Iron-catalyzed reductive coupling of nitroarenes with olefins: Intermediate of iron-nitroso complex

Article abstract of DOI:10.1021/acscatal.9b03604

Using a single half-sandwich iron(II) compound, CpFe(1,2-Ph₂PC₆H₄S)(NCMe) (Cp^- = C₅Me₅^-, 1) as a catalyst, reductive coupling of nitroarenes with olefins has been achieved by a well-defined iron(II)/(EtO)₃SiH system. Through either inter- or intramolecular reductive coupling, various branched amines and indole derivatives have been directly synthesized in one-pot. Mechanistic studies showed that the catalysis is initiated by activation of nitroarenes by the iron(II) catalyst with silane, generating iron-nitrosoarene intermediate for the C-N bond coupling.

Full text of DOI:10.1021/acscatal.9b03604

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Article
Iron-Catalyzed Reductive Coupling of Nitroarenes
with Olefins: Intermediate of Iron-Nitroso Complex
Heng Song, Zhuoyi Yang, Chen-Ho Tung, and Wenguang Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b03604 • Publication Date (Web): 22 Nov 2019
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Iron-Catalyzed Reductive Coupling of
Nitroarenes with Olefins: Intermediate of Iron-
Nitroso Complex
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ab
a
a
a
Heng Song, Zhuoyi Yang, Chen-Ho Tung, and Wenguang Wang*
aKey Lab of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry
and Chemical Engineering, Shandong University, No. 27 South Shanda Road, Jinan,
2
50100, P. R. China.
bSchool of Environment and Chemical Engineering, Jiangsu University of Science and
Technology, Zhenjiang 212003, Jiangsu, China
ABSTRACT. Using
a
single half-sandwich iron(II) compound, Cp*Fe(1,2-
−
−
Ph PC H S)(NCMe) (Cp* = C Me , 1) as a catalyst, reductive coupling of nitroarenes
2
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4
5
5
with olefins has been achieved by a well-defined iron(II)/(EtO) SiH system. Through
3
either inter- or intramolecular reductive coupling, various branched amines and indole
derivatives have been directly synthesized in one-pot. Mechanistic studies showed that
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the catalysis is initiated by activation of nitroarenes by the iron(II) catalyst with silane,
generating iron-nitrosoarene intermediate for the CN bond coupling.
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KEYWORDS ： Reductive CN bond coupling, iron catalysis, iron-nitrosoarene
intermediate, secondary amines, indoles
INTRODUCTION
(Hetero)aromatic amines have privileged scaffolds which are widely found in
natural products, pharmaceuticals, dyes and industrial fine chemicals.1-3
Hydroamination through intra- and intermolecular addition of amine NH bonds to
4
alkenes is an attractive strategy with which to construct new CN bonds and
5
manipulate (hetero)aryl amines, owing to the widespread availability of alkenes
(Scheme 1a). Although regioselective hydroamination via Markovnikov6 or anti-
7
Markovnikov process is challenging, significant progress has been made in producing
8
linear or branched amines based on transition-metal catalysis. Since aromatic amines
are usually obtained from reduction of nitro(hetero)arenes, direct use of
nitro(hetero)arenes as nitrogenous partners for the CN coupling has the great
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advantage of step economy and easy manipulation, which can offer an appealing
process for the construction of CN bonds.
1
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a) hydroamination of alkenes with amines
anti-Markovnikov
selectivity
H
N
Ar
R
reduction
H
N
R
Ar NO2
Ar
H
Cat.
Markovnikov Ar
selectivity
H
R
N
b) hydroamination of alipahtic alkenes with nitroarenes
i) Fe(acac)3 (30 mol%)
PhSiH3 (2 equiv)
R3
R2
R1
R2
R3
EtOH, 60 oC, 1 h;
Ar
Ar NO2
+
N
ii) Zn, HClaq, 60 oC, 1 h
H
R1
O-N
Cleavage
[
H]
Zn/HCl
Ar
R3
O
R3
Ar
N
R1
R2
R1
N
R2
R1
R2
O
Proposed radical pathway
Baran (2015)
R3
Me
Me
N
O
O
10 mol% NiI2, 11mol % L
Fe
Cl
O
6.0 equiv Me(MeO) SiH
2
Me
DMPU/DMA (1:1, 0.5 M)
5 oC 10 min, then 50 oC 12 h
2
2.0 mol% [Fe(III)]
1
.0-2.0 equiv PhSiH3
L =
EtOH (0.2 M), rt, 1-2 h;
ii) Zn, HClaq, 60 oC, 1 h
N
N
Shaver and Thomas (2016)
c) this work
Zhu (2018)
Ar
Ar NO2 +
Ar
Ar
N
H
Fe
S
O
or
1
N
via Ph2P
R1
3
4 equiv (EtO) SiH
R1
Ar
R2
THF, 40-60 o
C
R
R
R2
NO2
N
1
(ArNO)
H
Scheme 1. Approaches to Hydroamination of Alkenes with Nitroarenes
Using a single catalyst or catalyst precursor to realize the reduction of nitro groups,
coupled with hydroamination of alkenes in a one-pot protocol is challenging. A
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breakthrough was made by Baran et al. in 2015, who reported the first use of a simple
Fe(acac) /PhSiH catalytic system to achieve reductive CN bond formation between
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nitro(hetero)arenes and aliphatic alkenes, forming various secondary amines in
moderate to good yields (Scheme 1b). Subsequently, Thomas and Shaver11 reported
the selective reduction and formal hydroamination of functional nitroarenes with
aliphatic alkenes by using a bench-stable amine–bis(phenolate) iron(III) complex (2
mol%) and silane. In these transformations, nitroso compounds was often envisioned as
the key intermediate which couples with carbon-centered alkyl radicals generated in situ
from alkenes. Hu and co-workers reported reductive coupling of nitro(hetero)arenes with
numerous alkyl halides to synthesize various (hetero)aryl amines using a
12
FeCl ∙4H O/TMSCl system. In these iron-catalyzed transformations, Zn is required to
2
2
serve as additional reductant. Recently, Zhu et al. reported an elegant nickel-catalyzed
CN coupling system achieved by merging alkene isomerization and hydroamination.13
Although considerable progress have been made in reductive coupling of
nitro(hetero)arenes with alkenes, aryl olefins often do not perform well in the iron-
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catalyzed reductive C−N coupling system, and with the formation of (hetero)aromatic
amines in low yields.10 Using a neutral half-sandwich iron(II) compound, Cp*Fe(1,2-
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−
−
14
Ph PC H S)(NCMe) (Cp* = C Me , 1) as a catalyst, we report an well-defined
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4
5
5
iron(II)/(EtO) SiH
system
which
achieves
reductive
hydroamination
of
3
nitro(hetero)arenes with alkenes to produce secondary amines and indoles (Scheme
1c). The catalysis is initiated by activation of nitroarenes, generating the iron-nitroso
intermediate for the reductive CN bond coupling.
RESULTS AND DISCUSSION
Activation of Nitroarenes by 1. Metal-nitroso complexes can be derived from the
deoxygenation of organic nitro compounds by metal-phosphine15 or metal-carbonyl
precursors.16 At the outset of this study, we found that 1 reacts readily with
nitro(hetero)arenes, affording an iron-nitrosoarene complex that mediates the reduction
of nitroarenes to aryl amines by reaction with (EtO) SiH.
3
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NO2
(
Et3O)SiH/THF, rt
+
Fe
S
O
Fe
S
-(EtO)3SiOSi(EtO)3
NCMe
N
Ph2P
Ph2P
Cl
1
Cl
1
(ArNO)
THF, rt
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Fe
O
+
1(ArNO)
S
Ph2P
PPh2
S
1'
Scheme 2. Activation of Nitroarenes by 1 and the Structure of Iron-Nitrosoarene
Intermediate
An addition of 1 to a mixture of 1-chloro-4-nitrobenzene (2a) and an equimolar
amount of (EtO) SiH in THF caused the color of the solution to turn from brown to deep
3
red. According to GC-MS analysis, polysiloxane (EtO) SiOSi(EtO) (m/z = 342) and 4-
3
3
chloroaniline was produced, indicating the deoxygenation of the nitro group by
hydrosilane. The 31P NMR spectrum of the reaction mixture displayed only a single
phosphorus resonance at  75. The new iron(II)-nitrosoarene complex, 1(ArNO) was
1
isolated and identified. Crystallographic analysis revealed the structure of an η (N)-
1
7
nitroso complex, in which the N atom of 4-Cl-C H NO is coordinated to the iron center
6
4
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(
Scheme 2). The NO bond length of 1.282(5) Å is slightly increased relative to that in
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the free 4-Cl-C H NO molecule, 1.265(3) Å.
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In the absence of (EtO) SiH, 1 also reacts with 2a (Scheme 2). In addition to
3
1
(ArNO), however, a half-sandwich iron(III) species (1’) was also crystallized from the
reaction mixture (SI, Figure S6). This result indicates that ArNO can oxidize the iron
2
complex from Fe(II) to Fe(III), with the phosphine unit serving as a deoxygenation
agent.15 Importantly, 1(ArNO) reacts further with (EtO) SiH to produce p-chloroaniline
3
according to GC-MS analysis. The recovery of the organoiron species was deduced
from the ESI-MS spectrum, which showed a peak at m/z = 484.1147 for Cp*Fe(1,2-
Ph PC H S). Inspired by the reactivity of 1 and its nitrosoarene derivative, we examined
2
6
4
the catalytic reduction of 2a with (EtO) SiH using 1 as the catalyst. With 2 mol% of 1,
3
compound 2a was converted to the corresponding primary amine in 94% yield at room
temperature (eq 1).
1
(2 mol%)
NO2
NH2
(
EtO)3SiH (4 eq)
(1)
neat, rt, 10 h
Cl
Cl
2a
94% yield
H
NO2
1
(4 mol%)
EtO)3SiH (4 eq)
THF, 40 oC, 24 h
N
Ph
(2)
Ph
3a
(
Cl
Cl
2a
4a, 77% yield
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Catalytic Intermolecular C−N Bond Coupling. Subsequently, we examined the reductive
CN bond formation in the reaction between nitro(hetero)arenes and styrene type
olefins and found that 1 is capable of catalyzing this challenging transformation. Under
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the optimized reaction conditions using 4 mol% of 1 and 4 equivalents of (EtO) SiH in
3
THF, the intermolecular reductive coupling between 2a and styrene (3a) proceeded well
o
at 40 C, affording the branched chain amine (4a) which was obtained in 77% yield (eq
2). Formation of a linear product was not observed.
Table 1. Scope of Intermolecular C−N Bond Couplinga
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R1
R1 R2
R3
1 (4 mol%)
+
R2
Ar
H
Ar NO2
N
R4
(EtO)3SiH
H
R3 R4
THF, 40 °C, 24 h
2
3
4
Nitroarenes Scope:
H
H
H
R
N
Ph
H
N
Ph
N
Ph
N
Ph
R
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4a, R = Cl, 77%
4
e, R = 2-Me, 64%
f, R = 3-Me, 61%
g, R = 3-Cl, 56%
4
b, R = H, 69%
c, R = Me, 65%
4h, 58%
4i, 60%
4
4
4
4d, R = OMe, 68%
tBu
tBu
H
H
N
N
Ph
H
N
O
O
N
MeO
N
4
j, 57%
4k, 64%
4l, 67%
Olefins Scope:
R
H
N
H
N
H
N
Fe
H
N
4
m, R = CN, 55%
n, R = Br, 51%
4
4
q, 57%
4r, 64%
4s, 48%b
4o, R = tBu, 67%
4
p, R = OEt, 63%
H
N
H
N
3i
3j
4
t, 54%
4u, 57%
H
N
H
N
Ph
Ph
Ph
Ph
Et
3k
3l
4
v, 57%
4w, 53%
H
N
H
Ph
Ph
Ph
N
Ph
O
O
O
O
3
m
4x, 46%
3n
4y
, 49%
H
N
Ph
Ph
H
N
O
O
3p
3o
4z, 50%
4aa, 56%
aReaction conditions: nitro(hetero)arene (0.5 mmol), olefin (2.5 mmol), (EtO)
SiH (2 mmol, 4 equiv), 4 mol%
3
catalyst loading relative to the nitroarene in 0.5 mL of THF at 40 °C for 24 h. Isolated yields were given.
b60 °C for 8 h.
As shown in Table 1, a series of nitroarenes were subjected to reaction with
styrene, and the corresponding branched amines were obtained in moderate to good
yields. Various groups at the para, ortho or meta position on the aromatic ring do not
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affect this reaction (4a-4g). Notably, nitro-heteroarenes were used to deliver medicinally
relevant building blocks containing indole (4k) and pyridine (4l) ring systems, the latter
of which was confirmed by X-ray crystallographic analysis.
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Styrene type alkenes and several aliphatic alkenes are suitable substrates,
delivering the corresponding amines in moderate to good yields (Table 1). Electron-
donating groups (4o and 4p) at the para-position of the phenyl ring appeared to give
better yields than electron-withdrawing groups (4m and 4n). Although a cyano group
might coordinate to the Fe center and deactivate the catalyst, it is compatible with the
reaction, affording the coupled amine product (4m) in 55% yield. Aliphatic alkenes are
less reactive, and the conversion of cyclopentene (3h) provided the amine product (4s)
in 48% yield. Disubstituted alkenes are all suitable substrates, giving diverse amine
products (4t-4w) in moderate yields. With the substrates (3m-3o) containing both
aromatic and aliphatic alkene moieties, the CN coupling reaction occurs selectively at
the aromatic alkene sites. It is interesting that a 1,4-addition product (4aa) was formed
selectively in 56% yield when conjugated diene 3p was used as the substrate. Overall,
the present transformation allows the direct amination of aromatic alkenes with
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nitroarenes to afford branched amines bearing an aryl group in alpha position, which is
complementary to the previously described iron-catalyzed reductive coupling of
nitroarenes with aliphatic alkenes.10,11
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Catalytic Intramolecular CN coupling. Reductive CN bond coupling can lead to
derivatives of indole, one of the most important heterocycles in organic chemistry.19
Previous reactions often required strong reductants such as Grignard reagent20 or
TiCl321 together with toxic reagents. Therefore, developing efficient catalysts based on
earth-abundant metals and under mild reaction conditions is highly desirable.22 We
applied our protocol to the synthesis of indoles by using o‑nitrostyrenes as the starting
materials (Table 2). The intramolecular reductive CN bond coupling is so efficient that
even at 0.5 mol% catalyst loading the indole product 6a was produced in 88% yield
o
within 6 h at 60 C. A series of monosubstituted or disubstituted indole derivatives were
synthesized in good to excellent yields following this simple procedure.
Table 2. Scope of Intramolecular C−N Bond Couplinga
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R2
R2
R3
1
(0.5 mol%)
EtO)3SiH
R1
R1
R3
(
N
NO2
H
THF, 60 C
5
6
Me
R
R
Ph
Ph
N
H
N
N
Ph
R
H
H
N
H
6
6
6
6
a, R = H, 88%
b, R = OMe, 87%
c, R = F, 73%
d, R = Br, 86%
6e, R = CF3, 91%
6f, R = OMe, 91%
6g, R = CF , 89%
6h, R = F, 82%
6i, R = Cl, 58%
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6j, 81%
3
Ph
N
N
H
H
F
N
H
6m, 85% (10 h)b
6k, 64%
6l, 90% (5 h)b
Me
Ph
N
H
N
H
N
H
N
H
6
n, 93% (8 h)b
6o, 94% (5 h)
6p, 91% (5 h)b
6q, 66% (6 h)b
aReaction conditions: substrates (0.1 mmol), (EtO)
SiH (0.4 mmol), 0.5 mol% catalyst loading relative to
the substrates in 4 mL of THF at 60 °C for 6 h, and isolated yields were given. b80 °C in 1,2-
3
dimethoxyethane.
Functional groups such as −OMe, −F, −Br and −CF are all tolerated under the
3
reaction conditions. Nitro compounds with cyclic-alkenes with six to eight membered
rings are all suitable substrates, and the fused tricyclic indole derivatives (6l-6n) were
o
produced in excellent yields in 1,2-dimethoxyethane at 80 C. The structure of 6l
containing a seven membered ring was unambiguously established by single crystal X-
ray diffraction. Acyclic trisubstituted olefins were converted to 2,3-disubstituted indoles
(6o, 6p) in very good yields. 3-Phenyl indole (6q) was obtained from the corresponding
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alkene in 66% yield. The catalysis is amenable to a reductive NN coupling to access
23
an indazole (8) from the corresponding aldimine (eq 3).
1
1
1
1
1
1
1
1
1
1
2
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2
2
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2
2
2
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5
5
5
6
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2
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0
nBu
1
(0.5 mol%)
EtO)3SiH
THF, 60 °C
N
N
nBu (3)
(
NO2
N
7
8, 82%
Mechanistic Insights. Several control experiments were conducted to probe the
mechanism of the catalysis. When a radical trap, 1,1-diphenylethylene (9),24 was
subjected to intermolecular CN coupling with PhNO , the reaction was suppressed by
2
productions of 1,1-diphenylethane (10) and aniline (Scheme 3a). Such hydrogenation
reactions suggest that a stepwise addition of hydrogen radical is involved in the
catalysis, probably because the diphenyl stabilized radical (Int1a) is not sufficiently
active for the CN coupling with the nitroso intermediate.
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a) 1,1-Diphenylethylene as the alkene substrate
Ph
1
Ph
(
4 mol%)
+
PhNO2
+
PhNH2
Ph
(EtO)3SiH (4 eq)
THF, 40 °C, 24 h
Ph
9
2b
10
H
Ph
H
Ph
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8
9
0
Int1a
b) TEMPO as radical trap
Ph
1
(4 mol%)
OH
N
O
N
TEMPO (4 eq)
+
PhNO2
+
Ph
(EtO)3SiH (4 eq)
THF, 40 C, 6 h
2b
3a
11a
11b
c) Reductive C-N bond coupling catalyzed by iron-nitroso intermediate
Ph
1
(ArNO) (4 mol%)
EtO)3SiH (4 eq)
THF, 40 oC, 24 h
Ph
PhNO2 + Ph
N
H
(
2b
3a
4
b, 63%
Scheme 3. Control Experiments for Mechanistic Study
That the catalysis proceeds by a radical mechanism was further supported by the
addition of the radical quencher, 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO), to the
reductive coupling reaction of 2b and 3a. Judging by GC-MS analysis, the desired CN
coupling was indeed interrupted. The hydrogen radical and benzyl radical involved in
the coupling were trapped by TEMPO, forming 11a and 11b, respectively (Scheme 3b).
In contrast, no reaction was detected in the absence of the catalyst. Particularly, the
nitroso complex 1(ArNO) is capable of catalyzing the reductive amination reaction. As
examplified by the reaction of 2b with 3a (Scheme 3c), the reaction provided 4b in 63%
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yield, comparable to the 69% yield obtained from the reaction using catalyst 1. This
result is indicative of the intermediacy of 1(ArNO) in the CN bond coupling.
1
1
1
1
1
1
1
1
1
1
2
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2
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0
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0
ArNH2
(EtO)3SiH
ArNO2
Stage A
Fe(II)
1
Stage B
ArNO d
O
N
Ar
FeII
R
b
-
(EtO)3SiOH
OH
[1(ArNO)]
O
N Ar
O
FeII
N
R
Ar
OSi(EtO)3
R
b
Int1
OSi(EtO)3
N
a
Ar
O
FeIII
O
N
(EtO)3SiH
FeIII
Int2
H
Ar
Int3
R
OSi(EtO)3 (EtO)3SiH
H
N
3
(EtO) SiH
N
Ar
Ar
R
R
Scheme 4. Proposed Mechanism for the Iron-Catalyzed Reductive CN Coupling.
Accordingly, a radical mechanism was proposed for the intermolecular CN bond
coupling (Scheme 4). Based on the dissociation of the MeCN ligand from 1,14 we
propose that coordination of nitroarene to the iron(II) center leads to the formation of an
Fe(II)-substrate adduct (Int1) intermediate. In the absence of (EtO) SiH, Int 1 undergoes
3
deoxygenation reaction to produce 1’ and 1(ArNO) (Scheme 2). By the reaction of
25
(
EtO) SiH and Int 1, the Fe(III)-H intermediate and an aryl-NO radical (a) are
3
26
generated. Fe(III)-H can react with styrene by donating a hydrogen radical, forming an
alkyl radical (b) with recovery of the catalyst to the Fe(II) state. The Fe(III)-H could also
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donate a hydrogen radical in the reduction of a nitro group to the nitroso compound (d).
Recombination of Fe(II) with d leads to the Fe(II)-nitroso intermediate. The benzyl
radical then attacks 1(ArNO) resulting in the formation of an Fe(III) complex (Int3), and
0
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this is followed by reaction with (EtO) SiH to produce the amine product while
3
regenerating the Fe(III)-H species.
1
, 0.5 mol%
(EtO)3SiH (4 equiv)
Ph
Ph (4)
N
THF
60 C, 6 h
N
H
OH
6a, 80%
With regard to the intramolecular C−N bond coupling, Driver recently reported a
Fe(OAc) /4,7-(MeO) Phen/PhSiH system for reductive cyclization of o-nitrostyrenes
2
2
3
into indoles.22a They proposed that the catalysis is initiated by the reaction of the iron(II)
precursor with the silane to generate iron(II)-hydride species, which is responsible for
the reduction of nitrostyrene to nitrosostyrene. In the present case, we propose that the
nitrostyrene substrates is directly activated by 1 and subsequently reduced to the
nitroso intermediate by silane (Stage A). The intermediacy of N-hydroxyindole formed
by electrocyclization of nitrosostyrene is usually proposed for such intramolecular C−N
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bond coupling process.21,22 Indeed, 1 is capable of catalyzing the transformation of N-
hydroxyindole to the indole product. For instance, the reaction of N-hydroxy-2-
phenylindole under the catalytic conditions produced 6a in 80% yield (eq 4). In contrast,
the production of 6a was not observed in the absence of the iron catalyst.
1
1
1
1
1
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1
1
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6
0
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CONCLUSION
In summary, we have developed an iron(II)-catalyzed reductive coupling of nitro
compounds with styrenes under mild conditions. The intermolecular CN reductive
coupling led to diverse branched amines, while an intramolecular transformation
efficiently afforded a variety of indole derivatives. In the reported cases of iron-catalyzed
reductive CN couplings, reductive workup with Zn/HCl(aq) is necessary to cleave NO
bond forming the desired products. In the present case, (EtO) SiH is the only reducing
3
agent, and the catalysis is initiated by activation of nitroarenes by the iron(II) catalyst
with silane. The resultant iron-nitrosoarene intermediate is essential to promote the CN
coupling and a one-pot synthesis of (hetero)aromatic amines. This reaction offered a
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promising platform for the use of earth-abundant metals for these important radical-type
homogeneous transformations.27
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ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
*E-mail: wwg@sdu.edu.cn
ORCID
Chen-Ho Tung: 0000-0001-9999-9755
Wenguang Wang: 0000-0002-4108-7865
Notes
The authors declare no competing financial interest.
Supporting Information
This contains experimental details, characterization of the products, crystallographic
data and cif files (CCDC 1944762-1944765). This information is available free of charge
on the ACS Publications website.
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ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China (21871166 and 91427303)
for their financial support. We also thank Prof. Zhenghu Xu for helpful discussions.
1
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(
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4005449. (b) Surry, D. S.; Buchwald, S. L. Biaryl Phosphane Ligands in Palladium-
5
Catalyzed Amination. Angew. Chem. Int. Ed. 2008, 47, 63386361. (c) Hartwig, J. F.
Evolution of a Fourth Generation Catalyst for the Amination and Thioetherification of
Aryl Halides. Acc. Chem. Res. 2008, 41, 15341544. (d) Guillena, G.; Ramón, D. J.;
Yus, M. Hydrogen Autotransfer in the N-Alkylation of Amines and Related Compounds
Using Alcohols and Amines as Electrophiles. Chem. Rev. 2010, 110, 16111641.
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of Unactivated Terminal and Internal Alkenes with Primary and Secondary Amines. J.
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Catalyzed Asymmetric Intramolecular Hydroamination of Unactivated Alkenes. Angew.
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Table of Contents Graphic and Synopsis
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Cat. Fe
R
(EtO) SiH
3
Ar
Ar NO2
+
R
N
THF, 40 °C
H
O
Fe
S
N
Ph P
2
Ar
Cat. Fe
R1
R1
(
EtO) SiH
R2
3
R
R
R2
Iron-Nitroso
Intermediate
THF, 60 C
N
NO2
H
Reductive coupling of nitroarenes with olefins has been achieved by a well-defined
iron/(EtO) SiH catalytic system. By either inter- or intramolecular reductive coupling,
3
diverse branched amines and indole derivatives has been directly synthesized. The
catalysis is initiated by activation of nitroarenes, generating the iron-nitroso intermediate
for the reductive CN bond coupling.
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