The one-pot synthesis of quinolines: Via Co(iii)-catalyzed C-H activation/carbonylation/cyclization of anilines

Article abstract of DOI:10.1039/c7ob02310c

We herein disclose a novel Co(iii)-catalyzed C-H activation/carbonylation/cyclization of anilines with ketones using paraformaldehyde as the carbonyl source, giving direct access to diverse quinolines with broad functional group tolerance and in good to excellent yields. Moreover, exclusive site-/region-selectivity was observed in this versatile transformation.

Full text of DOI:10.1039/c7ob02310c

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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DOI: 10.1039/C7OB02310C
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One-pot synthesis of quinolines via Co(III)-catalyzed C–H
activation/carbonylation/cyclization of anilines
Received 00th January 20xx,
Xuefeng Xu,a Yurong Yang,b Xin Chen,a Xu Zhang*a and Wei Yi*b
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
efficient preparative methods for efficient synthesis of quinolines has
attracted considerable attention from synthetic chemists. More
recently, the high efficiency with respect to the docking of the
catalysts, especially for those that derived from the earth-abundant
and inexpensive first-row transition metal species, such as the Co
catalysts,9 has received special attention due to their outstanding
activities in C−H activation reaction. However, they are still lacking
in simplicity and generality.
We herein disclose
a
novel Co(III)-catalyzed C–H
activation/carbonylation/cyclization of anilines with ketones
using paraformaldehyde as the carbonyl source, giving direct
access to diverse quinolines with broad functional group
tolerance and in good to excellent yields. Moreover, exclusive
site-/region-selectivity was observed in this versatile
transformation.
Quinolines are commonly occurring structural motifs found in a
broad range of biologically active compounds and pharmaceuticals.1
Driven by their potential biological application, many versatile
methods toward the synthesis of quinolines have been developed,
and accordingly, a wide variety of multi-functionalized quinolines
has increased exponentially as bioactive agents. However, by
reviewing these developed traditional methods, such as the Skraup
reaction,2 Doebner-von Miller synthesis,3 Doebner synthesis,4
Combes synthesis,5 Pfitzinger synthesis,6 and Friedländer protocols
synthesis,7 they still suffer from one or more of the following
disadvantages, such as relatively harsh conditions, the compulsive
use of using toxic and corrosive reagents, and sometimes, giving a
limited substrate scope. From the view of green chemistry, recently
developed transition-metal-catalyzed C−H functionalization to
construct polysubstituted quinoline derivatives represents an atom-
and step-economical strategy, numerous synthetic protocols which
were catalyzed by several transition-metal such as Au, Pd, Cu and
Ag have been discolsed.8 Among these strategies, direct
functionalization of C−H bonds is one of the elegant ways to access
quinolines. However, it often requires the assistance of a proximal
directing group. Moreover, in most cases, the directing group only
exhibited a coordinative effect or acted as a simple nucleophile in
post coupling transformations. Consequently, the development of
In 2015, a beautiful example for the synthesis of quinolines
through the Co(III)-catalyzed redox-neutral annulative couplings of
amides with alkynes has been developed by the Li group (eqn (1)).10
Meanwhile, the Balaraman group has reported a rhodium-catalyzed
C−H activation strategy which employed unprotected anilines and
electron-deficient alkynes to build C−C bonded products, where
used CO gas or CO surrogates (e.g. paraformaldehyde) as the
carbonyl source (eqn (2)).11 Compared to CO gas,12 indeed,
paraformaldehyde is more desirable because it is cheap, stable, low
toxicity and simple to operate. As a result, the employment of
paraformaldehyde had been well explored in several Rh-catalyzed
CO gas-free carbonylation reactions of alkynes.13 Most recently,
Zhou and co-workers also disclosed an elegant Ru(II)-catalyzed
direct C-H hydroxymethylation reaction using paraformaldehyde as
the efficient coupling partner.14 However, to the best of our
knowledge, no example involving in Co(III)-catalyzed C−H
functionalization using paraformaldehyde as the carbonyl source has
been reported. Thus, we herein develop the first Co(III)-catalyzed
C–H activation/carbonylation/cyclization of anilines with ketones for
one-pot synthesis of quinolines, in which employed
a.College of Chemistry and Pharmaceutical Engineering, Nanyang Normal
University, Nanyang 473061, P. R. China
b.Key Laboratory of Molecular Clinical Pharmacology & Fifth Affiliated Hospital,
Guangzhou Medical University, Guangzhou, Guangdong 511436, P. R. China
E-mail: yiwei@gzhmu.edu.cn and zhangxuedu@126.com
Fax: +86-20-37104196; Tel: +86-20-37104196
Electronic Supplementary Information (ESI) available: Detailed experimental procedure
and characterization of all new products (H1 and C13 NMR spectra). See
DOI: 10.1039/x0xx00000x
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aReaction conditions: aniline 1a (0.5 mmol), acetophenone 2a (0.5 mmol),
paraformaldehyde as the carbonyl source for direct construction of
the C1 source (sp2) in quinoline products via a regioselective
migratory insertion and intramolecular cyclization and dehydration
cascade (eqn (3)).
paraformaldehyde (2.5 mmol), Cp*Co(CO)I2 (5.0 mol%), VaiedwdiAtirtviceleAOn(li1ne0
DOI: 10.1039/C7OB02310C
mol%), additive B (100 mol%), MeOH (2 mL), 120 °C, 8 h, reaction kettle
b
c
under air. Isolated yield after chromatography. 0.375 mmol of 1a and 0.5
d
mmol of 2a were used, repectively. 0.625 mmol of 1a and 0.5 mmol of 2a
were used, repectively. 0.75 mmol of 1a and 0.5 mmol of 2a were used,
We began our quinoline synthesis using aniline (1a),
acetophenone (2a) and paraformaldehyde as model substrates,
(Table 1). As shown in Table 1, when the reaction was conducted in
CH3OH using Cp*Co(CO)I2 (5.0 mol%) as the catalyst, AgSbF6 (10
mol%) and HOTf (100 mol%) as the co-catalytic additives at 120 °C,
to our delight, the desired quinoline 3a was obtained in 14% isolated
yield (entry 1). The brief screening of Ag salts (e.g. AgOAc, AgOTf,
Ag2CO3, and AgNTf2) showed that AgNTf2 was optimal, which the
isolated yield of 3a could be increased to 62% (entries 1-4). (entries
2-5). With this finding in hand, next various acidic additives were
explored (entries 6-10). The results displayed that HOTf gave the
highest efficiency (entry 5). Significantly reduced yield of the
product was found in the absence of the catalyst or co-catalytic
additives, which indicated that the introduction of both Co(III)
catalyst and Ag salt/HOTf additives are essential for the reaction
outcome (entries 11-13). Moreover, changing Cp*Co(CO)I2 to other
well-known cobalt species obviously inhibited the process (entries
14-16). Finally, we investigated the effects of substrate concentration
for further optimization of the reaction (entries 5 and 17-19). The
results showed that the molar ratio of 1a and 2a had a certain
influence on the reaction outcome.
e
repectively.
With the optimized catalytic system established, we next turned to
examine the scope with respect to aniline substrates, and the results
are summarized in Scheme 1. The reaction of diverse anilines with
2a and paraformaldehyde went through smoothly and the desired
quinoline products were obtained in moderate to good yields (44-
89%). Anilines bearing the either electron-donating (Me, t-Bu) or
electron-withdrawing (F, Br) substituents at the ortho-position
showed good compatibility, and the corresponding products 3b-e
were obtained in 51-65% yields. Tolerance to the F and Br is
particularly noteworthy since they can be further transformed to
other valuable functional groups through traditional cross-coupling
reactions. Similarly, meta- and para-substituted anilines also
underwent
the
Co(III)-catalyzed
C–H
activation/carbonylation/cyclization cascade successfully to afford
the corresponding quinoline products 3f-3p with considerable yields
(up to 78%). It is worth emphasizing that, for meta-substituted
anilines, the sole regioisomer was detected, in which the C–H
activation reaction exclusively occurred at less-hindered ortho-
position. Moreover, disubstituted anilines could be transformed to
the desired products in satisfactory yields (for 3q: 89% and for 3r:
72%), which indicated high steric tolerance of this catalytic system.
Notably, the reaction also tolerated the polyaromatic naphthalene
substrates, such as 1s, which provided the interesting 3s in 59% yield.
Table 1. Optimization of Reaction Conditionsa
Finally,
large-size
4-(piperidin-1-yl)benzenamine
and
4-
morpholinobenzenamine were also suitable substrates, which could
converted to their corresponding products 3t and 3u in 61% and 64%
yields, respectively. Taken together, these results further
demonstrated the versatility of this reaction.
Entry
catalyst
additive A
additive B
solvent
Yield (%)b
1
2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
--------
AgSbF6
AgOAc
AgOTf
Ag2CO3
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
--------
TfOH
TfOH
TfOH
TfOH
TfOH
Tf2O
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
14
0
3
32
0
4
5
62
0
6
7
TFA
10
0
8
PivOH
AcOH
TsOH
--------
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
9
0
10
11
12
13
14
15
16
17c
18d
19e
0
25
10
5
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
AgNTf2
Co(OAc)2
0
Co(acac)2
0
CoI2
0
Cp*Co(CO)I2
Cp*Co(CO)I2
Cp*Co(CO)I2
52
60
48
Scheme 1 Scope of anilines. Reaction conditions: aniline 1 (0.5 mmol),
acetophenone 2a (0.5 mmol), paraformaldehyde (2.5 mmol), Cp*Co(CO)I2 (5
mol%), AgNTf2 (10 mol%), HOTf (100 mol%), MeOH (2 mL), 120 °C, 8 h,
reaction kettle under air. Yields refer to isolated yields.
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aniline substrate slightly dominated the product formation (Scheme
DOI: 10.1039/C7OB02310C
3a), revealing that the C–H bond activation most likely occurs via an
internal electrophilic substitution (IES)-type mechanism and thus
argues against ring closure through the electrophilic aromatic
substitution (EAS) pathway. In addition, treatment of 2-
styrylphenylamine with paraformaldehyde provided the desired
We next investigated the scope with respect to ketone substrates.
As illustrated in Scheme 2, 2,4-dimethylbenzenamine 1r was readily
reacted with a wide variety of functionalized unsymmetrical aryl
ketones bearing the substituents either at ortho-, meta-, or para-
position on the aryl ring to give the corresponding products with
excellent regioselectivity and in moderate to excellent yields (54-
91%). Both electron-donating (Me, N(CH3)2) and -withdrawing (Cl,
Br, I, CF3) groups that attached at the aryl ring part of ketones were
all well tolerated. The reaction also performed smoothly with cyclic
alkyl ketones (1j-l). Interestingly, heteroaromatic furan (1m),
thiophene (1n), pyridine (1o) and indole (1p), polyaromatic
naphthalene (1q-r) and cycloaryl ketone (1s) substrates could be
accommodated in the catalytic system, giving their corresponding
products 4m-s in moderate to good yields. It should be noted that the
reaction also worked well with 2-phenylacetophenone (1t) and
styrene ketone (1u) substrates to give their corresponding products
4m and 4u in 73% and 91% yields, respectively.
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product
5 in 23% yields (Scheme 3b), suggesting that 2-
vinylbenzenamine species might be used as the key intermediate
form in this transformation. Furthermore, 9H-carbazole was
designated as a substrate for this reaction in the absence of
paraformaldehyde. As shown in Scheme 3c, the reaction of 9H-
carbazole and 1-p-tolylethanone gave 6 with the isolated yield of
42%, indicating that paraformaldehyde might play a vital role as the
C1 source (sp2) for the formation of desired quinoline products. To
further define the role of paraformaldehyde, CO gas was submitted
to the current Rh(III) catalysis in the absence of paraformaldehyde
(Scheme 3d). Gratefully, the reaction of 2-styrylphenylamine and
CO gas proceeded successfully to give the desired product 5, which
not only gave a powerful evidence to support our speculation that
paraformaldehyde was employed as the CO surrogate but also hinted
the reaction mechanism. No conversion was observed when N-
phenylformamide was treated with 2a under the otherwise identical
conditions (Scheme 3e), revealing that the present reaction pathway
should not be employed N-phenylformamide as the key intermediate.
Finally, the reversibility of the C−H activation step was defined by
running the reaction in CD3OD/DOTf in the absence of 2a and
paraformaldehyde. As shown in Scheme 3f, after stirring for 2 h,
significant deuteration of 1a at ortho-H position (82% D) was
observed, while the NH2 part retained no incorporation of deuterium.
The results revealed that the C−H bond activation was largely
reversible and also suggested that ortho-metallation of aniline might
be employed as a key reaction step for this transformation.
Scheme 2 Scope of ketones. Reaction conditions: aniline 1r (0.5 mmol),
ketones 2 (0.5 mmol), paraformaldehyde (2.5 mmol), Cp*Co(CO)I2 (5 mol%),
AgNTf2 (10 mol%), HOTf (100 mol%), MeOH (2 mL), 120 °C, 8 h, reaction
kettle under air. Yields refer to isolated yields.
To further gain insight into the reaction mechanism, a set of
additional experiments were conducted (Scheme 3). First, the
competition experiment using equimolar amounts of 1m and 1n
under the standard reaction conditions showed that the electron-rich
Scheme 3 Mechanistic studies.
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Theoclitou and L. A. Robinson, Tetrahedron Lett., 2002, 43, 3907; (d) J. J.
On the basis of these experimental observations and literature
precedents,11-15 a plausible mechanism is proposed in Scheme 4.
First, Cp*Co(CO)I2 is activated by AgNTf2 in the reaction, which
coordinated with 1a, followed by EAS pathway and then ortho-
metalation to afford the intermediate A. Regioselective migratory
insertion of the enol form of 2a into A gives the intermediate B. β-H
elimination provides the alkenyl intermediate C. Subsequently, the
reaction of intermediate C with carbon monoxide (in situ generated
from paraformaldehyde) leads to D, which goes through an
intramolecular cyclization to give a Co(III) alkoxide species E.
Protonolysis of the Co–O bond regenerates the active Co(III)
catalyst and produces a tertiary alcohol F. Finally, the intermediate F
undergoes an intramolecular dehydration to deliver the desired
product 3a.
Eisch and T. Dluzniewski, J. Org. Chem., 1989, 54, 1269.View Article Online
DOI: 10.1039/C7OB02310C
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In summary, we have developed the first Co(III)-catalyzed C–H
activation/carbonylation/cyclization of simple anilines and ketones
with paraformaldehyde for one-pot synthesis of diverse quinolines
with excellent substrate/functional group tolerance and in good to
excellent isolated yields, which only released H2O and H2 as by-
products. Exclusive site- or/and region-selectivity was also observed
when meta-substituted anilines and unsymmetrical ketones were
employed as viable substrates. Taken together, we believe that the
present protocol should have the potential for broad synthetic utility,
especially for the atom-/step-economical and environmentally
benign construction of biologically important quinoline framework.
We thank the Science Foundation of Nanyang Normal University
(QN2016018), and NSFC (21502100 and 81502909) for financial
support of this study.
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