Synthesis of (E)-2-Alkenylazaarenes via Dehydrogenative Coupling of (Hetero)aryl-fused 2-Alkylcyclic Amines and Aldehydes with a Cobalt Nanocatalyst

Article abstract of DOI:10.1002/cctc.201800202

To date, the synthesis of (E)-2-alkenylazaarenes via the condensation of 2-methyl N-heteroarenes with aldehydes or their equivalents has been well demonstrated. However, the direct formation of such a class of useful compounds from extensively distributed 2-alkylcyclic amine motifs remains an unresolved goal. Herein, by employing the nitrogen-silica-doped carbon (Vulcan XC-72R) as the support, we have developed a low-loading cobalt nanocatalyst (Co/N-Si-C). The combination of such a catalyst with p-nitrobenzoic acid and molecular O₂ exhibits excellent catalytic performance towards the dehydrogenative coupling of (hetero)aryl-fused 2-alkylcyclic amines with aldehydes to afford the (E)-2-alkenylazaarenes. In the reaction, effective capture of the partially dehydrogenated cyclic amine motifs appears to be the key strategy to address the issue of the chemoselectivity. The developed catalytic transformation proceeds with the merits of broad substrate scope, good functional group tolerance, high atom-efficiency, use of an earth-abundant and reusable cobalt catalyst and molecular O₂ as a green oxidant, which offers an important basis for the direct conversion of inert cyclic amine units into the functional frameworks.

Full text of DOI:10.1002/cctc.201800202

Accepted Article
Title: Synthesis of (E)-2-Alkenylazaarenes via Dehydrogenative
Coupling of (Hetero)aryl-fused 2-Alkylcyclic Amines and
Aldehydes with a Cobalt Nanocatalyst
Authors: Min Zhang, Changjian Zhou, Zhenda Tan, and Huanfeng
Jiang
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Synthesis of (E)-2-Alkenylazaarenes via Dehydrogenative
Coupling of (Hetero)aryl-fused 2-Alkylcyclic Amines and
Aldehydes with a Cobalt Nanocatalyst
Changjian Zhou,[a] Zhenda Tan,[a] Huanfeng Jiang,[a] and Min Zhang*[a]
To date, the synthesis of (E)-2-alkenylazaarenes via the
condensation of 2-methyl N-heteroarenes with aldehydes or its
equivalents has been well demonstrated. However, the direct
formation of such a class of useful compounds from extensively
distributed 2-alkylcyclic amine motifs remains an unresolved goal.
Herein, by employing the nitrogen-silica-doped carbon (Vulcan XC-
72R) as the support, we have developed a low-loading cobalt
nanocatalyst (Co/N–Si–C). The combination of such a catalyst with
p-nitrobenzoic acid and molecular O2 exhibits excellent catalytic
performance towards the dehydrogenative coupling of (hetero)aryl-
fused 2-alkylcyclic amines with aldehydes to produce the (E)-2-
alkenylazaarenes. In the reaction, effective capture of the partially
dehydrogenated cyclic amine motifs appears to be a key strategy
to address the issue of chemoselectivity. The developed catalytic
transformation proceeds with the merits of broad substrate scope,
good functional group tolerance, high atom-efficiency, use of an
earth-abundant and reusable cobalt catalyst and molecular O2 as a
green oxidant, which offers an important basis for the direct
conversion of inert cyclic amine units into the functional
frameworks.
Introduction
Azaarenes are distributed in numerous bioactive molecules,
functional materials, dyes, agrochemicals, pharmaceuticals,
and natural products. Meanwhile, the cyclic amines constitute
a class of important chemical raw materials, and extensively
exist as the core structures of a wide array of functional
products. Therefore, the conversion of cyclic amine motifs into
the azaarene-based frameworks is of high importance in
synthetic organic chemistry. During the past decade, much
attention has been devoted towards the direct dehydrogenation
of cyclic amines into the azaarenes by developing alternative
catalyst systems.[1] However, the construction of functional
Wang and the co-workers presented a catalyst-free synthesis
of the related compounds via a similar coupling procedure
(Scheme 1b).[6b] More recently, the condensation of 2-
alkylazaarenes with aldehydes have also been nicely
demonstrated to synthesize (E)-2-alkenylazaarenes in the
presence of Lewis acids[6c,d] or under catalyst-free
conditions[6e,f] (Scheme 1c). However, to the best of our
knowledge, the dehydrogenative coupling of (hetero)aryl-fused
2-alkylcyclic amines 1 with aldehydes 2 to give the (E)-2-
alkenylazaarenes 3 remains an unresolved goal (Scheme 2).
azaarenes
via
selective
capture
of
the
partially
dehydrogenated cyclic amine motifs by an additional coupling
partner remains a new challenging subject, although it would
offer the potential for the development of new approaches to
access valuable products in step- and atom-economic fashion.
As the structural combination of olefin and N-heterocycle
motifs, the 2-alkenylazaarenes have the potentials to be
employed for the discovery of new bioactive molecules[2] and
used as valuable synthetic building blocks[3]. Moreover, such
compounds have already been applied to develop new
optoelectronic materials[4] and fluorescent probes[5]. Despite
these interesting applications, there are only limited methods
for the synthesis of such compounds. Through tandem addition
of 2-methylazaarenes to N-sulfonyl imines and elimination of
amide, the Huang group reported an iron-catalyzed synthesis
of (E)-2-alkenylazaarenes in 2011 (Scheme 1a).[6a] Later,
Scheme 1. Existed methods for the synthesis of (E)-alkenylazaarenes.
As a part of our continuous interest in construction of
functional heterocycles by transfer hydrogenative[7] and
dehydrogenative coupling[8] strategies, we were inspired to
employ the partial dehydrogenation as a substrate-activating
strategy and achieve the above-described synthetic purpose.
As illustrated in Scheme 2, the presence of a suitable catalyst
(Cat) and oxidant leads to the first dehydrogenation of the
(hetero)aryl-fused 2-alkylcyclic amine (1) and generates imine
A-1 or its tautomer A-2. Due to the interaction of the N-atom
with the catalyst and influence of steric effect, the β-site of A-2
would selectively undergo nucleophilic addition to aldehyde 2
and form the transition state (TS) by H-bond. Then, the (E)-2-
[a]
C. J. Zhou, Z. D. Tan, Prof. H. F. Jiang, Prof. M. Zhang
Key Lab of Functional Molecular Engineering of Guangdong
Province, School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou 510641 P. R. China
E-mail: minzhang@scut.edu.cn
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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alkenylaraarene 3 is afforded via dehydration of TS to form the
thermodynamically more stable (E)-alkene B-1 (or its tautomer
B-2 or B-3) followed by the second dehydrogenation (path a).
Alternatively, product 3 is obtained via full dehydrogenation of
1 to 2-alkyl azaarene 1' followed by the condensation with
aldehyde 2 at the benzylic site (path b). In addition, it is
important to note that the enamine B-3 is able to trap another
molecule of aldehyde 2 to afford the 2-alkenyl-3-alkyl azaarene
3' by successive acid-promoted elimination of H2O and
tautomerization.
Results and Discussion
Typically, the catalyst was prepared as follows: The mixture of
Co(OAc)2·4H2O and 1,10-phenanthroline (1 : 3 molar ratio) in
o
ethanol was stirred at 100 C for 1 hour to generate the cobalt
complex. Silica was then introduced into the above solution by
in situ hydrolysis of the added Si(OC2H5)4 (TEOS) with
aqueous ammonia. After that, the carbon support (Vulcan XC-
72R) was added and refluxed for 5 h at 100 oC. Finally, the
catalyst material was afforded by successive removal of the
o
suspension, dried overnight, pyrolyzed at 800 C, and etched
the non-supported cobalt species with aqueous HCl, and it is
named as Co/N–Si–C (Figure 1). Similarly, the materials
prepared in the absence of TEOS, 1,10-phenanthroline and
metal source are named as Co/N–C, Co/Si–C and N–Si–C,
o
o
respectively. And the catalysts calcined at 400 C and 600 C
are denoted as Co/N–Si–C-400 and Co/N–Si–C-600,
respectively (for details, see Supporting Information (SI)).
Scheme 2. Envision dehydrogenative coupling strategy.
However, our initial attempts showed that the above idea
was not practical when different combinations of homogeneous
complexes (i.e., Ru, Ir, Co, Rh) with acid additives were
employed in the presence of molecular O2, as we mainly
obtained the mixture of the non-coupling azaarene 1' and (E)-
2-alkenylazaarene 3, which is rationalized as the result of
reaction path b (Scheme 2). So, to achieve chemoselective
synthesis of (E)-2-alkenylazaarene 3, the first dehydrogenation
of 1 should be slow enough to ensure that enamine A-2 is
effectively trapped by aldehyde 2 (from A-2 to B-1). This idea
led us to envisage that the development of a low-loading
heterogeneous catalyst might be able to address the existing
issue. Unlike the homogeneous catalysis, such catalyst
exhibits relatively low dehydrogenation efficiency due to the
low density of catalytic sites and the occurrence of reaction on
the solid surface. In consideration of the natural abundance of
cobalt and its capability in both dehydrogenation[9] and
reduction[10], as well as the potential of silica in modifying the
micropore size of carbon materials to favor the mass
transfer,[11] we herein report the preparation and
Figure 1. Procedure for the preparation of Co/N–Si–C.
With the catalyst materials prepared, we then evaluated their
catalytic performance by testing the dehydrogenative coupling
of 2-methyl-1,2,3,4-tetrahydroquinoline 1a with benzaldehyde
2a as a model reaction. First, by using molecular O2 as an
o
oxidant, the reaction in p-xylene was performed at 120 C by
using p-nitrobenzoic acid (p-NBA) as an additive. And six
prepared materials and Co(OAc)2·4H2O (Table 1, entries 1-7)
were tested. (E)-2-alkenylquinoline (3aa) was obtained as a
major product along with small portion of 3-alkyl-2-
alkenylquinoline 3aa’ in most cases, and Co/N–Si–C (calcined
at 800 oC) exhibited the best activity and selectivity in affording
compound 3aa (95% yield). The blank experiments showed
that the presence of Co species and p-nitrobenzoic acid was
essential to obtain a satisfactory yield (entries 8-9). Next, by
using Co/N–Si–C as a preferred catalyst, a series of protonic
and Lewis acids were tested. The results showed that they
were inferior to p-nitrobenzoic acid (entries 10-17). Further,
change of the reaction solvents was unable to increase the
product yields (entry 18). Hence, the optimal conditions
(standard conditions) are as shown in entry 6 of Table 1.
characterization of
a
low-loading cobalt nanocatalyst
employing the nitrogen-silica-doped carbon (Vulcan XC-72R)
as support, and describe, for the first time, its application
towards the dehydrogenative coupling of (hetero)aryl-fused 2-
alkylcyclic amines with aldehydes to produce (E)-2-alkenyl
azaarenes in a chemoselective manner.
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Table 1. Screening of optimal reaction conditions.[a]
conclusion. Table 1 and Table S1 clearly show that the Co is
the catalytic active ingredient. In addition, the N–Si–C has a
surface area of 161 m2g-1, pore volume of 0.27 cm3g-1, and
pore size of 6.6 nm. The addition of Co precursor to the N–Si–
C led to decrease of surface area, which shows that the Co
particles are introduced into the pore of the support (Table S2).
Then, the morphology of the Co/N–Si–C catalyst was analyzed
by TEM. As shown in Figure S3, Co/N–Si–C (800 oC) has
smaller Co particles (Figure 5a, 0.35 nm) than those of Co/N–
Si–C-400 and Co/N–Si–C-600 in the same area, which
rationalizes the relatively higher catalytic activity of Co/N–Si–C.
In addition, the elemental mapping results (Figure 2) show that
the signals of Co, N and C are completely overlapped and
interweaved with each other, suggesting that the Co is bonded
to the N-atom prior to pyrolysis, which facilitates the uniform
adsorption of cobalt complex on the hydrolyzed silica and
prevents it from agglomeration during the calcining process.[14]
3aa/3aa’ [%][b]
Conv. [%]
Entry
Catalyst (cat.)
Additive
1
Co/N–C
p-NBA
p-NBA
p-NBA
p-NBA
p-NBA
p-NBA
p-NBA
-
20
10
3
13/0
6/1
2
Co/Si–C
3
N–Si–C
0/0
35
46
100
15
4
Co/N–Si–C-400
Co/N–Si–C-600
Co/N–Si–C
Co(OAc)2·4H2O
Co/N–Si–C
-
30/1
40/1
95/2
10/1[c]
72/0
0/0
5
6
7
8
80
0
9
p-NBA
PhCO2H
AlCl3
80
5
10
11
12
13
14
15
16
17
Co/N–Si–C
Co/N–Si–C
Co/N–Si–C
Co/N–Si–C
Co/N–Si–C
Co/N–Si–C
Co/N–Si–C
75/0
0/0
6
CF3SO3Ag
CF3COOH
p-TSA
H2SO4
HCOOH
0/0
6
0/0
10
5
7/1
0/0
Figure 2. TEM image of Co/N–Si–C and the corresponding elemental
mapping of C, N and Co, respectively.
5
0/0
sulfamic
acid
13
Co/N–Si–C
Co/N–Si–C
10/0
Table 2. The binding energy and content of N in the catalyst.
(39/0, 0/0,
13/0)[d]
18
45, 3, 15
p-NBA
Binding Energy / eV (Area/%)
Catalyst
NCo-N
Npyrrol
Nammonia
[a] Unless otherwise stated, all the reactions were performed in p-xylene
(1.0 mL) with 1a (0.5 mmol), 2a (0.75 mmol), cat. (40 mg), additive (20
mol %) at 120 oC for 16 h under O2. p-NBA: p-nitrobenzoic acid. [b] GC
yield using hexadecane as an internal standard. [c] 2 mol % catalyst. [d]
Yields are with respect to chlorobenzene, DMSO and DMF as the solvents,
respectively.
Co/N–Si–C
398.8 (60.4)
400.6 (20.7)
402.5 (18.8)
Subsequently, we set out to study the features of the
optimal catalyst. Except for two peaks of carbon, the XRD
pattern of Co/N–Si–C (SI, Figure S1) did not show any signal
ascribed to the Co metal, whereas the existence of cobalt
metal was evidenced by EDX detection (Figure S2). The
results indicate that the cobalt species in the catalyst are highly
dispersed with low density.[12] The peaks of carbon at 26o and
43o are with respect to the (002) and (100) planes, the
diffraction pattern of graphite-like structure.[13] In comparison,
the carbon signal intensity of Co/N–C is much stronger than
those of Co/N–Si–C and Co/Si–C (Figure S1). The introduction
of silica into the carbon support improves both the specific
surface area and pore volume of the resulting materials (Table
S2). In order to figure out the single factor influence for the
catalysts, the pore structure for Co/C, N–C and Si–C were
shown in Table S2. The specific surface area of Si–C is almost
twice as big as the other two samples, which also has the
biggest pore volume, this result exactly coincides with above
Figure 3. XPS spectra of N1s in Co/N–Si–C.
The surface chemistry of the new developed catalyst (Co/N–
Si–C) was then analyzed by the X-ray photoelectron
spectroscopy (XPS). In good agreement with the XRD results,
it was hard to detect the Co species due to the extremely low
content, and only the valence of N could be detected, and it
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was deconvoluted into three peaks with Co–N (398.8 eV),
pyrrolic (400.6 eV) and ammonia (402.5 eV),
thus favoring the coupling process. Furthermore, alkyl
aldehyde 2r also underwent dehydrogenative coupling with 1a,
albeit the yield was somewhat low (3ar). Cinnamaldehyde 2s
could serve as an effective coupling partner to react with 1a,
N
N
respectively[11b] (Figure 3 and Table 2). The binding energy of
Co–N is ascribed to pyridinic-type N, while the pyrrolic N
originates from the calcination of the 1,10-phenanthroline.
Deconvolution shows that around 60.4% of the N-atoms are
bound to the cobalt metal (see Table 2, Co–N). Further, the
poisoning experiment by addition of thiocyanate ion (SCN-) to
the model reaction led to loss of the catalyst activity.[15] These
results support that the highly dispersed Co–N particles serve
as the catalytic active sites.
affording the (E,E)-2-dienylquinoline 3as as
a
single
stereoisomer in 60% yield.
In addition to the variation of aldehydes, we then focused
on the transformation of different (hetero)aryl-fused 2-
alkylcyclic amines. First, the reactions of both electron-rich and
electron-deficient 2-methyl-1,2,3,4-tetrahydroquinolines (1b-1f)
with aldehydes 2 were tested (Scheme 4). Gratifyingly, all the
substrates underwent efficient dehydrogenative coupling to
give the desired products in high isolated yields (3ba, 3bl, 3ca,
3da, 3dc, 3ea, 3fa). Further, other types of (hetero)aryl-fused
2-alkylcyclic amines such as 1,2,3,4-tetrahydroquinoxaline (1g)
and 1,2,3,4-tetrahydro-1,8-naphthyridine (1h) were also
amenable to the transformation to couple with aldehyde 2a,
affording the (E)-2-alkenyl azaarenes in moderate yields (3ga
and 3ha). Furthermore, 1,2,3,4-tetrahydroacridine 1i could be
efficiently converted in combination with 2a into the desired
product in 80% yield (3ia). As illustrated in Scheme 3 and 4, a
series of functional groups (i.e., –Me, alkoxy, amide, ester,
halogens, –NO2, –CF3, –CN) are well tolerated in the reaction,
which offers the potential for molecular complexity via further
chemical transformations.
Scheme 3. The coupling of 1a with aldehydes 2. Standard conditions: all the
reactions were performed in p-xylene (1.0 mL) with 1a (0.5 mmol), 2 (0.75
mmol), cat. (1.08 mol %, 40 mg), p-nitrobenzoic acid (20 mol %) at 120 oC
for 16 h with O2.
Scheme 4. Both variations of two coupling partners.
With the optimal conditions available, we then tested the
generality of the synthetic protocol under the optimal conditions.
First, the reactions of 1a with a wide range of (hetero)aryl
aldehydes (for structures, see Table S3 in the SI) were
examined. As shown in Scheme 3, all the reactions proceeded
In order to demonstrate the stability and reusability of the
new developed catalyst (Co/N–Si–C), it was recycled and
reused for 5 consecutive runs with the model reaction. As
shown in Figure 4, the catalytic activity maintains well, albeit
with a slight decline of yield. In comparison with the fresh
catalyst (Co/N–Si–C), the pore diameter of the used Co/N–Si–
C (after use of 5 runs) slightly changed, whereas its specific
surface area and pore volume decreased dramatically (Table
S2). In addition, the Co content dropped from 0.79 wt % to
0.68 wt % (determined by ICP-OES analysis). The TEM
images (Figure 5) showed that the average particle sizes of the
smoothly and furnished the (E)-2-alkenylquinolines
3 in
moderate to excellent isolated yields (3aa-3aq). The
substituents with different electronic property on the aryl ring of
benzaldehydes slightly affected the product yields. Especially,
the electron-withdrawing groups enabled to afford the
corresponding products (3ai-3am) in relatively higher yields
than those of electron-donating ones (3ad-3af). This
phenomenon is rationalized as the electron-deficient
substituents enhance the electrophilicity of the carbonyl carbon,
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fresh and used Co/N–Si–C catalysts are 0.35 ± 0.02 nm and
0.42 ± 0.02 nm, respectively. These results indicate that the
collapse of pore structure, aggregation and leaching of
nanoparticles occurred during the reaction, which count for the
slight deactivation of the catalyst.
Conclusions
In conclusion, by developing a low-loading cobalt nanocatalyst
with the nitrogen-silica-doped carbon (Vulcan XC-72R) as the
support
(Co/N–Si–C),
we
have
demonstrated
a
To gain mechanistic insights, the model reaction under
standard conditions was interrupted after 1 h to analyze the
product system (Scheme 5, eq 1). Except for the coupling
product 3aa detected in 21% yield, the fully dehydrogenated 2-
methyl quinoline 1a’ was observed in 1% yield. Then, the
reaction of 1a’ with aldehyde 2a under the standard conditions
only gave product 3aa in a 19% yield (eq 2). However, the
model reaction under the same conditions gave 3aa in almost
quantitative (see Table 1, entry 6: 95% yield) along with small
portion of 2-alkenyl-3-alkyl quinoline 3aa’ (2%). Thus, the
reaction involving 1a’ as a main intermediate is less likely, and
path a is a favored reaction way (Scheme 2).
dehydrogenative coupling of (hetero)aryl-fused 2-alkylcyclic
amines with aldehydes to give the (E)-2-alkenylazaarenes for
the first time. In the reaction, effective capture of the partially
dehydrogenated cyclic amine motifs appears to be a key
strategy to address the chemoselective issue. The developed
chemistry proceeds with the merits of broad substrate scope,
excellent functional group tolerance, high atom-efficiency, use
of an earth-abundant and reusable cobalt catalyst and
molecular O2 as a green oxidant, which offers an important
basis for direct conversion of inert cyclic amine units into the
functional frameworks.
Experimental Section
Procedure for the preparation of catalyst: The mixture of
Co(OAc)2·4H2O (186.8 mg, 0.75 mmol) and 1,10-phenanthroline (405
mg, 2.25 mmol) in ethanol (50 mL) was stirred for 1 hour at 100 oC.
Silica was then introduced into the above solution by in situ hydrolysis
of the added Si(OC2H5)4 (TEOS, 1.04 g, 5.0 mmol) with aqueous
ammonia (3.0 mL, 19.5 mmol) and the resulting mixture was then
refluxed for another 2 hours. After that, the Vulcan XC-72R carbon
support (1000 mg) was added and refluxed for 5 h at 100 oC. Then, the
solvent of the suspension was removed and the remained solid was
dried overnight at 60 oC under vacuum. The remaining sample was
grounded to fine powder and then pyrolyzed at 800 oC under a constant
argon flow for 2 hours. After cooling down to room temperature, the
catalyst Co/N–Si–C was afforded by treating the sample with HCl
solution to remove the non-supported cobalt particles.
Figure 4. The reusability of the Co/N–Si–C catalyst.
Typical procedure for the synthesis of compound 3aa: The mixture
of 2-methyl-1,2,3,4-tetrahydroquinoline 1a (0.5 mmol), benzaldehyde
2a (0.75 mmol), p-nitrobenzoic acid (20 mol %) and cobalt catalyst (Co:
o
1.08 mol %, 40 mg) in p-xylene (1.0 mL) was stirred at 120 C for 16 h
under O2 atmosphere. After cooling down to room temperature, the
resulting mixture was filtered and washed with ethyl acetate, and then
concentrated by removing the solvent under vacuum. Finally, the
residue was purified by preparative TLC on silica, eluting with
petroleum ether (60–90 oC) : ethyl acetate (20 : 1, v/v) to give N-
heteroaromatics 3aa.
Acknowledgements
Figure 5. TEM images and particle size distribution of fresh (a) and used (b)
Co/N–Si–C.
We thank the Fundamental Research Funds for the Central
Universities (2017ZD060), “1000 Youth Talents Plan”, Science
and Technology Program of GuangZhou (201607010306),
Science Foundation for Distinguished Young Scholars of
Guangdong (2014A030306018) for financial support.
Conflict of interest
Scheme 5. The control experiments.
The authors declare no conflict of interest.
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Keywords: cobalt nanocatalyst ∙ (hetero)aryl-fused 2-
alkylcyclic amines ∙ aldehydes ∙ (E)-2-alkenylazaarenes ∙
dehydrogenative coupling
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10.1002/cctc.201800202
ChemCatChem
FULL PAPER
FULL PAPER
An earth-abundant and reusable
cobalt nanocatalyst exhibits excellent
catalytic performance towards the
dehydrogenative coupling of
Changjian Zhou, Zhenda Tan, Huanfeng
Jiang, and Min Zhang*
(hetero)aryl-fused 2-alkylcyclic amines
with aldehydes to produce the (E)-2-
alkenylazaarenes.
Page No. – Page No.
Synthesis of (E)-2-Alkenylazaarenes
via Dehydrogenative Coupling of
(Hetero)aryl-fused 2-Alkylcyclic
Amines and Aldehydes with a Cobalt
Nanocatalyst
This article is protected by copyright. All rights reserved.