Oxidation of thioethers and sulfoxides with hydrogen peroxide using TS-1 as catalyst

Article abstract of DOI:10.1039/a907605k

A combined experimental and molecular simulation study of the oxidation of thioethers with H₂O₂ was conducted using titanium-containing zeolites as catalysts. Regioselectivity was investigated for the oxidation of allyl methyl thioether using TS-1 as catalyst. Only products for the oxidation of sulfur, such as sulfoxide and sulfolane, were found. Shape-selective oxidation was studied with four isomeric butyl methyl thioethers. For n-, iso-, and sec-butyl methyl thioethers, the principal product for the TS-1 catalyzed reaction was the sulfone, but that for tert-butyl methyl thioether was from partial oxidation to sulfoxide. The origin of this effect was evaluated using molecular simulations. Thioether oxidation to sulfoxide readily occurred via a non-catalyzed solution reaction, while the oxidation of sulfoxide to sulfone was only observed in the catalyzed reactions. The non-catalyzed reaction can be suppressed by conducting the catalyzed reaction in the presence of a base, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), that is too large to diffuse into the intracrystalline pore structure of TS-1. With DBU, the reaction rate with TS-1 as catalyst was much lower. The relative ratios of the sulfoxide and sulfone products can be elucidated through intramolecular steric hindrance and the shape selectivity of the zeolite. TS-1 has been used for the oxidation of alkenes, alcohols, allylic alcohols, phenols, and benzene.

Full text of DOI:10.1039/a907605k

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Oxidation of thioethers and sulfoxides with hydrogen peroxide using
TS-1 as catalyst
Denis J. Robinson,a Lucinda Davies,a Neil McGuire,a Darren F. Lee,¤a Paul McMorn,b
David J. Willock,b Graeme W. Watson,b Philip C. Bulman Page,c Donald Bethella and
Graham J. Hutchings*ab
a L everhulme Centre for Innovative Catalysis, Department of Chemistry, University of L iverpool,
L iverpool, UK L 69 3BX
b Department of Chemistry, Cardi† University, Cardi†, UK CF10 3T B
c Department of Chemistry, L oughborough University, L eicestershire, UK L E11 3T U
Received 20th September 1999, Accepted 11th February 2000
A combined experimental and molecular simulation study of the oxidation of thioethers with hydrogen
peroxide using Ti-containing zeolites as catalysts is described and discussed. Two aspects of reaction selectivity
were explored. First, regioselectivity was studied for the oxidation of allyl methyl thioether with TS-1 as
catalyst, and only products for the oxidation at sulfur, i.e., the sulfoxide and sulfone, were observed. Second,
shape-selective oxidation was studied using four isomeric butyl methyl thioethers. For n-, iso- and sec-butyl
methyl thioethers the dominant product in the TS-1 catalysed reaction was the sulfone, but for tert-butyl
methyl thioether, the dominant product was from partial oxidation to the sulfoxide. Molecular simulations
were used to investigate the origin of this e†ect. For all substrates used in this study, the oxidation of the
thioether to the sulfoxide was found to occur readily by a non-catalysed solution reaction and this was studied
in detail. However, the oxidation of the sulfoxide to the sulfone was only observed in the catalysed reactions. It
was observed that the non-catalysed reaction can be suppressed by carrying out the catalysed reaction in the
presence of a base, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), that is too large to di†use into the
intracrystalline pore structure of TS-1. In the presence of DBU, the rate of reaction with TS-1 as catalyst was
much lower and the relative ratios of the sulfoxide and sulfone products can be explained through a
combination of intramolecular steric hindrance and the shape selectivity of the zeolite.
containing zeolites20 to determine the e†ect of the solvent. In
Introduction
this paper, we present the results for the oxidation with hydro-
gen peroxide of a range of thioethers using TS-1 and Ti-zeolite
b (Ti-b) as catalysts. A particular aspect of this study was to
explore the possibility of shape-selective oxidation, a topic we
previously highlighted for the epoxidation of allyl alcohol.21
Shape selectivity for the oxidation of thioethers was explored
using four isomeric butyl methyl thioethers (n-, iso-, sec- and
tert-butyl methyl thioether). In addition, the topic of regiosel-
ectivity was explored using allyl methyl thioether as substrate.
For this molecule, two functional groups are available for oxi-
dation by hydrogen peroxide, namely oxidation at the alkenyl
double bond and oxidation at the sulfur atom. In addition, the
study combined the results of the experimental studies with
molecular simulations of the pore volume available to the sub-
strates and products through the intracrystalline pore struc-
ture of TS-1 and Ti-b to conÐrm the origin of the observed
shape selectivity e†ects.
Partial oxidation reactions are extensively used in industrial
processes to functionalise hydrocarbon substrates, increasing
their reactivity so that they can be utilised as intermediates in
the chemical industry. The discovery of the titanium silicalite
TS-1 by researchers at Enichem1,2 stimulated immense
research interest into the partial oxidation of an extensive
range of substrates under relatively mild reaction conditions
using microporous and mesoporous materials containing
cations known to be active for oxidation reactions,3,4 e.g.,
Ti,1,2 Cr5 and V.6 However, TS-1 remains the most widely
studied material since, owing to the hydrophobic nature of its
active sites, it can be used e†ectively with aqueous hydrogen
peroxide. To date, TS-1 has been used for the oxidation of
alkenes,7 alcohols,8h10 allylic alcohols,11 phenols12 and
benzene.13 Recently, the intramolecular rearrangements of
epoxides, that are typically a primary product for the oxida-
tion of alkenes, have been studied.14 These previous investiga-
tions concentrated on oxidation at carbon centres, and
relatively limited attention has been given to the oxidation at
other atom centres, e.g., sulfur in thioethers.15h18 The selective
oxidation of thioethers, in particular dialkyl thioethers, has
been studied by Hulea et al.19 and the oxidation of sulfoxides
to sulfones has been studied by the same group using Ti-
Experimental
TS-1 (containing 2.4 mol% Ti) was prepared using a modiÐed
version of the method of Taramasso et al.1 as described pre-
viously.11,22 The material was characterised using powder
X-ray di†raction, infrared spectroscopy and scanning electron
microscopy and was conÐrmed to be crystalline TS-1 with
uniform orthorhombic crystallites 0.3 lm in diameter. Low Al
Ti-b (containing 1.7 mol% Ti) was prepared following the
method of Camblor et al.23 The material was characterised
using powder X-ray di†raction.
¤ Present address: BNFL plc, SpringÐelds, Preston, Lancashire, UK
PR4 0XJ.
DOI: 10.1039/a907605k
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This journal is ( The Owner Societies 2000
1523

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Fig. 1 Oxidation of allyl methyl thioether with TS-1 at 21 ¡C. +,
Thioether conversion with TS-1; …, sulfoxide selectivity with TS-1;
L, sulfone selectivity with TS-1; =, thioether conversion for non-
catalysed reaction; >, sulfoxide selectivity for non-catalysed reaction;
|, sulfone selectivity for non-catalysed reaction.
Fig. 3 Oxidation of sec-butyl methyl thioether with TS-1 at 21 ¡C.
Symbols as in Fig. 1.
shape selectivity in the catalysed process. The results for the
catalysed and non-catalysed reactions are given in Fig. 2È5. As
with allyl methyl thioether, the TS-1 catalysed reaction gave
extensive formation of the sulfone, except for tert-butyl methyl
thioether, for which selectivity to the sulfoxide was 100% after
1 h (Fig. 5) and only 7.6% conversion to the fully oxidised
product was observed after reaction for 4 d. In contrast, in the
absence of catalyst, the oxidation of the butyl methyl thio-
ethers gave the sulfoxide almost exclusively and, even after
several days of reaction, only 2È4% of the sulfone was formed.
Reactions of thioethers with hydrogen peroxide were
carried out in a small scale batch reactor using standard con-
ditions. In a typical reaction, the substrate (4.5 mmol),
aqueous hydrogen peroxide (30%, 45 mmol), catalyst (0.015 g)
and methanol (45 ml) were stirred together at 21 ¡C. Samples
were removed at various times for analysis by gas chroma-
tography (GC) and GC coupled with mass spectrometry
(GC-MS).
Results
Oxidation of allyl methyl thioether using TS-1
The oxidation of allyl methyl thioether was investigated with
TS-1 as catalyst using aqueous hydrogen peroxide. Allyl
methyl thioether was selected for study as the molecule has
two functional groups for oxidation, namely the carbonÈ
carbon double bond and the sulfur atom. However, as can be
seen from the representative results given in Fig. 1, no oxida-
tion products of the carbonÈcarbon double bond were
observed with TS-1 as catalyst. In the presence of TS-1, allyl
methyl thioether is rapidly oxidised to the sulfoxide and
sequentially to the sulfone. The oxidation of allyl methyl
thioether in the absence of TS-1 catalyst was also investigated
under the same experimental conditions. The results (Fig. 1)
indicate that, again, no oxidation occurs at the carbonÈcarbon
double bond but also, in the absence of the catalyst, the for-
mation of the sulfone is signiÐcantly depressed.
Fig. 4 Oxidation of iso-butyl methyl thioether with TS-1 at 21 ¡C.
Symbols as in Fig. 1.
Oxidation of butyl methyl thioethers using TS-1
The oxidation of the four isomeric butyl methyl thioethers
was studied with TS-1 as catalyst to investigate the role of
Fig. 2 Oxidation of n-butyl methyl thioether with TS-1 at 21 ¡C.
Symbols as in Fig. 1.
Fig. 5 Oxidation of tert-butyl methyl thioether with TS-1 at 21 ¡C.
Symbols as in Fig. 1.
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Table 1 Oxidation of tert-butyl methyl thioether with TS-b at 21 ¡C
a†ected by DBU. Unfortunately, equimolar amounts of DBU
and hydrogen peroxide are required. In addition, it can be
expected that the use of DBU may change the mechanism by
which the oxidation proceeds since a di†erent oxidising agent
will be present. Hence the use of high pH and the di†erent
oxidising species can be expected to lead to a signiÐcant
change in the rate of oxidation, and this is observed. However,
the experiments with DBU were conducted to determine the
relative selectivities of the catalysed reaction within the micro-
pores of TS-1.
Selectivity (%)
Time/
min
Conversion
(%)
Sulfoxide
Sulfone
5
15
30
61.2
86.5
99.3
100
100
100
91.6
81.2
57.0
43.8
28.1
19.7
0
8.4
18.8
43.0
56.2
71.9
80.3
100
60
220
300
1440
100
The oxidation of the butyl methyl thioethers in the absence
of TS-1 in the presence of DBU (H O : DBU \ 1 : 1) was
2
2
investigated under the same conditions as described above
(Fig. 2È5). Even after a 24 h reaction time, no oxidation of the
thioethers was observed. This conÐrms that DBU suppresses
the non-catalysed oxidation of the thioether. The reactions
were then carried out in the presence of TS-1 and the results
are given in Table 2. The four butyl methyl thioethers all
exhibit similar conversions in the presence of DBU (ca. 50%
after 24 h of reaction), but there are signiÐcant di†erences in
the selectivity to the sulfoxide and the sulfone. The selectivity
to sulfone formation decreased in the order: n- [ iso- [ sec-
[ tert-butyl methyl thioether.
These results conÐrm that the oxidation of a thioether to a
sulfoxide can proceed readily in the absence of a catalyst but
the sequential oxidation of the sulfoxide to a sulfone requires
an e†ective catalyst.
Oxidation of tert-butyl methyl thioether using Ti-b as catalyst
To investigate the oxidation of thioethers in a larger pore
zeolite, tert-butyl methyl thioether was investigated with Ti-b
and the results are given in Table 1. Ti-b has a pore structure
with interlinking channels of cross-section 0.55 ] 0.55 nm
along the [011] direction and 0.76 ] 0.64 nm along the [100]
direction. It can, therefore, accommodate much larger mol-
ecules than can enter the pores of TS-1 (0.56 ] 0.53 nm). In
the more accessible pore volume of Ti-b, the sulfone is rapidly
formed from sequential oxidation of the sulfoxide and so these
results conÐrm the shape selectivity of the oxidation of tert-
butyl methyl thioether with TS-1.
Molecular simulations
To gain an improved understanding of the observed selectivity
in the catalytic reaction, we carried out a series of calculations
to assess the accessibility of the interior of the zeolite to the
reactant molecules and the oxides produced. The structure of
each molecule was initially optimised using MOPAC25 with
the PM3 Hamiltonian26 using the eigenvector follower
geometry optimiser. Structures were also obtained at the ab
initio HartreeÈFock level using the GAMESS27 program, with
the standard double zeta plus polarisation (DZP)28 basis func-
tions. This provides a much richer basis for the description of
the molecular orbitals compared with the MOPAC case,
which is e†ectively a parameterised minimal basis set tech-
nique. In particular, the DZP basis provides d-functions on
sulfur. The conformations used for each thioether are illus-
trated in Fig. 6 and the calculated geometry around the S
atom for each molecule is given in Table 3. In general, the
GAMESS-UK results give shorter SÈC and SÈO bonds
although the maximum di†erence between the results from the
two techniques for these molecules is only 0.05 A for SÈC and
0.06 A for SÈO. This indicates that even the simple semi-
empirical approach is able to give reasonable geometries for
thioethers and their oxides. Test calculations using a DZ basis
set in the ab initio calculation gave a general overestimation of
the bonds to the sulfur atom by up to 0.2 A compared with
the results in Table 3. This indicates that polarisation basis
functions are essential for obtaining reliable results from the
ab initio approach. The semi-empirical method does give rea-
sonable agreement with the higher level calculations and
requires several orders of magnitude less computer time.
Oxidation of butyl methyl thioethers using TS-1 in the presence
of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)
It is clear that oxidation of thioethers using H O as oxidant
2
2
with TS-1 or Ti-b as catalysts is complicated by a relatively
rapid non-catalysed oxidation of the thioether to the sulf-
oxide. However, the sulfoxide is relatively stable with respect
to non-catalysed oxidation. The non-catalysed reaction needs
to be suppressed if the true e†ects of shapeÈselective oxidation
using TS-1 are to be observed. This can be achieved if a strong
base is incorporated in the reaction mixture, since this will
deprotonate H O leading to the production of the hydro-
2
2
peroxy anion, which is incapable of acting as an oxidant for
these thioethers. Suitable strong bases include potassium car-
bonate and DBU.24 Potassium carbonate was considered to
be inappropriate for these TS-1 studies since the presence of
potassium cations in solution could ion exchange the weak
Bronsted site present in TS-1.22 DBU (pK B 13) is a very
a
bulky molecule and molecular simulation calculations showed
that DBU could not access the intracrystalline pore structure
of TS-1. It is, therefore, an ideal base to suppress the non-
catalysed reaction, since the shapeÈselective nature of TS-1
ensures that the active sites in the internal pores are not
The occupiable pore volume for each molecule based on the
GAMESS-UK DZP level molecular structures was calculated
using the MSI Solids Characterise module within the InsightII
package.29 Only the pure silica frameworks were considered,
Table 2 Oxidation of butyl methyl thioethers with TS-1 in the presence of DBU at 21 ¡C for 24 h
Selectivity (%)
Conversion
Product ratio,
sulfoxide : sulfone
Substrate
(%)
Sulfoxide
Sulfone
n-Butyl methyl thioether
iso-Butyl methyl thioether
sec-Butyl methyl thioether
tert-Butyl methyl thioether
52.1
49.5
47.8
51.7
79.3
88.9
90.4
93.8
20.7
11.1
9.6
3.8
8.0
9.4
6.2
15.1
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Fig. 6 Geometry optimised conformations from GAMESS-UK calculations on the isolated molecules: (a) n-butyl methyl thioether; (b) sec-butyl
thioether; (c) iso-butyl methyl thioether; (d) allyl methyl thioether; (e) tert-butyl methyl thioether.
so the e†ect of extra framework cations on the available pore
space was ignored, an approximation which should be valid
for these low Al materials. Since the silica form is being used,
we shall refer to the TS-1 structure by its generic structure
type, MFI. We also used the pure achiral polymorph structure
(polymorph B30) for zeolite b. Both frameworks were taken
from the MSI database. To calculate the occupiable volume,
the unit cell of each zeolite was divided into a series of grid
points with a separation of 0.5 A. The probe molecule was
then placed at each grid point in turn and rotated to attempt
to Ðnd an orientation in which there was no steric conÑict
between the probe and the zeolite. The total number of grid
points for which this is possible was then used to calculate the
total volume of the unit cell that is occupiable by the adsorb-
ate. The greater this volume, the easier it will be for the probe
molecule to access the interior of the microporous material
which contains the majority of the active sites. This method
gives the volume of the framework which can be occupied by
at least one molecular orientation. It di†ers from the idea of a
kinetic diameter for a molecule, which is the pore diameter
required to allow a molecule to occupy a location without
signiÐcantly impeding its thermal motion. Neither method
allows for the treatment of kinetic e†ects directly but simply
gives an indication of the degree of restriction placed upon a
molecule by the host. Full molecular dynamics calculations
have been performed for relatively mobile alkane adsorbates
in zeolites31 but the cost of such calculations for molecules
whose dimensions are similar to the pore diameter is prohibi-
tive.
The selectivity di†erences between these substrates for the
catalysed reactions will arise from a combination of the elec-
tronic e†ect of di†erently branched butyl groups on sulfur, the
e†ect of steric hinderance around sulfur due to substituent
groups32 and the shape selectivity imposed by the pore. The
pore access calculations can be used to assess the importance
of the latter a†ect.
In the case of MFI, each C substituted molecule shows a
4
decrease in the accessible pore volume on successive oxidation
at sulfur. This corresponds to the increasing steric bulk of the
molecules as oxygen atoms are added (Fig. 7). In the case of
allyl methyl thioether a small increase is observed for the Ðrst
oxidation step but the occupiable volume remains high for
both oxidation products. This indicates that the smallest
probe molecule would not be strongly a†ected by the shape
Fig. 7 Geometry optimised conformations from GAMESS-UK cal-
culations on the oxidation products of (a) n-butyl methyl thioether
and (b) tert-butyl methyl thioether. In each case the thioether is shown
at the top of the diagram with the sulfoxide and sulfone below.
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Table 3 Geometries of thioethers and their oxides from MOPAC and GAMESS calculationsa
Molecule
RÈSMe/A
RSÈMe/A
SÈO/A
CÈSÈC (¡)
n-Butyl methyl thioether
n-Butyl methyl sulfoxide
n-Butyl methyl sulfone
sec-Butyl methyl thioether
sec-Butyl methyl sulfoxide
sec-Butyl methyl sulfone
iso-Butyl methyl thioether
iso-Butyl methyl sulfoxide
iso-Butyl methyl sulfone
tert-Butyl methyl thioether
tert-Butyl methyl sulfoxide
tert-Butyl methyl sulfone
Allyl methyl thioether
Allyl methyl sulfoxide
Allyl methyl sulfone
M
G
M
G
M
G
M
G
M
G
M
G
M
G
M
G
M
G
M
G
M
G
M
G
M
G
M
G
M
G
1.826
1.820
1.846
1.812
1.828
1.789
1.847
1.835
1.872
1.831
1.846
1.800
1.824
1.826
1.854
1.811
1.827
1.791
1.863
1.846
1.898
1.847
1.875
1.824
1.828
1.820
1.851
1.821
1.824
1.788
1.801
1.812
1.819
1.803
1.794
1.779
1.801
1.813
1.818
1.804
1.795
1.782
1.801
1.812
1.819
1.804
1.797
1.781
1.801
1.815
1.820
1.805
1.795
1.785
1.801
1.810
1.819
1.802
1.795
1.782
È
È
1.557
1.500
1.468
1.447
È
101.7
101.3
98.8
98.9
99.1
104.6
102.6
103.4
99.8
101.5
100.0
106.9
103.9
101.3
99.6
È
1.558
1.501
1.469
1.448
È
È
1.560
1.500
1.469
1.447
È
98.5
98.0
103.5
104.9
103.9
100.4
102.1
98.0
108.8
101.6
99.5
98.5
98.6
98.8
103.2
È
1.559
1.502
1.471
1.449
È
È
1.556
1.498
1.466
1.445
a M indicates MOPAC results, G indicates GAMESS-UK results, for the dioxides the two S2O bond lengths were always found to be equivalent
to within 0.002 A or less and so only a single value is quoted.
restrictions imposed by the microporous host during the oxi-
dation process. The occupiable pore volume data (Table 4)
also indicates that n- and sec-butyl methyl thioether have
similar occupiable volumes for their sulfoxide and sulfones
again suggesting that no shape selectivity should be expected
in these cases. Only iso- and tert-butyl thioethers show signiÐ-
cantly smaller occupiable volumes for the sulfone compared
with the sulfoxide. This suggests that in these cases the pore
shape of MFI will tend to prevent the creation of the bulky
sulfone product and so inÑuence the product selectivities. In
the case of zeolite b all substrates show roughly equivalent
occupiable volumes for the two oxide products, suggesting
that no shape selectivity should be expected in this case.
to the sulfone is not observed. In the presence of TS-1, oxida-
tion to both the sulfoxide and sulfone is typically observed.
The reactions were carried out with a reactant ratio of thio-
ether to hydrogen peroxide of 1 : 10 and hence it is considered
that the oxidation to the sulfoxide and sulfone can be rep-
resented by two sequential pseudo-Ðrst-order reactions:
k1
k2
thioether ÈÈÈ sulfoxide ÈÈÈ sulfone
The calculated rate constants for the non-catalysed and
TS-1-catalysed reactions are given in Table 5. Since the catal-
ysed and non-catalysed reactions are carried out under identi-
cal conditions except for the addition of the catalyst, the total
observed rate constant (k ) for the catalysed reaction can be
1
considered to be a composite of the rate constant for the
Discussion
catalysed reaction [k (cat)] and the non-catalysed reaction
1
[k (non-cat)].
Oxidation of thioethers using hydrogen peroxide
1
k \ k (non-cat) ] k (cat)
Thioethers in the absence of TS-1 are oxidised by hydrogen
peroxide to form the sulfoxide via a rapid non-catalysed reac-
tion. The non-catalysed sequential oxidation of the sulfoxide
1
1
1
The non-catalysed reaction data gives the Ðrst of these
terms directly and can then be used along with the catalysed
Table 4 Pore access results
Cell
volume/
Available pore volume/A
Ž
3 M% of totalN
Ratio of %
volume for
R
A
Ž
3
Thioether
Sulfoxide
Sulfone
sulfoxide: sulfone
MFI structure
n-Butyl methyl
sec-Butyl methyl
iso-Butyl methyl
tert-Butyl methyl
Allyl methyl
Zeolite b structure
n-Butyl methyl
sec-Butyl methyl
iso-Butyl methyl
tert-Butyl methyl
Allyl methyl
5332
1584M29.7N
1449M27.2N
1496M28.1N
820M15.4N
1617M30.3N
1526M28.6N
1212M22.7N
1261M23.6N
713M13.4N
1622M30.4N
1512M28.4N
1174M22.0N
967M18.1N
590M11.1N
1574M29.5N
1.007
1.032
1.304
1.207
0.970
4096
1739M42.5N
1665M40.6N
1650M40.3N
1591M38.8N
1689M41.2N
1649M40.3N
1665M40.6N
1553M37.9N
1266M30.9N
1715M41.9N
1637M40.0N
1512M36.9N
1554M37.9N
1177M28.7N
1727M42.2N
1.008
1.100
1.000
1.077
0.993
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Table 5 Rate constants for TS-1 catalysed and non-catalysed oxidation of thioethers at 21 ¡C
Substrate
k (cat)/s~1
k (non-cat)/s~1
k /s~1
1
1
2
Allyl methyl thioether
9.0 ] 10~4
13.3 ] 10~4
2.2 ] 10~4
0.15 ] 10~4
5.8 ] 10~4
5.2 ] 10~4
8.8 ] 10~4
4.8 ] 10~4
4.5 ] 10~4
16.0 ] 10~4
1.6 ] 10~4
4.2 ] 10~4
0.6 ] 10~4
0.36 ] 10~4
Èa
n-Butyl methyl thioether
iso-Butyl methyl thioether
sec-Butyl methyl thioether
tert-Butyl methyl thioether
a Not determined, rate negligible.
reaction data to obtain the second term. Values of k (non-cat)
in the MFI structure of TS-1 are less than for the other three
butyl isomers (Fig. 7). In addition the space available to the
sulfoxide is 1.2 times greater than to the sulfone which will be
the most restricted of all the molecules studied. In the pres-
ence of DBU, oxidation will only take place in the pore space
of the zeolite and we observe a high product ratio of sulfoxide
to sulfone (Table 2) for tert-butyl methyl thioether compared
with the other substrates. From the calculated occupiable
volumes we suggest that this is due to the shape selectivity of
the TS-1 catalyst which prevents the formation of the sulfone
of the tert-butyl isomer. In the absence of DBU we have
established that the non-catalysed reaction only proceeds as
far as sulfoxide, and sulfone is only obtained as the major
product in the presence of TS-1 catalyst. In the case of tert-
butyl methyl thioether with TS-1 as catalyst, the second oxi-
dation step is again prevented by the shape selectivity of the
catalyst.
For Ti-b all the molecules studied have a much larger frac-
tional pore occupiable volume than in TS-1 and the sulfoxide-
to-sulfone ratios are all near to unity. This suggests that, even
for tert-butyl methyl thioether, no shape selectivity e†ects
would be expected. In the presence of Ti-b we Ðnd that the
conversion to sulfone for tert-butyl methyl thioether is
observed from the beginning of the experiment and is com-
plete after a 1 d reaction time (Table 1), conÐrming that shape
selectivity is not present in the larger pore system.
In the case of iso-butyl methyl thioether with TS-1, the cal-
culations indicate a ratio of 1.3 between the occupiable
volumes of the sulfoxide and sulfone, although the individual
occupiable volumes are larger than for the corresponding tert-
butyl isomers. In this case, too, then, we expect the shape
selectivity of TS-1 to contribute to the observed selectivity
toward the sulfoxide and it is interesting to note that the
larger occupiable volumes for the two products correspond to
a lower product ratio in Table 2.
The experimentally observed sulfoxide-to-sulfone product
ratio for the sec-butyl methyl thioether case is actually higher
than for the iso-butyl case and yet the occupiable volume cal-
culations suggest that the two oxide products would have
similar pore volumes available to them. In this case we would
not expect, then, the pore size to restrict the second oxidation
step. However, since the Ti catalytic centre in these materials
is incorporated into the framework, it is possible that local
steric interactions at the catalytic centre will inÑuence reaction
rate.32 Here the methyl group a to the sulfur atom in the sec-
butyl isomers will be more likely to inhibit reaction than the
b-methyl of the iso-butyl molecules, particularly in the second
oxidation for which the catalytic site will also have to accom-
1
and k (cat) for the reactions reported here are given in Table
1
5. Since sulfoxide oxidation is only observed for the catalytic
reaction, the rate constant k does not require compensation
2
in this manner.
It is apparent that the order of reactivity for the catalytic
formation of the sulfoxide from the butyl methyl thioether is
n- [ tert- [ iso- [ sec-butyl methyl thioether. For the non-
catalysed oxidation, tert-butyl methyl thioether is observed to
be the most reactive for sulfoxide formation and, indeed, apart
from n-butyl methyl thioether, the non-catalysed oxidation is
the dominant process. The reactivity of the oxidation of the
sulfoxide to the sulfone is very similar for the butyl methyl
thioethers apart from tert-butyl methyl thioether, for which no
sulfone is observed.
In the presence of DBU, the non-catalysed oxidation is
totally suppressed and now the four butyl thioethers investi-
gated have similar reactivities. The reaction rate is much lower
than in the absence of DBU since, even after 24 h, only ca.
50% conversion was observed (Table 2). This lower reactivity
is considered to be due to the high pH used which is known to
deactivate TS-1.11 It is clear that the hydroperoxy anion
(OOH~) is much less reactive as an oxidant for the TS-1
catalysed reaction than the electrophilic HOOH species.
However, these experiments are useful in demonstrating the
selectivity of the oxidation processes occurring wholly within
the micropores of TS-1.
Comments on regio- and shape selectivity
For the oxidation of allyl methyl thioether, only the products
from the oxidation at sulfur are observed and, hence, the reac-
tion is a clear example of regioselectivity. No oxidation of the
carbonÈcarbon bond is observed. This is in contrast to the
oxidation of allyl alcohol with TS-1,11,22 when it is found that
glycidol can be formed with 100% selectivity and 20% yield
under comparable reaction conditions. The pseudo-Ðrst-order
rate constants for the formation of glycidol from allyl alcohol
(21 ¡C, methanol as solvent, 10 : 1 hydrogen peroxide-to-sub-
strate ratio) is 2.3 ] 10~4 s~1. This is not signiÐcantly lower
than the pseudo-Ðrst-order rate constant for the oxidation of
allyl methyl thioether to the sulfoxide under the same condi-
tions (9.0 ] 10~4 s~1). In view of this, it is clear that the sulfur
deactivates the carbonÈcarbon double bond towards oxida-
tion by the electrophilic oxidant hydrogen peroxide.
The presence of a heteroatom allylic to the carbonÈcarbon
double bond is known to deactivate the substrate for TS-1
catalysed oxidation. For example, non-substituted alkenes
react several orders of magnitude more rapidly than allylic
alcohols.11,22 The relative ease with which sulfur is oxidised in
thioethers can be expected as the sulfur lone pair electrons are
more nucleophilic than the p electrons of the carbonÈcarbon
double bond. This is well known from the non-catalysed oxi-
dation of sulÐdes using monochloroperbenzoic acid as stoi-
chiometric oxidant.33 It is, however, interesting that, even
when the sulfone is formed, no oxidation at the double bond
is observed.
modate the sulfoxide group. This may explain the lower k
rate constant (Table 5) for sec-butyl methyl sulfoxide com-
2
pared with iso-butyl methyl sulfoxide despite the shape selec-
tivity expected in the latter case.
Conclusions
We have demonstrated that regioselectivity to oxidation at
sulfur is 100% for allyl methyl thioether. By a comparison of
experimental product ratios and calculated occupiable
volumes, we Ðnd that shape selectivity for tert-butyl and iso-
butyl methyl sulfoxide reactions in TS-1 inhibits the formation
The pore access calculations suggest that the occupiable
volumes for tert-butyl methyl thioether, sulfoxide and sulfone
1528
Phys. Chem. Chem. Phys., 2000, 2, 1523È1529

View Article Online
14 A. Bhaumits and T. Tatsumi, J. Catal., 1999, 182, 349.
of the bulkier sulfones. We also suggest that more localised
steric hindrence at the active site may explain the observed
selectivity in the case of sec-butyl methyl thioether oxidation.
Clearly this e†ect would also be expected in the case of tert-
butyl methyl thioether for reaction in Ti-b, where the shape-
selective restriction has been removed. However in this case
oxidation of the sulfoxide to sulfone is more rapid than in any
of the TS-1 catalysed reactions.
15 R. S. Reddy, R. Sudhakas, R. Kumar and P. Kumar, J. Chem.
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19 V. Hulea, P. Moreau and F. DiRenzo, Stud. Surf. Sci. Catal.,
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20 P. Moreau, V. Hulea, S. Gomez, D. Brunel and F. DiRenzo,
Appl. Catal., A, 1997, 155, 253.
Acknowledgements
21 G. J. Hutchings, P. G. Firth, D. F. Lee, P. McMorn, D. Bethell,
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We thank ICI, Robinson Bros., the EPSRC and the DTI/
LINK Scheme on Asymmetric Synthesis for Ðnancial support.
The calculations presented in this paper were carried out, in
part, using Glyndwr, a Silicon Graphics multiprocessor Origin
2000 machine at the Department of Chemistry, Cardi† Uni-
versity. This facility was purchased with support from the
EPSRC, ICI Katalco and OCF.
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