27. A. Vanossi, N. Manini, M. Urbakh, S. Zapperi, E. Tosatti,Rev. Mod. Phys. 85, 529–552 (2013).

ACKNOWLEDGMENTS

This work was supported in part by the Japan Science andTechnology Agency PRESTO program, under the project“Molecular technology and creation of new function;” the NationalCenter of Competence in Research Nanoscale Scienceprogram; grant CRSII2 136287/1 from the Swiss NationalScience Foundation; the Swiss Nanoscience Institute; COST

(European Cooperation in Science and Technology) ActionMP1303; the European Commission, under the Graphene Flagship(award no. CNECT-ICT-604391); the U.S. Office of NavalResearch Basic Research Challenge program; and Comunidadde Madrid, under the MAD2D-CM (S2013/MIT-3007) project.The Partnership for Advanced Computing in Europe (project2012071262) and the Empa high-performance computingfacility Hypatia are acknowledged for computational resources.All data are tabulated in the main text and the supplementarymaterials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/351/6276/957/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S16References (28–44)

3 September 2015; accepted 25 January 201610.1126/science.aad3569

ORGANIC CHEMISTRY

An aromatic ion platformfor enantioselective Brønstedacid catalysisChirag D. Gheewala, Bridget E. Collins, Tristan H. Lambert*

Chiral acid catalysts are useful for the synthesis of enantioenriched small molecules,but the standard catalysts require laborious and expensive preparations. Here, we describea chiral Brønsted acid prepared in one step from naturally occurring (–)-menthol andreadily available 1,2,3,4,5-pentacarbomethoxycyclopentadiene. Aromatic stabilizationserves as a key contributing factor to the potent acidity of the resulting compound, whichis shown to catalyze both Mukaiyama-Mannich and oxocarbenium aldol reactions withhigh efficiency and enantioselectivity. Catalyst loadings as low as 0.01 mole percent andpreparative scalability (25 grams) are demonstrated. Alternative amide catalysts are alsoshown to be promising platforms. In addition to proton catalysis, a chiral anion pathwayis demonstrated to be viable with this catalyst system.

Protonation dramatically alters the reactiv-ity of a molecule. As such, Brønsted acid(proton donor) catalysts have long pro-vided a potent strategy for the accelerationof a diverse array of chemical transforma-

tions (1, 2). In recent years, the invention of ef-fective chiral Brønsted acid catalysts has enabledthe development of numerous asymmetric reac-tions that furnish valuable chemicals in enan-tioenriched form (3–6). By and large, this area ofenantioselective Brønsted acid catalysis has beendominated by the binaphthol (BINOL) phosphoricacid class of catalysts originally developed byAkiyama (7) and Terada (8). Although the utilityof these catalysts is unquestionable, the majordrawback to substituted BINOL-based catalystsis the lengthy, laborious, and expensive protocolrequired for their synthesis (9), which complicatescatalyst optimization and limits their applicationon scale. Although alternative catalysts have beendeveloped (10–16), the majority of these still relyon binaphthyl or other unnatural frameworks[e.g., VAPOL (17) or SPINOL (18)] as the basisof their chirality. Thismonocultural reliance on asingle chiral scaffold is thus a major limitingfactor for this important area of catalysis, and theidentification of more readily accessible architec-tures is an important goal. Here, we describe a

carbon acid platform for enantioselective Brønstedacid catalysis that uses aromaticity as its centralacidifying element.Although carbon acids typically manifest rela-

tively high pKa values (where Ka is the acid disso-ciation constant), several structuralmodificationscan substantially increase the propensity of C–Hbonds to undergo ionization by stabilizing theanionic charge of the conjugate bases (19, 20)(Fig. 1A). Themost common of thesemodificationsinvolve s-delocalization (induction) by electro-negative elements or groups and p-delocalization(resonance) by conjugated functionality. A thirdmode of carbanion stabilization involves a spe-cial form of resonance known as aromaticity,epitomized by the dramatic increase (1017)in acidity of cyclopentadiene versus its acyclicanalog (21) due to the aromatic nature of thecyclopentadienyl anion. In the case of a 1,2,3,4,5-pentacarboxycyclopentadiene (PCCP) 1 (Fig. 1B),the three elements of induction-, resonance-, andaromaticity-induced acidification conspire to pro-duce extremely strong carbon acids that rival theacidity of the mineral acids (22–24). In fact, theextended conjugation of such substituted cyclo-pentadienes is such that their acidic protonsreside not on carbon but rather on oxygen ashydroxyfulvenes (compare3) (25).With such strongacidity, we reasoned that PCCPs could serve aspotent Brønsted acid catalysts and that ester oramide derivatives incorporating simple chiralalcohols or amines could offer synthetically ac-

cessible and effective enantioselective variants.Herein, we demonstrate the realization of thisvision with the use of chiral PCCPs as highly ef-ficient enantioselective Brønsted acid catalysts.1,2,3,4,5-Pentacarbomethoxycyclopentadiene

is readily available via the reaction of dimethylmalonate (4) and dimethylacetylene dicarbox-ylate (5) in pyridine-acetic acid, followed bytreatment with potassium acetate (KOAc) toyield, after acidic workup, the acid 6 (26) (Fig.2A). Compound 6 is a stable crystalline solid,and we have scaled this procedure to producemore than 50 g of this material. To access chiralderivatives, we found that refluxing 6with excess(–)-menthol in the presence of N-methylimidazolein toluene furnished the pentamenthyl ester 7in 95% yield. Because (–)-menthol is a naturallyoccurring commodity chemical, we calculate thatcatalyst 7 can be prepared for about US$4/g(<$5.50/g for the unnatural enantiomer). As analternative means to introduce chirality, we havealso found that treatment of 6 with 1.0 equivalentof a primary amine (e.g., sec-naphthethylamine)in refluxing toluene resulted in production ofthe corresponding monoamide product 8 ingood yield. Because in the area of Brønsted acidcatalysis, reaction rate has been shown to havea linear correlation with catalyst acidity (27), atleast to a first approximation, we determinedthe pKa (CH3CN) of both pentaester 6 (8.85 ±0.05) and monoamide 9 (11.7 ± 0.1) and foundthat they compared quite favorably with knownBINOL-phosphoric acids 10 (12–14) and otherderivatives (Fig. 2B).To evaluate the effectiveness of chiral PCCPs

as enantioselective catalysts, we examined theirperformance in a Mukaiyama-Mannich reaction,which was known to be amenable to this type ofasymmetric promotion (7) (Fig. 3A).We observedthat 1 mole percent (mol %) of menthol-derivedacid7 catalyzed the addition of silyl ketene acetal12 to imine 11 in ethyl acetate at –78°C in 1 hourto furnish the adduct 13 in 97% yield and 97%enantiomeric excess (ee). This result comparesquite favorably to BINOL-phosphoric acid cata-lyst 14, which at 10 mol % loading was reportedto catalyze the same reaction (in toluene) over24 hours to furnish 13 in 98% yield and 89% ee.The loading of cyclopentadienyl catalyst 7 couldbe reduced to 0.01 mol % without compromis-ing enantioselectivity. In terms of further catalystscreening, monoamides 8 and 15were also foundto induce appreciable enantioselectivity in thisreaction, suggesting such catalysts may also beworthy of further development. Nevertheless,pentaester catalysts have thus far proven optimal

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Department of Chemistry, Columbia University, New York, NY10027, USA.*Corresponding author. E-mail: tl2240@columbia.edu

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both in terms of reactivity and enantioselectivity,as further exemplified by the performance of the2-phenylcyclohexanol-derived catalyst 16.To investigate the preparative utility of 7, we

conducted aMannich reaction on scale to produce25 g of product 13 using 0.1 mol % of the acidcatalyst (Fig. 2B, entry 1), which resulted in yieldand enantioselectivity comparable to the smaller-scale reactions. In terms of additional substratescope, we found that a variety of aryl or vinylimines could be engaged efficiently and withhigh selectivities (entries 2 to 7). In addition, analiphatic imine substrate, which is typically chal-lenging for this type of chemistry, could also beproductively engaged (entry 8). An alternativesilyl ketene acetal bearing a cyclohexyl ring wasfound to react in high yield and with high se-lectivity (entry 9). Thus far, reactions that leadto diastereomeric mixtures have exhibited onlymodest selectivities with catalyst 7 (see the sup-plementary materials, p. S15).To demonstrate the potential of PCCPs tomove

beyond established capabilities, we investigatedthe enantioselective addition to oxocarbeniumions. In comparison to iminium ionadditions, enan-tioselective reactions with oxocarbenium ions arenotoriously difficult to achieve because they typ-ically lack the traditional organizational elements(e.g., hydrogen bonding groups) employed bymany asymmetric catalysts. Indeed, only recentlyhave successful approaches to enantioselectiveoxocarbenium ion chemistry begun to appear[see, e.g., (28–38)]. We probed the capacity ofcatalyst 7 to catalyze the addition of silyl keteneacetal 12 to oxocarbenium ion intermediates 29generated from salicylaldehyde acetals 27, a so-called Mukaiyama oxocarbenium aldol (Fig. 2C)(35). First, we attempted the reaction of benzal-dehyde dimethyl acetal, which in fact led toproduct 30 in high yield but, as expected, withessentially no enantioselectivity (entry 1). On theother hand, the dimethyl acetal of salicylaldehydeled to the production of adduct 31 with an en-couraging 71% ee (entry 2). Modification to thediethyl acetal resulted in an appreciable improve-ment in selectivity to 85% ee for adduct32 (entry3), whereas further increasing the steric demandof the acetal substituent to n-butyl or i-propyl(entries 4 and 5) led to comparable enantiose-lectivities for products 33 and 34. An alternativeketone product 35 could also be obtained in 80%ee with the use of a silyl enol ether nucleophile(entry 6). These results demonstrate that chiralPCCPs have the ability to address one of themore notable challenges in asymmetric catalysis,and it is a reasonable expectation that optimiza-tion of catalyst structure will enable further im-provements in this area.The mechanistic rationale for the operation

of catalyst 7 is shown in Fig. 4A, in which initialprotonation of acetal 36 produces intermediatesalt 37. Addition of the silyl ketene acetal 12to the highly electrophilic oxocarbenium ion(compare 38) then leads to intermediate 39. Silyltransfer to the alcohol produced in the initial ion-ization step would then furnish the ether product32 and return the acid 7 to the catalytic cycle.

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Fig. 1. Tuning carbon acids for catalysis. (A) Common strategies for increasing C–H acidity, includinginduction, resonance, and aromaticity, the latter epitomized by the relatively high acidity of cyclo-pentadiene. (B) PCCPs combine all three acidifying elements to produce powerful Brønsted acid catalysts.Me, methyl; R, generic substituent.

Fig. 2. Catalyst preparation and characterization. (A) Synthesis of 1,2,3,4,5-pentacarbomethoxycyclo-pentadiene 6 and chiral derivatives 7 and 8. (B) pKa values measured in acetonitrile. i-Pr, isopropyl;Ph, phenyl; NMI, N-methylimidazole.

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To investigate whether proton catalysis wasindeed the operative mode for catalyst 7, weran the Mannich reaction of 11 and 12 usingdeprotonated 7 as its sodium salt (Fig. 4B). Asexpected, no reaction was observed at –78°C,lending credence to the notion that the catalystoperates as a Brønsted acid (entry 1). Interest-

ingly, however, when the reactionwas performedat higher temperatures, product formation wasobserved, with appreciable enantioenrichment(entry 2). As there was no acidic catalyst protonin this case, this result suggests that perhaps asilylium-catalyzed process was operative, withthe cyclopentadienyl group serving as a chiral

anion. This notion is supported by the fact thatthe reaction worked with comparable efficiencyand enantioselectivity even in the presence of2,6-di-tert-butyl-4-methylpyridine (entry 3). Itis worth noting that chiral anion catalysis hasalso proven to be an effective strategy for asym-metric synthesis (36–38), and so the current

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Fig. 3. Exploration of substrate scope. (A) Catalyst screen for enantioselective Mukaiyama-Mannich reaction. (B) Substrate scope for acid-catalyzed Mannichreaction. (C) Enantioselective catalytic Mukaiyama oxocarbenium aldol. *5 mol % 7 was used. †Reaction was performed with the trimethylsilyl ketene acetalderived frommethyl cyclohexanecarboxylate. ‡Reaction was performed in ethyl acetate at room temperature for 72 hours. §Reaction was performedwith1-o-tolyl-1-trimethylsiloxy ethylene as the nucleophile. R, R1, and R2, generic substituent; TMS, trimethylsilyl; EtOAc, ethyl acetate.

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platformmay have utility in this burgeoning areaas well.A stereochemical rationale for this chemistry

is shown in Fig. 4C. The presence of multiplerotatable bonds in catalyst 7 renders a numberof transition-state organizations plausible. For theMannich reaction, one possible scenario involvesassociation of the protonated imine and one ofthe carbonyl oxygens of the catalyst via two hy-drogen bonds (Fig. 4C). To accommodate thisinteraction would require substantial torquingof the carboxyl substituent, thereby placingthe iminium in close proximity to an adjacentcarboxymenthyl substituent. Blocking of the reprochiral face of the iminiumwould then ration-alize the observed stereochemistry.A similar model can be invoked for the

oxocarbenium addition; however, in this caseonly a single hydrogen bonding substituent ispresent (Fig. 4D). We speculate that there maybe additional organizing interactions betweenthe C–H bonds adjacent to the oxygen of theoxocarbenium ion and the catalyst carbonyl and/

or the cyclopentadienyl ring. In this case, p-facialblocking as described above would lead to reac-tion via the siprochiral face,which corresponds tothe observed stereoselectivity for theR-enantiomer.We have obtained a single-crystal x-ray struc-

ture of catalyst 7 as its tetramethylammoniumsalt (Fig. 4E). The face view (cation removed)shows a gearing of the carboxyl substituents allin the same direction, presumably to minimizesteric repulsion of the menthyl substituents. Thealternative edge view shows the substantial di-hedral angle between each carbonyl and the cyclo-pentadienyl ring ranging from 30.6° to 52.1°,with an average of 43.8°, and the orientation ofeach of the carbonyl oxygens to the same side ofthe ring. In this view, two adjacent (and equiv-alent) cations are included, which shows that onehalf of each ammonium is embedded in a hy-drophobic pocket formed by thementhyl groups,whereas the other half is associated with the op-posite ring face toward which the carbonyls areoriented. The extent to which this structure isrelated to the catalytic operation of 7 is not cur-

rently known and will be the subject of furtherstudy.The ability to synthesize asymmetric catalysts

rapidly, inexpensively, and on scale is an impor-tant goal for the field of organic chemistry. Thecurrent platform offers a useful advance in thisregard by allowing preparation of an effectivechiral Brønsted acid catalyst in a single step froma readily available acid core and simple chiralalcohols or amines. The central feature of thisplatform, the cyclopentadienyl anion, enables thecooperation of multiple acidifying elements toproduce powerful acid catalysts.

REFERENCES AND NOTES

1. J. N. Brønsted, Rec. Trav. Chim. 42, 718–728 (1923).2. T. M. Lowry, Chem. Ind. 42, 43–47 (1923).3. D. Parmar, E. Sugiono, S. Raja, M. Rueping, Chem. Rev. 114,

9047–9153 (2014).4. T. Akiyama, J. Itoh, K. Fuchibe, Adv. Synth. Catal. 348,

999–1010 (2006).5. T. Akiyama, in Acid Catalysis in Modern Organic Synthesis,

H. Yamamoto, K. Ishihara, Eds. (Wiley-VCH, Weinheim, 2008),pp. 62–107.

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Fig. 4. Mechanisticconsiderations.(A) Mechanistic ratio-nale for Mukaiyamaoxocarbenium additionreaction. (B) Demon-stration of analternative chiral anioncatalysis pathway.(C and D) Stereo-chemical rationale for(C) the Mannich reac-tion catalyzed by acid7 and (D) the oxocar-benium aldol cata-lyzed by acid 7. *Threeof the carboxylates(at the end of graybonds) are not shownfor clarity. (E) Molecu-lar structure of7•NMe4. In the faceview, the cation is notshown. In the edgeview, two adjacentcations are shown.

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6. T. Hashimoto, in Asymmetric Organocatalysis 2: BrønstedBase and Acid Catalysts, and Additional Topics, K. Maruoka, Ed.(Georg Thieme Verlag, Stuttgart, New York, 2012),pp. 279–296.

7. T. Akiyama, J. Itoh, K. Yokota, K. Fuchibe, Angew. Chem. Int. Ed.43, 1566–1568 (2004).

8. D. Uraguchi, M. Terada, J. Am. Chem. Soc. 126, 5356–5357(2004).

9. I. Ahmed, D. A. Clark, Org. Lett. 16, 4332–4335 (2014).10. D. Kampen, C. M. Reisinger, B. List, Top. Curr. Chem. 291,

395–456 (2010).11. T. James, M. van Gemmeren, B. List, Chem. Rev. 115,

9388–9409 (2015).12. T. Akiyama, Chem. Rev. 107, 5744–5758 (2007).13. T. Akiyama, K. Mori, Chem. Rev. 115, 9277–9306 (2015).14. A. Berkessel, P. Christ, N. Leconte, J.-M. Neudörfl, M. Schäfer,

Eur. J. Org. Chem. 2010, 5165–5170 (2010).15. D. Nakashima, H. Yamamoto, J. Am. Chem. Soc. 128,

9626–9627 (2006).16. M. Treskow, J. Neudörfl, R. Giemoth, Eur. J. Org. Chem. 2009,

3693–3697 (2009).17. G. B. Rowland et al., J. Am. Chem. Soc. 127, 15696–15697

(2005).18. F. Xu et al., J. Org. Chem. 75, 8677–8680 (2010).19. H. Yanai, T. Taguchi, J. Fluor. Chem. 174, 108–119 (2015).20. H. Yamamoto, D. Nakashima, in Acid Catalysis in Modern

Organic Synthesis, vol. 1, H. Yamamoto, K. Ishihara, Eds.(Wiley-VCH, Weinheim, 2008), pp. 35–62.

21. F. G. Bordwell, G. E. Drucker, H. E. Fried, J. Org. Chem. 46,632–635 (1981).

22. M. I. Bruce, A. H. White, Aust. J. Chem. 43, 949–995(1990).

23. O. Diels, Ber. Dtsch. Chem. Ges. 75, 1452–1467 (1942).24. O. Diels, U. Kock, Liebigs Ann. Chem. 556, 38–50 (1944).25. I. E. Mikhailov, G. A. Dushenko, V. I. Minkin, L. P. Olekhnovich,

J. Org. Chem. USSR 20, 1509–1514 (1984).26. E. Le Goff, R. LaCount, J. Org. Chem. 29, 423–427 (1964).27. K. Kaupmees, N. Tolstoluzhsky, S. Raja, M. Rueping, I. Leito,

Angew. Chem. Int. Ed. 52, 11569–11572 (2013).28. S. E. Reisman, A. G. Doyle, E. N. Jacobsen, J. Am. Chem. Soc.

130, 7198–7199 (2008).29. N. Z. Burns, M. R. Witten, E. N. Jacobsen, J. Am. Chem. Soc.

133, 14578–14581 (2011).30. M. R. Witten, E. N. Jacobsen, Angew. Chem. Int. Ed. 53,

5912–5916 (2014).31. G. C. Tsui, L. Liu, B. List, Angew. Chem. Int. Ed. 54, 7703–7706

(2015).32. J. H. Kim, I. Čorić, S. Vellalath, B. List, Angew. Chem. Int. Ed.

52, 4474–4477 (2013).33. I. Čorić, B. List, Nature 483, 315–319 (2012).34. A. Borokiva, P.-I. Tang, S. Klapman, P. Nagorny, Angew. Chem.

Int. Ed. 52, 13424–13428 (2013).35. L. Ratjen, P. García-García, F. Lay, M. E. Beck, B. List,

Angew. Chem. Int. Ed. 50, 754–758 (2011).36. G. L. Hamilton, E. J. Kang, M. Mba, F. D. Toste, Science 317,

496–499 (2007).37. K. Brak, E. N. Jacobsen, Angew. Chem. Int. Ed. 52, 534–561

(2013).38. M. Mahlau, B. List, Angew. Chem. Int. Ed. 52, 518–533 (2013).

ACKNOWLEDGMENTS

Funding for this work was provided by the National ScienceFoundation under CHE-0953259. C.D.G. is grateful for aNational Science Foundation Graduate Fellowship. We thankP. Quinlivan and the Parkin group, as well as D. Paley, for x-raystructure determination and the National Science Foundation(CHE-0619638) for acquisition of an x-ray diffractometer.We are grateful to M. Vetticatt (State University of New York,Binghamton) and V. Roytman for assistance with thestereochemical models. Metrical parameters for the structuresof 7•NMe4 and the oxocarbenium Aldol product are availablefree of charge from the Cambridge Crystallographic Data Centreunder CCDC 1450055 and 1450056, respectively.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/351/6276/961/suppl/DC1Materials and MethodsFigs. S1 to S4Tables S1 to S4References (39–57)

20 July 2015; accepted 27 January 201610.1126/science.aad0591

CATALYSIS

Palladium-tin catalysts for the directsynthesis of H2O2 with high selectivitySimon J. Freakley,1*† Qian He,2,3† Jonathan H. Harrhy,1 Li Lu,2 David A. Crole,1

David J. Morgan,1 Edwin N. Ntainjua,1 Jennifer K. Edwards,1 Albert F. Carley,1

Albina Y. Borisevich,3,4 Christopher J. Kiely,2 Graham J. Hutchings1*

The direct synthesis of hydrogen peroxide (H2O2) from H2 and O2 represents a potentiallyatom-efficient alternative to the current industrial indirect process. We show that theaddition of tin to palladium catalysts coupled with an appropriate heat treatment cycleswitches off the sequential hydrogenation and decomposition reactions, enablingselectivities of >95% toward H2O2. This effect arises from a tin oxide surface layer thatencapsulates small Pd-rich particles while leaving larger Pd-Sn alloy particles exposed.We show that this effect is a general feature for oxide-supported Pd catalysts containing anappropriate secondmetal oxide component, and we set out the design principles for producinghigh-selectivity Pd-based catalysts for direct H2O2 production that do not contain gold.

Currently, the demand for H2O2 is met byan indirect process, which produces H2O2

through the sequential hydrogenation andoxidation of a substituted anthraquinone(1). For economic reasons, the process is

operated at large scale and produces concen-trated H2O2. In reality, many applications, suchas disinfection and water purification, requireonly dilute H2O2, which means that concen-trated H2O2 must be diluted at the point of use.Research into the direct synthesis of H2O2 fromH2 and O2 as a more suitable solution to small-scale, on-site H2O2 production has focused onpalladium (Pd)–based catalysts (2–4). However,H2O2 is itself highly reactive, and the presenceof H2 favors hydrogenation and decompositionreactions that formwater. The addition of strongacids and halides to the reaction medium cansuppress the sequential hydrogenation and deg-radation in supported Pd catalysts (5) but canalso promotemetal leaching and requires furtherpurification of the H2O2 before use.Bimetallic Au-Pd alloy catalysts have been ex-

tensively studied as catalysts for the direct H2O2

synthesis reaction on a number of support mate-rials, including TiO2, SiO2, and activated carbon(6–9). Yields comparable to monometallic Pd cat-alysts can be achieved without the need for acidand halide additives in the reactionmixture, and95% selectivity to H2O2 can be achieved with Au-Pd alloy nanoparticles (NPs) dispersed on an acid-pretreated activated carbon support material (10).Hydrogen peroxide hydrogenation could be de-coupled from H2O2 synthesis with an acid pre-treatment that blocked sites on the carbon supportmaterial responsible for H2O2 degradation (10).

Although this approach was very successful on anactivated carbon supportmaterial, the same block-ing effect could not be fully achieved on other com-mercial support materials such as SiO2 and TiO2.Because O2 dissociation is undesirable in the

direct synthesis of H2O2, the reaction can betreated as a selective hydrogenation of O2. Weexplored other Pd-metal combinations that areused for selective hydrogenation reactions as po-tential catalysts for H2O2 synthesis, focusing onnonprecious metals to lower costs. Tin (Sn) hasbeen used to modify hydrogenation catalysts inreactions such as the selective hydrogenation of1,3-butadiene (11). Further examples have beenreported for the liquid-phase hydrogenation ofhexa-1,3-diene and hexa-1,5-diene (12) as well asthe hydrogenation of unsaturated alcohols (13).The addition of Sn to Pd or Pt can alter the behav-ior of the catalyst during hydrogenation reactionsand, in particular, may have an effect on subse-quent reactions of the products with the catalyst.We report the development of Sn-containing

Pd catalysts on commercially available TiO2 andSiO2 supports that can achieve >95% selectivitytoward direct H2O2 synthesis. These catalysts,after being subjected to an appropriate heattreatment regimen, obviate the need for pre-treating the support with acids and contain farless precious metal than Au-Pd catalysts. Wealso present the general principles wherebyhigh-selectivity catalysts can be obtained withother Pd-metal combinations.Simple impregnation of Au and Pd metal salts

onto many catalyst supports has been shown togenerate highly active catalysts for direct H2O2

synthesis. In addition, high-temperature calcina-tion or reduction treatments are known to becrucial to improve the stability of the catalyst. Asa starting point, we used this simple catalystpreparationmethodology to prepare a 2.5 weightpercent (wt%) Pd–2.5 wt % Sn/TiO2 catalyst aswell as its monometallic analogs (8, 10). A syn-ergistic effect toward higher H2O2 productivitywas observed when both metals were present

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1Cardiff Catalysis Institute and School of Chemistry, CardiffUniversity, Cardiff CF10 3AT, UK. 2Department of MaterialsScience and Engineering, Lehigh University, Bethlehem, PA18015, USA. 3Materials Science and Technology Division, OakRidge National Laboratory, Oak Ridge, TN 37831, USA. 4Centerfor Nanophase Materials Sciences, Oak Ridge NationalLaboratory, Oak Ridge, TN 37831, USA.*Corresponding author. E-mail: hutch@cf.ac.uk (G.J.H.); freakleys@cf.ac.uk (S.J.F.) †These authors contributed equally to this work.

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An aromatic ion platform for enantioselective Brønsted acid catalysisChirag D. Gheewala, Bridget E. Collins and Tristan H. Lambert

DOI: 10.1126/science.aad0591 (6276), 961-965.351Science 

, this issue p. 961Scienceselective catalysts for asymmetric Mukaiyama Mannich and oxocarbenium aldol reactions.framework that are easily accessible from readily available precursors. The compounds are showcased as highly

report a class of chiral carbon acids based around a cyclopentadieneet al.conjugate base in acid catalysis. Gheewala just one of two mirror-image products. The motif was first applied as a ligand in metal catalysis, and more recently as a

The binaphthyl framework has proven extremely effective in biasing a broad range of chemical reactions towardCompetition for binaphthyl catalysts

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