the products, benzil 5 (traces), 2-oxo-1,2-diphenylethyl benzoate
(21%: observed also in the benzoin condensation catalysed by
the aforementioned facile formation of benzoic acid in the presence
6
of CO and air, the IL shows very low catalytic activity after
2
6c
NHCs generated via electrochemical reduction ) and benzoic acid
(22%) could be isolated, together with the residual benzaldehyde
Fig. 3).
recycling, which necessitates air contact. Addition of triethylamine
has, however, reactivated the catalytic process showing that the
non-desirable effect is indeed attributable to the presence of the
acid formed.
7
(
As a summary of the results described above, imidazolium
acetates are effective catalysts for benzoin condensation, and
also show catalytic activity in NHC catalyzed hydroacylation.
Moreover, oxidation reactions were also activated by the EMIM-
Ac used, and further investigation of this observation together
with other EMIM-Ac catalyzed processes is in progress in our lab-
oratory. Accordingly, imidazolium acetates uniting the beneficial
properties of ILs and organocatalysts are of high potential in such
processes, provided that no acid formation takes place.
2
Fig. 3 Oxidation of benzoin by air (CO ), and the subsequent hydroacy-
lation, catalyzed by EMIM-Ac.
Considering that all products obtained in this reaction (5,6
and 7) are oxidized, and no reduced organic product could be
observed, it is reasonable to assume that (one component of) air
Acknowledgements
11
We want to thank BASF SE and New Hungary Development
Plan (Project ID: TAMOP-4.2.1/B-09/1/KMR-2010-0002) for
the financial support. Thanks are due to Dr J o´ zsef Balla and
has served as an oxidizing agent. 4 is known to be stable under air,
◦
and it shows no sign of oxidation even by melting at 137 C, or
12
by heating in conventional organic solvents. Carbon dioxide was,
´
Dr Aron Sz o˝ ll o˝ si for measuring the GC-MS and NMR spectra,
1
3a,b
however, reported recently to oxidize aromatic aldehydes
to the
respectively.
13
corresponding acids by NHC catalysis, although it is debated,
13c-e
whether it is the oxidant in the reaction.
of the degassed EMIM-Ac sample under a CO
same products were obtained as in the presence of air, showing
the crucial role of CO in the oxidation process, resulting in
Repeating the reaction
2
atmosphere, the
Notes and references
1
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13
benzoic acid and apparently also in 5. To our best knowledge,
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14
by hydroacylation from 5 and benzaldehyde is also known, the
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15
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2
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2
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This journal is © The Royal Society of Chemistry 2011
Org. Biomol. Chem., 2011, 9, 5362–5364 | 5363