Paper
Catalysis Science & Technology
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2
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5
limited under the applied reaction conditions. On the one
hand, the electron withdrawing effect of the substituent
results in a lower electron density at the epoxy ring oxygen
atom. On the other hand, the chloro substituent by itself
could also interact with NbCl , thus competing with the epox-
5
ide. Therefore, the interaction with NbCl as a Lewis acid is
5
weaker compared to that of the other tested substrates, lead-
ing to a conversion of only 44%.
1
1
1
1
1
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Conclusions
In this work it was shown that imidazolium bromides, which
can act as catalysts by themselves for the cycloaddition of
CO
dem catalytic reaction with NbCl
conditions (r.t., 4 bar CO pressure). Therefore, NbCl acts as
2
and epoxides, offer the possibility to yield PC in a tan-
5
under very mild reaction
6 R. L. Paddock and S. T. Nguyen, J. Am. Chem. Soc., 2001,
2
5
1
23, 11498–11499.
an activator of the epoxide and the imidazolium as a nucleo-
phile for the ring opening of the pre-coordinated epoxide.
Different imidazolium bromides bearing aliphatic (1–13),
aliphatic/aromatic (14–25) and aromatic (26–31) side chain
substituents were synthesized, characterized and shown to be
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active towards the formation of PC with NbCl . A detailed
screening of the nucleophiles led to 3, 4, 9, 10, 11, 12, 13 and
5
2
2 as interesting alternatives to the commercially available
2
002, 4, 2561–2563.
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TBAB. These observations showed a positive effect on the
catalytic behaviour of the imidazolium bromides with only
aliphatic wingtips. In this set of compounds, 22 is the only
example where one side chain has an aromatic moiety. The
results helped to understand the role of the cation in the
catalytic behaviour and showed a positive trend for the sub-
stitution of the C2-position of the imidazolium moiety. Fur-
ther research efforts will focus on the development of tailor-
made imidazolium cations, which allow the reusability of the
2
2
2
2
6
2
4 E.-H. Lee, J.-Y. Ahn, M. M. Dharman, D.-W. Park, S.-W. Park
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NbCl –imidazolium bromide system, e.g. via coordination.
5
2
6 M. Anthofer, M. Wilhelm, M. Cokoja and F. Kühn,
in Transformation and Utilization of Carbon Dioxide,
ed. B. M. Bhanage and M. Arai, Springer, Berlin Heidelberg,
Considering these criteria, the system is a very effective and
cost-efficient way to yield PC under very mild conditions and
thus helps to minimise the carbon footprint of the reaction.
2
014, pp. 3–37.
2
2
2
3
7 J. Palgunadi, O. S. Kwon, H. Lee, J. Y. Bae, B. S. Ahn,
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Acknowledgements
MEW, MHA and RMR thank the TUM Graduate School for
financial support. The authors thank T. Hofmann for experi-
mental assistance.
6
, 613–616.
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Catal. Sci. Technol.
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