Oppenauer-type oxidation of secondary alcohols catalyzed by homogeneous water-soluble complexes

Article abstract of DOI:10.1139/v05-037

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, 1-heteroaromatic, aliphatic, and allylic secondary alcohols using catalyst:substrate ratios ranging from 0.4% to 2.5%. Sterically hindered allylic alcohols undergo selectively good conversions to the corresponding enones, while unhindered ones are completely isomerized to saturated ketones. Mercury tests indicate that the catalytic process is likely homogeneous. The mechanism proposed for this Oppenauer-type oxidation including the isomerization process is based on iridium-alkoxide species.

Full text of DOI:10.1139/v05-037

7
02
Oppenauer-type oxidation of secondary alcohols
catalyzed by homogeneous water-soluble
1
complexes
Abdelaziz Nait Ajjou and Jean-Louis Pinet
Abstract: The catalytic system composed of [Ir(COD)Cl] , 2,2′-biquinoline-4,4′-dicarboxylic acid dipotassium salt
2
(
BQC), and sodium carbonate is highly efficient for the selective oxidation of benzylic, 1-heteroaromatic, aliphatic, and
allylic secondary alcohols using catalyst:substrate ratios ranging from 0.4% to 2.5%. Sterically hindered allylic alcohols
undergo selectively good conversions to the corresponding enones, while unhindered ones are completely isomerized to
saturated ketones. Mercury tests indicate that the catalytic process is likely homogeneous. The mechanism proposed for
this Oppenauer-type oxidation including the isomerization process is based on iridium-alkoxide species.
Key words: Oppenauer oxidation, water, catalysis, isomerization, secondary alcohols.
Résumé : Le système catalytique composé de [Ir(COD)Cl] , 2,2′-biquinoléine-4,4′-dicarboxylate de potassium (BQC),
2
et du carbonate de sodium est très efficace pour l’oxydation sélective des alcools secondaires benzyliques, 1-
hétéroaromatiques, aliphatiques, et allyliques en cétones correspondantes en utilisant des quantités du catalyseur allant
de 0,4 % à 2,5 %, par rapport aux substrats. Les alcools allyliques encombrés sont transformés, sélectivement, en éno-
nes correspondantes, alors que les alcools non-encombrés sont complètement isomérisés en cétones saturées. Les tests
avec le mercure indiquent que le processus catalytique est, probablement, homogène. Le mécanisme proposé pour cette
oxydation de type Oppenauer ainsi que pour l’isomérisation des alcools allyliques est basé sur la formation de l’espèce
iridium-alkoxyde.
Mots clés : oxydation d’Oppenauer, eau, catalyse, isomérisation, alcools secondaires. Nait Ajjou and Pinet 710
Introduction
as a very useful alternative technology (5). The water-soluble
catalyst that operates and resides in water is, easily, sepa-
rated from the reaction products by simple decantation. In
addition, the products are not contaminated with traces of
metal catalyst, and the use of organic solvents, such as ben-
zene and chlorinated hydrocarbons is circumvented. Despite
the obvious economical and ecological advantages of
aqueous-phase catalysis, very few water-soluble catalysts
have been reported for the oxidation of alcohols in water (6).
Oppenauer oxidation is an elegant and highly selective
process for the oxidation of secondary alcohols to the corre-
sponding ketones, with aluminum alkoxides (7). It can be
carried out under mild conditions, and is compatible with
many functional groups such as carbon–carbon double and
triple bonds, halogens, amino groups, aldehydes, or sulphur
atom-containing groups. The advantages of Oppenauer oxi-
dation have led to the development of more efficient meth-
ods based on modified aluminum alkoxides (8) and the use
of transition-metal complexes as catalysts (9).
The oxidation of alcohols to the corresponding aldehydes
and ketones is one of the most fundamental and indispens-
able reactions in organic synthesis (1). For such transforma-
tions, the use of stoichiometric amounts of harmful inorganic
oxidants, mainly chromium (VI) reagents, remains wide-
spread (2). As a consequence for the increasing demand for
efficient, cleaner, and environmentally friendly methods (3),
numerous catalytic methods using small amounts of metallic
derivatives and clean oxidants have been developed (4).
Unfortunately, most of these catalytic transformations are
accomplished in costly and toxic organic solvents. Further-
more, in the homogeneous processes, separation of the cata-
lysts from the reaction products and their quantitative
recovery in an active form are still the most important goals
but practical problems remain. To overcome these disadvan-
tages, aqueous organometallic catalysis has, recently, emerged
Received 30 November 2004. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 14 June 2005.
In the past few years, we have developed different cata-
lytic transformations in water including the hydration of
nitriles to the corresponding amides catalyzed by
This paper is dedicated to Professor Howard Alper, a
trailblazer in catalysis.
[
Rh(COD)Cl] –P(m-C H SO Na) (TPPTS) (10), organic
2 6 4 3 3
solvent-free oxidation of alcohols with tert-butyl
hydroperoxide catalyzed by water-soluble copper complexes
2
A. Nait Ajjou and J.-L. Pinet. Department of Chemistry
and Biochemistry, University of Moncton, Moncton, NB
E1A 3E9, Canada.
(
11), and transfer hydrogenation of aldehydes and ketones
with isopropanol catalyzed by water-soluble rhodium com-
plexes (12). In the present work, we describe the results for
the hydrogen transfer oxidation (Oppenauer-type oxidation)
of secondary alcohols with acetone, as the oxidant, catalyzed
1This article is part of a Special Issue dedicated to Professor
Howard Alper.
2
Corresponding author (e-mail: naitaja@umoncton.ca).
Can. J. Chem. 83: 702–710 (2005)
doi: 10.1139/V05-037
© 2005 NRC Canada

Nait Ajjou and Pinet
703
Table 1. Oxidation of 1-phenylethanol by acetone catalyzed with
various catalytic systems.
a
Catalyst
precursor
Time
(h)
Yield
(%)
b
Entry
Ligand
1
[Rh(COD)Cl]2
TPPTS
BQC
BQC
TPPTS
BQC
BQC
BQC
BQC
—
2
2
7
15
57
9
2
[Rh(COD)Cl]2
3
[Rh(COD)Cl]2
[Ir(COD)Cl]2
17
2
4
5
[Ir(COD)Cl]2
2
82
98c
82
68
5
6
[Ir(COD)Cl]2
4
7d
8e
9f
[Ir(COD)Cl]2
4
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
[Ir(COD)Cl]2
4
4
1
0g
11h
BQ
4
4
BQ
4
39
a
6
4
3
3
Reaction conditions: 1-phenylethanol (2.5 mmol), [M(COD)Cl]2
0.01 mmol), ligand (0.15 mmol), Na CO (2.5 mmol), water (10 mL), ac-
(
2
3
etone (5 mL), 90 °C, N
.
2
b
Yields refer to isolated ketones.
c
Full conversion of the substrate was obtained.
d
Second cycle of entry 6.
e
Third cycle of entry 6.
f
Reaction conditions: 1-phenylethanol (2.5 mmol), [Ir(COD)Cl] (0.01
2
mmol), Na CO (2.5 mmol), acetone (10 mL), 90 °C, N .
2 3 2
g
The reaction conditions are the same as in entry 9 except that BQ
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.
reported that [Rh(COD)Cl] –TPPTS was found to be quite
2
an effective catalyst for transfer hydrogenation of aldehydes
and ketones with isopropanol, under basic conditions (12).
Treatment of 1-phenylethanol (2.5 mmol) in the presence
of Na CO (2.5 mmol), with catalytic amounts of
cated that acetophenone was extracted without leaching of
the catalyst. When the oxidation was performed with
[Ir(COD)Cl] alone or in combination with 2,2′-biquinoline
2
3
2
[
Rh(COD)Cl]2 (0.01 mmol) and BQC (0.15 mmol) in a
(BQ) in acetone 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 acetophenone
was isolated in 39% yield (Table 1, entry 11). An analogous
positive effect of water on the rate of this homogeneous
Oppenauer-type oxidation has already been observed for
RuCl (PPh ) (9a). These results, which demonstrate the su-
deoxygenated mixture of water (10 mL) and acetone (5 mL)
at 90 °C afforded, selectively, acetophenone in 15% yield af-
ter 2 h (Table 1, entry 2). Increasing the reaction time to
1
7 h lead to a 57% yield (Table 1, entry 3). Remarkably,
when [Rh(COD)Cl] was substituted for its iridium analog
2
much more active catalyst was generated. Indeed, aceto-
phenone was obtained in 82% and 98% yields, respectively,
after 2 and 4 h (Table 1, entries 5 and 6). This catalytic
system has been shown to be much less active than
2
3 3
periority of aqueous-phase catalysis over the homogeneous
one in this Oppenauer-type oxidation, are even more inter-
3
esting since BQC is cheaper than BQ.
[
Rh(COD)Cl] –TPPTS for transfer hydrogenation of car-
To demonstrate the synthetic utility and to explore the
2
bonyl groups with i-PrOH (12). In aqueous-phase catalysis,
the most important aspects are product separation as well as
catalyst recovery and recycling in further experiments. The
scope and limitations of the [Ir(COD)Cl] –BQC system, a
2
range of different alcohols were subjected to oxidation by
acetone in a series of batch autoclave experiments. A variety
of benzylic secondary alcohols were smoothly and selec-
tively oxidized in very high yields, with full conversions in
some cases within 4 h (Table 2, entries 1–7). Under the same
reaction conditions, 1-indanol led to 1-indanone with only a
53% yield. A better result was obtained by increasing the re-
action time to 17 h (Table 2, entry 8). With a sterically en-
cumbered alcohol, namely 2-methyl-1-phenyl-1-propanol, a
58% yield was achieved, while the substrate was sluggishly
oxidized after 4 h (Table 2, entry 9). A special situation was
observed in the case of 4-chromanol, which underwent, in
addition to an oxidation of the benzylic alcohol, a hydro-
durability of the [Ir(COD)Cl] –BQC system was tested by
2
carrying out three consecutive cycles with the same catalyst
aqueous solution, carefully separated from the organic phase
under a nitrogen atmosphere at the end of each run (Table 1,
entries 6–8). Only a slight decrease in catalytic activity was
observed among each of the three recycling experiments
demonstrating the continuing viability of the catalyst. It is
relevant to point out that no metallic iridium formation was
observed after the reactions and the aqueous phases re-
mained as clear-brown solutions, indicating that there was
no decomposition of the catalyst. Analysis of the organic
phases, which remained clear, by atomic absorption indi-
genolysis of the ArO—CH (Ar is the aromatic ring) bond
2
3
BQC and BQ cost $4166.70 and $5854.12CAN per mol, respectively, from the Aldrich Chemical Co.
©
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04
Can. J. Chem. Vol. 83, 2005
Table 2. Oxidation of different benzylic alcohols by acetone using water-soluble [Ir(COD)Cl] –BQC
2
as the catalyst.a
Run
Substrate
Time (h)
4
Product
Yield (%)b
OH
O
c
1
2
98
OH
O
4
4
80
OH
O
c
3
96
CH O
3
CH O
3
OH
O
4
4
4
4
90
Br
Br
OH
O
c
5
6
7
97
OH
O
O
c
96
OH
4
90
OH
OH
O
8
9
4 (17)
53 (70)
32 (58)
O
4 (17)
4
OH
O
O
1
0
99
OH
a
Reaction conditions: substrate (2.5 mmol), [Ir(COD)Cl] (0.01 mmol), BQC (0.15 mmol), Na CO (2.5 mmol),
2
2
3
water (10 mL), acetone (5 mL), 90 °C, N .
2
b
Yields refer to isolated ketones.
c
Full conversion of the substrate was obtained.
leading to ortho-hydroxypropiophenone as the sole product
Table 2, entry 10). When the reaction was performed in the
chromanol molecule liberates ortho-hydroxypropiophenone
and regenerates the iridium alkoxide species. The transfer of
(
absence of acetone the same product was obtained and no 4-
chromanone was detected. These results indicate that the
production of ortho-hydroxypropiophenone from 4-chromanol
is, probably due to an intramolecular transfer hydrogeno-
lysis. The catalytic process is envisioned as a reaction be-
tween 4-chromanol and the water-soluble iridium system
resulting in an iridium alkoxide complex, which, in turn,
hydride to ArO–CH did not occur in the case of 1-(4-
3
methoxyphenyl)ethanol (Table 2, entry 3) since the methoxy
group is located far from the metal hydride core.
To explore substrate scope further, we tested the effi-
ciency of the [Ir(COD)Cl] –BQC system to oxidize second-
2
ary 1-heteroaryl alcohols (Table 3). Good to excellent yields
were obtained in all cases, although the reaction time was
extended to 17 h for the oxidation of 1-(2-furyl)-2-methyl-1-
propanol and the thienyl analog, which gave, respectively,
74% and 65% yields (Table 3, entries 3 and 4). The slug-
gishness in these two cases is attributed to steric hindrance
from the isopropyl group.
2
undergoes β-hydride elimination to produce a η -ketone
hydride species, where the 4-chromanone is still coordinated
to the iridium. The transfer of iridium hydride to the methy-
lene group of ArO–CH of the coordinated 4-chromanone
2
(
intramolecular 1,3-hydride shift) leads to an iridium ortho-
propionylphenoxide complex. Protonation with another 4-
The reaction was, also, extended to a variety of secondary
©
2005 NRC Canada

Nait Ajjou and Pinet
Table 3. Oxidation of different 1-heteroaryl alcohols by acetone using water-soluble
705
a
[
Ir(COD)Cl] –BQC as the catalyst.
2
Run
Substrate
Time (h)
Product
Yield (%)b
OH
O
O
S
O
1
4 (17)
87 (87)
OH
OH
O
O
S
2
3
4 (17)
4 (17)
88 (90)
66 (74)
O
O
OH
O
S
S
4
4 (17)
48 (65)
a
Reaction conditions: substrate (2.5 mmol), [Ir(COD)Cl] (0.01 mmol), BQC (0.15 mmol), Na CO
3
2
2
(
2.5 mmol), water (10 mL), acetone (5 mL), 90 °C, N .
Yields refer to isolated ketones.
2
b
Table 4. Oxidation of different aliphatic alcohols by acetone using water-soluble [Ir(COD)Cl] –BQC
2
as the catalyst.a
Run
Substrate
Time (h)
Product
Yield (%)b
OH
O
1
4
76
2
OH
4 (17)
17
O
15 (25)
80
c
3
OH
OH
O
O
4
5
4 (17)
4 (17)
17
21 (31)
21 (30)
91
OH
OH
O
O
c
6
OH
OH
O
O
7
4
23
80
72
c
8
17
20
c
OH
O
9
a
Unless stated otherwise, the reaction conditions are: substrate (2.5 mmol), [Ir(COD)Cl] (0.01 mmol), BQC
2
(
0.15 mmol), Na CO (2.5 mmol), water (10 mL), acetone (5 mL), 90 °C, N .
2
3
2
b
Yields refer to isolated ketones.
Substrate (1 mmol), [Ir(COD)Cl] (0.025 mmol), and BQC (0.375 mmol).
c
2
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Can. J. Chem. Vol. 83, 2005
Table 5. Oxidation of different allylic alcohols by acetone using various water-soluble catalysts.a
Run
1
Substrate
Catalyst
Time (h)
–BQC
1
Product
Yield (%)b
[Ir(COD)Cl]
100
2
OH
O
O
O
O
O
2
3
4
5
[Ir(COD)Cl]
2
–TPPTS
4
4
4
4
100
98
OH
OH
OH
PtCl (COD)–BQC
2
PdCl
[Rh(COD)Cl]
(PhCN)
–BQC
100
100
2
2
–BQC
2
OH
OH
O
6
7
[Ir(COD)Cl]
[Ir(COD)Cl]
[Ir(COD)Cl]
[Ir(COD)Cl]
2
–BQC
1.5
100
50
OH
OH
OH
O
O
–BQC
–BQC
–BQC
17
17
17
2
2
2
c
8
100
13
O
9
OH
O
c
10
[Ir(COD)Cl]
2
–BQC
17
100
a
Unless stated otherwise, the reaction conditions are: substrate (2.5 mmol), [Ir(COD)Cl] (0.01 mmol), BQC
2
(
0.15 mmol), Na CO (0.5 mmol), water (10 mL), acetone (5 mL), 90 °C, N .
2
3
2
b
1
Yields were estimated by H NMR.
c
Substrate (1 mmol), [Ir(COD)Cl] (0.025 mmol), and BQC (0.375 mmol).
2
aliphatic alcohols (Table 4). However, these inactivated al-
cohols proved to be sluggish substrates under our standard
conditions, and only poor yields were obtained. Fortunately,
a higher catalyst:substrate ratio (2.5 mol%) and longer reac-
tion times greatly improve the yields (Table 4, entries 3, 6,
chemistry that still suffers from several drawbacks (14). Sur-
prisingly, the water-soluble catalytic systems reported for
the isomerization of allylic alcohols are rare, despite their
synthetic utility (15). Thus, other water-soluble catalysts
were tested for the oxidation of 1-octen-3-ol (Table 5, en-
tries 2–5). Excellent to full conversions to the isomerization
8
, and 9). Remarkably, cyclooctanol showed a higher reac-
tivity than cyclohexanol and the former was oxidized in
6% yield with only 0.4 mol% of [Ir(COD)Cl] after 4 h
product were obtained except in the case of RuCl –BQC,
3
7
which is inactive. Although these reactions were not opti-
2
(
Table 4, entry 1).
mized, the result obtained with [Ir(COD)Cl] –BQC is prom-
2
To investigate the extent of [Ir(COD)Cl] –BQC oxidation,
ising since its catalytic frequency for the isomerization of 1-
2
–
1
different allylic alcohols have been studied (Table 5). The
oxidation of 1-octen-3-ol afforded 3-octanone as the sole
product after only 1 h of reaction time (Table 5, entry 1).
The reaction performed in the absence of acetone led to the
same result after 4 h. However, this reaction was not opti-
mized. The isomerization of allylic alcohols to saturated car-
bonyl compounds is an important reaction in organic
octen-3-ol (125 h at 90 °C) is slightly better than the re-
ported activities for other water-soluble catalysts based on
6
–1
[RuCl (η -arene){P(CH OH) }] (4–67 h at 75 °C) (15a),
2
2
3
–
1
–1
Rh (SO ) (30 h at 80 °C) (15b), RhCl (60 h at 70 °C)
2
4 3
3
–
1
(15b), and (COD)RhCl(PAr ) (15–48 h at 104 °C) (15c).
3
We are currently continuing to investigate the isomerization
of allylic alcohols in the aqueous phase. The oxidation of 1-
©
2005 NRC Canada

Nait Ajjou and Pinet
707
Scheme 1. Proposed mechanism for the Oppenauer-type oxidation and isomerization of allylic alcohols.
phenyl-2-propen-1-ol, under the same conditions, also gave
propiophenone as the isomerization product, with full con-
version (Table 5, entry 6), whereas cyclic allylic alcohols
are, surprisingly, oxidized to their corresponding unsaturated
ketones albeit in low yields (Table 5, entries 7 and 9). Fortu-
nately, under the conditions used for the oxidation of
aliphatic alcohols (Table 4, entry 3), isophorol and 3-methyl-
Although the structure of the water-soluble active
complex is not identified, a rational mechanism for this
Oppenauer-type oxidation, including the isomerization pro-
cess, is presented in Scheme 1.
Since no iridium mirror (black metallic iridium) was ob-
served after the reactions and the aqueous phases remained
as clear-brown solutions, we carried out mercury poisoning
experiments (see the Experimental section) to rule out the
possibility of catalysis by colloidal metal. Mercury poison-
ing is a well-known and widely used test to distinguish be-
tween heterogeneous and homogeneous processes (16). The
addition of a large excess of mercury(0) (Hg:Ir molar ratio
of 872) with vigorous stirring did not suppress the catalysis.
2
-cyclohexenol were fully converted to their corresponding
unsaturated ketones (Table 5, entries 8 and 10). Bäckvall
and co-workers (9a) have also reported that 1-octen-3-ol
was isomerized under their reaction conditions to 3-octanone
using ruthenium complexes, while 2-cyclohexenol was con-
verted to an unsaturated ketone.
©
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Can. J. Chem. Vol. 83, 2005
Table 6. Oxidation of 1-phenylethanol by ace-
Fig. 1. Oxidation profiles of PhCH(OH)CH (+) and
3
tone catalyzed by water-soluble [Ir(COD)Cl] –
PhCD(OH)CH (o). Substrate (2.5 mmol), [Ir(COD)Cl]
2
3
2
a
BQC using different amounts of Na CO .
(0.01 mmol), BQC (0.15 mmol), Na CO (2.5 mmol), water
2
3
2
3
(
10 mL), acetone (5 mL), 90 °C, N2.
Run
Na CO (mmol)
Yield (%)b
2
3
7
6
5
4
0
0
0
0
1
2
3
4
5
6
7
a
0
0
88
98
100
100
100
100
0.25
0.5
1
1.5
2
2.5
Reaction conditions: substrate (2.5 mmol),
30
[
Ir(COD)Cl] (0.01 mmol), BQC (0.15 mmol),
2
Na CO (0–2.5 mmol), water (10 mL), acetone
2
3
2
1
0
0
0
(
5 mL), 90 °C, N , 4 h.
2
b
1
Yields were estimated by H NMR.
These results suggest that nanoparticles are not the key spe-
cies in the present system and that the process is likely ho-
mogeneous.
0
10
20
30
40
50
60
70
Time (min)
The first step of the proposed mechanism would involve
the formation of iridium alkoxide II (9a, 17). Stoichiometric
amounts of Na CO were used in our experiments. However,
enone hydride species IIIisom.. This latter is expected to
evolve in two different ways depending on the steric re-
quirements of the enone. In the case of bulky substrates such
as isophorol, IIIisom. releases the enone through path A and
follows the catalytic cycle for the Oppenauer-type oxidation.
This is in agreement with previous reports regarding the
isomerization of allylic alcohols catalyzed by ruthenium
complexes (21, 22). The steric unhindered enones, on the
other hand, are not released and allow IIIisom. to rearrange to
2
3
oxidation of 1-phenylethanol with smaller amounts of
Na CO also gave excellent results (Table 6). When the reac-
2
3
tion was performed in the absence of base, its remarkable ef-
fect was particularly noticeable because no reaction occurred
(
Table 6, entry 1). This effect of base in Oppenauer-type oxi-
dations has been observed in previous reports (9a, 9b). Con-
sequently, the primary role of the base is to facilitate
formation of iridium alkoxide complex II. This latter species
can undergo β-hydride elimination of the coordinated alkoxide
to produce the desired ketone and iridium hydride species
IV, via complex III where the ketone is still coordinated to
π-oxallyl species IV
through path B. Protonation with
another allylic alcohol molecule liberates the saturated
isom.
ketone and regenerates the initial species IIisom.
.
2
the metal (η -ketone ligand). β-hydride elimination from
metal alkoxides has been observed (17, 18) and proposed for
hydrogen transfer reactions (9a, 19). In our case, the produc-
tion of ortho-hydroxypropiophenone from 4-chromanol is in
agreement with the formation of complex III. Comparison
of the oxidation rates for PhCH(OH)CH and PhCD(OH)CH ,
Summary
In conclusion, the system composed of [Ir(COD)Cl] and
2
the cheap water-soluble ligand BQC is a new and highly ef-
ficient catalyst for Oppenauer-type oxidation of benzylic, 1-
heteroaromatic, aliphatic, and allylic secondary alcohols.
The system showed much better activity than the water-
insoluble analogs. The catalytic process is likely homoge-
neous and the mechanism proposed for this Oppenauer-type
oxidation including the isomerization process is based on
iridium-alkoxide species.
3
3
under the same conditions (Fig. 1), indicates that the rate-
limiting step does not involve β-hydride elimination from II
since the isotope effect is low (k /k = 1.2). The reaction of
H
D
acetone with IV would produce complex V, which leads to
iridium isopropoxide VI by insertion of acetone into the Ir—
H bond. An exchange of alkoxy moiety would regenerate
species II and liberate isopropanol. In the case of allylic al-
cohols, an additional pathway for the isomerization process
is proposed. Three mechanisms have been proposed for tran-
sition metal catalyzed isomerization of allylic alcohols. The
first one involves a π-allyl metal hydride complex (20),
while a second one is based on metal hydride addition–elim-
ination (21). A third mechanism, suggested by Trost and
Kulawiec (22), involves the coordination of allylic alcohol as
a bidentate ligand and the formation of metal alkoxide spe-
Experimental
Materials and instruments
All the substrates, BQ, and BQC were purchased from
Aldrich Chemical Co. and used without further purification.
[
Rh(COD)Cl] , RuCl ·xH O, PdCl (PhCN) , and [Ir(COD)Cl]
2 3 2 2 2 2
were acquired from Strem Chemical Co.
TPPTS was prepared according to the literature method
(23) and PhCD(OH)CH was prepared using LiAlD (24).
cies. Our results from oxidation with [Ir(COD)Cl] –BQC
2
3
4
(
Table 5) are consistent with the alkoxide mechanism. Thus,
Routine NMR measurements were performed on a Bruker
AC-200 spectrometer at 200 MHz and 50 MHz, respectively,
in the case of allylic alcohols, II is reorganised as IIisom. to
allow the coordination of Ir with R (R is an olefinic moi-
1
13
2
2
for H and C, using tetramethylsilane (TMS) as an internal
ety). IIisom. undergoes β-hydride elimination to produce the
standard and CDCl as the solvent.
3
©
2005 NRC Canada

Nait Ajjou and Pinet
709
Typical procedure for this Oppenauer type oxidation
with acetone
In the glass liner of a 45 mL autoclave, under an atmo-
sphere of nitrogen, [Ir(COD)Cl]2 (0.01 mmol), BQC
References
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(
0.15 mmol), and Na CO (2.5 mmol) were dissolved in de-
2 3
gassed water (10 mL) at room temperature. Next a solution
of the substrate (2.5 mmol) in degassed acetone (5 mL) was
introduced. The autoclave, was then flushed several times
2
. G. Cainelli and G. Cardillo. Chromium oxidations in organic
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2
an oil bath at 90 °C for the required reaction time. The auto-
clave was cooled to room temperature and the mixture was
extracted three times with degassed diethyl ether (20 mL).
3
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(
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for comparison. The H NMR spectra for all samples
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of 4-hydroxy-4-methyl-2-pentanone formed by aldol con-
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a fresh charge of solution consisting of the alcohol
(
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(
2.5 mmol) in degassed acetone (5 mL).
Mercury poisoning experiments
Three experiments were performed to determine the effect
of mercury (0) on the reaction.
A solution of [Ir(COD)Cl] (13.4 mg, 0.02 mmol) and
2
BQC (0.3 mmol, 142 mg) in water (20 mL) was stirred in
the presence of excess mercury (7 g, 34.9 mmol, ca.
8
72 equiv.) for 24 h. The solution was decanted from the
mercury and divided into two equal parts, which were used
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mixture was then heated at 90 °C for the remaining reaction
time. The two experiments showed the same excellent cata-
lytic activity.
A third experiment was done just to confirm the previous
ones. In this experiment, the oxidation was initiated in the
presence of excess Hg(0) (872 equiv.). The result was identi-
cal to the previous ones.
6
7
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9
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2
6
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1
1
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(
Acknowledgements
2
We are grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC) and to the Faculté
des études supérieures et de la recherche (FESR) of the Uni-
versity of Moncton for financial support of this research.
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©
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