Paper
Green Chemistry
performed catalytic tests were also analysed by ICP-OES (for ing (only 0.007% of total copper) and can be easily recycled by
the complete series of collected data see ESI†). Again, negli- just re-feeding the reactor with the reagents. The alkene excess
gible copper leaching was observed in all cases. Slightly higher can be easily separated from the reaction products and even-
contents were found in the case of methyl-2-furoate, where tually re-used. Future studies will be devoted to improving the
concentrations up to 122.5 ppb of copper were obtained. This ligand design in order to enhance the stereo-selective outcome
last observation demonstrates that, effectively, the presence of of the reaction. In principle, in fact, if we can obtain a single
an oxygen donor atom in the substrate might be detrimental diastereoisomer while still maintaining high chemoselectivity
to the catalyst stability.
under the present conditions, no further purification will be
needed to yield a pure product of high added synthetic value
containing a cyclopropane unit.
Post-catalysis characterisation
Consistent with our previous report12 on Pc-L* (pyridine con-
taining macrocyclic ligands) copper(I) complexes, DRIFT
spectra showed that the Cu complexes are grafted without any
modification of the ligand structure, since the IR absorption
bands, detected for 1/D, A, M and 2/D, did not show any
appreciable modification with respect to solid 1 or 2, either in
location or in intensity (see ESI, Fig. S2,† for a comparison of
DRIFT spectra of complex 2 and the supported catalyst 2/D).
Grafted complexes were investigated also after the catalytic
runs. DRIFT spectroscopy, again, confirmed that the structures
of Cu complexes are stable under the reaction conditions,
showing the presence of all the bands in the skeletal range of
the spectrum typical of 1 and 2 (see ESI, Fig. S4†).
Acknowledgements
One of the authors, B. C., is grateful to Erasmus Life-Learning
Program for a fellowship of three months at the University of
St. Andrews. V. D. S. and R. P. thank the Italian Ministry of
Education, University and Research for financial support
through the project “ItalNanoNet” (Rete Nazionale di Ricerca
sulle Nanoscenze; prot. no. RBPR05JH2P).
However, in most cases the occurrence of an intense
absorption band, located around 1750 cm−1, in samples col-
lected at the end of catalytic runs suggests the presence of
adsorbed reaction intermediates. Experiments, conducted in
dichloroethane solutions of the unsupported Cu complex,
revealed the occurrence of an interaction between maleate and
fumarate and the copper complex, which gave rise to a band at
1713 cm−1 (see ESI, Fig. S5 and S6†). Thus, products adsorbed
on the samples after reaction seem to be of a different nature,
and can be ascribed to diazoacetate polymers as already
reported for bis(oxazoline) copper complexes.29
Notes and references
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3 V. Dal Santo, M. Guidotti, R. Psaro, L. Marchese,
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468, 1904–1926.
4 D. J. Cole-Hamilton, Science, 2003, 299, 1702–1706.
5 C. Aprile, H. Garcia and P. P. Pescarmona, in Catalytic
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Conclusions
In summary, we have developed a new catalytic system based
on supported hydrogen-bonded (SHB) chiral copper(I) com-
plexes. These are competent catalysts for asymmetric cyclopro-
panation reactions allowing the use of more eco-sustainable
CO2 as a vector instead of organic solvents normally used for
these reactions. The heterogenised systems under flowing CO2
showed comparable or even higher chemoselectivities than the
homogeneous counterpart. In terms of enantioselection excel-
lent results were obtained also with non-activated alkenes like
1-octene where ees are comparable to those observed in the
homogeneous phase. Interestingly, all the data in CO2 were
obtained at 40 °C, while the best temperature for the cyclopro-
panation reaction in the homogeneous phase was 0 °C. More-
over, supported catalysts showed a good recyclability and the 10 P. Barbaro, C. Bianchini, G. Giambastiani, W. Oberhauser,
turnover number has been increased (up to 440). The catalysts
remained active up to 25 h without any loss in activity and
L. M. Bonzi, F. Rossi and V. Dal Santo, Dalton Trans., 2004,
1783–1784.
chemoselectivity improved upon prolonged reaction times. 11 D. Rechavi and M. Lemaire, Chem. Rev., 2002, 102, 3467–
Catalysts were stable and robust with negligible copper leach-
3494.
3208 | Green Chem., 2014, 16, 3202–3209
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