Vanadium-Catalyzed Dehydrogenation of N-Heterocycles in Water

Article abstract of DOI:10.1021/acs.orglett.8b01484

In this paper, the dehydrogenation of tetrahydroquinolines using oxovanadium(V) catalysts under mild conditions in water and oxygen atmosphere is described. This catalytic technology was successfully applied to a range of other structurally related N-heterocycles, and a reaction mechanism is proposed.

Full text of DOI:10.1021/acs.orglett.8b01484

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Vanadium-Catalyzed Dehydrogenation of N‑Heterocycles in Water
†,‡
,‡
,†
Nadine Zumbragel, Makoto Sako,‡ Shinobu Takizawa,‡ Hiroaki Sasai,* and Harald Groger*
̈
̈
†
̈
Chair of Organic Chemistry I, Faculty of Chemistry, Bielefeld University, Universitatsstraße 25, 33615 Bielefeld, Germany
‡The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka, Ibaraki, Osaka 567-0047, Japan
S
* Supporting Information
ABSTRACT: In this paper, the dehydrogenation of tetrahydroquinolines using oxovanadium(V) catalysts under mild
conditions in water and oxygen atmosphere is described. This catalytic technology was successfully applied to a range of other
structurally related N-heterocycles, and a reaction mechanism is proposed.
itrogen-containing aromatic compounds play an im-
V2O5).16 On the other hand, vanadium represents a very
attractive metal component for catalysts due to its abundant
availability in nature and corresponding low costs, as well as
low toxicity compared to other heavy metals.17
Attracted by these advantageous properties and the
successful use of mono- and dinuclear oxovanadium catalysts
for oxidative reactions under mild conditions,17c,18 we were
interested in examining the dehydrogenation of N-heterocycles
using mono- and dinuclear vanadium complexes, allowing a
fine-tuning of their catalytic properties by modification of the
ligands.
In the following paper, we report the dehydrogenation of
tetrahydroquinolines in the presence of vanadium catalysts
under mild conditions in aqueous media using oxygen as
oxidant. Furthermore, we demonstrate that this catalytic
technology can be applied successfully toward dehydrogen-
ation of other N-heterocycles and a reaction mechanism is
proposed.
In our initial experiments, we screened three commercially
available vanadium catalysts and seven oxovanadium(V)-type
catalysts (comprising mono- as well as dinuclear vanadium
complexes) A−G (Scheme 1) against 1,2,3,4-tetrahydroqui-
naldine (1a), which was chosen as model substrate due to the
importance of the resulting product as a key structural motif in
pharmaceuticals.2 The screening was performed at 70 and 50
°C under ambient air atmosphere in distilled water for 17 h
using 5 mol % (in case of dinuclear vanadium complexes) or
10 mol % (in case of mononuclear vanadium complexes)
(Scheme 1).
N
portant role in nature and medicine.1 Among other N-
heteroaromatic compounds, 2-methylquinoline (quinaldine) is
of particular importance, as it represents a key structural motif
in a range of pharmaceuticals.2 Examples for pharmaceuticals
bearing quinaldine as a key structural motif are, among others,
montelukast2a and mefloquine.2b
Dehydrogenation of the corresponding tetrahydroquinolines
represents an atom-efficient access toward quinolines and has
been studied extensively in the last years. As the dehydrogen-
ation of N-heterocycles represents a thermodynamically “uphill
process”, even though the nitrogen atom decreases the
endothermicity compared to cycloalkanes,3 harsh reaction
conditions are required in many cases. Yamaguchi et al.
pioneered the dehydrogenation/hydrogenation of N-hetero-
cycles using an iridium complex in p-xylene (under reflux
conditions for the dehydrogenation reaction).4 In the past
years, dehydrogenation reactions were studied using other
iridium catalysts5 as well as metal complexes based on iron,6
cobalt,7 nickel,8 ruthenium,9 and copper.10 In addition, the
Stahl group developed a method for the dehydrogenation of
cyclohexanones toward phenols using palladium catalysts.11
Furthermore, a method using Pd/C12 was developed for the
dehydrogenation of N-heterocycles and, recently, the success-
ful use of palladium nanoparticles13,14 as well as platinum and
rhodium nanoparticles14 for the dehydrogenation of N-
heterocycles was also reported. Furthermore, the Stahl group
succeeded in applying bioinspired o-quinone-based catalysts in
combination with zinc,15 ruthenium,9b and cobalt7b for the
dehydrogenation of N-heterocycles.
Among the three applied commercially available vanadium
catalysts, only VO(OEt)3 showed a conversion toward
quinaldine (2a) albeit being very low with only 3% after
However, until now, there has been only one report on a
vanadium-catalyzed dehydrogenation of tetrahydroquinolines
and other N-heterocycles, which is based on the utilization of
vanadium pentoxide. This reaction suffers from harsh reaction
conditions and high catalyst loading (up to 2.0 equiv of
Received: May 10, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01484
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
optimization is the simple access to this catalyst via a one-step
synthesis from commercially available starting materials (see
Supporting Information (SI)). All optimization experiments
were performed under ambient air atmosphere and at a
reaction temperature of 60 °C (determined as optimal
temperature) for 24 h (Table 2). It is noteworthy that in the
Scheme 1. Screening of Mono- and Dinuclear Vanadium
Complexes for Dehydrogenation of 1,2,3,4-
Tetrahydroquinaldine (1a)
Table 2. Optimization of Reaction Conditions for
Dehydrogenation of 1,2,3,4-Tetrahydroquinaldine (1a)
cat.
substrate
(M)
additive (5
mol %)
cosolvent (%
v/v)
conv
d
entry (mol %)
(%)
a
1
2
3
4
5
6
7
8
9
2.5
5
10
0.4
0.4
0.4
0.4
0.1
1.0
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
14
30
44
60
35
53
41
44
37
30
30
39
40
63
90
a
a
a
a
a
a
a
a
20
10
10
10
10
10
10
10
10
10
10
10
SDS
Triton X-100
TBAB
Table 1. Screening Results for the Dehydrogenation of
1,2,3,4-Tetrahydroquinaldine (1a)
a
a
a
a
b
c
10
11
12
13
14
15
MeOH (10)
MeOH (20)
DMSO (10)
DMSO (20)
a
entry
cat. (mol %)
VOSO4
NaVO3
temp (°C)
conv (%)
1
2
3
4
5
6
7
8
70
70
70
70
70
70
70
70
70
70
50
50
50
50
50
50
50
0
0
3
VO(OEt)3
(Ra,S,S)-A (5)
(Ra,S)-B (10)
rac-C (10)
(S)-D (10)
(S)-E (10)
(R*,R*)-/(R,S)-F (5)
(S)-G (10)
(Ra,S,S)-A (5)
(Ra,S)-B (10)
rac-C (10)
41
26
30
50
20
44
28
39
13
26
36
16
24
23
a
b
Variation of reaction time: 24 h. Variation of reaction time: 48 h.
c
d
Variation of reaction time: 72 h. Determined by 1H NMR
spectroscopy via comparison of substrate and product integrals.
9
initial experiments of the process optimization, we found a
significant effect of the type of reaction vessel on the reaction
course. Accordingly, all reactions were performed in Schlenk
tubes (see the SI).
10
11
12
13
14
15
16
17
Toward an optimization of the dehydrogenation of 1,2,3,4-
tetrahydroquinaldine (1a), the optimal catalyst loading was
examined in the first step (Table 2, entries 1−4). As expected,
the conversion of substrate 1a is higher with increased catalyst
loading. As a compromise between high conversion and an
economic use of the catalyst, 10 mol % catalyst loading was
chosen for further optimization, leading to 44% conversion of
the starting material 1a (Table 2, entry 3). Afterward, the
optimal substrate concentration for the dehydrogenation of 1a
was studied: Interestingly, while keeping the catalyst to
substrate ratio constant, a higher substrate concentration
leads to higher conversions (Table 2, entries 3, 5, and 6). A
substrate concentration of 0.4 M (Table 2, entry 3) was chosen
as optimal and applied in further optimization reactions. Due
to the low solubility of the mononuclear vanadium complex
(S)-D as well as the substrate 1a in water, the effect of a
catalytic amount of surfactants was examined: Therefore, we
added either anionic surfactant sodium dodecyl sulfate (SDS),
neutral surfactant polyoxyethylene p-tert-octylphenyl ether
(Triton X-100) or cationic surfactant tetrabutylammonium
bromide (TBAB). However, addition of surfactants had no
beneficial impact on the conversion of 1a (Table 2, entries 7−
9). Furthermore, addition of water-miscible cosolvents such as
methanol and dimethyl sulfoxide, did not give higher
(S)-D (10)
(S)-E (10)
(R*,R*)-/(R,S)-F (5)
(S)-G (10)
a
1
Determined by H NMR spectroscopy via comparison of substrate
and product integrals.
17 h reaction time at 70 °C (Table 1, entries 1−3). For the
examined vanadium complexes A−G bearing a chelating ligand
backbone we were pleased to observe successful trans-
formations of substrate 1a to quinaldine (2a) proceeding
without any side reaction under nonoptimized reaction
conditions (Table 1, entries 4−17). It should be noted that
a differentiation of the enantiomers of substrate 1a in the
dehydrogenation process (leading to a kinetic resolution) was
not observed, thus enabling theoretically a quantitative
conversion of tetrahydroquinaldine 1a to the desired product
2a under mild conditions.
For further optimization of this reaction, mononuclear
vanadium complex (S)-D was chosen, which showed 36%
conversion already in the initial experiments under non-
optimized conditions (17 h at 50 °C; Table 1, entry 14). A
further reason for our decision to use catalyst (S)-D for process
B
DOI: 10.1021/acs.orglett.8b01484
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Organic Letters
Letter
conversions of the starting material 1a (Table 2, entries 10−
13). In order to gain high to quantitative conversions for the
dehydrogenation of substrate 1a, the impact of the reaction
time was studied: longer reaction times of 48 h (Table 2, entry
14) or 72 h (Table 2, entry 15) showed higher conversions of
the starting material 1a to quinaldine 2a (up to 90%
conversion after 72 h) (Table 2, entry 13), indicating a high
stability of the catalyst as well.
Encouraged by these results on the dehydrogenation of
1,2,3,4-tetrahydroquinaldine (1a), we further investigated the
effect of oxidants on the dehydrogenation using vanadium
complex (S)-D (Table 3): the addition of 1.5 equiv of
Inspired by these encouraging results on the dehydrogen-
ation of 1,2,3,4-tetrahydroquinaldine (1a), we were interested
in proposing a plausible reaction mechanism for the
dehydrogenation of 1a using oxovanadium catalysts (S)-D.
When the dehydrogenation of 1a was performed under inert
atmosphere, nearly no conversion to quinaldine (2a) was
observed (see the SI). This result is in accordance with our
further finding that the progress of the reaction under ambient
air is effected by the reaction vessel (see the SI). This indicates
that oxygen is needed as cosubstrate to accept hydrogen and
H2O2/H2O is formed as coupling product, being in accordance
with previous reports.3,9b In the first step of the proposed
mechanism, the amine is coordinated by vanadium to give I,
followed by a single-electron transfer (SET) from the electron-
rich nitrogen center to the vanadium complex yielding II.
Subsequent homolytic C−H bond cleavage gives an imine
which is released from the oxovanadium complex prior to
isomerization via a 1,4-dihydroquinaldine to 1,2-dihydroqui-
naldine. In addition, oxidation of the oxovanadium species with
O2 furnishes regeneration of the catalyst (S)-D, which then can
coordinate the formed 1,2-dihydroquinaldine leading to
complex III. Such steps are in accordance with computational
calculations by Sun et al.13 A further SET then gives IV, which
is converted in the final step to the (regenerated) vanadium
complex (S)-D under consumption of O2 and release of
quinaldine (2a) (Scheme 3). The single-electron transfer
hypothesis for the vanadium-amine complexes is in accordance
with the hypothesis by Zhu et al.19 for the related vanadium-
catalyzed oxidative Strecker reaction.
Table 3. Effect of Oxidants on the Dehydrogenation of
1,2,3,4-Tetrahydroquinaldine (1a)
c
entry
oxidant (equiv/atm)
temp (°C)
conv (%)
a
1
ambient air (1 atm)
ambient air (1 atm)
H2O2 (1.5 equiv)
TBHP (1.5 equiv)
O2 (1 atm)
60
rt
rt
rt
60
60
44
10
51
15
76
91
a
2
a
3
a
4
a
5
b
6
O2 (1 atm)
a
b
Variation of reaction time: 24 h. Variation of reaction time: 48 h.
c
1
Determined by H NMR spectroscopy via comparison of substrate
In order to explore the substrate scope of this methodology,
we studied dehydrogenations of other tetrahydroquinolines,
tetrahydroquinoxalines, 9,10-dihydroacridine, 1,2,3,4-tetrahy-
droisoquinoline, and 2-methylpiperidine (Scheme 4). We were
pleased to find that 1,2,3,4-tetrahydroquinoline (1b) as well as
tetrahydroquinolines 1c and 1d, which contain different
electron-donating substituents at position 6, were dehydro-
genated by the vanadium complex (S)-D efficiently, leading to
excellent conversions under the optimized reaction conditions.
The electron-deficient tetrahydroquinoline 1e was also
dehydrogenated by (S)-D with quantitative conversion. In
contrast, conversion was lower for tetrahydroquinolines 1f,
which contained an electron-withdrawing and sterically
demanding substituent at position 6, giving 89% conversion
after 48 h of reaction time (Scheme 4).
Encouraged by the promising results for the dehydrogen-
ation of other tetrahydroquinoline derivatives, we tested two
different tetrahydroquinoxalines 3a and 3b bearing different
substituents at position 2. In these cases, both quinoxalines
were obtained in quantitative conversion under the optimized
reaction conditions for the dehydrogenation with the
vanadium complex (S)-D. In addition, 9,10-dihydroacridine
(4a) was dehydrogenated by this vanadium catalyst with good
conversion of 82%, thus further underlining the value and
applicability of this catalytic methodology (Scheme 4).
and product integrals.
hydrogen peroxide increased the conversion of substrate 1a
from 10% to 51% (Table 3, entries 2, 3). In contrast, the
addition of 1.5 equiv of tert-butyl hydroperoxide showed
almost no improvement (Table 3, entry 4). When performing
the dehydrogenation reaction under 1 atm of oxygen at 60 °C,
we were pleased to find that the conversion increased
significantly to 76% after 24 h (Table 3, entry 5).
As oxygen represents a mild, environmentally friendly, and
easy-to-handle oxidant, we chose oxygen as oxidant for further
experiments. When extending the reaction time to 48 h,
1,2,3,4-tetrahydroquinaldine (1a) was converted under oxygen
atmosphere with a further improved conversion of 91% (Table
3, entry 6). With these optimized reaction conditions in hand,
we performed the dehydrogenation of 1,2,3,4-tetrahydroqui-
naldine (1a) on an elevated laboratory scale of 1 mmol
(Scheme 2). After 48 h of reaction time under oxygen
atmosphere, the starting martial 1a was dehydrogenated with
86% conversion and quinaldine (2a) was isolated in 66% yield
by flash column chromatography (Scheme 2). These results
underline the applicability of the dehydrogenation of
tetrahydroquinaldine (1a) by vanadium complex (S)-D.
Furthermore, we tested if 1,2,3,4-tetrahydroisoquinoline and
2-methylpiperidine are substrates for the dehydrogenation
using vanadium complex (S)-D: Unfortunately for both
substrates no product formation was observed (Scheme 4).
Based on these results, we concluded that an aryl-substituted
amine moiety is required as a structural prerequisite for the
dehydrogenation using vanadium complex (S)-D (Scheme 4).
In conclusion, we developed an efficient catalytic technology
for the dehydrogenation of a range of N-heterocycles based on
Scheme 2. Dehydrogenation of 1,2,3,4-
Tetrahydroquinaldine (1a) on a 1 mmol Scale
C
DOI: 10.1021/acs.orglett.8b01484
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Organic Letters
Letter
Scheme 3. Proposed Reaction Mechanism for the Dehydrogenation of 1,2,3,4-Tetrahydroquinaldine (1a)
ORCID
Scheme 4. Substrate Scope for the Dehydrogenation of N-
Heterocycles using Vanadium Complex (S)-D
a
Shinobu Takizawa: 0000-0002-9668-1888
̈
Harald Groger: 0000-0001-8582-2107
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge generous support from the German
Federal Ministry of Education and Research (BMBF) within
̈
the project “Biotechnologie 2020+, Nachste Generation
biotechnologischer Verfahren” (Grant No. 031A184A), as
well as from the German Academic Exchange Service (DAAD)
and Japan Society for the Promotion of Science (JSPS) within
the joint DAAD-JSPS funding program “DAAD PPP Japan
2017/2018” (DAAD Grant No. 57345562).
a
Conversions were determined by 1H NMR spectroscopy via
comparison of substrate and product integrals.
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ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01484.
Experimental procedures, characterization data and
NMR spectra for new compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: sasai@sanken.osaka-u.ac.jp.
*E-mail: harald.groeger@uni-bielefeld.de.
D
DOI: 10.1021/acs.orglett.8b01484
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