Communication
hydrogen peroxide, which can oxidize ClÀ to Cl2 to drive the
chlorination under strongly acidic conditions. A possible mech-
anism for the overall process is outlined in Scheme 2.
close to being higher than that reported for the traditional
chlorination technology, and this method has considerable ad-
vantages (see below). Also, the yield for this level of conversion
reaches up to 95% (Table 1, entry 1) for toluene and a p-NO2-
toluene substrate appears to give similar results (Table 1,
entry 5). However, the reaction of iso-propylbenzene resulted
in the formation of 2-phenyl-2-propanol, resulting from hydrol-
ysis of the corresponding chlorination product (Table 1,
entry 3).
Although there has been no direct evidence for electron
transfer on the Ag@AgCl catalyst surface under visible light,
the morphology of the catalyst before and after reaction re-
veals that the interface of the nano Ag and AgCl undergoes
obvious changes. Figure 1b shows that different circular cavita-
tions appeared on the surface of AgCl after the photocatalytic
reaction occurred five times. The formation of these cavitations
was caused by the disappearance of AgCl at the interface of
the nano Ag, which absorbs the photons leading to the oxida-
tion of chloride ions to chlorine radicals. The photogenerated
reducing electrons produced in situ transform silver ions to
silver metal. The Ag0/AgI ratio could be demonstrated by X-ray
diffraction of the Ag@AgCl after the catalytic reaction was
complete. The relative content of Ag0 increased from 6 to
16 mol% (Supporting Information, Figure 5). It is clear that the
AgCl in the catalyst participates in the reaction and the relative
content of the Ag@AgCl changes; however, when the catalyst
was used repeatedly (4 further times), its catalytic activity re-
mained almost unchanged. The conversion of toluene was still
higher than 38% when the Ag@AgCl catalyst was used for
a fourth time.
We have developed an efficient photocatalytic chlorination
of the a-H of alkylarenes using NaCl/HCl as the chlorine
source, which occurs through a radical mechanism under
either sunlight or visible light conditions. The chlorine radical
is formed by electron transfer from chloride ion to O2 in air
through the bandgap hole of the AgCl semiconductor. This
chlorination protocol is characterized not only by the use of
natural sunlight or visible light, but also by mild conditions,
cheap and sustainable chlorine source, green solvent condi-
tions, and high selectivity. The conversion of toluene is the
highest of the substrates examined at 40%, which almost
equals traditional chlorination methods but has the major ad-
vantage of being cheaper, cleaner, more sustainable, and more
selective. In addition, the catalyst can be re-used four times
with little impact upon reactivity, and the subsequent yield of
benzyl chloride is 95%. This novel reaction process possibly
provides new insight into understanding the formation of or-
ganochlorine-containing compounds in nature. Further work
to this heterogeneous catalyst system, especially applied to
the halogenation of different substrates, including alkanes and
cycloalkanes, will be reported, and the turnover number (TON)
of the photocatalyst will be further examined and improved by
changing the preparation method and optimizing the
components.
After examining the chlorination reaction of the a-H of tolu-
ene with NaCl/HCl in the presence of Ag@AgCl and sunlight or
visible light, we then examined the chlorination of other alky-
larenes including substituted toluene, ethylbenzene, isopropyl-
benzene, and methylnaphthalene in order to confirm the
scope and utility of the method. As shown in Table 1, the con-
Table 1. The result of the photocatalytic chlorination of the alkylarene
with NaCl/HCl.
Experimental Section
In a typical case, AgNO3 (0.51 g) and polyvinyl pyrrolidone (PVP,
K30, 0.625 g) were dissolved in 1.4m nitric acid solution (50 mL)
with stirring at RT. Aqueous KCl solution (50 mL, 0.06m) was added
to the mixture. After 30 min, the mixture was heated at 808C for
3 h. The resulting precipitate was collected by centrifugation, dis-
persed in a solution of deionized water (50 mL) and AgNO3
(0.10 g), and was irradiated with UV irradiation for 20 min in the
presence of sodium formate (1 mL, 0.02m). The resulting product
was collected, washed thoroughly with deionized water, then abso-
lute ethanol, and then dried at 708C in air for 12 h.
Entry
Ar-
R1
R2
Conversion
[%][a]
Yield
[%][b]
Light
source[c]
1
2
3
Ph
Ph
Ph
H
H
40
23
38
20
31
8
27
41
20
29
17
95
94
92
85
42
38
87
93
90
88
85
lamp
sunlight
lamp
sunlight
lamp
sunlight
lamp
lamp
sunlight
lamp
Me
Me
H
Me
4
5
p-Cl-Ph
p-NO2-Ph
H
H
H
H
6
p-tBu-Ph
H
H
Chlorination procedure of alkylarene with NaCl/HCl under
visible light
sunlight
[a] Gas chromatography data using 6 mol% nano Ag@AgCl. [b] Isolated
yield. [c] 300 W xenon lamp equipped with an ultraviolet cut-off filter.
The phase-transfer catalyst (0.30 g) and the Ag/AgCl (containing
3.5–8.7 mol% nano Ag photocatalyst, 0.90 g) were added, with stir-
ring, to a photocatalytic reaction vessel (PLS-SXE300CUV) charged
with alkylarene (0.5 mol). Saturated sodium chloride solution
(120 mL) containing conc. hydrochloric acid (6 mL) was added to
the solution. The visible light produced by a 300 W xenon lamp
equipped with an ultraviolet cut-off filter was used as light source
to irradiate the reaction mixture for 5–9 h and the reaction was
monitored by GC. The reaction solution was filtered (to reclaim the
version of the different alkylarenes depends on the structure
of the starting material. GC analysis of the reaction solution
showed that the reaction was highly selective and multiple
chlorination products were not detected. Although these con-
versions were not higher than for toluene, the level of 40% is
Chem. Eur. J. 2015, 21, 9671 – 9675
9674
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