2934
Z.-Z. Yang et al. / Tetrahedron Letters 51 (2010) 2931–2934
MeOH is captured by intermediate I (step 3); further nucleophilic
substitution of intermediate II by CH3OÀ produces DMC and ethyl-
ene glycol anion (intermediate III) (step 4); ultimately, EG is gen-
erated by proton exchange between intermediate III and MeOH
(step 5). The whole process is assumed to be reversible. This pro-
posed mechanism could account for catalytic efficiency depen-
dence on the structure of the ILs (Table 1, entries 2–9), viz cation
and anion effect. Particularly, step 4 is proposed to be more diffi-
cult than step 2. Therefore it is reasonable that less basic catalyst
could give more monoester product, that is, intermediate II, and
low catalyst concentration leads to facile formation of HEMC.
Please also note that generated EG can be reused for EC synthesis.17
In conclusion, a kind of DABCO-based basic ionic liquids, for
example, [C4DABCO]OH was found to be a highly efficient and
recyclable catalyst for DMC synthesis through the transesterifica-
tion reaction of EC with methanol under mild reaction conditions.
The ionic liquids used in this study represent as air stable, easily
synthesized, cheap, extremely robust, and environmentally benign
catalysts, which can effectively activate methanol through tertiary
nitrogen in the cation part of the IL in combination with hydroxide
anion. The catalyst could be recycled for four times without loss of
yield and selectivity. Therefore, this green process could show
much potential application in industry. Further extending applica-
tion of Lewis basic ionic liquids toward other reactions is currently
under investigation in our laboratory.
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This work was supported by the National Natural Science
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Research Fellowship for International Young Scientists from
NSFC(20950110325) and the 111 project (B06005), and the Com-
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Supplementary data (experimental details and characterization
data and charts for the prepared catalysts and the intermediate)
associated with this article can be found, in the online version, at