Oxidation Potential Tunable Organic Molecules and Their Catalytic Application to Aerobic Dehydrogenation of Tetrahydroquinolines

Article abstract of DOI:10.1021/acs.orglett.8b02749

In this work, oxidation potential tunable organic molecules, alkyl 2-phenyl hydrazocarboxylates, were disclosed. The exquisite tuning of their oxidation potentials facilitated a catalytic dehydrogenation of 1,2,3,4-tetrahydroquinolines in the presence of Mn(Pc) and O₂.

Full text of DOI:10.1021/acs.orglett.8b02749

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Oxidation Potential Tunable Organic Molecules and Their Catalytic
Application to Aerobic Dehydrogenation of Tetrahydroquinolines
Dahyeon Jung, Seol Heui Jang, Taeeun Yim,* and Jinho Kim*
Department of Chemistry and Research Institute of Basic Sciences, Incheon National University, 119 Academy-ro, Yeonsu-gu,
Incheon 22012, Republic of Korea
S
* Supporting Information
ABSTRACT: In this work, oxidation potential tunable organic
molecules, alkyl 2-phenyl hydrazocarboxylates, were disclosed. The
exquisite tuning of their oxidation potentials facilitated a catalytic
dehydrogenation of 1,2,3,4-tetrahydroquinolines in the presence of
Mn(Pc) and O2.
elective oxidative transformations using molecular oxygen
pyridine (DMAP).11 However, these redox-active organic
Sare of importance in organic synthesis because molecular molecules exhibited limited redox potentials because the
synthesis of their derivatives having various electronic
environments was quite restricted.
oxygen is inexpensive and readily accessible and in many cases
produces only water as the byproduct.1 However, the direct
oxidation of an organic substrate by O2 is not facile due to the
high energy barrier for electron transfer from a singlet organic
substrate to triplet molecular oxygen. To circumvent the high
energy barrier, catalytic oxidation reactions using a substrate-
Our continued interest in developing new aerobic oxidative
transformations12 has prompted us to investigate the
possibility of alky 2-phenyl hydrazocarboxylates as oxidation
potential tunable catalysts in aerobic oxidative transformations
because of two reasons. First, various alkyl 2-phenyl
hydrazocarboxylates having different electronic environments
on the aromatic ring could be readily synthesized by the
coupling of the corresponding phenylhydrazines with alkyl
chloroformates (Scheme 1a).13 We envisioned that the
oxidation potentials of alkyl 2-phenyl hydrazocarboxylates
were controllable through the variation of the substituents on
the phenyl group. Second, the aerobic oxidation of alkyl 2-
phenyl hydrazocarboxylates was able to be catalyzed by
inexpensive metal complexes such as CuI and Fe(Pc) (Pc =
selective catalyst, which may often be a transition metal (Mn+2
/
Mn), have been developed.2 The used catalyst oxidizes the
organic substrates to the desired products, and the unreactive
reduced catalyst is generated. Then, the reduced catalyst is
subsequently reoxidized by molecular oxygen for the
regeneration of an active catalyst. Sometimes, more than one
coupled redox system with electron transfer mediators (ETMs)
was required to facilitate the regeneration of the used catalyst
by O2.3
Redox-active organic molecules could also be employed as
substrate-selective catalysts. Quinones are representative
redox-active organic molecules which are used as catalysts in
various aerobic oxidative transformations,4 predominantly
amine oxidations.5 For example, Wendlandt and Stahl
developed bioinspired 1,10-phenanthroline-5,6-dione (phd)-
catalyzed aerobic oxidation of secondary amines and nitrogen-
containing heterocycles.6 Nitroxyl radicals are also employed
as substrate-selective catalysts in aerobic alcohol oxidations.7 In
the catalytic cycle, oxoammonium salts oxidize organic
substrates,8 and the aerobic oxidation of the reduced hydroxyl
amines takes place through NOx redox.9,10 Recently, our group
has revealed that di-tert-butyl azodicarboxylate (DBAD) was
able to catalyze aerobic dehydrogenation of 1,2,3,4-tetrahy-
droquinolines in the presence of CuI and 4-dimethylamino
Scheme 1. (a) Synthesis of Various Alkyl 2-Phenyl
Hydrazocarboxylates and (b) Aerobic Oxidation of Alkyl 2-
Phenyl Hydrazocarboxylates Catalyzed by Fe(Pc) or Cu
Received: August 28, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02749
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
phthalocyanine) (Scheme 1b).14 Herein, we describe the
oxidation potentials of tunable organic molecules and their
catalytic application to aerobic dehydrogenation of 1,2,3,4-
tetrahydroquinolines.
Table 1. Dehydrogenation of 6-Methyl Tetrahydroquinoline
Mediated by Ethyl 2-Phenyl Azocarboxylates
a
To examine our hypothesis, a series of ethyl 2-phenyl
hydrazocarboxylates 1 having different substituents at the para
position of the phenyl group were synthesized, and their
electrochemical oxidation potentials were determined by linear
sweep voltammetry (LSV) (Figure 1).15,16 It was revealed that
c
yield (%)
b
E/V (vs Ag/Ag+)
σp
rt
70 °C
+
entry
R
1
2
3
4
5
6
7
8
OMe
Me
F
H
Br
CF3
CN
NO2
2a
2b
2c
2d
2e
2f
0.57
0.68
0.77
0.81
0.84
1.01
1.01
1.00
−0.78
−0.31
−0.07
0.00
0.15
0.61
0
1
3
7
16
34
37
74
99
99
99
4
11
36
98
100
2g
2h
0.66
0.79
a
Reaction conditions: 3a (0.5 mmol) and 2 (1.1 mmol) in CH3CN
b
(1.0 mL) under N2 at room temperature or 70 °C for 12 h. The
oxidation potentials of the corresponding hydrazocarboxylates were
c
obtained at the highest current. Yield determined by 1H NMR
spectroscopy (internal standard: 1,1,2,2-tetrachloroethane).
ethyl 2-(4-nitrophenyl)azocarboxylate 2h, for example, effi-
ciently mediated the dehydrogenation of 3a in a quantitative
yield at room temperature (entry 8). These results indicate
that the stoichiometric dehydrogenations of 1,2,3,4-tetrahy-
droquinolines were largely affected by the electrophilicities of
the used azo compounds.22
In order to achieve an aerobic dehydrogenation of 1,2,3,4-
tetrahydroquinolines using catalytic amounts of 2h, several
metal phthalocyanine complexes were examined for the aerobic
oxidation of ethyl 2-(4-nitrophenyl)hydrazocarboxylate 1h
(Table 2). It was observed that Mn(Pc) and Fe(Pc) showed
Figure 1. LSV for various ethyl 2-phenyl hydrazocarboxylates 1
(working electrode: glassy carbon, counter electrode: Pt wire,
reference electrode: Ag/Ag+, supporting electrolyte: 0.1 M lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI) in CH3CN, scan rate:
100 mV s−1, scan ranges: open-circuit potential to 2.0 V (vs Ag/
Ag+)).
the electrochemical oxidation potentials of 1 strongly
depended on the class of substituent: 1 having the more
electron-withdrawing substituent showed the higher oxidation
potential. Interestingly, approximately linear correlations (r =
0.9834) were observed between the obtained oxidation
potentials of 1 and Hammett constants (electrophilic
Table 2. Aerobic Oxidation of 2-(4-
a
Nitrophenyl)hydrazocarboxylate
17,18
+
substituent constants, σp ) (Figure 2).
These results
indicate that the oxidation potentials of 1 were tunable and
predictable on the basis of Hammett constants.19,20
b
entry
catalyst
Mn(Pc)
Fe(Pc)
Co(Pc)
Mn(acac)3
Mn(OAc)2·4H2O
MnO2
yield (%)
1
2
3
4
5
6
7
8
94
93
0
88
0
1
0
0
FeCl3
-
a
Reaction conditions: 1h (0.5 mmol) and catalyst (10 mol %) in
CH3CN (1.0 mL) under O2 balloon at room temperature for 12 h.
b
Yield determined by 1H NMR spectroscopy (internal standard:
Figure 2. Relationship between oxidation potentials of various 1 and
Hammett constants (electrophilic substituent constants, σp ).
+
1,1,2,2-tetrachloroethane).
Next, we tried to utilize 1 as catalysts in the aerobic
dehydrogenations of 1,2,3,4-tetrahydroquinolines. First of all,
the reactivity of the ethyl 2-phenyl azocarboxylates 2, which are
oxidized forms of 1,14c was investigated in the stoichiometric
dehydrogenation of 6-methyl-1,2,3,4-tetrahydroquinoline 3a
(Table 1).21 Interestingly, the dehydrogenating activity of 2
highly depended on the substituent of the phenyl group. It was
revealed that 2 having the more electron-withdrawing
substituent resulted in the higher product yield. The use of
excellent catalytic activity;14b however, Co(Pc) could not
catalyze the aerobic oxidation of 1h (entries 1−3). Among the
other manganese sources screened, Mn(acac)3 showed a
considerable yield, while the aerobic oxidations of 1h using
Mn(OAc)2·4H2O or MnO2 were sluggish (entries 4−6). The
use of FeCl3 as a catalyst could not catalyze the aerobic
oxidation of 1h (entry 7). No autoxidation of 1h was observed
for 12 h (entry 8).
B
DOI: 10.1021/acs.orglett.8b02749
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
We then investigated the cooperation between the
dehydrogenation of 1,2,3,4-tetrahydroquinoline mediated by
2h and the aerobic oxidative regeneration of 2h with three
potent catalysts including Mn(Pc), Fe(Pc), and Mn(acac)3
(Table 3). Gratifyingly, the coupled catalytic system with 1h,
The substrate scope of 1,2,3,4-tetrahydroquinolines using
the developed coupled catalytic system (Table 3, entry 7) is
elucidated in Scheme 3. Although the aerobic dehydrogen-
Scheme 3. Substrate Scope of Aerobic Dehydrogenation of
1,2,3,4-Tetrahydroquinolines Catalyzed by Mn(Pc) and 2-
a
Table 3. Optimization for Dehydrogenation of 6-Methyl
(4-Nitrophenyl) Hydrazocarboxylate
a
Tetrahydroquinoline with Coupled Catalytic System
b
entry
catalyst
hydrazine 1
solvent
temp (°C) yield (%)
1
2
3
4
5
6
7
8
9
Mn(Pc)
Fe(Pc)
Mn(acac)3
Mn(Pc)
Mn(Pc)
Mn(Pc)
Mn(Pc)
Mn(Pc)
Mn(Pc)
Mn(Pc)
-
1h
1h
1h
1h
1h
1h
1h
1f
1g
1h
1h
-
CH3CN
CH3CN
CH3CN
DMF
toluene
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
rt
rt
rt
rt
70
29
40
62
12
79
90
26
52
29
0
rt
50
70
70
70
70
70
70
70
c
10
11
12
a
Reaction conditions: 3 (0.5 mmol), Mn(Pc) (10 mol %), and 1h (10
mol %) in CH3CN (1.0 mL) under O2 balloon for 15 h. Isolated yield.
Mn(Pc)
Mn(Pc)
6
8
c
d
d
Increased amount of 1h was used (30 mol %). Yield determined by
13
1h
1H NMR spectroscopy (internal standard: 1,1,2,2-tetrachloroethane)
a
Reaction conditions: 3a (0.5 mmol), catalyst (10 mol %), and 1 (10
b
mol %) in solvent (1.0 mL) under O2 balloon for 15 h. Yield
determined by 1H NMR spectroscopy (internal standard: 1,1,2,2-
ations of 3a and 3b produced the corresponding quinolines in
good yield, other tetrahydroquinolines having a methyl group
at different positions showed poor results. Increased yields
were able to be obtained when increased amounts of hydrazine
were used (30 mol %) (4c−4e). Parent quinoline and other 6-
substitued quinolines were synthesized by the modified
reaction conditions in good yields (4f−4i). Various 2-phenyl
tetrahydroquinolines were investigated in the developed
dehydrogenation. Interestingly, it was revealed that the
substituents of the 2-phenyl group affected the reactivity.
While 2-phenyl tetrahydroquinolines which have no sub-
stituent or electron donating at the para position efficiently
underwent the present dehydrogenation (4j−4l), the dehy-
drogenation of 3m and 3n required increased amounts of 1h to
obtain acceptable yields. It was observed that 4-phenyl
tetrahydroquinolines, which were synthesized by reductive
amination and the sequential cyclization,24 were transformed
to the corresponding quinolines in the presence of 30 mol % of
1h (4o−4q). Acridine could be synthesized efficiently by the
present aerobic dehydrogenation of 9,10-dihydroacridine (4r).
The dehydrogenations of electron-deficient tetrahydroquno-
lines such as 3s and 3t exhibited poor conversions with good
mass balances, even in high loading of 1h (the mass balances of
3s and 3t were 91% and 96%, respectively). In the context of
the data in Table 1, these results imply that the electro-
philicities of azo compounds and the nucleophilicities of
1,2,3,4-tetrahydroquinolines are crucial in the present dehy-
drogenation.
c
d
tetrachloroethane). Under air. Under N2.
Mn(Pc), and O2 facilitated the dehydrogenation of 1,2,3,4-
tetrahydroquinoline,23 while both Fe(Pc) and Mn(acac)3
exhibited poor cooperations (entries 1−3). The use of other
solvents such as DMF or toluene, instead of CH3CN, did not
increase the product yield (entries 4 and 5). The higher yields
were obtained as the reaction temperature was increased
(entries 6 and 7). The use of 1f or 1g, instead of 1h, as a
catalyst produced 4a in 26% or 52% yields, respectively
(entries 8 and 9). When the present coupled catalytic system
was carried out under air, only 29% of quinoline was produced
(entry 10). In order to verify that 2h-mediated dehydrogen-
ation and aerobic regeneration of 2h were conducted
independently, control experiments were carried out. Without
Mn(Pc), no production of quinoline was observed (entry 11).
When the dehydrogenation of 3a was carried out without
hydrazine or O2, the poor yields of 4a were observed (entries
12 and 13). These results indicate that no autoxidation of 1h
occurred; Mn(Pc)-catalyzed direct oxidation of 3a was
sluggish; and molecular oxygen is an essential terminal oxidant.
The simplified mechanism proposal for the developed aerobic
dehydrogenation of 1,2,3,4-tetrahydroquinoline is depicted in
Scheme 2.
Scheme 2. Simplified Mechanism Proposal for the
Developed Coupled Catalytic System
In conclusion, we revealed that the oxidation potential of
ethyl 2-phenyl hydrazocarboxylates could be tunable by the
substitution of the phenyl group. The higher oxidation
potentials of ethyl 2-phenyl hydrazocarboxylates were observed
as stronger electron-withdrawing groups were substituted at
the para position of the phenyl ring, and the observed
oxidation potentials were highly correlated to Hammett para-
C
DOI: 10.1021/acs.orglett.8b02749
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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substituent constants. From the exquisite tuning of oxidation
potentials, we could develop a coupled catalytic system for the
aerobic dehydrogenation of 1,2,3,4-tetrahydroquinolines using
a Mn(Pc) and 1h redox couple. The control experiments
reveal that the catalytic cycles were conducted independently
and cooperatively. A variety of 1,2,3,4-tetrahydroquinolines
underwent dehydrogenation in the presence of a catalytic
amount of Mn(Pc) and 1h to produce the corresponding
quinolines. Further studies understanding mechanistic details
for each catalytic cycle are underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b02749.
(10) For aerobic oxidations catalyzed by less sterically hindered
bicyclic nitroxyl radicals, see: (a) Shibuya, M.; Osada, Y.; Sasano, Y.;
Tomizawa, M.; Iwabuchi, Y. J. Am. Chem. Soc. 2011, 133, 6497−6500.
(b) Kuang, Y.; Nabae, Y.; Hayakawa, T.; Kakimoto, M. Green Chem.
2011, 13, 1659−1663. (c) Lauber, M. B.; Stahl, S. S. ACS Catal. 2013,
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S.; Iwabuchi, Y. Chem. - Asian J. 2015, 10, 1004−1009. (e) Shibuya,
M.; Furukawa, K.; Yamamoto, Y. Synlett 2017, 28, 1554−1557.
(11) Jung, D.; Kim, M. H.; Kim, J. Org. Lett. 2016, 18, 6300−6303.
(12) (a) Noh, J.-H.; Kim, J. J. Org. Chem. 2015, 80, 11624−11628.
(b) Yoon, Y.; Kim, B. R.; Lee, C. Y.; Kim, J. Asian J. Org. Chem. 2016,
5, 746−749. (c) Kim, M. J.; Jung, Y. E.; Lee, C. Y.; Kim, J.
Tetrahedron Lett. 2018, 59, 2722−2725.
Experimental procedures, spectroscopic data, and copies
1
of H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yte0102@inu.ac.kr (T. Yim).
*E-mail: jinho@inu.ac.kr (J. Kim).
ORCID
̌
(13) Urankar, D.; Steinbucher, M.; Kosjek, J.; Kosmrlj, J.
̈
Tetrahedron 2010, 66, 2602−2613.
Taeeun Yim: 0000-0002-7057-9308
Jinho Kim: 0000-0002-3592-9026
Notes
(14) (a) Hirose, D.; Taniguchi, T.; Ishibashi, H. Angew. Chem., Int.
Ed. 2013, 52, 4613−4617. (b) Hashimoto, T.; Hirose, D.; Taniguchi,
T. Adv. Synth. Catal. 2015, 357, 3346−3352. (c) Kim, M. H.; Kim, J.
J. Org. Chem. 2018, 83, 1673−1679. For Pd-catalyzed aerobic
oxidation of alkyl 2-phenyl hydrazocarboxylates, see: (d) Gaviraghi,
G.; Pinza, M.; Pifferi, G. Synthesis 1981, 1981, 608−610.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (MSIP) (NRF-2018R1A1A1A05019774) and
(NRF-2017R1A6A1A06015181).
̌
̈
(15) Jurmann, G.; Tsubrik, O.; Tammeveski, K.; Maeorg, U. J. Chem.
̈
Res. 2005, 2005, 661−662.
(16) The oxidation potentials of ethyl 2-phenyl hydrazocarboxylates
were obtained at the highest current. The values of oxidation
potentials are listed in Table 1.
(17) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165−195.
(18) The r values of the correlation between the obtained oxidation
potentials of ethyl 2-phenyl hydrazocarboxylates and other Hammett
constants such as σp and σp− were shown as 0.9233 and 0.8242. See
the Supporting Information for details.
(19) For a review for the experimental redox potentials of various
organic molecules, see: Warren, J. J.; Tronic, T. A.; Mayer, J. M.
Chem. Rev. 2010, 110, 6961−7001.
(20) For computational calculation of the substituted quinones, see:
Huynh, M. T.; Anson, C. W.; Cavell, A. C.; Stahl, S. S.; Hammes-
Schiffer, S. J. Am. Chem. Soc. 2016, 138, 15903−15910.
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B.; Kim, J. Synth. Commun. 2018, 48, 1291−1298.
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DOI: 10.1021/acs.orglett.8b02749
Org. Lett. XXXX, XXX, XXX−XXX