Z. Zand et al. / Tetrahedron Letters 55 (2014) 338–341
339
Table 1
Table 2
Photoreduction of nitrobenzene to aniline in the presence of TiO2–P25 using solar
light
Photoreduction of nitrobenzene to aniline in the presence of TiO2–P25 using LED
NO2
NH2
TiO2
NO2
NH2
TiO2
EtOH (4 ml), LEDa
30 h
EtOH (4 ml), solar lightf
Argon
Entry
TiO2 (g)
Nitrobenzene (mmol)
Conversionb (%)
Entry
TiO2 (g)
Nitrobenzene (mmol)
Time (h)
Conversiona (%)
1c
2
3
0
2 Â 10À3
2 Â 10À3
2 Â 10À3
2 Â 10À3
2 Â 10À2
2 Â 10À3
0
45
100
100
15
1b
2
3
4
5
0
2 Â 10À3
2 Â 10À3
2 Â 10À3
2 Â 10À3
2 Â 10À2
2 Â 10À2
2 Â 10À1
2 Â 10À3
2 Â 10À3
2 Â 10À3
8
6
3
1
8
1.5
8
3
3
0
60
80
100
40
100
30
75
85
0
0.05
0.08
0.1
0.1
0.1
0.005
0.007
0.01
0.005
0.01
0.01
0.007
0.007
0.01
4
5
6d
0
6
7
a
b
c
Four blue LEDs with 3 W electrical power, 80 Lumens.
8c
9d
10e
Determined by integration of the signals in GC chromatograms.
Reduction of nitrobenzene without TiO2.
In the dark.
d
8
a
b
c
d
e
f
Determined by integration of the signals in GC chromatograms.
Reduction of nitrobenzene without TiO2.
In 2-propanol.
In methanol.
be more efficient than LED. This is most likely due to the high
intensity and UV absorption of solar light.
Kisch has proposed that a photocatalyst concentration around
the plateau onset should be selected in order to ensure optimal
light absorption.15 Only in this case is a comparison of apparent
quantum yields or rates of various photocatalytic reactions mean-
ingful. Therefore, we selected 0.01 g and 0.08 g as the optimum
amounts of TiO2–P25 for solar and LED conditions respectively.
The results (Tables 1 and 2) revealed the high photocatalytic
efficiency of TiO2–P25 under visible light. After optimization of
the reaction conditions for both solar light and LEDs, the substrate
scope was investigated. Table 3 summarizes the results for the
photoreductions of nitro compounds using both LED and solar
light. In general, the products obtained via LED irradiation were
similar to when using solar light (Table 3), but the photoactivity
of TiO2 was found to be higher in the presence of solar light
compared to blue light LEDs.
Nitro aromatic compounds possessing functional groups such
as carbonyl, cyano, and halide were reduced selectively into their
corresponding amines (Table 3, entries 4–7 and 9). 1,4-dinitroben-
zene and 1,2-dinitrobenzene gave both nitroaniline and diamine
(Table 3, entries 2 and 3). We found that 2-methoxynitrobenzene
has higher conversion under solar light irradiation than with LEDs
(Table 3, entry 11). Nitromethane failed to react under the
experimental conditions. Nitromethane is weak electron acceptor
compared to nitrobenzenes,16 this discrepancy is obvious in the
In the dark.
(8 am–4 pm, sunlight intensity between 80 and 10 Â 103 Lux).
nitrobenzene was obtained with 0.005 g of the photocatalyst
(Table 1, entry 2). Increasing amounts of TiO2–P25 led to a signifi-
cant enhancement of the photoreduction: 80% conversion using
0.007 g in 3 h, and 100% using 0.01 g in 1 h (Table 1, entries 3 and 4).
Next, we investigated the effect of increasing the concentration
of nitrobenzene on the conversion. The results showed a quantita-
tive conversion (100%) for 0.02 mmol nitrobenzene using 0.01 g
TiO2 after irradiation for 1.5 h. The results also showed neither
irradiation alone nor the catalyst without light irradiation afforded
any conversion of nitrobenzene (Table 1, entries 1 and 10). It is also
known that the alcohol solvent is a sacrificial reagent, the electron
hole in the valence band of an excited semiconductor transfer to
alcohol. This photocatalytic reaction of nitrobenzene with TiO2
was investigated in solvents such as methanol and isopropanol
with irradiation for 3 h and the results are given in Table 1 (entries
8 and 9). There was no significant difference between ethanol and
methanol, so we used ethanol owing to its low cost and toxicity.
Recently, visible light emitting diodes (LED) sensitized by low
power have been applied as an attractive means to initiate photo-
catalytic reactions. Several advantages are associated with the use
of LED lamps as a light source such as high photon efficiency, low
voltage of electricity, and power stability.14 Due to the high impor-
tance and advances in the application of visible light in photocata-
lytic reactions, we studied the reduction of nitro compounds using
LEDs. The reduction of nitrobenzene under blue light LED
(4 Â 3 W) irradiation at an initial concentration of 2 Â 10À3 mmol
was examined using TiO2–P25 over 30 h. We observed that 45%,
100%, and 100% conversions of nitrobenzene (2 Â 10À3 mmol) were
obtained when using 0.05 g, 0.08 g, and 0.1 g of TiO2–P25, respec-
tively (Table 2, entries 2–4). The photoreduction was also investi-
gated for 2 Â 10À2 mmol of nitrobenzene using 0.1 g of TiO2–P25.
The result showed that the efficiency of the reduction was
decreased when the concentration of nitrobenzene increased
(Table 2, entry 5). In general, the observations clearly confirm the
beneficial role of TiO2–P25 under LED irradiation in the reduction
of nitrobenzene (Table 2).
Next, we investigated the effect of the TiO2–P25 amount on the
conversion of nitrobenzene under solar light and LED irradiation.
The analysis revealed that high conversions of nitrobenzene were
possible using solar light and low amounts of TiO2–P25 (Fig. 1).
The trend observed with solar light and TiO2–P25 was found to
Figure 1. Effect of the TiO2 amount on the photocatalytic conversion of nitroben-
zene. Reaction conditions: nitrobenzene (2 Â 10À3 mmol) in ethanol (4 mL) under
LED irradiation for 30 h, solar light (1–6 h).