Cooperative catalysis with binary Lewis acid-Lewis base system for the coupling of carbon disulfide and epoxides

Article abstract of DOI:10.1002/aoc.2908

The cycloaddition of epoxides with carbon disulfide proceeds effectively by using a binary catalyst system of tetradentate Schiff-base aluminum complexes (designated as salenAlX) as Lewis acid in connection with a nucleophilic co-catalyst, such as quatern

Full text of DOI:10.1002/aoc.2908

Full Paper
Received: 11 June 2012
Revised: 9 July 2012
Accepted: 16 July 2012
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2908
Cooperative catalysis with binary Lewis acid–
Lewis base system for the coupling of carbon
disulfide and epoxides
Yi-Ming Wang, Bo Li, Hui Wang, Zhi-Chao Zhang and Xiao-Bing Lu*
The cycloaddition of epoxides with carbon disulfide proceeds effectively by using a binary catalyst system of tetradentate
Schiff-base aluminum complexes (designated as salenAlX) as Lewis acid in connection with a nucleophilic co-catalyst, such
as quaternary ammonium salt. The steric factor of the substituent groups on the aromatic rings of salenAlX and the anion
of quaternary ammonium salt all have significant effects on the activity of the binary catalyst system, as well as the selectiv-
ities of the resultant cyclic products. The effects of temperature, time and the molar ratio of reactants were also investigated in
detail with regard to propylene oxide with carbon disulfide, and a plausible mechanism was proposed. Copyright © 2012 John
Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: Thiocarbonates; carbon disulfide; tetradentate Schiff-base aluminum complexes; quaternary ammonium salt
of KOH and CaH2, and fractionally distilled under a nitrogen
atmosphere. 2-Chloromethyl-oxirane, 2-ethyl-oxirane, 2-phenyl-
Introduction
Cyclic thiocarbonates have received much attention in view of their
biological activity and material science.[1–3] An efficient method for
synthesizing cyclic thiocarbonates is performed by the coupling
reaction of carbon disulfide (CS2) with epoxides. Depending on
the catalysts and reaction conditions, five-membered cyclic
dithiocarbonate 2, its regioisomers 3 and 4, trithiocarbonate 5
and episulfide 6 have been reported to be formed (Scheme 1 ).[4–9]
In recent decades, many catalyst systems, including amines,
alkali metal salts and quaternary ammonium salts, potassium alco-
holates and transition metal complexes, have been developed for
this transformation.[4,5,10–13] Although the advances have been
significant, these systems suffered from low catalyst activities, the
use of solvent or the requirement for high catalyst loading.
Recently, binary or bifunctional electrophile–nucleophile catalyst
systems have proved to be very effective in cooperatively
catalyzing the coupling reaction of carbon dioxide and epoxide to
form carbonates under mild conditions.[14–19] Thus we expected
that the similar systems could also catalyze the reaction of epoxides
and CS2 for selective formation of cyclic dithiocarbonate.
In this paper, we present a study concerning the cycloaddition
of CS2 to epoxides using binary catalyst systems consisting of
metal–salen complexes and quaternary ammonium salts. The
influence of the substitute groups on the aromatic rings of salen
complexes and the anion of the quaternary ammonium salt on
catalyst activity will be investigated. Additionally, the effects of
temperature, reaction time and molar ratio of reactants were
studied in detail for this coupling reaction.
oxirane, 2-hexyl-oxirane, 2-butyl-oxirane, 2-hexenyl-oxirane, 2-
phenoxymethyl-oxirane and 2-allyloxymethyl-oxirane were used
as received from Acros Company. Triethyl-aluminum (Et3Al) was
fractionally distilled under reduced pressure in a nitrogen
atmosphere. Toluene was freshly distilled from sodium benzo-
phenone prior to use. 1 H NMR spectra were recorded by a Varian
INOVA-400 type spectrometer at 400 MHz. Chemical shifts were
determined (ppm) using tetramethylsilane as an internal
standard. IR spectra were obtained with a Nicolet 50X FT-IR
spectrophotometer.
The salenM(III)X complexes were prepared according to the
literature methods.[14,20,21] These complexes are all sensitive to
moisture and should be stored in a nitrogen atmosphere. The
representative procedure for these aluminum complexes are
available online as supporting information.
Representative Coupling Reaction
To a 50 ml Schlenk flask fitted with a three-way stopcock, nBu4NBr
(16.1 mg, 0.05 mmol) and then propylene oxide (3.5 ml, 50 mmol)
were successively added by means of a hypodermic syringe in a
nitrogen atmosphere. The quaternary ammonium could be easily
dissolved in propylene oxide, and then SalenAlEt, A1 (16.1 mg,
0.05 mmol) was added under dry nitrogen. When the catalysts
were completely dissolved, the mixture was charged into a
10 ml autoclave via a syringe in a nitrogen atmosphere; CS2
(0.6 ml, 10 mmol) was then also added to the autoclave via a
*
Correspondence to: X.-B. Lu, State Key Laboratory of Fine Chemicals, Dalian
University of Technology, Dalian 116024, China. E-mail: lxb-1999@163.com
Experimental
CS2 was dried over CaCl2 and distilled under nitrogen after
refluxing over CaH2. Propylene oxide was refluxed over a mixture
State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Dalian 116024, China
Appl. Organometal. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

Y.-M. Wang et al.
S
O
S
S
(entry 7), including dithiocarbonate 2 at 65%, trithiocarbonate
5 at 23% and monothiocarbonate 7 at 12%. Increasing the molar
ratio of epoxides to CS2 is beneficial in improving the selectivity
of 2. Notably, SalenAlEt itself could not catalyze this reaction
(entry 8), while Bu4NBr alone as catalyst only shows a relatively
low activity at 100 ꢀC (entry 9).
S
catalyst
O
S
S
S
S
S
O
O
S
CS2
+
R
R
R
R
R
R
1
n
2
3
5
6
4
Scheme 1. Reaction products of CS2 with epoxides in the presence
Several salenAlX complexes (A1–A5, Scheme 2) with varying
substitute groups on the aromatic rings were investigated as
catalysts for the cycloaddition of CS2 to propylene oxide (Table 2,
entries 7 and 10–13). Monomeric salenAlX has a structure of five-
coordinate square pyramidal Al3+ strongly bound to both the
oxygen and nitrogen ligand atoms,[14] while substitution on the
aromatic ring of salenAlX probably causes geometrical distortion
of square pyramidal structure, and thus may affect its property
significantly. The experimental results show that substitution
on the aromatic rings of salenAlX has a negative effect on its
catalytic activity for the cycloaddition reaction. Under the
employed conditions, catalytic activities of the series of
substituted aluminum–salen complexes are in the following
order: A5 > A1 > A4 > A2 > A3; which is in reverses order to
the size of the substituted groups. It is obvious that the steric
effect of these bulk substituted groups play a key role in the
catalytic property of these aluminum complexes.
of catalysts.
syringe in a nitrogen atmosphere. The autoclave was placed in an
oil bath, heated to the desired temperature and stirred for the
desired time. After the mixture was cooled to room temperature
and the volatile fraction was evaporated, the residue was
purified by silica gel column chromatography eluted with
hexane–chloroform (7/3, volume ratio).
Results and Discussion
Cycloaddition of CS2 with Epoxides in the Presence of
SalenAlX and Quaternary Ammonium Salt
Since all SalenMX complexes shown in Scheme 2 and quaternary
ammonium salt are easily dissolved in the neat epoxides
surveyed, the cycloaddition of CS2 to epoxides does not
require any organic solvent. However, the reaction rate of
epoxides with CS2 is much lower than with CO2 in the presence
of salenAlEt/nBu4NBr under the same conditions. The results are
shown in Table 1. The reaction at room temperature gave no
product (entry 1), and at 100 ꢀC gave cyclic products in 94% yield
Table 2 (entries 13–16) shows the effect of the quaternary
ammonium salt on the coupling reaction. The experimental results
show that under the employed conditions catalytic activities of var-
ious quaternary ammonium salts are in the following order: PPNCl,
n
PPNN3, Bu4NCl > nBu4NBr > > nBu4NI. Interestingly, the anion of
the quaternary ammonium salt has a significant effect on product
selectivity. nBu4NBr favors the formation of product 2a, while
nBu4NCl seems to be beneficial to forming products 4a and 8a.
Although the binary catalyst system of cobalt(III)–salen
complex C and nBu4NCl proved to be highly active in the
coupling of CO2 and epoxides,[22–24] no product was observed in
the CS2/epoxide coupling. We tentatively assume that the presence
of sulfur-based compounds (such as CS2) easily causes the
poisoning of transition metal catalysts. Very low activity was also
found in the binary systems based on salenCrCl complex B.
The binary catalyst system (A1/nBu4NCl) was applicable to a
variety of terminal epoxides, affording the corresponding cyclic
thiocarbonates in good yield (Table 3). Propylene oxide (1a)
was found to be the most reactive epoxide.
O
S
S
S
O
O
O
catalyst
S
S
S
O
S
S
O
S
O
O
O
S
CS2
+
R
R
R
R
R
R
R
1
8
7
2
3
5
4
A1: M = Al, X = Et, R1 = R2 = H;
A2: M = Al, X = Et, R1 = tBu, R2 = H;
A3: M = Al, X = Et, R1 = R2 = tBu;
A4: M = Al, X = Et, R1 = Br, R2 = H;
A5: M = Al, X = Cl, R1 = R2 = H;
B: M = Cr, X = Cl, R1 = R2 = tBu;
C: M = Co, X = Cl, R1 = R2 = tBu
N
N
O
M
R1
O
R1
X
R2
R2
Scheme 2. Coupling reaction of CS2 and epoxides catalyzed by salenM(III)X.
Table 1. Coupling CS2 with propylene oxide (PO) using binary A1/nBu4NBr catalysta
Entry
PO/CS2/catal. molar
ratio
Temp. (ꢀC)
Time (h)
Yield (%)
TOF (hÀ1
)
2a
4a
5a
7a
8a
Total
1
200/200/1
200/200/1
200/100/1
500/100/1
500/500/1
1000/200/1
1000/200/1
1000/200/1
1000/200/1
25
60
24
12
11
11
10
5
—
6
—
—
—
—
—
—
—
—
—
—
4
—
1
—
—
—
—
—
—
—
—
—
—
11
12
26
22
40
94
—
38
—
1.8
1.1
2.4
11.0
16.0
20.9
-
2
3
60
7
4
1
4
60
17
12
25
61
-
7
2
5
100
100
100
100
100
7
3
6
10
22
—
12
5
7
9
11
—
1
8b
9c
9
9
25
8.4
aA1/nBu4NBr = 1:1 (molar ratio).
bA1 alone as catalyst.
cnBu4NBr alone as catalyst.
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Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2012)

Cooperative catalysis for the coupling of carbon disulfide and epoxides
Table 2. Coupling reaction of propylene oxide with CS2 using various binary catalyst systemsa
TOF (hÀ1
)
Entry
Catalyst
Yield (%)
2a
4a
5a
7a
8a
Total
7
A1/nBu4NBr
A2/nBu4NBr
A3/nBu4NBr
A4/nBu4NBr
A5/nBu4NBr
A1/nBu4NCl
A1/nBu4NI
A1/PPNCl
61
22
9
—
—
—
—
13
24
—
20
30
—
—
22
15
6
11
3
—
—
—
—
16
45
—
45
13
—
—
94
40
19
83
99
98
15
99
99
—
—
20.9
8.9
4.2
18.4
22.0
21.8
3.3
22.0
22.0
-
10
11
12
13
14
15
16
17
18b
19
4
49
38
-
25
15
15
3
9
17
15
2
10
-
17
32
—
—
18
24
—
—
A1/PPNN3
B/nBu4NCl
C/nBu4NCl
-
-
-
-
aReaction conditions: temperature = 100 ꢀC, time = 9 h, PO/CS2/salenMX/quaternary ammonium salt = 1000/200/1/1.
bGave polymer with a yield of <5%.
Table 3. Coupling reaction of CS2 with various epoxides catalyzed by binary salenAlEt/nBu4NBr systema
O
S
S
S
O
O
O
catalyst
S
S
S
O
S
S O
R
S
O
R
O
O
S
CS2
+
R
R
R
R
R
1
8
7
3
2
5
4
Entry
1
Epoxide
Yield (%)
TOF (h-1)
20.9
2
4
5
7
8
Total
94
61
—
22
11
—
O
1a
2
3
40
21
—
—
12
5
5
—
—
57
26
12.7
5.6
O
1b
1c
Cl
—
O
4
29
—
26
5
—
60
13.3
O
1d
1e
Ph
5
6
36
50
—
—
10
23
3
5
—
—
49
78
10.9
17.4
O
O
1f
Ph
O
aReaction conditions: temperature = 100 ꢀC, time = 9 h, epoxide/CS2/salenAlEt/ nBu4NBr = 1000/200/1/1 (molar ratio).
Appl. Organometal. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
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Y.-M. Wang et al.
O
S
2
S
O
O
O
O
O
S
O
S
S
S
S
O
S
O
S
CS2
CS2
CS2
R
R
R
R
R
R
R
R
R
R
R
O
O
COS
3
O
4
O
CO2
CS2
R
R
S
Al
Al
Al
Al
S
S
O
O
O
R
R
R
nBu4NX
nBu4NX
nBu4NX
nBu4NX
8
7
5
Y
Y
Y
Al
Et
Y
R
nBu4NX
Y
R
R
Y = O, S
XNnBu4
Y
R
Al
Et
Y
XNnBu4
Al
Et
Y
Y
R
R
XNnBu4
Y
Et Al
CY2
Scheme 3. Possible mechanism for the formation of various five-membered compounds from the coupling of CS2 and epoxides.
Cycloaddition Mechanism
Conclusion
Based on some mechanistic aspects of Lewis acid/base
In summary, the tetradentate Schiff-base aluminum complexes in
conjunction with a quaternary ammonium salt are effective
catalysts for the coupling reaction of CS2 and epoxides, affording
five-membered cyclic products. Also, the selectivity for the cyclic
products could be controlled to a certain extent by changing the
catalyst system, the molar ratio of the reactants and the
reaction temperature.
co-catalysis,[4,5,11,15,25–27] a plausible mechanism for this cyclo-
addition is proposed, as shown in Scheme 3. Indeed, in the
presence of a Lewis acid or base, the atom exchange reaction
between CS2 and epoxide easily occurs, thus forming
various reactive compounds, such as episulfide, COS and
CO2 (Scheme 3). The Lewis acidic metal ion and nucleophilic
co-catalyst corporately catalyze the coupling reaction of
three-membered-ring compounds and CS2 or COS or CO2 to
give the corresponding cyclic thiocarbonates or cyclic car-
bonates. Usually, terminal three-membered-ring compounds
are easily ring-opened at the less substituted C-O or C-S
bond, which probably results from nucleophilicity of free XÀ
of nBu4NX and the electrophilic interaction of SalenAlEt
with three-membered-ring compounds. The nucleophilic ring-
opening of the coordinated three-membered-ring compound
by the anion of the quaternary ammonium salt produces
an intermediate, and then further forms the five-membered
compound via intramolecular cyclic elimination. Interestingly,
we did not observe the formation of compound 3. This means
the nucleophilic ring-opening of the coordinated epoxide selec-
tively occurs at the methylene C-O bond. Compounds 4 and 7
should be produced from the coupling reactions of COS with
episulfide and with epoxide, respectively. The coupling of CS2 with
episulfide affords compound 5, while the cycloaddition reaction
of CO2 with epoxide produces compound 8. It seems that the
use of nBu4Cl or nBu4N3 is beneficial for the atom exchange
reaction between CS2 and epoxide, thus providing compounds 5,
7 and 8.
Acknowledgements
Gratitude is expressed to the National Natural Science Foundation
of China (U1162101) and the National Basic Research Program of
China (973 Program: 2009CB825300) for financial support.
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