Convenient synthesis of quinolines from α-2-nitroaryl alcohols and alcohols via a ruthenium-catalyzed hydrogen transfer strategy

Article abstract of DOI:10.1002/cctc.201402832

A new and straightforward method for convenient synthesis of quinolines via a ruthenium-catalyzed hydrogen-transfer strategy has been demonstrated. By employing a commercially available ruthenium catalyst system, different α-2-nitroaryl alcohols were efficiently converted in combination with a variety of alcohols into various substituted products in reasonable to good yields upon isolation. The synthetic protocol is operationally simple with a broad substrate scope, and there is no need for the use of specialized reducing agents, making it a practical approach for versatile preparation of various quinoline derivatives.

Full text of DOI:10.1002/cctc.201402832

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DOI: 10.1002/cctc.201402832
Convenient Synthesis of Quinolines from a-2-Nitroaryl
Alcohols and Alcohols via a Ruthenium-catalyzed
Hydrogen Transfer Strategy
Feng Xie,[a, b] Min Zhang,*[a] Mengmeng Chen,[b] Wan Lv,[a] and Huanfeng Jiang[a]
A new and straightforward method for convenient synthesis of
quinolines via a ruthenium-catalyzed hydrogen-transfer strat-
egy has been demonstrated. By employing a commercially
available ruthenium catalyst system, different a-2-nitroaryl al-
cohols were efficiently converted in combination with a variety
of alcohols into various substituted products in reasonable to
good yields upon isolation. The synthetic protocol is opera-
tionally simple with a broad substrate scope, and there is no
need for the use of specialized reducing agents, making it
a practical approach for versatile preparation of various quino-
line derivatives.
Introduction
With the gradual depletion of fossil resources, the utilization of
abundant and sustainable alcohols as the feedstocks for chem-
ical production is highly desirable in sustainable chemistry. On
the one hand, alcohols could serve as alternative reductants
for reducing a number of functional groups.[1] Comparing to
the conventional hydrogenation processes using high-pressure
hydrogen gas, the transfer hydrogenation with alcohols do not
need for special equipment such as autoclaves, offering more
convenient and safer operations. On the other hand, alcohols
have been employed as coupling agents for various synthetic
purposes via an acceptorless dehydrogenative process.[2] More-
over, in line with the principles of green chemistry, the applica-
tion of alcohols as both hydrogen suppliers and coupling com-
ponents for the benign construction of carbon-carbon and
carbon-heteroatom bonds is, in synthetic chemistry, of signifi-
cant importance.[3–5]
cently established inter- and intramolecular cyclization reac-
tions have brought new access.[10] Nevertheless, many of these
methods require the use of special pre-functional or less envi-
ronmentally benign halogenated reagents, which would in-
crease the complexity of the work-up procedure, or result in
detrimental influence on environment. Therefore, the explora-
tion of straightforward methods for convenient synthesis of
quinolines from easily available and eco-compatible reagents
still remains a demanding goal.
Among various methods employed, the Friedlander annula-
tion represents one of the most facile routes to access quino-
line derivatives. However, the use of active functional sub-
strates could constantly result in preparation difficulties, stabili-
ty and chemoselectivity limitations.[11] In recent years, the rela-
tively stable a-2-aminoaryl alcohols have been successfully em-
ployed for dehydrogenative synthesis of quinolines.[2h,12]
Moreover, 2-nitroaryl aldehydes were also coupled with ke-
tones to realize the related synthetic goal.[13] Such a synthesis,
however, needs to undergo successive nitro-group reduction
using a Fe/HCl reduction system and base-promoted cycliza-
tion processes. Herein, via a ruthenium-catalyzed hydrogen
transfer strategy, we wish to report a new and straightforward
method for versatile synthesis of quinoline derivatives from
stable and easily available a-2-nitroaryl alcohols 1 and alcohols
2. In such as a synthetic protocol, two alcohol units and the
nitro-group serve as the hydrogen donors and hydrogen ac-
ceptor, respectively. Hence, there is no need for the use of spe-
cialized reducing agents (Scheme 1).
Quinoline derivatives constitute a highly important class of
nitrogen-containing heterocycles that exhibit diverse biological
activities.[6] Moreover, quinolines have been widely applied as
key building blocks for the preparation of various functional-
ized materials,[7] sensors and dyes.[8] Owing the multiple func-
tions of quinolines, the development of efficient methods for
the synthesis of this type compounds has long been a subject
of synthetic chemists. Except for the classical named reactions
such as Skraup, Friedlander, and Combes syntheses,[9] the re-
[a] F. Xie, Prof. Dr. M. Zhang, W. Lv, Prof. Dr. H. Jiang
School of Chemistry & Chemical Engineering
South China University of Technology
Wushan Rd 381, Guangzhou 510641 (P.R. China)
Fax: (+86)020-39925999
Results and Discussion
E-mail: minzhang@scut.edu.cn
[b] F. Xie, M. Chen
To determine an efficient reaction system, we chose the syn-
thesis of 2-phenylquinoline 3a from 2-nitrophenyl methanol
1a and 1-phenylethanol 2a, as a model reaction to evaluate
different reaction parameters. Upon addition of catalytic
amount of ruthenium catalyst, the reaction in t-amyl alcohol
School of Chemical & Material Engineering
Jiangnan University
Wuxi 214122 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402832.
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Table 1. Screening of optimized reaction conditions for the production
of 3a.[a]
Entry
Catalyst
Ligand
Base
Yield [%][b]
1
2
3
4
5
6
7
8
Cat 1
Cat 2
Cat 3
Cat 4
Cat 5
Cat 6
Cat 7
–
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
Cat 4
L1
L1
L1
L1
L1
L1
L1
L1
L2
L3
L4
L5
L6
L1
L1
L1
L1
L1
L1
L1
L1
L1
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
tBuOK
tBuONa
K3PO4·3H2O
NEt3
31
49
17
68
36
57
21
–
66
38
53
34
46
<10
35
74
Scheme 1.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
65
70
28
tBuOK
tBuOK
tBuOK
63, 45, <10[c]
82, 72[d]
81,[e] 62,[f] 82[g]
Scheme 2. Catalysts and ligands tested for screening of optimized reaction
conditions.
[a] Reaction conditions: Unless otherwise specified, all reactions were per-
formed under nitrogen atmosphere by using 1a (0.5 mmol), 2a
(2.0 mmol), catalyst (For Cat 1, Cat 2, Cat 3, Cat 5, and Cat 7, 3 mol%
was added; For Cat 4, 1 mol% was added; For Cat 6, 1.5 mol% was
added), ligand (3 mol%), t-amyl alcohol (1.5 mL), temperature (1508C),
base (0.1 mmol), reaction time: 15 h. [b] GC Yield using hexadecane as an
internal standard. [c] Yields are with respect to toluene, DMSO and water
as solvent, respectively. [d] Reaction time: 18 h, isolated yield. [e] Reaction
temperature: 1608C. [f] Reaction temperature: 1408C. [g] 2a (2.5 mmol).
was performed at 1508C by using K2CO3 as a base and dppf as
a ligand (Scheme 2, L1). Seven catalysts were initially tested
(Scheme 2, Cat 1–Cat 7), and Ru3(CO)12 (Cat 4) exhibited excel-
lent activity in the formation of product 3a (Table 1, entries 1–
7). However, the reaction in absence of catalyst failed to yield
any desired product (Table 1, entry 8). Then, Ru3(CO)12 in com-
bination with other five phosphine ligands (Scheme 2, L2–L6)
were tested, the results indicate these ligands are inferior to
dppf (Table 1, entries 9–13). Hence, we chose Ru3(CO)12 and
dppf as the preferred catalyst and ligand, respectively. Several
representative bases were further examined (Table 1, en-
tries 14–19), tBuOK was proven to be the best choice in the
formation of product. Next, other polar and less-polar solvent
were evaluated, the results showed that these solvents are less
effective in comparison with t-amyl alcohol (Table 1, entries 20).
Furthermore, prolonging the reaction time from 15 h to 18 h
gave a best product yield by applying Ru3(CO)12/dppf/tBuOK as
the catalyst system (Table 1, entry 21). Finally, the increase or
decrease of reaction temperature could not afford improved
product yields, and 4 equivalents of alcohol 2a are sufficient to
obtain a desirable yield (Table 1, entry 22). Based on these ob-
servations, the optimal reaction conditions can be as indicated
in entry 21 of Table 1.
proceeded smoothly and furnished the desired products in
moderate to good yields upon isolation. It was found that the
substituents having different electron properties on the aryl
ring of substrates 1 and 2 affected the product yields signifi-
cantly. Specially, the electron-rich 4,5-dimethoxy-2-nitrophenyl
methanol 1c afforded the corresponding products in higher
yields (Table 2, 3k–3o) than the relatively electron-poor 5-
chloro-2-nitrophenyl methanol 1b (3g–3j), attributing to the
electron-donating groups could enhance the nucleophilicity of
the amino group arising from the transfer hydrogenation of
the nitro group of 1, thus favoring the imination step of the
annulation process. On the contrary, 1-(4-(trifluoromethyl)phe-
nyl)ethanol 2e gave the desired products in much better
yields (3e, 3I, and 3n) than 1-p-tolylethanol 2b (see 3b, 3h,
and 3l). It is conceivable that the electron-withdrawing group
CF3- could increase the a-reactivity of the ketone derived from
dehydrogenation of 2e, which is in favor of the Aldol conden-
sation step of the annulation process. Owing to the aryl
groups are ortho to the nitrogen atom in the quinoline skele-
ton, the obtained products offer the potentials for further elab-
oration of complex molecules via direct CÀH bond functionali-
zation’s,[14] or can serve as CN ligands for the preparation of
cyclometalated complexes or materials.[15]
With the availability of the optimized reaction conditions in
hand, we then examined the scope of this ruthenium-catalyzed
synthetic protocol. First, we focused on the synthesis of 2-aryl
substituted quinolines. Several representative 2-nitroaryl meth-
anols 1 (1a–1c) were tested in combination with a variety of
a-aryl ethanols 2 (2a–2 f). As shown in Table 2, all the reactions
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Table 2. Synthesis of 2-aryl quinolines from 2-nitroaryl methanols and a-
aryl ethanols.[a]
Table 3. Synthesis of various substituted quinolones from a-2-nitroaryl al-
cohols and alcohols.[a]
[a] Reaction conditions: all reactions were performed under nitrogen at-
mosphere by using 1 (0.5 mmol), 2 (2 mmol), Ru3(CO)12 (1 mol%), dppf
(3 mol%), t-amyl alcohol (1.5 mL), temperature (150 8C), tBuOK
(0.1 mmol), reaction time: 18 h. [b] Isolated yield.
[a] Reaction conditions: all reactions were performed under nitrogen at-
mosphere by using 1 (0.5 mmol), 2 (2 mmol), Ru3(CO)12 (1 mol%), dppf
(3 mol%), t-amyl alcohol (1.5 mL), temperature (150 8C), tBuOK
(0.1 mmol), reaction time: 18 h. [b] Isolated yield. [c] Reaction time: 20 h.
reaction of direct use of 1-(2-aminophenyl)ethanone 1da
(molar ratio of 1da and 2 f is 1:1) yielded product 3u exclu-
sively in 68% yield (Scheme 3C). Moreover, the reaction of 1d
with 4’-chloroacetophenone 2 f’ also gave product 3u and 1-
(2-aminophenyl)ethanone
1da
in
detectable
yields
Next, we turned our attention to synthesize various substi-
tuted quinolines by employing different type of a-2-nitroaryl
alcohols 1 with alcohols 2. As shown in Table 3, all the hydro-
gen transferring cross-coupling reactions underwent efficiently,
and furnished the desired products in moderate to good
yields. The reactions of 1,2,3,4-tetrahydronaphthalen-1-ol 2g
with three representative 2-nitroaryl methanols gave the 2-
aryl-3-alkyl products in good isolated yields (Table 3, see 3p–
3r). These examples demonstrate the potential of the synthetic
protocol for further construction of various polycyclic com-
pounds. Gratifyingly, the more bulky a-2-nitrophenyl ethanol
1d could also be annulated with the various alcohols, afford-
ing the 2-aryl-4-alkyl products in moderate yields (Table 3, 3s–
3v). Similarly, less reactive cyclohexanol 2h as well as primary
alcohols such as n-hexanol 2i and 2-phenylethanol 2j in com-
bination with 1a resulted in the 2,3-dialkyl, 3-alkyl and 3-aryl
products in reasonable yields, respectively (Table 3, 3w–3y).
To gain insight into the quinoline formation process, several
verification experiments were interrupted after 8 h to analyze
the reaction intermediates. The reaction of 1-(2-nitrophenyl)e-
thanol 1d with 1-(4-chlorophenyl)ethanol 2 f gave both de-
sired quinoline 3u and 1-(2-aminophenyl)ethanone 1da in
27% and 46% yields, respectively (Scheme 3A). Also, 1-(2-nitro-
phenyl)ethanone 1 db coupled with 2 f to afford product 3u
and 1da in a ratio of around 1:2 (Scheme 3B). As expected, the
(Scheme 3D). Notably, we did not observe any nitroaryl inter-
mediates in Scheme 3A, B, or D. These results support that
1da, 1 db, and 2 f’ serve as the reaction intermediates in the
Scheme 3. Verification experiments. 1) Ru3(CO)12 (1 mol%), dppf (3 mol%),
tBuOK (20 mol%), temperature (150 8C), reaction time: 8 h. GC Yield using
hexadecane as the internal standard.
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Experimental Section
General information
All the obtained products were characterized by melting
points (m.p), 1H NMR, 13C NMR, infrared spectra (IR), and
mass spectra (MS), the NMR spectra of the known com-
pounds were found to be identical with the ones report-
ed in the literatures. Additionally, all the new compounds
were further characterized by high resolution mass spec-
tra (HRMS). Melting points were measured by using an
Electrothemal SGW-X4 microscopy digital melting point
apparatus and are uncorrected; IR spectra were recorded
1
by using a Brucker Vector 22 spectrometer; H NMR and
13C NMR spectra were obtained by using
a Bruker
Avance 400 MHz NMR spectrometer; Mass spectra were
recorded by using a Shimadzu GC-MS-QP5050 A spec-
trometer at an ionization voltage of 70 eV equipped with
a DB-WAX capillary column (internal diameter: 0.25 mm,
length: 30 m). High-resolution mass spectra (HRMS) were
recorded by using a Shimadazu LCMS-IT-TOF mass spec-
trometer. Chemical shifts were reported in parts per mil-
lion (ppm, d) downfield from tetramethylsilane. Proton
coupling patterns are described as singlet (s), doublet (d), triplet
(t), multiplet (m); TLC was performed using commercially prepared
100–400 mesh silica gel plates (GF254), and visualization was ef-
fected at 254 nm; All the reagents were purchased from commer-
cial sources (J&KChemic, TCI, Fluka, Acros, SCRC), and used without
further purification.
Scheme 4. Possible pathway for the formation of quinoline.
formation of quinoline 3u, and the transfer hydrogenation of
the nitro group can be ruled out as a rate-determining step in
the annulation process.
Based on the above-described results, as well as the Fried-
lander annulation process,[9a] a possible reaction pathway is de-
picted in Scheme 4. Initially, the dissociation of Ruthenium
cluster Ru3(CO)12 and the coordination of the oxygen atoms of
2-nitroaryl alcohol 1 to the metal center form a ruthenacycle
A1. Under ruthenium catalysis conditions, the relatively active
alcohol unit of 1 firstly undergoes a hydrogen transfer to form
a nitrosobenzene complex A2. And the presence of excess of
alcohol 2 would proceed through two successive hydrogen
transfers to give ketone A3, hydroxyamine complex A4 and 2-
aminoketone A5, respectively. Then, the a-Aldol condensation
or carbonyl imination of A5 with A3 gives intermediate A6 or
A6’; Finally, the intramolecular condensation of A6 or A6’ af-
fords the desired quinoline product 3 (Scheme 4).
Typical procedure for synthesis of 2-phenylquinoline 3a
Under nitrogen atmosphere, Ru3(CO)12 (3.2 mg, 0.005 mmol), dppf
(8.3 mg, 0.015 mmol), tBuOK (11 mg, 0.1 mmol), nitrobenzyl alcohol
(1a; 76 mg, 0.5 mmol), 1-phenylethanol (2a; 244 mg, 2.0 mmol)
and tert-amyl alcohol (1.5 mL) were added successively to a Schlenk
tube (50 mL) equipped with a magnetic stirrer bar, the Schlenk
tube was then closed and the resulting reaction mixture was
heated at 1508C for 18 h. After cooling to room temperature, the
solvent in the reaction mixture was removed under vacuum, then
it was directly purified by preparative TLC on silica, eluting with
petroleum ether (60–908C): ethyl acetate (20:1) to give 2-phenyl-
quinoline (3a) as a yellow solid (73.8 mg, 72%).
Typical experimental procedure for Scheme 3A
Under a nitrogen atmosphere, Ru3(CO)12 (3.2 mg, 0.005 mmol),
dppf (8.3 mg, 0.015 mmol), tBuOK (11 mg, 0.1 mmol), a-2-nitro-
phenyl ethanol (1d; 84 mg, 0.5 mmol), 1-(4-chlorophenyl)ethanol
(2 f; 312 mg, 2.0 mmol), and tert-amyl alcohol (1.5 mL) were added
successively to a Schlenk tube (50 mL) equipped with a magnetic
stirrer bar, the Schlenk tube was then closed and the resulting re-
action mixture was heated at 1508C for 8 h. The reaction was al-
lowed to cool to room temperature, ethyl acetate (10 mL) and hex-
adecane(30 mg) were added and fully mixed, the mix was allowed
to stand for a moment and the upper layer was extracted for GC-
MS analysis.
Conclusions
We have demonstrated a new and straightforward method for
convenient synthesis of quinolines via ruthenium-catalyzed hy-
drogen-transfer strategy. By employing a commercially avail-
able ruthenium catalyst system [Ru3(CO)12/dppf/tBuOK], differ-
ent a-2-nitroaryl alcohols were efficiently converted in combi-
nation with a variety of alcohols into various substituted prod-
ucts in reasonable to good isolated yields. The synthetic proto-
col is operationally simple with a broad substrate scope, there
is no need for the use of specialized reducing agents, making
it a practical approach for versatile preparation of various quin-
oline derivatives. Owing to the importance of quinolines in Acknowledgements
biology, organic, and material chemistry, the presented
method has the potential to be employed for various
applications.
The authors are grateful to the funds of the “National Natural
Science Foundation of China (21472052 and 21101080)”, “Funda-
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(2014ZZ0047)”, and “Distinguished talent program of SCUT”.
Keywords: alcohols · annulation · homogeneous catalysis ·
hydrogen transfer · quinolines · ruthenium
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Received: October 18, 2014
Published online on && &&, 0000
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 6
&
5
&
ÞÞ
These are not the final page numbers!

FULL PAPERS
F. Xie, M. Zhang,* M. Chen, W. Lv,
H. Jiang
&& – &&
Convenient Synthesis of Quinolines
from a-2-Nitroaryl Alcohols and
Alcohols via a Ruthenium-catalyzed
Hydrogen Transfer Strategy
From king maker to ring maker: A
new and straightforward method for
convenient synthesis of quinolines via
a ruthenium-catalyzed hydrogen-trans-
fer strategy has been demonstrated.
The synthetic protocol is operationally
simple with a broad substrate scope,
and there is no need for the use of spe-
cialized reducing agents, offering a prac-
tical approach for versatile preparation
of various quinoline derivatives.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 6
&
6
&
ÞÞ
These are not the final page numbers!