Catalytic Aerobic Dehydrogenation of Nitrogen Heterocycles Using Heterogeneous Cobalt Oxide Supported on Nitrogen-Doped Carbon

Article abstract of DOI:10.1021/acs.orglett.5b01790

Dehydrogenation of (partially) saturated heterocycles provides an important route to heteroaromatic compounds. A heterogeneous cobalt oxide catalyst, previously employed for aerobic oxidation of alcohols and amines, is shown to be effective for aerobic dehydrogenation of various 1,2,3,4-tetrahydroquinolines to the corresponding quinolines. The reactions proceed in good yields under mild conditions. Other N-heterocycles are also successfully oxidized to their aromatic counterparts.

Full text of DOI:10.1021/acs.orglett.5b01790

Letter
pubs.acs.org/OrgLett
Catalytic Aerobic Dehydrogenation of Nitrogen Heterocycles Using
Heterogeneous Cobalt Oxide Supported on Nitrogen-Doped Carbon
Andrei V. Iosub and Shannon S. Stahl*
Department of Chemistry, University of WisconsinMadison, 1101 University Avenue, Madison, Wisconsin 53706, United States
*
S Supporting Information
ABSTRACT: Dehydrogenation of (partially) saturated heterocycles
provides an important route to heteroaromatic compounds. A heteroge-
neous cobalt oxide catalyst, previously employed for aerobic oxidation of
alcohols and amines, is shown to be effective for aerobic dehydrogenation
of various 1,2,3,4-tetrahydroquinolines to the corresponding quinolines.
The reactions proceed in good yields under mild conditions. Other N-
heterocycles are also successfully oxidized to their aromatic counterparts.
(
Hetero)aromatic molecules are ubiquitous in pharmaceutics
Scheme 1. Dehydrogenation 1,2,3,4-Tetrahydroquinolines to
Quinolines
1
and other biologically active molecules. The introduction of
substituents on the aromatic framework is typically accom-
plished by sequential introduction of groups onto a preformed
aromatic ring. Recently, our group has been exploring aerobic
dehydrogenation of (partially) saturated cyclic compounds as a
complementary route to prepare substituted arenes and
2
heteroarenes. In many cases, these methods enable synthesis
of aromatic compounds with substitution patterns and/or
functional groups that are difficult to access via traditional
aromatic functionalization reactions. This strategy has been
employed by us and others in Pd-catalyzed methods for the
conversion of cyclohexanone and cyclohexenone derivatives
3
4
5
into substituted phenols, aryl ethers, or anilines and for the
6
conversion of cyclohexene derivatives into substituted arenes.
More recently, we began exploring methods for aerobic
dehydrogenation of partially saturated N-heterocycles to
heteroarenes. Our initial efforts to employ Pd-based catalysts
led to unsatisfactory results, and we turned our attention to
bioinspired ortho-quinone-based catalysts that are effective for
the aerobic dehydrogenative aromatization of (partially)
previously reported homogeneous Pd-based dehydrogenation
7
−9
3a,16
saturated nitrogen heterocycles.
Collectively, the aerobic
catalysts
were tested, but they led to only modest yields
dehydrogenation methods represent compelling alternatives to
established methods that use stoichiometric reagents, such as
(49−55%) with rather poor mass balance (Table 1, entries 1−
3). In an effort to improve the results, we considered other
catalysts, including heterogeneous variants. Heterogeneous Pd
catalysts have been successfully used for the dehydrogenation of
1
0
DDQ or sulfur. In light of the challenges we encountered in
the use of Pd-based catalysts for N-heterocycle dehydrogen-
ation, specifically to access quinoline products (Scheme 1A), we
elected to test heterogeneous Co-oxide-based catalysts for this
application. Here, we present the results of this effort.
3c,4b,5e
cyclohexanones and -hexenones.
Other heterogeneous
catalysts composed of noble metals (Ru, Au, Pd Pb) have also
3
been shown to be active for aerobic dehydrogenation of the
17
parent 1,2,3,4-tetrahydroquinoline; however, the reactions
proceed at high temperatures (>100 °C), they are accompanied
by catalyst deactivation, and the results were not extended to
substituted tetrahydroquinoline derivatives. During the course
of these initial studies, Beller and co-workers reported a unique
Co-oxide-based catalyst for aerobic alcohol oxidation (oxidative
For our initial studies we chose 1,2,3,4-tetrahydroquinaldine
(1a) as a test substrate (Table 1). The product of this
dehydrogenation, 2-methylquinoline or quinaldine (2a),
resembles the core of compounds that exhibit important
pharmaceutical activity, including those for the treatment of
11
schizophrenia (PF-2545920), HIV infection (styrylquino
1
2
13
14
lines), malaria (Mefloquine), asthma (Montelukast), and
1
5
pain (JTC-801). Therefore, it represents a simple proof-of-
concept for use in catalyst screening (Scheme 1B). Our
Received: June 20, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01790
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Low Temperature Dehydrogenation of 1,2,3,4-
Tetrahydroquinaldine
source and 1,10-phenanthroline (phen) as the nitrogen source,
provided a 92% yield of quinaldine in the presence of 1 equiv of
K CO as a base. The reaction was complete within 4 h in
2
3
MeOH at only 60 °C (Table 1, entry 4). Without any base, the
catalyst was still active, but led to a lower final yield (43%, entry
22
5
). Catalysts prepared analogously using other nitrogen-based
22
solvent
yield
ligands instead of phen were also investigated (entries 6−8).
b
entry
catalyst (mol %)
(concn [M])
temp
80
(conv)
With 2,2′-bipyridine (bpy) as the nitrogen source, no product
was obtained, while, with 2,6-di(2-pyridyl)pyridine (terpy) and
1
Pd(TFA) (5)/
DMSO (1)
52 (88)
2
2
-NMe -py (10)/
2
1
,10-phenanthroline-5,6-dione (phd), 82% and 67% yields were
pTsOH (20)
obtained, albeit lower than when using phen. Further
investigations revealed that the ratio of cobalt to phen in the
catalyst preparation procedure had little impact on this
2
3
Pd(TFA) (5)/
AcOH (0.2)
DMSO (0.2)
80
80
55 (100)
49 (61)
2
DMSO (10)
Pd(TFA) (5)/4,5-
2
diazafluoren-9-one (5)
22
dehydrogenation. Next, catalysts with metals other than
a
,
c
4
5
6
7
8
9
1
1
1
CoO -phen/AB
MeOH (0.125)
MeOH (0.125)
MeOH (0.125)
MeOH (0.125)
MeOH (0.125)
MeOH (0.125)
MeOH (0.125)
MeOH (0.125)
MeOH (0.125)
60
60
60
60
60
60
60
60
60
92 (94)
43 (43)
0 (1)
x
cobalt were prepared using phen as the nitrogen source and
acetylene black as the carbon source. These heterogeneous Mn,
Fe, and Ni oxide materials were tested under the reaction
a
CoO -phen/AB
x
a
a
a
a
,
,
,
,
c
c
c
c
CoO -bpy/AB
x
CoO -terpy/AB
82 (100)
67 (74)
16 (32)
0 (52)
x
conditions. The MnO -based catalyst provided low yields of
x
CoO -phd/AB
x
product, while the rest were inactive (entries 9−11). Upon
MnO -phen/AB
x
obtaining a sample of Vulcan XC72R, a Co(OAc) /phen-
2
a
,
,
c
c
0
1
2
FeO -phen/AB
x
derived catalyst was prepared with this carbon source (entry 12).
This catalyst led to a product yield similar to that obtained with
the catalyst supported on AB, but it showed increased initial
a
NiO -phen/AB
0 (13)
x
a
,
c
CoO -phen/
92 (100)
x
Vulcan XC72R
22
a
a
a
a
activity.
13
14
15
16
Pd/C
xylene (0.2)
60
60
60
60
0 (0)
Control reactions were also conducted to verify the necessity
Pd/C
MeOH (0.125)
xylene (0.2)
13 (30)
29 (66)
28 (63)
,
,
d
d
,
,
e
f
of the pyrolysis, heterogeneity, and “nitrogen-doping” of the
activated C
Co(salen)
22
catalyst. Co(OAc) alone was not active for the reaction, in the
EtOH (0.066)
2
a
presence of either a nitrogenous ligand or carbon black. The
carbon black support (AB) was also completely inactive for the
dehydrogenation, with or without pyrolysis and/or nitrogen
1
a (0.10 mmol), solvent, 2.5 mol % [cat.], 1 atm of O , orbital
2
b c
1
mixing, 60 °C, 4 h. % Yields and conv determined by H NMR. 1
equiv of K CO added. Magnetic stirring was employed. 100 wt %
catalyst, 24 h. 1 mol % catalyst was used, 24 h. C = carbon; AB =
Acetylene Black.
d
e
2
3
f
doping. Finally, a mixture of Co(OAc) /1,10-phenanthroline
2
impregnated on acetylene black, but not pyrolyzed, was also
inactive. Collectively, the results show that pyrolysis of
Co(OAc) /phen in a 1:2 mixture on AB and Vulcan XC72R
2
at 800 °C under Ar generates effective catalysts. These
compositions align with the catalyst previously optimized and
characterized by Beller and co-workers, and their nomenclature
“
Co O -NGr/C” is adopted below.
The Co O -NGr/C catalyst outperforms other commonly
3 4
10a
3 4
1
8,19
esterification of primary alcohols).
More recently, the same
researchers have applied these and related catalysts to the
used dehydrogenation catalysts, such as Pd/C and activated
carbon. Moreover, it exhibits good activity at 60 °C, which is
lower than temperatures typically required for dehydrogenation.
When Pd/C was tested under comparable conditions, none of
the dehydrogenation product was obtained in xylene and only a
low yield was observed in methanol (Table 1, entries 13, 14).
Activated carbon has been shown to promote dehydrogenation
20
conversion of alcohols and amines to nitriles. Overall, these
catalysts represent appealing base-metal alternatives to hetero-
geneous Pd/C and other supported noble metals, and we
reasoned that similar catalysts could promote aerobic
2
1
dehydrogenation of heterocycles.
The Co-oxide catalysts were prepared according to the
18
protocol reported by Beller and co-workers. Briefly, cobalt(II)
acetate was stirred with different bi- and tridentate nitrogen
containing ligands in ethanol for 30 min, then acetylene black
of 1,2,3,4-tetrahydroquinolines at 120 °C under O with a 100
2
23
wt % loading of carbon. At the lower temperatures used here,
however, use of 100 wt % activated carbon affords only a 29%
yield of product (entry 15). Co(salen) has been used as a
catalyst previously for dehydrogenation of the parent 1,2,3,4-
(
AB) or Vulcan XC72R was added as a conductive carbon black
support, and the mixture was refluxed. After drying, the black
powder was pyrolyzed under an argon flow at 800 °C for 2 h.
This procedure enabled the synthesis of cobalt based nano-
particles of varying sizes, encapsulated by nitrogen-rich
graphene-like layers. This specific architecture has been
suggested to account for the notable reactivity of these
24
tetrahydroquinoline at 60 °C, but the presence of the 2-methyl
group in tetrahydroquinaldine significantly attenuates the yield
(28%, entry 16).
The Co O -NGr/C catalyst was then tested in the
3
4
dehydrogenation of a variety of 1,2,3,4-tetrahydroquinolines
under analogous reaction conditions (Scheme 2). The parent
1
8
25
catalysts.
Using the procedure just noted, a number of related catalysts
were prepared, in which the identities of the nitrogen-containing
ligand, the metal source, and/or the carbon support was varied.
Our initial studies were performed with acetylene black (AB) as
the carbon support because it was already available in our lab. A
quinoline 2b was obtained in 90% yield and 2-phenylquinoline
2c in 92% yield after 6 h. The presence of functional groups on a
2-aryl substituent influenced the dehydrogenation reactivity.
Substrates containing −SMe and −CF groups reacted well (2d,
3
2e), while those containing −NO and −CN groups needed
2
catalyst prepared with this material, using Co(OAc) as the Co
more catalyst and longer reaction times to reach completion,
2
B
DOI: 10.1021/acs.orglett.5b01790
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Organic Letters
Letter
Scheme 2. Substrate Scope for the Dehydrogenation of
in good yield (77% for the −OMe and 72% for the −Me
derivatives 2s and 2t). The reactions involving substrates 1r−1t
were somewhat slower, presumably due to steric effects.
With these successful results in hand, we tested a number of
other partially saturated N-heterocycles containing secondary
amines for their reactivity toward dehydrogenation (Scheme 3).
a
1
,2,3,4-Tetrahydroquinolines to Quinolines
Scheme 3. Dehydrogenation of Other Classes of N-
,
a b
Heterocycles
a
Conditions: 1 (0.5 mmol), 4 mL of MeOH, 25 mg of catalyst (2.5
b
mol %), 1 equiv of K CO , 1 atm of O , orbital stirring, 60 °C. 50 mg
of catalyst were used. Reaction on 10 mmol scale of 1m, magnetic
2
3
2
c
stirring.
affording lower yields of the desired products (2f, 2g).
Substrates with 2-thienyl and 2-furyl groups led to high yields
of the corresponding quinolines (2h, 2i), showing no inhibition
by the heterocyclic substituent. The results with 2c−2i are
noteworthy in demonstrating the compatibility of the catalyst
with 2-arylquinolines, a motif present in pharmaceutically active
compounds. The reaction forming 3-methylquinoline (2j)
exhibited a lower mass balance and product yield; however, 4-
methyl derivatives were obtained in high yields (1k, 1l).
a
Conditions: Substrate (0.1 mmol), 0.8 mL of MeOH, 5 mg of
b
catalyst, 1 equiv of K CO , 1 atm of O , orbital stirring. Yields and
conv determined by H NMR. EtOH was used as solvent. Catalyst
prepared with Co:phen ratio of 1:1; 10 mg of catalyst used.
2
3
2
1
c
d
A number of tetrahydroquinolines with substitution at
positions 6−8 were investigated (Scheme 2). Derivatization at
these positions is usually accessible via selective electrophilic
aromatic substitution reactions of tetrahydroquinolines. In
contract, the reactivity of quinolines in similar reactions is
usually unselective, yielding multiple products. First, substitu-
tion at the 6-position was explored. Substrates with electron-
donating substituents, such as methoxy and methyl, underwent
dehydrogenation in high yields (2m, 2n). The dehydrogenation
of 1m was also carried out on 10 mmol scale (1.47 g), and the
product was obtained in a 94% yield. The presence of halogens
F−, Cl−, and Br− at the 6-position led to good yields of
dehydrogenation products 2o−2q. These observations highlight
the advantages of this Co-based catalyst over Pd/C catalysts 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. The
The reaction of 1,2,3,4-tetrahydroquinoxaline 3 under the
standard reaction conditions led to quinoxaline 4 in 97% yield.
The readily accessible substrates, indoline 5 and 2-methyl
indoline 7, exhibited excellent reactivity under the reaction
conditions and afforded high yields of products at room
temperature in a short time. This reactivity compares favorably
with the previously reported aerobic dehydrogenation of
26
indolines with 10 mol % of a molecular Co-salen catalyst.
Hayashi and co-workers also reported dehydrogenation of the
parent indoline 5 using 100 wt % of activated carbon, although a
27a
longer reaction time and higher temperature were required.
A
tertiary amine substrate, N-methyl indoline 9 was less reactive,
affording only a 33% yield of the desired indole. The oxidation
of Hantzsch ester 11 was also successful, exhibiting a yield of
95% after only 2 h at room temperature, which again compares
27b
6,7-benzo-fused derivative 1r underwent dehydrogenation in
favorably with results obtained with activated carbon. Finally,
the more challenging dehydrogenation of 2-phenylimidazoline
13 led to a 43% product yield, with increased catalyst loading.
good yield to the benzoquinoline 2r. Dehydrogenation of
substrates with substituents in the 8 position also led to products
C
DOI: 10.1021/acs.orglett.5b01790
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Empirical optimization efforts with this substrate showed that
the best catalyst was prepared with a 1:1 ratio of Co/phen. In
this case, activated carbon is a more effective reagent for
dehydrogenation, affording the desired imidazole in 84% yield at
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20 °C.
In conclusion, we have shown that the dehydrogenation of
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cycles can be achieved using a cobalt oxide based catalyst
supported on nitrogen-doped carbon under mild conditions.
This heterogeneous first-row transition-metal catalyst represents
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based on Pd for the synthesis of biologically active
heteroaromatic compounds.
(
9) For other homogeneous catalytic methods for dehydrogenation of
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(
ASSOCIATED CONTENT
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*
S
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D. R. Encyclopedia of Reagents for Organic Synthesis; John Wiley & Sons,
Inc.: New York, 2010.
Optimization data, experimental procedures, and spectro-
scopic characterization data (PDF)
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AUTHOR INFORMATION
Corresponding Author
■
(
12) Mouscadet, J.-F.; Desmael
̈
e, D. Molecules 2010, 15, 3048.
(13) Croft, A.; Garner, P. BMJ 1997, 315, 1412.
*
E-mail: stahl@chem.wisc.edu.
(14) Belley, M. L.; Leger, S.; Labelle, M.; Roy, P.; Xiang, Y. B.; Guay,
D. U.S. Patent 5,565,473, Oct. 15, 1996.
Notes
(
15) Sestili, I.; Borioni, A.; Mustazza, C.; Rodomonte, A.; Turchetto,
L.; Sbraccia, M.; Riitano, D.; Del Giudice, M. R. Eur. J. Med. Chem.
004, 39, 1047.
The authors declare no competing financial interest.
2
ACKNOWLEDGMENTS
We thank the following individuals for assistance: Dr. R. Pokhrel
synthesis of catalysts), Dr. E. J. Popczun, M. S. Ahmed, and Y.
Preger (pXRD), and J. C. King (XPS) (all from UW
Madison). This work was funded by the NIH (R01-
GM100143). NMR facilities were partially supported by the
NSF (CHE-0342998, CHE-1048642) and NIH (S10
RR08389). MS instrumentation was partially supported by the
NIH (S10 RR024601).
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b) Diao, T.; Wadzinski, T. J.; Stahl, S. S. Chem. Sci. 2012, 3, 887.
■
(
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(
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DOI: 10.1021/acs.orglett.5b01790
Org. Lett. XXXX, XXX, XXX−XXX