Homogeneous perdehydrogenation and perhydrogenation of fused bicyclic N-heterocycles catalyzed by iridium complexes bearing a functional bipyridonate ligand

Article abstract of DOI:10.1021/ja5001888

Homogeneous perdehydrogenation of saturated bicyclic 2,6-dimethyldecahydro- 1,5-naphthyridine and perhydrogenation of aromatic 2,6-dimethyl-1,5- naphthyridine with release and uptake of five molecules of H₂ are efficiently achieved by iridium complexes bearing a functional bipyridonate ligand. Successive perhydrogenation and perdehydrogenation of 2,6-dimethyl-1,5-naphthryridine using a single iridium complex also proceed with the reversible interconversion of the catalytic species, depending on the presence or absence of H₂.

Full text of DOI:10.1021/ja5001888

Communication
pubs.acs.org/JACS
Homogeneous Perdehydrogenation and Perhydrogenation of Fused
Bicyclic N‑Heterocycles Catalyzed by Iridium Complexes Bearing
a Functional Bipyridonate Ligand
Ken-ichi Fujita,* Yui Tanaka, Masato Kobayashi, and Ryohei Yamaguchi*
Graduate School of Human and Environmental Studies, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
S
* Supporting Information
system is rather low.10 Therefore, it is highly desirable to develop
ABSTRACT: Homogeneous perdehydrogenation of sa-
turated bicyclic 2,6-dimethyldecahydro-1,5-naphthyridine
and perhydrogenation of aromatic 2,6-dimethyl-1,5-naph-
thyridine with release and uptake of five molecules of H2
are efficiently achieved by iridium complexes bearing a func-
tional bipyridonate ligand. Successive perhydrogenation and
perdehydrogenation of 2,6-dimethyl-1,5-naphthryridine using
a single iridium complex also proceed with the reversible inter-
conversion of the catalytic species, depending on the pres-
ence or absence of H2.
a new catalytic system in which a completely saturated
N-heterocycle with higher hydrogen capacity is employed. In
order to achieve this catalytic system, it is considered essential
that perdehydrogenation of saturated N-heterocycles and
perhydrogenation of aromatic N-heterocycles are developed,
although it is generally very difficult to accomplish these catalytic
processes.11 We report here the efficient homogeneous
perdehydrogenation of 2,6-dimethyldecahydro-1,5-naph-
thyridine and perhydrogenation of 2,6-dimethyl-1,5-naph-
12
thyridine with release and uptake of five molecules of H2
catalyzed by Cp*Ir complexes bearing functional bipyridonate
ligands as a single precatalyst (eq 2).
atalytic dehydrogenation and hydrogenation of N-hetero-
Quite recently, we have reported the high catalytic activity
of Cp*Ir complexes (2) bearing a bipyridonate ligand for the
dehydrogenative oxidation of a variety of primary and secondary
alcohols.13d Thus, we first examined the catalytic performance
of Cp*Ir complexes 1 and 2a−2d in the dehydrogenation of
2-MeTHQ. The results are shown in Table 1. It was apparent
C
cycles are basic and important organic transformations.
In addition, these transformations have recently attracted
considerable attention from viewpoints of organic hydrides
for hydrogen storage systems,1,2 because the dehydrogenation of
N-heterocycles is more favorable as compared to that of
cycloalkanes by decreasing the endothermicity of the reac-
tions.3−5 Although many catalytic systems using heterogeneous
and homogeneous metal catalysts have been reported,4−8 most
of them utilize heterogeneous catalysts and each of the
transformations has been carried out by using different metal
catalysts. The catalytic dehydrogenation and hydrogenation
system using a single metal catalyst is very rare.9
Table 1. Dehydrogenation of 2-MeTHQ Catalyzed by Various
a
Ir Complexes
We have recently reported that a homogeneous catalytic
system for reversible dehydrogenation−hydrogenation between
2-methyl-1,2,3,4-tetrahydroquinoline (2-MeTHQ) and 2-
methylquinoline (2-MeQ) with release and uptake of two
molecules of H2 can be achieved by using a Cp*Ir complex (1)
bearing a 2-pyridonate ligand (Cp* = pentamethylcyclopenta-
dienyl) (eq 1).7a However, only the N-heterocyclic part of the
a
The reaction was carried out with 2-MeTHQ (2 mmol) and the
b
catalyst (1.0 mol %) under reflux in p-xylene for 20 h. Determined
by GC.
that the new complexes 2 exhibited higher catalytic activities than
the previous complex 1.
Having the complexes 2 with high catalytic activity for
dehydrogenation of 2-MeTHQ, we next investigated the
perdehydrogenation of saturated bicyclic 2,6-dimethyldeca-
hydro-1,5-naphthyridine (3) as the substrate, because 3 contains
bicyclic quinoline system participates in both transformations
and, consequently, the hydrogen gravimetric capacity of this
Received: January 8, 2014
Published: March 24, 2014
© 2014 American Chemical Society
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Communication
a
the ring-fused structure of the tetrahydro-N-heterocyclic part
of 2-MeTHQ and two nitrogen atoms are apart from each other
to prevent the bidentate coordination to the iridium center of
the complex. The starting substrate 3 was prepared by hydro-
genation of 2,6-dimethyl-1,5-naphthyridine (4) as a mixture of
two stereoisomers (vide infra, also see Supporting Information).
The results are summarized in Table 2. When dehydrogenation
Table 3. Perhydrogenation of 4 Catalyzed by 2
Table 2. Perdehydrogenation of 3 Catalyzed by Various Ir
a
Complexes
a
The reaction was carried out with 4 (1.0 mmol) and 2 (5.0 mol %)
b
under H2 (70 atm) in p-xylene at 130 °C for 20 h. Determined by GC.
a
Table 4. Perhydrogenation of 4 Catalyzed by 6
a
The reaction was carried out with 3 (0.25 mmol) and the catalyst
b
under reflux in p-xylene for 20 h. Determined by GC.
of 3 was carried out using 1 as a precatalyst, the perdehydrogenated
product 4 was obtained in only 2% yield together with partially
dehydrogenated 2,6-dimethyltetrahydro-1,5-naphthyridine (5)
(23%) (entry 1). However, we were pleased to find that the use
of complexes 2 greatly improved the perdehydrogenation process.
Thus, dehydrogenation of 3 catalyzed by 2a gave 4 in 69% yield
together with 25% of 5 (entry 2). Furthermore, the yield of 4
increased to 96% without formation of 5 when 5 mol % of 2a was
used (entry 3). Similarly, the reactions using complexes 2b−2d
produced 4 in excellent to almost quantitative yields (entries 4−6)
and 2b exhibited the highest catalytic activity (entry 4).14 Thus, the
perdehydrogenation of the saturated fused bicyclic N-heterocycle 3
was achieved by using complexes 2 at relatively low temperature
(p-xylene reflux, 138 °C) as compared to the analogous reaction of a
fused bicyclic cycloalkane, i.e. decalin.15,16
a
The reaction was carried out with 4 (1.0 mmol) and 6 (5.0 mol %)
b
under H2 (70 atm) in p-xylene at 130 °C for 20 h. Determined by GC.
The perdehydrogenation of 3 was also successfully conducted
by using complexes 6. Thus, the reactions of a stereoisomeric
mixture of 3 catalyzed by 6a and 6b proceeded smoothly under
the same conditions as previously mentioned to afford fused
bicyclic N-heteroaromatic compound 4 in 97% and 98% yields,
respectively (Scheme 1), demonstrating the high catalytic
Scheme 1. Perdehydrogenation of 3 Catalyzed by 6
Since it was found that the complexes 2 catalyzed the
perdehydrogenation of 3 to afford N-heteroaromatic compound
4 in almost quantitative yields, we next examined the reverse
reaction, that is, perhydrogenation of 4 to 3 using the same
complexes 2. The results are summarized in Table 3. When
hydrogenation of 4 was conducted using 2a as a precatalyst, a
stereoisomeric mixture of 2,6-dimethyldecahydro-1,5-naph-
thyridines (3a and 3b) was obtained in 82% total yield (entry 1).17
Among the complexes 2a−2d examined, the perhydrogenation of 4
using 2b resulted in the best result, giving a mixture of 3a and 3b in
92% total yield (entry 2).
performance of complexes 6 for the perdehydrogenation
reaction of 3.
Since it was found that the complex 6b showed better and the
best catalytic performance in the perdehydrogenation and
perhydrogenation processes, respectively, the successive inter-
conversion of 4 and 3 was examined by using 6b (Scheme 2).
Although the perhydrogenation of 4 was accomplished by
using 2b in high yield, it was found that insoluble precipitates
were formed when a solution of the catalyst 2a was kept under H2
(60 atm) at 100 °C, probably causing a slight deactivation of the
catalyst.18 Then, we turned our attention to the Cp*Ir complexes
(6a and 6b)13d bearing a rigid functional 1,10-phenanthroline-
2,9-dione ligand to prevent the rotation around the pyridyl−
pyridyl bond. The results of the perhydrogenation of 4 are sum-
marized in Table 4. Notably, the reactions using complexes
6 under the same conditions as mentioned above gave the
perhydrogenated products 3 in almost quantitative yields.19,20
Scheme 2. Successive Perhydrogenation and
Perdehydrogenation of 4 and 3 Catalyzed by 6b
When a solution of 4 in the presence of 6b in p-xylene was heated
at 110 °C under H2 (70 atm), 3 was produced in quantitative
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Communication
yield. Then, the reaction mixture was transferred to a Schlenk
flask and heated at reflux to afford the starting substrate 4 in 92%
yield, demonstrating the successive perhydrogenation (uptake of
5H2) and perdehydrogenation (release of 5H2) of 4 by using 6b
as the single precatalyst.
Scheme 3. Overall Catalyic Processes for Successive
Perhydrogenation and Perdehydrogenation Catalyzed by 6b
a
As mentioned in the reversible dehydrogenation and hydro-
genation of 2-MeTHQ catalyzed by 1, the complex 1 was
reversibly interconverted to [Cp*IrHCl]2, depending on the
absence or presence of H2.7a Therefore, it is highly probable that
6b could be converted to different catalytic species under the
perhydrogenation conditions shown above. Then, a solution of
6b in toluene-d8 was heated at 130 °C under H2 (70 atm) for 2 h,
and it was revealed by 1H NMR analysis that 6b was converted to
the tetrahydride Cp*Ir complex, Cp*IrH4 (7),21 in 87% yield
along with a precipitate, which was identified as the liberated
ligand 2,9-dihydroxy-1,10-phenanthroline (8) by 1H NMR
analysis in DMSO-d6 (eq 3). In addition, it was observed that
the reaction of equimolar amounts of 7 and 8 in p-xylene under
reflux for 1 h regenerated the complex 6b in 76% yield (eq 4).
a
Step a: hydrogenolysis of 6b to 7 and 8. Step b: perhydrogenation of 4
catalyzed by 7 and 8. Step c: regeneration of 6b from 7 and 8 releasing
H2. Step d: perdehydrogenation of 3 catalyzed by 6b
hydrogenolysis of 6b would generate tetrahydride complex 7
along with the liberated ligand 8 under H2 (70 atm) at 130 °C
(step a). Then, the complex 7 could catalyze the perhydrogena-
tion of 4 with the aid of the liberated 8 (step b). Subsequently,
the starting complex 6b is regenerated by recombination
of 7 and 8 when H2 is removed from the reaction atmosphere
(step c). Finally, the perdehydrogenation of 3 catalyzed by
the complex 6b could proceed through a similar cooperative
catalysis of the iridium center and the functional bipyridonate
ligand to that suggested in the dehydrogenative oxidation of
alcohols13d to afford 4 with concomitant evolution of 5 equiv
of H2 (step d), resuming the starting point of the catalytic
system.
In conclusion, we have developed for the first time the
homogeneous catalytic systems for the perdehydrogenation and
perhydrogenation of fused bicyclic N-heterocycles using the
Cp*Ir complexes bearing a functional bipyridonate ligand. The
perdehydrogenation of saturated 3 gave N-heteroaromatic 4 in
excellent yield with the release of five molecules of H2. The
perhydrogenation of 4 was also accomplished by using the same
complexes to afford 3 in excellent yield with the uptake of five
molecules of H2. In addition, the successive perhydrogenation
and perdehydrogenation process was achieved by using 6b as
a single precatalyst through the reversible interconversion of
the catalytic species, depending on the presence or absence of
H2. Further investigations on the more efficient homogeneous
catalytic systems for the perdehydrogenation and perhydroge-
nation of other fused N-heterocycles with high hydrogen content
are in progress.
Thus, it was apparent that 6b and 7 were interconverted to each
other under the reaction conditions with or without H2.
Next, we investigated the perhydrogenation of 4 catalyzed by 7
to clarify whether 7 could be a catalytically active species. The
results are summarized in Table 5. When a solution of 4 in
a
Table 5. Perhydrogenation of 4 Catalyzed by Cp*IrH4 (7)
a
The reaction was carried out with 4 (1.0 mmol) and 7 (5.0 mol %)
b
under H2 (70 atm) in p-xylene at 130 °C for 20 h. Determined by GC.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and X-ray crystallographic data for 2a,
3a, and 3b (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
■
p-xylene was heated at 130 °C in the presence of 7 (5 mol %)
under H2 (70 atm) for 20 h, 3 was obtained in only 19% total
yield (entry 1). On the other hand, the addition of phenol
(20 mol %) as a proton source improved the yield to 76% (entry 2).
Furthermore, it was found that the perhydrogenation of 4 pro-
ceeded in 92% total yield when conducted in the presence of 8
(entry 3). These results clearly indicated that protonation of 4 could
facilitate the perhydrogenation and that 8 would serve as a proton
donor.22 Thus, the functional ligand 8 could play crucial roles in
both perdehydrogenation and perhydrogenation processes.
S
AUTHOR INFORMATION
Corresponding Authors
yamaguchi.ryohei.75s@st.kyoto-u.ac.jp
fujita.kenichi.6a@kyoto-u.ac.jp
■
Considering the above experimental results, we propose the
following overall catalytic processes as shown in Scheme 3. First,
Notes
The authors declare no competing financial interest.
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(13) For our previous reports on dehydrogenation of alcohols:
(a) Fujita, K.; Tanino, N.; Yamaguchi, R. Org. Lett. 2007, 9, 109.
(b) Fujita, K.; Yoshida, T.; Imori, Y.; Yamaguchi, R. Org. Lett. 2011, 13,
2278. (c) Kawahara, R.; Fujita, K.; Yamaguchi, R. J. Am. Chem. Soc. 2012,
134, 3643. (d) Kawahara, R.; Fujita, K.; Yamaguchi, R. Angew. Chem., Int.
Ed. 2012, 51, 12790.
(14) Evolution of hydrogen was confirmed by dual reactions. See
Supporting Information.
(15) The perdehydrogenation of decalin by using heterogeneous
catalysts has been generally carried out at >200 °C. Biniwalea, R. B.;
Rayalua, S.; Devottaa, S.; Ichikawa, M. Intl. J. Hydrogen Energy 2008, 33,
360 and references cited therein.
(16) Dehydrogenation of decahydro-1,5-naphthyridine and 2-methyl-
piperidine under conditions similar to those previously mentioned gave
trace amounts of perdehydrogenated products (<4%).
(17) The structures of 3a and 3b were determined by X-ray analyses.
See Supporting Information.
ACKNOWLEDGMENTS
■
This work was partially supported by KAKENHI (No. 22550098).
We thank Takuya Aikawa for his technical assistance.
REFERENCES
■
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(19) It should be noted that complexes 6 exhibit much lower catalytic
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̈
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(10) The hydrogen gravimetric capacity of 2-MeTHQ is 2.7 wt % at a
maximum.
(11) All of the perdehydrogenation and perhydrogenation of N-
ethyldodecahydrocarbazole and N-ethylcarbazole have been achieved
by using heterogeneous metal catalyst at high temperatures (>150
°C).4,6a−c It should be noted that the catalytic dehydrogenation of N-
ethyldodecahydrocarbazole using homogeneous PCP pincer iridium
complexes at 200 °C results in the partial dehydrogenation.6d
(12) The hydrogen gravimetric capacity of 3 is 6.0 wt % at a maximum.
The DOE 2015 target is 5.5 wt %.
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