n) atalytic cleavage of C Obonds in aromatic ethers is an with Pd catalysts has not identified this reaction owing to C À important step for the conversion of oxygen-rich lignocellu- the use of non-aqueous[12] or water–alcohol[13] solvents (the losic plant biomass to deoxygenated fuels and commercial alcohol serving as hydrogen donor instead of H2), which chemicals[1] and is challenging because of the strength and disfavor reductive hydrolysis,making the hydrogenation and/ stability of these linkages.[2] Cleavage of C Obonds can occur or hydrolysis steps uncompetitive with hydrogenolysis. À through oxidation,[3] transfer hydrogenation,[4] hydrogenoly- Diphenyl ether was first tested as the simplest diaryl ether sis,[5] hydrolysis/solvolysis,[6] and radical-mediated[7] pathways, model, which also contains one of the strongest structural among others.[1c,8] Hydrogenolytic cleavage of strong aryl links in lignin, the 4-O-5 type linkage (bond dissociation 1 [1a, 14] C Obonds over heterogeneous metal catalysts requires high energy:314 kJmolÀ ). Thepathways for C Obond À À temperatures and H2 pressures and occurs along with arene cleavage of diphenyl ethers have been broadly classified reduction.[1a,2] In arecent breakthrough, Hartwig and Sergeev into hydrogenation, hydrogenolysis,and hydrolysis (reductive used homogeneous nickel complexes[5a] in the presence of or non-reductive). It should be noted that not all hydro- NaOtBu base to catalyze the selective cleavage of aryl C O genation events are counted towards the “hydrogenation” À bonds under relatively mild conditions in m-xylene as the category.Here,“hydrogenation” is limited to reactions that solvent without hydrogenating the arene rings and cleaving saturate the aromatic rings without changing the molecular aliphatic C Obonds.The reaction could also be accom- backbone (i.e., cyclohexyl phenyl ether and dicyclohexyl À plished using Ni nanoparticles.[9] Supported Ni or NiM (M = ether). Thekinetic primary products from hydrogenolysis are Ru, Rh, Au,orPd) bimetallic catalysts can catalyze this benzene and phenol (1:1) whereas two phenol molecules can cleavage at significantly higher rates in water, but always lead be generated from non-reductive hydrolysis of one ether also to some ring saturation.[5b,10] molecule by conventional hydrolysis[11] or the path proposed Hydrolysis of aromatic C Obonds is known to be for Ni.[10a] À challenging,requiring harsh conditions such as using water First, we evaluated three different supported metal near or above its supercritical point or strong acids/bases at catalysts (Pd/C,Pt/C,Ni/SiO2)inwater at 40 bar H2 high temperatures.[11] Supported Ni catalysts have been (Table 1, entries 1, 5, and 6). Thedominant pathways were reductive hydrolysis for Pd (80–88%) and hydrogenolysis for [*] M. Wang, Dr.H.Shi, Dr.D.M.Camaioni,Prof. Dr.J.A.Lercher Pt (40%) and Ni (60%) at quantitative conversions of Institute for Integrated Catalysis diphenyl ether (selectivities given as %carbon unless noted Pacific Northwest NationalLaboratory otherwise). Thestrong preference for reductive hydrolysis P.O. Box 999, Richland,WA99352 (USA) and the small extent of hydrogenolysis with the Pd catalyst Prof. Dr.J.A.Lercher were confirmed to result from intrinsic characteristics of Pd Department of Chemistry and Catalysis Research Institute by using other supported metallic Pd catalysts (e.g.,Pd/Al O , TU München 2 3 Lichtenbergstrasse 4, 85748 Garching (Germany) entry 2). Theproduct distribution was independent of the E-mail:[email protected] amount of Pd/C over awide range of diphenyl ether/Pd ratios Supportinginformation and the ORCID identification number(s) for (100–44000). Adding phosphoric acid together with Pd/C the author(s) of this article can be found under: only marginally changed the reactivity and selectivity of the http://dx.doi.org/10.1002/anie.201611076. reaction (see the Supporting Information, Figure S1), indicat- 2110 2017 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Angew.Chem.Int. Ed. 2017, 56,2110 –2114 Angewandte Communications Chemie Table 1: Reactions of diphenylether.[a] non-reductive hydrolysis.[11b] As Entry Catalyst Solvent/ t [h] Carbon selectivity[b] [%] phenol was hardly hydrogenated to atmosphere Hydrogenolysis Hydrolysis Hydrogenation cyclohexanone in the presence of diphenyl ether (Figure 1A and 15wt%Pd/C (10.0 mg) H O/H 22 88 10 2 2 Table S1), we dismissed the possi- 25wt%Pd/Al2O3 H2O/H2 12 48016 (10.0 mg) bility that hydrolysis of diphenyl 35wt%Pd/C (10.0 mg) decalin/H2 63 –97 ether first forms phenol, half of 45wt%Pd/C (10.0 mg) H2O/N2 12 000 which is then hydrogenated on the 55wt%Pt/C (10.0 mg) H2O/H2 0.5 40 30 30 Pd surface to cyclohexanone.The 664wt% Ni/SiO2 H2O/H2 360382 yields of these products increased (100 mg) linearly for conversions of up to [a] Reaction conditions: Diphenyl ether (1.70 g), catalyst, solvent (80 mL), H2 (40 bar) or N2 (4 bar), 20%, with constant selectivities of 2008C, stirring at 700 rpm. [b] Calculated at >92%conversion:hydrogenolysis = 2 (cyclohexene+ 50%cyclohexyl phenyl ether, 25% benzene);hydrolysis =(phenol+cyclohexanone + cyclohexanol) hydrogenolysis;hydrogenation = À phenol, and 25%cyclohexanone. (phenyl cyclohexylether +dicyclohexyl ether). Thus the initial selectivities toward reductive hydrolysis and hydroge- ing that acid-catalyzed pathways do not contribute to the nation were nearly 50%and 50%, respectively (Figure S3B). observed reductive hydrolysis of diphenyl ether on metals at As the reaction proceeded, the yield of cyclohexyl phenyl 2008C. ether increased to amaximum of 17%and then decreased to Thetemporal evolution of products during the conversion zero at 100%conversion (Figure 1A). Theselectivities to of diphenyl ether on Pd/C in water was investigated at 1908C cyclohexanone,cyclohexanol, and dicyclohexyl ether (Figures 1Aand S2A). Cyclohexyl phenyl ether,phenol, and increased at the expense of phenol and cyclohexyl phenyl cyclohexanone were the only primary kinetic products. ether. At 900 min, the selectivities to the hydrogenation and Importantly,phenol and cyclohexanone were initially reductive hydrolysis products became 47%cyclohexanone, formed in a1:1 yield ratio (Figure S3A) instead of the two 25%cyclohexanol, 17%phenol, and 8% dicyclohexyl ether. phenol molecules that would be expected from conventional, Theamount of hydrogenolysis products (cyclohexane and benzene) remained low (2–3%) during the entire course of the reaction. Theconversion pathways of cyclohexyl phenyl ether were explored independently (Figure 1B). Themajor products were cyclohexanol, cyclohexanone,and dicyclo- hexyl ether during the entire 540 min reaction, with negligible amounts of phenol, benzene,and cyclohexane.The initial conversion rate of cyclohexyl 1 phenyl ether (TOF = 0.53 sÀ )was lower 1 than that of diphenyl ether (TOF = 1.9 sÀ ). Theselectivities towards reductive hydrol- ysis and hydrogenation were relatively constant at 87%and 13%, respectively. Under the same conditions,however, no reactivity of dicyclohexyl ether was observed. Thepathways for C Obond À cleavage of diphenyl ether are summarized in Figure 1C,accounting for the remark- able increase in reductive hydrolysis with reaction time (Figure S3B). As for diphenyl ether,the primary products from palladium-mediated reductive hydrolysis of cyclohexyl phenyl ether were cyclohex- Figure 1. A,B) Product distributions for the reactions of diphenyl ether (A) and cyclohexyl anone and cyclohexanol (1:1), in contrast phenyl ether (B) over Pd/C as afunction of conversion. Reaction conditions for (A): Diphenyl to cyclohexanol and phenol from conven- 7 ether (1.70 g, 0.010 mol), 0.2 wt%Pd/C (40.0 mg, 2.3 10À mol of Pdsurface,prepared by tional hydrolysis. diluting 5wt% Pd/C with activated carbon), H2O(80 mL), H2 (40 bar), 1908C, stirring at To better understand the high selectiv- 700 rpm, 0–900 min. Reaction conditionsfor (B): Cyclohexylphenyl ether (0.18 g, 0.001 mol), 7 ity for ether hydrolysis on Pd/C,wealso 0.2 wt%Pd/C (30.0 mg, 1.7 10À mol of Pdsurface), H2O(80 mL), H2 (40 bar), 1908C, stirring at 700 rpm, 0–540 min.