Pd-Catalyzed Reductive Cyclization of Nitroarenes with CO2 as the CO Source

Article abstract of DOI:10.1002/ejoc.201901629

A reductive amination process that constructs indoles, carbazoles or benzimidazoles from nitroarenes – irrespective of their electronic or steric nature – was developed that uses CO₂ as the source of CO. The process is robust, tolerating common gaseous components of flue gas (H₂S, SO₂, NO and H₂O) without adversely affecting the reductive cyclization.

Full text of DOI:10.1002/ejoc.201901629

A Journal of
Accepted Article
Title: Development of a Pd-Catalyzed Reductive Cyclization of
Nitroarenes that Uses CO2 as the CO Progenitor
Authors: Xinyu Guan, Haoran Zhu, Yingwei Zhao, and Tom G Driver
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10.1002/ejoc.201901629
European Journal of Organic Chemistry
COMMUNICATION
Development of a Pd-Catalyzed Reductive Cyclization of
Nitroarenes that Uses CO2 as the CO Progenitor
Xinyu Guan,[a] Haoran Zhu,[a] Yingwei Zhao,[a,b] and Tom G. Driver*[a]
Dedication ((optional))
Abstract: A reductive amination process that constructs indoles,
carbazoles or benzimidazoles from nitroarenes—irrespective of their
electronic or steric nature—was developed that uses CO2 as the
source of CO. The process is robust tolerating common gaseous
components of flue gas (H2S, SO2, NO and H2O) without adversely
affecting the reductive cyclization.
molecules of CO2, we wondered if CO2 could be used as the
source of CO (Scheme 1). Recently, a variety of strategies that
employ homogeneous transition metal catalysts,[6] carbene
catalysts[7] or fluoride have emerged to convert CO2 to CO.[8] We
were curious if one of these technologies would be effective to
produce CO at a high enough pressure to trigger the reductive
cyclization of nitroarenes. Herein, we report the development of a
method where CO2 is deoxygenated using disilane and fluoride to
produce CO, which is used as the reductant for a Pd-catalyzed
reductive cyclization of nitroarenes to produce indoles,
carbazoles or benzimidazoles.
Introduction
Because of its role as a greenhouse gas, the development of CO2-
fixation reactions continues to inspire scientists.[1] While
significant efforts have been made to develop transition metal-
catalyzed reactions that use CO2 as a C1 source,[2] less attention
has been given towards converting CO2 into CO for use in
carbonylation reactions.[3] In addition to being a C1 source for
these reactions, carbon monoxide is also commonly employed as
Results and Discussion
To simplify method development, we examined the reactivity of
nitrostilbene in a multi-chamber glass reactor where CO would be
generated in a separate chamber from the palladium-catalyzed
reductive cyclization. After initial investigations established that a
two-chamber system was not viable,[9] the optimization
experiments were performed in a three-chamber apparatus,
which we envisioned would enable the systematic investigation of
difference sources of CO2 (vide infra) once the optimal conditions
for deoxygenation were determined. To minimize the
manipulation of CO2 gas, improve the reproducibility of the
optimization and identify deoxygenation conditions that tolerated
water vapor, we chose to generate CO2 by thawing a frozen
aqueous solution of H2SO4 and K2CO3 in chamber 1. In chamber
2, conditions for the reduction of CO2 to CO were surveyed, and
in chamber 3 we chose to use the combination of 5 mol % of
[Pd(OAc)2] and 10 mol % tetramethylphenanthroline to catalyze
the conversion of nitrostilbene 1a into indole 2a because of its
established effectiveness in reductive cyclization reactions.[4k,
10][11] Several potential deoxygenation reactions were screened in
chamber 2 (Table 1). Unfortunately, no reaction was observed
using an N-heterocyclic carbene catalyst and cinnamaldehyde as
the reductant,[7] or by generating in situ a carbodiphosphorane
and zinc bromide to activate CO2[6d] or employing a copper NHC
catalyst using B2pin2 as the terminal reductant (entries 1 – 3).[6a]
In contrast, switching to a fluoride catalyst with B2(OH)4 resulted
in trace reduction of nitrostilbene to aniline (entry 4), and following
the report of Lindhardt, Skrydstrup and co-workers using 4 mol %
of KF and (Ph2MeSi)2 as the terminal reductant produced indole
2a in 18%.[8] To improve the yield of the reductive cyclization, the
deoxygenation reaction was optimized by examining different
fluorides, silanes and solvents. While no reaction was observed
using HF·pyridine, Et3N·3HF or n-Bu4NF (entries 6 – 8), nearly
quantitative yield of 2a and 95% of disiloxane (Ph2MeSi)2O was
obtained using 20 mol % of CsF (entry 9). Increasing or
decreasing the loading of CsF lead to lower yields of indole
a
terminal reductant in reductive cyclization reactions of
nitroarenes that produce N-heterocycles.[4] The resulting indole-
and carbazole scaffolds are ubiquitous in biologically active
molecules, pharmaceuticals and materials.[5] Because the by-
product of the reductive cyclization reaction of nitrostryenes is two
[Pd(OAc)2] (5 mol %)
tmphen (10 mol %)
R
O
C
O
O
C
R
+
+
N
H
O
DMF, 100 °C
N
(2 atm)
(2 equiv)
1
2
O
Convert CO2 to CO for Pd-Catalyzed Reductive Cyclization
R
O
O
C O
O
E
E
N
1
2
O
[Pd]
?
R
E
E
C O
N
H
Scheme 1. Towards the Development of a Pd-Catalyzed Reductive Cyclization
Reaction to Access N-Heterocycles that uses CO2 as the source of CO.
[a]
[b]
X. Guan, H. Zhu, Dr. Y. Zhao, Prof. Dr. T. G. Driver
Department of Chemistry
University of Illinois at Chicago
845 West Taylor Street, Chicago, Illinois, USA 60607-7061
E-mail: tgd@uic.edu
Homepage: www2.chem.uic.edu/driver
College of Chemical Engineering
Huaqiao University, Xiamen
668 Jimei Boulevard, Xiamen, Fujian, 361021 People’s Republic of
China
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejoc.201901629
European Journal of Organic Chemistry
COMMUNICATION
Table 1. Optimization of Deoxygenation of CO2 for Pd-Catalyzed Reductive
Table 2. Scope and Limitations of the Three-Chamber Pd-Catalyzed
Cyclization for Indole Synthesis.
Reductive Indole Formation.
chamber condition result
chamber condition result
H2SO4 (8 equiv)
K2CO3 (4 equiv)
H2SO4 (8 equiv)
K2CO3 (4 equiv)
1
1
R4 R5
R4
R5
Ph
R3
R2
R6
R3
R2
CO2
CO
CsF (20 mol %)
(Ph2MeSi)2 (2 equiv)
DMF, 25 °C
[Pd(OAc)2] (5 mol %)
tmphen (10 mol %)
DMF, 100 °C
CO2
CO
Ph
2
3
conditions
N
H
R6
2
3
H
Me
H
Me
NO2
N
H
NO2
1a
[Pd(OAc)2] (5 mol %) NO2
tmphen (10 mol %)
2a
NO
2
R1
R1
1
2
DMF, 100 °C
het
het
Entry[a]
catalyst
(mol %)
reductant
(2 equiv)
solvent
T
(°C)
%,
yield
Entry[a]
#
R1
R2
R3
R4
R5
R6
%,
yield
1
IMesCl,
K2CO3
DMF
100
n.r.
1
a
b
c
d
e
f
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-
94
97
91
98
69
90
57
76
83
78
69
72
68
72
92
2
OMe
H
2
3
ZnBr2, CH2I2
Et3P
PhMe
THF
100
100
n.r.
n.r.
3
(IPr)CuOt-Bu
(1)
B2pin2
4
F
4
CsF (10)
B2(OH)4
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMSO
γ-VL
100
25
25
25
25
25
25
25
25
25
25
100
0[b]
18
5
Cl
5
KF (10)
(Ph2MeSi)2
(Ph2MeSi)2
(Ph2MeSi)2
(Ph2MeSi)2
(Ph2MeSi)2
(Ph2MeSi)2
(Ph2MeSi)2
(Me3Si)2
6
CO2Me
CF3
6
HF•py (20)
Et3N•3HF (20)
(n-Bu)4NF (20)
CsF (20)
n.r.
n.r.
n.r.
94
7
g
h
i
7
8
–OCH2O–
OMe
8
9
H
H
H
H
H
H
H
9
10
11
12
13[b]
14[b,c]
15
j
Me
F
10
11
12
13
14
15
CsF (40)
61
k
l
CsF (10)
10
CF3
H
CsF (20)
n.r.
90
m
n
CsF (20)
(Me3Si)3SiH
(Ph2MeSi)2
(Ph2MeSi)2
H
CsF (20)
n.r.
63[c]
o
H
MeOC6H4
4-FC6H4
4-CF3C6H4
n-Pr
CsF (20)
16
p
q
r
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
61
[a] Conditions: chamber 1: 1 M H2SO4 (8 equiv), K2CO3 (4 equiv) in 0.8 mL
of H2O; chamber 3: [Pd(OAc)2] (5 mol %), phen (10 mol %), 0.1 M, 14 h. [b]
trace aniline observed. [c] poor conversion seen at lower temperatures. phen
= phenanthroline. γ-VL = γ-valerolactone.
17[b]
18
H
70[d]
79
H
19
s
t
H
CO2Me
H
88
20
Ph
Et
86
(entries 10 and 11). Alternative silanes were examined and while
no reaction was observed using (Me3Si)2, (Me3Si)3SiH was almost
as effective as (Ph2MeSi)2 (entries 12 and 13). A solvent screen
was performed. While no reaction was observed using DMSO as
the solvent (entry 14), 63% of 2-phenylindole was obtained using
γ-valerolactone if chamber 2 was heated to 100 °C (entry 15).
These investigations identified that the combination of 20 mol %
CsF, 2 equivalents of (Ph2MeSi)2 in DMF at room temperature
were most effective to promote deoxygenation of CO2 to produce
CO for use in the reductive cyclization reaction.
21
u
Ph
81
[a] Conditions: chamber 1: 1 M H2SO4 (8 equiv), K2CO3 (4 equiv) in 0.8 mL
of H2O; chamber 2: CsF (20 mol %), (Ph2MeSi)2 (2 equiv), DMF, 25 °C;
chamber 3: [Pd(OAc)2] (5 mol %), phen (10 mol %), 0.1 M, 14 h. [b] 10
mol % of [Pd(OAc)2] and 20 mol % of tmphen used. [c] 8 equiv of K2CO3, 10
equiv of H2SO4, 4 equiv of (Ph2MeSi)2 and 40 mol % of CsF used. [d] 6% of
the N-OH indole obtained.
With these optimal conditions, the scope and limitations of the
reductive cyclization reaction were investigated (Table 2). We
found that the reaction was relatively insensitive to the electronic
nature of the nitroarene producing indole 2 irrespective of whether
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10.1002/ejoc.201901629
European Journal of Organic Chemistry
COMMUNICATION
an electron-donating or electron-withdrawing group was present
(entries 1 – 12). While moderate yields of indole 2 were observed
with strong electron-withdrawing CF3 substituents, nearly
quantitative yields were observed in the presence of electron-
releasing groups (entries 1 – 11). We found that the steric
environment could be increased around the ortho-styryl
substituent or the nitro group to construct indoles 2m and 2n if the
catalyst loading and CO pressure were increased (entries 13 and
14). Our reductive cyclization also tolerated a range of R6-
substituents (entries 15 – 19). While an electron-poor β-aryl
substituent required higher catalyst loading (entry 17), indoles 2o
– 2s bearing 2-aryl, 2-alkyl or even 2-carboxyl substituents were
smoothly produced. Our reaction is not limited to only forming 2-
substituted indoles, exposure of α-phenyl or α,β-disubstituted
nitrostyrenes to reaction conditions readily constructed 3-phenyl
or 2,3-disubstituted indoles 2t and 2u (entries 20 and 21).
After investigating the effect of modifying electronic- and steric
nature of the nitrostyrene on the efficiency of our reductive
amination reaction, we explored several nitroarenes to determine
if our process could access more complex N-heterocycles
(Scheme 2). We determined that carbazoles such as 7 could be
constructed from nitrobiarenes 3 through a site-selective reaction
if twice as much K2CO3 and disilane were used. The process was
also found to smoothly transform nitroarene 4a into
tetrahydropyrano[3,4-b]indole 8a. In addition to facilitating the
To showcase the capability of our process, we decided to test
the robustness of our system towards different sources of CO2
(Scheme 3). To reduce the emission of CO2 into the atmosphere,
cryogenic distillation of flue gas has been demonstrated as an
effective means to capture CO2 as dry ice.[12] To see if our process
could use this carbon-capture product from flue gas, dry ice was
examined in the three-chamber apparatus. To our delight, we
found it to be equally effective enabling the transformation of
nitrostilbene 1a into indole 2a in 86%. While the composition of
flue gas varies depending on its origin, pre- and post-combustion
processes produce H2O, SO2, NO and H2S in addition to
CO2.[13],[14] By producing CO2 from an aqueous solution of K2CO3
and H2SO4, we demonstrated that the reductive cyclization
reaction was impervious to water vapor (vide supra), and we were
curious to see if small quantities of H2S, SO2 or NO were tolerated
by the palladium catalyst. Towards this end, we added a several
salts to chamber 1, which would produce H2S, SO2 or NO to
approximate their composition in flue gas. In every case, we were
delighted to find that indole 2a smoothly formed with only a slight
attenuation in yield observed when SO2 was present.
chamber
1
conditions
Ph
CO2 source
additive
Ph
H
NO2
N
Me
Me
CsF (20 mol %)
(Ph2MeSi)2 (2 equiv)
DMF, 25 °C
H
1a
2a
2
3
synthesis
of
functionalized
indoles
and
carbazoles,
benzimidazoles could be constructed from nitroarenes such as 5.
Gratifyingly, we found that our reductive cyclization process could
be extended to transform β-nitrostyrenes into α-substituted
indoles. Because the reductive cyclization of 6 also required
doubling the quantity of K2CO3 and disilane, we conclude that a
higher pressure of CO is required for substrates where aromaticity
is disrupted in C–N bond formation. In combination with the
results in Table 2, the success of these substrates illustrates that
a range of N-heterocycles—irrespective of the steric- or electronic
nature of the nitroarene or nitroalkene—can be easily accessed
using CO2 as the source of CO.
[Pd(OAc)2] (5 mol %)
tmphen (10 mol %)
DMF, 100 °C
CO2 source (equiv)
gas % yield, 2a
additive (equiv)
dry ice (6)
…
…
H2S
SO2
86
90
75
86
K2CO3 (4) + H2SO4 (8)
K2CO3 (4) + H3SO4 (8)
K2CO3 (4) + H3SO4 (8)
K2S (0.04)
NaHSO3 (0.032)
[Cu ] (0.002) + HNO3 (0.008) NO
0
Scheme 3. Investigation of the Effect of the Origin and Composition of CO2 on
the Reductive Cyclization Reaction.
Hb
Conclusions
Hb
A three-chamber process that transforms nitroarenes into N-
heterocycles was developed that uses CO2 as the source of the
superstoichiometric CO. Deoxygenation of CO2 was
accomplished using the combination of disilane and a fluoride
catalyst. The resulting CO is used in a palladium-catalyzed
reductive amination reaction that constructs a broad range of
indoles, carbazoles, benzimidazoles irrespective of the steric- or
electronic nature of the nitroarene substrate. The process
tolerates the in situ generation of CO2 or the use of dry ice and
the atmosphere can be contaminated with H2S, SO2, NO or H2O
to still smoothly afford the N-heterocycle.
Ha
NO2
N
Ha
3
4
7, 88%
chamber condition result
O
O
H2SO4 (8 equiv)
K2CO3 (4 equiv)
1
H
NO2
N
CsF (20 mol %)
(Ph2MeSi)2 (2 equiv)
DMF, 25 °C
[Pd(OAc)2] (5 mol %)
tmphen (10 mol %)
DMF, 100 °C
CO2
CO
H
2
3
8, 86%
NO
2
N
N
N
Ph
H
Ph
H
het
H
NO2
H
Ph
5
6
9, 67%
Ph
N
NO2
H
2t, 59%
Experimental Section
Scheme 2. Construction of Carbazoles, Indoles or Benzimidazoles through Pd-
Catalyzed Reductive Cyclization of Nitroarenes or Nitroalkenes.
To a 30 mL 3-chamber reactor, K2CO3 (4 equiv) in water and a 1 M aq soln
of H2SO4 (8 equiv) was added and frozen sequentially in chamber 1, CsF
(0.02 equiv), disilane (0.2 equiv) and 0.8 mL of DMF were added to
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10.1002/ejoc.201901629
European Journal of Organic Chemistry
COMMUNICATION
chamber 2, 0.10 mmol of nitrostyrene, palladium acetate (5 mol %),
tetramethylphenathroline (10 mol %) and 1 mL of DMF were added to
chamber 3. The 3-chamber reactor was sealed from outer environment
while allowing gas exchange among each other. Then the frozen reaction
mixture in chamber 1 was allowed to thaw and stir until effervescence of
CO2 was no longer observed. Chamber 3 was heated at 100 °C while
chamber 1 and 2 were stirring at room temperature. After 14 h, the reaction
mixture was cooled to room temperature and filtered through a pad of silica
gel. The filtrate was concentrated in vacuo, and the residue was purified
using MPLC to afford the N-heterocycle.
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Acknowledgments
We are grateful to the National Science Foundation CHE-
1564959. We thank Mr. Furong Sun (UIUC) for HRMS data.
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Keywords: carbon dioxide • palladium • CO equivalent • N-
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10.1002/ejoc.201901629
European Journal of Organic Chemistry
COMMUNICATION
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Key Topic*
Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
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width: 5.5 cm; max. height: 5.0 cm;
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CO2 as CO Progenitor in Pd-Catalyzed Reductive Cyclization
E
Reductive Cyclization*
O
O
C O
MePh2Si
SiPh2Me
Xinyu Guan, Haoran Zhu, Yingwei Zhao
and Tom G. Driver*
O
N
O
[Pd]
CsF
E
Page No. – Page No.
C
O
MePh2Si SiPh2Me
• Broad scope — 25 examples
N
H
Development of a Pd-Catalyzed
Reductive Cyclization of Nitroarenes
using CO2 as the CO Progenitor.
• Robust — unaffected by flue gases H2S, SO2, NO or H2O
A reductive amination process that constructs N-heterocycles from nitroarenes was
developed that uses CO2 as the source of CO.
*one or two words that highlight the emphasis of the paper or the field of the study
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