NaI/PPh3-Mediated Photochemical Reduction and Amination of Nitroarenes

Article abstract of DOI:10.1021/acs.orglett.1c01654

A mild transition-metal- and photosensitizer-free photoredox system based on the combination of NaI and PPh3 was found to enable highly selective reduction of nitroarenes. This protocol tolerates a broad range of reducible functional groups such as halogen (Cl, Br, and even I), aldehyde, ketone, carboxyl, and cyano. Moreover, the photoredox catalysis with NaI and stoichiometric PPh3 provides also an alternative entry to Cadogan-type reductive amination when o-nitrobiarenes were used.

Full text of DOI:10.1021/acs.orglett.1c01654

pubs.acs.org/OrgLett
Letter
NaI/PPh3‑Mediated Photochemical Reduction and Amination of
Nitroarenes
Zhonghua Qu,# Xing Chen,# Shuai Zhong, Guo-Jun Deng, and Huawen Huang*
Cite This: Org. Lett. 2021, 23, 5349−5353
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: A mild transition-metal- and photosensitizer-free
photoredox system based on the combination of NaI and PPh3 was
found to enable highly selective reduction of nitroarenes. This
protocol tolerates a broad range of reducible functional groups
such as halogen (Cl, Br, and even I), aldehyde, ketone, carboxyl,
and cyano. Moreover, the photoredox catalysis with NaI and stoichiometric PPh3 provides also an alternative entry to Cadogan-type
reductive amination when o-nitrobiarenes were used.
nilines play fundamentally substantial roles in both
Aacademia and industry, which therefore has triggered
wide method development of nitroarene reduction.1 While
noncatalytic reduction of nitroarenes using stoichiometric
reducing agents provides currently main commercial access to
functionalized anilines, these methods suffer from poor
selectivity and serious ecological issues.2 Recent progress on
catalytic reduction of nitroarenes mainly depends on
transition-metal (TM) catalysis via direct hydrogenation or
hydrogen transfer,3 electrocatalysis along with water oxida-
tion,4 and sustainable visible-light-induced photocatalysis
(Scheme 1a).5 Of them, the mild photocatalytic process via
hole-driven hydrogen transfer with hydrogen donors or hole
scavenger is an attractive strategy for reduction of nitroarenes.6
Hence, three kinds of photocatalytic systems based on photon
absorber including plasmonic,7 semiconductor,8 and dye-
sensitized photocatalyst9 have been exploited. In most cases,
however, the use of transition metals such as copper,10 silver,11
and palladium6b also lead to the issues of functional group
tolerance with respect to, for example, dehalogen12 and
addition upon unsaturated bonds.13 Recently, Wu and co-
workers14 reported a facile but efficient visible-light-driven
photochemical homogeneous nitroarene reduction using eosin
Y as photocatalyst and triethanolamine (TEOA) as electron
donor. In this system, the aqueous conditions resulted in
complete hydrolysis of cyano group and highly excessive
amounts of TEOA are required, which may restrict its broader
application.
On the other hand, intramolecular reductive amination of 2-
nitrobiarenes, the so-called Cadogan reaction (Scheme 1b), are
traditionally achieved under thermal conditions with stoichio-
metric triphenylphosphine,15 with recent development of mild
process using Grignard reagents16 and catalytic protocols based
on transition-metal catalysts17 or phosphacycloalkane cata-
lyst.18 Recently, we discovered mild photocatalytic reductive
couplings19 including the Cadogan amination employing
4CzIPN as the photosensitizor.19a Further studies on this
project reveal that the photocatalytic reduction of nitroarenes
could proceed in the absence of photocatalyst. Herein, we
report on these findings to disclose a divergent NaI/PPh3-
mediated photochemical reduction and amination of nitro-
arenes20 (Scheme 1c). Salient features of the present reduction
protocol include no need for transition-metal catalysts, mild
Scheme 1. Catalytic Nitroarene Reduction and
Intramolecular Amination (Cadogan Reaction)
Received: May 16, 2021
Published: June 28, 2021
https://doi.org/10.1021/acs.orglett.1c01654
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5349−5353
5349

Organic Letters
pubs.acs.org/OrgLett
Letter
reaction conditions, ample substrate scope with broad
functional group tolerance, high chemoselectivities, and key
mechanistic insights in the combinational electron-donor−
acceptor (EDA) formation21 and phosphine catalysis.22
To commence our study, 2-nitrobiphenyl (1a) was selected
as model substrate to evaluate the reaction conditions of
divergent reduction upon hydrogenation and amination (Table
1). The synergistic NaI/PPh3 catalytic system combined with
absence of PhSiH3. To our delight, however, the carbazole
product (3a) via intramolecular Cadogan amination was
formed as the major product (Table 1, entry 16). In this
respect, a series of optimizations of reaction conditions were
performed for the reductive amination of 2-nitrobiarenes [see
the Supporting Information (SI) for details (Table S1)]. The
formation of 3a was obviously improved when the reaction was
conducted in 1,4-dioxane, with 2a further suppressed (Table 1,
entry 17). Control experiments reveal that the reaction in the
absence of NaI afforded 3a in only 14% yield (Table 1, entry
18).
a
Table 1. Optimization of Reaction Conditions
With the established reductive reactivities by the visible-
light-induced NaI/PPh3 catalytic system, we first extended it to
aniline formation from substituted nitroarenes especially with
reducible functional groups (Scheme 2). The substrates
b
yield (%)
Scheme 2. NaI/PPh3-Mediated Photochemical Reduction of
Nitroarenes
entry
PR3
PPh3
PPh3
PPh3
PPh3
P(o-tol)3
P(p-FPh)3
P(PMP)3
PCy3
PPh3
PPh3
reductant
solvent
2a
3a
1
2
3
4
5
6
7
8
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
TEOA
TEA
Et3SiH
PhSiH3
PhSiH3
PhSiH3
PhSiH3
DCM
Et2O
60
45
90
98
trace
80
75
trace
12
trace
10
<20
nd
trace
trace
nd
EtOAc
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
dioxane
dioxane
nd
trace
trace
trace
trace
trace
trace
trace
nd
nd
nd
nd
52
9
10
11
12
PPh3
PPh3
PPh3
c
d
13
14
15
16
17
24
trace
16
8
trace
e
PPh3
PPh3
PPh3
PPh3
f
f
82 (78)
14
ef
,
18
a
Reaction conditions: 1a (0.2 mmol), NaI (40 mol %), PPh3 (20 mol
%), reductant (2 equiv), solvent (2.0 mL), irradiation with a 35 W
b
blue LEDs at 60 °C for 72 h under argon atmosphere; Yield
determined by GC analysis of the crude reaction mixture using 1,3,5-
trimethylbenzene as the internal standard, and yield in parentheses
c
a
Isolated yield of 10 mmol scale reaction.
indicates isolated yield. NaCl, NaBr, KI, or NH4I instead of NaI.
d
e
f
Reaction in dark. Without NaI. 2.5 equiv of PPh3.
bearing either electron-withdrawing or electron-donating
groups worked well to exclusively afford the corresponding
aniline products. The reducible carbon−halogen bonds other
than nitro remained without any detectable collapse with Cl
(2c, 2s, 2u), Br (2h, 2l), or even I (2d, 2i, 2m). Among others
including aldehyde (2f), ketone (2j, 2o, 2v), carboxyl (2g, 2u),
and cyano (2e, 2k, 2n), the reduction also occurred only in the
nitro group. In other words, 100% nitro reductive selectivities
were observed in all examples of competitive reduction.
Incomplete conversion of some nitroarenes was observed to
result in lower than 90% yield (2i, 2k, 2q−2t). Notably,
nitropyridines bearing chloro and bromo also proceeded
through exclusive nitro reduction (2w, 2x). Finally, 8-
nitroquinoline worked with moderate conversion (2y).
However, other nitro heterocycles, such as those containing
thiazole, imidazole, indazole, etc., failed to give the
corresponding reduction products in this system.
PhSiH3 as hydrogen donor was found to be effective for the
reductive hydrogenation. Hence, the 2-aminobiphenyl product
(2a) was produced with yields ranging from 45% to 98% when
performing in different media including DCM, Et2O, EtOAc,
and CHCl3. While the sustainable EtOAc gave excellent yield,
CHCl3 was the best solvent (Table 1, entries 1−4). Among
other phosphines (Table 1, entries 5−7), while tri-o-
tolylphosphine did not work, tris(4-fluorophenyl)phosphine
and tris(4-methoxyphenyl)phosphine afforded 2a in good
yields. Aliphatic tricyclohexylphosphine also completely
prohibited the reduction (Table 1, entry 8). Then some
other reductants including TEOA, triethylamine (TEA), and
Et3SiH, dramatically reduced the yields (Table 1, entries 9−
11). We also tested other halides instead of NaI, where NaCl,
NaBr, KI, and NH4I were all inferior (Table 1, entry 12). The
results of control experiments suggested that NaI, PPh3, and
light stimulation are all critical for the reductive hydrogenation
(Table 1, entries 13−15). In comparison, the nitro reduction
also proceeded when stoichiometric PPh3 was utilized in the
Subsequently, the generality and substrate scope of the NaI/
PPh3-mediated reductive aminations of o-nitrobiarenes were
probed (Scheme 3). Moderate to good yields were obtained
5350
https://doi.org/10.1021/acs.orglett.1c01654
Org. Lett. 2021, 23, 5349−5353

Organic Letters
pubs.acs.org/OrgLett
Letter
Scheme 3. NaI/PPh3-Mediated Photochemically Reductive
Amination of 2-Nitrobiarenes
Figure 1. (a) UV−vis absorption spectra of reactant mixtures, (b)
Predicted excited states molecular orbital diagrams of Int-2.
leads to the formation of the atom transfer complex Int-1. It is
calculated to be endergonic by 2.2 kcal/mol, which is much
lower than the generation of dioxazaphosphetane intermediate
from trialkyl phosphines including Radosevich’s phospha-
cycle.15,22 On the other hand, the additional NaI seems to
affect the interaction of 1a and PPh3 slightly, but the
Coulombic interaction of it with both 1a and PPh3 forms a
new atom transfer complex (Int-2) (endergonic by 1.7 kcal/
mol). The complex Int-2 is expected to absorb photons to
initiate the first deoxygenation process.23 Moreover, the time-
dependent DFT (TD-DFT) calculation on the excited state of
Int-2 assigned a S0-to-S4 excitation, which was predicted to be
558 nm. This peak has almost exclusively charge-transfer
excitation characteristic (98%) from I-PPh3 fragment to
PhNO2 fragment (HOMO-3 to LUMO).
To obtain more mechanistic information on the NaI/PPh3-
mediated reduction of nitroarenes, some control experiments
were carried out. First, aniline (2z) was found to be the only
reduction product within 12 h with major nitrobenzene (1z)
recovered, while potential intermediate products such as
nitrosobenzene (4), azoxybenzene (5), and azobenzene (6)
were not observed (Scheme 4a). Moreover, when these
compounds instead of 1z were subjected to the standard
conditions, the target aniline was not formed in all cases with
or without light stimulation (Scheme 4b−d). These results
suggest that the presence of nitroarenes could probably be a
critical factor for the late-stage reduction of intermediate
products. Hence, compounds 4−6 were treated with additional
1-bromo-4-nitrobenzene in catalytic amounts (Scheme 4e−g).
Aniline was indeed generated as the major product in those
reactions. We speculate that the combination of nitrobenzene,
NaI, and PPh3 forms a EDA complex20a that absorbs photons
to not only decompose into nitrosobenzene or the Ph-N(OH)
intermediate but also transfer energy to enable late-stage
reduction of intermediate products.4a
when substituted o-nitrobiarenes were employed, with a range
of compatible functionalities including chloro, cyano, ester,
amide, and formyl (3a−3h). The C−F amination did not
occur when o-fluorophenyl was attached (3i). Then 11H-
benzo[a]carbazoles were generated in moderate to excellent
yields when the reactants bore a naphthyl moiety (3j−3m).
Notably, 2-(5-chloro-2-nitrophenyl)naphthalene afforded the
corresponding benzo[a]carbazole (3k) in almost quantitative
yield (96%). Among others, aza-carbazoles (3n and 3o) were
smoothly delivered from the starting 3-nitro-2-phenylpyridines,
albeit in relatively low yield. Moreover, aza-benzo[a]carbazole
3p produced in 35% yield. While o-nitrostilbenes did not work,
o-nitrochalcone could proceed through reductive amination to
afford the 2-benzoylindole product 3q. Finally, pyrido[1,2-
b]indazole products were generally formed in modest yields
when 2-(2-nitrophenyl)pyridines were employed (3r−3u).
Notably, majority of starting materials was recovered in the
cases of those with low yields such as 3c and 3n−3u. The
aniline side products were generally formed in trace amounts.
We also tried to modify the reaction conditions to improve the
reactivity of these reactants, but we failed.
With the established reactivities of our mild visible-light-
driven nitroarene reduction mediated by NaI/PPh3, we were
attracted to depict its photoactivating action models. First, we
performed UV−vis spectroscopic absorption experiments on
various combinations of 1a, PPh3, and NaI in a solution of the
same concentration as the real reaction mixture (Figure 1a).
The results reveal that the combination of PPh3 and NaI
features the same absorption with PPh3, only in the UV (<360
nm). With the participation of o-nitrobiphenyl, either single
component or combinations display an absorption in the
visible region with an onset around 440 nm. Further, we did
density functional theory (DFT) calculations to support the
proposed photoactivating action models (Figure 1b). Without
the iodide additive, the electron-poor nitroarene 1a and the
electron-rich PPh3 interact via a Coulombic attraction with a
P−O distance of 3.57 Å and a π−π interaction with a shortest
π-stacking distance of approximately 3.44 Å. This assembly
In summary, the combination of nitroarenes with NaI and
PPh3 was found to generate an EDA complex that absorbs
photons to enable reductive hydrogenation of nitroarenes. This
mild photoredox catalytic system has been demonstrated to be
highly selective for nitro reduction because a pad of reducible
functional groups are accommodated in it. With stoichiometric
5351
https://doi.org/10.1021/acs.orglett.1c01654
Org. Lett. 2021, 23, 5349−5353

Organic Letters
pubs.acs.org/OrgLett
Letter
Xing Chen − Key Laboratory for Green Organic Synthesis and
Application of Hunan Province, Key Laboratory of
Environmentally Friendly Chemistry and Application of
Ministry of Education, College of Chemistry, Xiangtan
University, Xiangtan 411105, China
Scheme 4. Control Experiments
Shuai Zhong − Key Laboratory for Green Organic Synthesis
and Application of Hunan Province, Key Laboratory of
Environmentally Friendly Chemistry and Application of
Ministry of Education, College of Chemistry, Xiangtan
University, Xiangtan 411105, China
Guo-Jun Deng − Key Laboratory for Green Organic Synthesis
and Application of Hunan Province, Key Laboratory of
Environmentally Friendly Chemistry and Application of
Ministry of Education, College of Chemistry, Xiangtan
University, Xiangtan 411105, China; orcid.org/0000-
0003-2759-0314
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01654
Author Contributions
#Z.Q. and X.C. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support by the National Natural Science Foundation of China
(22071211), the Science and Technology Planning Project of
Hunan Province (2019RS2039), Hunan Provincial Natural
Science Foundation of China (2020JJ3032), and the
Collaborative Innovation Center of New Chemical Technol-
ogies for Environmental Benignity and Efficient Resource
Utilization is gratefully acknowledged.
amounts of PPh3 playing dual roles of initiator for energy
transfer and electron donor, the photocatalyst-free NaI/PPh3-
based photocatalysis has also been used as an efficient method
for Cadogan-type reductive amination of o-nitrobiarenes.
REFERENCES
■
(1) Tafesh, A. M.; Weiguny, J. A Review of the Selective Catalytic
Reduction of Aromatic Nitro Compounds into Aromatic Amines,
Isocyanates, Carbamates, and Ureas Using CO. Chem. Rev. 1996, 96,
2035−2052.
ASSOCIATED CONTENT
* Supporting Information
■
(2) For recent examples, see: (a) Gao, Y.; Yang, S.; Huo, Y.; Hu, X.
Q. Recent Progress on Reductive Coupling of Nitroarenes by Using
Organosilanes as Convenient Reductants. Adv. Synth. Catal. 2020,
362, 3971−3986. (b) Hosoya, H.; Misal Castro, L. C.; Sultan, I.;
Nakajima, Y.; Ohmura, T.; Sato, K.; Tsurugi, H.; Suginome, M.;
Mashima, K. 4,4’-Bipyridyl-Catalyzed Reduction of Nitroarenes by
Bis(neopentylglycolato)diboron. Org. Lett. 2019, 21, 9812−9817.
(c) Roscales, S.; Csáky, A. G. Transition-Metal-Free Three-
Component Synthesis of Tertiary Aryl Amines from Nitro
Compounds, Boronic Acids, and Trialkyl Phosphites. Adv. Synth.
Catal. 2020, 362, 111−117.
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01654.
Experimental procedures, characterization data, compu-
1
tational details, and H NMR and 13C NMR spectra for
all products (PDF)
AUTHOR INFORMATION
Corresponding Author
■
(3) Formenti, D.; Ferretti, F.; Scharnagl, F. K.; Beller, M. Reduction
of Nitro Compounds Using 3d-Non-Noble Metal Catalysts. Chem.
Rev. 2019, 119, 2611−2680.
Huawen Huang − Key Laboratory for Green Organic
Synthesis and Application of Hunan Province, Key
(4) (a) Song, J.; Huang, Z.-F.; Pan, L.; Li, K.; Zhang, X.; Wang, L.;
Zou, J.-J. Review on selective hydrogenation of nitroarene by catalytic,
photocatalytic and electrocatalytic reactions. Appl. Catal., B 2018,
227, 386−408. (b) Luo, H.; Hu, J.; Qu, L.; Liu, G.; Zhang, R.; Lu, Y.;
Qi, J.; Hu, J.; Zeng, C. Efficient reduction of nitrobenzene by sulfate-
reducer enriched biocathode in microbial electrolysis cell. Sci. Total
Environ. 2019, 674, 336−343.
(5) (a) Guo, Q.; Ma, Z.; Zhou, C.; Ren, Z.; Yang, X. Single Molecule
Photocatalysis on TiO2 Surfaces. Chem. Rev. 2019, 119, 11020−
11041. (b) Li, A.; Zhu, W.; Li, C.; Wang, T.; Gong, J. Rational design
of yolk-shell nanostructures for photocatalysis. Chem. Soc. Rev. 2019,
48, 1874−1907. (c) Zhu, H. L.; Zeng, F. L.; Chen, X. L.; Sun, K.; Li,
Laboratory of Environmentally Friendly Chemistry and
Application of Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan 411105, China; orcid.org/
0000-0001-7079-1299; Email: hwhuang@xtu.edu.cn
Authors
Zhonghua Qu − Key Laboratory for Green Organic Synthesis
and Application of Hunan Province, Key Laboratory of
Environmentally Friendly Chemistry and Application of
Ministry of Education, College of Chemistry, Xiangtan
University, Xiangtan 411105, China
5352
https://doi.org/10.1021/acs.orglett.1c01654
Org. Lett. 2021, 23, 5349−5353

Organic Letters
pubs.acs.org/OrgLett
Letter
H. C.; Yuan, X. Y.; Qu, L. B.; Yu, B. Acyl Radicals from alpha-Keto
Acids: Metal-Free Visible-Light-Promoted Acylation of Heterocycles.
Org. Lett. 2021, 23, 2976−2980.
(19) (a) Qu, Z.; Wang, P.; Chen, X.; Deng, G.-J.; Huang, H. Visible-
light-driven Cadogan reaction. Chin. Chem. Lett. 2021, DOI: 10.1016/
j.cclet.2021.02.047. (b) Wang, Z.; Liu, Q.; Ji, X.; Deng, G.-J.; Huang,
H. Bromide-Promoted Visible-Light-Induced Reductive Minisci
Reaction with Aldehydes. ACS Catal. 2020, 10, 154−159.
(20) (a) Fu, M.-C.; Shang, R.; Zhao, B.; Wang, B.; Fu, Y.
Photocatalytic decarboxylative alkylations mediated by triphenylphos-
phine and sodium iodide. Science 2019, 363, 1429−1434. (b) Wang,
Y. T.; Fu, M. C.; Zhao, B.; Shang, R.; Fu, Y. Photocatalytic
decarboxylative alkenylation of alpha-amino and alpha-hydroxy acid-
derived redox active esters by NaI/PPh3 catalysis. Chem. Commun.
2020, 56, 2495−2498. (c) Fu, M.-C.; Wang, J.-X.; Shang, R.
Triphenylphosphine-Catalyzed Alkylative Iododecarboxylation with
Lithium Iodide under Visible Light. Org. Lett. 2020, 22, 8572−8577.
(21) Liu, Y.; Chen, X. L.; Sun, K.; Li, X. Y.; Zeng, F. L.; Liu, X. C.;
Qu, L. B.; Zhao, Y. F.; Yu, B. Visible-Light Induced Radical
Perfluoroalkylation/Cyclization Strategy To Access 2-Perfluoroalkyl-
benzothiazoles/Benzoselenazoles by EDA Complex. Org. Lett. 2019,
21, 4019−4024.
(6) (a) Xiao, Q.; Sarina, S.; Waclawik, E. R.; Jia, J.; Chang, J.; Riches,
J. D.; Wu, H.; Zheng, Z.; Zhu, H. Alloying Gold with Copper Makes
for a Highly Selective Visible-Light Photocatalyst for the Reduction of
Nitroaromatics to Anilines. ACS Catal. 2016, 6, 1744−1753.
(b) Tsutsumi, K.; Uchikawa, F.; Sakai, K.; Tabata, K. Photoinduced
Reduction of Nitroarenes Using a Transition-Metal-Loaded Silicon
Semiconductor under Visible Light Irradiation. ACS Catal. 2016, 6,
4394−4398. (c) Xu, S.; Tang, J.; Zhou, Q.; Du, J.; Li, H. Interfacing
Anatase with Carbon Layers for Photocatalytic Nitroarene Hydro-
genation. ACS Sustainable Chem. Eng. 2019, 7, 16190−16199.
(7) Wei, P. F.; Qi, M. Z.; Wang, Z. P.; Ding, S. Y.; Yu, W.; Liu, Q.;
Wang, L. K.; Wang, H. Z.; An, W. K.; Wang, W. Benzoxazole-Linked
Ultrastable Covalent Organic Frameworks for Photocatalysis. J. Am.
Chem. Soc. 2018, 140, 4623−4631.
(8) Lee, J.; Buyukcakir, O.; Kwon, T. W.; Coskun, A. Energy Band-
Gap Engineering of Conjugated Microporous Polymers via Acidity-
Dependent in Situ Cyclization. J. Am. Chem. Soc. 2018, 140, 10937−
10940.
(9) Bhadra, M.; Kandambeth, S.; Sahoo, M. K.; Addicoat, M.;
Balaraman, E.; Banerjee, R. Triazine Functionalized Porous Covalent
Organic Framework for Photo-organocatalytic E-Z Isomerization of
Olefins. J. Am. Chem. Soc. 2019, 141, 6152−6156.
(10) Park, Y.; Kim, Y.; Chang, S. Transition Metal-Catalyzed C-H
Amination: Scope, Mechanism, and Applications. Chem. Rev. 2017,
117, 9247−9301.
(22) Reichl, K. D.; Ess, D. H.; Radosevich, A. T. Catalyzing
pyramidal inversion: configurational lability of P-stereogenic
phosphines via single electron oxidation. J. Am. Chem. Soc. 2013,
135, 9354−9357.
(23) Kawasaki, T.; Ishida, N.; Murakami, M. Dehydrogenative
Coupling of Benzylic and Aldehydic C-H Bonds. J. Am. Chem. Soc.
2020, 142, 3366−3370.
(11) Mandi, U.; Roy, A. S.; Kundu, S. K.; Roy, S.; Bhaumik, A.;
Islam, S. M. Mesoporous polyacrylic acid supported silver nano-
particles as an efficient catalyst for reductive coupling of nitro-
benzenes and alcohols using glycerol as hydrogen source. J. Colloid
Interface Sci. 2016, 472, 202−209.
(12) Cheung, C. W.; Hu, X. Amine synthesis via iron-catalysed
reductive coupling of nitroarenes with alkyl halides. Nat. Commun.
2016, 7, 12494.
(13) (a) Musacchio, A. J.; Lainhart, B. C.; Zhang, X.; Naguib, S. G.;
Sherwood, T. C.; Knowles, R. R. Catalytic intermolecular hydro-
aminations of unactivated olefins with secondary alkyl amines. Science
2017, 355, 727−730. (b) Gui, J.; Pan, C.-M.; Jin, Y.; Qin, T.; Lo, J.
C.; Lee, B. J.; Spergel, S. H.; Mertzman, M. E.; Pitts, W. J.; Cruz, T. E.
L.; Schmidt, M. A.; Darvatkar, N.; Natarajan, S. R.; Baran, P. S.
Practical olefin hydroamination with nitroarenes. Science 2015, 348,
886−891. (c) Musacchio, A. J.; Nguyen, L. Q.; Beard, G. H.;
Knowles, R. R. Catalytic olefin hydroamination with aminium radical
cations: a photoredox method for direct C-N bond formation. J. Am.
Chem. Soc. 2014, 136, 12217−12220.
(14) Yang, X.-J.; Chen, B.; Zheng, L.-Q.; Wu, L.-Z.; Tung, C.-H.
Highly efficient and selective photocatalytic hydrogenation of
functionalized nitrobenzenes. Green Chem. 2014, 16, 1082−1086.
(15) Freeman, A. W.; Urvoy, M.; Criswell, M. E. Triphenylphos-
phine-Mediated Reductive Cyclization of 2-Nitrobiphenyls: A
Practical and Convenient Synthesis of Carbazoles. J. Org. Chem.
2005, 70, 5014−5019.
(16) Gao, H.; Xu, Q. L.; Yousufuddin, M.; Ess, D. H.; Kurti, L.
Rapid synthesis of fused N-heterocycles by transition-metal-free
electrophilic amination of arene C-H bonds. Angew. Chem., Int. Ed.
2014, 53, 2701−2705.
(17) Song, H.; Yang, Z.; Tung, C.-H.; Wang, W. Iron-Catalyzed
Reductive Coupling of Nitroarenes with Olefins: Intermediate of
Iron−Nitroso Complex. ACS Catal. 2020, 10, 276−281.
(18) (a) Nykaza, T. V.; Ramirez, A.; Harrison, T. S.; Luzung, M. R.;
Radosevich, A. T. Biphilic Organophosphorus-Catalyzed Intra-
molecular Csp(2)-H Amination: Evidence for a Nitrenoid in Catalytic
Cadogan Cyclizations. J. Am. Chem. Soc. 2018, 140, 3103−3113.
(b) Lu, C.; Su, Z.; Jing, D.; Jin, S.; Xie, L.; Li, L.; Zheng, K.
Intramolecular Reductive Cyclization of o-Nitroarenes via Biradical
Recombination. Org. Lett. 2019, 21, 1438−1443.
5353
https://doi.org/10.1021/acs.orglett.1c01654
Org. Lett. 2021, 23, 5349−5353