Alkylated polyethyleneimine/polyoxometalate synzymes as catalysts for the oxidation of hydrophobic substrates in water with hydrogen peroxide

Article abstract of DOI:10.1021/ja046349u

Combinations of alkylated polyethylenimine and polyoxometalates yield water-soluble synzymes with hydrophobic regions that allowed the aqueous biphasic selective oxidation of very hydrophobic, water-insoluble substrates with hydrogen peroxide. With the alkylated PEI/{PO₄[WO(O₂)₂]₄}³- highly effective C-C bond cleavage of alkenes to aldheydes was observed. The synzymes have both tertiary and quaternary amine centers as shown by a combination of ¹⁵N-¹H HMBC and XPS measurements. The existence of hydrophobic regions was concluded from the measurement of contact angles and a hypsochromic shift of a fluorescent probe. Copyright

Full text of DOI:10.1021/ja046349u

Published on Web 09/03/2004
Alkylated Polyethyleneimine/Polyoxometalate Synzymes as Catalysts for the
Oxidation of Hydrophobic Substrates in Water with Hydrogen Peroxide
†
‡
,†
Adina Haimov, Hagai Cohen, and Ronny Neumann*
Department of Organic Chemistry and Chemical Research Support, Weizmann Institute of Science,
RehoVot 76100, Israel
Received June 21, 2004; E-mail: Ronny.Neumann@weizmann.ac.il
Aqueous biphasic catalysis in the absence of organic solvent has
1
potential for environmentally benign synthesis. The technique,
while attractive, has limitations such as reduced reaction rates
because of slow mass transfer of the hydrophobic organic substrate
to the aqueous catalyst phase and the requisite synthesis of water-
soluble ligands for the organometallic catalysts. Mass transfer to
the aqueous phase may be facilitated by addition of amphiphiles.
2
Thus, reaction rates may be increased by formation of micelles,
(
3
micro)emulsions, and other stabilized biphasic systems. The field
of biomimetic catalysis using water-soluble catalysts also presents
intriguing possibilities for aqueous biphasic catalysis. One example
is the use of cyclodextrins as extracting agents for organic substrates
15
1
Figure 1. N- H HMBC NMR experiment for PEI (left) and Alk-PEI
right).
(
4
into an aqueous phase. Recently, we have shown that certain water-
soluble polyanionic polyoxometalates (POMs), with known catalytic
properties in the activation of hydrogen peroxide, can be used in
15
and secondary amines are observed at 18-20 ppm ( N). From the
spectrum for Alk-PEI (Figure 1, right), it is clear that 1° and 2°
5
aqueous biphasic catalysis for oxidation of alcohols, diols, and
amines in the original PEI have been alkylated. The peaks at 25-
0 ppm ( N) may be assigned to tertiary amines, and the peak at
40 ppm ( N) may be assigned to quaternary ammonium
moieties.
Further analysis of Alk-PEI by the XPS (Figure 2) showed a
nitrogen 1s line for the tertiary amine moieties at a binding energy
of 398.7 eV and nitrogen 1s lines for the quaternary ammonium
moieties at 401.0 and 401.8 eV. Measurement of the peak areas
reveals that approximately 50% of the nitrogen atoms are at
quaternary ammonium centers.
6
amines. The systems were largely inactive for aqueous biphasic
15
3
7
oxidation of hydrophobic substrates such as alkenes. In the context
15
∼
of this research, water-soluble randomly alkylated polyethylenimines
have been shown to have properties that are enzyme-like (synzymes)
10
8
for general acid-base catalytic reactions. Now, we show that
POMs can be electrostatically bonded to quaternary ammonium
sites of the alkylated polyethylenimine (Alk-PEI). These new
alkylated polyethyleneimine/polyoxometalate (Alk-PEI/POM) syn-
zymes (Scheme 1) also have hydrophobic regions enabling the
solubilization or binding of hydrophobic substrates and therefore
are active catalysts for the selective oxidation of such substrates in
water.
9,11
Scheme 1. Components of the Alk-PEI/POM Synzyme
Polyethylenimine (M
of 1-iodododecane and iodomethane at a ratio CH
CH I ) 10.5:1:7.4 in a literature-adapted procedure. From the
reaction yield and XPS measurements (see below), an ∼95%
efficiency in the alkylation reaction and ∼50% formation of
quaternary ammonium centers can be determined; Alk-PEI has an
w
≈ 10 000) was alkylated with a mixture
2
CH NH/C12H25I/
2
8e,9
3
Figure 2. XPS N (1s) line for Alk-PEI. Solid lines are curve fits of data.
The hydrophobicity of Alk-PEI versus the unmodified PEI
polymer was estimated by measuring the contact angle on a
goniometer. For unmodified PEI, the contact angle was 11 ( 4°,
indicating a very hydrophilic compound. On the other hand, the
contact angle for alkylated PEI was 30 ( 5°, indicating a more
hydrophobic material. Since Alk-PEI remained water-soluble, one
may assume that there are indeed hydrophobic regions in the
synzyme. This hypothesis was further tested by measurement of
the fluorescence spectra of 3-aminopyrene, a hydrophobic probe,
M
w
≈ 20 000. Under these random alkylation conditions, the
15 1
purified Alk-PEI was first analyzed by a N- H HMBC experi-
ment (Figure 1). One can see that for PEI (Figure 1, left) the
methylene protons correlate with two different types of nitrogen
15
atoms. Logically, primary amines are observed at 9-11 ppm ( N),
†
Department of Organic Chemistry.
Chemical Research Support.
‡
11762
9
J. AM. CHEM. SOC. 2004, 126, 11762-11763
10.1021/ja046349u CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Oxidation of Exemplary Substrates Catalyzed by
with no significant formation of carboxylic acids by autoxidation
reactions. Recycling of the Alk-PEI/POM catalysts is straight-
forward. After the completion of a reaction and removal of the
products by liquid-liquid phase separation, a new amount of
substrate and oxidant is added. In this way, cyclododecene was
oxidized thrice by Alk-PEI/A without apparent loss of activity.
Alk-PEI/POMa
conversion
mol %b
substrate
catalyst
productsc
diphenylsulfide Alk-PEI/A 98 (8) diphenylsufoxide/sulfoned
cyclooctene Alk-PEI/A 99 (7) cyclooctene oxide
cyclododecene Alk-PEI/A 99 (4) cyclododecene oxide
e
styrene
methyl oleate
Alk-PEI/B 96 (0) benzaldehyde
Acknowledgment. The research was supported by the European
Commission (G1RD-CT-2000-00347), the Israel Science Founda-
tion, and the Kimmel Center for Molecular Design. R.N. is the
Rebecca and Israel Sieff Professor of Organic Chemistry.
f
Alk-PEI/B 97 (0) nonanal, methyl 9-oxononanoate
a
Reaction conditions: (1) with Alk-PEI/A (A ) Na12[ZnWZn2(H2O)2-
ZnW9O34)2]); 6.1 mg of Alk-PEI, 1 µmol Na12[ZnWZn2(H2O)2(ZnW9O34)2],
(
0
.2 mL of H2O, 0.5 mmol substrate, 2 mmol H2O2 (60% aq), 22 °C, 9 h.
14
References
(
2) with Alk-PEI/B (B ) Na3{PO4[WO(O2)2]4} formed in situ); 6.1 mg
of Alk-PEI, 0.2 mmol substrate, 8.0 µmol Na2WO4, 10.0 µmol H3PO4, 2.0
mmol H2O2 (60% aq), 22 °C, 24 h. After the reaction, organic components
were collected by phase separation; analysis was by GC and GC-MS using
(
1) (a) Pinault, N.; Bruce, D. W. Coord. Chem. ReV. 2003, 241, 1-25. (b)
Vancheesan, S.; Jesudurai, D. Catalysis 2002, 311-337. (c) Joo, F. Acc.
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b
an external standard. The results in parentheses are for reactions with PEI
instead of Alk-PEI. With POM or Alk-PEI only there were no reactions.
c
The products given are the only ones obtained unless otherwise noted.
7
1998, 2, 121-127.
d
2% Ph2SO2, 28% Ph2SO. e 3% styrene oxide. 70 °C.
f
(
(
2) (a) Reinsborough, V. C. In Interfacial Catalysis; Volkov, A. G., Ed.;
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solubilized in a water solution of Alk-PEI. 3-Aminopyrene (0.5
µM) has an emission peak at λmax ) 441 nm in water. Dissolution
of 3-aminopyrene (0.5 µM) in aqueous PEI (3.8 mM) showed no
shift in the fluorescence spectrum. On the other hand, dissolution
of 3-amniopyrene (0.5 µM) in aqueous Alk-PEI (3.8 mM) resulted
in a blue-shifted spectrum, λmax ) 433 nm. This hypsochromic shift,
2
002; pp 835-841. (c) Rathman, J. F. Curr. Opin. Colloid Interface Sci.
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∆λ ) 8 nm, shows that the probe is dissolved in a hydrophobic
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region of the Alk-PEI. The same hypsochromic shift was observed
upon dissolution of 3-aminopyrene in cetyltrimethylammonium-
based micelles,12 lending credence to a hypothesis that Alk-PEI in
water has a structure reminiscent of an enzyme with hydrophobic
regions and a hydrophilic surface. It should be noted that in Alk-
PEI there was no formation of micelles, as evidenced by the lack
of light scattering, measured at 300-450 nm, at concentrations
ranging from 0.1 to 250 mM Alk-PEI.13
(
5) (a) Kozhevnikov, I. V. Catalysis by Polyoxometalates, Wiley: Chichester,
U.K., 2002. (b) Hill, C. L.; Prosser-McCartha, C. M. Coord. Chem. ReV.
1
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2
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(
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The utility of Alk-PEI/POM synzymes for oxidation in water at
room temperature with hydrogen peroxide was tested using several
very hydrophobic, water-insoluble substrates. Reactions tested
included (a) the oxidation of diphenylsulfide (eq 1) and the
epoxidation of cyclododocene and cyclooctene (eq 2), both
(
1971, 68, 263-266. (b) Shu, J.; Scarpa, I. S.; Klotz, I. M. J. Am. Chem.
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Hong, S. H. J. Am. Chem. Soc. 1998, 120, 12545-12552.
12-
catalyzed by Alk-PEI/[ZnWZn
2 2
(H O)
2
(ZnW
O
9 34
)
2
]
, and (b) the
selective oxidative carbon-carbon bond cleavage of styrene and
methyloleate to the corresponding aldehydes (eq 3) catalyzed by
(
9) PEI (M
(3.1 mL, 0.0126 mol) in the presence of diisopropylethylamine (35 mL)
and EtOH (130 mL) at reflux for 4 h. CH I (5.5 mL, 0.088 mol) was
w
≈ 10 000, 5.4 g, 0.126 mol CH
2 2 25
CH NH) was reacted with C12H I
3-
4 2 2 4
Alk-PEI/{PO [WO(O ) ] } .
3
added dropwise and then refluxed for 17 h. Volatiles were removed by
vacuum evaporation, and low molecular weight components were removed
by dialysis (12 h): 50% EtOH in 50 mM HCl (×3), 20% EtOH in 50
mM HCl (×3), 10% EtOH in 50 mM HCl (×3), 50 mM HCl (×3), water
(
×3). The Alk-PEI obtained, 10.2 g, was dried by lyophilization.
15 1
(
10) The N- H HMBC and XPS experiments show several types of tertiary
and quaternary amine species, respectively. This is probably due to primary
formation of such moieties by alkylation with iododecane and iodomethane
and some additional formation of such moieties during the purification
process (dialysis in the presence of HCl).
(
11) Carlson, T. A. Photoelectric and Auger Spectroscopy, Plenum: New York,
1
975.
The results show (Table 1) that reactions in the presence of Alk-
PEI and the polyoxmetalate catalyst were practically quantitative
and highly selective to the products indicated, whereas in the
presence of unmodified PEI the conversions were negligible,
especially in the case of the alkene bond cleavage oxidation.
Especially notable is the selective formation of aldehydes in the
oxidative carbon-carbon bond cleavage reaction, since normally
such bond cleavage with hydrogen peroxide requires more drastic
(
12) Sarpal, R. S.; Dogra, S. K. J. Chem. Soc., Faraday Trans. 1992, 88, 2725-
2
731.
(13) This has also been discussed in the past. cf. Klotz, I. In Enzyme
Mechanisms; Williams, A., Page, M., Eds.; Royal Society of Chemistry:
London; 1987, pp 14-34.
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3
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reaction conditions that mostly lead to formation of carboxylic acids
rather than aldehydes.15 In this case, possibly due to the intrinsic
8
high acidity of Alk-PEI, there was carbon-carbon bond cleavage
JA046349U
J. AM. CHEM. SOC.
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VOL. 126, NO. 38, 2004 11763