A new solid-phase reaction system utilizing a temperature-responsive catalyst: Oxidative cyclization with hydrogen peroxide

Article abstract of DOI:10.1002/adsc.200700114

A switchable catalyst based on temperature change provides a novel solid-phase reaction system in water. An increase in catalyst affinity for organic substrates at higher temperature led to efficient activity driving the solid inner-phase reaction, and loss of affinity at lower temperature allowed easy separation of organic products upon completion of the reaction. Application of this catalyst intelligence to design a novel catalytic system brought about an efficient oxidative cyclization with hydrogen peroxide, a useful method of accessing oxygen heterocycles.

Full text of DOI:10.1002/adsc.200700114

FULL PAPERS
DOI: 10.1002/adsc.200700114
A New Solid-Phase Reaction System Utilizing a Temperature-
Responsive Catalyst: Oxidative Cyclization with Hydrogen
Peroxide
Hiromi Hamamoto,a,b Yachiyo Suzuki,b Hideyo Takahashi,a and Shiro Ikegamia,*
a
School of Pharmaceutical Sciences, Teikyo University, Sagamiko, Kanagawa 199-0195, Japan
Fax : (+81)-426-85-3729 E-mail: somc@pharm.teikyo-u.ac.jp
Present address: School of Pharmaceutical Sciences, Kinki University, 3-4-1 Kowakae, Higashi, Osaka 577-8502, Japan
b
E-mail: hamamoto@phar.kindai.ac.jp
Received: March 1, 2007; Revised: July 23, 2007; Published online: November 21, 2007
Abstract: A switchable catalyst based on tempera- gence to design a novel catalytic system brought
ture change provides a novel solid-phase reaction about an efficient oxidative cyclization with hydro-
system in water. An increase in catalyst affinity for gen peroxide, a useful method of accessing oxygen
organic substrates at higher temperature led to effi- heterocycles.
cient activity driving the solid inner-phase reaction,
and loss of affinity at lower temperature allowed
easy separation of organic products upon completion Keywords: oxidation; oxygen heterocycles; poly-
of the reaction. Application of this catalyst intelli- mers; supported catalysts; tungsten
Introduction
systems. Here we describe a new approach to a solid-
phase reaction system in aqueous media utilizing a
Solid-phase catalysts offer a number of important ad- switchable solid-phase catalyst. The property of the
vantages over their homogeneous counterparts.[1,2] For solid phase in water is controlled by the temperature-
example, their separation from reaction products is responsive nature of the polymer support, and the re-
easy and their subsequent recycling steps have the po- action system driving the inner phase of the solid
tential for green chemical processes. In addition, the state based on hydrophobic effects was designed for
immobilization of the solid phase often stabilizes sen- efficient catalytic activity.
sitive catalysts and has potential applicability to intel-
ligent materials. In spite of these advantages, solid-
phase catalysts, in which the reaction occurs on the Results and Discussion
surface, showlowreactivity due to weak affinity be-
tween the substrate and catalyst under heterogeneous Due to their characteristic temperature-responsive in-
conditions. Because of this significant disadvantage, telligence, poly(N-isopropylacrylamide) (PNIPAAm)
the synthetic utility of solid-phase reaction systems is polymers and polymer gels have been applied in vari-
decreased in many cases. On the other hand, organic ous fields such as drug-delivery systems, column pack-
reactions in aqueous media may lead to several un- ing materials, and cell culture substrates using exter-
foreseen reactivities or selectivities due to strong af- nal temperature changes.[5–7] PNIPAAm polymers un-
finity between the substrate and catalyst based on hy- dergo thermally reversible changes between the
drophobic effects.[3,4] However, this strong interaction water-soluble and -insoluble states across a lower crit-
may cause difficulties in the separation of organic ma- ical solution temperature (LCST) from 30 to 408C.
terials when the reaction is completed. In addition, Thus, the polymer chains of PNIPAAm are hydrated
subsequent clean-up of the used water-containing sur- belowthe LCST and dehydrated with aggregation
factants and inorganic agents may also be problemat- above the LCST.[6] The efficient applications of solu-
ic.
ble PNIPAAm supported catalyst to biphasic system
Due to the points mentioned above, the use of an have already been reported by Bergbreiter.[8] We
intelligent solid phase that can change the affinity to have investigated the self-assembly of PNIPAAm po-
an organic substrate in aqueous media would be an lymer and an inorganic species (Figure 1A).[9] Recent-
ideal approach to form efficient, recyclable catalytic ly, we have found that the use of the intelligence of
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Hiromi Hamamoto et al.
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Figure 2. Photographs of a mixture of pent-4-en-1-ol and 1
in water (with magnetic stir bar). A: at room temperature;
B: at 608C. The oil phase was stained with dye (oil red O).
Table 1. Oxidative cyclization of pent-4-en-1-ol.
Figure 1. A: Simplified structure of networked PNIPAAm-
inorganic complex. B: Simplified representation of thermo-
morphic catalysis.
PNIPAAm is important in the creation of valuable
catalyst recycling strategies in water[10] and established
a facile oxidation system via the thermoregulated for-
mation of stable emulsion species.[10a,11] The present
study focused on the use of the temporarily formed
specific hydrophobic PNIPAAm gel phase holding the
flexible solid state as a specific organic reaction
medium to provide a novel recyclable catalytic oxida-
tion system in water (Figure 1B). The utilization of
the inner site of such a networked complex would be
more effective than the conventional solid-phase reac-
tion on the surface.
Entry
Catalyst
Solvent
Conversion [%][a]
1
2
3
4
5
6
none
H2O
H2O
H2O
H2O
CH3CN
tBuOH
<5
1
1
1
1
1
83[b]
87[b] (2nd use)
88[b] (3rd use)
25[b]
48[b]
[a]
1
Determined by H NMR (300 MHz) and GC.
Selectivity:>95%.
[b]
equivalents, and the effect of temperature on the
Previously, a small amount (0.1 mol%) of PNI- phase transformation was investigated. Although the
PAAm-based solid-phase catalyst 1 (Scheme 1), a organic substrate, pent-4-en-1-ol (2), was separately
complex of PNIPAAm polymer with a quaternary floating on the water surface at room temperature
ammonium and phosphotungstate anion (PW12O403À), (Figure 2A), the osmosis of organic substrate to 1
in the presence of organic substrates caused thermo- gradually proceeded with heating and full absorption
regulated formation of stable emulsion species at into the solid gel phase was achieved at around 608C
908C in water. To explore the phase transformation (Figure 2B), while additional heating[10] to more than
mode of 1 in greater detail, the quantity of 1 used was 708C tended to induce the generation of emulsion
increased from 0.1 mol% equivalent to 0.4 mol% species.
The attractive properties of 1 prompted us to devel-
op a novel reaction system using the PNIPAAm gel
phase in catalytic oxidative cyclization with hydrogen
peroxide, a useful method of accessing oxygen hetero-
cycles.[12] In the oxidation of 2 in water, the absorption
of organic materials in the catalyst phase was similarly
detected at 608C, and higher reactivity and selectivity
were observed although the oxidation barely proceed-
ed in the absence of catalyst (Table 1, entries 1 and 2).
The recovery of catalyst was easy; the organic materi-
als retained were released by cooling the reaction mix-
ture and the organic/aqueous/solid phases were cleanly
separated after the addition of diethyl ether. The re-
Scheme 1. Networked PNIPAAm-PW12O403À complex (1).
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Solid-Phase Reaction System Utilizing a Temperature-Responsive Catalyst
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alyst phase with heating and smoothly converted into
the corresponding oxygen heterocycles via oxidative
cyclization in each case.[16] Thus, this method provides
a useful tool for the process design of various types of
oxygen heterocycle syntheses.
Conclusions
We have devised a novel catalytic system in water
based on the intelligence of a thermomorphic catalyst.
The reaction driving the inner phase of the solid state
based on the thermoregulated control of affinity for
the organic substrate in water led to a unique acceler-
ation of the reactivity and ease in catalyst recycling in
the oxidative cyclization with hydrogen peroxide. The
utilization of intelligent polymeric catalysts may offer
many advantages in both biochemical and industrial
applications. In addition, the results described here
will enhance the utility of PNIPAAm polymers in the
field of organic synthesis. More detailed investigations
into the catalytic features of 1 and efforts to extend
this system to other types of reactions are currently
ongoing.
Figure 3. Oxidative cyclization of 2a in the presence of vari-
ous tungsten catalyst systems. Conditions: A) 1 (0.4 mol%),
10% aqueous H2O2, 6 h; B) CWP (0.4 mol%), 10% aque-
ous H2O2, 6 h; Na2WO4·2H2O (4.8 mol%), [CH3(n-
A
C8H17)3N] HSO4, NH2CH2PO3H, 10% aqueous H2O2, 6 h
(& = undesired product).
covered 1 could be used for subsequent reactions with-
out any significant loss of catalytic efficacy (Table 1,
entries 3 and 4). The present system afforded higher Experimental Section
reactivity than a homogeneous system utilizing acetoni-
1
All melting points are uncorrected. H NMR spectra were
recorded at 300 MHz. All NMR spectra were recorded in
CDCl3 with either TMS or residual CHCl3 as the internal
standard. IR absorption spectra were recorded as a KBr pel-
lets. Silica gel 60N (Kanto Chemical Company) was used for
column chromatography. The organic layer was dried with
anhydrous MgSO4 or Na2SO4. All chemicals were purchased
commercially and use as received. Poly(N-isopropylacryl-
amide) (PNIPAAm) based tungsten catalyst (1) was synthe-
sized by an improvement of the previously reported meth-
od.[9d]
trile or tert-butyl alcohol (Table 1, entries 5 and 6), in-
dicating that the utilization of the reaction with the
PNIPAAm phase via the thermoregulated formation is
effective in increasing the activity of 1 in water.
To estimate the utility of the present system, fur-
ther investigation of the effects of the PNIPAAm
chain was performed using other conventional tung-
sten oxidation systems, employing cetylpyridinium
peroxotungstophosphate (CWP)[13] and sodium tung-
state in the presence of surfactants[14] for comparison
(Figure 3). When the oxidation was carried out at
lower temperature, no significant dominance was ob-
served. On the other hand, catalyst 1 showed remark-
able acceleration in reactivity at higher temperature
(608C), with generation of the adsorption of 2a into
the PNIPAAm gel phase, whereas undesired products
were detected with additional heating (higher than
708C), attributable to the subsequent partial forma-
Typical Experimental Procedure for the 1-Catalyzed
Oxidative Cyclization in Aqueous Media
A mixture of 1 (160 mg, 0.02 mmol) and open-chain precur-
sor (5 mmol) in 30% aquous H2O2 (10–15 mmol) was
heated to 608C and stirred for 6–12 h. After the reaction
mixture had been allowed to cool, Et2O (5 mL) was added.
The organic and aqueous phases were removed and the re-
tion of emulsion species.[15] The results demonstrate covered 1 was washed with ether and distilled water, which
were then added to the aqueous phase. The recovered 1 was
reused for subsequent catalytic experiments. The aqueous
phase was extracted three times with Et2O and the com-
bined organic extracts were washed with brine. The organic
phase was then dried over MgSO4 and concentrated under
reduced pressure. The products were purified by on column
chromatography with silica gel and isolated yields were de-
termined.
that the acceleration caused by 1 in water is novel
and the intelligence of the polymer chain plays an im-
portant role in the activation. Thus, higher catalytic
activity exerted over other versatile tungsten systems
with the advantage of easy catalyst recycling was
noted, and this concept might be helpful in tailoring
other polyoxometalate reaction systems.
Tetrahydrofurfuryl alcohol (3a):[17] Colorless liquid;
The scope of this oxidation system was studied fur-
ther with various open-chain precursors (Table 2). Or-
1H NMR: d=1.55–1.67 (m, 1H), 1.83–1.93 (m, 3H), 3.46
ganic substrates were similarly absorbed onto the cat- (dd, J=6.1, 11.5 Hz, 1H), 3.63 (dd, J=3.2, 11.5 Hz, 1H),
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Hiromi Hamamoto et al.
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Table 2. Oxidative cyclization promoted by 1.[a]
Entry
1
Substrate
t [h]
Product
Yield[b] [%]
70
6
2
3
6
6
82
85
4
5
6
6
89
92
6
7
12
6
71
85
8
12
46[c]
[a]
1 (0.4 mol%), 30% H2O2 (2.0–3.0 equivs.), 608C.
[b]
[c]
Yield of isolated products.
[16]
See ref.
3.70–3.78 (m, 1H), 3.80–3.86 (m, 1H), 3.95–4.01 (m, 1H);
IR (KBr): n=3451, 2951, 2876, 1181, 1039 cmÀ1; CAS regis-
try number [97–99–4].
(m, 2H), 2.12 (dd, J=1.7, 11.2 Hz, 1H), 2.38–2.39 (m, 1H),
2.50 (dd, J=4.6, 11.2 Hz, 1H), 3.13–3.16 (m, 1H), 3.94 (s,
1H), 4.40 (d, J=5.1 Hz, 1H); IR (KBr): n=3320, 1782,
1348, 1304, 1178, 1059, 1022, 895 cmÀ1; CAS registry number
[92343–46–9].
Tetrahydropyran-2-methanol (3b):[18] Colorless liquid;
1H NMR: d=1.29–1.35 (m, 1H), 1.42–1.61 (m, 4H), 1.80–
1.85 (m, 1H), 3.40–3.60 (m, 4H), 4.00–4.11 (m, 1H), IR
(KBr): n=3455, 2945, 2862, 1203, 1091, 1047 cmÀ1; CAS reg-
istry number [100–72–1].
3-Carbomethoxy-5-hydroxy-2,6-norbornanecarbolactone
(5d):[20] Colorless liquid; 1H NMR: d=1.55–1.58 (m, 1H),
2.17–2.23 (m, 2H), 2.58–2.63 (m, 1H), 2.74 (dd, J=4.6,
10.7 Hz, 1H), 3.02 (dd, J=4.4, 10.7 Hz, 1H), 3.22–3.28 (m,
1H), 3.67 (s, 3H), 4.30–4.34 (m, 1H), 4.46–4.50 (m, 1H); IR
(KBr): n=3400, 2990, 1780, 1437, 1363, 1200, 1150,
1076 cmÀ1; CAS registry number [5826–29–9].
5-Hydroxymethylbutyrolactone (5a):[12d] Colorless liquid;
1H NMR: d=2.06–2.15 (m, 1H), 2.18–2.27 (m, 1H), 2.45–
2.63 (m, 2H), 3.60 (dd, J=4.6, 12.5 Hz, 1H), 3.85 (dd, J=
2.9, 12.5 Hz, 1H), 4.57–4.62 (m, 1H); IR (KBr): n=3400,
2941, 1740, 1460, 1358, 1200, 937, 806 cmÀ1; CAS registry
number [10374–51–3].
2,3-Dihydro-2-benzofuran-2-ylmethanol (7a):[21] Colorless
1
liquid; H NMR: d=2.50 (bs, 1H), 3.00 (dd, J=7.3, 15.6 Hz,
5-Hydroxymethyl-2-methylbutyrolactone (5b):[12d] Color-
1H), 3.23 (dd, J=9.5, 15.6 Hz, 1H), 3.60 (dd, J=6.3,
15.6 Hz, 1H), 3.82 (dd, J=4.0, 15.6 Hz, 1H), 4.86–4.92 (m,
1H), 6.75–6.85 (m, 2H), 7.07–7.12 (m, J=7.3 Hz, 2H); IR
(KBr): n=3426, 2932, 1780, 1597, 1481, 1460, 1099, 1053,
1014, 989, 752 cmÀ1; CAS registry number [66158–96–1].
erythro-Tetrahydro-2-furan-1-ethanol (7b):[22] Colorless
liquid; CAS registry number [16765–41–6].
1
less liquid; H NMR (cis and trans): d=1.21 and 1.22 (d, J=
7.3 and 7.1 Hz, 3H), 1.65–1.91 (m, 1H), 2.29–2.37 (m, 1H),
2.63–2.73 and 2.71–2.82 (m, 1H), 3.55–3.61 (m, 1H), 3.75–
3.85 (m, 1H), 4.42–4.48 and 4.52–4.57 (m, 1H); IR (KBr):
n=3495, 2936, 1751, 1456, 1379, 1201, 927 cmÀ1; CAS regis-
try number [51577–65–2].
5-Hydroxy-2,6-norbornanecarbolactone (5c):[12d] Colorless
6-Hydroxymethyltetrahydropyran-2-one (7c):[12d] Colorless
liquid; CAS registry number [81683–96–7].
1
solid; mp 150–1528C (lit. 157–1588C[19]); H NMR (cis and
trans): d=1.55–1.58 (m, 1H), 1.59–1.64 (m, 1H), 1.92–2.02
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Acknowledgements
This work was partially supported by a Grant-in-Aid from
the Ministry of Education, Culture, Sport, Science, and Tech-
nology of Japan. We thank Ms. J. Shimode and Ms. A.
Tonoki (Teikyo University) for performing spectroscopic
measurements.
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