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.