Synthesis of a series of iridium complexes bearing substituted 2-pyridonates and their catalytic performance for acceptorless dehydrogenation of alcohols under neutral conditions

Article abstract of DOI:10.1016/j.jorganchem.2017.05.018

A series of Cp*Ir complexes bearing 5- and 4,5-substituted 2-pyridonate ligands have been synthesized and their catalytic performance for acceptorless dehydrogenation of alcohols has been investigated under neutral conditions. Electron-withdrawing groups such as methoxycarbonyl, trifluoromethyl, cyano, and nitro groups at the 5-position promoted the acceptorless dehydrogenation of 1-phenylethanol, whereas electron-donating methyl group at the 5-position retarded the reaction. Furthermore, introduction of methyl group at the 4-position improved the catalytic performance. Thus, Cp*Ir(5-trifluoromethyl-4-methyl-2-pyridonate)Cl (2bc) exhibited the highest catalytic performance among the complexes examined, and also showed good catalytic performance for acceptorless dehydrogenation of primary alcohols.

Full text of DOI:10.1016/j.jorganchem.2017.05.018

Journal of Organometallic Chemistry 843 (2017) 14e19
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of a series of iridium complexes bearing substituted 2-
pyridonates and their catalytic performance for acceptorless
dehydrogenation of alcohols under neutral conditions
*
**
Ryohei Yamaguchi , Daiki Kobayashi, Mineyuki Shimizu, Ken-ichi Fujita
Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 January 2017
Received in revised form
A series of Cp*Ir complexes bearing 5- and 4,5-substituted 2-pyridonate ligands have been synthesized
and their catalytic performance for acceptorless dehydrogenation of alcohols has been investigated
under neutral conditions. Electron-withdrawing groups such as methoxycarbonyl, trifluoromethyl,
cyano, and nitro groups at the 5-position promoted the acceptorless dehydrogenation of 1-
phenylethanol, whereas electron-donating methyl group at the 5-position retarded the reaction.
Furthermore, introduction of methyl group at the 4-position improved the catalytic performance. Thus,
Cp*Ir(5-trifluoromethyl-4-methyl-2-pyridonate)Cl (2bc) exhibited the highest catalytic performance
among the complexes examined, and also showed good catalytic performance for acceptorless dehy-
drogenation of primary alcohols.
4
May 2017
Accepted 7 May 2017
Available online 8 May 2017
Keywords:
Iridium complex
2-Pyridonate ligand
Acceptorless dehydrogenation of alcohol
Catalytic performance
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
oxidation of alcohols catalysed by iridium complexes bearing
functional 2-hydroxypyridine- and 2,2 -dihydroxybipyridine-based
0
The oxidation of alcohols to carbonyl compounds such as alde-
hydes and ketones is one of the most important and fundamental
reactions in organic chemistry. Recently, oxidant-free or acceptor-
free oxidation based on the transition-metal catalyzed dehydro-
genative transformation of alcohols accompanied by the evolution
of hydrogen gas (acceptorless dehydrogenation) has attracted
considerable attention from the standpoint of atom economy and
environmental concerns [1e4]. Furthermore, acceptorless dehy-
drogenation of alcohols is important not only for the production of
synthetically useful aldehydes and ketones from easily available
alcohols with high atom efficiency [5e11], but also for the pro-
duction of hydrogen which is one of the most promising energy
carriers in future energy plans [12e15]. Thus, much effort has been
devoted to develop various homogeneous catalytic systems for the
acceptorless dehydrogenation of alcohols affording ketones and
aldehydes.
ligands (complexes A, B, and C, D), as shown in Fig. 1 [16e20], [21].
Since these catalytic systems would proceed by the activation of
an alcohol with the 2-pyridonate ligand by way of either the
stepwise pathway through
b-hydrogen elimination of an alkox-
oiridium intermediate [16] or the concreted pathway through cyclic
transition state [20,22], the intramolecular reaction of hydride on
iridium with protic hydrogen on the 2-hydroxypyridine-based
ligand (ligand-promoted dehydrogenation) could be a critical step
(Scheme 1). Thus, the 2-pyridonate ligand could act as a proton
acceptor in the activation step [23] and as a proton donor in the
dehydrogenation step [24,25], playing a dual role in the cooperative
catalysis. Therefore, it should be indispensable to investigate the
substituent effect of the 2-pyridonate ligand on the catalytic per-
formance in order to develop the more effective catalyst. We report
here the synthesis of a series of iridium complexes bearing
substituted 2-pyridonates and their catalytic performance of
acceptorless oxidation of alcohols.
We have previously reported several systems for acceptorless
2. Experimental
*
Corresponding author.
2.1. General
*
* Corresponding author.
E-mail addresses: yamaguchi.ryohei.75s@st.kyoto-u.ac.jp (R. Yamaguchi), fujita.
kenichi.6a@kyoto-u.ac.jp (K.-i. Fujita).
All reactions and manipulations were performed under
http://dx.doi.org/10.1016/j.jorganchem.2017.05.018
0
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R. Yamaguchi et al. / Journal of Organometallic Chemistry 843 (2017) 14e19
15
0
Fig. 1. Iridium complexes bearing functional 2-hydroxypyridine- and 2,2 -dihydrox-
ybipyridine-based ligands for acceptorless oxidation of alcohols.
argon atmosphere using standard Schlenk techniques. 1H and
13
1
C{ H} NMR spectra were recorded on JEOL ECX-500 and ECS-
Scheme 1. Dual role of 2-Pyridonate Ligand in Cooperative Catalysis.
4
00 spectrometers. Gas chromatography (GC) analyses were
performed on a GL-Sciences GC353B gas chromatograph with a
capillary column (GL-Sciences InertCap and InertCap
Pure WAX). Elemental analyses were carried out at the
Microanalysis Center of Kyoto University. Silica-gel column
5
(
d, 1H, J ¼ 8.8 Hz, Ar), 6.20 (d, 1H, J ¼ 8.8 Hz, Ar), 3.85 (s, 3H,
13
COOMe), 1.76 (s, 15H, Cp*); C NMR (100.5 MHz, CDCl
aromatic), 165.6 (CO), 145.8 (aromatic), 139.8 (aromatic), 114.7
(aromatic), 113.4 (aromatic), 84.3 (C Me ), 52.0 (OCH ), 9.5 (C Me ).
ClIr: C, 39.64;
3
): d 178.8
(
chromatography was carried out by using Wako-gel C-200. The
5
5
5
3
5
5
compounds, [Cp*IrCl
2
]
2
(Cp*
¼
h
-pentamethylcyclo-penta-
Found: C, 39.19; H, 3.98; N, 2.70. Calcd. For C17
H, 4.11; N, 2.72.
H21NO
3
dienyl), [Cp*Ir(2-pyridonate)Cl] (2aa) [16], and [Cp*Ir(5-methyl-
-pyridonate)Cl] (2ab) [25], [Cp*Ir(5-trifuloromethyl-2-
2
pyridonate)Cl] (2ac) [25] were prepared according to the
literature methods. Solvents were dried by standard procedures
prior to use. All other reagents are commercially available and
were used as received.
2
.2.2. [Cp*Ir(5-cyano-2-pyridonate)Cl] (2ae)
1
1
43 mg (56%): H NMR (400 MHz, CDCl
3
): d 8.18 (s, 1H, Ar), 7.42
(
d, 1H, J ¼ 9.2 Hz, Ar), 6.13 (d, 1H, J ¼ 9.2 Hz, Ar), 1.76 (s, 15H, Cp*);
1
3
C NMR: (100.5 MHz, CDCl
39.8 (aromatic), 118.4 (CN), 116.0 (aromatic), 93.5 (aromatic), 85.2
Me ), 9.5 (C Me ). Found: C, 39.04; H, 3.90; N, 5.43. Calcd for
OClIr$H O: C, 38.44; H, 4.03; N, 5.60.
3
): d 172.7 (aromatic), 151.6 (aromatic),
1
2.2. Procedures for synthesis of Cp*Ir(5- and 4,5-subsutituted 2-
(C
5
5
5
5
pyridonate)Cl (2)
C
16
H
18
N
2
2
In
0.5 mmol) and NaOEt (34 mg, 0.5 mmol) in EtOH were placed
and a solution was stirred for 30 min at room temperature. After the
a flask, 5- and 4,5-subsutituted 2-hydorxypyridines
2
.2.3. [Cp*Ir(5-nitro-2-pyridonate)Cl] (2af)
(
1
1
09 mg (54%): H NMR (400 MHz, CDCl
d, 1H, J ¼ 9.6 Hz, Ar), 6.24 (2a 1H, J ¼ 9.6 Hz, Ar), 1.74 (s, 15H, Cp*);
13
3
): d 8.91 (bs,1H, Ar), 8.16
(
solvent was removed under vacuo, [Cp*IrCl
and CH Cl
2
]
2
(199 mg, 0.25 mmol)
C NMR: (100.5 MHz, CDCl
3
):
d
173.9 (aromatic), 146.0 (aromatic),
Me ),
). Found: C, 34.36; H, 3.79; N, 5.47. Calcd for
ClIr$H O: C, 34.64; H, 3.88; N, 5.39.
ꢀ
2
2
(3 mL) were added and stirred at 5 C. When the color of
134.4 (aromatic), 133.9 (aromatic), 114.2 (aromatic), 86.3 (C
5
5
the solution changed from orange to yellowish brown, the solvent
was removed under vacuo. The residue was extracted by benzene
9
C
5 5
.6 (C Me
15
H
18
N
2
O
3
2
and, then, washed with a mixture of Et
Finally, the corresponding complex was obtained by recrystalliza-
tion from CH Cl /hexane. The spectral and analytical data are
summarized in Table 1.
2
O/THF/benzene (40: 1: 1).
2.2.4. [Cp*Ir(4-methyl-5-trifluoromethyl-2-pyridonate)Cl] (2bc)
2
2
1
1
04 mg (39%): H NMR (400 MHz, CDCl
s, 1H, Ar), 2.34 (s, 3H, Me), 1.76 (s, 15H, Cp*); C NMR: (100.5 MHz,
CDCl ):
177.9 (aromatic), 149.0 (aromatic), 124.5 (q, J ¼ 271 Hz,
CF ), 84.2
), 115.8 (aromatic), 114.4 (q, J ¼ 31 Hz, ipso-position of CF
(C Me ), 19.5 (CH ), 9.6 (C Me ). Found: C, 37.66; H, 3.73; N, 2.59.
3
): d 7.99 (s, 1H, Ar), 6.06
13
(
3
d
2
.2.1. [Cp*Ir(5-methoxycarbonyl-2-pyridonate)Cl] (2ad)
3
3
1
8
6 mg (56%): H NMR (400 MHz, CDCl
3
):
d
8.54 (s, 1H, Ar), 7.70
5
5
3
5
5

16
R. Yamaguchi et al. / Journal of Organometallic Chemistry 843 (2017) 14e19
Table 1
Spectral and analytical data of 2ad, 2ae, 2af, 2bc, 2bd, 2be, and 2bf.
Complex 1H NMR (400 MHz, CDCl
13C NMR (100.5 MHz, CDCl
Elemental analysis
3
)
3
)
d
d
2
ad
8.54 (s, 1H, Ar), 7.70 (d, 1H, J ¼ 8.8 Hz, Ar),
.20 (d, 1H, J ¼ 8.8 Hz, Ar), 3.85
s, 3H, COOMe), 1.76 (s, 15H, Cp*)
8.18 (s, 1H, Ar), 7.42 (d, 1H, J ¼ 9.2 Hz, Ar),
.13 (d, 1H, J ¼ 9.2 Hz, Ar), 1.76 (s, 15H, Cp*) 116.0 (Ar), 93.5 (Ar), 85.2 (C
173.9 (Ar), 146.0 (Ar), 134.4 (Ar), 133.9 (Ar),
Me ), 9.6 (C Me
178.8 (Ar), 165.6 (CO), 145.8 (Ar), 139.8 (Ar),
114.7 (Ar), 113.4 (Ar), 84.3 (C Me ),
52.0 (OCH ), 9.5 (C Me
172.7 (Ar), 151.6 (Ar), 139.8 (Ar), 118.4 (CN),
Me ), 9.5 (C Me
Found: C, 39.19; H, 3.98; N, 2.70.
Calcd. For C17H21NO ClIr: C, 39.64; H, 4.11; N, 2.72.
3
6
(
5
5
3
5
5
)
2
2
2
ae
Found: C, 39.04; H, 3.90; N, 5.43.
Calcd for C16 OClIr$H O: C, 38.44; H, 4.03; N, 5.60.
Found: C, 34.36; H, 3.79; N, 5.47.
Calcd for C15 ClIr$H O: C, 34.64; H, 3.88; N, 5.39.
), Found: C, 37.66; H, 3.73; N, 2.59.
Calcd. For C17 NOClIr: C, 37.88; H, 3.74; N, 2.60.
6
5
5
5
5
)
H
18
N
2
2
af
8.91 (bs, 1H, Ar), 8.16 (d, 1H, J ¼ 9.6 Hz, Ar),
6
.24 (2a 1H, J ¼ 9.6 Hz, Ar), 1.74 (s, 15H, Cp*) 114.2 (Ar), 86.3 (C
5
5
5
5
)
H
18
N
2
O
3
2
bc
7.99 (s, 1H, Ar), 6.06 (s, 1H, Ar),
177.9 (Ar), 149.0 (Ar), 124.5 (q, J ¼ 271 Hz, CF
115.8 (Ar), 114.4 (q, J ¼ 31 Hz, ipso-position
3
2.34 (s, 3H, Me), 1.76 (s, 15H, Cp*)
20 3
H F
3 5 5 3 5 5
of CF ), 84.2 (C Me ), 19.5 (CH ), 9.6 (C Me )
2
bd
8.49 (s, 1H, Ar), 6.02 (s, 1H, Ar), 3.82
s, 3H, COOMe), 2.51 (s, 3H, Me),
.76 (s, 15H, Cp*)
8.22 (bs, 1H, Ar), 6.10 (s, 1H, Ar),
.18(s, 3H, Me), 1.53(s, 15H, Cp*)
8.82 (bs, 1H, Ar), 6.04 (s, 1H, Ar),
.59 (s, 3H, Me), 1.76 (s, 15H, Cp*)
178.2 (Ar), 166.1 (CO), 153.3 (Ar), 146.5 (Ar),
115.2 (Ar), 114.8 (Ar), 84.1 (C Me ),
51.6 (OCH ), 22.5 (CH ), 9.6 (C Me
168.6 (Ar), 155.3 (Ar), 147.5 (Ar), 118.3 (CN),
115.6 (Ar), 94.4 (C Me ), 19.0 (CH ), 8.1 (C Me
173.1 (Ar), 147.1 (Ar), 146.0 (Ar), 135.1 (Ar),
116.0 (Ar), 85.3 (C Me ), 21.8 (CH ), 9.6 (C Me
Found: C, 40.17; H, 4.31; N, 2.52.
Calcd for C18 23NOClIr: C, 40.86; H, 4.38; N, 2.65.
(
1
5
5
H
3
3
5
5
)
2
be
bf
Found: C, 38.85; H, 4.06; N, 4.83.
Calcd for C17 OClIr$(H O) : C, 38.38; H, 4.55; N, 5.27.
Found: C, 37.07; H, 3.92; N, 5.39.
Calcd for C16 20ClN Ir: C, 37.24; H, 3.91; N, 5.43.
2
5
5
3
5
5
)
)
H
20
N
2
2
2
2
2
5
5
3
5
5
H
2 3
O
Calcd. For C17
H
20
F
3
NOClIr: C, 37.88; H, 3.74; N, 2.60.
CDCl
3
):
d
173.1 (aromatic), 147.1 (aromatic), 146.0 (aromatic), 135.1
Me ), 21.8 (CH ), 9.6 (C Me ).
20ClN Ir: C, 37.24;
(aromatic), 116.0 (aromatic), 85.3 (C
5
5
3
5
5
2.2.5. [Cp*Ir(5-methoxycarbonyl-4-methyl-2-pyridonate)Cl] (2bd)
Found: C, 37.07; H, 3.92; N, 5.39. Calcd for C16
H, 3.91; N, 5.43.
H
2 3
O
1
108 mg (68%): H NMR (400 MHz, CDCl
3
): d 8.49 (s, 1H, Ar), 6.02
(
s, 1H, Ar), 3.82 (s, 3H, COOMe), 2.51 (s, 3H, Me), 1.76 (s, 15H, Cp*);
1
3
C NMR (100.5 MHz, CDCl
aromatic), 146.5 (aromatic), 115.2 (aromatic), 114.8 (aromatic), 84.1
Me ), 51.6 (OCH ), 22.5 (CH ), 9.6 (C Me ). Found: C, 40.17; H,
.31; N, 2.52. Calcd for C18 23NOClIr: C, 40.86; H, 4.38; N, 2.65.
3
): d 178.2 (aromatic), 166.1 (CO), 153.3
(
(
2.3. X-ray structure analysis of 2ad, 2bc, and 2bf
C
5
5
3
3
5
5
4
H
Diffraction data were obtained with a Rigaku RAXIS RAPID in-
strument. Reflection data were corrected for Lorentz and polari-
zation effects. Empirical absorption corrections were applied. The
structure was solved by Patterson methods (DIRDIF99 PATTY)
[26,27] and refined anisotropically for non-hydrogen atoms by full-
matrix least-squares calculations. Atomic scattering factors and
anomalous dispersion terms were taken from the literature
[28e31]. Hydrogen atoms were located on the idealized positions,
however, hydrogen atoms of the coordinated water molecule were
not located. The calculations were performed using the program
system Crystal Structure [32,33]. ORTEP drawings of 2ad, 2bc, and
2bf are shown in Fig. 2 and the bond parameters are summarized in
Table 2. The crystal data and details are shown in CIF files.
2
.2.6. [Cp*Ir(5-cyano-4-methyl-2-pyridonate)Cl] (2be)
1
7
8 mg (32%): H NMR (500MHz, DMSO): d 8.22 (bs, 1H, Ar), 6.10
13
(
s, 1H, Ar), 2.18(s, 3H, Me), 1.53(s, 15H, Cp*); C NMR (125.65 MHz,
CDCl ): 168.6 (aromatic), 155.3 (aromatic), 147.5 (aromatic), 118.3
CN), 115.6 (aromatic), 94.4 (C Me ), 19.0 (CH ), 8.1 (C Me ). Found:
OClIr$(H O) : C, 38.38;
3
d
(
5
5
3
5
5
C, 38.85; H, 4.06; N, 4.83. Calcd for C17
H, 4.55; N, 5.27.
H
20
N
2
2
2
2.2.7. [Cp*Ir(5-nitro-4-methyl-2-pyridonate)Cl] (2bf)
1
6
8 mg (26%): H NMR (400MHz, CDCl
3
): d 8.82 (bs, 1H, Ar), 6.04
13
(
s, 1H, Ar), 2.59 (s, 3H, Me), 1.76 (s, 15H, Cp*); C NMR (100.5 MHz,
Fig. 2. ORTEP drawings of complexes 2ad (left), 2bc (center), and 2bf (right).

R. Yamaguchi et al. / Journal of Organometallic Chemistry 843 (2017) 14e19
17
Table 2
Table 4
Bond parameters (values in Å) of 2ad, 2bc, 2bf, and 2aa.
Acceptorless dehydrogenation of 1-phenylethanol catalyzed by 2ac, 2bc, 2bd, and
a
2be.
2ad
2bc
2bf
2aa [16]
IreN1
IreO1
IreCl1
N1eC11
O1eC11
2.130(4)
2.198(4)
2.3899(18)
1.358(7)
1.283(7)
2.098(6)
2.216(5)
2.3834(15)
1.348(10)
1.300(8)
2.119(4)
2.192(4)
2.3998(13)
1.373(6)
1.286(6)
2.118(4)
2.181(4)
2.3818(15)
1.366(8)
1.276(7)
Entry
Catalyst
Conversion (%)b
Yield (%)b
TONc
1
2
3
4
2ac
2bc
2bd
2be
78
83
53
59
78
83
52
59
1560
1660
1040
1180
2
.4. Procedures for acceptorless dehydrogenation of 1-
phenylethanol catalyzed by Cp*Ir(5- and 4,5-subsutituted 2-
pyrionate)Cl (2) (Table 3)
a
The reaction was carried out with 1-phenylethanol (20 mmol), catalyst
(
0.05 mol%) in toluene (6 mL) under reflux for 20 h.
b
A solution of 1-phenylethanol (1.221 g, 10 mmol) and catalysts 2
Determined by GC.
c
ꢀ
Turnover number.
(
0.01 mmol) in toluene (3 mL) was refluxed for 20 h at 135 C (oil
bath temperature). After the mixture was cooled to room temper-
ature, 1,4-diisopropylbenzene (500 mg) was added as an internal
standard. The mixture was then diluted with toluene (12 mL) and
analyzed by GC using authentic acetophenone. The results are
shown in Table 3.
2
.7. Procedures for acceptorless dehydrogenation of primary
alcohols catalyzed by 2bc (Table 5)
A solution of an alcohol (0.5 mmol) and catalyst 2bc (5.7 mg,
.01 mmol) in toluene (9 mL) was refluxed for 20 h at 135 C (oil
ꢀ
0
2.5. Procedures for acceptorless dehydrogenation of 1-
bath temperature). In the reaction of cyclohexylmethanol, p-xylene
phenylethanol catalyzed by 2ac, 2bc, 2bd, and 2be (Table 4)
was used as the solvent and the solution was refluxed for 20 h at
ꢀ
160 C (oil bath temperature). After the mixture was cooled to room
A solution of 1-phenylethanol (2.443 g, 20 mmol) and catalysts 2
temperature, biphenyl (25 mg) was added as an internal standard.
The mixture was analyzed by GC using an authentic sample. The
results are shown in Table 5.
ꢀ
0.01 mmol) in toluene (6 mL) was refluxed for 20 h at 135 C (oil
(
bath temperature). After the mixture was cooled to room temper-
ature, 1,4-diisopropylbenzene (1.0 g) was added as an internal
standard. The mixture was then diluted with toluene (24 mL) and
analyzed by GC using authentic acetophenone. The results are
shown in Table 4.
3. Results and discussion
3.1. Synthesis of iridium complexes bearing substituted 2-
pyridonates
2
.6. Procedures for acceptorless dehydrogenation of 2-octanol
We started to synthesize iridium complexes bearing various 5-
catalyzed by 2bc (eq (2))
susbstituted 2-pyridonates, because it is considered that the 5-
position (para-position to the hydroxyl group) must be the most
influential position for the electronic effect of the 2-pyridonate
ligand. Since it is expected that an electron withdrawing group
A solution of 2-octanol (652 mg, 20 mmol) and catalyst 2bc
(
1
5.7 mg, 0.01 mmol) in toluene (2 mL) was refluxed for 20 h at
ꢀ
35 C (oil bath temperature). After the mixture was cooled to
(EWG) could increase the basicity of the 2-pyridonate ligand as the
room temperature, biphenyl (256 mg) was added as an internal
standard. The mixture was then diluted with toluene (8 mL) and
analyzed by GC using authentic 2-ocatnone to give 91% yield of
the product.
proton acceptor in the activation step as well as the acidity of the 2-
hydroxyl group as the proton donor in the dehydrogenation step,
we have introduced several electron-withdrawing groups at the 5-
position. In addition, methyl group is introduced to the 4-position
(para-position to the nitrogen atom) as the electron-donating
Table 3
Acceptorless dehydrogenation of 1-phenylethanol catalyzed by various Cp*Ir 2-
group (EDG) in order to increase coordination ability of the ligand
pyridonate complexes.a
Table 5
Acceptorless dehydrogenation of primary alcohols catalyzed by 2bc.a
Entry
Catalyst
Conversion (%)b
Yield (%)b
1
2
3
4
5
6
7
8
9
2aa
2ab
2ac
2ad
2ae
2af
2bc
2bd
2be
2bf
84
37
98
92
70
68
99
95
97
87
84
35
97
91
70
68
99
95
97
87
Yield (%)b
Entry
R
1
2
3
4
5
Ph
4-BrC
4-F
4-MeO
c-C
94
76
52
57
60
36
6
H
4
3
CC
6
H
4
2 6 4
CC H
6
H
11
c
6
7
C H
15
10
a
The reaction was carried out with an alcohol (0.5 mmol), catalyst (2.0 mol%) in
a
toluene (9 mL) under reflux for 20 h.
The reaction was carried out with 1-phenylethanol (10 mmol), catalyst (0.1 mol
b
Determined by GC.
%
) in toluene (3 mL) under reflux for 20 h.
c
b
The reaction was conducted in p-xylene.
Determined by GC.

18
R. Yamaguchi et al. / Journal of Organometallic Chemistry 843 (2017) 14e19
apparent that introduction of methyl group at the 4-position
clearly improved the catalytic performance to give acetophe-
none in excellent yields (entries 3, 4, and 5 vs. entries 7, 8, and 9).
These results indicate that the prediction mentioned above is
valid for consideration of the catalytic performance of Cp*Ir(2-
pyiridonate)Cl complexes.
Fig. 3. Substituted 2-hydorxypyridines.
In order to precisely compare the catalytic activities of com-
plexes 2ac, 2bc, 2bd, and 2be, the reactions using a smaller amount
of the catalyst (0.05 mol%) were carried out. The results are sum-
marized in Table 4. It is obvious that complex 2bc demonstrates the
highest catalytic performance among the complexes examined, and
TON reached up to 1660 in 20 h (entry 2).
It should be added that acceptorless dehydrogenation of an
aliphatic secondary alcohol can also be effectively catalyzed by
complex 2bc. Thus, the reaction of 2-octanol was conducted by
using 0.2 mol% of 2bc under reflux in toluene for 20 h to give 2-
to iridium center (Fig. 3).
The synthetic route is depicted in eq. (1). Treatment of a variety
of substituted 2-hydroxylpyridines (1) with sodium ethoxide gave
sodium
(
salts,
which
then
reacted
with
2 2
[Cp*IrCl ]
Cp* ¼ pentamethylcyclopentadienyl) to give the corresponding
various Cp*Ir 2-pyridonate complexes (2) in 29e75% yields after
recrystallization.
(
1)
1
The structures of the complexes were determined by H and
octanone in 91% yield (equation (2)).
13
C NMR spectroscopy, and elemental analyses and those of
complexes 2ad, 2bc, and 2bf were further confirmed by X-ray
analyses (Fig. 2). The bond parameters are summarized in
Table 2. Bond lengths of Ir-N in complexes 2ad, 2bc, and 2bf
were 2.130(4), 2.098(6), 2.119(4) Å, respectively. Bond lengths of
Ir-O in complexes 2ad, 2bc, and 2bf were 2.198(4), 2.216(5),
(2)
2
.192(4) Å, respectively. Bond lengths of Ir-N and Ir-O in com-
plexes 2ad and 2bf were close to the corresponding bond
lengths in complex 2aa [Ir-N: 2.118(4) Å; Ir-O: 2.181(4) Å] [16].
On the other hand, the bond length of Ir-N in complex 2bc was
slightly shorter than that in complex 2aa, while Ir-O in 2bc was
much longer than that in 2aa, indicating stronger coordination
of the pyridonate ligand in 2bc.
3
2
.3. Acceptorless dehydrogenation of primary alcohols catalyzed by
bc
We have next examined the reactions of various primary al-
3
.2. Acceptorless dehydrogenation of 1-phenylethnol catalyzed by
cohols by using highly active catalyst 2bc, because acceptorless
dehydrogenations of primary alcohols are generally more diffi-
cult than those of secondary alcohols. The results are shown in
Table 5. While the reactions of benzylic alcohols carried out by
using 2.0 mol% of 2bc in refluxing toluene to give the corre-
sponding benzaldehydes in moderate to high yields (entries
1e4), the reactions of aliphatic primary alcohols gave lower
yields of aliphatic aldehydes (entries 5 and 6). These results
indicate that the catalytic performance of 2bc is higher than that
of catalysts A and B bearing 2-hydroxypyridine-based ligands,
because A did not exhibit high activity [16] for the dehydroge-
nation of primary alcohols and B showed high activity only under
basic conditions [17].
various Cp*Ir 2-pyridonate complexes
We next examined acceptorless dehydrogenation of 1-
phenylethanol catalyzed by a series of Cp*Ir 2-pyridonate com-
plexes (2aa-2bf) as a benchmark test to compare their catalytic
performance. The reactions were conducted by using 0.1 mol% of
the complexes in toluene at reflux temperature for 20 h. The
results are shown in Table 3. Electron-withdrawing groups such
as 5-trifluoromethyl and 5-methoxylcarobonyl groups increased
the yields of acetophenone (entries 3 and 4), whereas introduc-
tion of electron-donating methyl group at the 5-position signif-
icantly retarded the reaction (entry 2). Furthermore, it is

R. Yamaguchi et al. / Journal of Organometallic Chemistry 843 (2017) 14e19
19
4
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In summary, we have synthesized a series of Cp*Ir complexes
bearing 5-substituted 2-pyridonate ligands and compared their
catalytic performance for acceptorless dehydrogenation of 1-
phenylethanol. Among various electron-withdrawing groups such
as trifluoromethyl, methoxycarbonyl, cyano, and, nitro groups,
Cp*Ir complex 2ac bearing 5-trifluoromethyl-2-pyridonate ligand
exhibited the higher catalytic activity. Furthermore, we have dis-
closed that introduction of methyl group at the 4-position in pyr-
idonate ligand improved the catalytic performance. Thus, Cp*Ir
complex 2bc bearing 4-methyl-5-trifuloromethyl-2-pyridonate
ligand gave the best result. Furthermore, complex 2bc demon-
strated good catalytic activity for acceptorless dehydrogenation of
primary alcohols. These results may provide the valuable infor-
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