H
H
For our studies, two titanium-doped zeolites were used,
namely TS-1 (5.3 3 5.6 Å)10 and Ti-beta-Na (6.4 3 7.6 Å).10
Control experiments showed that no reaction occurred in the
absence of the titanium zeolite. This clearly indicates that the
oxidative species requires the titanium metal for the activation
of H2O2 and that direct oxidation of the silane by H2O2 does not
take place. For comparison with a homogeneous TiIV catalyst,
we employed the well-known Ti(OPri)4/ButOOH oxidant.
When Ti-beta was used as oxidation catalyst, good to
excellent conversions of the silanes 1a–e to the corresponding
silanols 2a–e were obtained (Table 1, entries 1–3,5–7,11,12). In
contrast, the conversion of the model substrate 1d with the
homogeneous system was lower by 20% (entry 10 compared to
entries 5,6). Notably, the silane 1f is not oxidised because it is
sterically too encumbered to enter the zeolite channels (entry
13). Steric hindrance is also the reason why the silane 1d is not
oxidised when the TS-1 zeolite is employed as catalyst (entry 4).
The latter results provide unequivocal evidence that these
oxidations take place inside the zeolite and not on the outer
surface. In addition, if any oxidation of the silanes were to occur
on the outer surface of the zeolite, significant amounts of the
corresponding disiloxane products should have been formed, in
analogy to the Si–H insertion in solution with Ti(OPri)4/
ButOOH (entry 10).
O
H
SiO
O
H
O
Ti
SiO
OSi
OSi
O
Ti
O
H
H
O
H
H
O
H
OH
Si
R3
R1
Si
R3
R2
R1
R2
Fig. 1 Proposed transition-state structure for the titanium-catalysed Si–H
oxidation.
has been assessed.8 In analogy, we propose also for the Si–H
insertion a five-membered ring peracid-type geometry, as
shown in Fig. 1.
In conclusion, the well-known Ti-beta/H2O2 oxidant is an
excellent system for the catalytic conversion of silanes
selectively into silanols. This catalytic oxidation takes place
inside the zeolite channels and the observed selectivity (no
disiloxane) is due to prevention of the dimerization of the
silanol.
The authors thank the Deutsche Forschungsgemeinschaft
(SFB 347 ‘Selektive Reaktionen Metall-aktivierter Moleküle’),
the Bayerische Forschungsstiftung (Bayerischer Forschungs-
verbund Katalyse - FORKAT), and the Fonds der Chemischen
Industrie for generous financial assistance. A gift of Ti-beta-Na
zeolite from Professor A. Corma is greatly appreciated.
A major advantage of the Ti-beta/oxidant system for the
transformation of silanes to silanols is the fact that excellent
product ratios of silanol versus disiloxane were obtained for all
the silanes studied. This is in stark contrast to the silanol/
disiloxane product distributions which have been observed in
most previous studies.4 A further benefit is the fact that the 85%
aq. H2O2 solution may be substituted by 35% aq. H2O2 solution
without any loss of selectivity and reactivity (entry 5 vs. 6 and
entry 11 vs. 12).
Notes and references
1 A. Ladenburg, Chem. Ber., 1871, 4, 901.
Since the inorganic framework of the zeolite is quite resistant
to oxidative degradation, it was of interest to explore the
possibility of catalyst recycling. Indeed, we found that the Ti-
beta zeolite may be re-used several times by heating the filtered
catalyst at 240 °C for several hours. Interestingly, the recycled
catalyst showed even higher catalytic activity (entry 7). It is
known that calcination reduces the coordination number of the
lattice-bound titanium11 from 5 or 6 to 4, and thus it seemed
likely that the increase in activity is due to the loss of
coordinated water; thus more activated sites for H2O2 are
generated. However, an independently calcinated (ca. 500 °C)
sample of Ti-beta did not show enhanced catalytic activity
(entry 8); moreover, a lower silanol selectivity was observed
(59:41). Furthermore, when a sample of Ti-beta was stirred in
MeCN with 1 equiv. H2O2 for 24 h and recycled as described
above, a moderate silanol selectivity (entry 9) was obtained. We
propose that the increased catalytic activity is due to in situ
silylation of free OH groups in the zeolite lattice. The beneficial
effect of silylation on the enhancement of catalytic activity has
previously been reported for Ti-MCM-4112 and was assigned to
a change to a less polar zeolite interior. The lower silanol
selectivity in entries 8 and 9 might be caused by a higher zeolite
acidity due to loss of coordination water on calcination, which
would promote silanol dimerisation.
2 E. G. Rochow and W. F. Gilliam, J. Am. Chem. Soc., 1941, 63, 798;
R. O. Sauer, J. Am. Chem. Soc., 1944, 66, 1707.
3 W. Adam, R. Mello and R. Curci, Angew. Chem., Int. Ed. Engl., 1990,
29, 890.
4 L. H. Sommer, J. E. Lyons, J. Am. Chem. Soc., 1969, 91, 7061; E.
Matarasso-Tchiroukhine, J. Chem. Soc., Chem. Commun., 1990, 681; C.
Egger and U. Schubert, Z. Naturforsch., Teil B, 1991, 46, 783; U.
Schubert and C. Lorenz, Inorg. Chem., 1997, 36, 1258.
5 W. Adam, C. M. Mitchell, C. R. Saha-Mo¨ller and O. Weichold, J. Am.
Chem. Soc., submitted.
6 A. Corma, M. A. Camblor, P. Esteve, A. Martínez and S. Valencia,
J. Catal., 1994, 145, 151.
7 I. W. C. E. Arends, R. A. Sheldon, M. Wallau and U. Schuchardt,
Angew. Chem., Int. Ed. Engl., 1997, 36, 1144.
8 W. Adam, A. Corma, T. I. Reddy and M. Renz, J. Org. Chem., 1997, 62,
3631.
9 P. Kumar, R. Kumar and P. Pandey, Synlett, 1995, 289; T. I. Reddy and
R. S. Varma, Chem. Commun., 1997, 471.
10 E. Höft, H. Kosslick, R. Fricke and H.-J. Hamann, J. Prakt. Chem.,
1996, 338, 1; M. A. Camblor, A. Corma, A. Martínez and J. Pérez-
Pariente, J. Chem. Soc., Chem. Commun., 1992, 589; M. A. Camblor, A.
Corma and J. Pérez-Pariente, Zeolites, 1993, 13, 82.
11 T. Blasco, M. A. Camblor, A. Corma and J. Pérez Pariente, J. Am. Chem.
Soc., 1993, 115, 11806.
12 T. Tatsumi, K.A. Koyano and N. Igarashi, Chem. Commun., 1998,
325.
Recently, a peracid-type transition-state structure for the Ti-
beta-catalysed epoxidation of chiral allylic alcohols with H2O2
Communication 8/07442I
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Chem. Commun., 1998, 2609–2610