Amphiphilic α-alkoxyalkyl hydroperoxide (α-AHP): A new oxidising reagent in aqueous media

Article abstract of DOI:10.1039/b005414n

Effective oxidation of benzyl sulfide to the corresponding sulfoxide in water was achieved by using amphiphilic α-alkoxyalkyl hydroperoxide (α-AHP) in the presence of a catalytic amount of MoO₂(acac)₂ under mild conditions.

Full text of DOI:10.1039/b005414n

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Amphiphilic a-alkoxyalkyl hydroperoxide (a-AHP): a new oxidising reagent in
aqueous media
Araki Masuyama,* Kazuyuki Fukuoka, Masanori Wakita and Masatomo Nojima
Department of Materials Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan.
E-mail: toratora@ap.chem.eng.osaka-u.ac.jp
Received (in Cambridge, UK) 6th July 2000, Accepted 1st August 2000
First published as an Advance Article on the web 22nd August 2000
Effective oxidation of benzyl sulfide to the corresponding
sulfoxide in water was achieved by using amphiphilic a-
alkoxyalkyl hydroperoxide (a-AHP) in the presence of a
synthesised as a reference because it was freely soluble in water
but showed no surface-active properties.
Differential thermal analyses (DTA/TG) of 1 and 2 were
carried out.§ Both compounds were stable up to about 100 °C
but gradually decomposed above that temperature. These results
indicate that this type of a-AHP is moderately thermostable.
Although careful treatment is still required, the a-AHP is easier
2 2
catalytic amount of MoO (acac) under mild conditions.
Exploration of organic reactions in aqueous media is now
becoming important from the standpoint of environmentally
benign processes.1 One of the major problems in these
reactions, however, is the poor solubility of most organic
substrates in water. The application of micellar or vesicular
systems made from amphiphilic molecules to organic reactions
in water is an effective device to overcome this substantial
1
13
to handle than TBHP. The H and C NMR spectra of a 5 wt%
O solution of 1 did not change at all after the solution was
D
2
kept at 60 °C for 24 h.
Oxidation of benzyl sulfide was chosen to determine the
effectiveness of a-AHP as an oxidising reagent in aqueous
media. The reaction was run at rt and the reaction products were
directly monitored by HPLC.¶ The oxidation reaction in each
system was carried out at least three times to confirm its
reproducibility. The results are summarised in Table 1 along
with data for the same reaction conducted in dichloro-
methane.
2
problem. For example, Jaeger et al. have studied in detail the
rate and/or regioselectivities of certain reactions, such as
monohalogenation of alkyl phenyl ethers or Diels–Alder
reactions, in specially designed micellar or vesicular systems.3
Kobayashi et al. have developed Lewis acid–surfactant com-
bined catalysts and have applied micellar systems made from
these functional surfactants to aldol or Mannich-type reactions.4
A number of other papers concerning organic reactions in
aqueous micellar systems have been published over the past
decade.5
It was confirmed that 1 certainly acted as an oxidising reagent
9
in the presence of MoO
2
(acac)
2
catalyst under homogeneous
conditions in dichloromethane (entry 8). In an aqueous system,
too, 1 could oxidise water-insoluble benzyl sulfide very
effectively (entries 1 and 2). The reaction proceeded cleanly. In
no cases, were any compounds other than unreacted substrate,
unreacted a-AHP, benzyl sulfoxide, (benzyl sulfone), alkyl
aldehyde and tetra(ethylene glycol) detected in the reaction
mixture after 24 h. Aldehyde and tetra(ethylene glycol) were
fragements derived from the corresponding a-AHP through the
For oxidation reactions in aqueous media, commercially
available 30–60% hydrogen peroxide has been used as an
oxidising reagent because its oxygen efficiency is excellent and
6
water is a resultant side product. tert-Butyl hydroperoxide
(
TBHP) is another choice of reagent in aqueous media.7
Because these hydroperoxides are potentially hazardous com-
pounds, however, they must be handled with special care. With
this in mind, we developed a unique amphiphilic a-alkoxyalkyl
hydroperoxide (a-AHP) to act both as a micellar-forming
reagent to solubilise organic substrates and as an oxidising
reagent. The compounds designed for this work could be easily
prepared by ozonation of a-olefins in the presence of oligo-
oxidation reaction. Addition of a catalytic amount of MoO
2
-
(acac) to the aqueous micellar system promoted the reaction. In
2
the presence of excess a-AHP, however, benzyl sulfone as well
as benzyl sulfoxide was formed in quantity (entry 4). In contrast
9
to the MoO
2
(acac)
2
system, addition of VO(acac)
2
caused no
acceleration of the reaction in aqueous media (entries 5 and 6).
The conversion of the substrate was only 16% when 2 was
employed as an oxidising reagent (entry 7). Compound 2 is
water-soluble but does not form micelles in water. Evidently, a
(ethylene glycol)s† (Scheme 1).
Fundamental surface-active properties of 1 were as follows:
2
3
T
cp (cloud point at 1 wt%) = 40 °C; cmc = 3.1 3 10 mol
2
3
dm ; gcmc (the surface tension at cmc, as an indication of the
8
effectiveness of adsorption at the air/water interface) = 30 mM
2
1
; pC20 (the efficiency of adsorption)8 = 3.7.‡ This cloud
Table 1 Oxidation of benzyl sulfide with a-AHP 1 or compound 2 at room
m
temperaturea
point means that 1 is soluble in water at rt and forms micelles at
any concentration above its cmc. Compound 2 was additionally
Molar ratio
Oxidant/
Entry mmol dm
Conver- sulfoxide+
23
Systemb Metal-catalystc sion (%) sulfone
1
2
3
4
5
6
7
8
a
1 (10)
1 (20)
1 (10)
1 (20)
1 (10)
1 (20)
2 (10)
1 (10)
A
A
A
A
A
A
A
B
None
None
52
89
72
100
52
88
100+0
100+0
100+0
80+20
100+0
100+0
100+0
80+20
MoO
MoO
2
(acac)
(acac)
2
2
2
VO(acac)
VO(acac)
2
2
MoO
MoO
2
(acac)
(acac)
2
16
90
2
2
In 10 mmol dm23 benzyl sulfide dispersion (or solution) of water (or
3
23
dichloromethane) (0.01 dm ) containing 10 or 20 mmol dm of oxidant, at
rt for 24 h. b A: in water and B: in dichloromethane. Three mol% toward
benzyl sulfide.
c
Scheme 1
DOI: 10.1039/b005414n
Chem. Commun., 2000, 1727–1728
This journal is © The Royal Society of Chemistry 2000
1727

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key feature of success in promotion of this oxidation in water is
the formation of micelles.
‡ Cmc, gcmc and pC20 values were obtained from a surface tension vs.
concentration (on a log scale) curve measured by the Wilhelmy method
(Kyowa CBVP-A3 model; platinum plate) at 20 °C.
When the concentration of 1 in water was 10 mmol dm23, the
maximum conversion was only 72% (entry 3). It is surmised
that the oxygen atom transfer from 1 to benzyle sulfide results
in the formation of the corresponding hemiacetal, which in turn
is easily decomposed into non-surface-active nonanal and
§
DTA/TG measurements were performed in a stream of nitrogen at 2 °C
21
min of programming rate (Seiko DTA/TG30 model).
Oxidation of benzyl sulfide in aqueous systems was conducted as follows:
¶
3
23
to a stirred aqueous solution of a-AHP (0.01 dm , 10 or 20 mmol dm ) in
a 0.05 dm3 sample vial equipped with a magnetic bar and a septum were
2
3
tetra(ethylene glycol). Because the cmc of 1 is 3.1 mmol dm
,
added benzyl sulfide (0.1 mmol) and metal catalyst (3 mol%). The reaction
3
most of the micelles will disappear at the final stage of the
reaction. This results in the loss of effective solubilisation of
substrate in the system. The appearance of insoluble solids in
the reaction system was actually observed after 18 h with the
mixture was stirred in dark at rt for 24 h. Then, acetonitrile (0.015 dm ) was
poured into the mixture and bromobenzene as an internal standard was
added to the solution. The resultant solution was injected directly into the
HPLC apparatus equipped with a TSKgel ODS-80Ts column (eluate:
24
3
21
water–acetonitrile = 2/3, flow rate: 8 3 10 dm min ). From the
comparison with authentic samples, formation of nonanal (in the case of a-
AHP 1) or pentanal (in the case of compound 2), and tetra(ethylene glycol)
was confirmed in the reaction mixture.
2
3
naked eye. However, when using excess 1 (20 mmol dm
)
micelles were still present after 24 h, so that the conversion of
substrate in each system was higher than in the corresponding
2
3
system containing 10 mmol dm of 1.
The precise mechanism of these micellar oxidation systems
has not yet been established, but it seems reasonable to surmise
that benzyl sulfide solubilised within the a-AHP micelles will
be oxidised effectively by hydroperoxy groups of a-AHP,
which will exist in the vicinity of the solubilised site.
In summary, effective oxidation of benzyl sufide to the
corresponding sulfoxide (and sulfone) in water has been
achieved by using micellar a-AHP 1 in the presence of a
1 C.-J. Li and T.-K. Chan, Organic Reactions in Aqueous Media, John
Wiley & Sons, New York, 1997; Organic Synthesis in Water, ed. P. A.
Grieco, Blackie Academic & Professional, London, 1998.
2
J. H. Fendler and E. J. Fendler, Catalysis in Micellar and Macro-
molecular Systems, Academic Press, London, 1975.
3
D. A. Jaeger, M. W. Clennan and J. Jamrozik, J. Am. Chem. Soc., 1990,
1
12, 1171; D. A. Jaeger and J. Wang, J. Org. Chem., 1993, 58, 6745;
D. A. Jaeger, D. Su, A. Zahar, B. Piknova and S. B. Hall, J. Am. Chem.
Soc., 2000, 122, 2749.
2 2
catalytic amount of MoO (acac) under very mild conditions.
Studies on the preparation of a series of a-AHPs and their
application to other oxidation reactions in these micellar
systems are now in progress.
This work was supported by a Grant-in-Aid for Scientific
Research on Basic Research Area (12650835) from the Ministry
of Education, Science, Culture and Sports of Japan.
4
5
S. Kobayashi, T. Wakabayashi, S. Nagayama and H. Oyamada,
Tetrahedron Lett., 1997, 38, 4559; S. Kobayashi and T. Wakabayashi,
Tetrahedron Lett., 1998, 39, 5389; S. Kobayashi, Eur. J. Org. Chem.,
1999, 3, 15; K. Manabe, Y. Mori and S. Kobayashi, Tetrahedron, 1999,
5
5, 11203; K. Manabe and S. Kobayashi, Chem. Commun., 2000, 669.
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Cerichelli, L. Luchetti, G. Mancini and G. Savelli, Tetrahedron, 1995, 51,
1
0 281; Y. Zhang and W. Wu, Tetrahedron: Assymetry, 1997, 8, 2723;
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535.
Notes and references
†
Over a solution of 1-alkene (10 mmol) and tetra(ethylene glycol) (80
3
mmol) in dichloromethane (0.04 dm ) was passed a slow stream of ozone
(
1.2 equiv.) at 278 °C. Then, the mixture was poured into aqueous NaHCO
3
6 K. Sato, M. Aoki, J. Takagi, K. Zimmermann and R. Noyori, Bull. Chem.
Soc. Jpn., 1999, 72, 2287; C. W. Jones, Applications of Hydrogen
Hydroperoxide and Derivatives, RSC, Cambridge, 1999.
7 M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph 186,
Washington, D.C., 1990, p. 9; Handbook of Reagents for Organic
Synthesis, Oxidizing and Reducing Agents, ed. S. D. Burke and R. L.
Danheiser, John Wiley & Sons, Chichester, 1999, pp. 61–68.
8 M. J. Rosen, Surfactants and Interfacial Phenomena, 2nd edn., John
Wiley & Sons, New York, 1989, ch. 2, 3 and 5.
9 R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidations of Organic
Compounds, Academic Press, New York, 1981, ch. 1, 3 and 13.
and was extracted with ether. After evaporation of the solvent, the crude
products were separated by column chromatography on silica gel. Elution
with ether–methanol (95+5, v/v) gave the target a-AHP as an oil. Selected
data for compound 1 (R = n-C
8
H
17), d
.15–1.45 (m, 12H), 1.60–1.85 (m, 2H), 3.1 (s, 1H), 3.52–3.90 (m, 16H),
.84 (t, 1H), 10.66 (s, 1H); d (67.5 MHz) 13.98, 22.53, 24.71, 29.07, 29.23,
9.32, 31.05, 31.71, 61.49, 65.12, 69.95, 70.02, 70.19, 70.42, 70.92, 72.38,
07.00 (Calc. for C17 : C, 57.93; H, 10.30. Found: C, 58.12; H,
0.09%). For compound 2 (R = n-C ), similar NMR data were recorded
: C, 52.69; H, 9.52. Found: C, 52.39; H, 9.37%).
H 3
(270 MHz; CDCl ) 0.88 (t, 3H),
1
4
2
1
1
C
36 7
H O
H
4 9
(Calc. for C13
H O
28 7
1728
Chem. Commun., 2000, 1727–1728