Highly efficient and selective photocatalytic hydrogenation of functionalized nitrobenzenes

Article abstract of DOI:10.1039/c3gc42042f

We report a simple but efficient photocatalytic nitrobenzene reduction method employing eosin Y as the photocatalyst and TEOA as the reducing agent. With green LED light irradiation, the nitro group in the nitrobenzenes containing other reducible groups was chemoselectively reduced into an amino group, and the corresponding anilines were isolated in quantitative yields. The photoinduced electron transfer mechanism suggests that the high chemoselectivity originates from the better electron-withdrawing ability of the nitro group.

Full text of DOI:10.1039/c3gc42042f

Green Chemistry
View Article Online
View Journal | View Issue
COMMUNICATION
Highly efficient and selective photocatalytic
hydrogenation of functionalized nitrobenzenes†
Xiu-Jie Yang,a Bin Chen,*b Li-Qiang Zheng,*a Li-Zhu Wu*b and Chen-Ho Tunga,b
Cite this: Green Chem., 2014, 16,
1082
Received 1st October 2013,
Accepted 5th November 2013
DOI: 10.1039/c3gc42042f
www.rsc.org/greenchem
We report a simple but efficient photocatalytic nitrobenzene high pressure are usually needed which raise a high require-
reduction method employing eosin Y as the photocatalyst and ment for the equipment and may cause potential safety pro-
TEOA as the reducing agent. With green LED light irradiation, the blems. Evidently, it is highly desirable to develop methods for
nitro group in the nitrobenzenes containing other reducible the hydrogenation of aromatic nitro compounds that are eco-
groups was chemoselectively reduced into an amino group, and nomically feasible without toxic waste production, and are
the corresponding anilines were isolated in quantitative yields. The performed with high chemoselectivity under mild conditions.
photoinduced electron transfer mechanism suggests that the high
Over the past few years, the use of visible-light photoredox
chemoselectivity originates from the better electron-withdrawing catalysis to initiate organic transformations has made great
ability of the nitro group.
progress and has become a powerful tool in synthetic organic
chemistry,11 because such catalytic reactions are environ-
mentally benign, easy to handle, and carried out under mild
conditions.11b,12 A wide range of chemical reactions promoted
by such catalysis have been elegantly devised and developed
into practical synthetic methods.11,12 Furthermore, several con-
venient photocatalysts such as organometallic complexes of
ruthenium(II)12a,d,13 and iridium(III),12b,c and purely organic
dyes14 have been successfully adopted in visible-light induced
organic transformations. Among these, eosin Y (EY)14c,15 is a
widely used photocatalyst because it is structurally simple,
inexpensive and effective. In addition, several metal-free
heterogeneous organophotocatalysis systems for organic func-
tional group transformations under visible light irradiation
have also been reported.16
In the present study, we examined the visible-light photo-
catalytic reduction of nitrobenzenes containing other reduci-
ble groups (carbon–carbon or carbon–nitrogen triple bonds,
carbonyl, Cl or Br substituents) by using EY as the photo-
catalyst and triethanolamine (TEOA) as the reducing agent. We
found that only the nitro group was chemoselectively reduced
to an amino group at full conversion for the examined nitro-
benzene derivatives and that the aminobenzenes were isolated
in almost quantitative yields. The photocatalytic reduction was
carried out in oxygen-free ethanol–water (3 : 2, v/v) solution at
room temperature and atmospheric pressure. Typically, 5 mL
of a solution of the substrate (0.2 mmol), the photocatalyst
(EY, 1 mol%) and the reducing agent (TEOA, 3–6 equiv.) in a
Functionalized anilines are important intermediates for the
manufacture of agrochemicals, pharmaceuticals, dyes and pig-
ments.1 Most functionalized anilines are produced by hydro-
genation of the corresponding aromatic nitro compounds. The
key problem in such hydrogenation is the chemoselectivity
when other reducible groups are present in the same molecule.
A variety of methods for reducing the nitro group in a selective
manner, involving noncatalytic2 and catalytic3 methods, and
different reducing agents3,4 have been developed. The noncata-
lytic processes use either Béchamp (with Fe/HCl as a reducing
system)2b or sulfide reduction (with H2S or NaSH as a reducing
agent)2a technology. Although there are still certain selectivity
problems that can only be solved by using these stoichiometric
methods, these processes create large amounts of environ-
mentally toxic waste.5 In catalytic methods, the most fre-
quently used catalysts are noble metals, Pt6 and Au7 supported
9
on active carbon,3a,4b,8 TiO2,6,7 SiO2 or Fe2O3,3a and Ni-based
systems.10 Whereas such conventional catalysts can adequately
solve some selectivity problems, more demanding processes
often require modified or tailored catalysts.6 Furthermore,
when H2 is used as the reducing agent, high temperature and
aKey Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of
Education, Jinan, Shandong 250100, P. R. China. E-mail: lqzheng@sdu.edu.cn
bKey Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technical Institute of Physics and Chemistry, The Chinese Academy of Science,
Beijing, 100190, P. R. China. E-mail: chenbin@mail.ipc.ac.cn, lzwu@mail.ipc.ac.cn
†Electronic supplementary information (ESI) available: Experimental details,
control experiment, transient absorption spectra, 1H NMR and 13C NMR spectra.
See DOI: 10.1039/c3gc42042f
Pyrex reactor was irradiated with
a 3 W green LED
light (525 nm). The progress of the reaction was followed by
UV-Vis absorption spectra of the sample. For example, in the
1082 | Green Chem., 2014, 16, 1082–1086
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Communication
Fig. 1 UV-Vis absorption spectral change in the course of irradiation of
the solution containing nitrobenzene (0.2 mmol), TEOA (20 equiv.) and
EY (1 mol%) in 5 mL of EtOH–H2O (3 : 2) at pH 8.50 with 525 nm LED
(measured after a dilution of 150 times).
Scheme 2 Possible reaction pathways for the reduction of an aromatic
nitro compound to the corresponding aniline.
the irradiation time. Upon performing the photocatalytic reac-
tion in a solution pH of 8.50 and using 3–6 equivalent of TEOA,
full conversion and excellent chemoselectivity can be achieved
in reasonable irradiation time. When using 1 mol% EY, the
catalytic turnover number of the photocatalyst EY is close to
100, with the isolated yield of aniline 93%. As shown in Fig. 1,
during the photocatalytic reduction the absorption of EY with
λmax at 520 nm remained unchanged, suggesting that this
photocatalyst is stable. The high stability might be ascribed to
the formation of EY cation radical (EY+•), and the electron
transfer from TEOA to EY+• can quickly regenerate EY.19
Indeed, after the conversion of nitrobenzene is completed, we
syringed additional nitrobenzene and TEOA to the reactor and
irradiated the sample. We observed that the added nitro-
benzene was reduced at a similar rate.
It is generally accepted that the reduction of nitrobenzene
to aniline proceeds either through the direct route via nitroso-
benzene and N-phenylhydroxylamine intermediates, or a con-
densation route via azoxybenzene, azobenzene and hydrazo-
benzene (Scheme 2).20 We applied 1H NMR spectroscopy to
follow the photocatalytic reduction and indeed detected nitro-
sobenzene and N-phenylhydroxylamine in the initial stage of
the reaction (Fig. S1†). No azoxybenzene, azobenzene and
hydrazobenzene were observed. Furthermore, we carried out
the photocatalytic reduction of azobenzene under similar con-
ditions, and only isolated hydrazobenzene. No aniline was
detected (Table S2†). These results suggest that the photo-
catalytic reduction of nitrobenzene to aniline proceeds via the
direct route.
Scheme 1 Oxidation of triethanolamine.
reduction of nitrobenzene, upon irradiation the absorption of
the substrate with λmax at 265 nm gradually decreased in inten-
sity accompanying the appearance of a new absorption with
λmax at 240 nm ascribable to aniline, and an isobestic point
at 256 nm was observed (Fig. 1). After irradiation, the
1
product was isolated and identified by H NMR spectroscopy,
MS spectrometry and on the basis of a known retention time
of an authentic compound on GC. Control experiments
showed that the photocatalyst (EY), the reducing agent (TEOA)
and light were all essential for the effective reduction of nitro-
benzene (Table S1,† entries 1–4).
Two factors could affect the efficiency of the photocatalytic
reduction of nitrobenzene. First, the photocatalytic reduction
is very dependent on the pH value of the sample, and the
optimal pH value is 8.50 (Table S1,† entries 4–8). In this photo-
catalytic system, TEOA acts as a sacrificial electron donor.17 As
shown in Scheme 1, in the course of the photocatalytic reac-
tion, TEOA will provide electrons and protons for the
reduction. In acidic solution, TEOA will be protonated which
would reduce its electron donating ability. On the other hand,
in strong basic solution, the reduction efficiency is also low as
protons are involved in the reduction process.15a Secondly, the
amount of TEOA used is crucial for efficient reduction
(Table S1†). According to Scheme 1, each TEOA molecule with
one H2O molecule can provide two electrons and two protons
for the reduction.18 The reduction of one molecule of nitro-
benzene to aniline requires 6 electrons and 6 protons. Thus,
one can expect that full conversion could be achieved when 3
equivalents of TEOA are used. As shown in Table S1† (entries
12, 13), this is indeed the case. In such conditions by prolong-
ing irradiation, the conversion of the starting material can
reach above 90%. Employing excess TEOA can greatly shorten
A visible-light photoredox catalytic reaction has been pro-
posed to originate from single electron transfer.14b,d To eluci-
date the mechanism for the photocatalytic reduction of
nitrobenzene, we examined the quenching of EY excited states
by nitrobenzene and TEOA. EY in a water–ethanol mixed solu-
tion shows fluorescence with λmax at 543 nm. Experiments
revealed that nitrobenzene or TEOA only slightly quenched
fluorescence of EY even at high concentrations, suggesting
that neither electron transfer between the EY singlet excited
state (1EY*) and nitrobenzene, nor that between 1EY* and
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 1082–1086 | 1083

View Article Online
Communication
Green Chemistry
Fig. 2 Transient absorption spectra of (a) EY (1 × 10−5 M); (c) EY (1 × 10−5 M) and nitrobenzene (NB, 4 × 10−4 M); (e) EY (1 × 10−5 M) and TEOA (2.4 ×
10−3 M); (g) EY (1 × 10−5 M), nitrobenzene (4 × 10−4 M) and TEOA (2.4 × 10−3 M) with 532 nm light in de-aerated EtOH–H2O (3 : 2); (b) kinetic traces
at 560 nm obtained on laser photolysis of EY (1 × 10−5 M) and nitrobenzene at different concentrations ranging from 0 M to 1 × 10−4 M at intervals
of 2 × 10−5 M and Stern–Volmer plots (inset); (d) decay and rise-time profiles at 560 nm and 460 nm respectively upon excitation of the solution of
EY (1 × 10−5 M) and nitrobenzene (4 × 10−4 M); (f) kinetic traces at 403 nm obtained on laser photolysis of EY (1 × 10−5 M) and TEOA (2.4 × 10−3 M)
with 532 nm light; (h) kinetic traces at 460 nm obtained on laser photolysis of the solution of EY (1 × 10−5 M), nitrobenzene (4 × 10−4) and TEOA at
different concentrations ranging from 0 M to 1.5 × 10−3 M and Stern–Volmer plots (inset).
TEOA is important in the photocatalytic reduction. Thus, we 3EY*. The Stern–Volmer plots give the electron transfer rate
carried out the flash-photolysis study in degassed water– constant (k2) to be 1.96 × 107 M−1 s−1 (Fig. S3†). This rate con-
ethanol solutions. Upon laser excitation at 532 nm, the solu- stant is two orders of magnitude smaller than that of electron
tion of EY alone exhibited a strong bleaching of the ground transfer from 3EY* to nitrobenzene (k1, 1.72 × 109 M−1 s−1).
state around 520 nm and the triplet state (3EY*) absorption Although the amount of TEOA used was 3–6 equivalents of
with λmax at 560 nm (Fig. 2a).21 The decay throughout the nitrobenzene in our photocatalytic reduction, the quenching
3
absorption region and the recovery of the bleach occurs simul- rate of EY* by nitrobenzene would be ca. 30–15 times that by
taneously and can be described by a monoexponential func- TEOA. Thus, we infer that the triplet excited state of EY is
tion with a lifetime of ca. 150 μs (Fig. 2b, curve 1). Upon quenched mainly by nitrobenzene via electron transfer.
addition of nitrobenzene into the solution of EY, the absorp-
The radical cation EY+• generated through electron transfer
3
tion at 560 nm was gradually replaced by a new absorption from EY* to nitrobenzene can undergo electron transfer with
centered at 460 nm (Fig. 2c). The new absorption is assigned TEOA producing TEOA+• and ground state EY. As evidenced in
to the absorption of EY+• based on the literature.21 The decay Fig. 2g, photolysis of the solution of EY, nitrobenzene and
of the triplet absorption and the rise of the radical cation TEOA immediately results in the triplet EY absorption at
occur in the same time scale, suggesting occurrence of the 560 nm. This absorption was gradually replaced by the transi-
electron transfer from the triplet EY to nitrobenzene (Fig. 2d). ent absorption of EY+• around 460 nm owing to electron trans-
3
As the concentration of nitrobenzene increases from 0 to fer from EY* to nitrobenzene. The EY+• absorption is greatly
1 × 10−4 M, the lifetime of 3EY* decreases from 150 μs to 5.6 μs quenched by TEOA. In the absence of TEOA, the lifetime of
(Fig. 2b). From the Stern–Volmer plots, the electron transfer EY+• is ca. 345 μs (Fig. 2h, curve 1), while in the presence of
3
rate constant from EY* to nitrobenzene (k1) was calculated to 1.5 × 10−3 M TEOA the lifetime of EY+• is shortened to 22.9 μs.
be 1.72 × 109 M−1 s−1 (Fig. 2b). The triplet EY can also be The Stern–Volmer plots in Fig. 2h give the electron transfer
quenched by TEOA. When the solution of EY and TEOA was rate constant (k3) from TEOA to EY+• to be 2.7 × 107 M−1 s−1
.
excited with a 532 nm laser pulse, the absorption of 3EY* On the basis of the above-mentioned results, the photo-
around 560 nm was immediately observed and gradually catalytic reduction mechanism is summarized in Scheme 3.
replaced by an absorption at 403 nm (Fig. 2e). The later The photoinduced electron transfer from the triplet EY to
absorption has a lifetime of 357 μs (Fig. 2f) and is ascribed to nitrobenzene (oxidative quenching) results in the formation of
the absorption of the EY radical anion (EY−•).21 Again the EY+• and the nitrobenzene radical anion. Subsequent electron
3
decay of EY* and the rise of EY−• occurred in the same time transfer from TEOA to EY+• regenerates EY and yields TEOA+•.
scale (Fig. S2†), indicating electron transfer from TEOA to Reaction of the nitrobenzene radical anion with the TEOA
1084 | Green Chem., 2014, 16, 1082–1086
This journal is © The Royal Society of Chemistry 2014

View Article Online
Green Chemistry
Communication
heterogeneous catalytic reduction. In classical heterogeneous
catalytic reduction, the adsorption–reaction–desorption
mechanism7b suggests that the chemoselectivity will depend
on the strength of the adsorption of the respective reducible
groups on the catalyst surface. Here, in the photocatalytic
reduction the chemoselectivity will be determined by the
ability of accepting electrons of the respective reducible
groups, as suggested by Scheme 3. The nitro group has much
greater ability of accepting electrons than the other reducible
groups in the substrates shown in Table 1. Thus, it is not
difficult to understand why the nitro group in the substrates is
selectively reduced in the photocatalytic reduction.
Scheme 3 Possible mechanism for the photocatalytic reduction of
nitrobenzenes.
In summary, we have developed an efficient green light
driven catalytic reduction of nitrobenzenes into industrially
important anilines by using a pure organic photocatalyst. This
reaction shows excellent chemoselectivity when other reduci-
ble groups are present in the same nitrobenzene molecule.
Flash-photolysis investigation revealed that the reaction was
initiated by single electron transfer from the photocatalyst
triplet excited state to the nitrobenzenes. The photocatalyst
was regenerated by receiving an electron from the reducing
agent. Reaction of the nitrobenzene radical anion with the
reducing agent radical cation produced the anilines.
Table 1 Photocatalytic reduction of nitrobenzenesa
Reducible
group : X
Conversionb
[%]
Selectivityc
[%]
Entry
Position
1
2
3
4
5
6
7
8
—
p-
p-
p-
p-
p-
p-
p-
p-
m-
o-
–H
–Cl
–Br
99
99
98
99
98
100
95
96
100
93
–CH3
–OCH3
–CHO
–COCH3
–CuCH
–CuN
–CuN
–CuN
100
100
100
>99
>99
>99
>99
99d
90
Acknowledgements
9
10
11
100
100
100d
We are grateful for funding from the 973 program (no.
2013CB834505 and 2013CB834804), the National Natural
Science Foundation of China (grant no. 91027041, 91127017,
and 21090343), Solar Energy Initiative of the Knowledge Inno-
vation Program of the Chinese Academy of Sciences.
a Light source: LED (green light, 525 nm); room temperature; substrate
amount: 0.2 mmol; EY: 1 mol%; TEOA: 6 equiv.; EtOH–H2O (3 : 2),
5 mL; pH = 8.50; irradiation time: 24 h. b Determined by 1H NMR
(CDCl3). c Selectivity
yielded as the product.
=
yield/conversion. d 2-Aminobenzamide was
Notes and references
radical cation in the presence of H2O gives glycolaldehyde and
diethanolamine (Scheme 1), and produces the intermediates
shown in Scheme 2. The intermediates further undergo photo-
catalytic reduction in a similar manner and eventually yield
aniline.
1 (a) R. S. Downing, P. J. Kunkeler and H. van Bekkum, Catal.
Today, 1997, 37, 121; (b) H. U. Blaser, H. Steiner and
M. Studer, ChemCatChem, 2009, 1, 210.
2 (a) R. F. Kovar and F. E. Armond, US. Patent 3, 975, 444,
1976; (b) M. Suchy, P. Winternitz and M. Zeller, WO Patent
91/00278, 1991; (c) J. Butera and J. Bagli, WO Patent
91/09023, 1991.
3 (a) A. Corma and P. Serna, Science, 2006, 313, 332;
(b) G. Wienhöfer, I. Sorribes, A. Boddien, F. Westerhaus,
K. Junge, H. Junge, R. Llusar and M. Beller, J. Am. Chem.
Soc., 2011, 133, 12875; (c) I. Sorribes, G. Wienhöfer,
C. Vicent, K. Junge, R. Llusar and M. Beller, Angew. Chem.,
Int. Ed., 2012, 51, 7794.
4 (a) A. Corma, P. Serna and H. García, J. Am. Chem. Soc.,
2007, 129, 6358; (b) R. V. Jagadeesh, G. Wienhöfer,
F. A. Westerhaus, A.-E. Surkus, M.-M. Pohl, H. Junge,
K. Junge and M. Beller, Chem. Commun., 2011, 47, 10972;
(c) R. V. Jagadeesh, G. Wienhöfer, F. A. Westerhaus,
A.-E. Surkus, H. Junge, K. Junge and M. Beller, Chem.–Eur.
J., 2011, 17, 14375; (d) D. Cantillo, M. Baghbanzadeh and
With the understanding of the reaction mechanism, the
photocatalytic reduction was further extended to the nitro-
benzene derivatives with reducible groups, and the chemo-
selectivity was particularly examined. Table 1 summarizes the
results for a variety of substrates. The nitro group in the sub-
strates bearing either an electron-donating (entries 4 and 5) or
an electron-withdrawing (entries 6–11) substituent could be
reduced to the corresponding amino group. The reducible
groups other than nitro existing in the substrates involve
carbonyl (aldehyde and ketone), carbon–carbon triple bond,
carbon–nitrogen triple bond, Cl and Br. For all the substrates
examined except ortho cyanonitrobenzene22 only the nitro
group was reduced, and the isolated yields for the corres-
ponding anilines were close to 100% at full conversion. We
note that the factors governing the chemoselectivity in such
photocatalytic reduction are different from the classical
This journal is © The Royal Society of Chemistry 2014
Green Chem., 2014, 16, 1082–1086 | 1085

View Article Online
Communication
Green Chemistry
C. O. Kappe, Angew. Chem., Int. Ed., 2012, 51, 10190;
(e) P. P. Sarmah and D. K. Dutta, Green Chem., 2012, 14,
1086.
C.-H. Tung and L.-Z. Wu, Adv. Synth. Catal., 2013, 355,
2158.
14 (a) A. W.-H. Mau, O. Johansen and W. H. F. Sasse, Photo-
chem. Photobiol., 1985, 41, 503; (b) M. Neumann,
S. Füldner, B. König and K. Zeitler, Angew. Chem., Int. Ed.,
2011, 50, 951; (c) S. Gazi and R. Ananthakrishnan, Appl.
Catal., B: Environ., 2011, 105, 317; (d) D. Ravelli and
M. Fagnoni, ChemCatChem, 2012, 4, 169; (e) Y. C. Teo,
Y. H. Pan and C. H. Tan, ChemCatChem, 2013, 5, 235.
15 (a) T. Lazarides, T. McCormick, P. W. Du, G. G. Luo,
B. Lindley and R. Eisenberg, J. Am. Chem. Soc., 2009, 131,
9192; (b) Q. Liu, Y.-N. Li, H.-H. Zhang, B. Chen, C.-H. Tung
and L.-Z. Wu, Chem.–Eur. J., 2012, 18, 620.
16 (a) F. Su, S. C. Mathew, G. Lipner, X. Fu, M. Antonietti,
S. Blechert and X. Wang, J. Am. Chem. Soc., 2010, 132,
16299; (b) F. Su, S. C. Mathew, L. Möhlmann, M. Antonietti,
X. Wang and S. Blechert, Angew. Chem., Int. Ed., 2011, 50,
657; (c) X.-H. Li, J.-S. Chen, X. Wang, J. Sun and
M. Antonietti, J. Am. Chem. Soc., 2011, 133, 8074;
(d) M. Cherevatskaya, M. Neumann, S. Füldner,
C. Harlander, S. Kümmel, S. Dankesreiter, A. Pfitzner,
K. Zeiter and B. König, Angew. Chem., Int. Ed., 2012, 51,
4062.
5 H. U. Blaser, U. Siegrist, H. Steiner and M. Studer, in Fine
Chemicals
through
Heterogeneous
Catalysis,
ed.
R. A. Sheldon and H. van Bekkum, Wiley-VCH, Weinheim,
2001, p. 389.
6 M. Makosch, W.-I. Lin, V. Bumbálek, J. Sá, J. W. Medlin,
K. Hungerbühler and J. A. van Bokhoven, ACS Catal., 2012,
2, 2079.
7 (a) M. Boronat, P. Concepción, A. Corma, S. González,
F. Illas and P. Serna, J. Am. Chem. Soc., 2007, 129, 16230;
(b) A. Corma, P. Serna, P. Concepción and J. J. Calvino,
J. Am. Chem. Soc., 2008, 130, 8748.
8 A. L. Goff, V. Artero, B. Jousselme, P. D. Tran, N. Guillet,
R. Métayé, A. Fihri, S. Palacin and M. Fontecave, Science,
2009, 326, 1384.
9 Y. Y. Chen, J. S. Qiu, X. K. Wang and J. H. Xiu, J. Catal.,
2006, 242, 227.
10 (a) X. C. Meng, H. Y. Cheng, S.-I. Fujita, Y. F. Hao,
Y. J. Shang, Y. C. Yu, S. X. Cai, F. Y. Zhao and M. Arai,
J. Catal., 2010, 269, 131; (b) W. W. Lin, H. Y. Cheng,
J. Ming, Y. C. Yu and F. Y. Zhao, J. Catal., 2012, 291,
149.
11 (a) J. Xuan and W. J. Xiao, Angew. Chem., Int. Ed., 2012, 51,
6828; (b) D. Ravelli, M. Fagnoni and A. Albini, Chem. Soc.
Rev., 2013, 42, 97; (c) C. K. Prier, D. A. Rankic and
D. W. C. MacMillan, Chem. Rev., 2013, 113, 5322.
17 (a) T. M. McCormick, B. D. Calitree, A. Orchard,
N. D. Kraut, F. V. Bright, M. R. Detty and R. Eisenberg,
J. Am. Chem. Soc., 2010, 132, 15480; (b) W. Zhang,
J. D. Hong, J. W. Zheng, Z. Y. Huang, J. R. Zhou and R. Xu,
J. Am. Chem. Soc., 2011, 133, 20680.
12 (a) D. A. Nicewicz and D. W. C. MacMillan, Science, 2008, 18 (a) A. J. Esswein and D. G. Nocera, Chem. Rev., 2007, 107,
322, 77; (b) D. A. Nagib, M. E. Scott and
D. W. C. MacMillan, J. Am. Chem. Soc., 2009, 131, 10875;
4022; (b) T. J. Yu, Y. Zeng, J. P. Chen, Y.-Y. Li, G. Q. Yang
and Y. Li, Angew. Chem., Int. Ed., 2013, 52, 5631.
(c) H. W. Shih, M. N. V. Wal, R. L. Grange and 19 (a) T. Shimidzu, T. Iyoda and Y. Koide, J. Am. Chem. Soc.,
D. W. C. MacMillan, J. Am. Chem. Soc., 2010, 132, 13600;
(d) T. P. Yoon, M. A. Ischay and J. Du, Nat. Chem., 2010, 2,
527; (e) G.-B. Deng, Z.-Q. Wang, J.-D. Xia, P.-C. Qian,
1985, 107, 35; (b) Z. Han, W. R. McNamara, M.-S. Eum,
P. L. Holland and R. Eisenberg, Angew. Chem., Int. Ed.,
2012, 51, 1667.
R.-J. Song, M. Hu, L.-B. Gong and J.-H. Li, Angew. Chem., 20 (a) F. Haber, Z. Elektrochem. Angew. Phys. Chem., 1898, 22,
Int. Ed., 2013, 52, 1535; (f) J.-J. Zhong, Q.-Y. Meng, 506; (b) H. U. Blaser, Science, 2006, 313, 312.
G.-X. Wang, Q. Liu, B. Chen, K. Feng, C.-H. Tung and 21 (a) E. Joselevich and I. Willner, J. Phys. Chem., 1995, 99,
L.-Z. Wu, Chem.–Eur. J., 2013, 19, 6443.
6903; (b) S. D. M. Islam, T. Konishi, M. Fujitsuka, O. Ito,
Y. Nakamura and Y. Usui, Photochem. Photobiol., 2000, 71,
675; (c) H.-H. Zhang, K. Feng, B. Chen, Q.-Y. Meng, Z.-J. Li,
C.-H. Tung and L.-Z. Wu, Catal. Sci. Technol., 2013, 3, 1815.
13 (a) I. Willner, W. E. Ford, J. W. Otvos and M. Calvin, Nature,
1979, 280, 823; (b) T. Hirao, J. Shiori and N. Okahata, Bull.
Chem. Soc. Jpn., 2004, 77, 1763; (c) Y. Y. Chen, A. S. Kamlet,
J. B. Steinman and D. R. Liu, Nat. Chem., 2011, 3, 146; 22 The reduction of ortho cyanonitrobenzene yields 2-amino-
(d) D. Kalyani, K. B. Mcmurtrey, S. R. Neufeldt and
M. S. Sanford, J. Am. Chem. Soc., 2011, 133, 18566;
(e) X.-W. Gao, Q.-Y. Meng, M. Xiang, B. Chen, K. Feng,
benzamide. This reaction has been reported. The nitrile
moiety in this case is not reduced but rather its hydrolysis
to an amide group. See ref. 1b.
1086 | Green Chem., 2014, 16, 1082–1086
This journal is © The Royal Society of Chemistry 2014