Pt-catalyzed oxidative rearrangement of cyclic tertiary allylic alcohols to enones using aqueous hydrogen peroxide

Article abstract of DOI:10.1246/cl.2012.744

An oxidative rearrangement of cyclic tertiary allylic alcohols to β-disubstituted α,β-unsaturated ketones by Pt black catalyst with aqueous hydrogen peroxide is described. The reaction proceeds under organic solvent- and halide-free conditions and gives only water as a coproduct. The Pt black catalyst is commercially available and can be reused at least four times.

Full text of DOI:10.1246/cl.2012.744

doi:10.1246/cl.2012.744
744
Published on the web June 30, 2012
Pt-Catalyzed Oxidative Rearrangement of Cyclic Tertiary Allylic Alcohols
to Enones Using Aqueous Hydrogen Peroxide
Takashi Nagamine, Yoshihiro Kon,* and Kazuhiko Sato*
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
(Received March 28, 2012; CL-120270; E-mail: y-kon@aist.go.jp)
An oxidative rearrangement of cyclic tertiary allylic alcohols
On the other hand, H2O2 is a very attractive oxidant for
liquid-phase reactions, and H2O2 oxidation is particularly useful
for the synthesis of high-value fine chemicals, pharmaceuticals,
agrochemicals, and electronic materials requiring high chemical
purity. Therefore, we have developed various oxidation reac-
tions by using aqueous H2O2.17 However, to our knowledge,
there have been no reports on the oxidative rearrangement of
tertiary allylic alcohols by using H2O2 as an oxidant. Herein, we
report the oxidative rearrangement of cyclic tertiary allylic
alcohols to ¢-substituted ¡,¢-unsaturated ketones by using
aqueous H2O2 as an oxidant and Pt black18 as a reusable catalyst
under organic solvent- and halide-free conditions.
The procedure is very simple. First, 30% aqueous H2O2 was
added during a period of 2.5 h to a stirring mixture of 1-phenyl-
2-cyclohexen-1-ol (1a) and Pt black19 (0.01 equiv) in H2O at
90 °C. The reaction mixture was continuously stirred for 0.5 h at
90 °C to give 3-phenyl-2-cyclohexen-1-one (2a) in 95% yield
with good selectivity (95%). Dimeric ether was also obtained in
1% yield, but other presumable by-products, such as 1-phenyl-
1,3-cyclohexadiene, were not observed. One of the greatest
advantages of this catalytic system is that the catalyst is easily
recyclable.18,20 After the first reaction, Pt black can be recovered
by simple filtration and washing, then be reused for the next
reaction. Four cycles of reaction could be catalyzed by Pt black
with over 90% yield of 2a (Table 1).
Pt black was the best among our tested catalysts for the
oxidative rearrangement of 1a using H2O2. We examined
various metal catalysts for this process, but these catalysts were
less effective (Table 2). Pd black failed to afford 2a. In the
reaction using Ru black and Ir black, the yields of 2a were low
and rearranged allylic alcohol 3a was obtained in moderate
yield. Other Pt catalysts, Pt/C and [Pt(PPh3)4], showed low
catalytic activity to afford 2a in 17% and 29% yields,
respectively, with several unidentified by-products. When no
catalyst was added, the reaction afforded a complex mixture
containing 2a in 10% yield. In the reaction without H2O2, 1a
was converted to 3a in 47% yield and to 2a in 2% yield with
31% recovered 1a.
to ¢-disubstituted ¡,¢-unsaturated ketones by Pt black catalyst
with aqueous hydrogen peroxide is described. The reaction
proceeds under organic solvent- and halide-free conditions and
gives only water as a coproduct. The Pt black catalyst is
commercially available and can be reused at least four times.
An oxidative rearrangement of tertiary allylic alcohols to
¢-disubstituted ¡,¢-unsaturated carbonyl compounds has been
studied in recent decades and has been widely used in organic
synthesis.1-4 This process is an especially powerful tool for
constructing ¢-substituted cyclic enones in natural product
synthesis.5 Moreover, ¢-disubstituted carbonyl compounds are
very important intermediates for the enantioselective construc-
tion of all-carbon quaternary stereocenters.6 However, until
recently, stoichiometric amounts of hazardous oxochromiun-
(VI)-based oxidants such as CrO3, PCC, and PDC have been
used exclusively as oxidants in the oxidative rearrangement of
tertiary allylic alcohols.
In 2004, Iwabuchi and co-workers reported that stoichio-
metric amounts of 2-iodylbenzoic acid (IBX) could be used
instead of the hazardous Cr(VI)-based oxidant for the oxidative
rearrangement of tertiary allylic alcohols.7 In 2008, the same
research group reported that 2,2,6,6-tetramethyl-1-piperidinyloxyl
(TEMPO)-derived oxoammonium salts were more effective
stoichiometric reagents for this process.8 The catalytic oxidative
rearrangement of tertiary allylic alcohols was also reported as a
powerful tool for synthesizing enones by Iwabuchi and co-
workers, Vatèle, and Ishihara and co-workers, independently.
Iwabuchi’s group demonstrated this process by using catalytic
amounts of TEMPO with NaIO4-SiO2 as a co-oxidant.9 Vatèle
developed a Lewis acid-assisted oxidative rearrangement of
tertiary allylic alcohols by using catalytic amounts of TEMPO
with iodosylbenzene (PhIO) as a co-oxidant.10,11 In contrast,
Ishihara’s group developed a 2-iodylbenzenesulfonic acid (IBS)-
catalyzed oxidative rearrangement of tertiary allylic alcohols
by using catalytic amounts of sodium 2-iodobenzenesulfonate
with Oxoneμ as a co-oxidant.12 However, the atom efficiency of
these oxidants is low, and they form equimolar amounts of the
deoxygenated compounds as waste.13,14 From environmental and
economical standpoints, molecular oxygen and hydrogen perox-
ide (H2O2) are ideal oxidants, because they have a high active
oxygen content and the coproduct of the reactions is only water.15
Very recently, Vatèle demonstrated an aerobic oxidative rear-
rangement of tertiary allylic alcohols.11,16 In Vatèle’s aerobic
oxidation procedure, catalytic amounts of CuCl2 (0.5 equiv) and
TEMPO (0.1 equiv) are used under molecular oxygen, although
high catalyst loadings are necessary to convert tertiary allylic
alcohols into corresponding enones in good yields.
Table 1. Pt-Catalyzed oxidative rearrangement of 1a by using
30% H2O2, and recycling of catalysta
O
Pt black (0.01 equiv)
30% H2O2 (3 equiv)
Ph
OH
1a
H2O, 90 °C, 3 h
Ph
2a
Run
Yield/%b
1st
95
2nd
93
3rd
92
4th
90
a1.74 g (10 mmol) scale, H2O (10 mL). bIsolated yield. See
Supporting Information for details.24
Chem. Lett. 2012, 41, 744-746
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

745
+
Table 2. Screening of metal source on oxidative rearrangement
of 1aa
Pt
R
OH
Metal Source (0.01 equiv)
30% H2O2 (3 equiv)
2a
1a
+
Pt
Pt
H2O, 90 °C, 3 h
8
Ph
O
H2O
OH
H2O
3a
oxidation
Yield/%b
R
Entry
Metal source
OH
2a
91
0c
3a
R
R
H
7
2
3
1
2
3
4
Pt black
Pd black
Ru black
Ir black
5% Pt/C
[Pt(PPh3)4]
®
0
9c
H2O
-H2O
13
31
17
29
10
50
27
5
1
5
47f
+
R
5
9
6
7d
8e
Scheme 1. Plausible reaction mechanisms of Pt-catalyzed
oxidative rearrangement of tertiary allylic alcohols using H2O2
as oxidant.
Pt black
2f
a1.0 mmol scale, H2O (1 mL). bIsolated yield. cStarting material
1a was recovered (90%). Metal source was not added. H2O2
was not added. fStarting material 1a was recovered (31%). See
Supporting Information for details.24
d
e
n-Pr
Pt black
(0.01 equiv)
30% H2O2
(3 equiv)
O
1a
(1 mmol)
2a
3%
3a
21%
+
+
Table 3. Scope of Pt-catalyzed oxidative rearrangementa
1-PrOH
(1 mL)
90 °C, 3 h
Ph
10
69%
O
OH
Pt black (0.01 equiv)
30% H2O2 (3 equiv)
+
( )
R
OH
H2O, 90 °C, 3 h
( )
( )
Scheme 2. Etherification of tertially alcohol using a combina-
tion of Pt black and H2O2.
n
R
R
n
n
2
3
1
Yield/%b
Entry
Substrate
n
R
gave rearranged allylic alcohol 6 in 35% yield with 50%
recovered 4.21,22
2
3
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1i
1
1
1
1
1
1
1
0
2
Ph
91
55
61
67
48
85
78
5
0
A plausible reaction mechanism is shown in Scheme 1. We
speculate that the platinum-catalyzed oxidative rearrangement
proceeds through two steps. The first is the rearrangement of the
hydroxy group of 1 to 3 through the ³-allylplatinum inter-
mediate 8.23 Considering the acidity of H2O2, the elimination of
the hydroxy group of 1 is assisted by H2O2 as a proton source.
The allylic cation 9 pathway may also proceed. The second step
is the oxidation of 3 using the combination of Pt black and
H2O2.18
4-CF3-C6H4
4-Cl-C6H4
4-Me-C6H4
4-MeO-C6H4
n-Bu
i-Pr
Ph
Ph
2
10
9
0
0
0
0
27
10
To ascertain the ³-allylplatinum intermediate, we performed
the following reaction (Scheme 2). To a stirring mixture of 1a
and Pt black (1 mol %) in 1-propanol at 90 °C was added 30%
aqueous H2O2 during a period of 2.5 h. Then the reaction
mixture was stirred for 0.5 h at 90 °C, to give ether 10 in 69%
yield and 3a in 21% yield. The reaction without Pt black catalyst
in 1-propanol gave 10 and 3a in low yields. These results
suggest that a ³-allylplatinum intermediate is formed in the
presence of Pt black and H2O2, and the ³-allylplatinum
intermediate is nucleophilically attacked by 1-propanol to give
ether 10.
In conclusion, we have developed an environmentally
benign and convenient process for the oxidative rearrangement
of tertiary allylic alcohols to enones by using Pt black catalyst
and 30% aqueous H2O2. This process proceeds under organic
solvent- and halide-free conditions, and the catalyst is easily
reusable with simple manipulation. In general, H2O2 oxidation
with a metal catalyst seems to be environmentally benign, but it
is difficult to apply a reaction that consists of multiple reaction
b
a1.0 mmol scale, H2O (1 mL). Isolated yield. See Supporting
Information for details.24
The scope and limitations of this reaction system are
demonstrated in Table 3. Six-membered cyclic substrates gave
the corresponding enones in moderate to high yields (Entries
1-7). Oxidations of aryl-substituted substrate, such as 4-tri-
fluoromethylphenyl-, 4-chlorophenyl-, 4-methylphenyl-, and
4-methoxyphenyl-substituted substrates gave corresponding
enones in moderate yields with several unidentified by-products
(Entries 2-5). Alkyl-substituted substrates afforded correspond-
ing enones in good yields (85% and 78% for Entries 6 and 7,
respectively). Unfortunately, oxidations of five- and seven-
membered cyclic substrate afforded a complex mixture, and the
desired enones were obtained in low yields (5% and 27% for
Entries 8 and 9, respectively). On the other hand, the reaction of
2-phenyl-3-octen-2-ol (4) as an example of acyclic substrate did
not give desired enone, 2-phenyl-2-octen-4-one (5) at all, but
Chem. Lett. 2012, 41, 744-746
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

746
steps that require strict control. In this study, we demonstrated
that H2O2 oxidation can be applied to a reaction with strict
control, such as oxidative rearrangement. Further mechanistic
studies and application to the broad substrate are ongoing.
10 J.-M. Vatèle, Synlett 2008, 1785.
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This work was partially supported by the New Energy and
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19 We used Pt black that was purchased from N. E. CHEMCAT
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21 In this system, the oxidative rearrangement of 4 to 5 was not
proceeded.
3
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Ph
O
OH
Pt black (0.01 equiv)
30% H2O2 (3 equiv)
Ph
5
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Me
Me
H2O, 90 °C, 3 h
5
4
22 Rearranged allylic alcohol 6 was obtained as isomeric
mixture (E:Z = 5:1). The E,Z-selectivity of allylic alcohol
1
formation in 6 was determined by integration of H NMR
spectra recorded on a CDCl3 solution of crude mixture. The
relevant signals of the hydroxy methyne protons appear at ¤
4.55 and 4.07, respectively.
Ph OH
Me
6
6
7
8
9
For a review, see: C. Hawner, A. Alexakis, Chem. Commun.
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24 Supporting Information is also available electronically on
the CSJ-Journal Web site, http://www.csj.jp/journals/chem-
lett/index.html.
Chem. Lett. 2012, 41, 744-746
© 2012 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/