Palladium Nanoparticles Stabilized by Metal–Carbon Covalent Bonds as an Expeditious Catalyst for the Oxidative Dehydrogenation of Nitrogen Heterocycles

Article abstract of DOI:10.1002/cctc.201700370

The first method for the dehydrogenation of nitrogen heterocycles catalyzed by a palladium nanocatalyst was developed. Carbon–metal covalent-bond-stabilized nanoparticles were found to be efficient for the dehydrogenation process in the presence of tert-butyl hydroperoxide. A variety of N-heterocycles were transformed into functionalized quinolines in medium to excellent yields in water as the solvent under mild conditions by a simple operation.

Full text of DOI:10.1002/cctc.201700370

Accepted Article
Title: Metal-Carbon Covalent Bonds Stabilized Palladium Nanoparticles
as Expeditious Heterogeneous Catalyst for Oxidative
Dehydrogenation of N-Heterocycles
Authors: Xiao-Tao Sun, Jie Zhu, Yun-Tao Xia, and Lei Wu
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To be cited as: ChemCatChem 10.1002/cctc.201700370
Link to VoR: http://dx.doi.org/10.1002/cctc.201700370
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10.1002/cctc.201700370
ChemCatChem
COMMUNICATION
Metal-Carbon Covalent Bonds Stabilized Palladium Nanoparticles
as Expeditious Heterogeneous Catalyst for Oxidative
Dehydrogenation of N-Heterocycles
Xiao-Tao Sun,+[a] Jie Zhu,+[a] Yun-Tao Xia, Lei Wu*[a,b]
Dedication ((optional))
[9]
Abstract: We reveal here the first dehydrogenation of nitrogen
heterocycles catalyzed by a palladium nanocatalyst. The carbon-
metal covalent bonds-stabilized nanoparticles were proved to be
efficient for the dehydrogenation process in the existence of TBHP.
A variety of N-heterocycles were transformed into functionalized
quinolines with medium to excellent yields in water solvent under
mild conditions with simple operation.
ruthenium
were further developed in addition to some other
iridium catalysts [10] for dehydrogenation systems. To be noted,
the first Cu-catalyzed dehydrogenation system was studied very
recently, consisting of homogeneous CuI, di-tert-butyl
azodicarboxylate (DBAD) and molecular oxygen (Scheme 1b).[11]
Despite of the great progress, most of these metal catalyzed
systems, require harsh conditions such as high temperature,
toxic organic solvents and show limited functional-group
tolerance. Considering the importance of quinoline and its
derivatives as well as the current deficiency, it is still urgent and
challenging to develop innovative catalytic de/hydragenation
transformations under mild and clean conditions with high
efficiency.
The quinoline ring is one of the most ubiquitous heterocycles
found in numerous natural products, forming the scaffold for
compounds that exhibit diverse biological and therapeutic
activities.[1]
Its
hydrogenation
product,
1,2,3,4-
tetrahydroquinoline, is also a core structural motif in alkaloids
and other bioactive molecules, displaying wide applications in
agricultural, pharmaceutical and fine chemistry.[2] Given the
importance of these nitrogen-containing heterocycles, the last
few years have witnessed the development of various methods
to produce compounds containing quinoline as well as 1,2,3,4-
tetrahydroquinoline skeletons. Catalytic dehydrogenation (CDH)
and hydrogenation reactions, most often catalyzed by metal
complexes, represent an atom-efficient way for the conversion
between these N-heterocycles with only H2 generated/used.[3]
Sometimes, CDH process is even more challenging and oxidant
may be necessary as acceptor for H2 to promote the equilibrium
in CDH reaction, as the dehydrogenation of N-heterocycles was
proved to be a thermodynamically uphill process at ambient
conditions.[4] Despite of the various studies on hydrogenations in
recent years, CDH process, especially for N-heterocycles, has
been less well established as harch condiitons are usually
required.
A pioneering work was reported by Fujita and Yamaguchi,
in which a homogeneous catalytic system involving [Cp*Ir]
catalyst was developed for dehydrogenation-hydrogenation
reactions of N-heterocycles in refluxing p-xylene (Scheme 1a).[5]
Though the reaction condition is not ideal, catalyst capable of
Scheme 1. Representative studies on metal catalyzed
dehydrogenation process.
both hydrogenation and dehydrogenation is rather rare. After
During the past decade, a growing interest has been
attracted on the development of palladium-based organometallic
catalysts for organic synthesis, with de/hydration transformations
also involved. For representative examples, Stahl and his
coworkers reported a Pd-catalyzed method for dehydrogenation
of cyclohexanones to phenols (Scheme 1c),[12] after which
analogous approaches were developed to access other
compounds including cyclohexenes,[13] aryl ethers,[14], esters and
nitriles,[15] aniline derivatives and so on.[16] But for N-heterocycles
and their derivatives, despite of the achievement gained in
hydrogenation process, examples are still scarce for CDH
transformations.[17] Recently, we prepared a palladium nano-
catalyst stabilized by carbon–metal covalent bonds (Pd=NPs),
with which hydrogenation of quinoline was enabled in water
under mild condition.[18] The nano-catalytic heterogeneous
[8]
that, metal complexes including iron,[6] cobalt,[4a,7] nickel
and
[a]
X.-T. Sun, J. Zhu, Y.-T. Xia, Prof. Dr. L. Wu Author(s)
Jiangsu Key Laboratory of Pesticide Science and Department of
Chemistry
College of Sciences, Nanjing Agricultural University
Nanjing 210095 (P. R. China)
E-mail: rickywu@njau.edu.cn
[b]
+
Prof. Dr. L. Wu
Beijing National Laboratory for Molecular Sciences
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P. R. China)
These authors contributed equally to this work.
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/cctc.201700370
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system was highlighted featuring in the high efficiency as well as
catalyst recycling.
18
19
Pd=NPs
Pd=NPs
TBHP/8
TBHP/8
THF
32
73
DCM
Inspired by the leading results, it was thought to be amazing
if the reverse reaction could be achieved with the same
nanocatalyst to obtain quinoline from dehydrogenation of
tetrahydroquinoline. Hence, as our continuous efforts to discover
challenging de/hydrogenation transformations,[19] and to pursue
clean, mild and efficient catalytic strategies for sustainable
chemistry, herein, we report a highly efficient dehydrogenation
process with Pd nanoparticles as catalyst and tert-butyl
hydroperoxide (TBHP) as oxidant for tetrahydroquinoline and its
derivatives in water (Scheme 1d). To the best of our knowledge,
this is the first example of palladium catalyzed dehydrogenation
of N- heterocycles at room temperature in water solvent.
In our initial evaluation of oxidative dehydrogenation
conditions, 2-methyl-1,2,3,4-tetrahydroquinoline was used as
model substrate with 4 mmol % Pd=NPs as catalyst and 2
equivalents of TBHP as oxidant (see Table S1 in the Supporting
Information for full screening data). It was delight to discover that
2-methyl-quinoline was generated in 31% yield in water at room
temperature. Excessive TBHP could further increase the result
and over 95% yield was achieved with eight equivalents TBHP
(Table 1, entries 1-3). TBHP was supposed to play an important
role serving as the acceptor of H2, so as to promote the
equilibrium for dehydrogenation. It is noteworthy that Pd=NPs
outperformed some other metal precursors such as Iridium
nano-catalyst,[20] palladium-based catalysts,[21] or copper
catalysts[22] (entries 4-12), suggesting the high efficiency of the
nano-catalyst. Afterwards, several oxidants of both organic and
inorganic were screened, with TBHP still exhibiting the best
performance (entries 13-15). In addition to water, organic
[a] Reaction conditions: 2-methyltetrahydroquinolines (0.3 mmol), H2O (2 mL),
Pd NP (15 mg, 0.0125 mmol, 4% of substrate), TBHP (8 equiv), r.t., 18 h;
[b] Yields were determined by 1H-NMR.
After the promising initial results, subsequent efforts were
devoted to examine the applicability of this protocol. Under the
optimized conditions, substrates were extended to various
structurally differently substituted tetrahydroquinolines with 8
equivalents of TBHP in water. As shown in Scheme 2,
tetrahydroquinloines bearing electron-donating substituents
(OMe, NH2, OH) on both pyridine ring and benzene ring
performed well in the system giving corresponding quinolines in
good to excellent yields (2a-2m). The outcome showed that the
dehydrogenation process was not significantly affected by
varying the position of the same substituent, e.g. a methyl group
on 2, 3, 4- position of the pyridine moiety or 6-, 7-, 8- position of
the benzene moiety. Electron-deficient groups, including
carboxylate-, bromo-, and chloro- substitutions, retarded the
dehydrogenation process dramatically, but still exhibited great
selectivity with these groups remained intact without hydrolysis
or cleavage (2n-2p). Only 39% yield of 8-chlorotetrahydro-
quinolines was observed even improving to temperature to 80oC
(2p). It was supposed that electron-deficient groups impair the
electron density of aromatic rings, leading to a more difficult
coordination of substrates with Pd NP catalyst. These
observations highlight the advantages of this palladium nano-
catalyst over Pd/C for dehydrogenations reactions, as the latter
can promote undesired reactivity with aryl halides, and the −Cl
and −Br substituents are appealing functional groups for
subsequent derivatization.[7b] 3-phenyl substituted tetraquinoline
also performed well in this system, giving the corresponding
quinoline in 76% yield.
solvents,
including
CH3OH,
toluene,
1,4-dioxane,
tetrahydrofuran, and dichloromethane were also tested and
though enabled the dehydrogenation, lower yields were obtained
ranging from 13 to 73% (entries 16-19). Eventually, the
observations indicated that water was the best solvent with
Pd=NPs as catalyst and TBHP as oxidant at room temperature,
giving the product in yield of over 95%.
Scheme 2. Scope of the dehydrogenation of quinoline
derivatives [a]
Table 1 Optimization of reaction conditions.[a]
Oxidant
/equiv
yield
(%)[b]
31
entry Catalyst
Solvent
1
2
3
4
5
6
7
Pd=NPs
Pd=NPs
Pd=NPs
Ir=NPs
TBHP/2
H2O
H2O
H2O
H2O
H2O
H2O
H2O
TBHP/5
TBHP/8
TBHP/8
TBHP/8
TBHP/8
TBHP/8
77
>95
trace
47
Pd/C
Pd(OAc)2
6
Pd(OAc)2/4,5-
diazafluoren-9-one
Pd(PPh3)2Cl2
13
8
TBHP/8
TBHP/8
TBHP/8
TBHP/8
TBHP/8
Ag2O/8
DTBP/8
H2O2/8
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
17
9
Pd(PPh3)4
trace
65
10
11
12
13
14
15
16
17
CuI/bpy/TEMPO
CuBr/bpy/TEMPO
CuCl/bpy/TEMPO
Pd=NPs
91
78
74
Pd=NPs
trace
59
Pd=NPs
Pd=NPs
TBHP/8
TBHP/8
CH3OH 47
toluene 13
Pd=NPs
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10.1002/cctc.201700370
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[a] Reaction conditions: tetrahydroquinolines (0.3 mmol), H2O (2 mL), Pd NP
(15 mg, 0.0125 mmol, 4% of substrate), TBHP (8 eq), r.t., 18 h;
[b] Isolated yields.
isomerization among 3a, 4a, 5a was available due to the small
energy gap and the dehydrogenation process occurred again at
2-position to give the target product. This also explains why
excessive amount of TBHP is necessary.
[c] The reaction was carried out at 80oC.
Encouraged by these results, the substrates scope with
regard to other N-heterocycles was further evaluated as listed in
Scheme
3.
Substituted
quinoxaline,
acridine,
1,10-
phenanthroline, benzoquinoline could be obtained in this
dehydrogenation process from their corresponding heterocycles.
Tetrabenzoquinoline gave excellent yields to dehydrogenation
product (2w). Tetrahydroacridine as well as tetrahydro-1,10-
phenanthroline enabled the reaction with comparable results to
tetrahydroquinoline of good to excellent yields (2x-2z). As for
Tetrahydroquinoxalines, the yields were relatively impaired and
high temperature was needed in some cases to achieve the
process (2r-2v). Only 49% yield was obtained for 2-phenyl
quinoxaline derivative even at 80oC, probably due to the steric
effect.
Scheme 3. Pd-NPs-catalyzed dehydrogenation of nitrogen
Scheme 4. A plausible route for dehydrogenation process.
heterocycles [a]
Eventually, the reusability of this nanocatalyst was
examined with 2-methylquinoline as model substrate. After the
termination of dehydrogenation process, the product or any
substrate remaining in the mixture was extracted with ether
before analysis by 1H NMR. The nano-catalyst was then
recovered by extraction with dichloromethane due to the
hydrophobicity of Pd=NPs, and reused without further treatment.
By the addition a new batch of 2-methylquinoline and TBHP, the
recovered nanocatalyst can be used for at least 5 times without
obvious loss of reactivity or selectivity.
In summary, a simple and expeditious dehydrogenation
process of tetraquinolines as well as derivative N-heterocycles
was disclosed here using a metal-carbon stabilized palladium
nanoparticles as catalyst. The reaction was achieved in mild
conditions in water solvent at room temperature. Great substrate
tolarance was exhibited including a broad range of quionlines,
quinoxaline, acridine, 1,10-phenanthroline, benzoquinoline. In
addition to its high efficiency, the Pd=NPs could be easily
recovered and reused owning to the heterogeneous nature. It is
believed that this protocol, together with the reverse
hydrogenation process, may have fundamental implications Pd
catalytic chemistry and potential for industrial purpose.
[a] Reaction conditions: tetrahydroquinolines (0.3 mmol), H2O (2 mL), Pd NP
(15 mg, 0.0125 mmol, 4% of substrate), TBHP (8 eq), r.t., 18 h;
[b] Isolated yield.
[c] The reaction was carried out at 80oC.
Elaboration of the detailed mechanistic pathway will
undoubtedly allow further optimization of this dehydrogenation
system. In the present case, a detailed mechanism for the
overall catalytic process cannot be clealy elucidated yet, but a
plausible route was proposed as outlined in Scheme 4. The
computational results showed that the energy gap between
tetraquinoline (1a) and 1,2-dihydroquinoline (3a) (120.8 kJ/mol)
is relatively lower compared with that of 2,3-dihydroquinoline
(4a) (121.8 kJ/mol) or 3,4-dihydroquinoline (5a) (128.9 kJ/mol). It
was deduced that the dehydrogenation started from the
coordination of Pd=NPs to nitrogen, followed by beta-hydride
elimination with TBHP as accpetor of hydrogen. The result
coinsides with the concept that nitrogen atoms strongly promote
the dehydrogenation with H2 evolution.[4,23] Adsorption is
considered to be a crucial and initial step for nano-catalysis and
the dehydrogenation was supposed to experience the
adsorption of substrate onto the surface of Pd=NPs, as almost
no reaction happened with Pd(OAc)2 catalyts. Afterwards, the
Experimental Section
Preparation of the metal-carbon bond stabilized palladium nanocatalyst
was prepared and characterized according to our previous studies.[18] For
the dehydrogenation process, to a 10 mL vial was added N-heterocycles
(0.3 mmol), water (1 mL), TBHP (8 eq, 303 mg) and Pd=NP (4% of the
substrate, 15 mg, 0.0125 mmol). The system was sealed and kept at
room temperature for 18 h. After being finished, the product or any
substrate left was extracted with ethyl acetate (1 mL×3) before further
purification on flash chromatography or analysis by 1H-NMR.
Acknowledgements ((optional))
We are grateful for financial support from the National Natural
Science Foundation of China (NSFC, Grant No. 21372118,
21602108), the Foundation Research Project of Jiangsu
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Xiao-Tao Sun,+ Jie Zhu,+ Yun-Tao Xia,
Lei Wu*
The first nano-Pd catalyzed
dehydrogenation of N-heterocycles
was reported, with mild reaction
conditions, good substrate tolerance
and efficient catalyst recycling.
Page No. – Page No.
Metal-Carbon Covalent Bonds
Stabilized Palladium Nanoparticles as
Expeditious Heterogeneous Catalyst
for Oxidative Dehydrogenation of N-
Heterocycles
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