Cobalt-catalyzed synthesis of N-containing heterocycles: Via cyclization of ortho -substituted anilines with CO2/H2

Article abstract of DOI:10.1039/c9gc00095j

The CO₂-involved synthesis of chemicals is of great significance from the green and sustainable chemistry viewpoint. Herein, we report a non-noble metal catalytic system composed of CoF₂, CsF and P(CH₂CH₂PPh₂)₃ (denoted as PP₃) for the synthesis of N-containing heterocycles from ortho-substituted anilines and CO₂/H₂. Mechanism investigation indicates that [Co(PP₃)H(CO₂)]⁺ is a catalytically active intermediate under working conditions; and CsF plays important roles in activating ortho-substituted anilines via hydrogen bond interactions, thus promoting the formation of the final products. This catalytic system is highly efficient, and allows a wide scope of ortho-substituted anilines, together with excellent functional group tolerance, affording various N-containing heterocycles in good to excellent yields.

Full text of DOI:10.1039/c9gc00095j

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DOI: 10.1039/C9GC00095J
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ARTICLE
Cobalt-Catalyzed Synthesis of N-containing Heterocycles via
Cyclization of Ortho-substituted Anilines with CO2/H2
Zhengang Ke,a, b Bo Yu,a Huan Wang,a, b Junfeng Xiang,a Juanjuan Han,a Yunyan Wu,a, b Zhenghui Liu,
Received 00th January 20xx,
Accepted 00th January 20xx
a, b Peng Yang,a, b Zhimin Liu*a, b,c
DOI: 10.1039/x0xx00000x
The CO2-involved synthesis of chemicals is of great significance from the green and sustainable chemistry viewpoint. Herein,
we report a non-noble metal catalytic system composed of CoF2, CsF and P(CH2CH2PPh2)3 (denoted as PP3) for synthesis of
N-containing heterocycles from ortho-substituted anilines and CO2/H2. Mechanism investigation indicates that
[Co(PP3)H(CO2)]+ is catalytically active intermediate under working conditions; and CsF plays important roles in activating
ortho-substituted anilines via hydrogen bond interaction, thus promoting the formation of the final products. This catalytic
system is highly efficient, and allows wide scopes of ortho-substituted anilines, together with excellent functional group
tolerance, affording various N-containing heterocycles in good to excellent yields.
www.rsc.org/
green route to N-containing heterocycles was realized via the
reaction of 2-nitroanilines with CO2/H2 catalyzed by Au/TiO2.46
Though such progress has been made, the non-noble metal
catalyzed synthesis of N-containing heterocycles from o-
phenylenediamines and CO2/H2 has not been reported yet.
In our previous work, cobalt-based catalysts were found to be
Introduction
Since carbon dioxide (CO2) is non-toxic, abundant and
renewable C1 feedstock, the CO2-involved synthesis of value-
added chemicals is of great significance both in academic and
industrial fields.1-11 To date, various kinds of chemicals have
been made from CO2 via constructing C-O, C-N, C-C, C-S bonds
with nucleophilic substrates, such as amines,12-22 olefins,23-28
halohydrocarbons29-32 and thiols.33-35 Building C-N bond from
CO2 with amines is one of promising routes for CO2 conversion.
N-containing heterocycles are a kind of important organic
chemicals, which can be used as organic and pharmaceutical
intermediates.36 Usually, N-containing heterocycles can be
synthesized via the cyclization of o-phenylenediamines with
carboxylic acids or their derivatives, esters, amides, lactones,
nitriles and aldehydes etc., at high temperature.37 Recently,
effective for the CO2-involved reductive reaction in the
48
presence of H2.47,
For example, CoF2 combined with
P(CH2CH2PPh2)3 (denoted as PP3) and K2CO3 catalyzed the
reductive coupling of primary amines and aldehydes with
CO2/H2, yielding a series of unsymmetrical N,N-disubstituted
formamides.48 Encouraged by these results, we extended the
cobalt catalyst to the CO2/H2-involved synthesis of chemicals via
reductive cyclization. Herein, we report the cobalt-catalyzed
synthesis of N-containing heterocycles from reductive
cyclization of ortho-substituted anilines with CO2 and H2, which
is achieved over a catalytic system composed of CoF2, PP3 and
CsF (Scheme 1). The roles of the catalytic species and the
reaction mechanism were explored.
39
40-42
aldehydes38,
and amides36,
have been applied in the
synthesis of N-containing heterocycles, which make the
synthetic routes relatively clean. More recently, the synthesis of
N-containing heterocycles from o-phenylenediamines and CO2
in the presence of reductants such as hydrosilanes,35,
borane44 and H2
XH
X
CoF2/PP3/CsF
140 oC, 24 h
CO2
H2
2H2O
43
N
NH2
XH=NH2, SH and OH
45, 46
has been reported. Especially, the
cyclization of o-phenylenediamines with CO2/H2 provides green
route for the synthesis of N-containing heterocycles.45 Another
Scheme 1. Synthesis of N-containing heterocycles from ortho-substituted anilines with
CO2/H2
a. Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid,
Interface and Thermodynamics, CAS Research/Education Centre for Excellence in
Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China
Experimental
Materials. All metal salts, ligands, chemicals and anhydrous
solvents were purchased from commercial sources and were
used without further purification.
Synthesis of N-containing heterocycles. The cyclization
reaction of ortho-substituted aniline with CO2/H2 was
performed in a 16 mL high-pressure reactor with a Teflon inner
container in batch operation mode. Typically, ortho-substituted
b. University of Chinese Academy of Sciences, Beijing 100049, China.
c. Physical Science Laboratory, Huairou National Comprehensive Science Center,
Beijing 101407, China.
E-mail: liuzm@iccas.ac.cn
†Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI:10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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aniline (1 mmol), CoF2 (10 mol %), PP3 (5 mol %), CsF (1.5 equiv.) The yield of 1b reached up to 99% at 24h, ViaewndArticfluerOtnhliener
DOI: 10.1039/C9GC00095J
and ethanol (3 mL) were loaded into the reactor in a glovebox. prolonging time the amount of HCOOH remained unchanged.
The autoclave was screwed up, and then charged with CO2 to 3
MPa, further with H2 up to total pressure of 6 MPa at room
Table 1. Optimization of the reaction conditions a
temperature. Subsequently, the reactor was moved to an oil
NH2
NH2
N
catalyst
bath of 140 oC, which was controlled by a Haake-D3 temperature
controller. After the desired reaction time, the reactor was
cooled down in ice-water and the gas inside was carefully
vented. The reaction solution was examined by 1H NMR, and the
product yield was determined using 1,1,2,2-tetrachloroethane
as the internal standard. The crude reaction mixture was
concentrated by a rotary evaporator, and purified by column
chromatography using ethyl acetate/dichloromethane (100:1-
10:1) to give the isolated compound.
CO2
H2
2H2O
solvent, 140 o
C
N
H
1b
1a
base
(equiv.)
CsF
t (h)
entry
catalyst
yield (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19b
Mn(OAc)2
Fe(OAc)2
Co(OAc)2·4H2O
Ni(OAc)2·4H2O
Cu(OAc)2·H2O
CoF2
15
15
15
15
15
15
15
15
15
24
24
24
24
24
24
24
24
24
40
2
4
54
0
2
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
1
Characterization. H and 13C NMR spectra were recorded on a
61
44
57
38
99
0
33
25
75
72
73
26
12
92
Bruker Advance III 400 HD at ambient temperature; 31P NMR
spectra were recorded on a Bruker AVANCE 600 at ambient
temperature; 19F, 2H and 1H-15N HMBC NMR spectra were
recorded on an AVANCE III HD 500 at ambient temperature. The
GC-MS analysis was performed using gas chromatography-mass
spectrometry (GC-MS, 7890A and 5975C, Agilent). The gaseous
mixture was collected and analyzed by gas chromatography
equipped with thermal conductivity detector (4890, Agilent
Technology) with Argon as the carrier gas. HR-ESI-MS were
performed using a Bruker FT-ICR-MS (Solarix 9.4T).
CoCl2
Co(BF4)2·6H2O
Co2(CO)8
CoF2
CsF
---
CoF2
CoF2
CoF2
CoF2
CoF2
CoF2
CoF2
CoF2
KOH
K3PO4
tBuOK
Cs2CO3
K2CO3
NEt3
CsCl
---
RuCl2(dppe)2
Results and discussion
a Reaction conditions: o-phenylenediamine (1.0 mmol), catalyst (10 mol %),
PP3 (5 mmol %), base (1.5 equiv.), solvent (3 mL), CO2 pressure of 3 MPa,
total pressure of 6 MPa, 140 oC, 24 h. The yield was determined by 1H NMR,
calibrated using 1,1,2,2-tetrachloroethane as the internal standard. o-
phenylenediamine (5.0 mmol), RuCl2(dppe)2 (0.2 mol %), H2 pressure of 6
MPa, total pressure of 15 MPa, 120 oC.
Initially, the reaction of o-phenylenediamine (1a) with CO2/H2
was selected as the model reaction to optimize conditions for
the synthesis of benzimidazole (1b), and the results are shown
in Table 1. It was demonstrated that all the cobalt salts were
effective for the reaction in the presence of base (e.g., CsF),
while the manganese, nickel, iron, copper salts were hardly
active (Table 1, entry 1-9). Among the examined non-noble
metal catalysts, CoF2 displayed the best performance in the
presence of CsF, affording the 1b yield up to 99% at 24h (Table
1, entry 10). Base is vital in this catalytic system, confirmed by
the fact that the reaction did not occur without base (Table 1,
entry 11). CsF displayed the highest activity, affording excellent
1b yield (e.g., 99%); tBuOK, CsCO3 and K2CO3 showed the
moderate activity (72-75%), while KOH, K3PO4, NEt3 and CsCl
showed low activity (< 34%) (Table 1, entries 12-18). Other
phosphine ligands were tested in the cyclization reaction, and it
was indicated that they showed hardly activity (Table S1,
Supporting Information (SI)). Compared to the Ru catalyst
reported previously (Table 1, entry 19) 45, this catalytic system
was effective for the cyclization of 1a with CO2/H2 at relatively
lower pressure.
b
Figure 1. Dependences of 1a conversion and 1b yield together with formic acid amount
on the reaction time. Reaction conditions: 1a (1.0 mmol), CoF2 (10 mol %), PP3 (5
mmol %), CsF (1.5 equiv.), EtOH (3 mL), CO2 pressure of 3 MPa at room temperature,
total pressure of 6 MPa at room temperature, 140 oC. 1a, 1b and HCOOH were
In the reaction solution of 1a reacting with CO2/H2 over the
CoF2/PP3/CsF catalytic system, formic acid was detected in a determined by 1H NMR, calibrated using 1,1,2,2-tetrachloroethane as the internal
relatively large amount, suggesting that this catalytic system
standard.
was effective for the formation of formic acid via CO2
From the above results (Table 1, entries 11-18), it is obvious
that CsF played an important role in the cyclization reaction. We
studied the effects of the CsF amount on the cyclization reaction
of 1a with CO2/H2. Figure 2 shows the dependence of the 1b
hydrogenation. Figure 1 illustrates the dependence of the yields
of 1b and formic acid on the reaction time in the range of 3 to
30h. It is clear that the yields of both formic acid and 1b
increased with the reaction time and showed a similar trend.
2 | J. Name., 2012, 00, 1-3
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(H3) to 6.52 (H1), 6.39 (H2), 4.45-4.30 ppm (H3) in V1iHew-1A5rNticlHe OMnBlinCe
spectra. These results indicate that the C, H and N atoms of 1a
became more electron-rich when 1a mixed with CsF, also
suggesting that 1a can be activated by CsF.
In the 19F NMR spectrum of the mixture of 1a and CsF, a new
peak appeared at -94.95 ppm (Figure S7, SI), which may be
ascribed to the F atom bonded with H atom of -NH2 of 1a rather
than the F atom from N-F bond49-51 because no splitting of
nitrogen atom of 1a was observed in 1H-15N HMBC spectrum of
the mixture (Figure S6, SI). Therefore, it can be deduced that the
amidogen of 1a can be activated by CsF through hydrogen
bonding, which make the –NH2 of 1a more electronegative, thus
yield, the amount of formic acid and the total of the converted
CO2 on the CsF amount, where the total of the converted CO2 is
equal to the sum of the moles of formic acid, formate and 1b
obtained. It is obvious that the yield of 1b and the amount of
formic acid and formate increased with the CsF amount,
suggesting that CsF promoted the generation of 1b
DOI: 10.1039/C9GC00095J
beneficial for N-formylation with CO2.
CoF2/PP3/CsF
EtOH, 140 oC, 24 h
a
HCOOH
CO2
H2
1.05 mmol
NH2
N
b
HCOOH
EtOH, 140 oC, 24 h
N
H
NH2
yield = 78%
N
NH2
NH2
Figure 2. The amount of CsF to the effect of the cyclization reaction of 1a with CO2/H2.
Reaction conditions: 1a (1.0 mmol), CoF2 (10 mol %), PP3 (5 mol %), CsF, EtOH (3 mL), CO2
(3 MPa), H2 (3 MPa), 140 oC, 24h. The amount of HCOOH, formate and 1b was
determined by 1H NMR, calibrated using 1,1,2,2-tetrachloroethane as the internal
standard.
CoF2/PP3/CsF
EtOH, 140 oC, 24 h
C13
H
13CO2
c
H2
N
H
Scheme 2. Control experiments of the cyclization reaction. Reaction conditions: (a) CoF2
(0.1 mmol), PP3 (0.05 mmol), CsF (1.5 mmol), EtOH (3 mL), CO2 (3 MPa), H2 (3 MPa), 140
oC, 24h. (b) 1a (1.0 mmol), HCOOH (1.5 mmol), EtOH (3 mL), 140 oC, 24h. (c) 1a (1.0 mmol),
CoF2 (10 mol %), PP3 (5 mol %), CsF (1.5 equiv.), 13CO2 (1.0 MPa), H2 (3.0 MPa), EtOH (3
mL), 140 oC, 24h. The amount of HCOOH and benzimidazole were determined by 1H NMR,
calibrated using 1,1,2,2-tetrachloroethane as the internal standard.
and HCOOH. It is noteworthy that there is no 1b detected
without CsF, and 19% of 1b was obtained when 0.2 mmol CsF
was added. This suggests that CsF promote the formation of 1b.
To verify whether CsF can activate 1a, the interaction
between CsF and 1a was studied by 1H NMR, 2H NMR, 13C NMR,
1H-15N HMBC and 19F NMR spectroscopy. In the 1H NMR
spectrum of the mixture of 1b and CsF, the signal assigning to –
NH2 became a double-peak with chemical shifts of 4.40 and 4.37
ppm from a single peak at 4.38 ppm for 1a, meanwhile the
number of hydrogen atom assigning to –NH2 decreased from 4
to 1.59 (relative to the hydrogen atoms of benzene ring) (Figure
S1, SI), which might be caused by the H-D exchange between 1a
and the solvent DMSO-d6. To confirm this hypothesis, the
number of H atoms in DMSO-d6 of the mixture was counted,
To reveal the reaction pathway of the cyclization reaction,
control experiments and isotope labeling experiment were
performed (Scheme 2). First, the hydrogenation of CO2
catalyzed by CoF2/PP3/CsF was carried out, and HCOOH was
obtained, suggesting that CoF2/PP3/CsF was effective for CO2
hydrogenation to HCOOH (Scheme 2a and Figure S8, SI). Next,
o
the reaction of 1a with HCOOH was performed at 140 C, and
78% benzimidazole was obtained after 24 h (Scheme 2b and
Figure S9, SI). These two control experiments suggest that 1b
can be obtained via 1a reacting with in situ-formed formic acid.
Using 13CO2 instead of CO2, the N-13CH=N group in 1b was
detected by GC-MS (Figure S10, SI), indicating that CO2 involved
and it was found that the H atoms of DMSO-d6 increased. The in the cyclization reaction.
increased number of H atoms in DMSO-d6 was almost equal to
the decreased number of H atoms in –NH2 of 1a (Figure S2, SI).
The 2H NMR analysis on the mixture of 1a and CsF indicated the
presence of N-D in 1a (Figure S3, SI). That is, the H/D exchange
reaction took place between the amidogen of 1a and the -CD3
of DMSO-d6 in the presence of CsF, suggesting that CsF can
activate the amidogen of 1a.
The 13C and 1H-15N HMBC NMR spectra gave more
information on the interaction between 1a and CsF (Figure S4-
S6, SI). In the 13C NMR spectrum of the mixture, the chemical
shifts of carbon atoms of 1a shifted upfield from 134.90 (C1),
117.33 (C2) and 114.59 ppm (C3) to 134.85 (C1), 117.32 (C2) and
114.52 ppm (C3), respectively; the chemical shift of the nitrogen
atom of 1a shift upfield from 51.44 (N) to 50.87 ppm (N) in1H-
Figure 3. ESI-MS spectra of the catalytically active intermediates [Co(PP3)H(CO2)]+.
15N HMBC spectra; and the chemical shifts of hydrogen atoms
of 1a shifted upfield from 6.57 (H1), 6.44 (H2), 4.47-4.32 ppm
This journal is © The Royal Society of Chemistry 20xx
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To explore the catalytic mechanism of the cyclization reaction,
Journal Name
substituted amines to produce heterocycles in good to excellent
View Article Online
the active intermediates under the working conditions were
detected by ESI-MS and NMR analysis. Substrate of 1a (1.0
mmol) and the catalyst CoF2 (0.1 mml), PP3 (0.05 mmol) and CsF
(1.5 mmol) were dissolved in EtOH-D6 (2 mL) to form catalytic
solution, which was then charged with H2, or 13CO2 or 13CO2/H2
respectively, and kept at the reaction temperature of 140C for
1.5 h. Subsequently, the reaction solutions were examined by
ESI-MS. It was found that under H2 atmosphere CsF existed in
the form of “CsF clusters” as (CsF)n (n=1-11) in the reaction
solution (Figure S11, SI), and no Co-hydrogen complexes were
observed, while in the case of introduction of 13CO2 into the
reaction solution, the “CsF cluster” was partially broken.
Interestingly, in the simultaneous presence of H2/13CO2, no “CsF
cluster” was detected, and a new peak at 775.1574 m/z was
observed in ESI-MS spectrum. These results suggest that there
exists interaction between CsF and 13CO2; and CsF promotes the
CO2 absorption and the formation of the intermediate with the
peak at 775.1574 m/z.
yields. Based on the above experimentalDrOesI:u1l0t.s10a3n9d/C9GC00095J
N
H2O
+
Co(PP3) (HCO2)
X
1b
H
O
N
+
Co(PP3) H(CO2)
ESI-MS & NMR detected
XH
+
H2O
Co(PP3) (H2)
CsF
HCOOH
Beller' report
H2
NH2
CO2
CsF
2
Co(PP3)
XH
CsF activated 1a through
hydrogen bond interaction
Scheme 3. Plausible reaction mechanism of the cyclization of ortho-substituted anilines
with CO2/H2.
To verify the chemical structure of the intermediate with the previous report,45, 52, 53 a plausible reaction pathway is proposed
peak at 775.1574 m/z in ESI-MS spectrum, 12CO2 instead of as shown in Scheme 3. Under the catalysis of CoF2/PP3/CsF, CO2
13CO2 was used in the same experiment and a peak at 774.1528 is hydrogenated to HCOOH, which subsequently reacts with
m/z was observed, which suggests that this intermediate ortho-substituted anilines to form intermediate 1i. Followed by
contains a CO2 molecule (Figure 3). 1H, 13C and 31P NMR analysis intramolecular cyclization and dehydration, the product 1b was
gave more information of this intermediate, as show in Figure formed finally.
4. In the 1H NMR spectrum of the catalyst solution, a peak at -
Taking the optimal conditions (amine,1.0 mmol; CoF2, 0.2
9.70 ppm was detected,
which may be assigned to mmol; PP3, 5 mol %; CsF, 1.5 equiv.; CO2, 3 MPa; total pressure,
o
monohydride species Co-H (-9.93 ppm) rather than dihydrogen 6 MPa; EtOH, 3.0 mL; 140 C, 24h) in hand, the CoF2/PP3/CsF
Co complex Co-H2 (-11.05 ppm) based on the reported results.52 catalytic system was applied to the reactions of other
Meanwhile, a new multiple-peak around 220.35-221.47 ppm structurally different ortho-substituted anilines with CO2/H2,
appeared in the 13C NMR spectrum, which may be ascribed to and the results are listed in Table 2. It was indicated that ortho-
Co-13CO2 according to the previous report.53 In contrast, the 31
P
amidogen, ortho-sulfydryl and ortho-hydroxy substituted
NMR spectrum of the catalyst solution changed accordingly anilines can react with CO2/H2 to produce corresponding N-
compared to that of the mixture of CoF2/PP3/CsF at N2 containing heterocycles. The isolated yield of benzimidazole
atmosphere, suggesting the interaction with other species in was up to 94% under the optimum reaction conditions (Table 2,
the reaction solution (Figure S14, SI).
entry 1). O-phenylenediamines with electron-donating and
electron-withdrawing groups were able to react with CO2/H2
over CoF2/PP3/CsF, producing corresponding N-containing
heterocycles in good to excellent yields in most cases. The
position of the substituent in the phenyl ring affected the
reactivity of the substrate due to the steric effect. For example,
4-methyl substituted o-phenylenediamine showed good
reactivity, affording the product in an excellent yield of 99%,
while 2-methyl substituted o-phenylenediamine offered a
product yield of 74% (Table 2, entry 3). The substrate with two
methyl at 3- and 4- position in the phenyl ring also showed good
reactivity, yielding the product in an excellent yield (Table 2,
entry 4). Other o-phenylenediamines substituted at 4-position
with substituents including methoxy, halogen groups (F, Cl, Br)
displayed good reactivity, giving rise to the products in good
yields (Table 2, entries 5-9). However, the 4-CF3 substituted
diamine showed very low product yield of 26%, similar to that
obtained over Ru catalyst45. The catalytic system was also active
for the diamines containing N-heterocycle. For example, 2, 3-
and 3, 4-diaminopyridines were able to react with CO2/H2 under
the experimental conditions, producing corresponding products
in good yields (Table 2, entries 12 and 13). The o-
phenylenediamine containing extended conjugated structure
Figure 4. NMR spectra of the catalytically active intermediates [Co(PP3)H(CO2)]+.
From the above results, we can conclude that the intermediate
with the peak at 774.1528 m/z in ESI-MS spectrum is
[Co(PP3)H(CO2)]+, which is different from [Co(PP3)H]+ or
[Co(PP3)(H2)]+ reported previously.52,
In this work, the
54
catalytic system composed of CoF2, PP3 and CsF could
simultaneously capture H2 and CO2 and activate them, which
can render the CO2 hydrogenation to proceed under relatively
mild conditions, and further realize the cyclization of o-
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NH2
Table 2. Scope of the cobalt-catalysed synthesis of N-containing heterocyclesa
N
View Article Online
21
22
23
41
21b
21a
DOI: 10.1039/C9GC00095J
O
F
F
F
F
OH
entry
1
substrate
product
yield (%)
99 (94b)
NH2
NH2
N
N
1b
19
22b
22a
1a
N
H
O
N
OH
NH2
NH2
Br
NH2
N
Br
99 (93b)
74
2b
12
23b
2
3
23a
2a
N
H
O
OH
NH2
NH2
N
Reaction condition: a substrate (1.0 mmol), CoF2 (10 mol %), PP3 (5 mol %),
CsF (1.5 equiv.), EtOH (3mL), CO2 (3 MPa), H2 (3 MPa), 140 oC, 24 h. The yield
was determined by 1H NMR, calibrated using 1,1,2,2-tetrachloroethane as
the internal standard. b Isolated yield.
3b
3a
N
H
NH2
NH2
N
4b
4
5
99
85 (80b)
95
4a
N
H
NH2
NH2
could proceed successfully as well, with a product yield up to 80%
(Table 2, entry 14). In addition, this cobalt catalytic system was
also suitable for both ester and ketone functionalized o-
diamines, giving product yields in 48-74% (Table 2, entries 15-
17). The above results indicate that the CoF2/PP3/CsF catalytic
system is highly efficient for the synthesis of benzimidazoles,
and a series of benzimidazoles could be produced in good yields.
Delighted by the excellent performance of CoF2/PP3/CsF for the
cyclization of o-phenylenediamines, we explored the
application of CoF2/PP3/CsF to convert CO2 with other ortho-
substituted anilines such as 2-aminothiophenol and 2-
aminophenol (Table 2, entries 18-21). It was found that 2-
aminothiophenol could be completely converted to
benzothiazole in a yield of 99% under the same reaction
condition (Table 2, entry 18). Noticeably, 2-aminophenol could
be transformed into benzoxazole in a yield of 77% (Table 2,
entry 19), and -F and -CH3 substituted 2-aminophenols can also
be converted to corresponding N-containing heterocycles
(Table 2, entries 20 and 21). For the F and Br substituted 2-
aminophenols at para-position of -OH group, the yields of
corresponding target products were lower than 20% (Table 2.
Entries 22 and 23). These results further proved that
CoF2/PP3/CsF is highly efficient for the cyclization of ortho-
substituted anilines with CO2 into value-added heterocyclic
compounds.
N
5b
5a
N
O
O
NH2
NH2
H
N
6b
6
6a
N
F
F
NH2
NH2
H
N
7b
7
86 (83b)
86
7a
N
Cl
Cl
NH2
NH2
H
N
8b
9b
8
8a
N
H
Br
Cl
Br
Cl
NH2
NH2
N
9
75
9a
N
H
Cl
Cl
NH2
NH2
N
10b
10
26
10a
N
H
F3C
F3C
NH2
NH
NH2
11
48
N
11b
11a
N
NH2
N
12b
12
13
14
70
94
80
12a
N
H
N
N
NH2
NH2
N
13b
13a
N
N
N
H
NH2
N
NH2
14b
14a
15a
N
H
NH2
NH2
N
15b
16b
O
O
O
15
16
17
N
74
48
57
Conclusions
NH2
H
O
O
In conclusion, we have developed a highly efficient cobalt
catalytic system for the reductive cyclization of ortho-
substituted anilines with CO2 and H2 to N-containing chemicals.
This is for the first time to realize non-noble metal catalyzed
cyclization of ortho-substituted anilines with CO2 using
molecular hydrogen. Mechanism investigation indicates that
[Co(PP3)H(CO2)]+ is the catalytic active intermediate under the
working catalytic condition; CsF plays important roles in
activating ortho-substituted anilines and promoting the
formation of catalytic active intermediate. This catalytic system
allows wide scopes of ortho-substituted anilines, together with
excellent functional group tolerance, affording various N-
containing chemicals in good to excellent yields.
NH2
NH2
N
16a
O
N
H
O
O
O
O
NH2
N
17a
17b
N
NH2
H
NH2
N
18
19
20
99 (92b)
77 (70b)
43
18b
18a
S
N
SH
NH2
19b
19a
O
OH
NH2
N
20a
20b
O
Conflicts of interest
OH
There are no conflicts to declare.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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