Catalysis Science & Technology
Communication
in situ from the oxidation of the solvent, reducing drastically
the number of side products and/or harmful chemicals. The
photo-catalyst is commercially available from the World Gold
Council.17 In conclusion, photo-catalysts can be tailored to
perform selective and multi-step reactions. This offers great
opportunities in the field of organic synthesis.
Scheme 2 Photo-cyclization of 5-aminopentanol over Au–TiO2.
Table 3 Performance of Au–TiO2 in the photo-cyclization of 5-aminopentanol
Acknowledgements
Yield of
d-valerolactam
(%)
Yield of
piperidine
(%)
Reaction
time/h
Conversion
(%)
The authors thank Dr Frank Krumeich from ETH for the TEM
measurements.
4
17
74.0
98.5
16.3
32.5
4.8
0.3
Notes and references
carried out with 10 bar of hydrogen and at a temperature
exceeding 60 1C,11d to ensure meaningful catalytic conversions.
Furthermore, the catalytic activity of the bare support is often
very low and the overall catalytic performance is not strongly
affected by the presence of substituents.12
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The other possible mechanism is the photo-reduction of nitro-
arene. Nitroarene reduction was proposed by Mahdavi et al.,13 who
demonstrated that TiO2 under UV light could efficiently reduce
nitroarenes to corresponding anilines. Addition of Ag to the catalyst
formulation improved the catalytic performance,14 which they
attributed to a decrease of charge-recombination, suggesting that
nitro reduction occurs primarily on TiO2. Furthermore, the activity
was found to be dependent on the nitroarene substituent identity
and position with respect to the nitro group,15 which is precisely
what the results summarized in Table 2 show. Therefore we assume
that conversion of nitro to amine occurs photo-catalytically on TiO2.
After the formation of aniline, the reaction proceeds as previously
mentioned.
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has to do with the nitroarene reaction rate to aniline. A plethora of
species were measured, however it should be pointed out that they
were all intermediate species of either aniline formation (slower
reaction rates in the nitroarene reduction) or N-alkylation, possibly
caused by the competition for the active site, i.e., products related to
other reactions were not detected. When some of the reactions were
allowed to proceed for 24 h, yields of 70% or better were measured,
suggesting that the low yields are related primarily to the short time
on stream. However for comparison purposes the reactions were
carried out for the same time as that of the alkylation of aniline.
N-heterocycles are important scaffolds for many drugs and
natural products.16 As a proof of concept, we tested our catalyst
in the photo-cyclization of amino-alcohols for the production of
commercially valuable lactams and heterocycle amines, as
depicted in Scheme 2 and shown in Table 3.
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´
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4. Conclusions
The presented results propose that photo-catalysis can be used
for the synthesis of secondary and tertiary aniline derivatives in
one-pot. The photo-catalysis process is very versatile since the
reaction can be carried out either with nitro- or amino-arenes.
The reported approach is facile, mild (ambient conditions) and
´
´
2873–2920; (e) V. V. Kouznetsov, L. Y. V. Mendez and C. M. M. Gomez,
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c
This journal is The Royal Society of Chemistry 2013
98 Catal. Sci. Technol., 2013, 3, 94--98