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Silver(I) as a widely applicable, homogeneouscatalyst for aerobic oxidation of aldehydes towardcarboxylic acids in water—“silver mirror”:From stoichiometric to catalyticMingxin Liu, Haining Wang, Huiying Zeng, Chao-Jun Li*

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The first example of a homogeneous silver(I)-catalyzed aerobic oxidation of aldehydes in water is reported. Morethan 50 examples of different aliphatic and aromatic aldehydes, including natural products, were tested, and allof them successfully underwent aerobic oxidation to give the corresponding carboxylic acids in extremely highyields. The reaction conditions are very mild and greener, requiring only a very low silver(I) catalyst loading, usingatmospheric oxygen as the oxidant and water as the solvent, and allowing gram-scale oxidation with only 2 mgof our catalyst. Chromatography is completely unnecessary for purification in most cases.

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Oxidation is a central task for organic chemists to achieve conversion ofdifferent organic compounds. Among them, oxidation of aldehydes togive carboxylic acids is one of the most well-known and most frequentlyused methodologies (1, 2), for example, by stoichiometrically using theCr(IV)-based Jones reagent (3, 4), the Ag(I)-based Tollen’s reaction (5),the Cu(II)-based Fehling’s reaction (6), and the permanganate reagents(7). Although it has long been known that aldehydes are very prone tooxidation, methods to achieve a highly efficient and clean transfor-mation of aldehydes to carboxylic acids under mild and greener con-ditions are still scarce. Even today, most such oxidations still requirestoichiometric amounts of hazardous oxidants (8–25) and often takeplace in harmful solvents.

With its natural abundance and inherent greener characteristics,water has been a desirable solvent for chemists (26–28). Although bio-logical oxidations in water using enzymes or microorganisms are wellrecognized (29–34), it was only in 2000 that Sheldon established anaqueous-phase homogeneous catalytic aerobic oxidation methodology(35, 36). Yet, the method still requires a precious metal (palladium), ahigh pressure (30 bar), and a large amount of additive (TEMPO). In2008, Tian et al. reported a heterogeneous catalytic aqueous-phaseoxidation of aldehydes using silver(I)/copper(II) oxide (37), but themethod suffers from a high catalyst loading, a very limited substratescope, and side reactions. In 2009, Yoshida and co-workers reported awater-soluble N-heterocyclic carbene (NHC)–catalyzed oxidation ofaldehyde by oxygen (38). However, this method still requires the re-action solvent to be a mixture of N,N′-dimethylformamide/H2O in 10:1ratio, which is far from a complete water-phase oxidation. Recently, in2014, Han and co-workers reported a multifunctional utilization ofsilver-NHC complex as catalyst to achieve different oxidation of alco-hol (39), but the method still relies on organic solvent and anhydrousconditions. Here, we wish to report a highly efficient, widely applicable,homogeneous silver(I)-catalyzed aerobic oxidation of a wide range ofaldehydes using only water as solvent, and performed under atmosphericpressure with oxygen gas or oxygen in the air as the oxidant under mildconditions (Fig. 1).

Department of Chemistry and FRQNT Centre in Green Chemistry and Catalysis, McGillUniversity, 801 Sherbrooke Street West, Montreal, Quebec H3A 0B8, Canada.*Corresponding author. E-mail: cj.li@mcgill.ca

Liu et al. Sci. Adv. 2015;1:e1500020 27 March 2015

We began our investigation by introducing various silver(I) salts orcomplexes to benzaldehyde as a standard in air at atmospheric pressurewithout a balloon in a sealed tube (Table 1). We found the efficiency ofthe transformation to be strongly affected by the presence of inorganicsalt (for example, entries 1 and 2). When the reaction was carried out with-out oxygen, a very low yield was obtained (entry 3), reflecting a stoichi-ometric aldehyde oxidation. The anions of the salt were then tested.Besides formate, only fluoride and tetrafluoroborate provided the ox-idation product (entries 4 to 7). Considering that tetrafluoroboratemight undergo hydrolysis to give fluoride in situ, fluoride was thus chosenas the standard anion to conduct further investigations. Upon examiningthe cation, surprisingly, it seemed to be the only one enabling the ox-idation (entries 8 to 11). Sodium fluoride was therefore selected as thesalt for optimizing the conditions. We then tested different ligands(entries 12 to 14) and found that the combination of chelating bipyridineas a ligand and a noncoordinating PF6

− as the counter-ion (entry 14)achieved a quantitative yield of the corresponding oxidation. Switchingfrom air to oxygen gas under the same atmospheric pressure also led toa quantitative isolated yield (entry 15). As a control experiment, only atrace amount of the product was detected in the absence of the silvercatalyst.

A series of common aldehydes, including both aliphatic and aro-matic examples with different functional groups, was then chosen to con-duct the scope investigation with this catalytic system (Table 2). Besidesbenzaldehyde, which gave a quantitative yield (entry 1, compound 1),aliphatic 1-octanal also gave a quantitative yield of the corresponding acid(entry 2, compound 38). Hydrocinnamaldehyde and 1-naphthaldehydegave very good yields of 86 and 88% (entries 3 and 4, compounds 49 and4), respectively. With 4-fluorobenzaldehyde, the reaction only gave a34% yield (entry 5, compound 16), whereas 4-chlorobenzaldehyde led toa 100% recovery of the starting material (entry 6, compound 18). Surpris-ingly, for the unsaturated cinnamaldehyde, even with all the starting ma-terial consumed, the reaction still did not give any desired product (entry 7,compound 47), whereas the unconjugated 4-allyloxy benzaldehyde ledto a low conversion of the starting material and no desired product (entry8, compound 12). p-Anisaldehyde and piperonal, bearing additionaloxygen-based functional groups, also led to 100% recovery of the startingmaterial (entries 9 and 10, compounds 6 and 5). We postulated thatthose substrates with inferior reactivity might be caused by the competing

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coordination of oxygen, nitrogen, C=C double bond, and so on, to theLewis acidic silver(I) center within the same molecule as the aldehyde car-bonyl. Thus, we rationalized that a stronger coordinating ligand may berequired to release the Ag(I) center from such coordination.

Using piperonal, unreactive under the above conditions, as themodel substrate, phosphorus-based and NHC ligands were examined(Table 3). Unless otherwise noted, all experiments were carried out inhouse-light conditions, without light sheltering. With [(CF3)2CHO]3P, avery electron-poor ligand, we only obtained a 21% yield. Furthermore,some decomposition of the piperonal’s formacetal structure was ob-served (entry 2). The combination of AgPF6 with more electron-richtrifurylphosphine gave a good 66% nuclear magnetic resonance yield(entry 3); however, some decomposition (ca. 15%) of the acetal was stillobserved. The catalyst generated from AgPF6 and the NHC ligand IPrgave a much lower yield (entry 4). To our surprise, when we switchedAgPF6 to Ag2O, an almost quantitative yield was obtained (entry 5). Iso-lation of the product from the reactionmixturewas very easy: The aqueousreactionmixturewas simplywashedwith commonnon–water-mixableor-ganic solvent and then acidified, followed by extraction using diethylether. Without needing to perform flash chromatography, the productwith an extremely high purity level was obtained.Meanwhile, a 50%yieldwas achievedwhen only 0.5 equivalent of the base was added, indicatingthat the presence of base is necessary to drive the reaction. The con-trolled experiment was then conducted to test how the reaction pro-ceeds in the absence of oxygen. Surprisingly, with the reaction conductedwith argon at 1 bar and using ordinary distilled water stored in air, thereaction was still capable of giving 66% yield (entry 10), whereas verylittle product was detected when the systemwas completely oxygen-free(entry 11). It should be noted that visible light does not alter the reactionefficiency.

Liu et al. Sci. Adv. 2015;1:e1500020 27 March 2015

With the optimized reaction conditions in hand, a muchmore di-verse series of aldehydes were selected to examine the substrate scope(Table 4). To our satisfaction, excellent yields were obtained with allaldehydes that we examined. Aromatic aldehydes where the –R group isa hydrocarbon (benzaldehyde, p-tolualdehyde, 5-indancarboxaldehyde,or 1-naphthaldehyde) were all transformed in quantitative or nearlyquantitative yields (compounds 1 to 4). All of the electron-rich aromaticaldehydes that we tested—mono-, di-, and tri-methoxyl–substitutedbenzaldehydes—gave quantitative or nearly quantitative yields (com-pounds 6 to 9) regardless of the location of the substituent. Please notethat piperonal, which was tested in our investigation of the reactionconditions (compound 5), also gave almost quantitative yield. Themore hydrophobic 4-(pentyloxy)benzaldehyde and 4-(hexyloxy)benzaldehyde also gave excellent 94 and 90% yields (compounds 10and 11), respectively. The 4-allyloxy-benzaldehyde gave quantitativeyield as well, with the terminal C=C double bond intact and no ob-servation of the Claisen rearrangement (compound 12), whereas the4-benzyloxy-benzaldehyde resulted in a reduced 65% yield, probablydue to the cleavage of the benzyloxy group (compound 13).

Other than those electron-rich aldehydes, only slightly reducedyields were obtained with 3-bromo-2,4-dimethoxybenzaldehyde and5-bromo-1,3-benzodioxole-4-carboxaldehyde inwhichabrominewasalsoattached to the aromatic ring (compounds 14 and 15), possibly due tochelation. All other halogenated aromatic aldehydes, including fluorine-,chlorine-, bromine-, and the pseudohalogen cyano-substituted ben-zaldehydes, resulted in quantitative conversions regardless of the locationof the substituent (compounds 16 to 24). With terephthalaldehyde, ex-clusive oxidation of only one of the aldehyde groups was obtained in aquantitative yield (compound 25). To verify this selectivity, introducing4-formylbenzoic acid to our standard reaction conditions led to a

O2

WATER

-Excellent efficiency-1 atm O2 50°C in water-Extremely low [Ag] cat. loadcapable of doing gram scale with2 mg of catalyst(in prolonged time)-Chromatography is unnecessary in almostall cases

Fig. 1. Highlights of our aerobic oxidation.

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quantitative recovery of the starting material. The presence of anothercarbonyl group, other than an acid, in the aldehyde does not affect theoxidation yield: both 4-acetylbenzaldehyde and 4-acetaminobenzaldehydegave quantitative yields (compounds 26 and 27). 4-Hydroxymethyl-benzaldehyde also gave a quantitative yield (compound 28). With 4-quinolinecarboxaldehyde, a decreased yield (57%) was observed(compound 29), possibly due to the strong coordination of quinolone

Liu et al. Sci. Adv. 2015;1:e1500020 27 March 2015

nitrogen. Other heterocyclic aromatic aldehydes such as furfural and2-thiophenecarboxaldehyde were also subjected to the oxidation, in whichfurfural gave a quantitative yield, whereas 2-thiophenecarboxaldehydegave a reduced 60% yield (compounds 30 and 31), possibly alsodue to the stronger coordination of the sulfur atom in thiophene.Most electron-poor aromatic aldehydes, represented by nitro- andtrifluoromethyl-substituted benzaldehydes, were highly efficient in

Table 1. Investigation of reaction conditions.

H

O [Ag]Additive

DIPEA1 ml water, 50oC, 12 h

O2

(Air)*

OH

O

+

Entry [Ag] Additive Starting material conversion NMR yield†

1 AgF/Cy2PPh 0 % 0 %

2 AgF/Cy2PPh NaCO2H 19 % 11 %

3 AgF/Cy2PPh NaCO2H 5 % < 3 %

4 AgF/Cy2PPh NaF 27 % 20 %

5 AgF/Cy2PPh NaCl 0 % 0 %

6 AgF/Cy2PPh NaBr 0 % 0 %

7 AgF/Cy2PPh NaBF4 30 % 20 %

8 AgF/Cy2PPh LiF 0 % 0 %

9 AgF/Cy2PPh KF 0 % 0 %

10 AgF/Cy2PPh MgF2 0 % 0 %

11 AgF/Cy2PPh AlF3 0 % 0 %

12 AgF/BINAP NaF 31 % 21 %

13 AgF/bipy NaF 30 % 22 %

14 AgPF6/bipy NaF 100 % > 99 %

15§ AgPF6/bipy NaF 100 % > 99% ¶

16§ NaF Trace Trace

(0.1 mmol)

N N

PPh2

PPh2

BINAP Bipy

P

Cy2PPh

* All reactions were carried out in sealed 10 ml reaction vessels filled with atmospheric air or pure oxygen. † 1H-NMR yield was determined using 1,3,5-me

-sitylene as an internal standard.

‡

‡

Reaction was carried out under atmospheric argon.§ Reactions were carried out under atmospheric pure oxygen.¶ Isolated yield.

5 mol %5 mol %

5 mol %

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Table 2. Investigation of substrate scope.

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Table 3. Studies on improving the compatibility of the reaction.

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Table 4. Substrate scope investigation.

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this oxidation and gave quantitative yields regardless of the locationof the substituent (compounds 32 to 35).

Various aliphatic aldehydes were also tested: Linear hexanal, hep-tanal, octanal, and even the extremely hydrophobic decanal all gavequantitative oxidation products (compounds 36 to 39). Similarly,branched 2-methylbutanal, 2-methylpentanal, 2-ethylbutanal, and2-ethyl hexanal also gave quantitative yields (compounds 40 to 43).With a C=C double bond being conjugated to the carbonyl, 3-methyl-2-butenal gave a slightly reduced yield of 77% (compound 44). Cit-ronellal gave a moderate 60% yield, whereas citral gave a good 86%yield (compounds 45 and 46), indicating that the C=C double bond

O OO

HR

O

HRO

OH

AgIPr

AgIPr

O OH

R H

O

O

OHR

O

OR

OH

O2

O

HR

OHR O

O

H2O

Oxygenactivation

cycle

MeCN

Ag2O + IPr HCl

AgOH

IPr-Ag-OH

IPr-Ag-H

IPr-Ag-O-OH

IPr-Ag-H

IPr-Ag-Cl

NaOH/H2O

O

R O OH

Ag HIPr

O

R OAg IPr

AFig. 4. Reaction mechanism. (A) Plausible mechanism and (B) ZPE-corregiven in kcal/mol and referred to the system AgOH(PMe3) + PhCHO. Gibb

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does interfere with the oxidation slightly. With aryl-substituted con-jugated cinnamaldehyde and a-methylcinnamaldehyde, the oxidationproceeded quantitatively (compounds 47 and 48). Hydrocinnamal-dehyde and phenylpropionaldehyde also gave quantitative yield(compounds 49 and 50). The 4-nitro–substituted cinnamaldehydegave a slightly reduced 91% yield (compound 51). As a natural prod-uct and a widely used source of food spice, perillaldehyde also gave aquantitative yield (compound 52). An even more complex tertiary al-dehyde, abietadien-18-al, was also successfully oxidized to abieticacid (compound 53) with an increased reaction temperature andpressure (Fig. 2).

R H

OA

OH

R

O

OH

AgIPr

H

R

O

H

AgIPr

OH

Hydrideextraction

cycle

cted energies from B3LYP/6-3s free energy is considered f

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Finally, a gram-scale oxidation wasconducted with benzaldehyde (Fig. 3).To be practical and economical, we low-ered both the amount of solvent and theamount of catalyst compared to the stan-dard conditions. With only 2 mg of oursilver(I) catalyst {equivalent to about0.036 mol % [360 ppm (parts per million)]catalyst loading}, 560 mg of sodium hy-droxide, and 1.4 ml of benzaldehyde in10 ml of water at 1 bar of oxygen using anattached balloon, the reaction gave an as-tonishing 82% isolated yield with morethan 1.4 g of analytically pure benzoic acidafter 48 hours at 50°C. This indicates thatour methodology can be readily scaled upto an industrial level.

On the basis of the results of our study,a plausible reaction mechanism that in-volves two catalytic cycles is proposed inFig. 3: one of the cycles is responsible forextracting the hydride from the aldehyde,whereas the other is responsible for acti-vating the dioxygen molecule in water.Each cycle consumes one molecule of

gIPr

B1G(d) and LANL2DZ for Ag. All values areor drawing.

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Fig. 3. Gram-scale reaction.

Fig. 2. Abietadien-18-al aerobic oxidation.

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aldehyde and generates one molecule of carboxylic acid. The twotandem cycles operate cooperatively to give the very efficient oxidationobserved. It has previously been reported that the combination of silver(I)oxide and NHC ligand gives an NHC-Ag(I)-Cl complex (40). After thechloride complex is introduced to our aqueous NaOH solution, the Cl−

of the complex is substituted with hydroxyl to give the suggestedNHC-Ag(I)-OH catalyst species. The catalyst then coordinates tothe C=O double bond of the aldehyde and exchanges its coordinated–OH with the –H of the aldehyde, possibly through either a nucleo-philic attack of –OH followed by b-hydride elimination (Fig. 4A) or afour-membered ring transition state where the exchange of –OHand–H occurred simultaneously (Fig. 4B). The catalyst then releases thecarboxylic acid as the product and a silver(I)-hydride species, whosepresence has been suggested bymany of our recent studies (41–43) andhas also been directly detected recently (44). We also conducted a briefcomputational study for the proposed mechanism (detailed in the Sup-plementaryMaterials). To reduce the complexity of calculation, we useda simplified molecule model that simplifies the complicated NHC lig-and into a simple trimethylphosphine ligand because of their electronicsimilarity. The result has shown that it is possible for the existence of asilver(I)-hydride intermediate when the silver center is coordinatedto a strong electron-donating ligand (Fig. 4B). The computationalresult shows that a four-membered ring transition state is favored; how-ever, considering the simplification of the calculation, there is still nosolid proof of such a pathway. We tentatively propose that the silver(I)-hydride is responsible for the activation of oxygen in water, gener-ating a silver(I)-hydroperoxyl intermediate. The hydroperoxide thennucleophilically attacks another carbonyl of the aldehyde. The hydro-gen of the aldehyde is then extracted by silver with a similar b-hydrideelimination (44). Then, the silver(I)-hydride is oxidized by the coor-dinating peroxyl acid to give the corresponding carboxylate and torelease a watermolecule. The remaining carboxylate is then substitutedby a hydroxide to release the carboxylate and regenerate the silver(I)-hydroxide catalyst.

In summary, we have discovered the first example of homogeneoussilver(I)-catalyzed aerobic oxidation of an aldehyde in water. The re-action occurs at a very mild temperature, using water as the solvent andatmospheric oxygen as the oxidant, and this reaction can proceed withan extremely low loading of the Ag(I)catalyst (360 ppm catalyst on agram-scale experiment). More than 50 different kinds of aldehydes weretested, and all underwent transformation to their corresponding carbox-ylic acids in mostly excellent to quantitative yields, indicating a goodversatility and a variety of possible applications for this reaction. Furtherinvestigation of the mechanism and other potential applications of thesilver(I) catalyst is currently under way in our laboratory.

SUPPLEMENTARY MATERIALS

Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/2/e1500020/DC1Materials and Methods

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Acknowledgments: We are grateful to the Canada Research Chair Foundation (to C.-J.L.),the Canadian Foundation for Innovation, FRQNT Centre in Green Chemistry and Catalysis,and the Natural Sciences and Engineering Research Council of Canada for support of ourresearch.

Submitted 8 January 2015Accepted 2 March 2015Published 27 March 201510.1126/sciadv.1500020

Citation: M. Liu, H. Wang, H. Zeng, C.-J. Li, Silver(I) as a widely applicable, homogeneouscatalyst for aerobic oxidation of aldehydes toward carboxylic acids in water—“silver mirror”:From stoichiometric to catalytic. Sci. Adv. 1, e1500020 (2015).

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''silver mirror'': From stoichiometric to catalytic−−toward carboxylic acids in water Silver(I) as a widely applicable, homogeneous catalyst for aerobic oxidation of aldehydes

Mingxin Liu, Haining Wang, Huiying Zeng and Chao-Jun Li

DOI: 10.1126/sciadv.1500020 (2), e1500020.1Sci Adv 

ARTICLE TOOLS http://advances.sciencemag.org/content/1/2/e1500020

MATERIALSSUPPLEMENTARY http://advances.sciencemag.org/content/suppl/2015/03/24/1.2.e1500020.DC1

REFERENCES

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