Reaction of dibutyltin oxide with methanol under CO2 pressure relevant to catalytic dimethyl carbonate synthesis

Article abstract of DOI:10.1016/j.jorganchem.2008.01.028

Dibutyltin oxide (Bu₂SnO)_n (3) reacts with methanol to give the tetrabutyl(dimethoxy)stannoxane dimer (4). The presence of 2,2-dimethoxypropane under a CO₂ pressure accelerates the reaction resulting in nearly quantitative yield. Complex 4 exhibits catalytic activity as high as that of {Bu₂Sn(OMe)₂}₂ (1) in the dimethyl carbonate (DMC) synthesis from CO₂ and methanol. These results indicate that the conversion of 3 to 4 occurs under catalytic DMC production conditions.

Full text of DOI:10.1016/j.jorganchem.2008.01.028

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Journal of Organometallic Chemistry 693 (2008) 1389–1392
www.elsevier.com/locate/jorganchem
Note
Reaction of dibutyltin oxide with methanol under CO2 pressure
relevant to catalytic dimethyl carbonate synthesis
Kazufumi Kohno, Jun-Chul Choi, Yoshihiro Ohshima, Abulimiti Yili,
Hiroyuki Yasuda, Toshiyasu Sakakura *
National Institute of Advanced Industrial Science and Technology (AIST), AIST Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received 7 November 2007; received in revised form 16 January 2008; accepted 17 January 2008
Available online 26 January 2008
Abstract
Dibutyltin oxide (Bu2SnO)n (3) reacts with methanol to give the tetrabutyl(dimethoxy)stannoxane dimer (4). The presence of 2,2-
dimethoxypropane under a CO2 pressure accelerates the reaction resulting in nearly quantitative yield. Complex 4 exhibits catalytic activ-
ity as high as that of {Bu2Sn(OMe)2}2 (1) in the dimethyl carbonate (DMC) synthesis from CO2 and methanol. These results indicate that
the conversion of 3 to 4 occurs under catalytic DMC production conditions.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Carbon dioxide; 2,2-Dimethoxypropane; Tetrabutyl(dimethoxy)stannoxane dimer
1. Introduction
carbonate, and a subsequent ester exchange with methanol,
and (4) direct dehydrative condensation of carbon dioxide
with methanol (Eq. (1)) [8,9]. Although the last procedure
(Eq. (1)) is the most attractive in view of green chemistry,
this reaction is thermodynamically quite unfavorable.
Another problem is deactivation of the catalyst by the
co-produced water.
Dimethyl carbonate (DMC) is a key compound in so-
called green chemistry. DMC itself is also useful as an elec-
trolyte for lithium ion batteries and a fuel additive [1].
Moreover, DMC is also very important as an environmen-
tally benign synthetic reagent because it works as a substi-
tute for toxic conventional methylating and carbonylating
reagents such as methyl iodide, methyl sulfate, carbon
monoxide, etc. [2,3]. It is an especially effective substitute
for phosgene; e.g. intermediates for polycarbonates or
polyurethanes [4,5].
Several processes have been proposed to industrially
synthesize DMC [6]: (1) reaction of phosgene with metha-
nol, (2) reaction of carbon monoxide with methanol in
the presence of oxygen (oxidative carbonylation) [7], (3)
reaction of ethylene oxide with CO2 to produce ethylene
O
+ H2O
2 MeOH
+
CO2
MeO
OMe
ð1Þ
In order to circumvent these problems, we have already
reported a {Bu2Sn(OMe)2}2-catalyzed DMC synthesis
starting from methanol and carbon dioxide in the presence
of dimethyl ketals as a recyclable organic dehydrating
agent [10–13]. A possible catalytic cycle has been postu-
lated based on stoichiometric studies (Scheme 1); namely
the conversion of 1 to 2 through CO2 insertion (step a)
[11,14], and DMC formation through methanolysis of
*
Corresponding author. Tel./fax: +81 29 861 4719.
E-mail address: t-sakakura@aist.go.jp (T. Sakakura).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.01.028

1390
K. Kohno et al. / Journal of Organometallic Chemistry 693 (2008) 1389–1392
Me
O
MeO
Bu Sn
Bu
Bu
Sn Bu
OMe
O
δ -174 ppm
δ -181 ppm
Me
or
1
MeO OMe
Bu
OMe
Sn
Bu
H2O
Bu
Bu
O
Sn OMe
c
3
Bu
Bu
O
S n
CO
2
MeO
O
S n
Bu
Bu
OMe
4
MeOH
[Bu2SnO]n
-160
-170
-180
-190
δ/ppm
a
Fig. 1. 119Sn NMR spectrum of the reaction mixtures found in the Sn-
catalyzed DMC synthesis.
(or [Bu2Sn(OMe)(OH)]n)
MeOCO2
Bu
b
Bu
O
Bu
O
O
Sn
Sn
OMe
stannoxane dimer 4 could be the real active intermediate
in the catalytic cycle (see Scheme 1).
Bu
MeO Sn
Bu
OMe
MeO
Sn
(DMC)
Bu
5
Bu
MeOH
Bu
OCO2Me
Catalyst
or
Me
O
O
(2 mmol)
2 MeOH
+
CO2
MeO2CO
Bu
MeO
OMe
Bu Sn Sn Bu
Bu
MeO OMe
(100 mmol)
(300 bar)
OCO2Me
O
Me
(50 mmol)
180 oC,24 h
2
Scheme 1. Possible catalytic cycle of the Sn-catalyzed DMC synthesis.
ð2Þ
In order to convert 3 to 4 in high yields, the presence of
CO2 is very important, as exhibited in Table 1. Although
the conversion of 3 to 4 (step c, Scheme 1) occurred even
in the absence of CO2 (Runs 5–7, Table 1), the reaction
was largely promoted by CO2. Thus, the yield of stannox-
ane 4 was only 1% in the absence of CO2 at 150 °C after
24 h (Run 7), but was 93% (Run 2) in the presence of
CO2 (60 bar). The acceleration by CO2 pressure is presum-
ably ascribed to the involvement of DMC as a mediator
(Scheme 2).
The lower yield of 4 at 180 °C (Run 1, Table 1) com-
pared to the yield at 150 °C (Run 2) under CO2 is ascribed
to the formation of by-products such as Bu4Sn through dis-
proportionation. Under CO2 pressure, the conversion of 3–
4 proceeded even at 100 °C (Run 4). It is noteworthy that
this temperature is much lower than the temperature
required for catalytic DMC synthesis [12,13,19]. Hence,
dinuclear carbonate complex 2 (step b) [11,14]. However,
regeneration of tin methoxide such as {Bu2Sn(OMe)2}2
(1) in the presence of dimethyl ketals (step c) has yet to
be fully elucidated. Although the formation of tin methox-
ide from a tin oxide or hydroxide (step c, Scheme 1) has
been investigated by several groups using various dehydrat-
ing procedures (e.g. azeotropic distillation [15], DMC addi-
tion [16], Si(OMe)4 addition [17], and zeolite addition [18]),
each procedure has limitations and is less practical than
dehydration with dimethyl ketals. In this paper, we report
the transformation of dibutyltin oxide (Bu2SnO)n (3) to tin
methoxide 4 in nearly quantitative yield. The transforma-
tion is promoted by a dimethyl ketal and a high CO2
pressure.
2. Results and discussion
Elemental analysis showed that the end product in
DMC formation by methanolysis of 2 (step b, Scheme 1)
had a formula close to (Bu2SnO)n (3) [17]. Hence, we inves-
tigated the transformation of (Bu2SnO)n (3) using metha-
nol under reaction conditions similar to those in catalytic
DMC synthesis (300 bar of CO2, 180 °C). After 24 h, the
tin species in the reaction mixture was analyzed by 119Sn
NMR. The spectrum showed two different peaks in a one
to one ratio (Fig. 1). Judging from Fig. 1, the major tin spe-
cies in the reaction mixture was {Bu2(MeO)Sn–O–Sn(O-
Me)Bu2}2 (4) [15–17] rather than the dialkoxide,
{Bu2Sn(OMe)2}2 (1) [16]. Because the catalytic activity of
4 in the DMC synthesis is as high as alkoxide 1 (Eq. (2)),
Table 1
Temperature and pressure effect on the conversion of [Bu2SnO]n (3) to
{Bu2(MeO)Sn–O–Sn(OMe)Bu2}2 (4)
Run
Temperature (°C)
PCO (bar)
Yield of 4 (%)
2
1
2
3
4
5
6
7
180
150
120
100
200
180
150
60
60
60
60
0
48
93
69
41
18
21
1
0
0
Reaction conditions: 3 (4 mmol), MeOH 100 mmol, 2,2-dimethoxypro-
pane (24 mmol), 24 h.

K. Kohno et al. / Journal of Organometallic Chemistry 693 (2008) 1389–1392
1391
Table 2
O
CO2
O
Tin Methoxide
4
MeOH
H2O
Effect of acetal amount on the conversion of [Bu2SnO]n (3) to
{Bu2(MeO)Sn–O–Sn(OMe)Bu2}2 (4)
Acetal/Sn (mol/mol)
Yield of 4 (%)
MeO
OMe
(R2SnO)n
3
MeO
OMe
0.5
1
3
5
86
95
93
DMC
Scheme 2. DMC-mediated conversion of tin oxides to tin methoxides.
6
Reaction conditions: 3 (4 mmol), MeOH 100 mmol, CO2 (60 bar), 2,2-
dimethoxypropane, 150 °C, 24 h.
regeneration of tin methoxide is probably not the rate-
determining step in the catalytic DMC synthesis (Scheme
1 and Table 1).
A more precise pressure effect in the lower pressure
region (0–60 bar) was also investigated (Fig. 2). A low
CO2 pressure around 7 bar effectively achieved a conver-
sion over 70%.
Table 3
Effect of reaction time on the conversion of [Bu2SnO]n (3) to
{Bu2(MeO)Sn–O–Sn(OMe)Bu2}2 (4)
Reaction time (h)
Yield of 4 (%)
Table 2 summarizes the effect of the amount of acetal in
the synthesis of 4. When the acetal/Sn ratio (mol/mol) was
less than one, the yield of 4 was miserable due to the
decomposition by water. One equivalent of acetal was suf-
ficient to obtain 4 in over 85% yield. Employing 3 equiv. of
acetal resulted in a nearly quantitative yield of 4. A few
other groups have reported the formation of 4 from 3
[15]. However, we have found that the reaction is acceler-
ated by the presence of acetal. The time course of the reac-
tion at 150 °C revealed that the conversion of 3 to 4 was
rather fast at this temperature (see Table 3). Although a
2 h reaction gave 4 in sufficient yield (88%), a 6 h reaction
resulted in a nearly quantitative yield.
In summary, we have developed an efficient synthesis of
stannoxane dimer (4) by the reaction of dibutyltin oxide (3)
and methanol in the presence of 2,2-dimethoxypropane
under CO2. A high pressure CO2, an excess of 2,2-dime-
thoxypropane, and a longer reaction time are preferable
for promoting the formation of stannoxane (4). Because
the conversion of 3 to 4 proceeds under relatively mild con-
ditions (even at 100 °C), stannoxane dimer 4 rather than 1
could be the active intermediate in the real catalytic cycle.
1.5
2
3
6
24
70
88
91
99
95
Reaction conditions: 3 (4 mmol), MeOH 100 mmol, CO2 (60 bar), 2,2-
dimethoxypropane (12 mmol), 150 °C.
3. Experimental
3.1. General procedures
All manipulations were conducted under purified argon
or nitrogen. 2,2-Dimethoxypropane, Bu2Sn(OCH3)2 (1),
Bu2SnO (3) and Et4Sn were purchased from Aldrich Co.
Carbon dioxide (Showa Tansan Co., Kawasaki, purity
>99.99%) was used without further purification. The stan-
dard compound {Bu2(MeO)Sn–O–Sn(OMe)Bu2}2 (4) was
synthesized from an equimolar mixture of Bu2Sn(OCH3)2
(1) and Bu2SnO (3) according to the literature method
[17]. Reaction products were analyzed by NMR and GC–
MS. H, 13C and 119Sn NMR spectra were measured on
1
a JEOL LA-400WB superconducting high-resolution spec-
trometer (400 MHz for 1H). 119Sn {1H} NMR spectra were
referenced to external Et4Sn. GC analysis was conducted
using capillary columns; GL Science TC-1 (60 m) on a Shi-
madzu GC-2010 gas chromatograph equipped with a flame
ionization detector (FID). All the volatile products were
also characterized with GC–MS using a Shimadzu GC-
17A gas chromatograph connected to a GCMS-QP 5000
mass spectrometer.
100
80
60
40
20
0
3.2. Typical reaction procedure
In a stainless steel autoclave (20 cm3 inner volume), car-
bon dioxide (20 bar) was added to a mixture of methanol
(100 mmol), Bu2SnO (3) (4 mmol), and 2,2-dimethoxypro-
pane (12 mmol) at room temperature. The initial pressure
was adjusted to 60 bar at 150 °C, and the autoclave was
heated at that temperature for 6 h. After cooling to room
temperature, the solvent was removed under vacuum.
The Et4Sn (10 mg) was added to the solution of the
0
1
4
7
20
30
60
CO2 Pressure(bar)
Fig. 2. Pressure effect on the conversion of [Bu2SnO]n (3) to
{Bu2(MeO)Sn–O–Sn(OMe)Bu2}2 (4). Reaction conditions: 3 (4 mmol),
MeOH 100 mmol, 2,2-dimethoxypropane (12 mmol), 150 °C, 3 h.

1392
K. Kohno et al. / Journal of Organometallic Chemistry 693 (2008) 1389–1392
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reaction product in CDCl3 as the internal standard in order
to determine product yield by NMR.
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3.3. Catalytic reaction procedures
In a stainless steel autoclave (20 cm3 inner volume), car-
bon dioxide (liquid, 65 bar) was added to a mixture of
methanol (100 mmol), 4 (2 mmol), and 2,2-dimethoxypro-
pane (50 mmol) at room temperature. The initial pressure
was adjusted to 300 bar at 180 °C, and the autoclave was
heated at that temperature for 24 h. After cooling, 1,3,5-
trimethylbenzene (100 lL) was added to the reaction mix-
ture as the internal standard for the GC analysis.
[15] A.G. Davies, D.C. Kleinschmidt, P.R. Palan, S.C. Vasishtha, J.
Chem. Soc. C (1971) 3972.
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