Transfer Hydrogenation of Ketones and Imines with Methanol under Base-Free Conditions Catalyzed by an Anionic Metal-Ligand Bifunctional Iridium Catalyst

Article abstract of DOI:10.1021/acs.joc.9b02957

An anionic iridium complex [Cp*Ir(2,2′-bpyO)(OH)][Na] was found to be a general and highly efficient catalyst for transfer hydrogenation of ketones and imines with methanol under base-free conditions. Readily reducible or labile substituents, such as nitro, cyano, and ester groups, were tolerated under present reaction conditions. Notably, this study exhibits the unique potential of anionic metal-ligand bifunctional iridium catalysts for transfer hydrogenation with methanol as a hydrogen source.

Full text of DOI:10.1021/acs.joc.9b02957

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Article
Transfer Hydrogenation of Ketones and Imines with
Methanol under Base-Free Conditions Catalyzed by
an Anionic Metal-Ligand Bifunctional Iridium Catalyst
Rongzhou Wang, Xingyou Han, Jing Xu, Peng Liu, and Feng Li
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02957 • Publication Date (Web): 08 Jan 2020
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Transfer Hydrogenation of Ketones and Imines with
Methanol under Base-Free Conditions Catalyzed by
an Anionic Metal-Ligand Bifunctional Iridium
Catalyst
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Rongzhou Wang,†,§ Xingyou Han,†,§ Jing Xu,† Peng Liu,† and Feng Li*†,‡
†School of Chemical Engineering, Nanjing University of Science & Technology, Nanjing
210094, People’s Republic of China
‡State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024,
People’s Republic of China
[Na]
O
OH
R2
O
Ir OH
N
R1
R2
R2
R1
O
N
[Cp*Ir(2,2'-bpyO)(OH)][Na] (1 mol %)
under base-free conditions
functional group friendly
up to 96% yield
H
MeOH, 66 oC, 12 h
N
N
R2
R1
R1
45 examples
Anionic metal-ligand bifunctional catalyst
ABSTRACT:
An anionic iridium complex [Cp*Ir(2,2’-bpyO)(OH)][Na] was found to be a general and
highly efficient catalyst for transfer hydrogenation of ketones and imines with methanol under
base-free conditions. Readily reducible or labile substituents, such as nitro, cyano and ester
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groups, were tolerated under present reaction conditions. Notably, this study exhibited unique
potential of anionic metal-ligand bifunctional iridium catalysts for transfer hydrogenation with
methanol as a hydrogen source.
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■INTRODUCTION
The reduction of ketones and imines represents one of the most important transformations both
in the laboratory and in industry. With increasing global environmental concerns, extensive
effort has been devoted to transition metal-catalyzed transfer hydrogenation of ketones and
imines using 2-isopropal, formic acid or its derivatives as hydrogen sources1 due to that it avoids
the use of stoichiometric amount of reducing reagents such as LiAlH4 and NaBH4,2 and
hazardous molecular hydrogen and high-pressure equipment.3 Methanol, the simplest alcohol,
can be from natural gas, coal and, carbon dioxide and renewable biomass. Although methanol
has been utilized as a fuel, chemical feedstock and energy storage media,4 it was used as a
hydrogen source for transition metal-catalyzed transfer hydrogenation remained less explored
despite excellent hydrogen carrier ability (about 12.5 wt% hydrogen). Compared with higher
alcohols, the dehydrogenation of methanol required relatively high energy.5 Recently, Garcia and
co-workers reported Nickel-catalyzed transfer hydrogenation of α,β-unsaturated enones with
o
methanol at 180 C.6 Chen and co-worker described transfer hydrogenation of biomass-based
furfural and 5-hydroxymethylfurfural with methanol over hydrotalcite-derived copper catalysts
o
at > 200 C.7 More recently, Xiao and co-workers demonstrated transfer hydrogenation of
o
aromatic aldehydes with methanol at 90 C catalyzed by a cyclometalated rhodium complex in
the presence of 0.5 equiv of base.8 To the best of our knowledge, Crabtree and workers explored
the only an example of transfer hydrogenation of ketones and imines with methanol catalyzed by
a bis-NHC iridium complex bearing CO ligands to date.9 However, this procedure required high
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catalytic loading (5 mol%), large amount of strong base (KOH, 1-5 equiv), high temperature
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(120 C) and microwave irradiation, and still suffer from highly limited scope of substrates and
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low yields. Therefore, the development of a general and efficient organometallic catalyst for
transfer hydrogenation of ketones and imines with methanol under environmentally more
friendly conditions is still an extreme challenging subject.
In recent years, Fujita and co-workers synthesized a range of iridium complexes bearing a
bipyridine or a bipyridonate ligand, which were found to be highly efficient catalysts for
acceptorless dehydrogenation of alcohols and N-heterocycles,10 and hydrogen production from a
methanol-water solution under basic conditions.11 We reported also that these complexes are
effective metal-ligand bifunctional catalysts for hydrogen auto-transfer process,12 acceptorless
dehydrogenative cyclization13 and transfer hydrogenation of aldehydes with isopropal.14 As a
continuing interest in developing environmentally friendly reaction,12-15 we herein wish to report
transfer hydrogenation of ketones and imines with methanol under base-free conditions .
■RESULTS AND DISCUSSION
Initially, the transfer hydrogenation of acetophenone (1a) with methanol was selected as a model
to explore the feasibility of reaction. As shown in Scheme 1, a series of iridium complexes, such
as [Cp*IrCl2]2 (Cp* = pentamethylcyclopentadienyl) (cat. 1), [Cp*Ir(H2O)3][OTf]2 (cat. 2),
[Cp*Ir(NH3)3][Cl]2 (cat. 3), [Cp*Ir(bpy)Cl)][Cl] (cat. 4), [Cp*Ir(6,6’-(OMe)2-2,2’-
bpy)(H2O)][OTf]2 (cat. 5), [Cp*Ir(2-(OH)py)]Cl2 (cat. 6), [Cp*Ir(6,6’-(OH)2-2,2’-
bpy)(H2O)][OTf]2 (cat. 7), [Cp*Ir(2,2’-bpyO)(H2O)] (cat. 8) and [Cp*Ir(2,2’-bpyO)(OH)][Na]
(cat. 9), were examined for their catalytic activity for this model reaction. In the presence of cat.
1-7 (1 mol%), the reaction of 1a (1 mmol) with methanol (2 mL) as both solvent and hydrogen
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donor was carried out at 66 oC for 12 h, and none of product was detected. When Cp*Ir complex
bearing a bipyridonate ligand [Cp*Ir(2,2’-bpyO) (cat. 8) was used as a catalyst, the reaction gabe
product 2a in 16% yield. To our surprise, the product 2a was obtained in 93% yield when an
anionic iridium complex [Cp*Ir(2,2’-bpyO)(OH)][Na] (cat. 9) was used as an alternative catalyst.
Inspired by above promising result, the scope of reaction with a range of ketones (1) under
optimal conditions was investigated and these results are shown in Scheme 2. Reactions of
acetophenones bearing an electron-donating substituent afforded desired products 2b-2e in 78-
84% yields. As acetophenones bearing one or two halogen were utilized as substrates,
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Scheme 1. Transfer Hydrogenation of Acetophenone with Methanol using a series of Iridium
Catalysts.a,b
O
OH
catalyst (1 mol %)
MeOH, 66 oC, 12 h
1a
2a
[OTf]2
2Cl-
Cl
Cl
Cl
Ir
Ir
Ir
Ir
H2O
H2O
H3N
H3N
Cl
OH2
NH3
cat. 1, n.d.
cat. 2, n.d.
cat. 3, n.d.
Cl-
[OTf]2
MeO
HO
Ir Cl
N
Ir H2O
Ir Cl
Cl
N
OMe
N
N
N
cat. 4, <5 %
[OTf]2
cat. 5, n.d.
cat. 6, n.d.
-
Na+
HO
O
O
Ir H2O
Ir OH
Ir H2O
OH
N
O
O
N
N
N
N
N
cat. 7, n.d.
cat. 8, 16%
cat. 9, 93% (87%)c
aReactions conditions: 1a (1 mmol), MeOH (2 mL), catalyst (1 mol %), 66 oC, 12 h, under N2.
bNMR Yield was determined based on the 1H NMR spectrum of the crude reaction. cIsolated
yield.
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reactions proceeded to give corresponding products 2f-2m in 85-94% yields. Strong electron-
withdrawing substituents, such as trifluoromethyl, nitro, cyano and ester groups, were also
tolerated and desired products 2n-2q could be obtained in 87-93% yields. Furthermore, 2-
acetylpyridine and 2-acetonaphthone were successfully converted to corresponding products 2r
and 2s in 96% and 90% yields, respectively. This catalytic system was also proven to be
effective to non-methyl ketones, such as propiophenone, butyrophenone and benzophenone,
affording desired products 2t-2v in 87-93% yields. For aliphatic ketones, such as 2-dodecanone
and cyclohexanone, corresponding products 2w and 2x were obtained in 78% and 80% yields,
respectively. Interestingly, when unsaturated ketones, such as (E)-chalcone and (E)-4-Phenyl-3-
buten-2-one, were conducted, desired products 2y and 2z were obtained in 83% and 80% yields,
respectively, indicating C=O and C=C bonds were simultaneously hydrogenated under present
conditions. When benzylaldehyde as a substrate was examined, the product 2za was obtained in
the 55% yield with mehyl benzoate as a by-product (33% yield).16
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Scheme 2. Transfer Hydrogenation of a Variety of Ketones with Methanola,b
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9
O
OH
R2
cat. 9 (1 mol %)
R1
R2
R1
MeOH, 66 oC, 12 h
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1
2
OH
OH
OH
Me
Me
Me
2b, 78% yield
2c, 84% yield
2d, 81% yield
OH
OH
OH
MeO
F
F
2e, 82% yield
2f, 94% yield
2g, 90% yield
OH
Cl
OH
OH
Br
Cl
Br
Cl
I
2h, 92% yield
2i,.94% yield
2j, 88% yield
OH
OH
OH
I
2k, 92% yield
2l, 85% yield
2m, 87% yield
OH
OH
OH
O2N
NC
2p, 93% yield
F3C
2n, 90% yield
2o, 92% yield
OH
OH
OH
N
MeO2C
2q, 86% yield
2r, 96% yield
2s, 90% yield
OH
OH
OH
2t, 88% yield
2u, 87% yield
2v, 93% yield
OH
OH
OH
9
2w, 78% yield
2x, 80% yield
2y, 83% yield
OH
OH
2z, 80% yield
2za, 55% yield
aReactions conditions: 1 (1 mmol), MeOH (2 mL), cat. 9 (1 mol %), 66 oC, under N2, 12 h.
bIsolated yield.
Ttransfer hydrogenation of a series of imines with methanol was then examined (Scheme 3).
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The reaction of N-benzylideneaniline afforded the corresponding product 4a in 81% yield.
Similarly, N-benzylideneanilines bearing an electron-donating group were converted to desired
products 4b-4f in 75-83% yields. This system was also proven to be effective to N-
benzylideneanilines bearing one or two halogens, or a strong electron-withdrawing substituent,
affording corresponding products 4g-4n in 71-88% yield. Furthermere, highly catalytic activities
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were
found
when
(E)-N-benzylidenenaphthalen-1-amine,
(E)-N-(pyridin-2-
ylmethylene)benzenamine, (E)-N-benzylidene(phenyl)methanamine and aliphatic benzylidene-
butylamine were used as substrates and corresponding products 4o-4r were obtained in 79%-
86% yields.
Scheme 3. Transfer Hydrogenation of a Series of Imines with Methanola,b
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8
H
N
R2
cat. 9 (1 mol %)
N
R2
R1
R1
MeOH, 66 oC, 12 h
4
3
9
H
H
H
N
N
N
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Me
Me
4c, 76% yield
4a, 81% yield
4b, 75% yield
OMe
Me
OMe
H
H
N
H
N
N
4d, 75% yield
4e, 80% yield
4f, 83% yield
F
Cl
Br
H
N
H
N
H
N
F
4g, 77% yield
4h, 72% yield
4i, 81% yield
Cl
H
H
N
H
N
N
Cl
Br
Cl
4j, 80% yield
4k, 88% yield
4l, 81% yield
H
N
H
N
H
N
F3C
4m, 71% yield
4n, 87% yield
4o, 86% yield
N
H
H
N
N
N
H
4p, 86% yield
4q, 83% yield
4r, 79% yield
aReaction conditions: 3 (1 mmol), MeOH, cat. 9 (1 mol %), 66 oC, under N2, 12 h. bIsolated yield
.
A possible mechanism for this transfer hydrogenation of ketones and imines with methanol
under base-free conditions was proposed (Scheme 4). Initially, anionic methoxo species A were
generated via the reaction of cat. 9 with methanol. With ß-hydrogen elimination of species A,
iridium hydride species B were formed and formaldehyde was released. The protonation of
bipyridonate ligand by methanol afforded neutral hydrido species C. Furthermore, simultaneous
transfer of the proton on the hydroxy and the hydride on iridium to C=O or C=N bonds of
ketones or imines took place, resulting in the liberation of alcohols or amines as products and the
formation of unsaturated species D.17 Finally, catalytic species A were regenerated by the
reaction of species D with methoxide anion. Ligand-promoted simultaneous delivery of proton
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and hydride to ketones or imides was also proposed for other metal-ligand bifunctional catalysts,
such as Ru-TsDPEN systems18 and Shvo’s catalyst.19
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Scheme 4. Proposed Reaction Mechanism
_
Cp*
O
Ir OH
O
N
N
cat. 9
MeOH
H2O
_
Cp*
O
-OMe
O
Ir OMe
O
N
H
H
_
N
A
Cp*
H
Cp*
O
O
Ir
Ir
N
O
N
O
N
N
R2
O
Cp*
D
B
H
R1
H
O
Ir
N
O
N
MeOH
OH
R2
Cp*
H
O
R1
Ir
C
or
N
OH
N
or
-OMe
H
R2
N
R2
R1
Cp*
H
O
O
N
R1
Ir
C
R1
R2
N
O
N
or
H
N
R2
R1
: vacant site
C
To obtain further the information of reaction mechanism, kinetic studies were undertaken
(Scheme 5). Under standard reaction conditions, two parallel reactions of 1v with CH3OH and
CD3OD were proceeded and kinetic isotope effect (KIE) (KH/KD = 1.65) was found (Scheme 5).
This result suggested that C-H bond cleavage of methanol may be involved in the rate-
determining step.
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Scheme 5. Kinetic Isotope Effect Studies
9
OH
O
OD
D
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cat. 9 (1 mol %)
CH3OH, 66 o
cat. 9 (1 mol %)
C
CD3OD, 66 o
C
2v
1v
5
KH/KD = 1.65
parallel reactions
The practical potential of this methodology was explored. The gram-scale hydrogenation of 1a
(20 mmol) was performed in the presence of cat. 9 (0.5 mol %) to give the corresponding
product 2a in 85% yield (Scheme 6).
Scheme 6. Large-Scale Hydrogenation of 1a with Methanol
OH
O
cat. 9 (1 mol %)
MeOH, 66 oC, 12 h
1a (20 mmol)
2a, 85% yield
Furthermore, the synthesis of γ-valerolactone (GVL) via transfer hydrogenation of levulinate
ester (LE),20 one of the important biomass-derived chemicals, with methanol was represented. In
the presence of cat. 9 (1 mol %), the reaction of 6 was carried out at 130 °C for 12 h to give the
desired product 7 in 70% yield (Scheme 7).
Scheme 7. Synthesis of GVL via Transfer Hydrogenation of LE with Methanol
O
O
cat. 9 (1 mol %)
O
OCH3
MeOH, 130 oC, 12 h
O
6
7, 70% yield
■CONCLUSIONS
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We have demonstrated that an anionic iridium complex [Cp*Ir(2,2’-bpyO)(OH)][Na] is a
general and highly efficient catalyst for transfer hydrogenation of ketones and imines with
methanol under base-free conditions. Readily reducible or labile substituents, such as nitro,
cyano and ester groups, were tolerated under present reaction conditions. Furthermore, this
catalytic system was also applied to the gram-scale reaction and the biomass conversion. Notably,
this study exhibited unique potential of anionic metal-ligand bifunctional iridium catalysts for
transfer hydrogenation with methanol as a hydrogen source.
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■EXPERIMENTAL SECTION
General Experimental Details. Melting points were measured on a X-6 micro-melting
apparatus. 1H NMR spectra were recorded on a 500 spectrometer. Chemical shifts are reported in
delta (δ) units, parts per million (ppm) downfield from tetramethylsilane or ppm relative to the
center of the singlet at 7.26 ppm for CDCl3. Coupling constants J values are reported in Hertz
(Hz), and the splitting patterns were designated as follows: s, singlet; d, doublet; t, triplet; m,
multiplet; b, broad. 13C{1H} NMR spectra were recorded on a 125 MHz spectrophotometer with
1
broadband H decoupling. Chemical shifts are reported in delta (δ) units, ppm relative to the
center of the triplet at 77.0 ppm for CDCl3. All reactions were run under an atmosphere of
nitrogen, unless otherwise indicated. Analytical thin-layer chromatography (TLC) was carried
out using 0.2-mm commercial silica gel plates.
[Cp*IrCl2]2 (Cp* = pentamethylcyclopentadienyl) (cat. 1),21 [Cp*Ir(H2O)3][OTf]2 (cat. 2),22
[Cp*Ir(NH3)3][Cl]2 (cat. 3),23 [Cp*Ir(bpy)Cl)][Cl] (cat. 4),24 [Cp*Ir(6,6’-(OMe)2-2,2’-
bpy)(H2O)][OTf]2 (cat. 5),25 [Cp*Ir(2-(OH)py)]Cl2 (cat. 6),10a [Cp*Ir(6,6’-(OH)2-2,2’-
bpy)(H2O)][OTf]2 (cat. 7),10b [Cp*Ir(2,2’-bpyO)(H2O)] (cat. 8)10c and [Cp*Ir(2,2’-
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bpyO)(OH)][Na] (cat. 9)11 were synthesized according the previous reports.
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General procedure for transfer hydrogenation of ketones and imines catalyzed by
[Cp*Ir(2,2’-bpyO)(H2O)][Na] (Schemes 1-3). In a 25-mL Schlenk tube, ketones or imines (1
mmol), methnaol (2 mL), cat. 9 (5.7 mg, 1 mol %) were placed under an N2 atmosphere, and the
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reaction mixture was heated at 66 C in an oil bath for 12 h. The mixture was then cooled to
ambient temperature, concentrated in vacuo and purified by flash column chromatography
(hexanes/ethyl acetate = 10/1, v/v) to afford the corresponding products.
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1-Phenylethanol (2a).26 Light yellow oil; 87% yield (106 mg); H NMR (500 MHz, CDCl3) δ
7.37-7.33 (m, 4H), 7.28-7.25 (m, 1H), 4.89 (q, J = 6.3 Hz, 1H), 1.96 (br s, 1H), 1.49 (d, J = 6.5
Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 145.8, 128.5, 127.4, 125.3, 70.4, 25.1.
1-(m-Tolyl)ethanol (2b).26 Light yellow oil; 78% yield (106 mg); 1H NMR (500 MHz, CDCl3) δ
7.24-7.22 (m, 1H), 7.17 (s, 1H), 7.15 (d, J = 7.7 Hz, 1H), 7.08 (d, J = 7.5 Hz, 1H), 4.84-4.81 (m,
1H), 2.35 (s, 3H), 2.03 (br s, 1H), 1.47 (d, J = 6.5 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ
145.8, 138.1, 128.4, 128.1, 126.1,122.4, 70.3, 25.0, 21.4.
1-(p-Tolyl)ethanol (2c).27 Light yellow oil; 84% yield (114 mg); 1H NMR (500 MHz, CDCl3) δ
7.25 (d, J = 8.0 Hz, 2H), 7.15 (d, J = 7.8 Hz, 2H), 4.95-4.83 (m, 1H), 2.33 (s, 3H), 1.95 (br s,
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1H), 1.47 (d, J = 6.5 Hz, 3H); C {1H} NMR (125 MHz, CDCl3) δ 142.8, 137.1, 129.1, 125.3,
70.2, 25.0, 21.0.
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1-(4-Ethylphenyl)ethanol (2d).26 Light yellow oil; 81% yield (121 mg); H NMR (500 MHz,
CDCl3) δ 7.30 (d, J = 8.0 Hz, 2H), 7.19 (d, J = 8.0 Hz, 2H), 4.88-4.86 (m, 1H), 2.65 (q, J = 7.6
Hz, 2H), 1.79 (br s, 1H), 1.49 (d, J = 6.5 Hz, 3H), 1.23 (t, J = 7.6 Hz, 3H); 13C {1H} NMR (125
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MHz, CDCl3) δ 143.6, 143.1, 128.0, 125.4, 70.3, 28.5, 25.0, 15.6.
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1-(3-Methoxyphenyl)ethanol (2e).26 Light yellow oil; 82% yield (125 mg); 1H NMR (500 MHz,
CDCl3) δ 7.28-7.25 (m, 1H), 6.94 (s, 1H), 6.82 (d, J = 8.1 Hz, 1H), 4.89-4.87 (m, 1H), 3.82 (s,
3H), 1.86 (br s, 1H), 1.50 (d, J = 6.5 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 159.7, 147.6,
129.4, 117.6, 112.8, 110.9, 70.2, 55.1, 25.1.
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1-(3-Fluorophenyl)ethanol (2f).26 Light yellow oil; 94% yield (132 mg); H NMR (500 MHz,
CDCl3) δ 7.32-7.28 (m, 1H), 7.13-7.08 (m, 2H), 6.95 (td, d, J = 8.4 and 2.3 Hz, 1H), 4.91-4.86
(m, 1H), 2.02 (br s, 1H), 1.48 (d, J = 6.5 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 164.0 (d,
JC-F = 244.5 Hz), 148.5 (d, J = 6.4 Hz), 130.0 (d, J = 8.1 Hz), 120.9, 114.3 (d, J = 21.1 Hz),
112.4 (d, J = 21.7 Hz), 69.8, 25.2.
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1-(4-Fluorophenyl)ethanol (2g).26 Light yellow oil; 90% yield (126 mg); H NMR (500 MHz,
CDCl3) δ 7.33-7.31 (m, 2H), 7.03-7.00 (m, 2H), 4.87-4.86 (m, 1H), 2.09 (br s, 1H), 1.47 (d, J =
6.5 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 163.0 (d, JC-F = 243.8 Hz), 141.5, 127.0 (d, J
C-F = 8.0 Hz), 115.3 (d, JC-F = 21.1 Hz), 69.7, 25.2.
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1-(4-Chlorophenyl)ethanol (2h).26 Light yellow oil; 92% yield (144 mg); H NMR (500 MHz,
CDCl3) δ 7.32-7.29 (m, 4H), 4.90-4.85 (m, 1H), 1.93 (br s, 1H), 1.47 (d, J = 6.4 Hz, 3H); 13C
{1H} NMR (125 MHz, CDCl3) δ 144.2, 133.1, 128.6, 126.8, 68.7, 25.2.
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1-(2,4-Dichlorophenyl)ethanol (2i).27 Light yellow oil; 94% yield (179 mg); H NMR (500
MHz, CDCl3) δ 7.54 (d, J = 8.4 Hz, 1H), 7.34 (d, J = 2.0 Hz, 1H), 7.28-7.26 (m, 1H), 5.24-5.22
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(m, 1H), 2.10 (br s, 1H), 1.46 (d, J = 6.4 Hz, 3H); C {1H} NMR (125 MHz, CDCl3) δ 141.7,
133.4, 132.1,129.1, 127.5, 127.4, 66.6, 23.6.
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1-(3-Bromophenyl)ethanol (2j).26 Light yellow oil; 88% yield (176 mg); H NMR (500 MHz,
CDCl3) δ 7.53 (s, 1H), 7.40 (d, J = 7.9 Hz, 1H), 7.29 (d, J = 7.7 Hz, 1H), 7.21 (t, J = 7.8 Hz, 1H),
4.87-4.85 (m, 1H), 1.95 (br s, 1H), 1.48 (dd, J = 6.5 and 1.5 Hz, 3H); 13C {1H} NMR (125 MHz,
CDCl3) δ 148.1, 130.5, 130.1, 128.6, 124.0, 122.6, 69.7, 25.2.
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1-(4-Bromophenyl)ethanol (2k).26 Light yellow oil; 92% yield (185 mg); H NMR (500 MHz,
CDCl3) δ 7.47 (d, J = 8.4 Hz, 2H), 7.24 (d, J = 8.3 Hz, 2H), 4.86-4.84 (m, 1H), 2.0 (br s, 1H),
1.46 (d, J = 6.5 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 144.7, 131.5, 1127.1, 121.1, 69.7,
25.2.
1-(3-iodophenyl)ethanol (2l).28 Light yellow oil; 85% (211 mg); H NMR (500 MHz, CDCl3)
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7.7 (s, 1H), 7.58 (d, J = 7.85, 1H), 7.28 (d, J = 7.7 Hz, 1H), 7.06 (t, J = 7.78 Hz, 1H), 4.80 (q, J =
6.95 Hz, 1H), 2.80 (s, 1H), 1.42 (d, J = 6.50 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 148.1,
136.3, 134.4, 130.2, 124.6, 94.4, 69.4, 25.1.
1-(4-Iodophenyl)ethanol (2m).29 Light yellow oil, 87% (216 mg); 1H NMR (500 MHz, CDCl3)
δ 7.63-7.61 (d, J = 8.3 Hz, 2H), 7.05 (d, J = 8.0 Hz, 2H), 4.75 (m, J = 6.0 Hz, 1H), 2.22 (br s,
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1H), 1.43 (d, J = 6.5 Hz, 3H); C {1H} NMR (125 MHz, CDCl3) δ 145.3, 137.3, 127.3, 92.6,
69.6 , 25.1.
1-(4-(Trifluoromethyl)phenyl)ethanol (2n).30 Light yellow oil; 90% yield (171 mg); H NMR
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(500 MHz, CDCl3) δ 7.59 (d, J = 6.7 Hz, 2H), 7.45 (d, J = 8.2 Hz, 2H), 4.92 (q, J = 6.5 Hz, 1H),
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2.51 (br s, 1H), 1.47 (d, J = 6.5 Hz, 3H); C {1H} NMR (125 MHz, CDCl3) δ 149.7, 129.7 (q,
JC-F = 32.2 Hz), 125.6, 125.4, 123.1 (q, JC-F = 270.3 Hz), 69.7, 25.2.
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1-(4-Nitrophenyl)ethanol (2o).30 Light yellow oil; 92% yield (154 mg); H NMR (500 MHz,
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CDCl3) δ 8.21 (d, J = 8.6 Hz, 2H), 7.56 (d, J = 8.6 Hz, 2H), 5.04-5.02 (m. 1H), 2.08 (br s, 1H),
1.53 (d, J = 6.6 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 153.0, 147.2, 126.1, 123.7, 69.5,
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4-(1-Hydroxyethyl)benzonitrile (2p).30 Light yellow oil; 93% yield (136 mg); H NMR (500
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MHz, CDCl3) δ 7.64 (d, J = 8.2 Hz, 2H), 7.50 (d, J = 8.2 Hz, 2H), 4.97-4.95 (m, 1H), 2.17 (br s,
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1H), 1.50 (d, J = 6.6 Hz, 3H); C {1H} NMR (125 MHz, CDCl3) δ 151.1, 132.3, 126.0, 118.8,
111.0, 69.6, 25.4.
Methyl 4-(1-hydroxyethyl)benzoate (2q).31 Pale yellow oil; 86% yield (157 mg); 1H NMR (500
MHz,CDCl3) δ 7.95 (d, J = 8.3 Hz, 2H), 7.39 (d, J = 8.2Hz, 2H), 4.90 (q, J = 5.9 Hz, 1H), 3.88 (s,
3H), 2.91 (br s, 1H), 1.46 (d, J = 6.5 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 167.0, 151.0,
129.7, 128.8, 125.2, 69.7, 52.0, 25.1.
1-(Pyridin-2-yl)ethanol (2r).31 Light yellow oil; 96% yield (118 mg); H NMR (500 MHz,
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CDCl3) δ 8.51 (d, J = 4.1 Hz, 1H), 7.68 (td, J = 7.6 and 1.5 Hz, 1H), 7.32 (d, J = 8.1 Hz, 1H),
7.19-7.16 (m, 1H), 4.92-4.87 (m, 1H), 4.63 (br s, 1H), 1.51 (d, J = 6.9 Hz, 3H); 13C {1H} NMR
(125 MHz, CDCl3) δ 163.3, 148.0, 136.7, 122.1, 119.7, 68.9, 24.1.
1-(Naphthalen-2-yl)ethanol (2s).26 White solid; 90% yield (154 mg); mp 76-77 °C; H NMR
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(500 MHz, CDCl3) δ 7.85-7.81 (m, 4H), 7.51-7.46 (m, 3H), 5.10-5.06 (m, 1H), 1.91 (br s, 1H),
1.59 (d, J = 6.5 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 143.2, 133.3, 132.9, 128.3, 127.9,
127.6, 126.1, 125.8, 123.78, 123.77, 70.5, 25.1.
1-Phenylpropan-1-ol (2t).26 Light yellow oil; 88% yield (119 mg); 1H NMR (500 MHz, CDCl3)
δ 7.35-7.34 (m, 4H), 7.29-7.26 (m, 1H), 4.61-4.68 (m, 1H), 1.90 (br s, 1H), 1.85-1.72 (m, 2H),
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0.91 (t, J = 7.4 Hz, 3H); C {1H} NMR (125 MHz, CDCl3) δ 144.6, 128.4, 127.5, 125.9, 76.0,
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1-Phenylbutan-1-ol (2u).32 Light yellow oil; 87% yield (131 mg); 1H NMR (500 MHz, CDCl3) δ
7.34-7.33 (m, 4H), 7.28-7.25 (m, 1H), 4.67 (t, J = 6.7 Hz, 1H), 1.91 (br s, 1H), 1.82-1.75 (m, 1H),
1.71-1.64 (m, 1H), 1.47-1.39 (m, 1H), 1.35-1.25 (m, 1H), 0.93 (t, J = 7.4 Hz, 3H); 13C {1H}
NMR (125 MHz, CDCl3) δ 144.9, 128.4, 127.4, 125.9, 74.4, 41.2, 19.0, 13.9.
Diphenylmethanol (2v).26 White solid; 93% yield (171 mg); mp 65-66 °C; 1H NMR (500 MHz,
CDCl3) δ 7.38-7.31 (m, 8H), 7.27-7.24 (m, 2H), 5.83 (s, 1H), 2.24 (br s, 1H); 13C {1H} NMR
(125 MHz, CDCl3) δ 143.8, 128.5, 127.6, 126.5, 76.3.
Dodecan-2-ol (2w).33 Light yellow oil; 78% yield (145 mg); 1H NMR (500 MHz, CDCl3) δ 3.82-
3.76 (m, 1H), 1.46-1.40 (m, 4H), 1.31-1.26 (m, 15H), 1.19 (d, J = 6.2 Hz, 3H), 0.88 (t, J = 6.9
Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3)
δ
68.3, 39.4, 32.0, 29.8, 29.7
(three peaks overlapping with each other), 29.5, 25.9, 23.5, 22.8. These spectroscopic datas
correspond to reported datas.
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Cyclohexanol (2x).26 Colorless oil; 80% yield (80 mg); H NMR (500MHz, CDCl3) δ 3.57 (m,
1H), 3.20 (br s, 1H), 1.89 (s, 2H), 1.72 (s, 2H), 1.54 (m, 1H), 1.24 (m, 4H), 1.16 (m, 1H); 13C
{1H} NMR (125 MHz, CDCl3) δ 69.9, 35.2, 25.3, 24.0.
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1,3-Diphenylpropan-1-ol (2y).34 Light yellow oil; 83% yield (176 mg); H NMR (500 MHz,
CDCl3) δ 7.35-7.34 (m, 4H), 7.28-7.24 (m, 3H), 7.19-7.16 (m, 3H), 4.69-4.66 (m, 1H), 2.77-2.63
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(m, 2H), 2.16-2.00 (m, 2H), 1.92 (br s, 1H); C {1H} NMR (125 MHz, CDCl3) δ 144.6, 141.8,
128.5, 128.4, 128.4, 127.6, 125.9, 125.8, 73.9, 40.5, 32.0.
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4-Phenylbutan-2-ol (2z).35 Light yellow oil; 80% yield (120 mg); 1H NMR (500 MHz, CDCl3) δ
7.29-7.26 (m, 2H), 7.20-7.16 (m, 3H), 3.85-3.79 (m, 1H), 2.78-2.63 (m, 2H), 1.81-1.71 (m, 2H),
1.57 (br s, 1H), 1.23 (d, J = 6.2 Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 142.0, 128.3
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(two peaks overlapping with each other),
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These spectroscopic datas correspond to reported datas.
Phenylmethanol (2xa).35 Light yellow oil; 55% yield (59 mg); 1H NMR (500 MHz, CDCl3)
δ 7.35-7.30 (m, 4H), 7.27-7.25 (m, 1H), 4.60 (s, 2H), 2.47 (br s, 1H); 13C {1H} NMR (125 MHz,
CDCl3) δ 140.4, 128.0, 127.0, 126.5, 64.3.
N-Benzylaniline (4a).36 Light yellow oil; 81% yield (148 mg); H NMR (500 MHz, CDCl3) δ
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7.37-7.31 (m, 4H), 7.28-7.25 (m, 1H), 7.17 (d, J = 7.9 Hz, 2H), 6.71 (t, J = 7.3 Hz, 1H), 6.64-
6.62 (d, J = 7.9 Hz, 2H), 4.32 (s, 2H), 4.01 (br s, 1H); 13C {1H} NMR (125 MHz, CDCl3) δ
148.1, 139.4, 129.2, 128.6, 127.5, 127.2, 117.5, 112.8, 48.3.
N-(3-Methylbenzyl)aniline (4b).36 Light yellow oil; 75% yield (148 mg); H NMR (500 MHz,
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CDCl3) δ 7.23 (d, J = 7.5 Hz, 1H), 7.19-7.16 (m, 4H), 7.10 (d, J = 7.5 Hz, 1H), 6.71 (t, J = 7.3
Hz, 1H), 6.65 (d, J = 7.8 Hz, 2H), 4.28 (s, 2H), 3.99 (br s, 1H), 2.35 (s, 3H); 13C {1H} NMR
(125 MHz, CDCl3) δ 148.2, 139.3, 138.3, 129.2, 128.5, 128.3, 128.0, 124.6, 117.5, 112.8, 48.3,
21.4.
N-Benzyl-4-methylaniline (4c).36 Light yellow oil; 76% yield (150 mg); H NMR (500 MHz,
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CDCl3) δ 7.37-7.31 (m, 4H), 7.26-7.25 (m, 1H), 6.99-6.97 (m, 2H), 6.57-6.55 (m, 2H), 4.30 (s,
2H), 3.89 (br s, 1H), 2.23 (s, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 145.9, 139.6, 129.7,
128.6, 127.5, 127.1, 126.7, 113.0, 48.6, 20.4.
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N-(4-Ethylbenzyl)aniline (4d).36 Light yellow oil; 75% yield (158 mg); H NMR (500 MHz,
CDCl3) δ 7.29-7.27 (m, 2H), 7.18-7.15 (m, 4H), 6.73-6.69 (m, 1H), 6.64-6.62 (m, 2H), 4.27 (m,
2H), 3.96 (br s, 1H), 2.66-2.61 (m, 2H), 1.26-1.21 (m, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ
148.2, 143.3, 136.6, 129.2, 128.1, 127.6, 117.4, 112.8, 48.1, 28.5, 15.6.
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N-(4-Methoxybenzyl)aniline (4e).36 Light yellow solid; 80% yield (171 mg); mp 61-62 °C; H
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NMR (500 MHz, CDCl3) δ 7.30 (d, J = 8.6 Hz, 2H), 7.17 (t, J = 7.9 Hz, 2H), 6.89 (d, J = 8.6 Hz,
2H), 6.71 (t, J = 7.3 Hz, 1H), 6.64 (d, J = 7.8 Hz, 2H), 4.25 (s, 2H), 3.94 (br s, 1H), 3.80 (s, 3H);
13C {1H} NMR (125 MHz, CDCl3) δ 158.8, 148.2, 131.4, 129.2, 128.8, 117.5, 114.0, 112.8, 55.3,
47.8.
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N-Benzyl-2-methoxyaniline (4f).36 Light yellow oil; 83% yield (177 mg); H NMR (500 MHz,
CDCl3) δ 7.38-7.31 (m, 4H), 7.26 (t, J = 6.6 Hz, 1H), 6.84-6.77 (m, 2H), 6.68-6.65 (m, 1H), 6.59
(d, J = 7.5 Hz, 1H), 4.61 (br s, 1H), 4.34 (s, 2H), 3.83 (s, 3H); 13C {1H} NMR (125 MHz, CDCl3)
δ 146.8, 139.6, 138.1, 128.5, 127.5, 127.1, 121.3, 116.6, 110.0, 109.4, 55.4, 48.0.
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N-(4-Fluorobenzyl)aniline (4g).36 Light yellow oil; 77% yield (155 mg); H NMR (500 MHz,
CDCl3) δ 7.31 (t, J = 6.8 Hz, 2H), 7.17 (t, J = 7.3 Hz, 2H), 7.01 (t, J = 8.6 Hz, 2H), 6.72 (t, J =
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6.8 Hz, 1H), 6.62 (d, J = 8.5 Hz, 2H), 4.28 (s, 2H), 3.99 (br s, 1H); C {1H} NMR (125 MHz,
CDCl3) δ 163.0 (d, JC-F = 243.6 Hz), 147.9, 135.1, 129.2, 129.0 (d, JC-F = 7.9 Hz), 117.7, 115.5
(d, JC-F = 21.2 Hz), 112.8, 47.5.
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N-Benzyl-4-fluoroaniline (4h).36 Light yellow oil; 72% yield (145 mg); H NMR (500 MHz,
CDCl3) δ 7.36-7.32 (m, 4H), 7.28-7.25 (m, 1H), 6.86 (t, J = 8.8 Hz, 2H), 6.55-6.53 (m, 2H), 4.27
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(s, 2H), 3.90 (br s, 1H); C {1H} NMR (125 MHz, CDCl3) δ 156.8 (d, JC-F = 237.5 Hz), 144.5,
139.2, 128.6, 127.4, 127.3, 115.7 (d, JC-F = 22.2 Hz), 113.6 (d, JC-F = 7.3 Hz), 48.9.
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N-(4-Chlorobenzyl)aniline (4i).36 Light yellow oil; 81% yield (176 mg); H NMR (500 MHz,
CDCl3) δ 7.31-7.27 (m, 4H), 7.16 (t, J = 7.9 Hz, 2H), 6.72 (t, J = 7.3 Hz, 1H), 6.60 (d, J = 7.7 Hz,
2H), 4.29 (s, 2H), 4.04 (br s, 1H); 13C {1H} NMR (125 MHz, CDCl3) δ 147.8, 138.0, 132.8,
129.3, 128.7, 128.7, 117.8, 112.8, 47.6.
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N-Benzyl-4-chloroaniline (4j).36 Light yellow oil; 80% yield (173 mg); H NMR (500 MHz,
CDCl3) δ 7.37-7.29 (m, 5H), 7.13-7.10 (m, 2H), 6.56-6.55 (m, 2H), 4.31 (s, 2H), 4.07 (br s, 1H);
13C {1H} NMR (125 MHz, CDCl3) δ 146.7, 138.9, 129.1, 128.7, 127.41, 127.37, 122.1, 113.9,
48.3.
N-Benzyl-2,4-dichloroaniline (4k).37 Light yellow oil; 88% yield (221 mg); 1H NMR (500 MHz,
CDCl3) δ 7.36-7.32 (m, 4H), 7.29-7.25 (m, 2H), 7.03 (dd, J = 8.8 and 2.4 Hz, 1H), 6.52 (d, J =
8.8 Hz, 1H), 4.71 (br s, 1H), 4.37 (d, J = 5.6 Hz, 2H); 13C {1H} NMR (125 MHz, CDCl3) δ
142.5, 138.2, 128.8, 128.7, 127.7, 127.5, 127.1, 121.3, 119.3, 112.0, 47.8.
N-(4-Bromobenzyl)aniline (4l).38 Light yellow solid; 81% yield (213 mg); mp 51-52 °C; H
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NMR (500 MHz, CDCl3) δ 7.46 (d, J = 8.4 Hz, 2H), 7.25-7.24 (m, 2H), 7.17 (t, J = 7.9 Hz, 2H),
13
6.72 (t, J = 7.3 Hz, 1H), 6.61 (d, J = 7.8 Hz, 2H), 4.29 (s, 2H), 4.06 (br s, 1H); C {1H} NMR
(125 MHz, CDCl3) δ 147.8, 138.5, 131.6, 129.3, 129.0, 120.9, 117.8, 112.8, 47.6.
1
N-Benzyl-4-bromoaniline (4m).36 Light yellow oil; 71% yield (184 mg); H NMR (500 MHz,
CDCl3) δ 7.35 (d, J = 4.5 Hz, 4H), 7.30-7.23 (m, 3H), 6.51 (d, J = 8.8 Hz, 2H), 4.30 (s, 2H), 4.08
(br s, 1H); 13C {1H} NMR (125 MHz, CDCl3) δ 147.0, 138.8, 131.9, 128.7, 127.4
(two peaks overlapping with each other),
114.4,
109.1,
48.2.
These spectroscopic datas correspond to reported datas.
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N-Benzyl-4-(trifluoromethyl)aniline (4n).38 Light yellow solid; 87% yield (219 mg); mp 53-54
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°C; H NMR (500 MHz, CDCl3) δ 7.38-7.26 (m, 7H), 6.59 (d, JC-F = 8.6 Hz, 2H), 4.32 (s, 3H);
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13C {1H} NMR (125 MHz, CDCl3) δ 150.5, 138.4, 128.8, 127.5, 127.3 (q, JC-F = 3.4 Hz), 126.1
(q, JC-F = 268.7 Hz), 119.1 (q, JC-F = 32.2 Hz), 119.9, 47.7.
N-Benzylnaphthalen-1-amine (4o).36 Light yellow solid; 86% yield (199 mg); mp 69-70 °C; 1H
NMR (500 MHz, CDCl3) δ 7.77-7.73 (m, 2H), 7.43-7.22 (m, 9H), 6.59 (d, J = 7.4 Hz, 1H), 4.62
13
(br s, 1H), 4.42 (s, 2H); C {1H} NMR (125 MHz, CDCl3) δ 143.2, 139.0, 134.2, 128.7, 127.7,
127.3, 126.6, 125.7, 124.7, 123.3, 119.9, 117.6, 104.7, 48.5.
1
N-(Pyridin-2-ylmethyl)aniline (4p).39 Light yellow oil; 86% yield (159 mg); H NMR (500
MHz, CDCl3) δ 8.57 (d, J = 4.6 Hz, 1H), 7.60 (t, J = 7.7 Hz, 1H), 7.31 (d, J = 7.8 Hz, 1H), 7.18-
7.14 (m, 3H), 6.71 (t, J = 7.3 Hz, 1H), 6.66 (d, J = 7.8 Hz, 2H), 4.78 (br s, 1H), 4.44 (s, 2H); 13C
{1H} NMR (125 MHz, CDCl3) δ 158.5, 149.1, 147.8, 136.5, 129.2, 122.0, 121.5, 117.5, 112.9,
49.2.
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N-(1-Phenylethyl)aniline (4q).40 Light yellow oil; 83% yield (163 mg); H NMR (500 MHz,
CDCl3) δ 7.36 (d, J = 7.5 Hz, 2H), 7.30 (t, J = 7.6 Hz, 2H), 7.21 (t, J = 7.3 Hz, 1H), 7.08 (t, J =
7.9 Hz, 2H), 6.63 (t, J = 7.3 Hz, 1H), 6.51 (d, J = 8.1 Hz, 2H), 4.48 (q, J = 6.7 Hz, 1H), 4.00 (br s,
13
1H), 1.51 (d, J = 6.8 Hz, 3H); C {1H} NMR (125 MHz, CDCl3) δ 147.2, 145.2, 129.1, 128.6,
126.8, 125.8, 117.2, 113.2, 53.4, 25.0.
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Benzyl-butyl-amine (4r).41 Light yellow oil; 79% yield (130 mg); H NMR (500 MHz, CDCl3)
δ 7.37-7.23 (m, 5 H), 3.80 (s, 2 H), 3.03 (s, 1H), 2.63 (t, J = 7.5 Hz, 2 H), 1.53 (q, J = 7.4 Hz, 2
H), 1.34 (q, J = 7.3 Hz, 2 H), 0.90 (t, J = 7.5 Hz, 3 H). 13C {1H} NMR (125 MHz, CDCl3) δ
139.3, 128.3, 128.2, 127.0, 53.6, 48.7, 31.6, 20.3, 13.9.
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Kinetic Isotope Effect Studies (Scheme 5). Parallel reactions for the transfer hydrogenation of
1v with CH3OH and CD3OD catalyzed by cat. 9 under standard conditions following the general
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procedure. And the progress of the reaction was analysed by H NMR. All the reactions were
repeated twice and the average data were plotted as yield (%) vs time (h).
Diphenylmethanol-d2 (5).42 White solid, H NMR (500 MHz, CDCl3) δ 7.37-7.31 (m, 8H),
1
7.27-7.23 (m, 2H); C {1H} NMR (125 MHz, CDCl3) δ 143.6, 128.4, 127.5, 126.4, 75.7 (t, J =
13
25 Hz).
Procedure for large-scale transfer hydrogenation of acetophenone (1a) with methanol
catalyzed by [Cp*Ir(2,2’-bpyO)(H2O)][Na] (Scheme 6). In a 250 mL Schlenk tube, 1a (2400
mg, 20 mmol), methanol (30 mL), and cat. 9 (57 mg, 0.1 mmol, 0.5 mol %) were placed under
an N2 atmosphere, and the reaction mixture was heated at 66 °C in an oil bath for 12 h. The
mixture was then cooled to ambient temperature, concentrated in vacuo, and purified by flash
column chromatography (hexanes/ethyl acetate = 10/1, v/v) to afford the corresponding product
2a (2074 mg, 17 mmol, 85% yield).
Procedure for the synthesis of γ-valerolactone (GVL) via transfer hydrogenation of
levulinate ester (LE) with methanol catalyzed by [Cp*Ir(2,2’-bpyO)(H2O)][Na] (Scheme 7).
In a 25 mL Schlenk tube, methyl levulate 6 (130 mg, 1 mmol), methanol (2 mL), and cat. 9 (5.7
mg, 1 mol %) were placed under an N2 atmosphere, and the reaction mixture was heated at 130
°C in an oil bath for 12 h. The mixture was then cooled to ambient temperature, concentrated in
vacuo, and purified by flash column chromatography (hexanes/ethyl acetate = 10/1, v/v) to
afford the corresponding product.
Caution: This temperature is more than two times to boiling point of the methanol and a reaction
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at this temperature in a low boiling solvent should be conducted in a sealable vessel rated for
pressure (Teflon sealable flask, pressure tube etc.).
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γ-Valerolactone (7).43 Colorless oil; 70% yield (70 mg); 1H NMR (500 MHz, CDCl3) δ 4.65 (sxt,
J = 6.6 Hz, 1H), 2.57-2.53 (m, 2H), 2.37 (sxt, J = 6.6Hz, 1H), 1.87-1.78 (m, 1H), 1.42 (d, J = 6.3
Hz, 3H); 13C {1H} NMR (125 MHz, CDCl3) δ 176.9, 76.9, 29.3, 28.7, 20.6.
■ASSOCIATED CONTENT
*S Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.
1H NMR and 13C NMR spectra of the products (PDF)
■AUTHOR INFORMATION
Corresponding Author
*E-mail: fengli@njust.edu.cn.
ORCID
Feng Li: 0000-0003-3288-4069
Author Contributions
§ These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
■ACKNOWLEDGEMENTS
Financial support from the National Natural Science Foundation of China (21272115), the
Natural Science Foundation of Jiangsu Province (BK20161494) and the State Key Laboratory of
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under Extremely Mild Conditions by Novel Iridium(III) Complexes: [(η5-Me5C5)Ir(bpy)Cl]+,
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