Synthesis and reactivity studies of a new reagent, ethyltriphenylphosphonium tribromide

Article abstract of DOI:10.1080/00397910903531912

A new reagent, ethyltriphenyl phosphonium tribromide (ETPPTB), has been synthesized and studied. Results show that the reagent is quite efficient for various reactions such as organic bominations, acylations, and isothiocyanate preparation. Copyright

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Synthesis and Reactivity
Studies of a New Reagent,
Ethyltriphenylphosphonium Tribromide
Latonglila Jamir a , B. Alimenla a , Anil Kumar a , Dipak Sinha a
Upasana B. Sinha a
&
a Department of Chemistry, Nagaland University, Nagaland, India
Version of record first published: 14 Dec 2010.
To cite this article: Latonglila Jamir, B. Alimenla, Anil Kumar, Dipak Sinha & Upasana B. Sinha (2010):
Synthesis and Reactivity Studies of a New Reagent, Ethyltriphenylphosphonium Tribromide, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
41:1, 147-155
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Synthetic Communications1, 41: 147–155, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903531912
SYNTHESIS AND REACTIVITY STUDIES OF A NEW
REAGENT, ETHYLTRIPHENYLPHOSPHONIUM
TRIBROMIDE
Latonglila Jamir, B. Alimenla, Anil Kumar, Dipak Sinha, and
Upasana B. Sinha
Department of Chemistry, Nagaland University, Nagaland, India
A new reagent, ethyltriphenyl phosphonium tribromide (ETPPTB), has been synthesized
and studied. Results show that the reagent is quite efficient for various reactions such as
organic bominations, acylations, and isothiocyanate preparation.
Keywords: Acylation; bromination; desulfurization; dithiacarbamate; ethyltriphenyl phosphonium tribro-
mide; isothiocyanates
INTRODUCTION
Organic phosphonium tribromides are the structural analogs of organic
ammonium tribromides, with expectedly similar properties.[1] However, while organic
ammonium tribromides have become very popular in recent years and a number of
reports are available discussing the importance of these reagents in various types of
organic transformations,[2] unfortunately the phosphonium tribromides do not seem
to have received a similar kind of attention as yet. Nevertheless, these reagents merit
an amount of investigation, especially because they are reported to have milder reac-
tivity than organic ammonium tribromides.[3] Accordingly, an attempt was made to
synthesize one such phosphonium tribromide and then study its reactivity profile.
Thus, through this article we report the first synthesis of ethyltriphenyl phosphonium
tribromide (ETPPTB), its crystal structure,[4] and its efficacy in various reactions such
as bromination reactions, acylation reactions, and the synthesis of isothiocyanates.
RESULTS AND DISCUSSION
Ethyl triphenylphosphine tribromide (ETPPTB), molecular formula C20H20PBr3,
is a deep orange, solid crystalline compound that melts at 135 ꢀC. It is soluble in
polar aprotic solvents such as acetonitrile and dimethylformamide (DMF) as well
as nonpolar aprotic solvents such as chloroform and dichloromethane (DCM) and
is sparingly soluble in polar protic solvents such as ethanol and methanol. It is
Received September 29, 2009.
Address correspondence to Upasana B. Sinha, Department of Chemistry, Nagaland University,
Lumami Campus 798601, Nagaland, India. E-mail: u.borasinha@gmail.com
147

148
L. JAMIR ET AL.
Figure 1. ORTEP view of ETPPTB with atom numbering scheme.
Table 1. Bromination reactions with ethyl triphenyl phosphonium tribromidea
Entry
1
Substrate
Time (min)
Productb
Yield (%)c
78
20
2
3
4
5
6
15
15
15
35
25
81
84
61
73
76
7
8
35
15
20
45
58
68
76
65
9
10
aReactions were monitored by TLC.
bProducts were characterized by IR, 1H NMR, and 13C NMR.
cIsolated yield.

ETPPTB AND ITS REACTIVITY
149
nonhygroscopic, air stable, and has a prolonged shelf life at room temperature
without loss of activity. The crystal structure of the compound reveals a monocli-
nic structure with space group P2(1)=n and an independent, nearly symmetric, and
linear tribromide ion and an independent ethyltriphenylphosphonium ion. The tri-
˚
˚
bromide ion has bond lengths of 2.525 A (Br1–Br2) and 2.563 A (Br1–Br3) and a
˚
bond angle of 178.43 A (Fig. 1).
To study the versatility of ETPPTB as brominating agent, representative exam-
ples of different types of organic substrates were taken (Table 1). It was observed
that bromination reactions were quite facile, and the products were obtained in mod-
erate to excellent yields. The results have been summarized in Table 1. The products
were identified by comparison of their melting points, infrared (IR) absorption, and
NMR spectra with the authentic samples.[5]
Acylations of protic nucleophiles such as alcohols, amines, and thiols are
important in synthetic organic chemistry because the resulting esters, amides,
and thioesters serve as important functional components and=or intermediates in
synthetic chemistry.[6] Thus, some acylation reactions were attempted (Table 2).
The results were encouraging, revealing that ETPPTB can act well as an acylating
reagent.
Table 2. Acylation of alcohols with ethyl triphenyl phosphonium tribromidea
Entry
Substrate
Time (min)
Productb
Yield (%)c
1
2
15
15
82
78
3
4
5
25
15
15
75
79
80
6
7
8
45
45
55
73
70
67
9
50
10
74
69
10
aReactions were monitored by TLC.
bProducts were characterized by IR, 1H NMR, and 13C NMR.
cIsolated yield.

150
L. JAMIR ET AL.
Isothiocyanates are some of the most important synthetic intermediates for the
preparation of both sulfur- and nitrogen-containing organic compounds, especially
for heterocycles.[7] The functionality is frequently encountered in natural products
including marine sesquiterpenes. Additionally, synthetic isothiocyanates have been
proved to have some biological activity and act as antiproliferatives[8] and enzyme
inhibitors for the HIV virus.[9] Numerous methods for the preparation of isothiocya-
nates have been reported, and the most widely used procedure seems to be the
decomposition of dithiocarbamates using heavy metals,[10] thiophosgene iodine,[11]
ethyl chlorocarbonate,[12] and claycop.[13] However, most of the methods suffer from
poor yields and the use of environmentally unattractive reagents.
Through this work, we observed that ETPPTB works well as a desulfurizing
agent for synthesis of isothiocyanate from the corresponding dithiocarbamate pre-
cursor (Table 3). The dithiocarbamic acid salts were prepared following a modified
Table 3. Preparation of isothiocyanates with ethyl triphenyl phosphonium tribromidea
Entry
1
Substrate
Productb
Yield (%)c
83
2
3
69
71
4
5
70
68
6
7
8
74
87
70
9
67
10
11
65
68
aReactions were monitored by TLC.
bProducts were characterized by IR, 1H NMR, and 13C NMR.
cIsolated yield.

ETPPTB AND ITS REACTIVITY
151
Scheme 1. Mechanism for acylation of alcohols.
Scheme 2. Preparation of dithiocarbamate salt.
Scheme 3. Proposed mechanism for formation of isothiocyanate.
procedure of the reported methodology,[14] which can be represented as shown in
Scheme 2.
The proposed mechanism for the transformation is given in Scheme 3. The
precipitation of elemental sulfur supports the proposed mechanism.[15]
CONCLUSION
ETPPTB has been synthesized and its reactivity studied. Its ease of prep-
aration, mildness, and efficacy in organic reactions such as bromination, acylation
and isothiocyanate preparation shows that the reagent could be a useful addition
to the existing lot of reagents.
EXPERIMENTAL
Preparation of the Reagent
An amount of V2O5(10 mg, 0.1 mmol) was added to 30% H2O2(9 mL,
79.74 mmol) and stirred in a precooled beaker at 0–4 ꢀC until V2O5 completely dis-
solved and the solution attained a clear reddish brown color. To this content, 110 mL
of water were added. Then, a solution of ETPPB (5 g, 13 mmol) and potassium bro-
mide (KBr) (3.57 g, 30 mmol) in 100 mL of water was added. Later, 40 mL of 1 M
H2SO4 was added in small portions, and the mixture was stirred for another 3h.
ETPPTB precipitated out as fine yellow microcrystals, which were filtered under
suction using Whatman 40 filter paper and dried in a vacuum desiccator using

152
L. JAMIR ET AL.
self-indicating coarse silica gel. It was further recrystallized in ethyl acetate=hexane
(99:1%). The yield was 92%.
Typical Procedure for Bromination Reaction
In a typical reaction, the phenol (1, Table 1) (282 mg, 3 mmol) was taken in
acetonitrile (3 mL) and ETPPTB (1.59 g, 3 mmol), and dissolved acetonitrile
(5 mL) was added dropwise with constant stirring at room temperature. The progress
of the reaction was monitored by TLC. After completion of the reaction, the mixture
was worked up to remove the spent reagent. The crude product thus obtained was
concentrated and then subjected to column chromatography over a pad of silica
gel to get 78% of the product.
Typical Procedure for Acylation Reaction
ETPPTB (159 mg, 0.3 mmol) was added to a solution of octadecyl alcohol
(1, Table 2) (810 mg, 3 mmol) in acetic acid (3 mL). The reaction mixture was
refluxed, and the progress of the reaction was monitored by TLC. After completion
of the reaction, the reaction mixture was poured into a saturated solution of
NaHCO3 (15 mL) and then extracted with ethyl acetate (2 ꢁ 15 mL). The organic
layer was separated and dried over anhydrous Na2SO3, and the crude product was
isolated. Further purification was achieved by passing the compound through a short
column of silica gel. The product yield was 82%.
Typical Procedure for Preparation of Isothiocyanate from
Dithiocarbamate Salt
Carbondisulfide (1.5 mL, 25mmol) was added dropwise to an ice-cooled mixture
of the aniline (930mg, 10mmol) and triethylamine (4.15 mL, 30mmol) with constant
stirring. The reaction mixture was stirred further at room temperature for 2h. The
resultant salt was filtered, washed with hexane=ethylacetate (9=1%), and dried in air.
Triethylamine (622 mL, 4.5 mmol) was added to a stirred, ice-cooled suspension
of freshly prepared phenyl dithiocarbamate salt (1, Table 3) (810 mg, 3 mmol) in
acetonitrile (5 mL). ETPPTB (1.59 g, 3 mmol) dissolved in acetonitrile (5 mL) was
added dropwise over a period of 15 min. During the addition of the reagent, sulfur
precipitated out as a light-yellow compound along with the spent reagent. After the
completion of the reaction, the precipitated sulfur and spent reagent were filtered off.
The organic layer was evaporated and admixed=extracted with hexane (15 mL),
which was further washed with 1 N HCl (2 ꢁ 5 mL) and water (1 ꢁ 5 mL). The
organic layer was dried over anhydrous Na2SO4, concentrated under reduced press-
ure, and purified over a short column of silica gel (100% hexane) to give 83% yield of
the product (1a).
Crystallographic Description
Crystal data were collected with a Bruker Smart Apex-II CCD diffractometer
˚
using graphite monochromated MoKa radiation (k ¼ 0.71073 A) at 298 K. Cell

ETPPTB AND ITS REACTIVITY
153
parameters were retrieved using SMART software and refined with SAINT on all
observed reflections. Data reduction was performed with the SAINT software and
corrected for Lorentz and polarization effects. Absorption corrections were applied
with the program SADABS. The structure was solved by direct methods implemen-
ted in SHELX-97 program and refined by full-matrix least-squares methods on F2.
All nonhydrogen atomic positions were located in difference Fourier maps and
refined anisotropically. The hydrogen atoms were placed in their geometrically
generated positions. The orange crystal was isolated in rectangular shapes from
acetonitrile=hexane (9:1) at room temperature.
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