DOI: 10.1002/adsc.200505166

Silica-Supported Vanadium-Catalyzed N-Oxidation of TertiaryAmines with Aqueous Hydrogen Peroxide

Laxmidhar Rout, Tharmalingam Punniyamurthy*Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, IndiaFax: (þ91)-361-269-0762, e-mail: tpunni@iitg.ernet.in

Received: April 21, 2005; Accepted: August 30, 2005

Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.

Abstract: A recyclable silica supported vanadium 1catalyzes the oxidation of tertiray amines to the cor-responding N-oxides with 30% H2O2 in high yield.

Keywords: amine N-oxides; hydrogen peroxide; oxi-dation; recyclable catalyst; supported catalysts; terti-ary amines

Amine N-oxides hold a key position in the chemistry ofheterocycles offering functional group manipulationand structuralmodification possibilities which are not ac-cessiblebyanyothermethods.[1]Oxidation reactions suchas osmium-catalyzed dihydroxylation of alkenes,[2a]

ruthenium-catalyzed oxidation of alcohols[2b] and Mn-salen-catalyzed oxidation of hydroxylamines[2c] employamine N-oxides either as the stoichiometric oxidant oran additive to complete the catalytic cycle. Amine N-ox-ides are alsomajor components for ubiquitously usedma-terials such as toilet soaps, toothpastes, detergent pow-ders, shampoos and cosmetics aswell as in biomedical ap-plications.[3] These oxides are usually prepared via a non-catalytic oxidation process using activatedH2O2,

[4a]a-azohydroperoxides,[4b] CaroAs acid (H2SO5),

[4c] dioxiranes,[4d]

magnesium monophthalate[4e] and peracids.[4f] Most ofthese reagents are not only expensive, but also generatelarge amounts of effluent during the reaction processand demand a laborious work-up procedure. From anenvironmental andeconomic standpoint, catalyticoxida-tion processes are thus valuable and those employingatom-efficient oxidants such as aqueous H2O2, which isless expensive and generates water as the only by-prod-uct, are particularly attractive. Accordingly, methyl-trioxorhenium,[5a,b] manganese porphyrin,[5c] flavin,[5d,e]

TS-1,[5f] tungstate-exchanged Mg/Al-layered doublehydroxide acid,[5g] tungsten sulfide,[5h] tungsten-basedpolyperoxometalates,[5i] selenium[5j] and molybdenum[5k]

oxides have been shown to catalyze the N-oxidation oftertiary amines in the presence of aqueous H2O2.

The use of heterogeneous catalysts in the liquid phaseoffers several advantages over homogeneous ones, suchas ease of recovery and recycling, atom utility and en-hanced stability.[6]During the course of our investigationon the oxidation of organic compounds,[6c,7] we havefound that the readily accessible VxSi4xO6.4x (1) is recy-clable and catalyzes efficiently the oxidation of tertiaryamines to the correspondingN-oxideswith 30%H2O2 inhigh yields (Scheme 1). This is a simple and clean tech-nology process where compounds are formed freefrom any contaminating by-products. Aromatic as wellas aliphatic amines could be oxidized to the correspond-ing N-oxides.Preparation of the catalyst, VxSi4xO6.4x (1), was carried

out by stirring a 1 :1 (w/w) mixture of silica gel (Davis

grade, 600 mesh) and VO(acac)2 in dichloromethaneat ambient temperature for 24 h. Scanning electron mi-croscopic (SEM) and high resolution optical microscop-ic (OM) analysis of 1 indicated that the particles with anaverage size of 37.24 mm are uniformly dispersed.[8]

Electron diffractionX-ray fluorescence (EDXRF) anal-ysis showed the composition of the catalyst 1 to be in ac-cord with the formulaVxSi4xO6.4x.

[9] X-Ray diffractionanalysis indicated the amorphous nature[10] of the cata-lyst 1. Magnetic susceptibility measurements of 1[11] re-corded within the temperature range 80–293 K showedthe oxidation state of vanadium as þ4. The FT-IR spec-trum of 1 showed no peaks at 1200–1600 cm�1 indicat-ing the absence of acetylacetone in the catalyst. The pro-posed structure of the catalyst 1 is shown in Figure 1.[12]

Scheme 1.

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1958 G 2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Adv. Synth. Catal. 2005, 347, 1958 – 1960

The catalyst 1 was then investigated for the oxidationof triethylamine in the presence of 30%H2O2. We werepleased to find that the oxidation occurred to afford tri-ethylamineN-oxide in quantitative yield when the reac-tion mixture was allowed to stir in the presence of3.7 mol % of 1 and 3 equivs. of 30% H2O2 at 80 8C for4 h in acetonitrile.[13] A control experiment without thecatalyst 1 showed no reaction. Solvents such as dichloro-methane, toluene andmethanol were also employed butthey were less effective compared to acetonitrile.To study the scope of this procedure, the oxidation of

other amines was studied next (Table 1). N,N-Diethyl-

aniline was oxidized to the corresponding N-oxide inhigh yield. Similarly, pyridine, 3-methylpyridine, 3-cya-nopyridine, pyridine-3-carboxylic acid amide, quinolineand 8-hydroxyquinoline could be oxidized to the respec-tiveN-oxides. In the case of bipyridine, both theN atomswere oxidized. These results clearly reveal that the pres-ent method could be applied for the oxidation a varietyof tertiary amines with 30% H2O2.The catalyst 1 could be filtered and recycled without

loss of activity. After completion of the oxidation of tri-ethylamine, the catalyst 1 was filtered and the filtrate[13]

was concentrated on a rotary evaporator to provide aresidue, which was passed through a short pad of neutralsilica gel usingmethanol and dichloromethane as eluentto afford analytically pure triethylamine oxide. The fil-tered catalyst was reused for the oxidation of triethyl-amine in the presence of fresh 30% H2O2 for threeruns and no loss of activity was observed providing thecorresponding N-oxide in high yield. Furthermore, thecatalyst 1 was heated in acetonitrile at 80 8C for 3 hand the solid was filtered. The filtrate was used for theoxidation of triethylamine in the presence of 3 equivs.of 30% H2O2 and no reaction was observed. This indi-cates that the active ingredient has not leached out ofthe solid catalyst during the reaction.In conclusion, theN-oxidationof tertiary amines is de-

scribedusingVxSi4xO6.4x (1) in the presence of 30%H2O2

in high yield. It is a clean process and the catalyst 1 is re-cyclable without loss of activity.

Experimental Section

Preparation of the Catalyst 1

A 1:1 mixture (w/w) of VO(acac)2 (100 mg) and silica gel(100 mg, Devis grade 600 mesh) was stirred in dichlorome-thane (5 mL) for 24 h at ambient temperature. The supportedcatalyst, VxSi4xO6.4x (1), was filtered and washed with dichloro-methane (3�5 mL) to afford a green-colored solid; yield:127.8 mg.

Typical Procedure for Oxidation

Amine (1 mmol), catalyst (3.7 mol %, 10 mg) and 30% H2O2

(3 mmol) were stirred in acetonitrile (2 mL) at 80 8C for the ap-propriate time (Table 1). After completion of the reaction, thecatalyst was filtered and the filtrate was concentrated under re-duced pressure to afford a residue, which was passed through ashort pad of neutral silica gel (60–120 mesh) using methanoland dichloromethane as eluent to provide analytically pureamine oxide.[13] The catalyst could be recycled without loss ofactivity. The amineN-oxideswere characterized by IR,meltingpoint, mass and elemental analysis.

Figure 1. Proposed structure for VxSi4xO6.4x (1).

Table 1. Silica-supported vanadium/1-catalyzed oxidation oftertiary amines with 30% H2O2.

[a]

[a] Amine (1 mmol), catalyst 1 (3.7 mol %, 10 mg) and 30%H2O2 (3 mmol) were stirred at 80 8C in acetonitrile(2 mL).

[b] Yield of isolated product.[c] 6 mmol H2O2 used.

Silica-Supported Vanadium-Catalyzed N-Oxidation of Tertiary Amines COMMUNICATIONS

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Acknowledgements

This workwas supported byCouncil of Scientific and IndustrialResearch (Sanction No. 01(1804)/02/EMR-II), New Delhi.

References and Notes

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[13] After completion of the oxidation, the reaction mixturewas analyzed by titrating with KMnO4 and no H2O2

was found.[14] This indicates that the excess H2O2 is de-composed.

[14] Vogel8s Textbook for Quantitative Inorganic Analysis,(Eds.: J. Mendham, R. C. Denney, J. D. Barnes, M. J. K.Thomas), Pearson Education, Singapore, 6th edn.,2004, pp. 420–421.

COMMUNICATIONS Laxmidhar Rout, Tharmalingam Punniyamurthy*

1960 asc.wiley-vch.de G 2005 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim Adv. Synth. Catal. 2005, 347, 1958 – 1960