Highly efficient selective oxidation of alcohols to carbonyl compounds catalyzed by ruthenium (III) meso-tetraphenylporphyrin chloride in the presence of molecular oxygen

Article abstract of DOI:10.1016/j.bmcl.2007.08.063

Efficient selective oxidation of alcohols to carbonyl compounds by molecular oxygen with isobutyraldehyde as oxygen acceptor in the presence of metalloporphyrins has been reported. Ruthenium (III) meso-tetraphenylporphyrin chloride (Ru(TPP)Cl) showed exce

Full text of DOI:10.1016/j.bmcl.2007.08.063

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 17 (2007) 6364–6368
Highly efficient selective oxidation of alcohols to carbonyl
compounds catalyzed by ruthenium (III) meso-tetraphenylporphyrin
chloride in the presence of molecular oxygen
Hong-Bing Ji,* Qiu-Lan Yuan, Xian-Tai Zhou, Li-Xia Pei and Le-Fu Wang
School of Chemical and Energy Engineering, South China University of Technology, Guangzhou 510640, China
Received 6 July 2007; revised 19 August 2007; accepted 24 August 2007
Available online 28 August 2007
Abstract—Efficient selective oxidation of alcohols to carbonyl compounds by molecular oxygen with isobutyraldehyde as oxygen
acceptor in the presence of metalloporphyrins has been reported. Ruthenium (III) meso-tetraphenylporphyrin chloride (Ru(TPP)Cl)
showed excellent activity and selectivity for oxidation of various alcohols under mild conditions. Moreover, different factors influ-
encing alcohols oxidation, for example, catalyst, solvent, temperature, and oxidant, have been investigated. In large-scale oxidation
of benzyl alcohol, the isolated yield of benzaldehyde of 89% was observed.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Oxidation of alcohols to the corresponding carbonyl
compounds is a very important step for organic synthe-
sis.1 From an environmental and cost-effective view-
point, catalytic oxidation processes with molecular
oxygen or air are extremely valuable and particularly
attractive.2 Accordingly, variety of transition metal
based catalysts (mainly cobalt,3 vanadium,4 ruthenium,5
palladium,6 and copper7) have been intensively investi-
gated for aerobic oxidation of alcohols so far.
(48%) after 94 h in refluxing chlorobenzene.5a Kato ever
reported heterogeneous aerobic oxidation of benzyl alco-
hol by using microporous dinuclear ruthenium (II, III)
carboxylate tetrafluoroborate containing porphyrin,
[Ru2II;III(H2TCPP)]BF4(H2TCPP = 4,4,4,4-(21H, 23H-por-
phine-5,10,15,20-tetrayl)tetrakis benzoic acid), as catalyst,
in which high selectivity of benzaldehyde (95%) was
obtained after 24 h and the TON (turnover number) of
catalyst was 21.15b
As model catalysts of cytochrome P-450, metalloporphy-
rins could be used as the intermediate of oxygen carrier to
biological systems, and have been widely used as catalysts
for various oxidation reactions, for example, hydroxyl-
ation of hydrocarbon and epoxidation of olefins under
mild conditions.8 Although metalloporphyrins have been
used to catalyze the oxidation of alcohols with PhIO,9
Cl2PyNO,10 t-BuOOH,11 KHSO5,12 Bu4NHSO5 (tetra-
butylammonium peroxymonosulfate),13 and m-CPBA
(m-chloroperbenzoic acid)14 as oxidants, few studies on
metalloporphyrins-catalyzed oxidation of alcohols by
molecular oxygen were reported.15 For example, Woo
and co-authors ever reported the aerobic homogeneous
oxidation of benzyl alcohol with oxotitanium porphyrin
(TTP)Ti@O, which gave benzaldehyde in modest yields
In our previous studies, metalloporphyins exhibited high
catalytic performance for oxidation of alkanes, olefins,
and sulfides with molecular oxygen.16 We also devel-
oped procedures for highly selective oxidation of alco-
hols to carbonyl compounds with b-cyclodextrin,17
a
ruthenium cation combined with microcrystals of cobalt
hydroxide and cerium oxide18 and nanoparticle spinel as
catalysts.19 As part of our ongoing interests in metallo-
porphyrins-catalyzed oxidations with dioxygen, the aer-
obic oxidation of alcohols to carbonyl compounds
catalyzed by ruthenium (III) meso-tetraphenylporphyrin
chloride (Ru(TPP)Cl) in the presence of isobutyralde-
hyde has been developed (Scheme 1).20 The catalytic sys-
tem has been proved to be efficient for oxidation of
alcohols with high yields for carbonyl compounds under
mild conditions.
Keywords: Ruthenium porphyrin; Alcohol; Carbonyl compound;
Dioxygen.
The catalytic activity and selectivity of different metallo-
porphyrins for alcohol oxidation by molecular oxygen
were investigated with benzyl alcohol as model com-
*
Corresponding author. Tel./fax: +86 20 87114136; e-mail: cehbji@
scut.edu.cn
0960-894X/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.08.063

H.-B. Ji et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6364–6368
6365
N
N
N
Ru
Cl N
R
R-CH2OH
or
C O
H
or
R
benzotrifluoride, isobutyraldehyde, O2(1 atm), 60 oC
R
CH OH
C O
R'
R'
Scheme 1. Aerobic oxidation of alcohols catalyzed by ruthenium (III) meso-tetraphenylporphyrin chloride.
pound.21 The engaged catalysts for the oxidation reac-
Table 2. The effect of solvent and temperature on the oxidation of
benzyl alcohol by molecular oxygen in the presence of Ru(TPP)Cla
tions were simple structural metalloporphyrins that have
the same ligand, meso-tetraphenylporphyrin (TPP), but
different metals including Ru(TPP)Cl, Co(TPP)Cl,
Mn(TPP)Cl, and Fe(TPP)Cl. The results are summa-
rized in Table 1.
Entry
Solvent
T (ꢁC)
Conv. (%)
Yield (%)b
1
2
3
4
5
6
Benzotrifluoride
Benzotrifluoride
Benzotrifluoride
Toluene
60
50
40
60
60
60
>99
61
>99
61
43
73
43
61(12)
60(14)
18(8)
From Table 1, the catalytic activity of metalloporphy-
rins appeared to be dependent on the nature of central
ions for benzyl alcohol oxidation. Comparing with co-
balt, manganese, and iron porphyrin, ruthenium por-
phyrin was the most effective catalyst for the alcohol
oxidation system since benzyl alcohol could be com-
pletely converted to benzaldehyde. Cobalt porphyrin
also presented similar high activity toward benzyl alco-
hol oxidation (entry 2), but poor selectivity for benzal-
dehyde was obtained after the reaction was carried
out for 30 min. Similar results of alcohol oxidation
were obtained with other cobalt catalysts, for example,
Benzene
Acetonitrile
74
26
a Benzyl alcohol (1 mmol), Ru(TPP)Cl (1 · 10À3 mmol), isobutyralde-
hyde (3 mmol), solvent (5 mL), O2 bubbling (1 atm), 60 ꢁC, 0.5 h.
b The numbers in parentheses indicate the yields of benzoic acid.
electrophilic effect of benzotrifluoride was favorable
to the oxidation of benzyl alcohol, which gave 99%
yield of benzaldehyde. When toluene and benzene were
used instead, only about 60% benzaldehyde could be
obtained, together with a little of benzoic acid. Very
low yield of benzaldehyde (18%) could be obtained
when acetonitrile was used as solvent in the oxidation
system (entry 6). The distinct results should be attrib-
uted to the solvation effect of each solvent. The three
fluorine atoms of the benzotrifluoride resulted in better
solvation effect,25 which could make the O–H bond
cleavage much easier.
Co(acac)3 and cobalt Schiff base.23 It seems that man-
22
ganese and iron porphyrins were not effective catalysts
for benzyl alcohol oxidation by molecular oxygen
(entries 3 and 4). The catalytic activity and selectivity
of different metalloporphyrins were probably influenced
by stability of different valences of metal atoms and their
electric potential.24
Aerobic oxidation of benzyl alcohol with Ru(TPP)Cl
catalyst using various solvents and the effect of reaction
temperature on its conversion were also investigated and
the results are summarized in Table 2.
The effect of temperature on oxidation of benzyl alcohol
was also investigated. It was shown that the conversion
was increased with raising temperature from 40 to
60 ꢁC. When the temperature was 60 ꢁC, benzyl alcohol
could be oxidized to benzaldehyde stoichiometrically by
molecular oxygen with 1 · 10À3 mmol Ru(TPP)Cl cata-
lyst. It is notable that the increasing temperature could
hardly influence the selectivity of benzaldehyde, that
is, no over-oxidation product, for example, benzoic acid
could be detected as the temperature was raised.
As shown in Table 2, it seemed that solvent played
an important role in the oxidation system. The strong
Table 1. Oxidation of benzyl alcohol by molecular oxygen in the
presence of various metalloporphyrin catalystsa
Entry
Catalyst
Conv. (%)
Yield (%)b
The effects of oxidant on oxidation of benzyl alcohol
were also investigated, and the results are summarized
in Table 3. Table 3 shows that dioxygen was more effec-
tive than H2O2, t-BuOOH, and NaOCl for oxidation of
benzyl alcohol catalyzed by Ru(TPP)Cl/isobutyralde-
hyde (entries 1, 4, 7, and 10), indicating that the metal-
loporphyrins have excellent performance for activating
1
2
3
4
Ru(TPP)Cl
Co(TPP)Cl
Mn(TPP)Cl
Fe(TPP)Cl
>99
>99
45
>99
42(58)b
33(12)b
32
32
a Benzyl alcohol (1 mmol), catalyst (1 · 10À3 mmol), isobutyraldehyde
(3 mmol), benzotrifluoride (5 mL), O2 bubbling (1 atm), 60 ꢁC, 0.5 h.
b The numbers in parentheses indicate the yields of benzoic acid.

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H.-B. Ji et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6364–6368
Table 3. The effect of oxidant on the oxidation of benzyl alcohol
catalyzed by molecular oxygen in the presence of Ru(TPP)Cla
and ketones in high yields. It seems that the catalytic
activity is dependent on the electronic property of
substrates (entries 2–4). Secondary alcohols could be
easily converted to the corresponding ketones in high
yield (entries 5–8). In the case of 2-adamantanol, the
catalytic system shows high activity, which gave
ketone yield of 72% when the reaction continued
for 30 min despite the hindrance. In the cases of sat-
urated primary aliphatic alcohols, for example, 1-oct-
anol and 1-hexanol, further oxidation of aldehydes to
the corresponding carboxylic acids was observed
under the same reaction conditions (entries 9 and
10). It is consistent with Han’s result using m-CPBA
(m-chloroperbenzoic acid) as oxidant and iron (III)
porphyrin as catalyst.14
Entry
Oxidant
Conv. (%)
Yield (%)
1
2b
3c
O2
O2
>99
28
<1
22
18
3
>99
28
<1
22
18
3
O2
H2O2
4
5b
6c
H2O2
H2O2
7
t-BuOOH
t-BuOOH
t-BuOOH
NaOCl
NaOCl
NaOCl
20
26
26
15
13
70
20
26
26
9
8b
9c
10
11b
12c
6
70
a Benzyl alcohol (1 mmol), Ru(TPP)Cl (1 · 10À3 mmol), isobutyralde-
hyde (3 mmol), benzotrifluoride (5 mL), 60 ꢁC, 0.5 h, O2 bubbling
(1 atm), amount of H2O2, t-BuOOH, NaOCl was 2 mL, respectively.
b Without catalyst.
The alkyl length between benzene ring and –OH
group could significantly influence the oxidation
mode since only 27% of 2-phenylethanol was con-
verted after running reaction for 30 min (entry 11).
Furthermore, scission of C–C bond leading to loss
of one methylene group was also observed in the oxi-
dation of 2-phenylethanol, which gave a mean yield
of phenylacetaldehyde and benzaldehyde. Similar C–
C bond cleavage could also be found when cinnamyl
alcohol was subjected to the reaction system (entry
12), in which, cinnamyl alcohol could be converted
completely and benzaldehyde was the main product
owing to the existence of C@C bond. The catalytic
system also exhibited high activity for the oxidation
of diol, for example, 1-phenyl-1,2-ethanediol (entry
13). The main product benzaldehyde resulted from
the oxidative cleavage of C–C bonds. Diols that
could be cleaved by molecular oxygen to the corre-
sponding aldehydes were also reported with
Ru(PPh3)3Cl2 as catalyst.27 Okamoto reported that
an alkoxide was the intermediate in 1,2-diol oxidation
in the presence of molecular oxygen.28 So, in the
present study of C–C bond cleavage, alkoxide com-
plex was formed by the reaction of alcohol with high
valent Ru(V) porphyrin. Then the carbonyl product
could be formed by the thermal decomposition of
alkoxide.
c Without isobutyraldehyde.
dioxygen due to their special structure. This could be
further supported by the fact that the conversion was
only 28% in the absence of catalyst (entry 2), while no
obvious difference on catalytic activities could be
observed with H2O2, t-BuOOH, and NaOCl as oxidants
in the presence/absence of catalyst (entries 4/5, 7/8, and
10/11).
From Table 3, it could also be found that isobutyralde-
hyde played an important role in the benzyl alcohol oxi-
dation system when dioxygen and hydrogen peroxide
were employed as oxidants. The oxidation reaction with
molecular oxygen stopped suddenly in the absence of
isobutyraldehyde (entry 3). Similar phenomena could
be observed for H2O2 oxidation system either, since
the yield of benzaldehyde sharply decreased from 22%
to 3% (entry 6). However, similar results could not be
found for the oxidation system with t-BuOOH or NaO-
Cl as oxidant in the absence of isobutyraldehyde. It
should be attributed to the different formation mecha-
nisms of high valent metal-oxo intermediates with differ-
ent oxidants in the presence of isobutyraldehyde. In the
case of dioxygen system, high valent metal-oxo interme-
diate was generated in the radical chain from isobutyral-
dehyde.26 In addition, for the catalytic system with
NaOCl as an oxidant, high yield of benzaldehyde could
be obtained without isobutyraldehyde (entry 12). But
only 15% of benzyl alcohol could be oxidized in the
presence of isobutyraldehyde (entry 10). This is because
that in the presence of isobutyraldehyde, isobutyralde-
hyde was preferentially oxidized to propanoic acid by
the strong oxidant, which resulted in insufficient amount
of oxidant for benzyl alcohol oxidation.
A large-scale oxidation of benzyl alcohol was carried
out in the presence of molecular oxygen as shown in
Scheme 2.29
When the amount of Ru(TPP)Cl catalyst was 1 · 10À3
mmol, benzaldehyde could be obtained with the
isolated yield of 89% by conducting the reaction for
1.5 h. It should be mentioned that the turnover num-
ber of the present catalyst could reach up nearly nine
thousand.
To evaluate the scope of the catalytic system, oxidation
of various alcohols, including benzylic alcohols, second-
ary alcohols, and primary alcohols, by atmospheric
dioxygen in the presence of Ru(TPP)Cl under mild con-
ditions, has been investigated (Table 4).
Conclusions. In conclusion, ruthenium (III) meso-tetra-
phenylporphyrin chloride has been proven to be an
excellent catalyst for oxidation of alcohols in the pres-
ence of molecular oxygen and isobutyraldehyde. All
the factors that effected benzyl alcohol oxidation were
well investigated. Various alcohols including benzylic
alcohols, primary alcohols, and secondary alcohols
could be successfully oxidized.
As shown in Table 4, most alcohols could be
smoothly converted to the corresponding aldehydes

H.-B. Ji et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6364–6368
6367
Table 4. Oxidation of various alcohols by molecular oxygen in the presence of Ru(TPP)Cla
Entry
Alcohol
Product
Time (h)
0.5
Conv. (%)
>99
Yield (%)
OH
O
1
>99
35
OH
O
2
3
4
5
1.0
0.5
1.0
0.5
35
91
17
94
MeO
MeO
OH
O
91
17
94
Cl
Cl
OH
OH
O
N
N
O
OH
O
6
7
1.5
1.0
93
94
93
94
OH
O
O
OH
8
9
0.5
1.0
91
93
72
71
O
OH
OH
O
O
10
11
1.0
0.5
90
27
70
14
OH
OH
OH
O
O
13
OH
12b
1.0
0.5
100
74
17
O
OH
O
O
13
>99
88
8
OH
O
a Substrate (1 mmol), Ru(TPP)Cl (1 · 10À3 mmol), isobutyraldehyde (3 mmol), benzotrifluoride (5 mL), O2 bubbling (1 atm), 60 ꢁC, 0.5 h.
b Toluene (5 mL) as solvent.
-3
Ru(TPP)Cl (1x10 mmol), isobutyraldehyde(0.03 mol)
OH
O
o
benzotrifluoride(50 mL), O (1atm), 1.5 h, 60 C
2
10 mmol
Isolated Yield: 89%
TON: 8, 900
Scheme 2. Large-scale aerobic oxidation of benzyl alcohol by molecular oxygen in the presence of Ru(TPP)Cl.

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H.-B. Ji et al. / Bioorg. Med. Chem. Lett. 17 (2007) 6364–6368
13. Rezaeifard, A.; Jafarpour, M.; Moghaddam, G. K.;
Acknowledgments
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The authors thank the National Natural Science Foun-
dation of China (20576045) and the Program for New
Century Excellent Talents in University (NCET-06-
740) for providing financial support for this project.
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29. General procedure for the large-scale oxidation of benzyl
alcohol to benzaldehyde. Dioxygen was bubbled through a
solution of benzotrifluoride (50 mL), benzyl alcohol
(10 mmol), isobutyraldehyde (0.03 mol), and Ru(TPP)Cl
(1 · 10À3 mmol). The mixture was stirred at 60 ꢁC for
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