First example of water-soluble transition-metal catalysts for Oppenauer-type oxidation of secondary alcohols

Article abstract of DOI:10.1016/S0040-4039(00)01882-7

The first water-soluble transition-metal catalysts for Oppenauer-type oxidation of secondary alcohols have been developed. The catalytic system composed of [Ir(COD)Cl]2, 2,2'-biquinoline-4,4'-dicarboxylic acid dipotassium salt (BQC) and sodium carbonate is highly efficient for the selective oxidation of benzylic and aliphatic secondary alcohols to the corresponding ketones with catalyst/substrate ratios ranging from 0.4 to 2.5 percent. The substitution of [Ir(COD)Cl]2 by its rhodium analog [Rh(COD)Cl]2 generates a less active catalytic system. [Ir(COD)Cl]2/BQC was also found to be more active than its water-insoluble analog system [Ir(COD)Cl]2/2,2'-biquinoline (BC).

Full text of DOI:10.1016/S0040-4039(00)01882-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 13–15
First example of water-soluble transition-metal catalysts for
Oppenauer-type oxidation of secondary alcohols
Abdelaziz Nait Ajjou
Department of Chemistry and Biochemistry, University of Moncton, Moncton, New Brunswick, Canada E1A 3E9
Received 6 September 2000; revised 19 October 2000; accepted 23 October 2000
Abstract—The first water-soluble transition-metal catalysts for Oppenauer-type oxidation of secondary alcohols have been
developed. The catalytic system composed of [Ir(COD)Cl]2, 2,2%-biquinoline-4,4%-dicarboxylic acid dipotassium salt (BQC) and
sodium carbonate is highly efficient for the selective oxidation of benzylic and aliphatic secondary alcohols to the corresponding
ketones with catalyst/substrate ratios ranging from 0.4 to 2.5%. The substitution of [Ir(COD)Cl]2 by its rhodium analog
[Rh(COD)Cl]2 generates a less active catalytic system. [Ir(COD)Cl]2/BQC was also found to be more active than its water-
insoluble analog system [Ir(COD)Cl]2/2,2%-biquinoline (BC). © 2000 Elsevier Science Ltd. All rights reserved.
Catalyst recovery and recycling as well as product
separation are still the most important and difficult
problems of homogeneous catalysis, especially when
application to industry is considered. High-temperature
distillation, usually applied for separation, frequently
leads to catalyst decomposition or at least significant
deactivation. Aqueous organometallic catalysis has re-
Scheme 1.
cently emerged as a very useful alternative technology
to overcome these difficulties.1 Water-soluble transi-
A variety of catalytic systems based on different cata-
tion-metal complexes have been used as catalysts in
several important processes.2 Recently, we reported the
discovery of a novel catalytic system consisting of
[Rh(COD)Cl]2/P(m-C6H4SO3Na)3 (TPPTS) which cata-
lyzes efficiently the hydration of nitriles under basic
conditions.3
lyst precursors and the water-soluble ligand P(m-
C6H4SO3Na)3 (TPPTS) or BQC were investigated for
the oxidation of 1-phenylethanol, which was chosen as
the model substrate. The first experiments showed that
the presence of the catalyst precursor, acetone and
sodium carbonate together is required to achieve the
oxidation of 1-phenylethanol. While RuCl3·xH2O and
PdCl2(PhCN)2 exhibited no catalytic activity,
[Rh(COD)Cl]2 and [Ir(COD)Cl]2 gave rise to active
water-soluble systems (Table 1). The results shown in
Table 1 indicate that the least active catalytic systems
were provided by TPPTS (Table 1, entries 1 and 4).
Treatment of 1-phenylethanol (2.5 mmol) in the pres-
ence of Na2CO3 (2.5 mmol) and catalytic amounts of
[Rh(COD)Cl]2 (0.01 mmol) and BQC (0.15 mmol) in a
deoxygenated mixture of water (10 mL) and acetone (5
mL) at 90°C selectively afforded acetophenone in 15%
yield after 2 hours (Table 1, entry 2). An increase in the
reaction time to 17 hours lead to 57% yield (Table 1,
entry 3). A much more active catalyst was generated
when [Rh(COD)Cl]2 was substituted by its iridium
analog. Indeed, acetophenone was obtained in 82 and
98% yields, respectively, after 2 and 4 hours (Table 1,
The oxidation of alcohols to the corresponding car-
bonyl compounds is one of the most important reac-
tions in organic synthesis. As a consequence due to the
increasing demand for efficient and environmental
friendly methods,4 numerous transition-metal-catalyzed
oxidation reactions of alcohols have been developed.5
In this communication I am pleased to report the first
water-soluble iridium and rhodium complexes contain-
ing 2,2%-biquinoline-4,4%-dicarboxylic acid dipotassium
salt (BQC), which catalyze the oxidation of secondary
alcohols with acetone as the oxidant (Scheme 1).6
Keywords: Oppenauer oxidation; secondary alcohols; water-soluble
catalyst; ketones.
0040-4039/01/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01882-7

14
A. Nait Ajjou / Tetrahedron Letters 42 (2001) 13–15
Table 1. Oxidation of 1-phenylethanol by acetone cata-
catalytic activity was observed between each of the
three recycling experiments demonstrating a good sta-
bility of the catalyst. It is relevant to point out that no
metallic iridium formation was observed after the reac-
tions and the aqueous phases remained clear-brown
solutions, indicating no decomposition of the catalyst.
When the oxidation was performed with [Ir(COD)Cl]2
alone or combined with 2,2%-biquinoline (BC) in ace-
tone containing only 0.5% water very low yields were
obtained (Table 1, entries 9 and 10). Addition of water
(10 mL) increased the rate of oxidation and acetophe-
none was isolated in 39% yield (Table 1, entry 11). A
similar positive effect of water on the rate of homoge-
neous Oppenauer-type oxidation has already been ob-
served for RuCl2(PPh3)3.5a These results which
demonstrate the superiority of aqueous-phase catalysis
over the homogeneous one in this Oppenauer-type oxi-
dation are even more interesting since BQC is cheaper
that 2,2%-biquinoline (BC).8
lyzed with various catalytic systemsa
Entry
Catalyst precursor
Ligand
Time (h)
Yield (%)b
1
2
3
4
5
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Rh(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
TPPTS
BQC
BQC
TPPTS
BQC
BQC
BQC
BQC
–
2
2
17
2
2
4
4
4
4
4
7
15
57
9
82
98c
82
68
5
6
7d
8e
9f
10g
11h
BQ
BQ
4
39
4
a Reaction conditions: 1-phenylethanol (2.5 mmol), [M(COD)Cl]2
(0.01 mmol), ligand (0.15 mmol), Na2CO3 (2.5 mmol), water (10 mL),
acetone (5 mL), 90°C, N2.
b Yields refer to isolated ketones.
c Full conversion of the substrate was obtained.
d Second cycle of entry 6.7
To demonstrate the efficiency of the [Ir(COD)Cl]2/BQC
system, a range of different alcohols were subjected to
oxidation by acetone in a batch autoclave experiment
(Table 2).9 The highest yields with full conversions, in
most cases, were obtained when benzylic alcohols were
used as starting materials (Table 2, entries 1–7). The
catalytic system was also effective for the oxidation of
aliphatic alcohols. However, higher catalyst/substrate
ratios (2.5 mol%) and longer reaction times are needed
(Table 2, entries 9–16). Cyclooctanol showed higher
reactivity than cyclohexanol and was oxidized in 76%
yield with only 0.4 mol% of [Ir(COD)Cl]2 after 4 hours
(Table 1, entry 8).
e Third cycle of entry 6.7
f Reaction conditions: 1-phenylethanol (2.5 mmol), [Ir(COD)Cl]2
(0.01 mmol), Na2CO3 (2.5 mmol), acetone (10 mL), 90°C, N2.
g The reaction conditions are the same as in entry 9 except that
2,2%-biquinoline (0.15 mmol) was added to the mixture.
h The reaction conditions are the same as in entry 10 except that
water (10 mL) was added to the mixture.
entries 5 and 6). Since one of the important aspects of
aqueous-phase catalysis is the possibility of easily sepa-
rating the catalyst from the reaction medium and recy-
cling it in further experiments, the durability of the
[Ir(COD)Cl]2/BQC system was tested by carrying out
three consecutive cycles with the same catalyst in
aqueous solution, carefully separated from the organic
phase under a nitrogen atmosphere at the end of each
run (Table 1, entries 6–8).7 A slight decrease in the
In conclusion, the first water-soluble transition-metal-
catalyzed oxidation of secondary alcohols with acetone
has been revealed. The system composed of [Ir(-
COD)Cl]2 and the cheap water-soluble ligand 2,2%-
Table 2. Oxidation of different alcohols by acetone using water-soluble [Ir(COD)Cl]2/BQC as the catalysta
Run
Substrate
Time (h)
Product
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11d
12
13
14
15
16d
1-Phenylethanol
1-Phenyl-1-propanol
1-(4-Methoxyphenyl)ethanol
1-(4-Bromophenyl)ethanol
1-Tetralol
9-Hydroxyfluorene
Benzhydrol
Cyclooctanol
Cyclohexanol
Cyclohexanol
Cyclohexanol
2-Octanol
2-Octanol
2-Decanol
4
4
4
4
4
4
4
4
4
17
17
4
17
4
Acetophenone
Propiophenone
4%-Methoxyacetophenone
4%-Bromoacetophenone
1-Tetralone
9-Fluorenone
Benzophenone
Cyclooctanone
Cyclohexanone
Cyclohexanone
Cyclohexanone
2-Octanone
98c
80
96c
90
97c
96c
90
76
15
25
80
21
31
21
30
91
2-Octanone
2-Decanone
2-Decanone
2-Decanone
2-Decanol
2-Decanol
4
4
a Unless otherwise stated, the reaction conditions are: substrate (2.5 mmol), [Ir(COD)Cl]2 (0.01 mmol), BQC (0.15 mmol), Na2CO3 (2.5 mmol),
water (10 mL), acetone (5 mL), 90°C, N2.
b Yields refer to isolated ketones.
c Full conversion of the substrate was obtained.
d Substrate (1 mmol), [Ir(COD)Cl]2 (0.025 mmol) and BQC (0.375 mmol).

A. Nait Ajjou / Tetrahedron Letters 42 (2001) 13–15
15
biquinoline-4,4%-dicarboxylic acid dipotassium salt
(BQC) catalyzes the oxidation of secondary benzylic
and aliphatic alcohols selectively and efficiently. The
system is much more reactive than the water-insoluble
analogs. I am currently examining the scope of this
catalytic system using various substrates. Attempts to
improve the catalyst recycling and a study of the oxida-
tion mechanism are currently in progress.
5. (a) Almeida, M. L. S.; Beller, M.; Wang, G.-Z.; Ba¨ckvall,
J. E. Chem. Eur. J. 1996, 2, 1533 and references cited
therein. (b) A¨ıt-Mohand, S.; He´nin, F.; Muzart, J. Tetra-
hedron Lett. 1995, 36, 2473 and references cited therein.
(c) Muzart, J.; Nait Ajjou, A.; A¨ıt-Mohand, S. Tetra-
hedron Lett. 1994, 35, 1989. (d) Riahi, A.; He´nin, F.;
Muzart, J. Tetrahedron Lett. 1999, 40, 2303. (e) Mura-
hashi, S. I.; Naota, T.; Oda, Y.; Hirai, N. Synlett 1995,
733. (f) Marko´, I. E.; Giles, P. R.; Tsukazaki, M.; Chelle´-
Regnaut, I.; Gautier, A.; Brown, S. M.; Urch, C. J. J.
Org. Chem. 1999, 64, 2433.
Acknowledgements
6. For recent publications on Oppenauer oxidations, see: (a)
Graauw, C. F.; Peters, J. A.; Bekkum, H.; Huskens, J.
Synthesis 1994, 1007. (b) Ishihara, K.; Kurihara, H.;
Yamamoto, H. J. Org. Chem. 1997, 62, 5664. (c) Aka-
manchi, K. G.; Chaudhari, B. A. Tetrahedron Lett. 1997,
38, 6925. (d) Krohn, K.; Knauer, B.; Ku¨pke, J.; Seebach,
D.; Beck, A. K.; Hayakawa, M. Synthesis 1996, 1341.
7. The aqueous phase obtained after removal of solvents
was re-used with a fresh charge of a solution of the
alcohol (2.5 mmol) in degassed acetone (5 mL).
I am grateful to the FESR and Chemistry & Biochem-
istry department for financial support of this research.
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