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Novel Method of Synthesis ofQuaternary Ammonium Tribromidesand Investigation of Catalytic Role ofBenzyltrimethylammonium Tribromidein Oxidation of Alcohols to CarbonylCompoundsMadhudeepa Dey a , Siddhartha Sankar Dhar a & Mukul Kalita aa Department of Chemistry, National Institute of Technology, Silchar,Silchar, Assam, IndiaAccepted author version posted online: 08 Aug 2012.Version ofrecord first published: 06 Mar 2013.

To cite this article: Madhudeepa Dey , Siddhartha Sankar Dhar & Mukul Kalita (2013): NovelMethod of Synthesis of Quaternary Ammonium Tribromides and Investigation of Catalytic Role ofBenzyltrimethylammonium Tribromide in Oxidation of Alcohols to Carbonyl Compounds, SyntheticCommunications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,43:12, 1734-1742

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NOVEL METHOD OF SYNTHESIS OF QUATERNARYAMMONIUM TRIBROMIDES AND INVESTIGATION OFCATALYTIC ROLE OF BENZYLTRIMETHYLAMMONIUMTRIBROMIDE IN OXIDATION OF ALCOHOLS TOCARBONYL COMPOUNDS

Madhudeepa Dey, Siddhartha Sankar Dhar, and Mukul KalitaDepartment of Chemistry, National Institute of Technology, Silchar,Silchar, Assam, India

GRAPHICAL ABSTRACT

Abstract Stable crystalline organic quaternary ammonium tribromides (QATBs) have been

easily synthesized by the oxidation of the corresponding organic ammonium bromides

(QABs) with ammonium persulfate. The reactions have been performed under solvent-free

conditions in the presence of sulfuric acid and silica as supporting agent. Two equivalents

of potassium bromide have been used as the source of additional bromides for quantitative

conversion of QABs to QATBs. Ammonium persulfate, a cheap and readily available oxidant,

carries out the bromide oxidation to tribromide very effectively under solvent-free conditions.

The synthesized QATBs have been shown to catalyze the oxidation of alcohols to carbonyl

compounds with hydrogen peroxide as oxidant in good yields under mild reaction conditions.

Keywords Ammonium persulfate; green synthesis; organic ammonium tribromides;

oxidation

INTRODUCTION

Classical bromination involves the direct or indirect use of elemental bromineunder harsh conditions.[1] However, handling of liquid bromine is cumbersomebecause of its hazardous nature, and special care is required for its storage andtransport.[2–6] Moreover reactions carried out with elemental bromine reduce theatom efficiency to 50%.[7] Environmental legislation against the use of detrimentalchemicals and solvents causes major concern for the use of liquid bromine in classical

Received January 11, 2012.

Address correspondence to S. S. Dhar, NIT Silchar, Silchar 788010, Assam, India. E-mail: ssd_

iitg@hotmail.com

Synthetic Communications1, 43: 1734–1742, 2013

Copyright # Taylor & Francis Group, LLC

ISSN: 0039-7911 print=1532-2432 online

DOI: 10.1080/00397911.2012.666694

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brominations.[8–11] Therefore, some environmentally safer procedures have beenenvisioned. Taking cues from various literatures, it is seen that organic ammoniumtribromides constitute a convenient source of bromine and find increasing applicationsas substituents for molecular bromine for a variety of organic transformationreactions.[12–14] Hence, they are regarded as highly efficient, versatile brominating andoxidizing agents.[15–17] However, most of their preparations invariably involve elementalbromine and in some cases HBr, which leads to environmental pollution.[12–14,18,19]

In the recent past, a few environmentally benign syntheses of tribromides havebeen documented, including contributions from our own group.[12,15,16] Despite theplethora of methods available in the literature, the search for a simple, convenient,and environmentally safer synthesis of tribromides is still a challenge. Therefore,as part of our research interest aimed towards the development of greener syntheticprotocols in organic synthesis, we report herein the syntheses of quaternaryammonium tribromides (QATBs) using a benign oxidant ammonium persulfateand application of one of the QATBs viz., BTMATB for carrying out the oxidationof a number of primary and secondary alcohols.

Ammonium persulfate is relatively strong oxidant in comparison to mostoxidants employed for bringing out organic transformation reactions.[20] It isinexpensive, stable, and easily handled. Its use as an oxidant is also safe fromenvironmental point of view, and it is readily commercially available, which makesammonium persulfate attractive for carrying out organic syntheses. Moreover, itsuse as an oxidant for tribromide syntheses is hitherto unknown, which promptedus to utilize it for the oxidative transformation of ammonium bromides toammonium tribromides. The most common application of this reagent in the chemi-cal industry is for bleaching, for alkene oxidation[21] and substituted aromatics,[22]

for cyclization, and for deoximation of steroidal oximes,[23] but its general applica-bility in organic syntheses has largely been unexplored.[24–26]

RESULTS AND DISCUSSION

The syntheses of QATBs involve the grinding of benzyltrimethylammoniumbromide (BTMAB) together with potassium bromide and ammonium persulfate asoxidant in the presence of dilute sulfuric acid. A little amount of silica was used asa supporting agent (Scheme 1). The color of the mixture changed from white toorange-yellow, indicating the formation of tribromide. The UV-Visible spectrum ofthe product showed an intense absorption peak at 279 nm, typical of the tribromide(Br�3 ) anion (Fig. 1).[13] Similar methodology was applied for the preparation of othertribromides viz., tetrabutylammonium tribromide TBATB (87%), cetyltrimethylam-monium tribromide (CTMATB) (85%), tetramethylammonium tribromide (TMATB)(81%), and tetraethylammonium tribromide (TEATB) (87%).

The plausible mechanistic pathway for the oxidative transformation of bromidesto tribromides by persulfate under acidic condition may be rationalized from the reac-tion shown in Scheme 2. It is evident from the reaction that the amount of sulfuric acidrequired is twice the amount of the oxidant used for carrying out the reaction.

Selective oxidation of alcohols is a fundamental reaction in organic chemis-try.[27–29] A variety of transition-metal-based[30–32] as well as metal-free oxidation

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methodologies have been reported in the literature as alternative efficient routes toaccomplish this transformation. In this context, the importance of bromine cataly-sis in the auto-oxidation for a variety of industrial processes is welldocumented.[33,34] In one of the reported methodologies, liquid bromine is usedfor the catalytic oxidation of alcohols to carbonyl compounds with H2O2 as theoxidant.[35] A recent report showed use of N-methylpyrrodin-2-one hydrotribro-mide (MPHT) as a catalyst for carrying out the oxidation of alcohols withH2O2.

[36] However, the preparation of MPHT itself requires bromine,[37] which

Scheme 2. Proposed mechanism of formation of Br�3 from Br� using ammonium persulfate.

Figure 1. UV-Visible spectrum of Br�3 . (Figure is provided in color online.)

Scheme 1. Oxidation of ammonium bromides to ammonium tribromides using (NH4)2S2O8.

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Table 1. Benzyltrimethylammonium tribromide–catalyzed oxidation of alcohols to carbonyl compoundsa

Entry Substrate Product Reaction time (h) Yield (%)b

1 1 97

2 2 88

3 1.75 90

4 1.5 93

5 2.5 72c

6 4 78c

7 3.5 78c

8 6 72

9 3 88

10 2.5 85

aReaction conditions: Substrate (1mmol), BTMATB (10mol%), aqueous hydrogen peroxide (2mmol,

30wt%), acetonitrile (5mL) at 60 �C.bIsolated yields.cExperiment carried out using (50mol%) of BTMATB in the presence of hydrogen peroxide (2mmol,

30wt%) at 60 �C.

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does not comply with the norms of green chemistry. In another similar work,poly(ethylene glycol)–embedded potassium tribromide (PEG �KBr3) has been usedas a catalyst for oxidation of alcohols,[38] but according to this protocol the regen-eration of the catalyst involves the addition of molecular bromine, thereby makingthe protocol unfavorable from environmental viewpoints. This has prompted us toexplore the effectiveness of one of the synthesized tribromides (BTMATB) as cat-alyst=oxidant in transformation of alcohols to carbonyl compounds in the presenceof H2O2.

The results of oxidation of a variety of alcohols, their reactions conditions, andyields are summarized in Table 1.

When the oxidation of alcohols was carried out at room temperature, the reac-tion took longer to complete and the yield of the product was very low. For example,yield of the product for oxidation of 4-methoxybenzylalcohol was only �20%. Alsodropwise addition of hydrogen peroxide resulted in better yield than addition of allat a time. During the course of the investigation, it was further observed that if thereaction was carried out in the absence of the catalyst, no product was isolated. Itmay be further mentioned that when oxidation of alcohols was carried out with onlycatalytic amount of BTMATB in the absence of the oxidant (30% aqueous hydrogenperoxide), only a trace amount of the corresponding aldehydes were obtained in somecases, whereas for others the reaction did not take place at all. Similarly the reactionof the alcohols with a stoichiometric amount of BTMATB without using hydrogenperoxide did not occur effectively, indicating that BTMATB=H2O2 is an efficientcatalytic system for this transformation. Acetonitrile proved to be the best solventfor this oxidative transformation. Among the various benzyl alcohols studied, unsub-stituted benzyl alcohol (Table 1, entry 1) was found to be more reactive than theothers. Furthermore, it was observed that aromatic-substituted alcohols (Table 1,entries 1–6) were more reactive than the aliphatic or alicyclic alcohols (Table 1, entries7–11). Again, alcohols substituted with electron-donating groups (Table 1, entries2–4) were more reactive than alcohols bearing electron-withdrawing groups, as theyfacilitate attack of electrophile on the alcoholic carbon (Table 1, entries 5 and 6).However, for some of the substrates (Table 1, entries 5–7) that comparativelyafforded lower yields, the catalyst amount was increased to 50mol% to obtainsubstantial amounts of the product.

Scheme 3. Proposed mechanism for Br�3 -catalyzed transformation of alcohols to carbonyl compounds.

(Figure is provided in color online.)

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Although the exact mechanism is not comprehensible at this stage, it probablyinvolves the in situ generation of molecular Br2 from tribromide Br�3 , which thenreacts with hydrogen peroxide to give hypobromous acid. The generated hypobro-mous acid subsequently reacts with the alcoholic group to form the reactive hypo-bromite species, which finally gets converted to the desired carbonyl compound.The proposed mechanism is depicted in Scheme 3.

CONCLUSION

Thus in conclusion, a simple and useful method has been devised for the syn-thesis of QATBs from quaternary ammonium bromides by employing ammoniumpersulfate as oxidant under very mild and environmentally safe reaction conditions.Its role as an effective catalyst has been utilized in the oxidation of alcohols to car-bonyl compounds. This methodology for the quaternary ammonium tribromidessynthesis represents an easy and alternative route to the rather hazardous classicalpreparation of such reagents.

EXPERIMENTAL

All the alcohols were purchased from Fisher Scientific and used without furtherpurification. Melting points were determined in open capillaries and are uncorrected.The completion of the reaction was monitored by thin-layer chromatography (TLC).Infrared (IR) spectra were recorded on KBr matrix with Perkin-Elmer BX-FTIRspectrometer. 1H NMR spectra were recorded in dimethylsulfoxide (DMSO-d6)=CDCl3 using tetramethylsilane (TMS) as internal standard on a 400MHz Varianspectrometer.

Synthesis of Quaternary Ammonium Tribromides

Typical methodology of synthesis of QATBs involves the grinding of BTMAB(1mmol, 0.23 g), KBr (2mmol, 0.24 g), ammonium persulfate (1mmol, 0.23 g), 4Nsulfuric acid (0.1mL), and a little silica in a motor for ca. 5–10min at room tempera-ture (Scheme 1). The color of the mixture changed from white to orange-yellow,indicating the formation of the tribromide salt. The product was extracted with ethylacetate (5mL) and concentrated under reduced pressure to get the tribromide inexcellent yield (88%) with high purity. Analytical data for benzyltrimetylammoniumtribromide: C10H16NBr3, mp: 99 �C, lit.[15](99–101 �C), Calc: C, 30.8; H, 4.14; N,3.59%. Found: C, 30.6; H, 4.19; N, 3.61%.

General Procedure for the Oxidation of Alcohols

Substrate (1mmol) was added to a solution of 5mL of acetonitrile and magneti-cally stirred, followed by the addition of BTMATB (10mol%).Hydrogen peroxide(30%, 0.3mL, �2mmol) was then added dropwise, and the resulting mixture washeated at 60 �C. The progress of the reaction was monitored by TLC using ethylacetate–hexane (6:4) as eluent. After the completion of the reaction, the reaction mix-ture was extracted with diethyl ether to get the corresponding carbonyl compound.

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The ether layer was dried over anhydrous sodium sulfate and concentrated underreduced pressure to give the desired oxidized products.

ACKNOWLEDGMENT

S. S. Dhar acknowledges the Department of Science and Technology (DST),New Delhi, for funding the work through DST-SERC Fast Track Project No.SR=FTP=CS-100=2007.

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