Palladium‐Catalyzed Hydrolytic Cleavage of Aromatic C−O Bonds

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-

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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 dicyclohexyl 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 hydrolysis 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. The corresponding yield–time plots are shown in Figure S2. performed the reactions in decalin (H2 C) Reaction pathways and selectivities for diphenyl ether.The TOF of cyclohexylphenyl ether atmosphere) and in water (N2 atmos- was calculated from separate experiments with cyclohexylphenyl ether. phere). When decalin was used as the

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solvent, hydrogenation of the aromatic rings accounted for

97%selectivity.C6 oxygenates were produced in exactly the same quantity as the sum of benzene and cyclohexane, apparently as aresult of the lack of water to initiate hydrolysis pathways.

Reactions did not occur in water under 4bar N2 even after 12 h(Table 1, entry 4). In acontrol experiment (Figure S1), where the reaction mixture (ether,Pd/C,water) was first

heated at 2008Cfor 12 hunder N2 atmosphere and then

reacted for another 10 min under H2,the conversion and product distribution were identical to those obtained without

the first 12 hunder N2.Inanother control experiment with areversed order of operation, where the mixture was first

reacted at 2008Cfor 10 min under H2 and then heated for

another 12 hunder N2,hardly any reactions took place while

under N2 atmosphere (Figure S1). Neither hydrogenation nor

hydrogenolysis occurred in the absence of H2.The fact that Figure 2. Postulated mechanistic pathway for the reductive hydrolysis even hydrolysis did not proceed in the absence of H suggests 2 of aryl ethers on Pd surfaces. R =phenyl, cyclohexyl, phenylethyl, and that water does not directly attack the aryl C Obond of À n-butyl in this work. diphenyl ether to initiate hydrolysis.The results of these control experiments further indicate that no reactive inter-

mediate forms and accumulates in the absence of H2,and that Thehydrolysis of enol ethers in aqueous phase is known to the reactive intermediate for hydrolysis is also not any of the occur even at ambient temperature by rate-limiting proto- detectable products.Most likely,hydrolysis of the ether bond nation and fast water addition to form ahemiacetal;inthe has to follow partial hydrogenation of diphenyl ether to next, fast step,the hemiacetal eliminates R OH (one of the À undetectable reactive species that are presents in amounts stable primary products) and forms an enol, which quickly below the detection limit. tautomerizes to aketone.[15] In the case of diphenyl ether,this One such partially hydrogenated, highly reactive inter- mechanistic framework predicts equimolar formation of mediate could be cyclohex-1-enyl phenyl ether, which was phenol (R OH) and cyclohexanone along the initial reduc- À used as astarting material under avariety of conditions tive hydrolysis pathway whereas for cyclohexyl phenyl ether, (Table S2). In contrast to diphenyl ether,this enol aryl ether cyclohexanol (R OH) and cyclohexanone will be the stable À did not undergo hydrogenation, but was hydrolyzed rapidly, primary products from reductive hydrolysis,which is fully forming equimolar amounts of cyclohexanone and phenol consistent with the experimental observations discussed [Eq. (1)] with atotal selectivity of 99.9%under all conditions above (Figures 1and S1). Without partial hydrogenation of tested (with or without Pd/C,H or N ). Thereaction already the aromatic ring, acid-catalyzed hydrolysis of the aryl C O 2 2 À proceeded considerably in water at 1008Cwithout any bond cannot occur at these temperatures (e.g., 2008C). external catalyst, but was significantly promoted by the On metals such as Pd, olefinic moieties are highly reactive presence of Pd/C (Table S2). At 1908C, hydrolysis of cyclo- under hydrogenating conditions,preventing direct chromato- hex-1-enyl phenyl ether was also much faster, requiring no graphic and spectroscopic observation of the partially hydro-

catalyst or H2,than the conversion of diphenyl ether. genated intermediates (excepting cyclohexanone). Therefore, additional experiments were performed to provide evidence for the postulated reductive hydrolysis pathway. As shown in Figure 2, the mechanism predicts that the initially formed cyclohexanone should contain oxygen exclusively from water while phenol should contain oxygen solely from the ether.This was confirmed by isotope labeling 18 Taken together, the above results led us to propose experiments with H2 Oand unlabeled diphenyl ether. After anovel pathway for the reductive hydrolysis of diphenyl ether 30 min at 1908C(< 5% conversion;Table 2, entry 1) no 18O on Pd in aqueous phase (R = Ph, Figure 2). Stepwise hydro- had been incorporated into the phenol whereas the cyclo- gen addition events first occur at one of the aromatic rings hexanone contained > 90% 18O. However, the observation of forming two types of ether intermediates.Inone,the ether 18O-labeled cyclohexanone is not sufficient proof for our oxygen atom is connected to avinylic carbon atom (e.g., hypothesis as subjecting cyclohexanone directly to the same cyclohex-1-enyl phenyl ether), while in the other one,the conditions also led to the incorporation of 18O, presumably via ether oxygen atom is connected to an alkyl carbon atom (e.g., rapid equilibration with the geminal diol (Figure S4).[16] On cyclohex-3-enyl phenyl ether). In principle,these intermedi- the other hand, the phenol being completely unlabeled is ates can be further hydrogenated to stable ether products entirely consistent with the proposed mechanism. If,asan 18 (cyclohexyl phenyl ether and dicyclohexyl ether). However, alternative mechanism, ether cleavage preceded H2 Oaddi- as shown above,enol ether intermediates almost exclusively tion or 18OHC addition (by dissociative water adsorption) to undergo hydrolysis. the phenyl and phenoxy fragments,half of the initially formed

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Table 2: Initial product selectivities of the Pd/C-catalyzed transformation bond, rather than the weaker aliphatic C Obond, in contrast 18 [a] À of various aryl ethers in H2 O. to the cleavage pattern observed for aryl alkyl ethers on Ni/ [10a] SiO2 in aqueous phase. This should not be seen as aviolation of the bond dissociation energy;instead, it is due to partial hydrogenation of the aromatic ring, which leads to enol ether moieties that are much more reactive towards Entry Rgroup Product selectivity [mol%] water attack. 12345 In conclusion, we have demonstrated that Pd catalysts are 1phenyl 31 26 41 11[b] active and highly selective (up to 90%atquantitative 2cyclohexyl46468 ––conversion) towards the reductive hydrolysis of diaryl and 3[c] phenylethyl 32 32 533aryl alkyl ethers in aqueous phase under relatively mild 4 n-butyl 45 48 7––conditions and lead to very limited hydrogenolysis.Wehave [a] Reaction conditions: Ether (50.0 mg), 0.2 wt%Pd/C (1.0 mg, pre- identified anovel pathway for the Pd-catalyzed reductive 18 pared by diluting 5wt%Pd/C with activated carbon), H2 O(1.0 mL), H2 hydrolysis of aryl ethers,which is initiated by partial hydro- (40 bar at room temperature), 1908C, 0.5 h. [b] As 2 and 5 were identical genation of the arene ring to enol ether intermediates that are in this case, the selectivity to phenol from hydrogenolysis was inferred highly susceptible to water attack. This pathway contrasts the from that of 4.[c] The selectivity towards hydrogenation of the Rgroup often postulated acid-catalyzed ether cleavage pathway, (phenylethyl) was 25%. which does not require H2.Itisalso distinct from metal-

mediated direct ether cleavage (without direct H2 participa- phenol would contain 18O, in contradiction with the observation) followed by recombination of the fragments with surface tion. HC and COH radicals from water dissociation at the metal [10a] Additionally,weinvestigated the H2 pressure dependence surface. We currently think that the reason for why Pd is of the hydrogenation and reductive hydrolysis pathways with better at hydrolysis than Ni or Pt is related to the activity of diphenyl ether. Theproduct selectivities did not change with the metals to catalyze hydrogen addition to C=Cbonds.Niis

H2 pressure.The rates of hydrogenation and reductive the slowest in converting diphenyl ether, giving little hydro- hydrolysis were both shown to be first order with respect to genation product, whereas Pt is fastest at converting diphenyl the H2 pressure (Figure S5). An identical H2 pressure depend- ether and gives the largest amount of dicyclohexyl ether. encyfor hydrogenation and reductive hydrolysis is possible as Future work will be directed towards exploring the effects of the conversion is proposed to be initiated by aseries of ring substituents and understanding the origin of the prefer- common hydrogen addition steps prior to the branching of the ence for reductive hydrolysis pathways on Pd. pathways (Figure 2). Thedisparate activation energies mea- sured for the two pathways (Figure S6) clearly reject the possibility of acommon rate-determining step (RDS). Addi- Acknowledgements tional insight comes from the constant reaction order in H2 despite eightfold variation in H2 pressure (10–80 bar). Con- This work was supported by the U.S. Department of Energy, sidering that the adsorption of ethers and phenol onto aPd Office of Science,Office of Basic Energy Sciences,Division of surface is stronger than for Hadatoms,[12a, 17] we conclude that Chemical Sciences,Geosciences,and Biosciences.Portions of the (sub)surface Hcoverage is relatively low on Pd under the the work were performed at the William R. Wiley Environ- reaction conditions.Atlow Hcoverages,the kinetic observa- mental Molecular Science Laboratory,anational scientific tions,namely reductive hydrolysis and hydrogenation path- user facility sponsored by the DOEsOffice of Biological and ways showing the same first-order dependence on H2 but Environmental Research located at Pacific Northwest different activation energies,are consistent with amechanistic National Laboratory,amulti-program national laboratory scenario in which the second Haddition is the RDS for the operated for DOE by Battelle Memorial Institute. hydrogenation pathway while the RDS for reductive hydrolysis occurs after the addition of at least two hydrogen atoms (see the Supporting Information for detailed derivations of Conflict of interest the rate expressions). DFT calculations are being undertaken to validate this hypothesis. Theauthors declare no conflict of interest. To explore the generality of the novel mechanistic framework, we analyzed reactions of aryl ethers (Ar O R) Keywords: aryl ethers ·hydrogenation ·palladium · À À containing Rgroups other than phenyl, namely cyclohexyl, reductive hydrolysis ·selective cleavage 18 2-phenylethyl, and n-butyl, all in H2 Oatconversions < 5% to minimize the impact of secondary reactions (Table 2, Howtocite: Angew.Chem. Int. Ed. 2017, 56,2110–2114 entries 2–4). All of these ether substrates produced Angew.Chem. 2017, 129,2142–2146 [18O]cyclohexanone and R16OH as the initial products in amolar ratio of nearly 1:1, confirming that the mechanism depicted in Figure 2also applies to these aryl ethers. [1] a) J. Zakzeski, P. C. A. Bruijnincx,A.L.Jongerius,B.M.Weck- Remarkably,reductive hydrolysis of ethers with C O aromaticÀ À huysen, Chem. Rev. 2010, 110,3552 –3599;b)C.Xu, R. A. D. C linkages always occurred at the stronger aryl C O Arancon, J. Labidi, R. Luque, Chem. Soc.Rev. 2014, 43,7485 – aliphatic À

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