NaY zeolite as host for the selective heterogeneous oxidation of silanes and olefins with hydrogen peroxide catalyzed by methyltrioxorhenium

Article abstract of DOI:10.1021/jo991908e

The methyltrioxorhenium(MTO)-catalyzed oxidation of silanes to silanols and the epoxidation of various olefins by aqueous 85% H₂O₂ proceed in high yields and excellent product selectivities (no disiloxanes, diols) in the presence of the zeolite NaY. The oxidative species is located inside the 12- A supercages. This prevents the bimolecular condensation of the silanol to disiloxane by steric means and the Lewis-acid assisted hydrolysis of the epoxide to the diol.

Full text of DOI:10.1021/jo991908e

J . Org. Chem. 2000, 65, 2897-2899
2897
Na Y Zeolite a s Host for th e Selective Heter ogen eou s Oxid a tion of
Sila n es a n d Olefin s w ith Hyd r ogen P er oxid e Ca ta lyzed by
Meth yltr ioxor h en iu m
Waldemar Adam, Chantu R. Saha-M o¨ ller, and Oliver Weichold*
Institut f u¨ r Organische Chemie, Universit a¨ t W u¨ rzburg, Am Hubland, D-97074 W u¨ rzburg, Germany
Received December 14, 1999
The methyltrioxorhenium(MTO)-catalyzed oxidation of silanes to silanols and the epoxidation of
various olefins by aqueous 85% H
disiloxanes, diols) in the presence of the zeolite NaY. The oxidative species is located inside the
2-Å supercages. This prevents the bimolecular condensation of the silanol to disiloxane by steric
means and the Lewis-acid assisted hydrolysis of the epoxide to the diol.
2 2
O proceed in high yields and excellent product selectivities (no
1
In tr od u ction
As an alternative approach to circumvent such detri-
mental side reactions by the MTO/H oxidant, we have
recently shown the efficacy of the hydrogen peroxide
2
O
2
Methyltrioxorhenium (MTO), the so-called Herrmann
1
catalyst, has been the subject of intensive investigations
adduct (UHP) as oxygen source in oxyfunctionalizations
since its discovery.2 This versatile and highly active
rhenium complex catalyzes numerous oxyfunctionaliza-
3a,6
catalyzed by MTO.
Thus, the heterogeneous MTO/
UHP oxidant (UHP is not soluble in the organic reaction
medium, e.g., CH Cl ) also exhibits high conversions and
3
4
tions such as epoxidation, heteroatom oxidation, and
2
2
5
6
3a,6
C-H as well as Si-H oxidations by activating the cheap
and environmentally benign but reluctant oxidant H
Despite these advantages, the drawbacks of the MTO/
oxidation system are low conversions and ap-
selectivities in olefin epoxidations and silane oxidations.
9
2
O
2
.
Instead of the buffering action of the MTO/pyridine
combination, host-guest chemistry was proposed for the
MTO/UHP system, for which the oxidation takes place
inside the urea channels and formation of the disiloxane
2 2
H O
preciable formation of side products, due to the facile
decomposition of MTO to the perrhenate. For example,
8
6,12
side product is prevented due to spatial constraints.
in epoxidations the epoxide ring is opened quite readily
To place this (for urea) novel concept on a rigorous
experimental basis, it was decided to employ a zeolite as
host for MTO/H O oxidations. Herein we report our
2 2
results on NaY as a heterogeneous host for MTO-
catalyzed epoxidations and silane oxidations; this zeolite
is widely used for shape-selective oxidations with other
organometallic compounds.13 The similarity of the NaY/
MTO/H O and the MTO/UHP oxidants, i.e., high ef-
2 2
ficiency and selectivity, establishes host-guest chemistry
in these heterogeneous oxyfunctionalizations.
7
to the diol, which suffers pinacol-type rearrangement
and cleavage.3a In silane oxidations, the undesirable
6
disiloxane is frequently formed as the major product
9
through acid-catalyzed condensation. Pyridine and pyra-
zole10 as Lewis acid buffers overcome these drawbacks
by coordination to the metal center, thereby reducing the
Lewis acidity of the catalyst and promote stability against
decomposition to the perrhenate. Consequently, such
additives greatly enhance the efficiency and selectivity
of MTO-catalyzed oxidations.11
Resu lts a n d Discu ssion
*
To whom correspondence should be addressed. Fax: +49 931 888
756. E-mail: adam@chemie.uni-wuerzburg.de. Internet: http://www-
organik.chemie.uni-wuerzburg.de.
1) Herrmann, W. A.; Fischer, R. W.; Marz, D. W. Angew. Chem.,
4
Sila n e Oxid a tion s. The MTO-catalyzed reactions
were carried out at ambient temperature (ca. 20 °C) in
methylene chloride in the presence of unactivated zeolite.
In contrast to the previously reported preparation of NaY/
MTO by vacuum sublimation in the presence of a highly
(
Int. Ed. Engl. 1991, 30, 1638-1641.
(
2) Gable, K. P. Adv. Organomet. Chem. 1997, 41, 127-161.
(3) (a) Adam, W.; Mitchell, C. M. Angew. Chem., Int. Ed. Engl. 1996,
3
4
5, 533-534. (b) Tan, H.; Espenson, J . H. Inorg. Chem. 1998, 37, 467-
72.
1
4
activated zeolite under strictly anhydrous conditions,
2
we found that mixing the unactivated zeolite with H O ,
(
4) (a) Adam, W.; Mitchell, C. M.; Saha-M o¨ ller, C. R. Tetrahedron
2
1
994, 50, 13121-13124. (b) Murray, R. W.; Iyanar, K.; Chen, J .;
the substrate, and MTO was beneficial for the conversion
and product distribution in the silane oxidations. Activa-
Wearing, J . T. Tetrahedron Lett. 1996, 37, 805-808. (c) Zhu, Z.;
Espenson, J . H. J . Org. Chem. 1995, 60, 1326-1332.
(5) Murray, R. W.; Iyanar, K.; Chen, J .; Wearing, J . T. Tetrahedron
Lett. 1995, 36, 6415-6418.
6) Adam, W.; Mitchell, C. M.; Saha-M o¨ ller, C. R.; Weichold, O. J .
Am. Chem. Soc. 1999, 121, 2097-2103.
7) (a) Al-Ajlouni, A. M.; Espenson, J . H. J . Org. Chem. 1996, 61,
969-3976. (b) Herrmann, W. A.; Fischer, R. W.; Scherer, W.; Rauch,
M. H. J . Mol. Catal. A 1994, 86, 243-266.
8) (a) Herrmann, W. A. J . Organomet. Chem. 1995, 500, 149-174.
b) Abu-Omar, M. M.; Hansen, P. J .; Espenson, J . H. J . Am. Chem.
Soc. 1996, 118, 4966-4974.
9) Rudolph, J .; Reddy, K. L.; Chiang, J . P.; Sharpless, K. B. J . Am.
Chem. Soc. 1997, 119, 6189-6190.
10) Herrmann, W. A.; Kratzer, R. M.; Ding, H.; Thiel, W. R.; Glas,
H. J . Organomet. Chem. 1998, 555, 293-295.
(
(11) Adolfsson, H.; Converso, A.; Sharpless, K. B. Tetrahedron Lett.
1999, 40, 3991-3994.
(
(12) Adam, W.; Mitchell, C. M.; Saha-M o¨ ller, C. R.; Weichold, O. J .
Chem. Soc., Chem. Commun. 1998, 2609-2610.
(13) (a) Armengol, E.; Corma, A.; Fornes, V.; Garc ´ı a, H.; Primo, J .
Apl. Catal. A 1999, 181, 305-312. (b) W o¨ ltinger, J .; B a¨ ckvall, J . E.;
Zsigmond, A. Chem., Eur. J . 1999, 5, 1460-1467. (c) Langhendvies,
G.; Baron, G. V.; Neys, P. E.; J acobs, P. A. Chem. Eng. Sci. 1999, 54,
3563-3568.
3
(
(
(
(14) Malek, A.; Ozin, G. Adv. Mater. 1995, 7, 160-163. Bein, T.;
Huber, C.; Moller, K.; Wu, C.-G.; Xu, L. Chem. Mater. 1997, 9, 2252-
2254.
(
1
0.1021/jo991908e CCC: $19.00 © 2000 American Chemical Society
Published on Web 04/14/2000

2
898 J . Org. Chem., Vol. 65, No. 10, 2000
Adam et al.
Ta ble 1. Ma ss Ba la n ces, Con ver sion s, a n d P r od u ct Selectivities for th e MTO-Ca ta lyzed Oxid a tion of Sila n es by
Aqu eou s H2O2 in th e P r esen ce of Na Y Zeolitea
b
convnb (%)
c
entry
substrate
[Ox.]
solvent
m.b. (%)
selectivity 2:3
1
2
3
4
5
6
7
8
9
EtMe2SiH (1a )
EtMe2SiH (1a )
tBuMe2SiH (1b)
cHexMe2SiH (1c)
PhMe2SiH (1d )
PhMe2SiH (1d )
PhMe2SiH (1d )
PhMe2SiH (1d )
PhMe2SiH (1d )
PhMe2SiH (1d )
Et3SiH (1e)
NaY/85% H2O2
UHP
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeOH
>95
77
85
72
>95
>99
>99
>99
87
26
28
17
31
>99:1
94:6
>99:1
>99:1
>99:1
98:2
20:80
17:83
99:1
d
NaY/85% H2O2
NaY/85% H2O2
NaY/85% H2O2
80
>95
>99
>99
82
91
77
d
UHP
d
85% H2O2
SiO2/85% H2O2d
NaY/30% H2O2
NaY/85% H2O2
NaY/85% H2O2
1
1
1
0
1
2
94:6
99:1
>99:1
CH2Cl2
CH2Cl2
74
76
86
78
d
Et3SiH (1e)
UHP
a
Reaction performed with 330 µmol of substrate, 2 mol % of MTO, and 2 equiv of H2O2 in the presence of 35 mg of NaY zeolite.
b
1
Entries 1 and 2: determined by H NMR spectroscopy directly on the crude reaction mixture, error (5% of the stated values. Entries
4
-12: mass balances and conversions were determined by gas chromatography versus an internal standard (IS), error (2% of the stated
values. Determined by gas chromatography versus an internal standard (IS), error (2% of the stated values. d Data taken from ref 6.
c
tion of the zeolite results in low silane conversion and
poor product selectivity (silanol versus disiloxane), even
when pyridine was used to neutralize acidic sites in the
zeolite. We ascribe this to the strong acidity of the
calcinated zeolite, which presumably leads to decomposi-
tion of the MTO catalyst. As test substrate, dimethylphe-
nylsilane (1d ) was chosen to optimize the reaction
conditions in order to achieve high silane conversions and
good product selectivities (eq 1).
neous conditions, only a modest conversion to the disi-
loxane 3d as the main product was found. These results
establish that the oxidation takes place inside the 12 Å
supercages rather than on the outer surface of the zeolite.
Force-field calculations revealed that the dimensions of
the silanol 2d are 6.5 × 4.8 Å and those for the silanol-
to-disiloxane condensation ca. 9.1 × 8.8 Å (core to core
distances). These data suggest that two silanol molecules
fit into the same supercage and that disiloxane formation
inside the zeolite cavity should be feasible; however, there
is a 20-fold excess of silane relative to the number of
supercages and GC analysis shows that almost all of the
organic material is in solution during the reaction, which
makes it unlikely for two silanol molecules to meet in
the same cavity. Moreover, the silanol does not dimerize
spontaneously and requires activation by Lewis acids, but
coordination of a rhenium species for such purpose would
exceed the available space. Therefore, the high silanol
selectivity (little disiloxane formation) is evidently due
to the fact that two silanol molecules will not meet in
the same cavity and if they did, acid-catalyzed condensa-
tion is prevented by space limitations. Further support
For this purpose, the ratios of catalyst to oxidant,
catalyst to substrate, and the amount of zeolite were
varied. The optimal conditions are 2 mol % of MTO, 2
2 2
equiv of H O , and 5.3 mg of zeolite per 1 µmol of MTO
catalyst, which were used unless otherwise stated. No
oxidation was observed without MTO. Furthermore, no
decomposition of MTO to perrhenate, which efficiently
catalyzes the silanol condensation,6 was found even
during prolonged reaction times and in the presence of
hydrogen peroxide. The order of addition of the four
2 2
is provided by our previous studies on the Ti-â/H O -
catalyzed silane oxidation,12 for which the same high
silanol selectivities have been found. Like in the present
case, space limitations of the 7.4 Å wide channel in the
Ti-â zeolite prevent the silanol condensation.
reaction partners (zeolite, MTO, H
varied, and higher conversion and better selectivity were
obtained when the zeolite and H were added to the
silane before the MTO catalyst. The latter is very soluble
in CH Cl and has, therefore, no tendency to enter the
polar interior of the zeolite. Thus, when the H is added
2 2
O , and silane) was
2
O
2
To demonstrate the general scope of the catalytic NaY/
2
2
MTO/H
were oxidized (Table 1). Excellent conversions and silanol
selectivities were found for NaY and CH Cl with 85%
(entries 1, 3-5, and 11) as well as for the urea-
adduct (UHP, entries 2, 6, and 12), both heteroge-
2 2
O oxidation system, some selected silanes 1a -e
2
O
2
last, considerable oxidation (and also catalyst decomposi-
tion) will take place in solution, which leads to low
2
2
6
H
H
2
2
O
O
2
2
conversions and high amounts of disiloxane. In contrast,
2 2
when H O is added first, it is absorbed by the zeolite
neous conditions. In contrast, the homogeneous MTO-
catalyzed oxidation of the silane 1d led mainly to the
disiloxane 3d (entry 7). However, when 30% H O was
2 2
and the oxidation takes place inside the zeolite, which is
beneficial for high substrate (silane) conversions and
product (silanol) selectivities.
used, the conversion dropped dramatically to 17%, al-
though the selectivity remained excellent (entry 9).
Moreover, the change of the solvent from methylene
chloride to methanol also resulted in a drastic drop in
the conversion to 31%, but the selectivity remained as
well high at 94:6 (entry 10). This trend has been previ-
To assess whether the oxidation takes place inside the
zeolite, a prestirred slurry of NaY/MTO/H O was re-
2 2
moved by filtration and the silane 1d added to the
filtrate; no conversion was observed even after 24 h, as
confirmed by GC analysis. To explore the effect of the
outer zeolite surface, an experiment with SiO (32-63
2
µm) was conducted, which mimics the outer surface of
the zeolite (Table 1, entry 8).15 Under these heteroge-
(15) Corma, A. Chem. Rev. 1997, 97, 2373-2419.

Heterogeneous Oxidation of Silanes and Olefins
J . Org. Chem., Vol. 65, No. 10, 2000 2899
Ta ble 2. Ma ss Ba la n ces, Con ver sion s, a n d Selectivities
for th e MTO-Ca ta lyzed Ep oxid a tion of Alk en es by 85%
H2O2 in th e P r esen ce of Na Y Zeolite
the oxidation of silanes (Table 2). In addition, MTO/H
pyridine was also employed for the oxidation of the olefins
b,c, but with 85% instead of the normally used 30%
2 2
O /
4
H
H
2
O
O
2
, since for the silanes it was shown that with 30%
no oxidation is observed in the zeolite. The conver-
2
2
sions were in most cases excellent; however, even after
exhaustive washing with methanol, only moderate mass
balances have been obtained for olefins 4b (entry 3) with
NaY/MTO/85% H
6
2
O
2
/pyridine and olefins 4d ,h (entries
and 10) with NaY/MTO/85% H
Little (<5%) diol was found for the NaY/MTO/85%
2 2
O conditions, except for 1-methylcyclohexene (4b),
2 2
O .
H
3
whose epoxide 5b is known to be readily hydrolyzed.
Even with 85% aqueous H , the usual cleavage and
2
O
2
rearrangement products were not observed. This may be
explained by recalling some control experiments from
silane oxidations. These showed that most of the organic
material is in the organic phase. Furthermore, the
supernatant liquid does not show any oxidation activity
even in the presence of pyridine; i.e., the rhenium species
is absorbed in the supercages of the zeolite. The combina-
2 2
tion of MTO/H O , however, has previously been shown
7
to cause epoxide hydrolysis. Since oxidant and epoxide
are topologically separated (MTO is absorbed in the
zeolite, the epoxide is in solution), formation of the diol
side-product is therefore prevented.
These results represent a definite improvement in the
1
selectivity and yields compared to MTO/H
2
O
2
and dis-
play significantly increased epoxide selectivities com-
pared to the previously published heterogeneous MTO
system on modified silica.17 Moreover, our data compare
3
favorably with those obtained for MTO/UHP and MTO/
9
3
0%H
2
O
2
/pyridine, albeit the conversions are lower. This
drawback of the NaY/MTO/85% H
remedied by the addition of catalytic amounts of pyridine
entries 3 and 5), which also improves the selectivity. It
2 2
O system may be
(
9
is evident that the beneficial pyridine effect in solution
is also manifested in the heterogeneous epoxidation; the
zeolite merely serves as microscopic reaction vessel.
a
Mass balance, conversion, and product ratios determined by
1
H NMR spectroscopy directly on the crude reaction mixture, error
5% of the stated values. In the presence of 12 mol % pyridine.
Exo/endo ratio 82:18.
b
(
c
Con clu sion
The present results demonstrate the general ap-
plicability of the NaY zeolite as host for heterogeneous
MTO reactions. For both, silane oxidation and epoxida-
tion, high product selectivities have been obtained with
aqueous 85% H O as oxidant only in the presence of the
ously observed and ascribed to polarity effects, since the
peroxo complexes are formed faster in aprotic media.16
The present zeolite results are mechanistically relevant
to our recent report, in which we have proposed that the
efficient and selective silane oxidation by MTO/UHP
2
2
zeolite, which acts as absorbent for the aqueous phase
and prevents the MTO catalyst from decomposition.
6
takes place inside the urea channels. The efficiencies and
selectivities of the previous process match those for the
Si-H insertion in the NaY zeolite as host of the present
study (Table 1). In view of this similarity and the fact
that the heterogeneous zeolite oxidation must take place
in its interior, the highly efficient and selective MTO/
UHP heterogeneous process evidently also involves also
host-guest chemistry.
Ep oxid a tion s. A selection of olefins, which encom-
passes the cis- and trans-substituted 4a ,c,g,h , the 1,1-
disubstituted 4e,f, the trisubstituted 4b, and the tetra-
substituted 4d ones, was oxidized in the presence of the
NaY zeolite under the optimized conditions developed for
Ack n ow led gm en t. The authors thank the Deutsche
Forschungsgemeinschaft (SFB 347, “Selektive Reak-
tionen Metall-aktivierter Molek u¨ le“) and the Fonds der
Chemischen Industrie for generous support.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedure for the MTO-catalyzed oxidation of the silanes 1a -e
and olefins 4a -h ; GC conditions (Table 3); order of the
addition of the zeolite, H O , MTO, and silane (Table 4); and
2
2
the amounts of zeolite (Table 5). This material is available free
of charge via the Internet at http://pubs.acs.org.
J O991908E
(
16) Wang, W.-D.; Espenson, J . H. J . Am. Chem. Soc. 1998, 120,
(17) Neumann, R.; Wang, T.-J . J . Chem. Soc., Chem. Commun. 1997,
1915-1916.
1
1335-11341.