Synthesis and characterization of 2-carboxyethyltriphenyl phosphonium tribromide and its application as catalyst in silylation of alcohols and thiols under solvent-free condition

Article abstract of DOI:10.1080/00397911.2014.897356

A phosphonium-based catalyst, 2-carboxyethyltriphenyl phosphonium tribromide (CTPTB), has been synthesized by reacting triphenyl phosphine with acrylic acid and potassium bromide under solvent-free condition followed by oxidation of Br^- to with KMnO₄. This hitherto unknown tribromide reagent is characterized by ultraviolet-visible, Fourier transform-infrared, ¹H NMR, and ³¹P NMR spectroscopy. Its efficacy as catalyst is established by investigating its catalytic activity in silylation of alcohols and thiols by hexamethyl disilazane (HMDS). It is found to be a very good catalyst and selectively and efficiently catalyzes the silylation reactions. Copyright

Full text of DOI:10.1080/00397911.2014.897356

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Synthesis and Characterization of 2-
Carboxyethyltriphenyl Phosphonium
Tribromide and Its Application as
Catalyst in Silylation of Alcohols and
Thiols Under Solvent-Free Condition
Rupa Rani Dey a & Siddhartha Sankar Dhar a
a Department of Chemistry , National Institute of Technology ,
Silchar , Assam , India
Accepted author version posted online: 15 Apr 2014.Published
online: 13 Jun 2014.
To cite this article: Rupa Rani Dey & Siddhartha Sankar Dhar (2014) Synthesis and Characterization
of 2-Carboxyethyltriphenyl Phosphonium Tribromide and Its Application as Catalyst in Silylation
of Alcohols and Thiols Under Solvent-Free Condition, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 44:16, 2355-2363, DOI:
10.1080/00397911.2014.897356
To link to this article: http://dx.doi.org/10.1080/00397911.2014.897356
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Synthetic Communications1, 44: 2355–2363, 2014
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2014.897356
SYNTHESIS AND CHARACTERIZATION OF
2-CARBOXYETHYLTRIPHENYL PHOSPHONIUM
TRIBROMIDE AND ITS APPLICATION AS CATALYST IN
SILYLATION OF ALCOHOLS AND THIOLS UNDER
SOLVENT-FREE CONDITION
Rupa Rani Dey and Siddhartha Sankar Dhar
Department of Chemistry, National Institute of Technology, Silchar,
Assam, India
GRAPHICAL ABSTRACT
Abstract A phosphonium-based catalyst, 2-carboxyethyltriphenyl phosphonium tribromide
(CTPTB), has been synthesized by reacting triphenyl phosphine with acrylic acid and
potassium bromide under solvent-free condition followed by oxidation of Brꢀ to Br3 with
ꢀ
KMnO4. This hitherto unknown tribromide reagent is characterized by ultraviolet–visible,
Fourier transform–infrared, 1H NMR, and 31P NMR spectroscopy. Its efficacy as catalyst
is established by investigating its catalytic activity in silylation of alcohols and thiols by
hexamethyl disilazane (HMDS). It is found to be a very good catalyst and selectively
and efficiently catalyzes the silylation reactions.
Keywords Acrylic acid; alcohols; hexamethyl disilazane (HMDS); oxidation; potassium
bromide; silylation; tribromide reagent
INTRODUCTION
The exploration of new reagents and development of green methodologies to
curb environmental hazards have become challenging tasks in synthetic organic
chemistry.[1] Recent years have witnessed a large number of reports on the synthesis
of various reagents=catalysts, especially organic ammonium tribromides (OATBs),
and their application in organic reactions.[2] The most frequently used quaternary
ammonium tribromide reagents are tetrabutyl ammonium tribromide (TBATB),[3]
cetyltriethyl ammonium tribromide (CTETB),[4] 1,2-dipyridinium ditribromide
(DPTBE),[5] N-methylpyrrolidine-2-one hydrotribromide (MPHT),[6] quinoline
Received November 4, 2013.
Address correspondence to S. S. Dhar, Department of Chemistry, National Institute of
Technology, Silchar, Assam 788010, India. E-mail: ssd_iitg@hotmail.com
Color versions of one or more of the figures in the article can be found online at www.tandfonline.
com/lsyc.
2355

2356
R. R. DEY AND S. S. DHAR
hydrotribromide (QHTB),[7] benzyltriphenyl phosphonium tribromide,[8] tridecyl-
methyl phosphonium tribromide,[9] ethyltriphenyl phosphonium tribromide,[10] and
crown ether entrapped tribromide.[11] These are preferable as reagents=catalysts
because they are highly efficient, less hazardous, easy to handle, and usually give
good yields and selectivity of products. As a part of our continuous endeavour
to develop new green protocols for synthesis of organic ammonium tribromides as well
as biologically important compounds,[12] we report herein the synthesis and charac-
terization of a new tribromide compound, 2-carboxyethyltriphenyl phosphonium
tribromide (CTPTB). With successful synthesis of this new reagent, it becomes
imperative to study the efficiency of this compound as a catalyst in the silylation
of alcohols and thiols. It may be emphasized that protection of reactive hydroxyl
groups are indispensible in multistep reactions for the synthesis of polyfunctional
compounds. One of the most common and easy methods for protection of hydroxyl
and thiol groups is silylation by converting them into corresponding silyl ethers.[13]
The reagents that are used in the traditional and conventional processes are
[15]
R3SiCl,[14] MeSiN C(Me)OSiMe,
Me3SiCl, or Me3SiOTf in the presence of
=
base.[16] However, many of these methods have demerits, such as lack of reactivity,
long reaction times, and difficult-to-remove by-products and co-bases during
the reaction.[17] HMDS is commercially available, cheap, noncorrosive, and easy
to handle for the formation of trimethyl silyl ether, which generates only ammonia
gas as by-product.[18] A variety of catalysts, such as Cu(SO4) ꢁ 5H2O, Al(OTf)3,
trichloroiso cyanuric acid, and I2, have been already reported efficient synthesis
of silylated products.[18(b),19] However, no report on solvent-free silylation using
CTPTB as a catalyst has been observed. The solvent-free condition is preferrable
for environmental issues and also for obtaning good yields and pure products.[20]
RESULTS AND DISCUSSION
The present protocol is an easy and clean synthesis of 2-carboxyethyltriphenyl
phosphonium tribromide under solvent-free condition. The crystalline compound
is stable for several days at room temperature without decomposition. The reaction
involves the nucleophilic attack of triphenyl phosphine[21] on b-carbon of acrylic acid
to generate an ylide I. In the next step, KMnO4 serves as good oxidant[12(a)]
in the presence of sulfuric acid to convert Brꢀ to Br3ꢀ. The plausible mechanism
is depicted in Scheme 1.
The formation of tribromide was confirmed by electronic absorption
spectroscopy and FT-IR technique (Scheme 1). An intense band at 282 nm (Fig. 1)
is characteristic of tribromide anion (Br3ꢀ).[22] The infrared (IR) spectra of CTPTB
(Fig. 2) exhibits characteristic (Br3ꢀ) bands at 44 (n1) and 188 (n2) cmꢀ1 for bending
Scheme 1. Reaction showing the formation of 2-carboxyethyltriphenyl phosphonium tribromide.

2-CARBOXYETHYLTRIPHENYL PHOSPHONIUM TRIBROMIDE
2357
Figure 1. UV-vis spectra of CTPTB.
and asymmetric stretching, respectively.[23] The tribromide gives two peaks, one sing-
1
let (d ¼ 29.18) and one triplet (d ¼ 22.55, 2H), in its 31P f Hg NMR spectrum, which
is consistent with literature information.[24] Parish et al. found that PPh3 shows sing-
let with d values ꢀ7. However tetra-phenylphosphonium cations Ph3PHþ and Ph4Pþ
give signals at d ꢀ 1 ppm and d þ 21 ppm respectively.[25] The downfield shift (to
d ¼ 23) in the synthesized tribromide is assumed to be for the fourth acrylic acid
group attached to phosphorous atom.
The silylation of ROH and RSH is investigated under solvent-free conditions
by using HMDS and a catalytic amount of CTPTB (Scheme 2). A variety of com-
pounds were chosen for this purpose (Table 1). These include aromatic, aliphatic,
and alicyclic alcohols and thiols. The silylated products were characterized by
comparing their melting points, FTIR, and 1H NMR spectra with those of authentic
Figure 2. FT-IR spectra of CTPTB.

2358
R. R. DEY AND S. S. DHAR
Scheme 2. Silylation of alcohols and thiols.
Table 1. Silylation of alcohols and thiols by using HMDS and catalytic amount of CTPTB
Catalyst amount Time Yieldc,d
Entry
Substratea
Productsb
(mmol)
(min)
(%)
1
2
0
45
65
0.1
4
87
3
4
5
0.1
0.1
0.1
10
5
80
81
90
6
6
7
0.1
0.1
4
7
85
75
8
9
0.1
0.1
7
4
75
78
10
0
30
67
11
12
0.1
0.1
8
8
80
81
(Continued )

2-CARBOXYETHYLTRIPHENYL PHOSPHONIUM TRIBROMIDE
2359
Table 1. Continued
Catalyst amount Time Yieldc,d
Entry
13
Substratea
Productsb
(mmol)
(min)
(%)
0.1
6
68
14
0.1
7
70
aAlcohol=thiols (20 mmol), HMDS (10 mmol), CTPTB (0.1 mmol).
bProducts were identified by their IR and NMR spectral data and comparing them with published data.
cIsolated yield.
dReactions were monitored by TLC and products were extracted by ethyl acetate.
Scheme 3. Catalytic loop showing the plausible mechanism of silylation by CTPTB.
samples.[26] The plausible mechanism is represented using a catalytic cycle (Scheme 3)
in which the function of CTPTB and the evolution of ammonia have been depicted.
It is believed that intermediate II reacts with ROH and=or RSH to produce Br2 and
NH3 alongside the silylated products. The Br2 thus produced reacts with tetraalkyl
phosphonium bromide to form the CTPTB. The reactions, carried out without
a catalyst, taking butanol and thiol as substrate (Table 1), also resulted in the
formation of silylated products; however, such noncatalytic reactions were observed
to give poor yield in relatively long reaction time.[27]
CONCLUSION
In conclusion, a new phosphonium-based tribromide reagent, 2-carboxytriphenyl
phosphonium tribromide, has been synthesized by reacting triphenyl phosphine with

2360
R. R. DEY AND S. S. DHAR
acrylic acid and KBr in presence of mildly acidic KMnO4 solution. The compound
is found to be good and efficient catalyst for silylation of alcohols and thiols by
HMDS under solvent-free condition. The method is simple, mild, nonhazardous,
and less time-consuming. Although several methods are known for the protection
of hydroxyl and thiol groups via silyaltion, the present methodology offers a new
and useful alternative for such reactions.
EXPERIMENTAL
All the chemicals are of analytical grade and used without further purification.
The completion of reaction was monitored by thin-layer chromatography (TLC).
The synthesized tribromide was charaterized with UV-visible, FT-IR, and 31P
NMR spectroscopy. The IR spectra were recorded on KBr with Magna 550 FTIR
spectrometer. NMR spectra were recorded in CDCl3 on Mercury Plus 400-MHz NMR
spectrometer.
Synthesis of CTPTB
In a typical procedure, CTPTB was synthesized by mixing 2.62 g triphenyl
phosphine (10 mmol) and 1 ml acrylic acid (10 mmol) in a magnetic stirrer for
10 min. To the mixture 3.57 g KBr (30 mmol) and 1.58 g of KMnO4 (10 mmol)
in 10 ml of 4 N H2SO4 solution were added. The whole mixture was again stirred
for 10 min until an orange compound was formed. The product was extracted with
ethylacetate (2 ꢂ 5 ml) and then dried in vacuum. The crude product was recry-
stallized to get better yield with high purity; yield 85%, mp: 138–140 ꢃC, UV-vis:
282 nm; FT-IR: 3779, 2957, 2667, 1563, 1374, 1110, 883, 740, 188, 44 cmꢀ1
;
31P NMR d: 29.17, 22.8–23.2; 1H NMR d: 10.7 (s, 1H), 7.2–7.4 (m, 15H), 2.5
(t, 2H), 1.3 (t, 2H).
Representative Procedure for Silylation of Alcohols and Thiols
Silylation of butanol. In a representative procedure, 20 mmol of butanol was
mixed with a catalytic amount of CTPTB (0.1 mmol) and a little amount of silica gel
in a mortar. To the mixture, 10 mmol of hexamethyl disilazane was added drop by
drop and ground well for 4 min at room temperature until the product formed. After
monitoring the completion of reaction by TLC, the products were extracted
with ethyl acetate (2 ꢂ 5 ml) and evaporated under vacuum to obtain pure butylox-
ytrimethylsilane. Yield: 87%; FT-IR: 2960, 2765, 1483, 1291, 1168, 1082, 948,
1
687 cmꢀ1; H NMR (400 MHz, CDCl3) d: 0.09 (s, 9H), 0.92 (t, 3H), 1.35–1.45 (m,
4H), 3.87 (m, 2H).
Silylation of thiophenol. In a representative procedure, 20 mmol of thiophe-
nol was mixed with a catalytic amount of CTPTB (0.1 mmol, 0.05 g) and a little
amount of silica gel in a mortar. To the mixture, 10 mmol of hexamethyl disilazane
was added drop by drop and ground well for 6 min at room temperature. The
completion of the reaction was monitored by TLC. After the completion, the pro-
ducts were extracted with ethyl acetate and evaporated under vacuum to obtain pure

2-CARBOXYETHYLTRIPHENYL PHOSPHONIUM TRIBROMIDE
2361
1
phenylthiotrimethylsilane. Yield: 68%; H NMR (400 MHz, CDCl3) d: 0.09 (s, 9H),
1
7.2–7.1 (m, 5H). FT-IR: 3286, 2578, 1579, 1442, 1025, 734, 689 cmꢀ1; H NMR
(400 MHz, CDCl3) d: 0.09 (s, 9H), 7.2–7.1 (m, 5H).
ACKNOWLEDGMENT
The authors are thankful to the Sophisticated Analytical Instrument Facility,
Indian Istitute of Technology, Bombay, for providing the spectral analyses of the
compounds.
FUNDING
R. D. acknowledges NIT Silchar for financial assistance through a GATE
fellowship.
SUPPORTING INFORMATION
1
Full experimental details and data of FT-IR and H NMR spectra of silylated
products can be accessed on the publisher’s website.
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