Chlorine-radical-mediated photocatalytic activation of C-H bonds with visible light

Article abstract of DOI:10.1002/anie.201207904

Going green on the cheap: A highly effective photocatalytic reaction mode involving chlorine radicals generated on chlorinated BiOBr/TiO₂ upon irradiation with visible light enabled selective transformations of alkanes into functionalized products through the formation of C-O and C-C bonds (see scheme). This process provides a sustainable strategy for direct C-H functionalization under mild conditions. Copyright

Full text of DOI:10.1002/anie.201207904

Angewandte
Chemie
DOI: 10.1002/anie.201207904
Photocatalysis
À
Chlorine-Radical-Mediated Photocatalytic Activation of C H Bonds
with Visible Light**
Rusheng Yuan,* Shaolong Fan, Huaxi Zhou, Zhengxin Ding, Sen Lin, Zhaohui Li,*
Zizhong Zhang, Chao Xu, Ling Wu, Xuxu Wang, and Xianzhi Fu
3
À
The selective activation and transformation of C(sp ) H
bonds in alkanes to produce higher-value products is of great
importance in both fundamental and applied chemistry.[1]
Since these bonds are both thermodynamically strong and
kinetically inert, conventional catalytic strategies for the
activation step require the use of toxic, aggressive, and
expensive reagents under rather stringent conditions.[2] Het-
erogeneous semiconductor photocatalysis is considered to be
tants by irradiation with UV light. Recently, our research
group found that TiO2-based photocatalysts with chlorine
chemisorbed on their surface showed a remarkable improve-
ment in photocatalytic activity in the degradation of gaseous
organic substrates under irradiation with visible light.[7] This
finding makes photocatalytic reactions more practical owing
the availability of endlessly renewable solar energy. Most
interestingly, the behavior of these chlorinated TiO2-based
photocatalysts in the degradation of aromatic alkanes
depended on whether UV or visible light was used. Under
UV irradiation, both high conversion and extensive mineral-
ization of toluene were observed, whereas under irradiation
with visible light, high conversion and rather low mineraliza-
tion were observed. In the latter case, the low mineralization
suggested that toluene had been transformed into oxygenated
products other than CO2. This result led us to reason that this
chlorine-radical-mediated strategy might be more promising
for the selective transformation of alkanes: a widely sought
yet elusive process in organic synthesis.
À
a promising alternative for the activation of C H bonds under
mild conditions.[3] The use of cheap and abundant molecular
oxygen as the oxidant and light as the driving force makes
such processes especially appealing for green chemical
reactions for industrial applications. However, most photo-
catalytic reactions require high-energy ultraviolet radiation
and show extremely low visible-light activity.[4] Therefore, the
development of an environmentally friendly and highly
efficient photocatalytic system that can transform inert
alkanes into functionalized products under visible light is
significant and highly desirable.
3
À
Although the C(sp ) H bonds in alkanes are strong and
Herein, we report the application of surface-chlorinated
3
À
difficult to cleave directly, they react readily with extremely
reactive species, such as free radicals.[5] For example, the
aggressive hydroxyl radical usually generated in photocatal-
BiOBr/TiO2 (CBT) for the selective activation of C(sp ) H
bonds in alkanes under irradiation with visible light. The
generation of chlorine radicals and their pivotal role in these
heterogeneous photocatalytic processes were revealed for the
first time. The use of a cheap, environmentally friendly
inorganic semiconductor photocatalyst and solar energy
make this method more green and sustainable than conven-
À
ysis has been the typical initiator of C H cleavage, but
mineralizes nearly all organic compounds owing to its poor
selectivity. The chlorine radical, which shows similar hydro-
gen-abstraction behavior, was reported by the Ollis research
group to enhance activity in the photocatalytic degradation of
gaseous contaminants under UV irradiation, although no
direct evidence for its formation was provided.[6] However,
the idea of chlorine-assisted photocatalysis has not drawn
much attention, since its practical application has been
restricted to the photodegradation of gas-phase trace pollu-
À
tional C H bond-activation strategies.
The oxygenation of hydrocarbons was initially carried out
in benzotrifluoride (Btf) over BiOBr/TiO2 (BT) under
irradiation with visible light (l > 420 nm) with a moderate
light intensity of 250 mWcmÀ2 (see Table S1 in the Supporting
Information). Although it occurred with almost 100%
selectivity for the formation of the corresponding aldehyde
or ketone, the conversion of toluene and cyclohexane was low
[*] Dr. R. S. Yuan, S. L. Fan, H. X. Zhou, Dr. Z. X. Ding, Dr. S. Lin,
Prof. Dr. Z. H. Li, Dr. Z. Z. Zhang, Dr. C. Xu, Prof. L. Wu,
Prof. X. X. Wang, Prof. X. Z. Fu
À
(ca. 1%), since the photocatalytic activation of C H bonds by
semiconductors usually shows extremely low activity under
visible-light irradiation. Surprisingly, when CBT was used as
the photocatalyst under similar conditions, a significantly
improved conversion ratio was observed. The conversion of
toluene and cyclohexane reached 18.5 and 11.8%, respec-
tively, which was about 18 and 13 times that observed for
toluene and cyclohexane over BT. Meanwhile, the chlorinated
system maintained good selectivity for conversion into
benzaldehyde (80.2%) and cyclohexanone (95.2%). The
CBT catalyst was also active for the transformation of other
hydrocarbons, with 55.0% conversion of ethylbenzene and
33.3% conversion of p-xylene. In all cases, no chlorinated
products were detected by GC.
Research Institute of Photocatalysis
Fujian Provincial Key Laboratory of Photocatalysis, State Key
Laboratory Breeding Base, Fuzhou University
Fuzhou, 350002 (P. R. China)
E-mail: yuanrs2002@yahoo.com.cn
zhaohuili1969@yahoo.com
[**] This research was supported financially by the 973 Program
(2011CB612314 and 2013CB632405), the NSFC (21077023,
21273035, and 21033003), and the NSF of Fujian Province
(2010J01035, JA10008, and JK2011001). Z.H.L. thanks the Award
Program for a Minjiang Scholar Professorship.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201207904.
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Inspired by these initial promising results, we further
investigated the photocatalytic oxygenation of hydrocarbons
over CBT in the absence of an organic solvent, since solvent-
free conditions are environmentally friendly and more
practical for the separation of the desired products
(Table 1). Like in Btf, the catalyst CBT also showed highly
absence of a solvent seems lower than that in Btf, the absolute
yield of the corresponding aldehyde or ketone and the TON
are far greater.
We also explored the effects of different variables on the
activity over CBT. In all cases, no chlorinated products were
detected. An increase in the reaction time from 4 to 12 h led
to high conversion of the substrates as well as good selectivity
for the formation of the corresponding aldehyde or ketone
(Table 1, entries 11 and 12). When we increased the light
intensity from 250 to 450 mWcmÀ2 to increase the number of
incident photons, the conversion of all substrates increased;
however, the selectivity for the formation of the correspond-
ing aldehyde or ketone decreased slightly (Table 1, entries 15
and 16). The CBT chlorine content was also found to play
a critical role in the selective transformation of toluene and
cyclohexane. The use of CBT catalysts obtained by treating
BT with higher concentrations of HCl led to the oxygenated
products in higher yields (Figure 1a; see also Table S1). The
Table 1: Photocatalytic oxygenation of several typical hydrocarbons over
chlorinated (CBT) and nonchlorinated (BT) BiOBr/TiO2 catalysts under
irradiation with visible light and solvent-free conditions.[a]
Entry
Substrate
Cat.
Yield[b]
[mmol]
Conv.
[%]
Sel.
[%]
TON
1
2
3
4
toluene
toluene
BT
CBT
BT
CBT
CBT
CBT
BT
CBT
BT
CBT
CBT
CBT
CBT
CBT
CBT
CBT
CBT
CBT
9.8
85.1
2.6
0.12
1.00
0.03
0.47
1.03
0.40
0.48
1.90
0.38
1.32
1.80
0.85
3.60
2.23
1.86
0.68
4.28
2.83
86.7
90.6
100
–
20.9
–
9.7
21.5
8.3
–
34.5
–
26.4
37.4
17.5
65.3
44.9
38.9
14.1
77.5
56.8
cyclohexane
cyclohexane
toluene
cyclohexane
ethylbenzene
ethylbenzene
p-xylene
p-xylene
toluene
cyclohexane
ethylbenzene
p-xylene
36.0
82.8
87.4
81.3
93.8
86.2
96.1
91.6
94.0
87.0
96.7
90.6
86.2
83.6
87.4
86.2
5[c]
6[c]
7
423.0
151.0
36.7
133.7
34.3
109.1
158.8
68.3
284.4
183.3
151.0
53.0
8
9
10
11[d]
12[d]
13[d]
14[d]
15[e]
16[e]
17[e]
18[e]
toluene
cyclohexane
ethylbenzene
p-xylene
305.1
221.0
[a] General reaction conditions: photocatalyst (20 mg), substrate (1 mL),
room temperature, O2 atmosphere (0.1 MPa), visible-light irradiation
(420<l<780 nm), light intensity: 250 mWcmÀ2, irradiation time: 4 h.
[b] Yield of benzaldehyde, cyclohexanone, acetophenone, or p-tolualde-
hyde. [c] The reaction was carried out on a larger scale (toluene: 5 mL,
photocatalyst: 100 mg). [d] The reaction was carried out with an
irradiation time of 12 h. [e] The reaction was carried out with a light
intensity of 450 mWcmÀ2
.
improved performance under solvent-free conditions with
respect to that of BT for the oxidation of pure hydrocarbons.
Upon irradiation for 4 h, the conversion of toluene over CBT
reached 1.0%, which was about 8.3 times that observed for
the reaction over BT (Table 1, entries 1 and 2). The amount of
benzaldehyde produced in 4 h was 85.1 mmol per milliliter,
which corresponds to a high selectivity of 90.6% and a high
turnover number (TON) of 20.9. In the case of cyclohexane,
the conversion observed over CBT was 15.7 times more than
that over BT, whereas the selectivity for the formation of
cyclohexanone was still high at 82.8% (Table 1, entries 3 and
4). When the reaction was scaled up by a factor of 5, toluene
and cyclohexane were also converted into benzaldehyde and
cyclohexanone in high yield (423.0 and 151.0 mmol, respec-
tively; Table 1, entries 5 and 6). Both the yield per gram of the
photocatalyst and the reaction rate for the oxidation of
toluene and cyclohexane with excitation by visible light (l >
420 nm) are superior to those observed with previously
reported photocatalytic systems.[4f–i] CBT also showed signifi-
cantly improved activity for the conversion of ethylbenzene
and p-xylene into their oxygenated products (Table 1,
entries 8 and 10). Although the conversion over CBT in the
Figure 1. Influence of a) the chlorine content in the photocatalyst and
b) the wavelength of the incident light on the yield of oxygenated
products derived from the photocatalyzed conversion of toluene. The
results in (b) were obtained with the catalyst CBT, a photograph of
which is shown.
dependence of the activity on the intensity and wavelength of
the incident light indicates that the transformation of hydro-
carbons over CBT is typically photocatalytic (Figure 1b). The
estimated apparent quantum efficiency (AQE) for the trans-
formation of hydrocarbons into oxygenated products reached
15.3 and 6.7% (at l = (430 Æ 25) nm) for toluene and
cyclohexane, respectively (see Figure S1 in the Supporting
Information). Although CBT can only absorb light with
a wavelength lower than 550 nm, it still catalyzed the
formation of benzaldehyde from toluene (2 mL) in satisfying
yield (251.9 mmol) with high selectivity (83.1%) under natural
solar radiation with a low intensity of 80 mWcmÀ2 (see
Table S2). All of these results indicate that the oxygenation of
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hydrocarbons over irradiated CBT is controllable, chemically
productive, and even practically applicable (see Table S3).
In a control reaction over HF-treated BT, almost no
improvement in activity was observed for the oxygenation of
toluene with respect to the equivalent reaction over BT (see
Table S1). This result ruled out the possible contribution of
the introduced Brønsted acid to the enhanced activity[8] and
together with the dependence of the activity on the chlorine
content suggested that chlorine was involved in the oxygen-
ation process over CBT. The electron spin resonance (ESR)
spectrum of irradiated CBT (in the absence of O2) in the
presence of N-tert-butyl-a-phenylnitrone (PBN) as a trapping
agent showed a large double-quartet splitting corresponding
to the Cl-PBNC adduct[9] and thus confirmed the formation of
chlorine radicals during the selective oxidation of hydro-
carbons over CBT (Figure 2a). In contrast, no signal corre-
sponding to Cl-PBNC was observed when BT was irradiated.
This result implies that the chlorine radicals formed over the
irradiated CBT system are responsible for its higher photo-
catalytic activity in hydrocarbon oxygenation. Moreover,
when the CBT catalyst was irradiated with higher-energy
UV light (l > 320 nm), the intensity of the signals for the
chlorine radical increased greatly (Figure 2b), and the con-
version of hydrocarbons into oxygenated products was higher
(see Table S3).
aldehyde or ketone was poor in this homogeneous system,
since both oxygenated and chlorinated products were
obtained. It was also found that the introduction of CCl4
into reaction systems containing the BT photocatalyst led
a significant enhancement of the conversion of both toluene
(1.9-fold) and cyclohexane (32-fold) in Btf under UV
irradiation in comparison with the use of BT alone (see
Table S4). However, in systems containing CCl4, the forma-
tion of chlorinated products was unavoidable. The extremely
low conversion of toluene observed under irradiation with
visible light in the absence of chlorine radicals (see Figure S3)
also demonstrated their dominant role in the transformation
of alkanes in the BT/CCl4 system. The direct participation of
the semiconductor photocatalyst in the hydrocarbon oxygen-
ation over CBT irradiated with visible light was also
confirmed by an experiment in which chlorine radicals were
quenched. The addition of PBN to the reaction system led to
a significant decrease in the formation of the aldehyde or
ketone, but did not totally inhibit the reaction (Figure 2c).
Therefore, it is clear that the oxygenation of hydrocarbons
over CBT is induced by both chlorine radicals and direct
semiconductor photocatalysis.
The proposed existence of a dominant pathway involving
radicals in the oxygenation of hydrocarbons over the current
CBT system was corroborated by the addition of tetrame-
thylpiperidine N-oxide (TEMPO), a typical radical scavenger,
to the reaction system. The transformation of a substrate into
the corresponding oxygenated products decreased as the
amount of TEMPO was increased, and when TEMPO was
added at a concentration of 64 mmollÀ1, the conversion of the
substrate was almost completely inhibited (Figure 2d; see
also Table S6). A comparison of the transformations of
toluene, cyclohexane, ethylbenzene, and p-xylene revealed
that the conversion activity over CBT is inversely propor-
To further corroborate the important role of chlorine
radicals in the reaction, we added CCl4, which is believed to
generate chlorine radicals through homolytic cleavage upon
UV irradiation (see Figure S2), to a reaction system contain-
ing a substrate (toluene or cyclohexane), a solvent, and O2,
but without a photocatalyst. Both oxygenated and chlorinated
products as well as HCl were obtained under UV irradiation
(see Tables S4 and S5). However, in the absence of CCl4 or
under irradiation with visible light, no products were
detected, which suggests that chlorine radicals produced by
À
tional to the C H bond strength, which suggests that the
À
À
the UV irradiation of CCl4 are essential for C H activation in
cleavage of the C H bond in the hydrocarbon is an important
this system. Selectivity for the formation of the corresponding
step in the oxygenation reactions. An experiment carried out
to determine the kinetic isotopic effect (KIE) showed
a significant kH/kD value of 4.6 for the oxidation of toluene
in our CBT system (see Table S7). We could therefore
À
conclude that C H bond cleavage is probably the rate-
determining step. Furthermore, although a range of substi-
tuted toluene derivatives underwent oxygenation to form the
corresponding aldehydes, the reaction rate was higher for
a toluene derivative with an electron-donating para substitu-
ent (CH3) than for substrates with electron-withdrawing para
substituents (Cl, Br, and CF3; see Table S8). Thus, more
strongly electron withdrawing substituents on the benzene
ring drastically deactivate the aromatic system for hydrogen
abstraction.
On the basis of the above experimental evidence, the
following mechanism is proposed for the selective oxidation
of hydrocarbons over the CBT catalyst (Scheme 1): BiOBr
with a narrow band gap (Eg) can be excited to produce
electron and hole pairs under irradiation with visible light.[10]
When BiOBr is coupled to TiO2 and chlorinated, the photo-
generated holes can transfer to TiO2 as a result of the more
negative valence band of BiOBr. On TiO2 these holes are
trapped by chlorine or hydroxy groups chemisorbed on the
Figure 2. a,b) Evidence for the formation of chlorine radicals on the
CBT photocatalyst under irradiation with visible light (a) and UV light
(b). c,d) Influence of the trapping agents PBN (c) and TEMPO (d) on
the formation of oxygenated products in the photocatalytic reaction
(the concentration of the trapping agent is given on the x axis).
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Scheme 1. Proposed mechanism for the chlorine-radical-mediated photocatalytic activation of hydrocarbons and their further functionalization.
CB=conduction band, NHE=normal hydrogen electrode, VB=valence band.
catalyst surface to produce chlorine radicals or hydroxyl
radicals. Both types of radicals can abstract hydrogen from
alkanes to give alkyl radicals, but in organic media the
production of hydroxyl radicals is very limited. Since chlorine
radicals are more reactive than hydroxyl radicals in hydrogen-
abstraction reactions,[11] the presence of chlorine radicals
during the aerobic oxidation of alkanes will lead to the
production of more alkyl radicals. These alkyl radicals can
either react with O2 to form ROOC or react with chlorine
radicals to give alkyl halides. In the CBT system, these
chlorine radicals remain surface-bound in a similar way to
that described for hydroxyl radicals by Xu et al.[12] Sur-
rounded by a large excess of the alkane, most of the chlorine
radicals are consumed to form alkyl radicals and stable HCl at
the solid–liquid interface. Owing to the presence of an ample
O2 supply, including the adsorbed O2 on the catalyst surface
and a continuous supply of O2 from the solution at the
interface (see Figure S4), the possibility of the reaction
between chlorine radicals and alkyl radicals to form alkyl
halides is highly suppressed, and oxygenated products are
obtained exclusively, although DFT calculations imply that
the alkyl halides should be the main products (see Figure S5
and Table S12). In contrast, in the reactions involving CCl4
under UV irradiation, collisions between chlorine radicals
and alkyl radicals to form alkyl halides can not be avoided,
since the chlorine radicals generated homogeneously in
solution can migrate freely. As a result, lower selectivity for
the formation of the oxygenated products is observed.
The ROOC radicals formed can either combine to form
ROOOOR or be reduced by the photogenerated electrons on
the photocatalyst and undergo reaction with H+ to yield
ROOH.[4c] The former pathway leads to the formation of
alcohols and ketones in almost equal amounts through the
decomposition of ROOOOR and is commonly observed in
selective oxygenation reactions. However, the formation of
aldehydes or ketones as the dominant products with the CBT
catalyst indicates that in our system the ROOC radical favors
the reductive pathway to form ROOH. DFT calculations
revealed that the process to form ROOH is very exothermic,
whereas the formation of ROOOOR is slightly endothermic.
Furthermore, the elevated concentration of protons derived
from preadsorbed HCl and generated through the reaction of
chlorine radicals with the hydrocarbon should help to direct
the reaction to yield ROOH. The dehydration of ROOH to
yield the aldehyde or ketone can occur readily, since this
process is highly exothermic. A clearly enhanced yield of the
alcohol product was observed when the preferred reductive
pathway to form ROOH via ROOC was blocked by the
addition of a typical electron acceptor (quinine) as a result of
the consumption of the photogenerated electrons by quinine.
The surface chlorination of other semiconductors, such as
WO3 and WO3/TiO2, also led to significant improvements in
these photocatalysts for the selective oxygenation of both
toluene and cyclohexane (see Table S11).
Since the chlorine radicals generated over irradiated CBT
can promote the formation of alkyl radicals, we expected that
À
the CBT photocatalytic system could also be applied to C C
coupling reactions involving alkyl radicals: another important
À
application of C H activation. In the absence of O2, toluene
was transformed over CBT irradiated with visible light into
a range of coupling products dominated by 1-benzyl-4-
methylbenzene, 1-benzyl-2-methylbenzene, and 1,2-diphenyl-
ethane, whereas the coupling reaction between toluene and
benzene led to diphenylmethane as the major product (see
Figure S8). In contrast, only a trace amount of the desired
coupling product was obtained if CBT was replaced with BT.
Thus, the surface chlorination of BT plays a pivotal role in the
reaction. These preliminary studies on this propagation
reaction pathway involving aryl radicals show that our
À
reaction mode provides an alternative approach to C C
bond construction directly from hydrocarbons without pre-
functionalization of the starting materials.[13] The optimiza-
tion of this synthetic route and further detailed mechanistic
studies are under way.
In summary, we have developed a highly effective photo-
catalytic reaction mode involving chlorine radicals generated
on a semiconductor system upon irradiation with visible light
for the transformation of alkanes into oxygenated products,
even under solvent-free conditions. This system can also be
applied to other synthetic reactions involving alkyl radicals.
The current study represents a conceptual breakthrough in
À
direct C H functionalization by photocatalysis.
Received: October 1, 2012
Published online: November 26, 2012
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À
Phys. Chem. C 2011, 115, 19320; h) D. Tsukamoto, A. Shiro, Y.
Keywords: C H activation · chlorine radicals · photocatalysis ·
solvent-free conditions · visible light
.
Shiraishi, T. Hirai, J. Phys. Chem. C 2011, 115, 19782; i) H.
Hattori, Y. Ide, S. Ogo, K. Inumaru, M. Sadakane, T. Sano, ACS
Catal. 2012, 2, 1910.
[5] A. A. Fokin, P. R. Schreiner, Chem. Rev. 2002, 102, 1551.
[6] a) Y. Luo, D. F. Ollis, J. Catal. 1996, 163, 1; b) O. dꢁHennezel,
D. F. Ollis, J. Catal. 1997, 167, 118; c) M. Lewandowski, D. F.
Ollis, J. Catal. 2003, 217, 38; d) O. dꢁHennezel, P. Pichat, D. F.
Ollis, J. Photochem. Photobiol. A 1998, 118, 197.
[7] R. Yuan, T. Chen, E. Fei, J. Lin, Z. Ding, J. Long, Z. Zhang, X.
Fu, L. Wu, X. Wang, ACS Catal. 2011, 1, 200.
[8] Q. Wang, M. Zhang, C. Chen, W. Ma, J. Zhao, Angew. Chem.
2010, 122, 8148; Angew. Chem. Int. Ed. 2010, 49, 7976.
[9] E. G. Janzen, B. R. Knauer, L. T. Williams, W. B. Harrison, J.
Phys. Chem. 1970, 74, 3025.
[10] Z. Jiang, F. Yang, G. Yang, L. Kong, M. O. Jones, T. Xiao, P. P.
Edwards, J. Photochem. Photobiol. A 2010, 212, 8.
[11] A. K. Croft, H. M. Howard-Jones, C. E. Skates, C. C. Wood, Org.
Biomol. Chem. 2011, 9, 7439.
[12] Y. Xu, K. Lv, Z. Xiong, W. Leng, W. Du, D. Liu, X. Xue, J. Phys.
Chem. C 2007, 111, 19024.
[13] a) G. Deng, K. Ueda, S. Yanagisawa, K. Itami, C.-J. Li, Chem.
Eur. J. 2009, 15, 333; b) C.-L. Sun, H. Li, D.-G. Yu, M. Yu, X.
Zhou, X.-Y. Lu, K. Hang, S.-F. Zheng, B.-J. Li, Z.-J. Shi, Nat.
Chem. 2010, 2, 1044; c) K. Fagnou, M. Lautens, Chem. Rev. 2003,
103, 169.
[1] a) C. Copꢀret, Chem. Rev. 2010, 110, 656; b) J. A. Labinger, J. E.
Bercaw, Nature 2002, 417, 507.
[2] a) A. E. Shilov, G. B. Shulꢁpin, Chem. Rev. 1997, 97, 2879; b) D.
Balcells, E. Clot, O. Eisenstein, Chem. Rev. 2010, 110, 749;
c) S. E. Bromberg, H. Yang, M. C. Asplund, T. Lian, B. K.
McNamara, K. T. Kotz, J. S. Yeston, M. Wilkens, H. Frei, R. G.
Bergman, C. B. Harris, Science 1997, 278, 260; d) L. Kesavan, R.
Tiruvalam, M. H. A. Rahim, M. I. B. Saiman, D. I. Enache, R. L.
Jenkin, N. Dimitratos, J. A. Loppez-Sanchez, S. H. Taylor, D. W.
Knight, C. J. Kiely, G. J. Hutchings, Science 2011, 331, 195.
[3] a) A. Maldotti, A. Molinari, R. Amadelli, Chem. Rev. 2002, 102,
3811; b) M. Fagnoni, D. Dondi, D. Ravelli, A. Albini, Chem. Rev.
2007, 107, 2725.
[4] a) T. P. Yoon, M. A. Ischay, J. Du, Nat. Chem. 2010, 2, 527; b) F.
Su, S. C. Mathew, G. Lipner, X. Fu, M. Antonietti, S. Blechert, X.
Wang, J. Am. Chem. Soc. 2010, 132, 16299; c) K. Ohkubo, K.
Suga, K. Morikawa, S. Fukuzumi, J. Am. Chem. Soc. 2003, 125,
12850; d) R. Li, H. Kobayashi, J. Guo, J. Fan, J. Phys. Chem. C
2011, 115, 23408; e) M. Zhang, C. Chen, W. Ma, J. Zhao, Angew.
Chem. 2008, 120, 9876; Angew. Chem. Int. Ed. 2008, 47, 9730;
f) S. Sarina, H. Zhu, Z. Zheng, S. Bottle, J. Chang, X. Ke, J. Zhao,
Y. Huang, A. Sutrisno, M. Willans, G. Li, Chem. Sci. 2012, 3,
2138; g) S. Furukawa, T. Shishido, K. Teramura, T. Tanaka, J.
Angew. Chem. Int. Ed. 2013, 52, 1035 –1039
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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