Hydrogen transfer type oxidation of alcohols by rhodium and ruthenium catalyst under microwave irradiation

Article abstract of DOI:10.1016/j.tetlet.2003.10.032

Secondary alcohols were converted into the corresponding ketones by methyl acrylate and rhodium catalyst efficiently under microwave irradiation. Treatment of primary alcohols with the same condition resulted in the recovery of the starting materials. Primary alcohols were converted into aldehydes by hydrogen transfer reaction using methyl vinyl ketone and ruthenium catalyst under microwave irradiation.

Full text of DOI:10.1016/j.tetlet.2003.10.032

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 9201–9203
Hydrogen transfer type oxidation of alcohols by rhodium and
ruthenium catalyst under microwave irradiation
Masaaki Takahashi, Koichiro Oshima and Seijiro Matsubara*
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoutodaigaku-katsura, Nishikyo,
Kyoto 615-8510, Japan
Received 5 September 2003; revised 30 September 2003; accepted 3 October 2003
Abstract—Secondary alcohols were converted into the corresponding ketones by methyl acrylate and rhodium catalyst efficiently
under microwave irradiation. Treatment of primary alcohols with the same condition resulted in the recovery of the starting
materials. Primary alcohols were converted into aldehydes by hydrogen transfer reaction using methyl vinyl ketone and ruthenium
catalyst under microwave irradiation.
© 2003 Elsevier Ltd. All rights reserved.
An oxidation of alcohols has required the efficiency and
selectivity, and also been argued from the point of view
of safety. The use of stoichiometric amount of haz-
ardous oxidizing reagent should be avoided.1 The sol-
vent is also tried to change to non-hazardous solvent
including water. For these reasons, transition metal
catalyzed reaction using air or oxygen has been
reported as a preferable system.2 Hydrogen transfer
type oxidation catalyzed with a transition metal com-
pound may also be desirable in some cases, as the use
of alkene as hydrogen acceptor diminishes a difficulty
and danger of oxidation reaction. Hayashi reported
palladium catalyzed oxidation by ethene.3 The method
is really attractive, but unfortunately only applicable
for benzylic and allylic alcohols. The catalytic ability of
transition metal complex for hydrogen transfer from
alkanol to alkene had been well investigated and shown
that rhodium complex was more active than ruthenium,
platinum, and palladium complexes.4 We planned to
use rhodium complex as a catalyst and tried to oxidize
secondary aliphatic alcohol with alkene. For comple-
tion of these hydrogen transfer, microwave irradiation
was proved to be effective.5
Oxidation of alkanol was examined as follows (Fig. 1).
As microwave system, we used Discover system from
CEM corporation.6 In a pressure 10 ml vial which can
be sealed by a septum, a mixture of alkanol (1, 1.0
mmol),
methyl
acrylate
(2.0
mmol),
and
RhCl(CO)(PPh3)2 (0.05 mmol) in water (3.0 ml) and
N,N-dimethylformamide (1.0 ml) was placed.
Microwave (200W) was irradiated maintaining 140°C
and 200 psi for 15 min. After being cooled, the reaction
mixture was extracted with hexane. The combined hex-
ane solution was passed through a short silica gel
column and concentrated in vacuo. The residue was
1
analysed by H NMR and purified by a short silica gel
column, as the need arose.
Results of the reaction in Figure 1 are shown in Table
1. The secondary alcohols were oxidized to the corre-
sponding ketones (entries 1–4, and 7). Without
microwave irradiation, these reactions resulted in the
recovery of starting alcohols. Primary alcohols, both of
benzylic and saturated ones, were not oxidized and
resulted in recovery of starting material (entries 5 and
6). As shown in Figure 2, a mixture of primary and
secondary alcohols was treated with the above condi-
tion. Only secondary alcohol was oxidized and the
primary alcohol was recovered unchanged. In Figure 3,
the diol 3, which contained primary and secondary
alcohol moieties, was also converted into keto alkanol 4
in good yield.7
Figure 1. Oxidation of secondary alcohols by Rh(I) and
methyl acrylate.
* Corresponding author. Tel.: +81-75-383-2441; fax: +81-75-383-2438;
e-mail: matsubar@mc.kyoto-u.ac.jp
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.032

9202
M. Takahashi et al. / Tetrahedron Letters 44 (2003) 9201–9203
Table 1. Oxidation of secondary alcohols via Figure 1a
Entry
Alcohol 1
Product 2
Yieldb
1
2
3
4
5
6
7
5-Tetradecanol
Cyclododecanol
Cyclooctanol
4-t-Butylcyclohexanol
n-Tridecanol
1a
1b
1c
1d
1e
1f
5-Tetradecanone
Cyclododecanone
Cyclooctanone
4-t-Butylcyclohexanone
Recovery of alcohol
Recovery of alcohol
p-Tolyl pentyl ketone
2a
2b
2c
2d
–
86%
99
68
27
ꢀ0
ꢀ0
99
Benzyl alcohol
1-p-Tolyl-1-hexanol
–
2g
1g
a Reaction was conducted via Figure 1. The procedure was shown in text.
b Isolated yields.
As shown in Figure 4, primary alcohols were tried to
oxidize into aldehydes. A procedure for the oxidation
reaction of primary alcohol was as follows. In a pres-
sure 10 ml vial which can be sealed by septum, a
mixture of alkanol (1.0 mmol), methyl vinyl ketone (2.0
mmol), and RuCl2(PPh3)3 (0.025 mmol) was placed
without solvent. Microwave (200 W) was irradiated
maintaining 120°C and 200 psi for 15 min. The results
are summarized in Table 2.
Figure 2. Selective oxidation of a secondary alcohol in the
presence of a primary one.
In both reactions in Figures 1 and 4, an oxidation of a
substrate carrying CꢀC double bond contained some
difficulty. A migration of the CꢀC double bond was
observed in both cases. As shown in Figure 5, treat-
ment of 2-undecen-4-ol (5) with the rhodium catalyst
and methyl acrylate gave a mixture of 4-undecanone
and 2-undecen-4-one. In the ruthenium-catalyzed reac-
tion, the migration was also observed as shown in entry
5 of Table 2.
Figure 3. Conversion of a diol into keto alcohol.
When the migration of CꢀC double bond via hydride
shift reaches the a-carbon of the hydroxyl group,
ketone was obtained via enol form. The isomerization
reaction may proceed completely dependant on the
catalyst. The efficiency of the catalyst in Figure 6 was
examined as shown in Table 3. A procedure for the
isomerization reaction was as follows. In a pressure 10
ml vial which can be sealed by a septum, a mixture of
alkanol (1.0 mmol) and catalyst (0.025 mmol) in water
(3.0 ml) and N,N-dimethylformamide (1.0 ml) was
placed. Microwave (200W) was irradiated maintaining
180°C and 200 psi for 15 min.
Figure 4. Oxidation of primary alcohols by Ru(II) and
methyl vinyl ketone.
We tried to overcome the difficulties of the oxidation of
primary alcohol under the conditions in Figure 1. Pro-
longed reaction time and use of methyl vinyl ketone
instead of methyl acrylate were not effective. The ruthe-
nium-catalyzed reaction, however, was effective for
hydrogen transfer type oxidation of primary alcohol.8
Some of the transition metal compounds have an abil-
ity of CꢀC double bond migration via hydride shift
Table 2. Oxidation of primary alcohols via Figure 4a
Entry
Alcohol
Product
Yieldb
1
2
3
4
5
6
n-Heptanol
1h
1e
1f
1i
1j
1b
Heptanal
Tridecanal
Benzaldehyde
Heptanedial
Citroneralc
2h
2e
2f
2i
2j
2b
45%
77
54
53
53
n-Tridecanol
Benzyl alcohol
1,7-Heptanediol
Citronerol
(Cyclododecanol
Cyclododecanone
54)
a Reaction was conducted via Figure 4. The procedure was shown in text.
b Isolated yields.
c Accompanying the aldehydes with a migrated CꢀC double bond in 21% yield.

M. Takahashi et al. / Tetrahedron Letters 44 (2003) 9201–9203
9203
for Scientific Research from The Ministry of Education,
Science, Sports, and Culture. The financial supports from
Chugai Pharmaceutical Company and Takahashi Indus-
trial and Economic Research Foundation are also
acknowledged. The authors also deeply grateful to Pro-
fessor M. Hayashi (Kobe University) for valuable discus-
sions.
Figure 5. Treatment of allylic alcohol 5 with rhodium catalyst
and methyl acrylate.
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The transition metal catalyzed intermolecular hydride
shift from alkanol to enone was promoted effectively by
microwave irradiation. In the same way, ruthenium-cat-
alyzed intramolecular hydride shift was also accelerated
by microwave irradiation. The procedures are simple and
will give a way for microwave chemistry concerning
organometallic chemistry.
Acknowledgements
This work was supported financially by a Grant-in-Aid