Green synthesis of dichlorophenylphosphine sulfide using chloroaluminate ionic liquids as a catalyst

Article abstract of DOI:10.1080/10426500600892735

Triethylhydrogenammonium chloride-XAlCl₃ Ionic Liquids (ILs) were used as a catalyst for the clean synthesis of dichlorophenylphosphine sulfide. Two synthesis routes were investigated; one of them was a reaction of sulfur and Dichlorophenylphosphine (DCPP), and another was a one-pot reaction of benzene, phosphorus, chloride, and sulfur. Effects of the ILs composition and reaction time and the quantity of the ILs on the reactions were studied. A simple product isolation procedure was achieved. The ILs showed reusable and low-consumption characters in the reaction of sulfur and DCPP. Thus the triethylhydrogenammonium chloride-XAlCl₃ ILs gave this reaction a green character. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/10426500600892735

Phosphorus, Sulfur, and Silicon, 182:227–236, 2007
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500600892735
Green Synthesis of Dichlorophenylphosphine Sulfide
Using Chloroaluminate Ionic Liquids as a Catalyst
Li-Sheng Wang
Hui-Bao Kang
Zhong-Wei Wang
Chong-Gang Wang
School of Chemical and Environment Engineering, Beijing Institute
of Technology, Beijing, People’s Republic of China
Triethylhydrogenammonium chloride-XAlCl Ionic Liquids (ILs) were used as
3
a catalyst for the clean synthesis of dichlorophenylphosphine sulfide. Two syn-
thesis routes were investigated; one of them was a reaction of sulfur and
Dichlorophenylphosphine (DCPP), and another was a one-pot reaction of benzene,
phosphorus, chloride, and sulfur. Effects of the ILs composition and reaction time
and the quantity of the ILs on the reactions were studied. A simple product isolation
procedure was achieved. The ILs showed reusable and low-consumption characters
in the reaction of sulfur and DCPP. Thus the triethylhydrogenammonium chloride-
XAlCl ILs gave this reaction a green character.
3
Keywords Clean synthesis; Dichlorophenylphosphine sulfide; ionic liquids; Lewis acid
INTRODUCTION
Dichlorophenylphosphine Sulfide (DCPPS) is an important industrial
material that is widely used in the manufacture of flameretardants,
insecticides, and plasticizers. There are many literatures discovering
various ways for its preparation. Among these methods, there are two
that are commonly discussed. One is that the reaction mixture of ben-
zene and phosphorus trichloride (PCl3) catalyzed by aluminium chlo-
ride (AlCl3) at the first step was then sulfurized without isolation of the
catalyst.1 In this method, the catalyst consumption is large and non-
reusable, and the DCPPS has to be isolated by hydrolysis, which will
contaminate the environment. The other commonly used method to pre-
pare DCPPS is to react phosphorus thiochloride and benzene catalyzed
Received March 31, 2006; accepted June 26, 2006.
Address correspondence to Li-Sheng Wang, Beijing Institute of Technology, School
of Chemical and Environment Engineering, Beijing, 10081 People’s Republic of China.
E-mail: lishengwang@btamail.net.cn
227

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L.-S. Wang et al.
by a Lewis acid.2−6 With this method, there still are some problems
of large catalyst consumption and difficulty in isolation, and in some
cases, high temperature and pressure was involved.2,3 Therefore, there
is a strong incentive to find a convenient and clean synthesis to prepare
DCPPS.
Recently, Ionic Liquids (ILs) are receiving an upsurge of interest as
green solvents or catalysts, and they are widely studied as replace-
ments for conventional media in chemical proceses. There are liter-
atures discovering that the aluminum halide-alkylammonium halide
ILs can replace the Lewis-acid catalyzed reactions, such as a Friedel-
Crafts reaction, and show green characters; they are free of volatile
organic compounds; they can be readily recycled; they have profound
effects on the activity and selectivity in reactions, and, in most of
the cases, they facilitate product isolation.7,8 In previous reports, we
found [BuPy]Cl-AlCl3 (BuPy = N-butylpyridinium cation),9 [trEHA]Cl-
XAlCl3 (trEHAm = triethylhydrogenammonium cation),10 and Et4NBr-
XAlCl3 (Et4NBr = tetraethylammonium bromide)11 ILs (where X is the
mole ratio of AlCl3 in the IL) could catalyze the Friedel-Crafts reaction
of PCl3 and benzene and render this reaction cleaner characters. We
are wondering if ILs could replace the Lewis acid in the preparation
of DCPPS and render the reaction a clean synthesis and convenient
isolation procedure.
In this article; we use the readily prepared ILs [trEHAm]Cl-XAlCl3
to catalyze the reaction of sulfur and dichlorophenylphosphine via an
one-pot reaction to prepare DCPPS (see Scheme 1). Effects of the quan-
tity of ILs used, the composition of the ILs (different X values), and
reaction time were investigated. ILs were recycled, and their reusabil-
ity was also investigated.
SCHEME 1
EXPERIMENTAL
Materials
Triethylhydrogenammonium chloride ([trEHAm]Cl), phosphorus
trichloride, aluminum chloride, sulfur, and petroleum ether (60–90◦C)
were all from VAS Chemical Reagent Corporation Ltd. in Beijing.

Green Synthesis of Dichlorophenylphosphine Sulfide
229
Dichlorophenylphosphine was prepared according to our previous
described procedures.11 To ensure reaction conditions were free of
water, the preparation of ILs and sulfurization were carried out in a
nitrogen atmosphere.
Preparation of [trEHAm]Cl-2AlCl3 ILs
In a round-bottomed flask equipped with a magnetic stirrer and a
gas inlet valve, 26.67 g (0.2 mol) anhydrous AlCl3 was slowly added
to 13.8 g (0.1 mol) triethylhydrogenammonium chloride. The mixture
was stirred for 3 h to ensure a complete reaction. Other compositions
of [trEHAm]Cl-XAlCl3 (X = 1, 1.5, 2.3) were prepared according to the
previously discussed procedure, with appropriate amounts of AlCl3 and
triethylhydrogenammonium chloride.
Preparation of DCPPS from PCl3, Benzene, and Sulfur
Benzene 65 g (0.833 mol) was added dropwise to a refluxing mixture of
PCl3 343.7 g (2.5 mol) and [trEHAm]Cl-2AlCl3 30.8 g (0.076 mol) for 1 h
and then was refluxed at 78–80◦C for 4 h. The mixture was then cooled
to r.t.; the stirrer was stopped, and then the solution was settled for
30 min; a biphasic system was obtained (in the case of 31P, it was nec-
essary to measure NMR, and the clear upper portion of the solution was
withdrawn to another measuring flask). The solution was then stirred
and heated to distill the excess PCl3 at 40◦C and reduce pressure. Then
the residue was cooled to 30◦C, and 28.4 g sulfur powder (0.887 mol) was
added partially to keep the reaction at about 80◦C. After the addition
of the sulfur, the mixture was reacted for 1.5 h at 80◦C. Then the reac-
tion mixture was cooled, and petroleum ether was added to the solution
and stirred for 30 min in order to extract the DCPPS. After the stir-
rer was stopped, phase separation was observed. The upper layer was
the petroleum ether phase, and the lower layer was the IL phase. The
DCPPS was partially dissolved in the petroleum ether. The petroleum-
ether layer was separated and collected. The operation was repeated
thrice, and the collected petroleum ether was combined. Then the ILS
in the lower layer were separated and saved for another reaction. The
rough DCPPS-petroleum ether solution was then washed with cold
water to remove the unreacted Dichlorophenylphosphine (DCPP) and
was dried with anhydrous CaCl2. The petroleum ether in the organic
layer was then recovered by distillation at atmospheric pressure at first
and then at reduced pressure. The DCPPS yield 68% based on ben-
zene was distilled in reduced pressure with b.p. temperature 105◦C at
267 Pa and 150◦C at 1333 Pa, respectively (literature 150◦C/3.47 kPa.12

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L.-S. Wang et al.
95–110◦C/0.27–0.40 kPa,1,2 87–88◦C/0.40 kPa.17 76–77◦C/0.08 kPa14).
Density at 25◦C (d25) was 1.390 (g/cm3) (literature,13 d20 = 1.4042). Re-
flection index (n2D0) was 1.6178–1.6182 (literature,13 n2D0 = 1.6250). Ele-
mental analysis: experimental value of elemental content, %: Cl, 33.94;
S, 15.01; C, 34.31; H, 2.55. Calculated value: Cl, 33.64; S, 15.16; C, 34.12;
H, 2.37. Figure 1 shows the flowing chart and material conservation of
the previously discussed procedure.
Preparation of DCPPS from DCPP and Sulfur
In a round-bottomed four-necked flask equipped with a stirrer, a ther-
mometer, and a gas inlet valve, the designed amount of DCPP was added
to a weighed quantity of [trEHAm]Cl-XAlCl3. The mixture was heated
to 30◦C, and an equal mole of sulfur based on the DCPP was added.
The reaction was exothermic, and sulfur was added partially to keep
the reaction temperature between 30–40◦C. After addition of sulfur, the
reaction was kept for the scheduled time. Then the reaction mixture was
cooled, petroleum ether was added to extract the DCPPS thrice, and the
petroleum ether layers were combined. The rough DCPPS-petroleum
ether solution was then washed with cold water to remove the unreacted
DCPP and was dried with anhydrous CaCl2. The low-boiling liquid in
the upper layer was removed by distillation at atmospheric pressure;
DCPPS was received using the method previously discussed. The yield
was calculated based on DCPP.
The IL recovered in the lower layer was separated and saved for an-
other reaction. Before use, the IL was kept at 60◦C at reduced pressure
for about 1 h to remove the dissolved petroleum ether.
RESULTS AND DISCUSSION
31P NMR Analysis
The 31P NMR analysis was carried out for the solution sampled in the
preparation of DCPPS from PCl3, benzene, and sulfur. Chemical shifts
are referenced to 85% H3PO4, with positive values downfield. The origi-
nal 31P NMR signal of the PCl3 was detected at δ + 221.2 ppm (literature
221 ppm). A signal that belonged to DCPP at δ 162.4 ppm (literature15
161.2 ppm) was also detected from this solution. In our previous work,11
we have shown that the complex (DCPP · AlCl3) was not dissolved in
either PCl3 or petroleum ether. It agrees with the 31P NMR analysis
results obtained in this work. The complex (DCPP · AlCl3) formed dur-
ing the reaction in the IL phase was dissociated by an[AlCl4]− anion.
This helped the DCPP to be dissolved into the PCl3 Phase. A sample

Green Synthesis of Dichlorophenylphosphine Sulfide
231
FIGURE 1 Flowing chart of DCPPS synthesis from phosphorus trichlo-
ride, benzene, and sulfur catalyzed with triethylhydrogenammonium chloride-
2AlCl3 IL with PCl3:benzene:sulfur:IL = 33:11:11:1 (molar ratio).
from a solution prepared from 1 mol PCl3 and 0.33 mol AlCl3 was also
determined by the 31P NMR; three signals were observed. The strong
signal at δ + 221.2 ppm assigned to PCl3 was obvious, while the two
weak signals δ + 106.0 and δ + 37.8 were assigned to the interaction of
PCl3 with AlCl3; one belonged to Cl3P—AlCl3, and another belonged to

232
L.-S. Wang et al.
Cl2P—Cl—AlCl3. These NMR assignments corresponding to two forms
of a PCl3/AlCl3 interaction were not reported in the literature. There-
fore, one of the mechanisms can be assumed that the AlCl3-PCl3 interac-
tion formed an ionic complex (Cl2P+ AlCl4−), and that the cation was the
species attacting the aromatics. The mechanisms of the Friedel–crafts
reaction of the phosphylation of benzene catalyzed by the [trEHAm]Cl-
2AlCl3 IL can assumed as in Scheme 2.
SCHEME 2
Catalytic Activity
This study showed that the reaction of sulfur and DCPP would not take
place without a catalyst. While the reactions using the [trEHAm]Cl-
2AlCl3 ILs as a catalyst proceeded smoothly. In the one-pot reaction us-
ing PCl3, benzene, and sulfur as raw materials, we found that the acidic
ILs catalyzed the reaction with yields of about 60%∼78%, while the ba-
sic and neutral ILs showed nearly no catalytic activity. It was found that
a small amount of ILs catalyzed the reaction with fair yields. For exam-
ple, with PCl3:benzene:sulfur:ILs = 3:1:1:0.08, a yield of 62.1% DCPPS
was received. A yield of 78.8% was obtained in the case of a 10:10:1 mole
ratio of DCPP:sulfur:ILs, and a yield of 70.0% was obtained in the case
of a 30:30:1 mole ratio of DCPP:sulfur:ILs. Another interesting result
is that the use of large mole ratios of ILs decreased the yields. The ILs
layers were weighed after each reaction and distillation. The weight of
the residue of the IL’s layer was increased as compared with the weight
of the catalyst, and the amount of increase showed a direct ratio to the
amount of ILs used in the reactions. We believe that the product was
dissolved in the ILs, since the ILs were miscible with DCPPS. Benzene

Green Synthesis of Dichlorophenylphosphine Sulfide
233
TABLE I Catalytic Activity
of [trEHAm]Cl-XAlCl3 ILsa
X
DCPP:S:ILs/mole ratio
Yield (%)
1
10:10:1
3:3:1
5:5:1
10:10:1
15:15:1
30:30:1
3:3:1
8.6
10.6
24.1
63.8
69.4
66.1
23.7
37.6
78.8
70.2
70.0
32.6
41.2
84.3
76.9
74.5
1.5
1.5
1.5
1.5
1.5
2
2
2
2
2
2.3
2.3
2.3
2.3
2.3
5:5:1
10:10:1
15:15:1
30:30:1
3:3:1
5:5:1
10:10:1
15:15:1
30:30:1
aReactions were completed at 40◦C for 5 min.
and PCl3 were partially miscible, and petroleum ether was immiscible
with ILs.16
Reaction cases using various amounts of ILs with different ILs com-
positions (a different X value in [trEHAm]Cl-XAlCl3 ILs) were carried
out to investigate the catalytic activity of [trEHAm]Cl-XAlCl3 ILs in
the reaction of DCPP and sulfur, and the results are summarized in
Table I.
The acidic (X > 1) [trEHAm]Cl-XAlCl3 ILs showed fair catalytic ac-
tivity in the reaction; the catalytic activity increased as their acidity in-
creased. Those basic (X = 0.67) and neutral (X = 1) [trEHAm]Cl-XAlCl3
ILs, however, showed nearly no catalytic activity.
Product Isolation
In all cases, DCPPS was extracted directly from the reaction mixture.
This isolation method is different from those in the literature. In the
preparation methods using AlCl3 as a catalyst and PCl3 and benzene
as starting materials, the DCPPS was not isolated directly, and the
reaction mixtures had to be hydrolyzed in ice water. Thus the isolation
procedures in our experiments provide a more convenient and more
environmental way to get the product. It is apparent that the DCPPS
was not in the complex form with AlCl3 because the Lewis bacity of
DCPPS was lower than DCPP.

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L.-S. Wang et al.
TABLE II Effects of Reaction Time
and Temperaturea
Temperature (◦C)
Reaction time (min) Yield (%)
30–40
30–40
30–40
50–60
50–60
50–60
70–80
70–80
70–80
10
15
30
10
15
30
10
15
30
79.3
77.5
79.6
78.1
80.2
78.3
78.5
79.2
80.4
aDCPP:S:ILs(X = 2) = 10:10:1.
Effects of Reaction Time and Temperature
The effects of reaction time and temperature on the reaction of DCPP
and sulfur were investigated, and the results were summarized in
Table II.
From Table II it is shown that the reaction was completed at a low
temperature and in short time. This is different from that described in
the literature. In the literature methods, reactions were carried out at
about 80◦C in about 3 h. The reaction in our experiments took place at
about 30–40◦C and were be completed within 10 min after an addition
of sulfur. A lower reaction temperature and shorter reaction time in
this reaction showed good catalysis activity of the ILs.
TABLE III Reusable Characters
of [trEHAm]Cl-2AlCl3 ILs
DCPP:S:ILs
(mole ratio)
Runsa
Yield (%)
1
2
3
4
5
6
1
2
3
4
5
6
78.8
77.7
78.4
76.6
77.8
73.6
70.3
71.2
71.4
69.6
67.4
70.2
10:10:1
15:15:1
aReactions were carried out at 30–40◦C for 5 min.

Green Synthesis of Dichlorophenylphosphine Sulfide
235
Reusable Character of ILs
The isolated ILs were used directly in the followed runs to investigate
their reusable characters, and the results are summarized in Table III.
From Table III it can be seen that the ILs still showed good catalytic
activity, as they were used another 5 times, which means the ILs can
be recycled. Thus a yield of 78.8% was obtained in the first run (with
DCPP:S:ILs equal to 10:10:1) and a yield of 76.7% was obtained at the
sixth reaction when the ILs were reused. In the preparation of DCPPS
directly from PCl3, benzene, and sulfur, similar results were obtained.
While in preparation of DCPP from benzene and PCl3 with the catalysis
of same ILs, after the catalyst was recycled three times, no activity was
observed.
CONCLUSIONS
[trEHAm]Cl-XAlCl3 ILs were used as a catalyst for the clean synthe-
sis DCPPS from the reaction of sulfur and DCPP and from the one-pot
reaction of benzene, phosphorus trichloride, and sulfur. A simple prod-
uct isolation procedure was achieved. The experimental phenomenon
indicated that the DCPPS did not form a complex in the [trEHAm]Cl-
XAlCl3 ILs but only simply dissolved in it. The effects of the IL’s com-
position and reaction time and the quantity of the ILs on the reactions
were studied. The ILs showed reusable, low consumption, and high
activity characters in the reaction of sulfur and DCPP. Thus
[trEHAm]Cl-XAlCl3 ILs gave this reaction a green character. Compared
with literature methods for the preparation of DCPPS, the reaction us-
ing [trEHAm]Cl-XAlCl3 ILs as a catalyst exhibited a simple product
isolation procedure, a short reaction time, and less and reusable cata-
lyst consumption, which contribute to a clean synthesis of DCPPS.
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