Efficient selective oxidation of alcohols to carbonyl compounds catalyzed by Ru-terpyridine complexes with molecular oxygen

Article abstract of DOI:10.1016/j.inoche.2019.107544

The oxidation of alcohols with molecular oxygen is a promising approach to produce corresponding carbonyl compounds. In this work, efficient aerobic oxidation of alcohols to carbonyl compounds catalyzed by ruthenium-terpyridine [(tpy-PhCH₃)RuCl₃] with isobutyraldehyde as co-substrate was developed. Various alcohols including primary and secondary alcohols are smoothly converted to corresponding carbonyl compounds in good yield. In a 100 times large-scale oxidation of benzyl alcohol, benzaldehyde was obtained with 92% isolated yield. Moreover, a plausible mechanism involving high-valence ruthenium species was proposed based on in situ UV–vis spectroscopy.

Full text of DOI:10.1016/j.inoche.2019.107544

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Efficient selective oxidation of alcohols to carbonyl compounds
catalyzed by Ru-terpyridine complexes with molecular oxygen
Qi Han, Xiao-Xuan Guo, Xian-Tai Zhou, Hong-Bing Ji
PII:
S1387-7003(19)30576-3
DOI:
https://doi.org/10.1016/j.inoche.2019.107544
INOCHE 107544
Reference:
To appear in:
Inorganic Chemistry Communications
Received date:
Revised date:
Accepted date:
8 June 2019
13 August 2019
15 August 2019
Please cite this article as: Q. Han, X.-X. Guo, X.-T. Zhou, et al., Efficient selective
oxidation of alcohols to carbonyl compounds catalyzed by Ru-terpyridine complexes with
molecular oxygen, Inorganic Chemistry Communications (2018), https://doi.org/10.1016/
j.inoche.2019.107544
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Efficient selective oxidation of alcohols to carbonyl compounds
catalyzed by Ru-terpyridine complexes with molecular oxygen
Qi Hana, Xiao-Xuan Guoa, Xian-Tai Zhou*a, Hong-Bing Jib,c
aSchool of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China
bFine Chemical Industry Research Institute, the Key Laboratory of Low-carbon Chemistry &
Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University,
Guangzhou 510275, China
c School of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming
525000, P.R.China
*Corresponding author. Tel.: +86-20-84113653; fax: +86-20-84113654.
E-mail: zhouxtai@mail.sysu.edu.cn (X. T. Zhou)
Abstract: The oxidation of alcohols with molecular oxygen is a promising approach
to produce corresponding carbonyl compounds. In this work, efficient aerobic
oxidation of alcohols to carbonyl compounds catalyzed by ruthenium-terpyridine
[(tpy-PhCH3)RuCl3] with isobutyraldehyde as co-substrate was developed. Various
alcohols including primary and secondary alcohols are smoothly converted to
corresponding carbonyl compounds in good yield. In a 100 times large-scale
oxidation of benzyl alcohol, benzaldehyde was obtained with 92% isolated yield.
Moreover, a plausible mechanism involving high-valence ruthenium species was
proposed based on in situ UV-vis spectroscopy.
Keywords: Alcohol oxidation; Carbonyl compounds; Ruthenium-terpyridine;
Molecular oxygen; Mild conditions
Introduction
The selective oxidation of alcohols to corresponding carbonyl compounds is one
of the most important processes because aldehydes or ketones is a fundamental
transformation in organic synthesis [1-3]. Traditional methods for this selective
oxidation involve the use of a stoichiometric amount of oxidants such as MnO2 [4-6],
chromium oxides [7], which produce a large amount of toxic waste. Due to the
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growing concerns for environmental preservation and cost effective, catalytic
oxidation processes with molecular oxygen or air are extremely valuable and
particularly attractive [8]. A variety of methods reported in the literature for the
aerobic oxidation of alcohols are based on metal catalysts such as manganese [9-11],
cobalt [12-14], ruthenium [15-17] and Au nanoparticles or Au/CuO-hydroxyapatite
[18-20] etc. Among these catalytic systems, based on nitroxyl radical like TEMPO
and NHPI in combination with catalysts are the most reported [21-23].
Aldehyde acts as a reducing agent for the reductive activation of dioxygen is
widely reported in the oxidation, in which the catalysts involved metalloporphyrins
[24] and Mn salen complexes [25] etc. Rahimi et al. reported the effective alcohol
oxidation in the presence of Cu(II) meso-tetraphenylporphyrin and isobutyraldehyde
[26]. We ever reported an efficient oxidation process of alcohol catalyzed ruthenium
tetraphenylporphyrins in the presence of molecular oxygen and isobutyraldehyde [27].
Recently, organic-inorganic hybrid cobalt phthalocyanine exhibited good activity for
the selective oxidation of alcohol by using isobutyraldehyde as co-substrate [28].
Ruthenium complexes with nitrogen-based ligands such as pyridine have been
intensively investigated in order to develop catalysts for organic oxidation processes.
Nishiyama first reported the asymmetric epoxidation by one kind of ruthenium
complex based on bis(oxazolinyl)pyridine [29]. Although this kind ruthenium
compounds were used in the oxidation of alcohol, hydrogen peroxide and TBHP
(tert-butylhydrogen peroxide) were mostly used as oxidant [30]. The selective
oxidation of alcohols catalyzed by ruthenium(II) complexes of pyridine ligand
complexes with molecular oxygen as oxidant is still limited.
As part of our ongoing interests in green oxidations process with dioxygen, the
aerobic oxidation of alcohols to carbonyl compounds catalyzed by
ruthenium-terpyridine [(tpy-PhCH3)RuCl3] in the presence of isobutyraldehyde has
been developed in this work (Scheme 1). The catalytic system has been proved to be
efficient for oxidation of alcohols with high yields for carbonyl compounds under
mild conditions.
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N
N
Cl
N
Ru
Cl
Cl
O
OH
R2
[(tpy-PhCH3)RuCl3]
1,2-dichloroethane, isobutyraldehyde, O2(1 atm), 60oC
R1
R2
R1
Scheme 1 Aerobic oxidation of alcohols catalyzed by [(tpy-PhCH3)RuCl3]
2. Experimental
2.1 Reagents and methods
Different para-substituted alcohols, aldehydes and RuCl3·3H2O were purchased
from Sigma. Other Chemicals were of analytical grade and purchased from Aldrich
without further purification unless indicated. Solvents were of analytical purity and
used as received. The precursor that used for the synthesis of the ligand
4′-(p-methylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCH3) was synthesized following
reported procedures [31]. [(tpy-PhCH3)RuCl3] was prepared by refluxing RuCl3·3H2O
and tpy-PhCH3 (1:1 molar ratio) in EtOH according to the previous procedures [32].
Mass spectra were obtained on a Shimadzu LCMS-2010A. Elemental analyses
1
were carried out with an Elementar vario EL elemental analyzer. HNMR was
recorded on a Bruker AVANCE 400 spectrometer (500MHz). IR spectra were
recorded on a Bruker 550 FT-IR spectrometer. UV spectra were recorded on a
Shimadzu UV-2450 spectrophotometer. The in situ UV–vis spectra of ruthenium
complex were recorded on the AvaSpec-2048 spectrometer, which was equipped with
a high-pressure, high-temperature probe and connected to the stainless-steel reactor.
1
The structure characterization data of [(tpy-PhCH3)RuCl3]: H NMR: (500MHz,
CDCl3): δ 10.59(s, 2H), 10.46 (m, 4H), 9.66 (s, 2H), 9.37(m, 2H), 9.13 (m, 4H), 3.98
(m, 3H). IR (KBr)/cm-1: 3032, 1620, 1473, 1093, 820, 784. UV-vis, CH3CN, max
nm(log): 205(0.31), 286(0.18), 514(0.05). EI-MS: m/z=530. Elemental analysis
Calcd(%). for C22H17Cl3N3Ru: C, 49.76; H, 3.23; N, 7.92. Found: C, 48.61; H, 3.51;
N, 7.43.
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2.2 Catalytic oxidation of alcohol
The catalytic oxidation of alcohol was carried out in a magnetically stirred glass
reaction tube fitted with a reflux condenser. A typical procedure was as follows using
benzyl alcohol as a model substrate: A solution of 1,2-dichloroethane (DCE, 5 mL),
benzyl alcohol (1 mmol), isobutyraldehyde (2 mmol), [(tpy-PhCH3)RuCl3] (1×10-3
mmol, 0.1mol% based substrate) and 0.5 mmol naphthalene (inert internal standard)
were added in the tube. Then the mixture was stirred at 60C for 60 min under
dioxygen balloon. The consumption of the starting alcohols and formation of products
were monitored by GC and GC-MS. GC analyses were performed on a Shimadzu
GC-2010 plus chromatography equipped with Rtx-5 capillary column
(30m×0.25mm×0.25µm). GC-MS analyses were recorded on a Shimadzu
GCMS-QP2010 equipped with Rxi-5ms capillary column (30m×0.25 mm×0.25µm).
3. Results and discussion
3.1 Alcohol oxidation under different conditions
By using benzyl alcohol as the model substrate, the reaction conditions were
optimized with [(tpy-PhCH3)RuCl3] as the catalyst. As shown in Table 1, In control
experiments without the catalyst, only 13% benzyl alcohol was converted (entry 1 in
Table 1). As the oxidation was conducted in the absence of isobutyraldehyde as
co-substrate, no yield of benzaldehyde was obtained (entry 2 in Table 1). This results
also indicate that isobutyraldehyde plays a significant role in the oxidation of benzyl
alcohol. Higher yield of benzaldehyde could be obtained with the usage of
isobutyraldehyde increasing (entries 3~7 in Table 1). However, no significant
difference was observed when the amount of isobutyraldehyde reached up to 2 equiv.
for the present system.
In addition, the role of isobutyraldehyde is to activate dioxygen through free
radical chains propagation in the catalytic system [33]. As another kind of aldehyde,
the oxidation product benzaldehyde maybe also promote the catalytic efficiency.
However, almost no oxidation occurred when benzaldehyde was used as co-substrate
(entry 8 in Table 1). Hence, the possible that benzaldehyde participated in the
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oxidation could be excluded. Moreover, from the viewpoint of cost-effective, catalytic
oxidation with air is extremely valuable and attractive for industrialization. When the
oxidation was conducted under atmospheric pressure air, 52% yield of benzaldehyde
was obtained (entry 9 in Table 1). But the catalytic efficiency was remarkably
enhanced when the air was pressured to 1.0 MPa (entry 10 in Table 3).
Table 1. Aerobic oxidation of benzyl alcohol catalyzed by [(tpy-PhCH3)RuCl3] with
isobutyraldehydea
[(tpy-PhCH3)RuCl3]/isobutyraldehyde
OH
O
DCE, 60oC,O2 (1 atm), 60 min
Entry
Catalyst
isobutyraldehyde
(equiv.)
Conv. (%)
Select. (%)
1
2
-
2.0
13
<1
51
73
87
99
99
3
>99
-
[(tpy-PhCH3)RuCl3]
[(tpy-PhCH3)RuCl3]
[(tpy-PhCH3)RuCl3]
[(tpy-PhCH3)RuCl3]
[(tpy-PhCH3)RuCl3]
[(tpy-PhCH3)RuCl3]
[(tpy-PhCH3)RuCl3]
[(tpy-PhCH3)RuCl3]
[(tpy-PhCH3)RuCl3]
-
3
0.5
1.0
1.5
2.0
3.0
-
>99
>99
>99
4
5
6
>99
>99
7
8b
9c
10d
>99
>99
>99
2.0
52
2.0
96
a
Benzyl alcohol (1mmol), [(tpy-PhCH3)RuCl3] catalyst (0.1 mol%), DCE (5 mL), 60oC, 60 min,
under O2 balloon.
b Benzaldehyde (2 mmol) was used as co-substrate.
c Atmospheric air as oxidant.
c Air as oxidant, air pressure was 1.0 MPa.
Aerobic oxidation of benzyl alcohol with [(tpy-PhCH3)RuCl3] catalyst using
various solvents and the effect of reaction temperature on its conversion were also
investigated and the results were summarized in Table 2.
Table 2. Effect of solvent and temperature on the oxidation of benzyl alcohola
Entry
Solvent
Conv. (%)
Select. (%)b
T (C)
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1
2
3
4
5
6
7
8
9
Cyclohexane
60
60
60
60
60
60
40
50
70
83
86
99
51
35
26
46
72
>99
>99
>99
Toluene
1,2-Dichloroethane
Ethyl acetate
>99
87(13)
90(10)
78(22)
>99
Acetonitrile
Methanol
1,2-Dichloroethane
1,2-Dichloroethane
1,2-Dichloroethane
>99
>99
aBenzyl alcohol (1mmol), [(tpy-PhCH3)RuCl3] (0.1 mol%), isobutyraldehyde (2 equiv.), solvent (5
mL), 60oC, 60 min, under O2 balloon.
b The numbers in parentheses indicate the selectivity of benzoic acid.
It seems that the solvent plays a key role in the oxidation system. Compared with
cyclohexane and toluene, DCE (1,2-dichloroethane) was more favorable to the
oxidation of benzyl alcohol, which gave 99% yield of benzaldehyde (entries 1~3 in
Table 2). When stronger polarity solvents such as ethyl acetate, acetonitrile, and
methanol were used instead, the significantly declined efficiency could be obtained,
companies with the formation of benzoic acid (entries 4~6 in Table 2).
The effect of temperature on oxidation of benzyl alcohol was also investigated.
The conversion was increased with raising the temperature from 40oC to 70oC (entries
7~9 in Table 2). It is notable that the increasing temperature could hardly influence
the selectivity towards benzaldehyde, i.e., no over-oxidation product such as benzoic
acid was observed.
Subsequently, the catalyst amount was examined for the selective oxidation of
benzyl alcohol. As shown in Figure 1, lower catalysts usage present less efficient
under the standard reaction conditions. The yield of benzaldehyde increased with the
rising amount of catalyst. But no significant difference in efficiency was observed
when the amount of catalyst was increased up to 1.0 mol% (based on the substrate).
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However, an excess amount of catalyst caused a decrease of selectivity towards
benzaldehyde, which resulted in the over-oxidation to generate by-product benzoic
acid.
100
80
60
40
20
0
A
B
C
D
Figure 1. Effect of catalyst amount on benzyl alcohol oxidation (A: 0.001 mol%; B:
0.01 mol%; C: 0.1 mol/%; D: 1.0 mol/%). Benzyl alcohol (1mmol),
[(tpy-PhCH3)RuCl3], isobutyraldehyde (2 equiv.), DCE (5 mL), 60oC, 60 min, O2
balloon.
3.2 Aerobic oxidation of other alcohols by [(tpy-PhCH3)RuCl3]
To evaluate the scope of the catalytic system, oxidation of various alcohols,
including benzylic alcohols, secondary alcohols, and primary alcohols, by dioxygen in
the presence of [(tpy-PhCH3)RuCl3] under mild conditions has been investigated
(Table 3).
Table 3. Oxidation of various alcohols with dioxygen catalyzed by
[(tpy-PhCH3)RuCl3]a
Entry
Substrate
Product
Time/min Conv./% Seletc./%
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1
2
3
4
5
6
60
60
98
99
99
65
69
60
>99
>99
>99
90
60
120
60
OH
O
92
Cl
Cl
120
99
7b
8
120
60
92
99
>99
>99
O
OH
OH
O
9
60
60
>99
79
>99
>99
OH
O
10
Cl
Cl
OH
O
11
12
60
60
98
92
>99
>99
O
OH
OH
O
13
14
60
60
>99
97
>99
>99
OH
O
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OH
O
15c
16d
60
60
98
12(88)
90(10)
OH
OH
OH
>99
aSubstrate (1 mmol), [(tpy-PhCH3)RuCl3] (0.1 mol%), isobutyraldehyde (2 equiv.), DCE (5 mL),
60oC, O2 balloon.
b[(tpy-PhCH3)RuCl3] (0.2 mol%).
cThe number in parentheses was the selectivity of benzaldehyde.
dThe number in parentheses was the selectivity of cinnamaldehyde.
It could be known that the catalytic system was efficient for most primary
alcohol, which alcohols were smoothly converted to corresponding carbonyl
compounds with a high conversion and excellent selectivity (entries 1~8 in Table 3).
It seems that the catalytic activity is dependent on the electronic property of substrates.
The catalytic system was more effective for the substrates with electron-withdrawing
groups at para position (entries 2-5 in Table 3). It was inefficient for basic substrates
like 4-pyridinemethanol (entry 6 in Table 3), which only 60% conversion was
obtained even prolonged reaction time to 120 min. However, the catalytic efficiency
could be improved by increasing amount of catalyst for 4-pyridinemethanol (entry 7
in Table 3). The catalytic system is also efficient for the oxidation of saturated
primary aliphatic alcohols such as 1-octanol (entry 8 in Table 3).
Secondary alcohols could be easily converted to the corresponding ketones in
high yields (entries 9-14 in Table 3). The influence of the electronic property was
confirmed further from the secondary alcohols oxidation (entries 10 and 11 in Table
3). The catalytic system also were apparently more conducive for the secondary
alcohols with electron-withdrawing groups than that of electron-donating groups. As
for 2-adamantanol, the catalytic system shows high activity, which gave ketone yield
of 92% despite the hindrance (entry 12 in Table 3).
Furthermore, the catalytic system also exhibited high activity for the oxidation of
diol, such as 1-phenyl-1,2-ethanediol (entry 15 in Table 3). The main product was
benzaldehyde that resulted from the oxidative cleavage of C-C bonds. Diols could be
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cleaved to the corresponding aldehydes by ruthenium porphyrins with molecular
oxygen as oxidant [27]. Similar C=C bond cleavage could also be found when
cinnamyl alcohol was carried out (entry 16 in Table 3). Cinnamyl alcohol could be
converted completely and benzaldehyde was the main product. This phenomenon is
similar to that of MnTPPCl-catalyzed aerobic oxidation of cinnamyl alcohol [34].
3.3 Large-scale oxidation of benzyl alcohol
To further confirm the practicality and efficiency of this catalytic system, a 100
times large-scale experiment for the oxidation of benzyl alcohol was conducted under
standard conditions. The reaction solution mixture with dioxygen bubbling was stirred
at 60 oC for 60 min. After removing of the solvent under reduced pressure, the crude
products were purified via column chromatographic (silica gel, petroleum ether as
eluting agent). Pure benzaldehyde was obtained with the yield of 92% by removing
solvent (Scheme 2). The result suggested that this catalytic system was amenable to
scale up.
[(tpy-PhCH )RuCl ]
DCE, isobutyraldehyde (2 equiv.), O2(1 atm), 60oC, 60 min
(0.1 mol%)
3
3
OH
O
(100 mmol)
Isolated yield: 92%
Scheme 2. Large-scale oxidation of benzyl alcohol
3.4 Plausible mechanism
As reported previously, the activation of molecular oxygen by aldehyde in
oxidation proceeded via a free radical process. In order to verify the free-radical
mechanism, 2,6-di-tert-butylphenol serving as the free-radical inhibitor was used in
the oxidation solution. After the addition of this inhibitor, the oxidation was
subsequently quenched.
The ruthenium complexes catalyst was monitored by in situ UV-vis spectroscopy
(AvaSpec-204814 with a fiber optic probe) during the reaction with
o
isobutyraldehyde and dioxygen at 60 C. Figure 2 shows an initial characteristic
absorption peak of [(tpy-PhCH3)RuCl3] at 205 nm. Note that the spectrophotometer
was programmed to acquire UV–vis spectra every 10 min. As the reaction proceeded,
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the peak at 205 nm gradually decreased in intensity, accompanied by the increase of
the peak at 228nm.
Figure 2. In situ UV-vis spectra of [(tpy-PhCH3)RuCl3] catalyst during aerobic
oxidation of benzyl alcohol in presence of isobutyraldehyde (interval 10 min), benzyl
alcohol (10 mmol), catalyst (1×10-2 mmol), DCE (50 mL), isobutyraldehyde (2.0
equiv.) and 60 oC.
This change was attributed to the generation of high-valent ruthenium complex
during oxidation. The recorded spectroscopic features supported the conclusion that
the complex was converted into ruthenium-oxo species [35]. As reported previously
by Beller, ruthenium oxo complex was the active catalyst species in the asymmetric
epoxidation system [36]. As to [(tpy-PhCH3)RuCl3] in presence of isobutyraldehyde
and O2, the reaction mechanism could also involve the participation of Ru-oxo species
generated from the reaction between [(tpy-PhCH3)RuCl3] and dioxygen through a
series radical chain. The formation of carbonyl compounds was attributed to the
reaction of the alcohol with Ru-oxo species, followed by -hydride elimination.
Based on the above discussion, a plausible mechanism was proposed for the
oxidation of benzyl alcohol catalyzed by [(tpy-PhCH3)RuCl3] in presence of dioxygen
and isobutyraldehyde, and it is shown in Scheme 3.
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OH
R2
O
RCHO
V
R1
(L
)R
u
c
O
H
R2
R1
OH
V
(L
)R
u
.
O
O
R－ C
O
d
III
b
(L
)R
u
H
O2
O
.
O
R－ C－O－O
O
(L)RuIII
R－ C－O－OH
R1
R2
a
R: isobutyl
Scheme 3. Plausible mechanism of [(tpy-PhCH3)RuCl3]-catalyzed oxidation of
alcohol in presence of isobutyraldehyde and molecular oxygen.
An acyl radical is derived from the thermal abstraction of hydrogen at the
carbonyl position at first. Peroxyacid was generated by radical propagation. Then, the
reaction should be initiated by the smooth interaction between Ru(III) complex (a)
and peroxy acid to generate Ru(III)-OOH (b) [37-38]. Ru-oxo (c) was then generated
from heterolytic cleavage of O-O bond of species (b). Due to the electrophilic
character of Ru-oxo (c), the formation of carbonyl compounds was attributed to the
reaction of the alcohol with Ru-oxo species through transition species (d), followed
by -hydride elimination.
4. Conclusions
In summary, ruthenium complex [(tpy-PhCH3)RuCl3] has been proven to be an
excellent catalyst for oxidation of alcohols in the presence of molecular oxygen and
isobutyraldehyde. The catalytic system exhibited excellent substrate tolerance, both
primary and secondary alcohols were oxidized into their corresponding carbonyl
compounds in good yield. The influence of various reaction parameters such as
solvent, catalyst and oxidant amount on the activity and selectivity was evaluated. In a
100 times large-scale oxidation of benzyl alcohol, benzaldehyde was obtained with 92%
isolated yield.
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Acknowledgments
This study was supported by the National Key Research and Development
Program of China (2016YFA0602900), the National Natural Science Foundation of
China (No. 21576302, 21878344 and 21425627), Guangdong Technology Research
Center for Synthesis and Separation of Thermosensitive Chemicals (2015B090903061)
and Science and Technology Innovation Teams Project of Huizhou
(20131226121851953).
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Journal Pre-proof
Graphical Abstract
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Journal Pre-proof
Highlights
► Ruthenium-terpyridine presented excellent efficiency for alcohol oxidation.
► Aerobic oxidation of alcohols were carried out under mild conditions.
► The catalytic protocol could be amenable to scale up.
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