NCP-Type Pincer Iridium Complexes Catalyzed Transfer-Dehydrogenation of Alkanes and Heterocycles?

Article abstract of DOI:10.1002/cjoc.202000097

A series of NCP-type pincer iridium complexes, (^RNC^CP)IrHCl (2a—2c) and (^BQ-NC^OP)IrHCl 3, have been studied for catalytic transfer alkane dehydrogenation. Complex 3 containing a rigid benzoquinoline backbone exhibits high activity and robustness in dehydrogenation of alkanes to form alkenes. Even more importantly, this catalyst system was also highly effective in the dehydrogenation of a wide range of heterocycles to furnish heteroarenes.

Full text of DOI:10.1002/cjoc.202000097

Chinese Journal
of Chemistry
CJC 中 国 化 学 - An International Journal
Accepted Article
Title: NCP-Type Pincer Iridium Complexes Catalyzed Transfer-Dehydrogenation of
Alkanes and Heterocycles
Authors: Yulei Wang, Lu Qian, Zhidao Huang, Guixia Liu* and Zheng Huang*
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NCP-Type Pincer Iridium Complexes Catalyzed Transfer-Dehydrogenation
of Alkanes and Heterocycles
a
a
a
,a
,a,b,c
Yulei Wang, Lu Qian, Zhidao Huang , Guixia Liu* and Zheng Huang*
a
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of
Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China.
b
China School of Chemistry and Material Sciences, Hangzhou Institute of Advanced Study, University of Chinese Academy of Sciences, 1 Sub-lane
Xiangshan, Hangzhou 310024, China.
C
Chang-Kung Chuang Institute, East China Normal University, Shanghai 200062, China.
R
C
BQ-
O
Summary of main observation and conclusion A series of NCP-type pincer iridium complexes, ( NC P)IrHCl (2a-c) and ( NC P)IrHCl 3, had been
studied for catalytic transfer alkane dehydrogenation. Complex 3 containing a rigid benzoquinoline backbone exhibits high activity and robustness in
dehydrogenation of alkanes to form alkenes. Even more importantly, this catalyst system was also highly effective in the dehydrogenation of a wide range
of heterocycles to furnish heteroarenes.
Background and Originality Content
Catalytic dehydrogenation of abundant and low-cost alkanes
to alkenes is one of the most important transformation, since
[
1]
alkenes are highly versatile intermediates in organic synthesis.
Substantial efforts have been devoted to the development of
alkane dehydrogenation (AD) using soluble molecular catalysts
due to relatively mild reaction conditions and the ability to control
[
2]
selectivity. Among the known homogeneous AD catalysts, pincer
Ir complexes have been extensively studied. In the mid-1990s,
Kaska, Jensen, and Goldman discovered that thermally stable
Figure 1. NCP-type pincer iridium complexes.
Driven by our interest in developing more efficient and
(
PCP)Ir is a robust and effective catalyst for dehydrogenation of
[
7,16]
[3]
selective dehydrogenation catalysts,
we previously
cyclic and linear alkanes. Following that, diverse pincer Ir
complexes have been developed through the modification of
R O
synthesized a series of iridium complexes ( NC P)IrHCl with
[
17]
[
4]
pyridine and phosphinite arms (Figure 1, 1a-c). Upon activation
with NaOtBu, the least sterically congested 1a shows moderate
catalytic activity for transfer-dehydrogenation of alkanes using
tert-butylethylene (TBE) as the hydrogen acceptor with turnover
pincer ligands by changing the phosphinoalkyl substituents, the
[
5]
coordination atoms, the linkers between the coordination
[
6,7]
[8-10]
atoms
and the backbone.
A few of these complexes afford
catalytic efficiency that are higher than that of the parent (PCP)Ir
complex.
[17a]
[
2]
numbers (TONs) up to 582.
NC P)IrHCl (2a-c) with a “CH
Subsequently, the analogous
”, instead of oxygen, as the linker,
and ( NC P)IrHCl 3 featuring a rigid benzoquinoline backbone
were synthesized. These complexes exhibit outstanding catalytic
The efforts on the structural and electronic
R
C
(
2
modification of pincer ligands not only enrich the variety of AD
catalysts, but also shed lights into the structure-activity
relationship and provide hints for the development of more
effective catalysts.
BQ-
O
[
18]
activity in regio- and stereoselective olefin isomerization and
transfer hydrogenation of alkenes and alkynes using EtOH as the
The application of homogenous AD catalysts could be
extended to dehydrogenation of functionalized substrates, such as
[
19]
hydrogen donor. Herein, we wish to report the catalytic activity
R
C
BQ-
O
[11]
[12]
[13]
of complexes ( NC P)IrHCl 2a-c and ( NC P)IrHCl 3 in the
transfer dehydrogenation of alkanes and heterocycles. Complex 3
not only displays catalytic activity superior to other (NCP)Ir
complexes in the dehydrogenation of simple alkanes, but also
enables the efficient dehydrogenation of a wide variety of O-, and
N-heteroarenes, including piperidines.
amines,
ethers
and heterocycles.
Among them, the
dehydrogenation of saturated N- and O-heterocycles represents a
straightforward and efficient way to construct heteroaromatic
[
14]
products such as indoles, quinolines, benzofuran. However, the
unfavorable interaction between the metal centers and the
heteroatoms may inhibit the catalytic activity. For this reason,
most known AD catalysts suffered from poor functional group
tolerance. In particular, the dehydrogenation of piperidine to Results and Discussion
pyridine is of challenge because of the strong binding affinity of
We initially tested the catalytic performance of (NCP)Ir
pyridine to the metal center. As a result, despite tremendous
advances in transition-metal catalyzed dehydrogenation of
heterocycles, the dehydrogenation of piperidine to pyridine
complexes 2a-c and 3 in the benchmark reaction, that is
transfer-dehydrogenation of cyclooctane (COA) with TBE as the
hydrogen acceptor (Table 1). The reactions were conducted in a
COA solution containing TBE with (NCP)IrHCl as the precatalyst
[
7a,15]
remains underdeveloped.
H
C
and NaOtBu as the catalyst activator. The use of ( NC P)IrHCl 2a as
the precatalyst provided 26 turnovers after 0.5 h at 150 °C (entry
*E-mail: huangzh@sioc.ac.cn, guixia@sioc.ac.cn
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First author et al.
a
1
), which was much less active than the O-linked analog
Table 2. TONs for transfer-dehydrogenation of COA/TBE catalyzed by 3
H
O
[17a]
(
NC P)IrHCl 1a.
The TONs grew to 114 at elevated
temperature (200 °C, entry 2). Increasing the TBE concentration (1
M) or prolonging the reaction time had little impact on the TONs
(
entries 3 and 4). Compared with 2a, the sterically more
congested 2b and 2c with Me and tBu substituents at the pyridine
rings of the NCP ligand, are far less effective (entries 5 and 6). To
BQ-
O
our delight, the activity of complex ( NC P)IrHCl 3 with a rigid
R
C
benzoquinoline backbone is much higher than ( NC P)IrHCl 2,
o
producing 807 turnovers after 4 h at 150 C (entry 7).
Table 1 TONs for transfer-dehydrogenation of COA/TBE catalyzed by 2a-2c
a
and 3
a
TONs were calculated on the basis of TBE conversion determined by GC.
transfer-dehydrogenation reactions using (POCOP)Ir and (PCP)Ir as
tBu
tBu
the precatalysts: while ( POCOP)Ir is more active than ( PCP)Ir
for the transfer-dehydrogenation of COA, the latter shows greater
[2]
activity for n-octane dehydrogenation. The more crowded
complexes 2b and 2c show poor activity in the n-octane/TBE
transfer-dehydrogenation (entries 3 and 4). As observed in the
reaction
with
COA,
studies
on
the
n-octane
a
TONs were calculated on the basis of TBE conversion determined by GC.
transfer-dehydrogenation revealed that 3 is more effective than
either 1a or 2a, affording 89 and 164 TOs in 1 h and 12 h,
Subsequently, the catalytic efficiency of complex 3 for the
COA/TBE dehydrogenation was extensively investigated under
various conditions. The results are summarized in Table 2. The
activity of complex 3 was significantly deteriorated with high
concentration of TBE ([TBE]): while increasing [TBE] to 3.9 M
afforded 231 TOs in 18 h at 150 °C (entry 2), the TONs increased
to 938 when the [TBE] was reduced to 0.5 M (entry 3). With the
respectively (entries
5 and 6). Similar to the known
dehydrogenation reactions of linear alkanes, the reaction shows
low selectivity for the formation of 1-octene, which is presumably
[
20]
attributed to facile olefin isomerization.
Table 3. Ir-catalyzed transfer dehydrogenation of n-octane with TBE.
[
1
Ir]/TBE ratio fixed at 1/2000, varying the TBE concentrations from
.0 M to 0.75 M and 0.5 M delivered 1436, 1747 and 1892 TOs,
respectively (entry 4-6). These results further indicated that
lowering the [TBE] favored the transfer-dehydrogenation reaction,
suggesting an inhibition effect of this hydrogen acceptor on the
catalysis. The catalytic turnover is also influenced by the catalyst
loadings. The TONs increased as the ratio of [Ir]/TBE decreased:
9
38 TOs at 1/1000 ratio, 1892 TOs at 1/2000, 2703 TOs at 1/3000,
and 2992 TOs at 1/4000 (entries 3, 6-8). Moreover, the
BQ-
O
(
NC P)IrHCl complex 3 exhibits high thermal stability and
catalytic activity at 200 °C. When the [Ir]/TBE ratio was 1/4000,
o
TONs up to 3440 were achieved at 200 C in 18 h (entry 9). Note
the catalytic activity is comparable to the relevant bisphosphinite
o
POCOP complex that afford ~2000 TONs at 200 C in the COA/TBE
[6a]
transfer-dehydrogenation reaction.
a
Next, we investigated the activity of the (NCP)Ir complexes in
the transfer-dehydrogenation of a linear alkane (n-octane). The
results are summarized in Table 3. Using 2a as the precatalyst, the
TONs were calculated on the basis of conversion of TBE determined by GC.
Values in parentheses are the percentage of 1-octene relative to total
b
octenes.
o
reaction at 150 C in 1 h gave 52 turnovers (entry 1). Prolonging
The high activity and relatively mild reaction conditions in
alkane dehydrogenation encouraged us to examine the viability of
our NCP-typed complexes in the dehydrogenation of heterocycles.
We first examined the catalytic performance of complexes 2a−2c
and 3 in the dehydrogenation of tetrahydroquinoline at 150 C
with a 1.0 mol% catalyst loading (Table 4). While 2a-2c showed
moderate or low activity, complex 3 gave almost quantitative
the reaction time to 5 h increased the TONs to 78 (entry 2). Note
H
C
that although complex ( NC P)IrHCl 2a is much less active than its
H
O
O-linked analog ( NC P)IrHCl 1a in the catalytic COA
transfer-dehydrogenation, the two complexes exhibit comparable
o
[17a]
activity in n-octane dehydrogenation reaction.
phenomenon is in line with that observed in the
This
2
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Running title
Chin. J. Chem.
Table 4. Iridium catalytic transfer-dehydrogenation of heterocycles with
TBE
conversion, producing quinoline with 93% isolated yield after 24
h.
a
The dehydrogenation of a diverse series of saturated
heterocycles using 3 as the precatalyst was then explored. It is
important to note that 3, in most cases, is a more effective
precatalyst than previously reported PCP-type Ir precatalysts for
[
7a]
N-heterocycles dehydrogenation. The substrates with fluoro (4c)
and methyl (4d) groups in the benzene rings were
dehydrogenated smoothly. In addition, 2- and 4-methyl
tetrahydroquinoline are suitable substrates, albeit with increased
catalyst loadings (entries 7-8). The dehydrogenation of
tetrahydroisoquinoline 4f and tetrahydroquinoxaline 4g with TBE
(
4 equiv) gave the corresponding products in 90% and 77%
isolated yields, respectively (entries 9 and 10).
The dehydrogenation of piperidine 4h to pyridine is a rather
challenging task given the strong coordination ability of the
BQ-
O
product to the metal center. To our delight, this ( NC P)Ir/TBE
system allowed this transformation in 61% yield by using 5 mol%
o
of 3 at elevated temperature (200 C, entry 11). Treatment of
4-phenyl piperidine 4j also gave a moderate yield of 4-phenyl
pyridine. The dehydrogenation of 4i with TBE (6 equiv) using Ir (5
mol%) gave 2,5-dimethylpyridine 5i in 88% yield after 24 h. The
relatively poor coordination ability of steric hindered 5i could
contribute to the high yield.
Indoline 4k underwent dehydrogenation smoothly to afford
quantitative yield of indole 5k using only 0.2 mol% of 3 under neat
o
conditions at a relatively low temperature (120 C). Treatment of
2
-methylindoline 4l with 2 equiv TBE formed the product in 96%
isolated yield.
This transfer-dehydrogenation system is also effective for
some O-heterocycles. The dehydrogenation of
,3-dihydrobenzofuran (4m) occurred readily even with a catalyst
2
loading as low as 0.1 mol% under neat conditions. However,
isochromane (4n) and 3,4-dihydrocoumarin (4o) are barely
reactive under the current catalytic conditions.
Conclusions
R
C
In summary, NCP-type pincer Ir complexes ( NC P)IrHCl (2)
BQ-
O
and
(
NC P)IrHCl (3) have been studied for catalytic
transfer-dehydrogenation of alkanes and heterocycles. Evidently,
complex 3 with a rigid benzoquinoline backbone is much for
R
O
R
C
effective than ( NC P)IrHCl (1) and ( NC P)IrHCl (2) in the
dehydrogenation of alkanes. This can be attributed to the fact that
the adoption of benzoquinoline backbone in 3 prevents the
cyclometalation, which can occur in the case of 1 and 2. Upon
dehydrochlorination of complexes 1 and 2, the subsequent
2
rotation of the pyridine subunit and cyclometalation via C(sp )-H
oxidative addition would lead to catalytically inactive Ir(III) hydride
[
19a]
BQ-
O
species.
Moreover, complex ( NC P)IrHCl 3 was found to be
effective for heterocycles dehydrogenation, producing a wide
range of heteroarenes in high efficiency.
Experimental
General Description. All manipulations were carried out in an
argon-filled glovebox or under an atmosphere of dry argon using
standard Schlenk techniques, unless stated otherwise.
Cyclooctane (COA), tert-butylethylene (TBE), n-octane were
4
purchased from Alfa Aesar and dried over LiAlH overnight under
an atmosphere of dry argon, then distilled prior to use and stored
in an argon atmosphere glovebox. p-Xylene was distilled from
sodium benzophenone ketyl prior to use. 4a-4f, 4h-4o were
a
R
C
[18]
Substrate (1 mmol), p-xylene (1 mL), TBE (4 equiv), GC yield, numbers in
distilled from CaH prior to use. Complexes ( NC P)IrHCl (2a-2c)
2
b
c
d
BQ-
O
[19]
parentheses are isolated yields. TBE (6 equiv). TBE (2 equiv). Neat
conditions.
and ( NC P)IrHCl (3) were prepared according to previously
reported procedures. All other reagents and solvents used in this
study were purchased from commercial sources and used as
Chin. J. Chem. 2019, 37, XXX－XXX
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3

Report
First author et al.
received. NMR spectra were recorded on Agilent 400 MHz and
Varian 400 MHz at ambient temperature. The residual peak of
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13
deuterated solvent was used as a reference for H and
C
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chemical shifts. GC analysis was acquired on Agilent 7890A gas
chromatograph equipped with a flame-ionization detector.
General procedure for transfer-dehydrogenation of alkanes with
TBE. In an argon-filled glove-box, an oven-dried 10 mL thick-wall
glass tube was charged with an iridium complex (5 µmol), NaOtBu
(
11.0 µmol, 1.1 mg), tert-butylethene (2.5 mol, 0.5 M) and
n-octane (4.7 mL). The tube was sealed with a Teflon plug, and the
solution was stirred in a 150 °C oil bath. Periodically, the flask was
removed from the bath and cooled to room temperature. An
aliquot was removed from the flask and analyzed by GC.
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General procedure for transfer-dehydrogenation of heterocycles.
In an argon-filled glovebox, complex 3 (10 µmol, 5.7 mg) was
added to a solution of p-xylene (1 mL), tetrahydroquinoline (1.0
mmol, 133.2 mg) and TBE (4 mmol, 0.49 mL) in a 5 mL thick-wall
glass tube, and then NaOtBu (22.0 µmol, 2.1 mg) was added. The
tube was sealed by a Teflon plug and heated in a preheated
oil-bath at 150 °C for 24 h. After that, the tube was cooled to
room temperature and the sample was analyzed by GC, GC-MS.
Removal of the solvent in vacuum and the residue was purified by
chromatography on silica gel.
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The supporting information for this article is available on the
WWW under https://doi.org/10.1002/cjoc.2018xxxxx.
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Acknowledgement (optional)
This work was supported by the National Basic Research
Program of China (2016YFA0202900), the National Natural
Science Foundation of China (21825109, 21821002, 21572255,
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The following will be filled in by the editorial staff)
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Manuscript revised: XXXX, 2019
Manuscript accepted: XXXX, 2019
Accepted manuscript online: XXXX, 2019
Version of record online: XXXX, 2019
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Entry for the Table of Contents
Page No.
NCP-Type Pincer Iridium Complexes Catalyzed
Transfer-Dehydrogenation of Alkanes and
Heterocycles
Yulei Wang, Lu Qian, Zhidao Huang, Guixia Liu*
and Zheng Huang*
.
a
c
Department, Institution, Address 1
Department, Institution, Address 3
E-mail:
E-mail:
b
Department, Institution, Address 2
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Chin. J. Chem. 2018, template
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