Oxidative dimerization of primary alcohols to esters catalyzed by iridium complexes

Article abstract of DOI:10.1016/j.tetlet.2006.10.144

Primary alcohols undergo efficiently oxidative dimerization by iridium complexes under air without any solvent to form esters in fair to good yields. For instance, the reaction of 1-dodecanol in the presence of [IrCl(coe)₂]₂ (3 mol %) at 95 °C for 15 h produced dodecyl dodecanoate in 91% isolated yield. This is the first successful Ir-catalyzed oxidative dimerization of primary alcohols to esters using air as an oxidant. Various primary alcohols are converted to the corresponding esters in fair to good yields.

Full text of DOI:10.1016/j.tetlet.2006.10.144

Tetrahedron Letters 47 (2006) 9199–9201
Oxidative dimerization of primary alcohols to esters catalyzed
by iridium complexes
Aki Izumi, Yasushi Obora, Satoshi Sakaguchi and Yasutaka Ishii*
Department of Applied Chemistry and High Technology Research Center, Faculty of Engineering, Kansai University,
Suita, Osaka 564-8680, Japan
Received 11 October 2006; revised 24 October 2006; accepted 26 October 2006
Abstract—Primary alcohols undergo efficiently oxidative dimerization by iridium complexes under air without any solvent to form
esters in fair to good yields. For instance, the reaction of 1-dodecanol in the presence of [IrCl(coe) ] (3 mol %) at 95 °C for 15 h
2
2
produced dodecyl dodecanoate in 91% isolated yield. This is the first successful Ir-catalyzed oxidative dimerization of primary alco-
hols to esters using air as an oxidant. Various primary alcohols are converted to the corresponding esters in fair to good yields.
Ó 2006 Elsevier Ltd. All rights reserved.
The homogeneous catalytic transformation of primary
hydrogenation, for example, 1-butanol to 2-ethyl-1-hex-
anol. The achievement of the Ir-catalyzed Guerbet
1
7
alcohols to esters has been carried out using Pd(OAc)
2
2
and Ru (CO)
in the presence of bromobenzene and
reaction of primary alcohols which involves the forma-
tion of aldehydes as a transient intermediate prompted
us to design a reaction system for the oxidative transfor-
mation of primary alcohols to esters. Quite recently, it
3
12
tolane, respectively. The first systematic study of the cat-
alytic oxidative condensation of alcohols was made by
Murahashi and co-workers using RuH (PPh ) as a
2
3 4
3
*
catalyst. Thereafter, Cho and co-workers reported the
a-alkylation of ketones with alcohols by the same ruthe-
nium complex in the presence of stoichiometric amounts
of a hydrogen acceptor like 1-dodecene and a base like
has been reported that the Cp Ir aminoalkoxide com-
plex promotes the oxidative dimerization of primary
alcohols to esters in the presence of K CO and excess
2
3
ketone (2.7 equiv) like 2-butanone which serves as
4
8
KOH using dioxane as a solvent. In contrast to the
hydrogen acceptor. In this letter, we wish to report
Ru-catalyzed a-alkylation of ketones with alcohols
which calls for the presence of a hydrogen acceptor
the Ir-catalyzed oxidative dimerization of alcohols to
esters using air as an oxidant without any solvent and
base (Eq. 1).
and the use of dioxane as a solvent, the [IrCl(cod)] -cat-
2
alyzed a-alkylation of ketones with alcohols was
achieved without both a hydrogen acceptor and a sol-
vent. This reaction provides a very convenient clean
route to aliphatic ketones to which a carbonyl function
can be introduced into the desired position by choosing
cat.
[
IrCl(coe)2]2(3 mol%)
O
R
OH
°
C, 15 h
ð1Þ
95
R
O
R
without base and solvent
open air
1
2
5
ketones and alcohols employed. This strategy has been
extended to the reaction between secondary alcohols
The reaction of 1-octanol (1a) to octyl octanoate (2a)
was chosen as a model reaction and carried out under
the influence of several iridium complexes (Table 1).
and primary alcohols leading to b-alkylated secondary
6
alcohols. Recently, we have found that Ir complexes
like [IrOH(cod)] in the presence of a small amount of
2
KOH catalyze the Guerbet reaction of primary alcohols
to higher molecular alcohols through the aldol conden-
sation of the resulting aldehydes and the subsequent
The reaction afforded 2a as a major product together
with small amounts of octanal (3) and octanoic acid
(
4). Among the catalysts examined, [IrCl(coe) ] and
2 2
[
IrCl(cod)] were found to be good catalysts for the
2
transformation of 1a to 2a (entries 1 and 2). It is impor-
tant to note that the Ir-catalyzed dimerization could be
achieved using air as an oxidant without a solvent at
Keywords: Iridium; Primary alcohol; Ester.
*
Corresponding author. Tel.: +81 6 6368 0793; fax: +81 6 6339
026; e-mail: ishii@ipcku.kansai-u.ac.jp
4
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.144

9200
A. Izumi et al. / Tetrahedron Letters 47 (2006) 9199–9201
Table 1. Oxidative dimerization of 1-octanol (1a) to octyl octanoate
when phthalyl alcohol (1l) was employed as a substrate,
the reaction afforded phthalide (2l) in low yield (10%).
However, the reaction took place smoothly in the pres-
ence of Na CO (5 mol %) and PPh (10 mol %) in tolu-
a
(
2) by Ir-catalyst under various conditions
b
Entry
Catalyst
Conv. (%)
Yield (%)
2
3
3
1
2
3
4
5
[IrCl(coe)
[IrCl(cod)]
2
]
2
99
99
66
91
27
91
39
82
76
22 (36)
61
3
58
0
ene to afford 2l in 86% yield (entry 11).
2
c
[IrOH(cod)]
2
þ
ꢁ
4
½IrðcoeÞ ꢀ BF
The reaction is rationally explained by the following
reaction pathway (Scheme 1). The reaction is initiated
2
*
[Cp IrCl
2
]
2
d
5,10
6
e
[IrCl(coe)
[IrCl(coe)
2
]
]
by the in situ generation of an Ir–dihydride complex
7
2
2
via hydrogen transfer from alcohols (1) to an Ir-
complex, which affords aldehydes. Subsequent dehydro-
a
Compound 1a (2 mmol) was reacted in the presence of Ir-catalyst
3 mol %) under open air at 95 °C for 15 h.
(
b
GLC yields. The yields were determined based on the alcohol (1a)
Table 2. Oxidative dimerization of primary alcohols (1) to esters (2) by
used.
Yield of heptane.
Toluene (1 mL) was used as solvent.
The reaction was carried out under Ar.
a
c
d
e
Ir-catalyst
O
cat.
[IrCl(coe) ] (3 mol%)
2
2
R
OH
R
O
R
9
5 °C,15 h
open air
1
relatively lower temperature, although the ruthenium-
catalyzed reactions are carried out at higher temperature
2
2
3
of 147 °C for Ru (CO) and 180 °C for RuH (PPh ) .
Entry Alcohol (1)
Conv. Ester
3
12
2
3 4
b
(
%)
(2) (%)
In addition, the Ir-catalyst was not deactivated by the
water generated during the reaction course to promote
smoothly the dimerization of 1a to 2a, since the in situ
generated water is often responsible for prevention of
the desired reaction and decomposition of the catalyst.
c
OH
1
97
50
1
b
OH
2
3
4
5
6
94
88
1
c
The reaction by the [IrOH(cod)] complex, which pro-
2
motes efficiently the Guerbet reaction of primary alco-
OH
>99
99
90 (86)
76
7
hols, gave 2 in low yield (22%), and a considerable
1d
amount of heptane (36%) was detected after the reac-
tion. The formation of heptane can be explained by
the following sequential reactions, that is, the hydro-
genation of heptene derived from the decarbonylation
of an acyl Ir-complex generated by the oxidative addi-
tion of octanal to an Ir-complex. The formation of hep-
tane shows that 1a is smoothly dehydrogenated to
OH
1
e
OH
99
94 (91)
80 (68)
1f
OH
d
e
98
1
g
octanal by the [IrOH(cod)] complex, but this Ir-com-
2
plex would tend to induce decarbonylation rather than
7
8
98
97
51
56
oxidative dimerization of the octanal and 1a (entry 3).
OH
þ
ꢁ
1
h
A cationic complex, ½IrðcodÞ ꢀ BF , indicated moder-
2
4
ate activity to give 2 in 61% yield (entry 4). However,
the [Cp IrCl ] complex having excellent dehydrogena-
d,e
*
OH
2
2
9
1
i
tion ability for alcohols was inert for the present reac-
tion (entry 5). When toluene was used as a solvent, the
yield of 2 was decreased to 58% (entry 6). It was found
that the reaction did not take place under Ar atmo-
sphere without air (entry 7), in contrast to the
RuH (PPh ) -catalyzed reaction where alcohols are con-
OH
9
99
82
76
61
1
j
OH
2
3 4
3
verted into esters with the evolution of hydrogen.
10
11
1
k
On the basis of these results, several primary alcohols
were allowed to react in the presence of [IrCl(coe)2]2
under air at 95 °C for 15 h (Table 2). Higher alcohols
were more selectively dimerized than lower carbon-
numbered alcohols to give the corresponding esters in
good yields. For example, 1-dodecanol (1f) was con-
verted to dodecyl dodecanoate (2f) in 94% yield (entry
OH
OH
f
>99
86
1
l
a
Alcohol (2 mmol) was reacted in the presence of [IrCl(coe)
3 mol %) under open air at 95 °C for 15 h.
GLC yields. The numbers in parentheses show isolated yields. The
yields were determined based on the alcohol (1) used.
Toluene (0.1 mL) was used as solvent.
The stereochemistry is not determined.
Reaction time was 20 h.
The reaction was performed in the presence of Na CO3 (5 mol %),
PPh (10 mol %), and toluene (1 mL).
3
2 2
]
(
b
5
). Branched primary alcohols like 2-ethylhexanol (1g)
were also oxidatively dimerized to 2-ethylhexyl 2-ethyl-
hexanoate (2g) in good yield (entry 6). Cyclohexylmeth-
anol (1j) and 2-phenylethanol (1k) were also converted
to the corresponding esters, 2j and 2k, respectively, in
substantial yields (entries 9 and 10). On the other hand,
c
d
e
f
2

A. Izumi et al. / Tetrahedron Letters 47 (2006) 9199–9201
9201
LnIr
The conversions and yields of products were estimated
from the peak areas based on the internal standard tech-
H
2
O
3a
12
3a
13
R
OH
O
nique using GLC. The products 2a–b, 2c, 2d, 2e,
1
14
15
3a
16
2
f, 2g, 2h–k, and 2l were reported previously.
1/2O2
Acknowledgements
R
H
LnIrH
2
R
OH
O
1
This work was supported by ‘High-Tech Research
Center’ Project for Private Universities: matching fund
subsidy from the Ministry of Education, Culture,
Sports, Science and Technology, 2005–2009.
R
O
2
R
OH
LnIr
R
O
R
References and notes
Scheme 1. A possible reaction pathway for the Ir-catalyzed reaction of
to 2.
1
2
3
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2
2
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2
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1
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1
1
1
8
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mixture of 1a (2 mmol) and [IrCl(coe) ] (0.06 mmol)
2
2
was stirred at 95 °C for 15 h under open air. The
product was isolated by column chromatography
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