RuCl 3-DCHA catalyst system: A selective aerobic oxidation of primary benzylic alcohols under mild conditions

Article abstract of DOI:10.1080/00397911.2011.555905

The aerobic oxidation of benzylic alcohols to their corresponding aldehydes was performed in a RuCl₃ · 3H₂O-dicyclohexylamine (DCHA) catalyst system under ambient atmosphere at room temperature. It is noteworthy that the RuCl₃ · 3H₂O-DCHA system displayed a preference for the primary versus secondary benzylic alcohols in both intermolecular and intramolecular competition experiments.

Full text of DOI:10.1080/00397911.2011.555905

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RuCl3-DCHA Catalyst System: A Selective
Aerobic Oxidation of Primary Benzylic
Alcohols Under Mild Conditions
Kaihui Yu a , Dongnai Ye a , Lei Shu a , Shiyong Zhang a , Qiaosheng
Hu a & Liangxian Liu a
a Department of Chemistry and Biology , Gannan Normal University ,
Ganzhou , Jiangxi , China
Accepted author version posted online: 19 Dec 2011.Published
online: 09 Apr 2012.
To cite this article: Kaihui Yu , Dongnai Ye , Lei Shu , Shiyong Zhang , Qiaosheng Hu & Liangxian Liu
(2012) RuCl3-DCHA Catalyst System: A Selective Aerobic Oxidation of Primary Benzylic Alcohols Under
Mild Conditions, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 42:15, 2318-2326, DOI: 10.1080/00397911.2011.555905
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Synthetic Communications1, 42: 2318–2326, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.555905
RuCl3-DCHA CATALYST SYSTEM: A SELECTIVE
AEROBIC OXIDATION OF PRIMARY BENZYLIC
ALCOHOLS UNDER MILD CONDITIONS
Kaihui Yu, Dongnai Ye, Lei Shu, Shiyong Zhang, Qiaosheng
Hu, and Liangxian Liu
Department of Chemistry and Biology, Gannan Normal University, Ganzhou,
Jiangxi, China
GRAPHICAL ABSTRACT
Abstract The aerobic oxidation of benzylic alcohols to their corresponding aldehydes was
performed in a RuCl3 ꢀ 3H2O–dicyclohexylamine (DCHA) catalyst system under ambient
atmosphere at room temperature. It is noteworthy that the RuCl3 ꢀ 3H2O–DCHA system
displayed a preference for the primary versus secondary benzylic alcohols in both intermo-
lecular and intramolecular competition experiments.
Keywords Benzylic alcohol; oxidation; RuCl3 ꢀ 3 H2O
INTRODUCTION
The oxidation of alcohols to the corresponding aldehydes or ketones is an
important process in organic synthesis. Many traditional reagents for converting
alcohols to carbonyl compounds have been reported over the past years, such as
hypochlorite,[1] chromium(VI) oxide,[2] dichromate,[3] manganese(IV) oxide,[4] per-
manganate,[5] ruthenium(VIII) oxide,[6] iodosobenzene,[7] and cerium ammonium
nitrate (CAN).[8] Unfortunately, these hazardous or toxic oxidizing agents often
requires usage in stoichiometric amounts leading to the presence of heavy metals
in waste. This can cause profoundly harmful effects on the environment. To reduce
the damage inflicted on the environment, it is necessary to develop an environmen-
tally friendly process for the oxidation of alcohols. With this aim in mind, many
Received July 10, 2010.
Address correspondence to Liangxian Liu, Department of Chemistry and Biology, Gannan
Normal University, Ganzhou, Jiangxi 341000, China. E-mail: lxliu@xmu.edu.cn
2318

AEROBIC OXIDATION OF PRIMARY BENZYLALCOHOLS
2319
metal-catalyzed ‘‘green’’ aerobic oxidations of alcohols to the corresponding car-
bonyl compounds have been proposed. The benign effects on the environment as
well as the potential utility of such reactions are significant advances.[9] In recent
years, many catalytic methods for aerobic oxidation of alcohols have been reported,
displaying a large array of transition-metal catalysts (e.g., Fe, Co, Cu, Au, Ru, Pd,
Os).[10] The palladium-based catalysts[11] and ruthenium compounds[12] are parti-
cularly useful for this process, but reports of palladium- or ruthenium-catalyzed
alcohol oxidations employing oxygen (or air) as the sole oxidant are limited,
especially those displaying a preference for the primary versus the secondary
hydroxyl group of either an aliphatic or a benzylic diol.[12d]
In our research on the oxidation of benzyl alcohol to benzaldehyde, we found
an interesting oxidation process based on RuCl3 ꢀ 3H2O catalyst. We report herein a
simple and general procedure for the aerobic oxidation of benzylic alcohols to the
corresponding aldehydes.
In our initial attempts, the oxidation of benzyl alcohol was selected as a model
reaction, and various parameters were optimized. We first investigated the oxidation
of benzyl alcohol to benzaldehyde using a RuCl3 ꢀ 3H2O=base catalyst system, and
the results are summarized in Table 1.
As shown in Table 1, amines can play an important role in the RuCl3-catalyzed
aerobic oxidation of benzyl alcohol to benzaldehyde and have a dramatic influence
on the benzaldehyde yield. For example, when amines were not employed as the base
in the reactions, the benzaldehyde product was afforded in less than 5% yield
(Table 1, entry 1). Furthermore, the desired product was not detected when
K2CO3 was used as the base (Table 1, entry 6). To our delight, the oxidation reaction
was greatly enhanced by certain amines, with dicyclohexylamine (DCHA) being the
most effective amine base. When 5 mol% RuCl3 ꢀ 3H2O was used in tetrahydrofuran
(THF) at rt for 48 h with DCHA as a base (Table 1, entry 5), an excellent yield of
Table 1. RuCl3-catalyzed aerobic oxidation of benzyl alcohola
Entry
Base
–
RuCl3 ꢀ 3H2O (mol%)
Isolated yield (%)b
1
2
3
4
5
6
7
8
9
5
5
5
5
5
5
2
3
6
<5
Et3N
19 (26)
30 (32)
59 (60)
85 (82)
0
Diethylamine
TMEDA
DCHA
K2CO3
DCHA
33 (31)
37 (38)
86 (82)
DCHA
DCHA
aThe reaction was performed with benzyl alcohol (1 mmol) and base (1.0 mmol) in 1 mL of THF at rt for
48 h.
bThe values in parentheses are the yields determined by 1H NMR using nitromethane as internal
standard.

2320
K. YU ET AL.
85% was afforded. To further examine the effects of the amines on the reaction, a
variety of amines were chosen as the base. Many other amines, including trimethy-
lamine, diisopropylamine, aniline, N,N-dimethyl aniline, N,N-diethyl ethylenedia-
mine, imidazole, triethanolamine, 3-amino-1,2,4-trizole, tri-n-propylamine, tri-n-
butylamine, pyridine, and pyridine hydrochloride, were also screened, but the results
were unsatisfactory. The influence of the loading of RuCl3 ꢀ 3H2O was also evalu-
ated. Good results were obtained when 5 mol% RuCl3 ꢀ 3H2O catalyst was used.
Interestingly, when more than 5 mol% of the RuCl3 ꢀ 3H2O (Table 1, entry 9) catalyst
was employed, there was no visible effect on the reaction yield. However, when the
catalytic loading was decreased to 2 mol%, the yield was significantly decreased to
33% (Table 1, entry 7).
Efforts were next directed toward investigating the solvent effects. After exam-
ining different solvents including THF, ethylene glycol dimethyl ether (GDE),
CH2Cl2, C6H6, and CHCl3, THF was found to be the optimal solvent (Table 2,
entry 1). The reaction provided an excellent yield of 88% when THF was used. Sub-
sequently, the influence of the amount of DCHA was evaluated. As shown in Table 2,
the amount of DCHA had a significant impact on the yield. Good results were
obtained when 1.0 equiv of DCHA was used. However, when the amount of DCHA
was decreased to 0.2 equiv, the yield significantly decreased to 55% (Table 2, entry 8),
indicating that any further decrease in the amount of DCHA will most likely lead to
lower yields. To improve the reaction rate, the oxidation of benzylalcohol was
treated at 40 ꢁC, and the yield significantly decreased (Table 2, entries 9 and 10).
Table 2. Optimization of the reaction conditionsa
Entry
Solvent
DCHA
Isolated yield (%)b
1
2
THF
1.0 equiv
1.0 equiv
1.0 equiv
1.0 equiv
1.0 equiv
1.2 equiv
0.8 equiv
0.2 equiv
1.0 equiv
1.0 equiv
88 (85)
83 (79)
82 (83)
75 (67)
50 (55)
86 (84)
83 (76)
55 (53)
54 (52)d
52 (53)e
GDEc
CH2Cl2
C6H6
CHCl3
THF
3
4
5
6
7
THF
THF
8
9
10
THF
THF
aThe reaction was performed with benzyl alcohol (1 mmol) and RuCl3 ꢀ 3H2O (0.05 mmol) in 1 mL of
solvent at rt for 4 days.
bThe values in parentheses are the yields determined by 1H NMR using nitromethane as internal
standard.
cEthylene glycol dimethyl ether.
dThe reaction was performed with benzylalcohol (1 mmol) and RuCl3 ꢀ 3H2O (0.05 mmol) in 1 mL of
solvent at 40 ꢁC for 2 days.
eThe reaction was performed with benzylalcohol (1 mmol) and RuCl3 ꢀ 3H2O (0.05 mmol) in 1 mL of
solvent at 40 ꢁC for 4 days.

AEROBIC OXIDATION OF PRIMARY BENZYLALCOHOLS
2321
Based on these observations, the optimum reaction conditions for the oxi-
dation were found to be 0.05 equiv of RuCl3 ꢀ 3H2O, 1.0 equiv of DCHA, with
THF as the solvent at rt.
Using the optimal reaction conditions, the scope of the reaction was next
examined, and the results are summarized in Table 3. As shown in Table 3, many
benzylic alcohols were oxidized to their corresponding aldehydes in moderate to
excellent yields under the optimized conditions. It is worthy of note that this catalyst
system showed compatibility with various groups such as Cl, Br, CH3, CH3O, CF3,
CN, NO2, and NHCOCH3. The system is especially favorable with an OH group
substituted on the benzene ring. The results have shown that electronegativities of
the substituents played a major role in governing the reactivity of the substrates.
With electron-donating substituents on the benzylic alcohols, increased yields of pro-
ducts were observed (Table 3, entries 3–7). For example, p-methoxylbenzyl alcohol
was oxidized to p-methoxyl benzaldehyde with a yield of 90% (Table 3, entry 6).
Electron-withdrawing groups (e.g., NO2, CN, CF3) on the benzylic alcohols
Table 3. RuCl3-catalyzed aerobic oxidation of benzylic alcoholsa
Entry
Compound
R1
R2
Time (days)
Yield (%)b
1
2
2a
2b
2c
2d
2e
2f
H
o-CH3
m-CH3
p-CH3
m-OCH3
p-OCH3
p-OH
H
H
2
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
82c
72
86
88
89
90
85
70
82
80
40
44
36
45
43
41
70
75
30
26
3
H
4
H
5
H
6
H
7
2g
2h
2i
H
8
m-Cl
p-Cl
H
9
H
10
11
12
13
14
15
16
17
18
19
20
2j
o-Br
H
2k
2l
o-NO2
m-NO2
p-NO2
m-CN
p-CN
H
H
2m
2n
2o
2p
2q
2r
2s
2t
H
H
H
p-CF3
H
m-NHCOCH3
p-NHCOCH3
H
H
H
CH3
Ph
H
aThe reaction was performed with benzyl alcohol (1 mmol) and RuCl3 ꢀ 3H2O (0.05 mmol) in 1 mL of
THF at rt.
bIsolated yield.
cYield was determined by 1H NMR using nitromethane as internal standard.

2322
K. YU ET AL.
Scheme 1. Oxidation of cinnamic alcohol and furfuryl alcohol.
inhibited the oxidation, affording the corresponding aldehydes in poor yields
(Table 3, entries 11–16). For instance, the yield of p-nitrobenzaldehyde decreased
to 36% when p-nitrobenzyl alcohol was oxidized under the same reaction conditions.
Although the oxidation took place at a slow pace, it was conducted under mild
conditions. It is noteworthy that the RuCl3 ꢀ 3H2O=DCHA system displayed a pref-
erence for the primary versus secondary benzylic alcohols. As observed in the oxi-
dation of the secondary benzylic alcohols, 2-phenylethanol (Table 3, entry 19),
and benzohydrol (Table 3, entry 20), poor yields were obtained under the same reac-
tion conditions. Our results suggested that steric nature of the substrates played an
important role in the oxidation process.
In addition, as shown in Scheme 1, cinnamic alcohol was resistant to oxidation
and the RuCl3 ꢀ 3H2O=DCHA system is a poor catalyst for the aerobic oxidation of
furfuryl alcohol to furfural under standard reaction conditions. These results showed
again that the RuCl3 ꢀ 3H2O=DCHA system has selectivity for benzylic alcohols.
To obtain more information about the selectivity of RuCl3 ꢀ 3H2O=DCHA sys-
tem in oxidation of primary benzylic alcohols and secondary benzylic alcohols, the
intermolecular and intramolecular competition experiments between primary and sec-
ondary benzylic alcohols were designed. As seen from Scheme 2, RuCl3 ꢀ 3H2O=
DCHA system oxidized primary benzylic alcohols with high selectivity. In addition
to the intermolecular competition experiment, the same behavior was observed in
the intramolecular competition of two primary=secondary diols, 4-(10-hydroxyethyl)-
benzyl alcohol 3 and 3-(10-hydroxyethyl)benzyl alcohol 6. As shown in Scheme 3, the
RuCl3 ꢀ 3H2O=DCHA system again displayed a preference for the primary versus the
Scheme 2. Intermolecular competition oxidation.

AEROBIC OXIDATION OF PRIMARY BENZYLALCOHOLS
2323
Scheme 3. Intramolecular competition oxidation.
secondary hydroxyl group of a benzylic diol. Notably, we observed that the selectivity
for the primary hydroxyl group of 6 was greater than 3.
In conclusion, we have developed a mild procedure for the direct aerobic oxi-
dation of benzylic alcohols using RuCl3 ꢀ 3H2O=DCHA, a simple combination of
commercially available reagents. Under ambient atmosphere and at room tempera-
ture, primary benzylic alcohols were transformed to the corresponding aldehydes in
moderate to excellent yields. It is noteworthy that the RuCl3 ꢀ 3H2O–DCHA system
displayed a preference for the primary versus secondary benzylic alcohols in both
intermolecular and intramolecular competition experiments. The present protocol
is mild and selective as compared to previously reported literature methods. Further
investigations are under way to formulate a plausible reaction mechanism.
EXPERIMENTAL
1H and 13C NMR spectra were recorded in CDCl3 on a Bruker 400 spec-
trometer with tetramethylsilane (TMS) as an internal standard. Chemical shifts
are expressed in d (ppm) units downfield from TMS. RuCl3 ꢀ 3H2O, DCHA, alcohols
and other reagents were all purchased from commercial sources.
Typical Experimental Procedure for Oxidation of Benzylic Alcolols
Using RuCl3 ꢀ 3H2O/DCHA
DCHA (1.0 mmol) was added dropwise to a solution of benzylic alcohol
(1.0 mmol) and RuCl3 ꢀ 3H2O (0.05 mmol) in THF (1 mL) under an air atmosphere,
and the mixture was stirred at room temperature (monitored by thin-layer chroma-
tography, TLC). Then, the solvent was removed under reduced pressure, and the
crude product was purified by silica-gel column chromatography to provide the
desired product.
Characterization Data of Typical Products
1
3-Cyanobenzaldehyde (2n). Colorless oil. H NMR (400 MHz, CDCl3): d
7.70 (t, J ¼ 7.7 Hz, 1H, Ar-H), 7.92 (dt, J ¼ 7.7, 1.2 Hz, 1H, Ar-H), 8.13 (dt,

2324
K. YU ET AL.
J ¼ 7.7, 1.2 Hz, 1H, Ar-H), 8.17 (t, J ¼ 1.2 Hz, 1H, Ar-H), 10.05 (s, 1H); 13C NMR
(100 MHz, CDCl3): d 113.7, 117.6, 130.1, 133.1, 133.3, 136.9, 137.2, 190.0 (CO).
1
4-Cyanobenzaldehyde (2o). Colorless oil. H NMR (400 MHz, CDCl3): d
7.84 (dd, J ¼ 6.8, 1.6 Hz, 2H, Ar-H), 7.99 (dd, J ¼ 6.8, 1.6 Hz, 2H, Ar-H), 10.09 (s,
1H); 13C NMR (100 MHz, CDCl3): d 117.6, 117.7, 129.9, 132.9, 138.7, 190.7 (CO).
1
4-(Trifluoromethyl)benzaldehyde (2p). Colorless oil. H NMR (400 MHz,
CDCl3): d 7.89 (d, J ¼ 8.0 Hz, 2H, Ar-H), 8.00 (d, J ¼ 8.0 Hz, 2H, Ar-H), 10.09 (s,
1H); 13C NMR (100 MHz, CDCl3): d 123.4 (q, J ¼ 272.9 Hz, CF3), 126.0 (q,
J ¼ 3.7 Hz), 129.9, 135.5 (q, J ¼ 32.5 Hz), 138.6, 191.1 (CO).
1
3-Acetamidobenzaldehyde (2q). White waxy solid. H NMR (400 MHz,
CDCl3): d 2.21 (s, 3H, CH3), 7.45 (t, J ¼ 7.9 Hz, 1H, Ar-H), 7.59 (d, J ¼ 7.9 Hz,
1H, Ar-H), 7.88 (d, J ¼ 7.9 Hz, 1H, Ar-H), 8.04 (s, 1H, Ar-H), 8.45–8.55 (br s,
1H, NH), 9.94 (s, 1H); 13C NMR (100 MHz, CDCl3): d 24.4, 120.4, 125.7, 125.9,
129.7, 136.9, 139.1, 169.4, 192.4.
4-Acetamidobenzaldehyde (2r). White waxy solid. 1H NMR (400 MHz,
CDCl3): d 2.24 (s, 3H, CH3), 7.72 (d, J ¼ 8.5 Hz, 2H, Ar-H), 7.76–7.83 (br s, 1H,
NH), 7.86 (d, J ¼ 8.5 Hz, 2H, Ar-H), 9.93 (s, 1H); 13C NMR (100 MHz, CDCl3): d
24.8, 119.2 (2C), 131.2 (2C), 132.2, 143.6, 168.7, 191.1.
1
4-(1’-Hydroxyethyl)benzaldehyde (4). Colorless oil. H NMR (400 MHz,
CDCl3): d 1.49 (d, J ¼ 6.5 Hz, 3H, CH3), 2.53–2.56 (br s, 1H, OH), 4.96 (q,
J ¼ 6.5 Hz, 1H), 7.51 (d, J ¼ 8.2 Hz, 2H, Ar-H), 7.82 (d, J ¼ 8.2 Hz, 2H, Ar-H),
9.94 (s, 1H). 13C NMR (100 MHz, CDCl3): d 25.3, 69.8, 125.9 (2C), 130.1 (2C),
135.5, 152.9, 192.2 (CO).
1
4-Acetylbenzaldehyde (5). Colorless oil. H NMR (400 MHz, CDCl3): d
2.68 (s, 3H, CH3), 8.00 (d, J ¼ 8.0 Hz, 2H, Ar-H), 8.12 (d, J ¼ 8.0 Hz, 2H, Ar-H),
10.1 (s, 1H). 13C NMR (100 MHz, CDCl3): d 27.0, 128.8 (2C), 129.8 (2C), 139.1,
141.2, 191.6 (CHO), 197.4 (CO).
1
3-(1’-Hydroxyethyl)benzaldehyde (7). Colorless oil. H NMR (400 MHz,
CDCl3): d 1.51 (d, J ¼ 6.5 Hz, 3H, CH3), 2.55 (s, 1H, OH), 4.97 (q, J ¼ 6.5 Hz,
1H), 7.50 (t, J ¼ 7.6 Hz, 1H, Ar-H), 7.65 (dt, J ¼ 7.6, 1.2 Hz, 1H, Ar-H), 7.76 (dt,
J ¼ 7.6, 1.2 Hz, 1H, Ar-H), 7.87 (t, J ¼ 1.2 Hz, 1H, Ar-H), 9.98 (s, 1H). 13C NMR
(100 MHz, CDCl3): d 25.4, 69.7, 126.5, 128.9, 129.2, 131.7, 136.5, 147.0, 193.5 (CO).
1
3-Acetylbenzaldehyde (8). Colorless oil. H NMR (400 MHz, CDCl3): d
2.69 (s, 3H, CH3), 7.68 (t, J ¼ 7.7 Hz, 1H, Ar-H), 8.11 (dt, J ¼ 7.7, 1.4 Hz, 1H,
Ar-H), 8.25 (dt, J ¼ 7.7, 1.4 Hz, 1H, Ar-H), 7.87 (t, J ¼ 1.4 Hz, 1H, Ar-H), 10.1 (s,
1H). 13C NMR (100 MHz, CDCl3): d 26.7, 129.5, 129.6, 133.7, 133.8, 136.6, 137.8,
191.5, 197.0.
ACKNOWLEDGMENTS
The authors are grateful to the National Science Foundation of China (No.
21162001), the Natural Science Foundation of Jiangxi Province (No. 2009GZH0013),

AEROBIC OXIDATION OF PRIMARY BENZYLALCOHOLS
2325
and the National Key Technology R&D Program of China (No. 2009BAI78B01) for
financial support.
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