Novel method of synthesis of quaternary ammonium tribromides and investigation of catalytic role of benzyltrimethylammonium tribromide in oxidation of alcohols to carbonyl compounds

Article abstract of DOI:10.1080/00397911.2012.666694

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. Copyright Taylor & Francis Group, LLC.

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Novel Method of Synthesis of
Quaternary Ammonium Tribromides
and Investigation of Catalytic Role of
Benzyltrimethylammonium Tribromide
in Oxidation of Alcohols to Carbonyl
Compounds
Madhudeepa Dey a , Siddhartha Sankar Dhar a & Mukul Kalita a
a Department of Chemistry, National Institute of Technology, Silchar,
Silchar, Assam, India
Accepted author version posted online: 08 Aug 2012.Version of
record first published: 06 Mar 2013.
To cite this article: Madhudeepa Dey , Siddhartha Sankar Dhar & Mukul Kalita (2013): Novel
Method of Synthesis of Quaternary Ammonium Tribromides and Investigation of Catalytic Role of
Benzyltrimethylammonium Tribromide in Oxidation of Alcohols to Carbonyl Compounds, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
43:12, 1734-1742
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Synthetic Communications1, 43: 1734–1742, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.666694
NOVEL METHOD OF SYNTHESIS OF QUATERNARY
AMMONIUM TRIBROMIDES AND INVESTIGATION OF
CATALYTIC ROLE OF BENZYLTRIMETHYLAMMONIUM
TRIBROMIDE IN OXIDATION OF ALCOHOLS TO
CARBONYL COMPOUNDS
Madhudeepa Dey, Siddhartha Sankar Dhar, and Mukul Kalita
Department 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 bromine
under harsh conditions.[1] However, handling of liquid bromine is cumbersome
because of its hazardous nature, and special care is required for its storage and
transport.[2–6] Moreover reactions carried out with elemental bromine reduce the
atom efficiency to 50%.[7] Environmental legislation against the use of detrimental
chemicals 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
1734

QUATERNARY AMMONIUM TRIBROMIDES
1735
brominations.[8–11] Therefore, some environmentally safer procedures have been
envisioned. Taking cues from various literatures, it is seen that organic ammonium
tribromides constitute a convenient source of bromine and find increasing applications
as substituents for molecular bromine for a variety of organic transformation
reactions.[12–14] Hence, they are regarded as highly efficient, versatile brominating and
oxidizing agents.[15–17] However, most of their preparations invariably involve elemental
bromine 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 have
been documented, including contributions from our own group.[12,15,16] Despite the
plethora 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 synthetic
protocols in organic synthesis, we report herein the syntheses of quaternary
ammonium tribromides (QATBs) using a benign oxidant ammonium persulfate
and application of one of the QATBs viz., BTMATB for carrying out the oxidation
of a number of primary and secondary alcohols.
Ammonium persulfate is relatively strong oxidant in comparison to most
oxidants employed for bringing out organic transformation reactions.[20] It is
inexpensive, stable, and easily handled. Its use as an oxidant is also safe from
environmental point of view, and it is readily commercially available, which makes
ammonium persulfate attractive for carrying out organic syntheses. Moreover, its
use as an oxidant for tribromide syntheses is hitherto unknown, which prompted
us to utilize it for the oxidative transformation of ammonium bromides to
ammonium 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 benzyltrimethylammonium
bromide (BTMAB) together with potassium bromide and ammonium persulfate as
oxidant in the presence of dilute sulfuric acid. A little amount of silica was used as
a supporting agent (Scheme 1). The color of the mixture changed from white to
orange-yellow, indicating the formation of tribromide. The UV-Visible spectrum of
the product showed an intense absorption peak at 279 nm, typical of the tribromide
(BrÀ3 ) anion (Fig. 1).[13] Similar methodology was applied for the preparation of other
tribromides 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 bromides
to 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 acid
required 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

1736
M. DEY, S. S. DHAR, AND M. KALITA
Scheme 1. Oxidation of ammonium bromides to ammonium tribromides using (NH4)2S2O8.
methodologies have been reported in the literature as alternative efficient routes to
accomplish this transformation. In this context, the importance of bromine cataly-
sis in the auto-oxidation for
a variety of industrial processes is well
documented.[33,34] In one of the reported methodologies, liquid bromine is used
for the catalytic oxidation of alcohols to carbonyl compounds with H2O2 as the
oxidant.[35] A recent report showed use of N-methylpyrrodin-2-one hydrotribro-
mide (MPHT) as a catalyst for carrying out the oxidation of alcohols with
H2O2.[36] However, the preparation of MPHT itself requires bromine,[37] which
Figure 1. UV-Visible spectrum of Br3À. (Figure is provided in color online.)
Scheme 2. Proposed mechanism of formation of BrÀ3 from BrÀ using ammonium persulfate.

QUATERNARY AMMONIUM TRIBROMIDES
1737
Table 1. Benzyltrimethylammonium tribromide–catalyzed oxidation of alcohols to carbonyl compoundsa
Entry
1
Substrate
Product
Reaction time (h)
Yield (%)b
1
97
2
3
4
5
6
2
1.75
1.5
2.5
4
88
90
93
72c
78c
7
8
3.5
6
78c
72
9
3
88
85
10
2.5
aReaction conditions: Substrate (1 mmol), BTMATB (10 mol%), aqueous hydrogen peroxide (2 mmol,
30 wt%), acetonitrile (5 mL) at 60 ꢀC.
bIsolated yields.
cExperiment carried out using (50 mol%) of BTMATB in the presence of hydrogen peroxide (2 mmol,
30 wt%) at 60 ꢀC.

1738
M. DEY, S. S. DHAR, AND M. KALITA
does not comply with the norms of green chemistry. In another similar work,
poly(ethylene glycol)–embedded potassium tribromide (PEG Á KBr3) has been used
as 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 making
the protocol unfavorable from environmental viewpoints. This has prompted us to
explore the effectiveness of one of the synthesized tribromides (BTMATB) as cat-
alyst=oxidant in transformation of alcohols to carbonyl compounds in the presence
of H2O2.
The results of oxidation of a variety of alcohols, their reactions conditions, and
yields 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%. Also
dropwise addition of hydrogen peroxide resulted in better yield than addition of all
at a time. During the course of the investigation, it was further observed that if the
reaction was carried out in the absence of the catalyst, no product was isolated. It
may be further mentioned that when oxidation of alcohols was carried out with only
catalytic amount of BTMATB in the absence of the oxidant (30% aqueous hydrogen
peroxide), only a trace amount of the corresponding aldehydes were obtained in some
cases, whereas for others the reaction did not take place at all. Similarly the reaction
of the alcohols with a stoichiometric amount of BTMATB without using hydrogen
peroxide did not occur effectively, indicating that BTMATB=H2O2 is an efficient
catalytic system for this transformation. Acetonitrile proved to be the best solvent
for this oxidative transformation. Among the various benzyl alcohols studied, unsub-
stituted benzyl alcohol (Table 1, entry 1) was found to be more reactive than the
others. Furthermore, it was observed that aromatic-substituted alcohols (Table 1,
entries 1–6) were more reactive than the aliphatic or alicyclic alcohols (Table 1, entries
7–11). Again, alcohols substituted with electron-donating groups (Table 1, entries
2–4) were more reactive than alcohols bearing electron-withdrawing groups, as they
facilitate 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 comparatively
afforded lower yields, the catalyst amount was increased to 50 mol% to obtain
substantial amounts of the product.
Scheme 3. Proposed mechanism for BrÀ3 -catalyzed transformation of alcohols to carbonyl compounds.
(Figure is provided in color online.)

QUATERNARY AMMONIUM TRIBROMIDES
1739
Although the exact mechanism is not comprehensible at this stage, it probably
involves the in situ generation of molecular Br2 from tribromide BrÀ3 , which then
reacts 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 ammonium
persulfate 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 tribromides
synthesis represents an easy and alternative route to the rather hazardous classical
preparation of such reagents.
EXPERIMENTAL
All the alcohols were purchased from Fisher Scientific and used without further
purification. 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-FTIR
1
spectrometer. H NMR spectra were recorded in dimethylsulfoxide (DMSO-d6)=
CDCl3 using tetramethylsilane (TMS) as internal standard on a 400 MHz Varian
spectrometer.
Synthesis of Quaternary Ammonium Tribromides
Typical methodology of synthesis of QATBs involves the grinding of BTMAB
(1 mmol, 0.23 g), KBr (2 mmol, 0.24 g), ammonium persulfate (1 mmol, 0.23 g), 4 N
sulfuric acid (0.1 mL), and a little silica in a motor for ca. 5–10 min 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 ethyl
acetate (5 mL) and concentrated under reduced pressure to get the tribromide in
excellent yield (88%) with high purity. Analytical data for benzyltrimetylammonium
tribromide: 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 (1 mmol) was added to a solution of 5 mL of acetonitrile and magneti-
cally stirred, followed by the addition of BTMATB (10 mol%).Hydrogen peroxide
(30%, 0.3 mL, ꢁ2 mmol) was then added dropwise, and the resulting mixture was
heated at 60 ꢀC. The progress of the reaction was monitored by TLC using ethyl
acetate–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.

1740
M. DEY, S. S. DHAR, AND M. KALITA
The ether layer was dried over anhydrous sodium sulfate and concentrated under
reduced 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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