J . Org. Chem. 1998, 63, 7095-7096
7095
Selective Con ver sion of Ca r bon Dioxid e to
Dim eth yl Ca r bon a te by Molecu la r
Ca ta lysis
Toshiyasu Sakakura,* Yuko Saito, Makoto Okano,
J un-Chul Choi, and Takeshi Sako
Hence, we need some new methodologies to improve
the efficiency of catalytic CO2 conversion. Our idea is
rather simple. Instead of removing water after the
reaction (eq 1), we first dehydrate MeOH and then react
the resulting dehydrated derivatives with CO2 to produce
DMC. Typical dehydrated derivatives of methanol are
dimethyl ether, dimethyl acetal (ketal), and trimethyl
ortho ester. Since reactivity should be increased in this
order, we first tried the reaction of the most reactive
compound, the ortho ester.
We report here the selective DMC synthesis via the
reaction of supercritical CO2 and trimethyl orthoacetate.
The reaction pattern can be summarized in eq 3. The
desired reaction gives DMC and methyl acetate, while
the side reaction results in the formation of dimethyl
ether and methyl acetate. Although complex-catalyzed
reactions of CO2 with high-energy chemicals such as
oxiranes, oxetanes, diynes, and dienes are well docu-
mented,1,3 reactions with saturated compounds have not
been well developed except for hydrogenation.8
National Institute of Materials and Chemical Research,
Higashi, Tsukuba, Ibaraki 305, J apan
Received March 11, 1998
The development of an environmentally friendly in-
dustrial process utilizing carbon dioxide, which is the
most abundant carbon resource and a greenhouse com-
pound, as a cheap and safe C1 building block as well as
a nontoxic reaction media has drawn much interest.1,2
Especially, the possibility as a phosgene alternative is
quite attractive.3 Our synthetic target starting from
carbon dioxide is dimethyl carbonate (DMC). Applica-
tions of DMC range from polycarbonate synthesis via
diphenyl carbonate to methylating reagents, carbonylat-
ing reagents, solvents, octane boosters in gasoline, carbon-
particle reducing agents for diesel engines, and so on.4
The historical synthetic route to DMC is based on the
methanolysis of toxic and corrosive phosgene. Recently,
another route to DMC was developed using CO under
oxidative conditions.4,5 However, CO is also highly toxic
and flammable. Hence, CO2 is obviously the ideal
carbonyl source for carbonates.
A considerable amount of effort has already been
devoted to producing DMC from methanol and CO2, and
some of the reactions were catalyzed by organotin alkox-
ides (eq 1).6 However, the catalytic activities so far
obtained have been very low (less than a few turnovers)
due to the decomposition of the catalysts by water
(coproduct) (eq 2). In the last several years, researchers
in the Czech Republic intensively reinvestigated this
reaction with regard to various promoters and also many
kinds of drying agents, resulting in no remarkable
progress.6a-c Although several other approaches for
carbonate synthesis from CO2 were also proposed, the
performance of those processes is still unsatisfactory.7
On the basis of the previous results of the DMC
synthesis from methanol and CO2,6,7 we examined the
combination of metal alkoxides and some promoters as
catalytic systems. The results are summarized in Table
1 (Ts, p-toluenesulfonyl; Cp, cyclopentadienyl, Cp*, pen-
tamethylcyclopentadienyl).9 In all the cases, material
balance was satisfactory: the sum of remaining ortho
ester and produced methyl acetate was nearly 100%
based on the starting ortho ester. Hence, the DMC/
AcOMe ratio reasonably represents the selectivity of the
reaction.
The reaction strongly depends on the structure of metal
alkoxides. For example, although dibutyltin dimethoxide
catalyzes the reaction selectively (entry 1), use of tri-
butyltin methoxide resulted in virtually no reaction
(entry 2). Dimethyltin dimethoxide was less effective
compared with the dibutyltin derivative. On the other
hand, addition of tetrabutylammonium p-toluenesulfonate
(1) (a) Leitner, W. Coord. Chem. Rev. 1996, 153, 257-284. (b)
Applied Homogeneous Catalysis with Organometallic Compounds;
Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim, 1996; pp 1048-
1072.
(2) As for the general properties of supercritical CO2 as a solvent
for metal-catalyzed reactions, see: (a) J essop, P. G.; Ikariya, T.; Noyori,
R. Science 1995, 269, 1065-1069. (b) Tumas, W.; et al. In Green
Chemistry; Anastas, P. T., Williamson, T. C., Eds.; ACS Symposium
Series 626; American Chemical Society, Washington, DC, 1996; pp
132-151.
(3) (a) Aresta, M.; Quaranta, E. CHEMTECH 1997, 30-40. (b)
Shaikh, A.-A.; Sivaram, S. Chem. Rev. 1996, 96, 951-976.
(4) Pacheco, M. A.; Marshall, C. L. Energy Fuels 1997, 11, 2-29.
(5) Applied Homogeneous Catalysis with Organometallic Com-
pounds; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim, 1996;
pp 176-178.
(8) (a) J essop, P. G.; Ikariya, T.; Noyori, R. Chem. Rev. 1995, 95,
259-272. (b) Leitner, W. Angew. Chem., Int. Ed. Engl. 1995, 34, 2207-
2221.
(6) (a) Kizlink, J .; Pastucha, I. Collect. Czech. Chem. Commun. 1995,
60, 687-692. (b) Kizlink, J .; Pastucha, I. Collect. Czech. Chem.
Commun. 1994, 59, 2116-2118. (c) Kizlink, J . Collect. Czech. Chem.
Commun. 1993, 58, 1399-1402. (d) Yamazaki, N.; Nakahama, S.;
Higashi, F. Rep. Asahi Glass Found. Ind. Technol. 1978, 33, 31-45.
(7) (a) Ruf, M.; Schell, F. A.; Walz, R.; Vahrenkamp, H. Chem. Ber./
Recl. 1997, 130, 101-104. (b) Fang, S.; Fujimoto, K. Appl. Catal. A
1996, 142, L1-L3. (c) Sasaki, Y. Chem. Lett. 1996, 825-826. (d)
McGhee, W.; Riley, D. J . Org. Chem. 1995, 60, 6205-6207.
(9) (a) A part of this work was presented at the 9th IUPAC
Symposium for Organometallic Chemistry Directed towards Organic
Synthesis (OMCOS-9), Go¨ttingen, Germany, J uly 20-25, 1997; Inter-
national Union of Pure and Applied Chemistry; Abstract SL-25. (b)
We have also found that onium salts alone promote eq 3; see: 44th
Symposium on Organometallic Chemistry, Osaka, J apan, September
20-21, 1997; Kinki Chemical Society; Abstract B209. (c) A relevant
patent has appeared recently: Ko, K.; Ogata, F. (Showa Denko) J P-
A2 07224010 [Chem. Abstr. 1996, 124, 86370].
S0022-3263(98)00460-5 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/16/1998