Dehydrogenation of 1,2,3,4-tetrahydroquinoline and its related compounds: Comparison of Pd/C-ethylene system and activated carbon-O2 system

Article abstract of DOI:10.1016/j.tetlet.2010.06.118

Dehydrogenation of substituted 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroisoquinoline, and 1,2,3,4-tetrahydrocarbazole proceeded using Pd/C-ethylene system (method A) or activated carbon-O₂ system (method B) to give the corresponding heteroaromatic compounds.

Full text of DOI:10.1016/j.tetlet.2010.06.118

Tetrahedron Letters 51 (2010) 4633–4635
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Tetrahedron Letters
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Dehydrogenation of 1,2,3,4-tetrahydroquinoline and its related compounds:
comparison of Pd/C–ethylene system and activated carbon–O2 system
*
Takanori Tanaka, Ken-ichi Okunaga, Masahiko Hayashi
Department of Chemistry, Graduate School of Science, Kobe University, Kobe 657-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Dehydrogenation of substituted 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydroisoquinoline, and 1,2,3,4-
tetrahydrocarbazole proceeded using Pd/C–ethylene system (method A) or activated carbon–O2 system
(method B) to give the corresponding heteroaromatic compounds.
Received 22 June 2010
Accepted 25 June 2010
Available online 30 June 2010
Ó 2010 Published by Elsevier Ltd.
Keywords:
Activated carbon
Oxidation
Heteroaromatic compounds
We previously reported the oxidation method of benzylic and
allylic alcohols using Pd/C–ethylene system.1 After that, we found
some kinds of activated carbons themselves promoted the oxidation
including oxidative aromatization, carbonylation, and oxidation of
benzylic alcohols.2 In the course of the above study, we briefly men-
tioneddehydrogenationof1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetra-
hydroisoquinoline, and 1,2,3,4-tetrahydrocarbazole proceeded using
Pd/C–ethylene system to afford the corresponding heteroaromatic
compounds in 2002.3 Recently, Luo and Crabtree reported
dehydrogenation reaction of nitrogen- and oxygen-containing het-
erocycles in 2006.4 Furthermore, there have been some reports on
dehydrogenative oxidation of amines using transition metal such as
Cp*Ir complex,5 Cu-salene complex,6 Co3O4-supported hydrous
RuO2,7 supported ruthenium hydroxide catalysis (Ru(OH)x/Al2O3),8
and dirhodium caprolactamate.9 Here, we reported that
dehydrogenation of 1,2,3,4-tetrahydroquinoline, 1,2,3,4-tetrahydro-
isoquinoline, and 1,2,3,4-tetrahydrocarbazole proceeded using Pd/
C–ethylene system (method A). Furthermore, we have revealed that
only activated carbon promotes this reaction in the presence of oxy-
gen (method B) (Scheme 1). As for oxidative conversion of 1,2,3,4-tet-
rahydroquinoline (1) to quinoline (7), Pratt and McGovern reported
the method using 8 equiv of MnO2 in benzene at 81 °C to give 7 in
79% yield.10 Concerning oxidative conversion of the compound 5,
Wolthuis reported the method using 50 wt % of 5% Pd/C in mesitylene
at reflux temperature (bp 162–164 °C) to give 11 in 72% yield.11
Pd/C–ethylene system (method A): When 1,2,3,4-tetrahydro-
quinolines (1, 2, and 3), 1,2,3,4-tetrahydroisoquinoline (4), and
1,2,3,4-tetrahydrocarbazoles (5 and 6) were treated with 50 wt %
of 10% Pd/C in acetonitrile under ethylene atmosphere, the corre-
sponding hetero aromatic compounds (7–12), that is, quinolines,
isoquinolines, and carbazoles were obtained in high yield (Table 1).
We suggested that method A included hydrogen transfer reac-
tion. To confirm this, in the Pd/C–ethylene system, we actually de-
tected the generation of ethane by GC analysis (Hewlett-Packard
5890 series II, Porapak R column: 40 °C: retention time (tR) of eth-
ylene, 2.3 min; tR of ethane, 3.3 min) (Scheme 2). Excess amount of
ethylene was also observed.
It should be mentioned that in the presence of ethylene, the
product was only heteroaromatic compound, whereas, in the ab-
sence of ethylene, the reaction gave some by-products. These re-
sults will be consistent with the above results, that is, ethylene is
smoothly hydrogenated to ethane, therefore, we judge that method
A involves hydrogenation transfer reaction.
Activated carbon–O2 system (method B): We found that the con-
version of 1,2,3,4-tetrahydroquinolines (1, 2, and 3), 1,2,3,4-tetra-
method A
R
R
method B
N
H
N
method A
method B
NH
N
R
method A
method B
R
N
N
H
H
method A: Pd/C__ ethylene, method B: activated carbon__O2
Scheme 1. Dehydrogenation of nitrogen-containing 1,2,3,4-tetrahydroaromatic
compounds using Pd/C–ethylene system (method A) or activated carbon–O2 system
(method B).
* Corresponding author. Tel.: +81 78 803 5687; fax: +81 78 803 5688.
E-mail address: mhayashi@kobe-u.ac.jp (M. Hayashi).
0040-4039/$ - see front matter Ó 2010 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2010.06.118

4634
T. Tanaka et al. / Tetrahedron Letters 51 (2010) 4633–4635
Table 1
In summary, dehydrogenation of substituted 1,2,3,4-tetrahy-
droquinoline (1, 2, and 3), 1,2,3,4-tetrahydroisoquinoline (4), and
1,2,3,4-tetrahydrocarbazoles (5 and 6) proceeded using Pd/C–eth-
ylene system (method A) or activated carbon–O2 system (method
B) to give the corresponding heteroaromatic compounds. When
we compared method A with method B, for the oxidation of
1,2,3,4-tetrahydroquinolines (1, 2, and 3) and 1,2,3,4-tetrahydro-
isoquinoline (4), method B was found to be superior in the point
of reactivity. On the other hand, for 1,2,3,4-tetrahydrocarbazoles
(5 and 6), method A afforded higher yield compared with method
B.12–16
Oxidation of 1,2,3,4-tetrahydroquinolines (1, 2, and 3), 1,2,3,4-tetrahydroisoquinoline
(4), and 1,2,3,4-tetrahydrocarbazols (5 and 6) using Pd/C–ethylene system (method A)
and activated carbon–O2 system (method B).
Entry Substrate
Product
Yielda (%)
Method Ab Method Bc
1
2
75
40
86
86
N
H
N
1
7
Me
Me
Acknowledgments
N
H
N
2
8
This work was supported by a Grant-in-Aid for Scientific Re-
search on Priority Areas ‘Advanced Molecular Transformations of
Carbon Resources’ and No. B17340020 from the Ministry of Educa-
tion, Culture, Sports, Science, and Technology, Japan.
MeO
MeO
3
4
39
50
87
N
H
N
3
9
References and notes
NH
NH
75d
1. (a) Hayashi, M.; Yamada, K.; Arikita, O. Tetrahedron Lett. 1999, 40, 1171–1174;
(b) Hayashi, M.; Yamada, K.; Arikita, O. Tetrahedron 1999, 55, 8331–8340; (c)
Hayashi, M.; Yamada, K.; Nakayama, S.; Hayashi, H.; Yamazaki, S. Green Chem.
2000, 257–260.
4
10
2. Hayashi, M. Chem. Rec. 2008, 8, 252–267. and references cited therein.
3. Hayashi, M.; Kawabata, H. J. Synth. Org. Chem. Jpn. 2002, 60, 137–144.
4. Moores, A.; Poyatos, M.; Luo, Y.; Crabtree, R. H. New J. Chem. 2006, 30, 1675–
1678.
5. Yamaguchi, R.; Ikeda, C.; Takahashi, Y.; Fujita, K. J. Am. Chem. Soc. 2009, 131,
8410–8412.
6. Wu, X.; Gorden, A. E. V. Eur. J. Org. Chem. 2009, 503–509.
7. Li, F.; Chen, J.; Zhang, Q.; Wang, Y. Green Chem. 2008, 10, 553–562.
8. Mizuno, N.; Yamaguchi, K. Catal. Today 2008, 132, 18–26.
5
6
92e
77
N
N
H
H
5
11
Cl
Cl
69e
53e
N
N
H
H
9. Choi, H.; Doyle, M. P. Chem. Commun. 2007, 745–747.
6
10. Pratt, E. F.; McGovern, T. P. J. Org. Chem. 1964, 29, 1540–1543.
11. Wolthuis, E. J. Chem. Educ. 1979, 56, 343.
12
a
b
c
Isolated yield by silica gel column chromatography unless otherwise noted.
Method A: Pd/C–ethylene, in CH3CN, 50 wt % of 10% Pd/C, 100 °C, 96 h.
Method B: activated carbon, in xylene, 100 wt % activated carbon, 120 °C, 24 h.
96 h.
12. Typical procedure for the oxidation of tetrahydroheteroaromatic compounds to
heteroaromatic compounds by activated Pd/C–ethylene system: Synthesis of
quinoline (7). A dry Schlenk tube containing stirring bar under an ethylene
atmosphere was charged with 1,2,3,4-tetrahydroquinoline (266.4 mg, 2 mmol)
and acetonitrile (1.5 mL). To this mixture was added 50 wt % of 10% Pd/C and
the mixture was stirred vigorously at 100 °C for 96 h under ethylene
atmosphere using a balloon. After confirmation of the completion of the
reaction by TLC, the palladium precipitate was filtered off. After removal of the
solvent, the product was purified by silica gel column chromatography to give
quinoline in 194.1 mg (75%). All spectral data of the products were identical
with those of commercially available authentic samples. Rf = 059 (hexane/ethyl
acetate = 1:2); IR (KBr): vmax (cmÀ1) 3056, 3036, 1932, 1620, 1596, 1529, 1570,
d
e
Isolated yield by recrystallization.
1501, 1313, 1118, 939, 805, 611; 1H NMR (400 MHz, CDCl3):
d 8.92 (d,
J = 4.0 Hz, 1H), 8.15 (d, J = 8.0 Hz, 1H), 8.11 (d, J = 8.8 Hz, 1H), 7.82 (d, J = 7.6 Hz,
1H), 7.73 (t, J = 7.2 Hz, 1H), 7.56 (d, J = 7.2 Hz, 1H), 7.40 (dd, J = 4.4, 4.4 Hz, 1H).
13. Typical procedure for the oxidation of 1,2,3,4-tetrahydroquinoline to quinoline by
activated carbon–O2 system: Synthesis of quinoline (7). A mixture of 1,2,3,4-
tetrahydroquinoline (133.2 mg, 1 mmol), 100 wt % of activated carbon
(Charcoal Activated, TOKYO CHEMICAL INDUSTRY CO., LTD (TCI)), and
anhydrous xylene (5 mL) was placed in
a three-necked flask under an
Scheme 2. Detection of the generation of ethane using Pd/C–ethylene system.
oxygen atmosphere using a balloon. The whole was heated to 120 °C and
stirred for 24 h at this temperature. After confirmation of the completion of the
reaction by TLC analysis (hexane/ethyl acetate = 1:2), activated carbon was
filtered off using Celite. After washing with ethyl acetate, the filtrate was
evaporated and the resulting liquid was purified by silica gel column
chromatography. Quinoline was obtained in 111.1 mg (86% yield).
hydroisoquinoline (4), and 1,2,3,4-tetrahydrocarbazoles (5 and 6)
to the corresponding heteroaromatic compounds also proceeded
by the aid of only activated carbon under the oxygen atmosphere.
The results are summarized in Table 1. As for the role of activated
carbon as a promoter, after examining the effects on reactivity in
the oxidative aromatization, we found that the essential role of
activated carbon in this oxidation system is concerned not only
with specific surface area, pore volume, mean pore diameter and
surface oxygen groups evolving as CO2 at 900 °C but also with sur-
face oxygen groups evolving as CO during heating at 900 °C, such
as carbonyl groups on the surface of activated carbon.
14. Synthesis of isoquinoline (10).
A
mixture of various 1,2,3,4-
tetrahydroisoquinoline (133 mg, 1 mmol), 100 wt % of activated carbon
(Charcoal Activated, TOKYO CHEMICAL INDUSTRY CO., LTD (TCI)), and
anhydrous xylene (5 mL) was placed in
a three-necked flask under an
oxygen atmosphere using a balloon. The whole was heated to 120 °C and
stirred for 96 h at this temperature. After confirmation of the completion of the
reaction by TLC analysis (hexane/ethyl acetate = 1:2), activated carbon was
filtered off using Celite. After washing with ethyl acetate, the filtrate was
evaporated and the resulting liquid was purified by silica gel column
chromatography. Isoquinoline was obtained in 96.9 mg (75% yield). Rf = 0.4
(hexane/ethyl acetate = 1:2); IR (KBr): vmax (cmÀ1) 3402, 3056, 1924, 1627,
1588, 1498, 1382, 1273, 1141, 1014, 944, 862, 827, 742, 638; 1H NMR
(400 MHz, CDCl3): d 9.20 (s, 1H), 8.52 (d, J = 6.0 Hz, 1H), 7.96 (d, J = 8.4 Hz, 1H),
7.81 (d, J = 8.4 Hz, 1H), 7.7–7.6 (m, 3H).
This reaction would involve dehydration (not dehydrogenation)
via radical species. It should be noted that in the absence of acti-
vated carbon the reaction of tetrahydroquinoline 1 did not take
place even under O2 atmosphere at 120 °C for 24 h.
15. Synthesis of carbazole (11). A mixture of various 1,2,3,4-tetrahydrocarbazole
(171 mg, 1 mmol), 100 wt % of activated carbon (Charcoal Activated, TOKYO
CHEMICAL INDUSTRY CO., LTD (TCI)), and anhydrous xylene (5 mL) was placed

T. Tanaka et al. / Tetrahedron Letters 51 (2010) 4633–4635
4635
in a three-necked flask under an oxygen atmosphere using a balloon. The
whole was heated to 120 °C and stirred for 24 h at this temperature. After
confirmation of the completion of the reaction by TLC analysis (hexane/ethyl
acetate = 2:1), activated carbon was filtered off using Celite. After washing with
ethyl acetate, the filtrate was evaporated and the resulting solid was purified
by silica gel column chromatography. Carbazole was obtained in 128.8 mg
(77%). Rf = 0.62 (hexane/ethyl acetate = 2:1); mp 244–246 °C (lit.17
243.5–246 °C); IR (KBr): vmax (cmÀ1) 3418, 3049, 2924, 1601, 1494, 1451,
1327, 1237, 928, 748, 725; 1H NMR (400 MHz, CDCl3): d 11.3 (br s, 1H), 8.10 (d,
J = 7.6 Hz, 2H), 7.47 (d, J = 8.0 Hz, 2H), 7.38 (t, J = 8.0 Hz, 2H), 7.15 (t, J = 7.6 Hz,
1H).
16. For all other products, IR and 1H NMR spectra were identified with authentic
compounds.
17. Akermark, B.; Eberson, L.; Jonsson, E.; Pettersson, E. J. Org. Chem. 1975, 40,
1365–1367.