K.M. Deshmukh et al. / Catalysis Communications 12 (2010) 207–211
211
in conjunction with dibutyltin oxide, fourfold increase in yield of DPC
with 84.2% selectivity (Table 1, entry 3) was observed, indicating that
ionic liquids are excellent promoter for the reaction. It has been
observed that Brønsted acidic ionic liquid [HMim][pTSA] showed a
similar result as of the [NMP][HSO ] (Table 1, entry 4). To investigate
4
the effect of alkyl chain length present in ionic liquids on
−OCH
99.9% selectivity for MPC and DPC formation (Table 3, entry 1, 3).
However, when −CH , −OCH at ortho position was used as a
substrate conversion was found to be 8.2% and 6.1% with a selectivity
of 99.4% and 99.9% for MPC and DPC formation respectively (Table 3,
entry 2, 4). In general, any substitution at ortho position leads to lower
conversion and selectivity due to the steric hindrance .
3
at para position provides conversion of 10.5% and 13.4% with
3
3
transesterification reaction, we replaced the hydrogen of [HMim]
[
pTSA] with the butyl (C
4
) and observed no significant effect on the
Phenols with electron withdrawing substituent like chloro (−Cl)
at para and ortho positions provide aryl methyl carbonate with a very
low conversion (Table 3, entries 5–6). A strong electron withdrawing
DPC formation while conversion of phenol toward MPC formation
increased (Table 1, entry 5).
Similarly, the effects of the various Lewis acidic ionic liquid based
imidazolium and choline chloride cations were also studied (Table 1,
entries 6–12). As shown in Table 1, imidazolium based ionic liquids 1-
substituent such as (−NO
2
) at ortho or para positions of phenol was
not compatible for the present protocol as no conversion obtained.
butyl-3-methyl imidazolium zinc chloride ([BMim]Cl[ZnCl
ed 38.5% conversion of phenol with an increase in the yield of DPC to
7.8% and 0.1% formation of anisole (Table 1, entry 6), which is in
agreement to the prior literature [25]. Increase in molar ratio of
[BMim]Cl[ZnCl ) to 1:2 has decreased the selectivity toward DPC
2
]) provid-
4
. Conclusion
1
Transesterification reaction of dimethyl carbonate with phenol to
methylphenyl carbonate and diphenyl carbonate using dibutyltin
oxide catalyst in conjunction with Brønsted and Lewis acidic ionic
liquids was studied. It was observed that the use of ionic liquids which
acts as a promoter significantly enhances the yield of diphenyl
carbonate. Furthermore, Lewis acidity order of the ionic liquid was
determined using pyridine as probe by IR spectroscopic method. The
developed protocol was further studied for various substituted
phenols to check the efficiency of catalytic system.
(
2 2
]
formation (Table 1, entry 7). Similar results were obtained for the
choline chloride based Lewis acidic ionic liquids (Table 1, entries 8–
1
2). The probable reason for enhancement of catalytic activity of
dibutyltin oxide in the presence of Brønsted and Lewis acidic ionic
liquids can be ascribed to the in situ formation of highly active tin
species containing anionic site of the ionic liquid acting as a ligand
[
26]. Present protocol was also compared with the previously
reported catalytic system [18,19]. It can be seen that these catalyst
are more selective to MPC rather than DPC. Thus dibutyltin oxide in
Acknowledgement
4 2
combination with [NMP][HSO ] or ([ChCl][ZnCl ]) was found to
provide the best yield of DPC, and hence was used for further
optimization.
The financial assistance from the Indira Gandhi Center for Atomic
Research (IGCAR, India) is kindly acknowledged.
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3
.2. Influence of catalyst loading of ionic liquid on transesterification
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3
conversion and selectivity. Electron donating substituent like −CH ,