In situ coupled oxidation cycle catalyzed by highly active and reusable Pt-catalysts: Dehydrogenative oxidation reactions in the presence of a catalytic amount of o-chloranil using molecular oxygen as the terminal oxidant

Article abstract of DOI:10.1039/c0cc02865g

An in situ coupled oxidation cycle that allows catalytic oxidation of a substrate with catalytic amounts of o-chloranil and novel reusable polymer-immobilized platinum nanocluster catalysts using molecular oxygen as the terminal oxidant was developed.

Full text of DOI:10.1039/c0cc02865g

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In situ coupled oxidation cycle catalyzed by highly active and reusable
Pt-catalysts: dehydrogenative oxidation reactions in the presence
of a catalytic amount of o-chloranil using molecular oxygen
as the terminal oxidantw
Hiroyuki Miyamura, Kanako Maehata and Shu Kobayashi*
Received 28th July 2010, Accepted 13th September 2010
DOI: 10.1039/c0cc02865g
An in situ coupled oxidation cycle that allows catalytic oxidation
of a substrate with catalytic amounts of o-chloranil and novel
reusable polymer-immobilized platinum nanocluster catalysts
using molecular oxygen as the terminal oxidant was developed.
Scheme 1 In situ coupled oxidation cycle using molecular oxygen as
Oxidation is a basic and very important process in organic
the terminal oxidant.
synthesis and many methodologies and reagents for oxidation
have been developed so far.1 Quinones with high oxidation
platinum nanocluster catalysts using molecular oxygen as the
terminal oxidant. To the best of our knowledge, this is the first
example of in situ regeneration of quinones with high
oxidation potential using molecular oxygen under ambient
conditions.
potential, such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ) and chloranils, are versatile and important oxidizers in
organic synthesis (Fig. 1).2 However, stoichiometric use of
these quinones often leads to difficulty in removing concomitant
by-products, hydroquinones. The high cost of these expensive
oxidizers is another problem. Such issues may be overcome by
using a catalytic amount of quinone and a stoichiometric
amount of co-oxidant.3 Of all co-oxidants, molecular oxygen
is an ideal oxidizer from environmental and economic points
of view, because molecular oxygen is abundant in air and
water is the sole by-product after oxidation.
Recently, we have developed aerobic oxidation of hydro-
quinones using polymer-incarcerated gold and platinum
nanocluster catalysts.7 This project commenced with the
development of polymer-incarcerated platinum catalysts,
which were effective for oxidation of hydroquinones with
both electron-donating and electron-withdrawing groups.
However, the structure of this polymer contains benzyloxy
moieties, which may be oxidized by o-chloranil, leading to
partial decomposition of the catalyst.9 We addressed this issue
by preparing two types of novel durable platinum cluster
catalysts under oxidative conditions using organic–inorganic
hybrid copolymer 1, which contains trimethoxysilyl groups for
cross-linking,8 and polystyrene based polymer 2 without
benzyloxy moieties (Scheme 2). Copolymer 1 and sodium
borohydride were dissolved in diglyme at room temperature
and a diglyme solution of sodium hexachloroplatinate(IV) was
added dropwise. After the solution was stirred for several
hours at room temperature, a mixture of aqueous sodium
hydroxide solution and isopropanol was added, and micro-
capsules were formed immediately. The solution containing
microcapsules was heated at 90 1C for 3 h, and then successive
filtration, washing, grinding, and heating at 120 1C without
solvent afforded an organic–inorganic hybrid platinum
nanocluster catalyst (HB Pt) as a gray powder. On the other
hand, oxidation-resistant polymer-incarcerated platinum
catalyst (RPI Pt) was prepared from copolymer 2, which does
not have benzyloxy moieties, according to the procedure for
the preparation of PI Pt.7 The metal loadings were analyzed by
inductively coupled plasma (ICP) analysis, and the size of
the platinum nanoclusters was determined by transmission
electron microscopy (TEM) (3–9 nm).9
However, because molecular oxygen is a triplet, highly
selective direct reactions between molecular oxygen and
common singlet organic compounds are difficult under mild
conditions in spite of the high oxidative potential of molecular
oxygen.4 An alternative approach is transfer of the oxidative
potential of molecular oxygen to singlet organic compounds
that can easily react with common organic substrates in the
presence of a reusable catalyst. Under these conditions, a more
abundant and environmentally benign oxidation system could
be realized (Scheme 1).5,6
Here, we report an in situ coupled oxidation cycle that
allows catalytic oxidation of a substrate with catalytic
amounts of o-chloranil and novel reusable, polymer-immobilized
Fig. 1 Quinones with high oxidation potential.
Department of Chemistry, School of Science and Graduate School of
Pharmaceutical Sciences, The University of Tokyo,
The HFRE Division, ERATO, Japan Science Technology Agency (JST),
Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
Hantzsch’s pyridine synthesis, preparation of pyridines via
oxidation of 1,4-dihydropyridines that are synthesized by
w Electronic supplementary information (ESI) available: Experimental
details and TEM images of the catalysts. See DOI: 10.1039/c0cc02865g
c
8052 Chem. Commun., 2010, 46, 8052–8054
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obtain excellent yields (quantitative conversions) after further
optimization (entries 5 and 8). Moreover, no leaching of
platinum to the reaction mixture was observed after filtration
of Pt catalysts (entries 4 and 6), and atmospheric air can be
used as an oxidant to afford the product in good yield while
longer reaction time was required (entry 7). We also tested
other quinones both with electron withdrawing groups (EWG)
and with electron donating groups (EDG). Quinones with
EWG showed higher reactivity than those with EDG, and
o-chloranil was found to be the best.9
Recovery and reuse of the HB Pt catalyst was examined
using 0.25 mol% of HB Pt and 2.5 mol% of o-chloranil.
Remarkably, a high yield of the desired product was obtained
until the seventh use, although the remaining starting material
was detected in the eighth and ninth uses (Table 2). TEM
analysis of HB Pt after the ninth use showed that the sizes of
the platinum nanoclusters increased from a range of 3–9 nm to
5–15 nm.9 Deactivation of the HB Pt catalyst could be
attributed to aggregation of platinum nanoclusters.
Scheme 2 Preparation of an organic–inorganic hybrid platinum
nanocluster catalyst (HB Pt) and an oxidation-resistant polymer-
incarcerated platinum catalyst (RPI Pt).
The substrate scope for this reaction system was then
investigated. Dihydropyridine substituted by both aromatic
and alkyl groups at the 4-position was successfully oxidized to
the corresponding pyridines in good to excellent yields
(Table 3, entries 1–10). Substrates bearing both electron-
rich and electron-poor aromatic substituents were oxidized
(entries 5–9). Both HB Pt and RPI Pt worked well for this
oxidation, although slight difference of activities depending on
substrates was observed. Nonsymmetric dihydropyridines,
whose preparation method was reported by Manning and
Davies,12 were also good substrates for this oxidation system
as well as symmetric dihydropyridines (entries 11–13).
The oxidation system was applicable to other reactions.
Indoline could also be oxidized efficiently to indole in high
yield (Scheme 3).13 A p-methoxybenzyl (PMB) protected
alcohol could be deprotected with catalytic amount of RPI
Pt and o-chloranil to afford the desired alcohol in good yield
(Scheme 4). It is noteworthy that when a stoichiometric
amount of o-chloranil was used for oxidation of a tetra-
hydroquinoline derivative,14 a mixture of ketone and ketal
derived from tetrachlorocatechol was obtained as a major
product; however, the use of a catalytic amount of o-chloranil
provided the desired ketone in high yield as the sole product
(Scheme 5).
Table 1 Aerobic oxidation with catalytic amounts of o-chloranil and
Pt catalysts
CH2Cl2/H2O
(mL/mmol)
Entry Pt cat. x/y
Time/h Yielda (%)
1
2
3
4
5
6
7
8
HB
HB
HB
HB
HB
RPI
RPI
RPI
0/110
2.5/0
1.0/10
0.25/2.5
0.25/0.50 2/0.1
0.25/0.50 2/0.1
0.25/2.5 2/0.1
0.10/0.10 2/0.1
4/0.2
4/0.2
4/0.2
4/0.2
2
>99
0
2.5
2.5
6
>99
>99b
95
24
5
21
24
97b
92c
93
a
b
Isolated yield. No leaching of Pt was confirmed by ICP analysis
c
(detection limit: 0.21 ppm; no peak of Pt was observed). Air (1 atm)
was used instead of O2 (1 atm).
condensation of 1,3-dicarbonyl compounds, aldehydes and
ammonia, is a traditional but highly efficient method for
production of symmetric pyridines, in which a stoichiometric
amount of DDQ is usually used in the oxidation step from
dihydropyridine to pyridine.10,11 To probe the viability of the
designed in situ coupled oxidation cycle, the oxidation
of Hantzsch’s dihydropyridine to pyridine using catalytic
amounts of o-chloranil and Pt catalysts under an oxygen
atmosphere was investigated (Table 1). A stoichiometric
amount of o-chloranil rapidly oxidized dihydropyridine 3a to
pyridine 5a (entry 1). On the other hand, HB Pt was not
effective for direct aerobic oxidation of dihydropyridine (entry 2).
When catalytic amounts of both o-chloranil and HB Pt
were used under atmospheric oxygen, the reaction proceeded
smoothly to give the desired product quantitatively
(entries 3–5). We also found that RPI Pt was effective for this
reaction (entries 6 and 7). It is noted that the amounts of
o-chloranil/HB Pt and o-chloranil/RPI Pt could be reduced to
0.25 mol%/0.5 mol% and 0.1 mol%/0.1 mol% respectively, to
In conclusion, we have developed an in situ coupled
oxidation cycle that was applied to dihydropyridine, indoline,
PMB, and tetrahydroquinoline oxidation. Novel platinum
nanocluster catalysts were developed for this system. It
is noted that the oxidations proceeded smoothly in the
Table 2 Recovery and reuse of HB Pt
Run
1
2
3
4
5
6
7
8
9
Yield (%)
99
>99
99
94
98
>99
97
90
85
a
Recovered HB Pt was washed after each use in a mixture of 1 N
NaOH and isopropanol (1 : 1).
c
This journal is The Royal Society of Chemistry 2010
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Table 3 Substrate scope for aerobic oxidation of dihydropyridines
presence of a catalytic amount of o-chloranil under very mild
conditions. Further investigations to apply this aerobic
oxidation system to other reactions are now in progress.
This work was partially supported by a Grant-in-Aid for
Science Research from the Japan Society for the Promotion of
Science (JSPS), Global COE Program, The University of
Tokyo, MEXT, Japan, and NEDO. H. M. thanks the JSPS
fellowship for Japanese Junior Scientist. Special thanks to
Mr. Kuramitsu (The University of Tokyo) for electronic
microscopy analysis.
Time/ Yielda
Solventb
h
(%)
Notes and references
Entry Substrate R
Pt cat. x/y
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London, 1988, vol. 1; (b) M. Hundlucky, Oxidation in Organic
Chemistry, American Chemical Society, Washington, DC, 1990.
2 Reviews: (a) D. Walker and J. D. Hiebert, Chem. Rev., 1967, 67,
153; (b) P. P. Fu and R. G. Harvey, Chem. Rev., 1978, 78, 317;
(c) C.-J. Li, Acc. Chem. Res., 2009, 42, 335.
3 (a) S. Chandrasekhar, G. Sumlthra and J. S. Yadav, Tetrahedron
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(c) G. V. M. Sharma, B. Lavanya, A. K. Mahalingam and
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1
2
3
4
5
6
7
8
3b
3c
3c
3c
3d
3e
3e
3f
Et
Ph
Ph
Ph
HB
HB
HB
0.5/5 4/0.2
0.5/5 4/0.2
0.5/5 4/0.2
64
36
36
20
82
>99
>99
>99
>99
97
>99
90
>99
>99
91
RPI 1/10 8/0.4
HB
4-ClC6H4
4-MeOC6H4 HB
0.5/10 16/0.8 24
1/10 16/0.8 21
20
1/10 16/0.8 33
20
1/10 16/0.8 36
1/10 8/0.4
1/10 8/0.4
1/10 8/0.4
4-MeOC6H4 RPI 1/10 8/0.4
HB
RPI 1/10 8/0.4
4-CF3C6H4
4-CF3C6H4
9
3f
10
11
12
13
3g
4a
4b
4c
–CHQCH–Ph HB
Ph HB
4-MeOC6H4 HB
4-CF3C6H4 HB
6
6
6
4 Review: T. Punniyamurthy, S. Velusamy and J. Iqbal, Chem. Rev.,
2005, 105, 2329.
84
87
5 Review: J. Piera and J.-E. Backvall, Angew. Chem., Int. Ed., 2008,
47, 3506.
¨
a
b
Isolated yield. CH2Cl2/H2O (mL/mmol).
6 While multistep electron transfer systems using catalytic amount of
benzoquinone and molecular oxygen as a terminal oxidant have
been developed, examples using quinones with high redox potential
such as DDQ and chloranils are rare: (a) J.-E. Backvall and
¨
R. B. Hopkins, Tetrahedron Lett., 1988, 29, 2885;
(b) J.-E. Backvall, R. B. Hopkins, H. Grennberg, M. M. Mader
¨
and A. K. Awasthi, J. Am. Chem. Soc., 1990, 112, 5160;
¨ ¨
(c) J. Woltinger, J.-E. Backvall and A. Zsigmond, Chem.–Eur. J.,
1999, 5, 1460; (d) J.-E. Backvall, A. K. Awasthi and Z. D. Renko,
¨
J. Am. Chem. Soc., 1987, 109, 4750; (e) J. Piera, K. Narshi and
Scheme 3 Aerobic oxidation of an indoline derivative.
¨
J.-E. Backvall, Angew. Chem., Int. Ed., 2006, 45, 6914; (f) Z. An,
¨
X. Pan, X. Liu, X. Han and X. Bao, J. Am. Chem. Soc., 2006, 128,
16028; (g) K. Bergstad, H. Grennberg and J.-E. Backvall,
¨
Organometallics, 1998, 17, 45; (h) H. Grennberg, K. Bergstad
and J.-E. Backvall, J. Mol. Catal. A: Chem., 1996, 113, 355;
¨
(i) T. Yokota, S. Fujibayashi, Y. Nishiyama, S. Sakaguchi and
Y. Ishii, J. Mol. Catal. A: Chem., 1996, 114, 113; (j) J.-E. Backvall,
¨
R. L. Chowdhury and U. Karlsson, J. Chem. Soc., Chem.
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J.-E. Backvall, J. Chem. Soc., Chem. Commun., 1994, 1037;
¨
(l) G. Gsjernyik, A. H. Ell, L. Fadini, B. Pugin and
Scheme 4 Deprotection of PMB group using catalytic amount of RPI
Pt and o-chloranil.
J.-E. Backvall, J. Org. Chem., 2002, 67, 1657. Quite recently, an
¨
oxidation system including in situ regeneration of p-chloranil by
molecular oxygen under relatively harsh conditions (0.5 MPa O2,
140 1C) was reported: (m) Y. Fan, Z. An, X. Pan, X. Liu and
X. Bas, Chem. Commun., 2009, 7489.
7 (a) H. Miyamura, M. Shiramizu, R. Matsubara and S. Kobayashi,
Angew. Chem., Int. Ed., 2008, 47, 8093; (b) H. Miyamura,
M. Shiramizu, R. Matsubara and S. Kobayashi, Chem. Lett.,
2008, 37, 360.
8 We have already developed polymer immobilized ruthenium
catalyst using this polymer: T. Matsumoto, M. Ueno, N. Wang
and S. Kobayashi, Chem.–Asian J., 2008, 3, 239.
9 See ESIw.
10 Review: D. M. Stout and A. I. Meyers, Chem. Rev., 1982, 82, 223.
11 Review: G. D. Henry, Tetrahedron, 2004, 60, 6043.
12 J. R. Manning and H. M. L. Davies, J. Am. Chem. Soc., 2008, 130,
8602.
13 D. Zhang and L. S. Liebeskind, J. Org. Chem., 1996, 61, 2594.
14 S. Kobayashi and S. Nagayama, J. Am. Chem. Soc., 1996, 118,
8977.
Scheme 5 Comparison between a catalytic amount and a stoichiometric
amount of o-chloranil in the oxidation of tetrahydroquinoline
derivative. Conditions a: HB Pt (2 mol%), o-chloranil (20 mol%),
CH2Cl2/H2O, rt, O2, 96 h. b: o-chloranil (3 equiv.), CH2Cl2/H2O, rt,
O2, 24 h. ND = not detected.
c
8054 Chem. Commun., 2010, 46, 8052–8054
This journal is The Royal Society of Chemistry 2010