K. Selvam, M. Swaminathan / Catalysis Communications 12 (2011) 389–393
393
Table 2
process has significant advantages when compared with other
methods (i) a cheap and stable reactant (alcohol), (ii) it does not
require the use of acids, oxidants or reductants, and (iii) the reaction
proceeds under milder ambient conditions. Therefore, this process has
the potential to enable a more sustainable quinaldine synthesis from
nitroarenes. The combination of photocatalytic redox-cyclization
reaction presented here may help to develop a new strategy towards
the development of photocatalysis-based organic synthesis.
Supplementary materials related to this article can be found online
at doi:10.1016/j.catcom.2010.11.004.
Effect of water content on photocatalytic synthesis.
Run
Conditionsa
Yield of quinaldine [%]
Conversion [%]
1
2
3
4
Neat ethanol
75
24
14
6
99
99
99
85
Ethanol–H2O (99/1)
Ethanol–H2O (98/2)
Ethanol–H2O (96/4)
All reactions were performed with a 25 mM alcoholic solution of a reactant 50 mg of
Au–TiO2 suspension, IUV =1.381×10−6 Einstein L−1 s−1, irradiation time—5 h.
photogenerated conduction band electron on the Au–TiO2 surface.
Simultaneously the alcohol is oxidized to the corresponding aldehyde
consuming the photogenerated valance band holes on TiO2. The
photocatalytic formation of aldehyde from alcohol was well estab-
lished [19]. These initial steps of oxidation and reduction are more
facilitated with Au–TiO2 when compared to pure TiO2. Pathways A
and B (Scheme 1) suggest the formation of an imine (Schiff base) via
the reaction between the aniline and acetaldehyde, whereas pathway
C requires the formation of crotonaldehyde, the product of an aldol
condensation of two acetaldehyde units. These pathways are
proposed on the basis of earlier report on the conversion of nitroarene
to tetrahydroquinoline using TiO2 with a co-catalyst p-toluene
sulfonic acid [12]. Crotonaldehyde was produced in low concentration
and consumed readily. Hence it could not be detected by GC–MS
under the experimental conditions of this work. In fact, when
crotonaldehyde was made to react with N-ethylideneaniline ther-
mally, the reaction was completed within 4 h.
More evidence for the reaction pathway B is the formation of
tetrahydroquinoline, which is also detected by GC–MS (Fig. 5). 2-
Methyl-N-phenyl-1,2,3,4-tetrahydroquinolin-4-amine (I) can be
formed via cycloaddition of the imine (II) to its enamine tautomer.
Subsequent elimination of an aniline molecule from I yields the
dihydroquinoline (III) as intermediate, which on dehydration gives
quinaldine. In the reaction pathway A, titanium enolate formed on the
surface of TiO2 condenses with the aniline to give IV, which on further
condensation and oxidation yield corresponding quinaldines. Forma-
tion of enolate with TiO2 was reported by Chen et al. [20].
Acknowledgements
Authors thank Prof. P.V. Satyam, IOP, Bhubaneswar, India for HR-
TEM measurements. The authors also thank Professor M.S. Hegde,
IISC, Bangalore for the XPS measurements. The author (M.S) thanks
the Council of Scientific and Industrial Research (CSIR), New Delhi, for
the financial support through research grant no. 21 (0799)/10/EMR-II.
One of the authors, K. Selvam is thankful to CSIR, New Delhi, for the
award of Senior Research Fellowship.
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