Efficient synthesis of quinoxalines from 2-nitroanilines and vicinal diols via a rutheniumcatalyzed hydrogen transfer strategy

Article abstract of DOI:10.1039/c4gc01316f

Via a ruthenium-catalyzed hydrogen transfer strategy, we have demonstrated a one-pot method for efficient synthesis of quinoxalines from 2-nitroanilines and biomass-derived vicinal diols for the first time. In such a synthetic protocol, the diols and the nitro group serve as the hydrogen suppliers and acceptors, respectively. Hence, there is no need for the use of external reducing agents. Moreover, it has the advantages of operational simplicity, broad substrate scope and the use of renewable reactants, offering an important basis for accessing various quinoxaline derivatives.

Full text of DOI:10.1039/c4gc01316f

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Efficient synthesis of quinoxalines from
2-nitroanilines and vicinal diols via a ruthenium-
catalyzed hydrogen transfer strategy†
Cite this: DOI: 10.1039/c4gc01316f
Feng Xie,a,b Min Zhang,*a Huanfeng Jiang,a Mengmeng Chen,a,b Wan Lv,a
Aibin Zhenga and Xiujuan Jiana
Via a ruthenium-catalyzed hydrogen transfer strategy, we have demonstrated a one-pot method for
efficient synthesis of quinoxalines from 2-nitroanilines and biomass-derived vicinal diols for the first time.
In such a synthetic protocol, the diols and the nitro group serve as the hydrogen suppliers and acceptors,
respectively. Hence, there is no need for the use of external reducing agents. Moreover, it has the advantages
of operational simplicity, broad substrate scope and the use of renewable reactants, offering an important
basis for accessing various quinoxaline derivatives.
Received 12th July 2014,
Accepted 15th August 2014
DOI: 10.1039/c4gc01316f
www.rsc.org/greenchem
semiconductors,15 chemically controllable switches,16 dehydro-
Introduction
annulenes,17 etc. Due to the interesting functions, the
Concerning the gradual depletion of fossil resources, the utiliz- development of efficient methods for accessing quinoxalines
ation of renewable reactants for chemical production has has long been a subject of synthetic chemists. Conventionally,
become a key issue in sustainable chemistry. As a result, the quinoxalines could be prepared via a double condensation of
biomass-derived alcohols have emerged as desirable candi- 1,2-phenylenediamines with 1,2-diketones (Scheme 1, method
dates for various synthetic purposes: (1) condensation reac- A).18 Other elegant contributions mainly involve the oxidative
tions with the use of in situ formed carbonyl intermediates via trapping of vicinal diols or α-hydroxy ketones with 1,2-di-
transition-metal catalysed alcohol dehydrogenation;1 (2) amines (method B),19,20 1,4-addition of 1,2-diamines to
reduction reactions using alcohols as alternative hydrogen diazenylbutenes (method C),21 the coupling of epoxides with
sources for reducing a number of organic chemicals;2 (3) in ene-1,2-diamines (method D),22 2-nitroanilines with phenethyl-
line with the principles of green chemistry, the application of amines (method E),23 alkynes or ketones with 1,2-diamines via
alcohols as both hydrogen suppliers and coupling components a key oxidation process (method F),24 and the sequential
for benign construction of carbon–carbon and carbon–hetero- reductive coupling and cyclization of polymer-linked 2-nitro-
atom bonds is, in synthetic chemistry, of particular
importance.3–5 In these processes, the screening of a suitable
catalyst system that is compatible with both dehydrogenation
of alcohols and hydrogen transfer processes is considered as
the main challenging point.
Quinoxaline derivatives constitute an important class of
N-containing heterocycles that exhibit diverse biological activities
such as antitumor,6 antiviral,7 antibacterial,8 anti-inflamma-
tory,9 anti-HIV,10 and anticancer activities.11 Moreover, quin-
oxalines have been widely applied as building blocks for the
preparation of dyes,12 cavitands,13 luminescent materials,14
aSchool of Chemistry & Chemical Engineering, South China University of Technology,
Wushan Rd-381, Guangzhou 510641, People’s Republic of China.
E-mail: minzhang@scut.edu.cn; Fax: (+86)-020-39925999; Tel: (+86)-13424037838
bSchool of Chemical & Material Engineering, Jiangnan University, Wuxi 214122,
People’s Republic of China
†Electronic supplementary information (ESI) available: Images of 1H and 13C
NMR of all products. See DOI: 10.1039/c4gc01316f
Scheme 1 Various methods for accessing quinoxalines.
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phenyl carbamate with α-bromoketones (method G).25 Never-
theless, many of these methods require the addition of exces-
sive additives, the use of special pre-functionalized or less
environmentally benign halogenated reagents, which could
constantly result in preparation difficulties and/or have a detri-
mental influence on the environment. In 2012, the Corma and
Iborra group demonstrated an interesting synthesis of quin-
oxalines from glycols with 1,2-phenylenediamines or 1,2-di-
nitrobenzenes by employing heterogeneous catalysis.26 However,
the use of 1,2-dinitrobenzenes need to undergo the nitro
group reduction using an external high-pressure hydrogen
source and the oxidative cyclization processes (method H).
From the viewpoint of step-economy concern, the development
of environmentally friendly shortcuts for the synthesis of
quinoxalines from renewable reactants would be of important
significance.
Herein, via a ruthenium-catalyzed hydrogen transfer strat-
egy, we report a straightforward method for efficient synthesis
of quinoxalines from biomass-derived glycols27 and stable
2-nitroanilines for the first time. In such a synthetic protocol,
the vicinal diols and the nitro group of 2-nitroanilines serve as
the hydrogen donors and hydrogen acceptor, respectively.
Hence, there is no need for the use of external reducing agents
(Scheme 1, method I).
Table 1 Optimization of reaction conditionsa
Entry
Catalyst
Ligand
Base
3a, Yieldb %
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
Cat 4
Cat 4
L1
L1
L1
L1
L1
L1
L1
L1
—
L2
L3
L4
L5
L6
L7
L4
L4
L4
L4
L4
L4
L4
L4
L4
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
t-BuOK
K2CO3
12
16
25
62
45
31
14
—
8
23
57
68
61
16
<10
64
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Cs2CO3
CsOH·H2O
KOH
75
79
63
22
NEt3
CsOH·H2O
CsOH·H2O
CsOH·H2O
CsOH·H2O
[65, 45, 72]c
62d
83e, 83f
87g
a Reaction conditions: all reactions were carried out under a nitrogen
atmosphere by using 1a (0.5 mmol), 2a (3 equiv.), catalyst (1 mol%),
ligand (3 mol%), solvent (1.5 mL), temperature (150 °C), base (20 mol%),
reaction time (8 h). b GC yield using hexadecane as an internal
standard. c Yields are with respect to toluene, DMSO and diglyme used
as the reaction solvents, respectively. d Reaction temperature (140 °C).
e Base (50 mol%). f Base (70 mol%). g 2a: (4 equiv.), base (50 mol%).
Results and discussion
We initiated our investigations by choosing the synthesis of
2,3-dimethylquinoxaline 3a from 2-nitroaniline 1a and butane-
2,3-diol 2a as a model reaction to determine different reaction
parameters. First, 7 ruthenium catalysts (Scheme 2, see Cat 1–
Cat 7) were tested by performing the reaction at 150 °C for 8 h
using t-BuOK as the base and t-amyl alcohol as the solvent
(Table 1, entries 1–7). Ru3(CO)12 (Cat 4) exhibited the highest
activity in the formation of product 3a. The absence of ruthe-
nium catalyst failed to give any desired product (Table 1, entry
8). Further investigation showed that the ligand played an
important role in the reaction yield (Table 1, entry 9). By using
Cat 4, the diphosphine L4 was proven to be the most effective
one among various phosphine ligands tested (Scheme 2:
L1–L7 and Table 1: entries 10–15). Subsequently, several in-
organic and organic bases were examined, CsOH·H2O was the
best choice (Table 1, entries 16–20). Other polar and less-polar
solvents were proven to be inferior to t-amyl alcohol (Table 1,
entry 21). Finally, we employed the combination of Cat 4, L4,
CsOH·H2O and t-amyl alcohol, a decrease of reaction tempera-
ture led to a decreased product yield (Table 1, entry 22). And
50 mol% of the base was sufficient to obtain a desirable yield
(Table 1, entry 23). An increase in the diol amount to 2 mmol
resulted in the best product yield (Table 1, entry 24). Hence,
the optimal reaction conditions can be as indicated in entry 24
of Table 1 (Table 1, entry 24).
With the availability of optimized reaction conditions, we
then examined the generality of the synthetic protocol. First,
we focused on the synthesis of quinoxaline 3 by testing a
Scheme 2 Catalysts and ligands employed for the optimization of reac-
tion conditions.
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variety of 2-nitroanilines 1 with symmetrical vicinal diols 2. As
shown in Table 2, both alkyl (i.e. 2a, 2b) and aryl (i.e. 2c) sub-
stituted vicinal diols underwent smooth cyclization to afford
the 2,3-dialkyl and 2,3-diaryl quinoxalines in moderate to
excellent yields upon isolation (Table 2, entries 1–16). The
ortho-substituent of 2-nitroaniline 1d has little influence in
affording the desired product 3d (Table 2, entry 4). Cyclo-
hexane-1,2-diol 2b resulted in the tricyclic products efficiently
(entries 8–12), these examples demonstrate the potential of the
methodology for further construction of polycyclic products.
Interestingly, ethylene glycol can also be applied for the prepa-
ration of 2,3-non-substituted products in reasonable yields
(entries 17–19). Among all the examples examined, it was
found that the electronic properties of the substituents on the
aryl ring of substrate 1 influenced the product yields signifi-
cantly. Specially, the electron-donating groups (i.e., –Me,
–OMe) containing 2-nitroanilines (Table 2, entries 2–4, 9, 10,
14, 17 and 18) afforded the products in higher yield than the
electron-deficient ones (i.e., –Cl, –F) (Table 2, entries 5, 6, 11,
15 and 19). This phenomenon can be rationalized as the elec-
tron-donating groups could enhance the nucleophilicity of the
anilines 1, thus favoring the imination step of the annulation
process. It is noteworthy that owing to the aryl groups are
ortho to the nitrogen atom of the quinoxalines, the 2,3-diaryl
quinoxalines could be applied as the C^N ligands for
the preparation of organometallic complexes or materials28
(Table 2, entries 13–16).
Table 2 Ruthenium-catalyzed synthesis of quinoxaline from nitro-
aniline and symmetrical vicinal diolsa
Entry
1
1
2
Product 3
Yieldb %
1a
2a
3a, 82
2
3
4
1b
1c
1d
2a
2a
2a
3b, 86
3c, 71
3d, 80
Subsequently, we turned our attention to employ unsymme-
trical vicinal diols with our synthetic protocol. Representative
substrates such as 1-phenylethane-1,2-diol 2e and propane-1,2-
diol 2f in combination with various 2-nitroanilines were
tested. All the reactions underwent efficient cyclization to
afford the desired products in moderate to good isolated yields
(Table 3, entries 1–9). Similar to the results described in
Table 2, the electron-rich 2-nitroanilines could give the pro-
ducts in relatively higher yields (Table 3, entries 2, 4 and 7)
than the electron-poor ones (Table 3, entries 3 and 5). Based
on 1H-NMR analysis, the reactions of 4-methoxy-2-nitrobenzen-
amine 1b and 4-chloro-2-nitrobenzenamine 1e with 2e gave
two regioisomers in ratios of 43 : 57 and 52 : 48, respectively
(Table 3, entries 4 and 5). Interestingly, glycerol 2g could also
be transformed in combination with 2-nitroanilines into the
2-methyl quinoxalines in reasonable yields (Table 3, entries 8
and 9), indicating that glycerol can be utilized as an alternative
of propane-1,2-diol 2f.
Upon the GC and GC-MS analyses, it was found that the
vicinal diols undergo partial decomposition to form aldehydes
under the standard reaction conditions. It is noteworthy that
the aldehydes can be easily trapped by 1,2-phenylenediamine
to form benzimidazoles.24 Interestingly, in all of our tested
examples (Tables 2 and 3), we did not observe any benzimid-
azole by-products. Further, the reaction of 1a and 2a was inter-
rupted after 3 h to analyze the reaction intermediates. We
detected only the product 3a in 42% yield without observation
of any 1,2-phenylenediamine (Scheme 3, eqn (1)). Moreover,
the reaction of 4-chlorobenzene-1,2-diamine 1e′ with 2e gave
5
1e
1f
2a
2a
2a
2b
2b
2b
2b
2b
3e, 68
3f, 61
3g, 45c
3h, 79
3i, 83
3j, 84
3k, 56
3l, 43c
6
7
1g
1a
1b
1c
1e
1g
8
9
10
11
12
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Table 2 (Contd.)
Table 3 Synthesis of quinoxaline using unsymmetrical vicinal diolsa
Entry
1
1
2
Product 3
Yieldb (%)
1a
2e
3t, 78
Entry
13
1
2
Product 3
Yieldb %
1a
2c
3m, 84
2
3
4
1c
1h
1b
2e
2e
2e
3u, 70
3v, 40c
14
1b
2c
3n, 87
(3w : 3w′ = 43 : 57), 75
15
16
17
1e
1g
1b
2c
2c
2d
3o, 70
3p, 63c
3q, 65
5
6
7
8
9
1e
1a
1c
1a
1c
2e
2f
(3x : 3x′ = 52 : 48), 61
3y, 74
2f
3z, 69
18
19
1c
1e
2d
2d
3r, 68
3s, 58
2g
2g
3y, 36c
3z, 38c
a Reaction conditions: all reactions were carried out under a nitrogen
atmosphere by using 1a (0.5 mmol), 2a (4 equiv.), catalyst (1 mol%),
ligand (3 mol%), solvent (1.5 mL), temperature (150 °C), base (50 mol%),
reaction time (8 h). b Isolated yield. c Reaction time (12 h).
a Reaction conditions: all reactions were carried out under a nitrogen
atmosphere by using 1a (0.5 mmol), 2a (4 equiv.), catalyst (1 mol%),
ligand (3 mol%), solvent (1.5 mL), temperature (150 °C), base
(50 mol%), reaction time (8 h). b Isolated yield. c Reaction time (12 h).
1
products 3x and 3x′ in a ratio of 30 : 70 upon H-NMR analysis
(Scheme 3, eqn (2)), which is inconsistent with the result of
the reaction of 1e with 2e (Table 3, entry 5). These results
suggest that the reactions involving 1,2-phenylenediamine
intermediates are less likely, and the imination of the amino
group of 2-nitroanilines should occur prior to the reduction of
the nitro group.
On the basis of the above-described results as well as the
related processes,19,20,24
a possible reaction pathway is
depicted in Scheme 4, which comprises the following tandem
sequences: (1) the reaction initiates with the dehydrogenation
of vicinal diol 2 via cooperative actions of the ruthenium cata-
lyst and base;3 (2) then, the imination of 2-nitroaniline 1 gave
α-hydroxy imine A1 or A2; (3) the transfer hydrogenation of the Scheme 3 Verification experiments.
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a commercially available catalyst system [Ru3(CO)12/dppp/
CsOH·H2O], different 2-nitroanilines were efficiently converted
in combination with a variety of biomass-derived glycols into
various substituted products in moderate to excellent isolated
yields. The synthetic protocol has the advantages that there is no
need for the use of external reducing agents, and has operational
simplicity, broad substrate scope and the utilization of renew-
able reactants, offering an important basis for accessing
quinoxaline derivatives.
Scheme 4 Possible pathway for the formation of quinoxalines.
nitro group and tautomerization result in intermediate A3 or
A4; (4) finally, the intramolecular condensation of A3 or A4
and dehydrogenative aromatization would afford desired
product 3 or 3′ (Scheme 4).
Acknowledgements
The authors are grateful to the funds of the “National Natural
Science Foundation of China (21472052 and 21101080)”, “Fun-
damental Research Funds for the Central Universities of China
(2014ZZ0047)”, and “Distinguished talent program of SCUT”.
Experimental
General information
Notes and references
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 compounds were
found to be identical with the ones reported in the literature.
Melting points were measured on an Electrothermal
SGW-X4 microscope digital melting point apparatus and are
uncorrected; IR spectra were recorded on a FTLA2000 spectro-
meter; 1H-NMR and 13C-NMR spectra were obtained on
Bruker-400; mass spectra were recorded on Trace DSQ GC/MS.
Chemical shifts were reported in parts per million (ppm, δ)
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
effected at 254 nm; All the reagents were purchased from com-
mercial sources (J&KChemic, TCI, Fluka, Acros, SCRC), and
used without further purification.
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was then purged. Under nitrogen atmosphere 2, 3-butanediol
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