Titanium silsesquioxanes grafted on three-dimensionally netted polysiloxanes: Catalytic ensembles for epoxidation of alkenes with aqueous hydrogen peroxide

Article abstract of DOI:10.1002/1521-3773(20020215)41:4<637::AID-ANIE637>3.0.CO;2-I

Catalytic ensembles suitable for alkene epoxidation with aqueous hydrogen peroxide are now accessible. They are made by grafting robust titanium silsesquioxane sites onto dimethylsiloxane polymers (see picture) followed by cross-linking to three-dimensionally netted polymers.

Full text of DOI:10.1002/1521-3773(20020215)41:4<637::AID-ANIE637>3.0.CO;2-I

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these complexes are stable in aqueous media,[7] none could
perform alkene epoxidation with hydrogen peroxide. It is
tempting to ascribe this to the lack of a combination of the
active titanium site with a suitable hydrophobic environment.
The same phenomenon can acount for the lack of catalytic
activity in a study by Sherrington and Alder on TiIV-grafted
polysiloxane networks prepared from silanol-rich supports.
These materials, showed no activity in alkene epoxidation
with 30% aqueous H2O2.[8] This can be ascribed to either the
presence of residual silanol groups that make the materials
more hydrophilic thus rendering unsuitable catalytic ensem-
bles or to the presence of Ti centers with inappropriate
structures.
Herein, epoxidation catalysts are reported that, like TS-1,
epoxidize 1-octene but also substrates that are too large for
TS-1 such as cyclooctene and cyclododecene. In this ap-
proach, ensembles are made from robust titanium silsesqui-
oxane complexes. These are grafted on commercially avail-
able linear methylhydrosiloxane dimethylsiloxane copoly-
mers and then cross-linked by reaction with vinyl-terminated
polydimethylsiloxanes. The resulting titanium polysiloxane
materials are hydrophobic, three-dimensionally netted poly-
mers that enclose the titanium sites in cavities that can, in
principle, be varied according to the choice of the starting
materials. Since titanium polysiloxanes are found to epoxidize
alkenes with aqueous hydrogen peroxide, while titanium
silsesquioxane complexes alone do not have this ability, we
demonstrate the need for catalytic ensembles in this area of
science.
[18] Chiral nonracemic terminal propargylic epoxides have been described
in: a) M. Lopp, T. Kanger, A. M¸raus, T. Pehk, ‹. Lille, Tetrahedron:
Asymmetry 1991, 2, 943; b) T. Kanger, P. Niidas, A.-M. M¸¸risepp, T.
Pehk, M. Lopp, Tetrahedron: Asymmetry 1998, 9, 2499 (one example
¬
derived from tartaric acid derivative is given); c) R. Sanchez-
¬
Obregon, B. Ortiz, F. Walls, F. Yuste, J. L. GarcÌa Ruano, Tetrahedron:
Asymmetry 1999, 10, 947 (3 examples with ee×s up to 88% are given);
synthetically useful enantioselectivities with terminal olefins are yet to
be achieved with chiral (salen)Mniii systems: E. N. Jacobsen in
Catalytic Asymmetric Synthesis (Ed.: I. Ojima), Wiley-VCH, New
York, 1993, p. 178.
[19] K. Takahashi, T. Minami, Y. Ohara, T. Hiyama, Bull. Chem. Soc. Jpn.
1995, 68, 2649.
[20] Alcohols (R)- and (S)-2a were converted into the corresponding E-
and Z allylic alcohols by using Red-Al or Lindlar catalyst, respec-
tively. The olefinic products were obtained in high yield, without
racemization (ee >99%) and excellent E/Z ratios (Red-Al:
E/Z 100:0; Lindlar catalyst: E/Z 5:95). Thus, another class of hardly
known highly functionalized synthetic intermediates is easily acces-
sible; T. Schubert, M. M¸ller, unpublished results.
[21] The ee values were determined by conversion of epoxide (S)-3a and
rac-3a into the terminal nitrile, by treatment with a threefold excess of
KCN in ethanol/water (4:1) at 508C for 4 h. The nitrile obtained was
separated by means of HPLC on a Chiralcel OB column (208C,
0.5 mLminÀ1, −Iso-hexane×/2-propanol 95:5), Rt ˆ 68.6 min ((R)-3a),
79.3 min ((S)-3a).
Titanium Silsesquioxanes Grafted on Three-
Dimensionally Netted Polysiloxanes: Catalytic
Ensembles for Epoxidation of Alkenes with
Aqueous Hydrogen Peroxide**
ˆ
Vinyl silsesquioxane trisilanol [(H2C CH)(c-C6H11)6-
Si7O9(OH)3] (1)[9] can be easily converted to new titanium
ˆ
Maria D. Skowronska-Ptasinska,
Martinus L. W. Vorstenbosch, Rutger A. van Santen,
and Hendrikus C. L. Abbenhuis*
silsesquioxane complexes [(H2C CH)(c-C6H11)6Si7O12TiX]
(2a, X ˆ h5-C5H5 (Cp), 2b, X ˆ OiPr) by reaction with
[Cl3TiCp] or [Ti(OiPr)4], respectively. These reactions are
Few catalysts have been truly efficient in alkene epoxida-
tion with aqueous hydrogen peroxide. Development of such
catalysts is an important goal since with regard to desirability,
this oxidant comes only second to oxygen itself.[1] Currently,
the best catalyst in this field is the synthetic titanium-
containing zeolite, titanium silicalite-1 (TS-1),[2] which is
active for a wide range of oxidation reactions, including
epoxidation.[3] For TS-1, activity seems to originate from a
combination of a robust active Ti(OSi )n site (n ˆ 3, 4),[4] and
ꢀ
its location in a hydrophobic channel or cavity in the MFI
(ZSM-5) structure.[5] The resulting catalytic ensemble pre-
vents poisoning of the active site by water as well as
unproductive decomposition of the oxidant. A series of
titanium silsesquioxane complexes with structural elements
that are very similar to the active TS-1 site have been reported
to function as homogeneous catalysts.[6] Although some of
similar to those previously described for the related, unfunc-
tionalized
silsesquioxane
trisilanol
[(c-C6H11)7Si7O9-
(OH)3].[6b, 10] For both new complexes, the 13C NMR spectra
(400 MHz, CDCl3) are particularly informative, showing four
peaks for the cyclohexyl methine carbon atoms (ratio 2:1:1:2
for 2a and 1:1:2:2 for 2b) characteristic for C2-symmetric,
monomeric silsesquioxane titanium species. Attempts to
obtain crystals of 2 suitable for X-ray analysis were unsuc-
cessful thus far.
[*] Dr. H. C. L. Abbenhuis, Dr. M. D. Skowronska-Ptasinska,
M. L. W. Vorstenbosch, Prof. Dr. R. A. van Santen
Schuit Institute of Catalysis
Eindhoven University of Technology
P. O. Box 513, 5600 MB Eindhoven (The Netherlands)
Fax : (‡31)40-24-55-05-4
E-mail: H.C.L.Abbenhuis@chem.tue.nl
Vinyl-bearing metallosilsesquioxanes readily undergo plat-
inum-catalyzed hydrosilylation,[11] thus for the immobilization
[**] We thank Degussa/Creavis GmbH for financial support.
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of 2, linear methylhydrosiloxane dimethylsiloxane copoly-
mers were chosen as a support. These copolymers are
commercially available over a wide range of molecular weight
and reactive silane content. Both 2a and 2b can be smoothly
grafted on these copolymers, followed by cross-linking with
the vinyl-terminated polydimethylsiloxanes DMS-V05, also
conveniently available (Scheme 1), to form brittle organosili-
ceous materials 3.
508C). From these experiments, it is clear that only grafted
titanium derivatives yield truly heterogeneous catalysts. After
reaction times of 24 h, 73 and 55% conversion of cyclooctene
to cyclooctene epoxide was found for 3a and 3b, respectively,
with >95% selectivity. For comparison, a material 4b related
to 3b resulting from a known, non-vinyl derivative [(c-
C6H11)7Si7O12TiOiPr],[6b] that is only physically ™trapped∫ in
the polysiloxane, was prepared by using the same polymer-
ization procedure as for 3b. For the system 4b, using the same
conditions as above, 61% conversion of cyclooctene to
cyclooctene epoxide was observed after 24 h. Hot filtration
experiments, however, reveal that only materials 3 are truly
heterogeneous catalysts, as the epoxidation stops when the
polymer is filtered out of the reaction mixture. In the case of
4b epoxidation proceeds further after filtration, due to
leaching of [(c-C6H11)7Si7O12TiOiPr] from the polymer.
Materials 3 were subsequently tested in the catalytic
epoxidation of neat cyclooctene with 35% aqueous hydrogen
peroxide (H2O2/alkene ˆ 1:4, 0.33 mol% Ti, 508C); the re-
sults are presented in Figure 1. Epoxide yields are based on
Figure 1. Heterogeneous epoxidation of cyclooctene with hydrogen per-
oxide catalyzed by 2 4; epoxide yields are based on the amount of
peroxide used.
the amount of peroxide used. From these experiments, it is
clear that only grafted-titanium derivatives yield active
catalysts in application with aqueous hydrogen peroxide.
Material 4b is less active than its fully heterogeneous
analogue 3b, despite having at least the same Ti content as
3b (Figure 1). The lower catalytic activity of 4b can be easily
explained by leaching of [(c-C6H11)7Si7O12TiOiPr] from the
polymer, the unsupported complex being inactive in epox-
idation with H2O2.
The reaction scope of the most active catalyst 3b was found
to cover n-alkenes and large cyclic alkenes. For cyclododecene
(H2O2/alkene ˆ 1:4, 0.33 mol% Ti, 508C) a maximum epoxide
yield of 45% was reached; the remaining portion of the
peroxide decomposed unproductively to O2. Catalytic epox-
idation with 3b of 1-octene was optimized to give the epoxide
in 62% yield in 1.5 h (H2O2/alkene ˆ 1:12, 2 mol% Ti, 808C);
this is equivalent to a turnover frequency of about 20 hÀ1.
Reported turnover frequencies for 1-octene epoxidation by
TS-1 vary from 4 to about 80 hÀ1 and generally increase
Scheme 1. Synthesis of three-dimensionally netted polymeric catalysts 3.
The first step of the hydrosilylation of 2 with the methyl-
hydrosiloxane dimethylsiloxane copolymer HMS-301 can be
monitored by 1 H NMR and 13C NMR spectroscopy. Already
half an hour after addition of the Pt hydrosilylation catalyst
(ppm amounts) to the reaction mixture of 2 and HMS-301 in
deuterated benzene (at the concentrations applied for poly-
mer preparation), resonances assignable to vinyl groups have
disappeared in both 1H NMR and 13C NMR spectra, and
simultaneous changes of the cyclohexyl methine carbon
pattern of the silsesquioxanes have taken place as observed
by 13C NMR spectroscopy.
Catalytic activity of materials 3 was first studied in the
epoxidation of cyclooctene with equimolar amounts of tert-
butylhydroperoxide (TBHP, 1.8m in isooctane, 0.33 mol% Ti,
638
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significantly with increasing alkene/H2O2 ratios.[12] This is also
valid for our systems. Since the synthesis of catalysts 3 allows
an interesting degree of freedom with regard to support
properties, such as tether length and the nature of the
immobilized silsesquioxane complex, more active catalysts
may result from further optimization. Such work is currently
in progress together with comparative experiments using
EURO TS-1.[12]
The results presented demonstrate that grafting of func-
tionalized titanium silsesquioxanes on polysiloxanes provides
a way to realize the formation of a catalytic ensemble that is
capable of performing epoxidation with aqueous hydrogen
peroxide. Clearly, the entire system is capable of outperform-
ing the sum of its parts; it is the synergy between active site
and its environment that allows the catalysts presented here,
and likewise TS-1, or even metalloenzymes[13] to achieve their
desirable performances.
mixture was stirred for additional 15 min, poured into a Petri plate to
evaporate the solvent and cured at 808C overnight. The brittle polymer was
ground, and continuously extracted with diethyl ether for two days to
remove ungrafted titanium. After filtration and drying polymer 3a (5.1 g)
was obtained. Elemental analysis: calcd: Ti 5.5; found: Ti 5.1 mggÀ1. The
same procedure was applied using 2b (720 mg) to achieve the same initial
Ti content (5.5 mg TigÀ1 of reacting components) to give the polymer 3b
(4.8 g).
Alkene epoxidation: The epoxidation tests were carried out in 2mL stirred
batch reactors by using 3 (ca. 60 mg). For epoxidation with TBHP, TBHP
(1.8 mmol) and cyclooctene (1.8 mmol) in isooctane were used. Epoxida-
tion with aqueous H2O2 was carried out using H2O2 (35% aqueous
solution) and neat alkene in the molecular ratios specified above. Excess of
alkene is used here to suppress unproductive decomposition of H2O2 to O2.
Samples taken from the reaction mixtures were analyzed by GC analysis
using a Carlo Erba GC6000 Vega Series apparatus equipped with a
capillary DB-1 collumn and a FID (flame ionisation detector). For this
purpose all samples contained 1,3,5-trimethylbenzene (25 mL; >98%,
Merck) as GC internal standard. Epoxide yields are based on the amount of
peroxide used.
Received: April 17, 2001
Revised: November 29, 2001 [Z16952]
Experimental Section
ˆ
Synthesis of [(H2C CH)(c-C6H11)6Si7O12TiCp] (2a): [CpTiCl3] (0.658 g)
[1] G. Strukul, Catalytic Oxidations with Hydrogen Peroxide as Oxidant,
Kluwer, Dordrecht, 1992.
[2] G. Perego, M. Taramasso, B. Notari (SNAM Progetti S. p. A.) BE
886812, 1981 [Chem. Abstr. 1981, 95, 206272k].
ˆ
was added to [(H2C CH)(c-C6H11)6Si7O9(OH)3] (2.75 g, 3.00 mmol) in
toluene (35 mL). Then an excess of dry pyridine (1.5 mL) was added to the
mixture, which was stirred for 1 h at room temperature, filtered, and the
filtrate was concentrated to about 10 mL. After addition of acetonitrile
(10 mL), 2a (2.35 g, 76% yield) was isolated as white crystals. 1H NMR
[3] B. Notari, Catal. Today 1993, 18, 163.
[4] a) G. Bellussi, M. S. Rigutto, Stud. Surf. Sci. Catal. 1994, 85, 177;
b) A. H. Joustra, W. de Bruijn, E. Drent, W. G. Reman, EP 0345856
(Shell) 1989 [Chem. Abstr. 1990, 113, 23675q]; c) S. Bordiga, S.
Coluccia, C. Lamberti, L. Marchese, A. Zecchina, F. Boscherini, F.
Buffa, F. Genoni, G. Leofanti, G. Petrini, G. Vlaic, J. Phys. Chem.
1994, 98, 4125; S. Bordiga, S. Coluccia, C. Lamberti, L. Marchese, A.
Zecchina, F. Boscherini, F. Buffa, F. Genoni, G. Leofanti, G. Petrini, G.
Vlaic, J. Phys. Chem. B 1998, 102, 6382.
[5] a) B. Notari, Adv. Catal. 1996, 41, 253; b) U. Romano, A. Esposito, F.
Maspero, C. Neri, M. G. Clerici, Chim. Ind. 1990, 72, 610; b) W. M.
Meier, D. H. Olson, Atlas of Zeolite Structure Types, Butterworth-
Heinemann, London, 1992.
[6] a) F. J. Feher T. A. Budzichowski, Polyhedron 1995, 14, 32 39; b) T.
Maschmeyer, M. C. Klunduk, C. M. Martin, D. S. Shephard, J. M.
Thomas B. F. G. Johnson, Chem. Commun. 1997, 1847; c) M. Crocker,
R. H. M. Herold, A. G. Orpen, M. T. A. Overgang, J. Chem. Soc.
Dalton Trans. 1999, 3791.
ˆ
(CDCl3, 400 MHz): d ˆ 6.53 (s, 5H; C5H5), 6.0 (m, 3H; CH2 CH-), 1.75 (m,
30H; CH2-C6H11), 1.26 (m, 30H; CH2-C6H11), 0.76 (m, 6H; CH-C6H11);
13C NMR (CDCl3): d ˆ 134.89 (CH2-vinyl), 130.97 (s, CH-vinyl), 116.26 (s,
CH-Cp), 27.64, 27.57, 27.55, 27.52, 27.11, 27.01, 26.93, 26.89, 26.83, 26.77, 26.65
(s, CH2-C6H11), 23.43, 23.22, 23.20, 23.15 (s, CH-C6H11, ratio 2:1:1:2); 29Si
NMR (CDCl3): d ˆ À66.14, À68.69, À69.33, À69.52(s, Si-CH, 2:1:2:1
ratio), 78.01 (s, Si-vinyl); elemental analysis calcd (%) for C43H74O12Si7Ti: C
50.26, H 7.26; found: C 50.19, H 7.37.
ˆ
Synthesis of [(H2C CH)(c-C6H11)6Si7O12TiOiPr] (2b): [Ti(OiPr)4]
(0.91 mL, 3.00 mmol)) was added to [(H2C CH)(c-C6H11)6Si7O9(OH)3]
(2.75 g, 3.00 mmol) in hexane (25 mL). The mixture was stirred at 508C for
ˆ
1.5 h and the solvent was evaporated in vacuo to yield 2b (3.03 g) as a crude
white powder. 1 H NMR (CDCl3, 400 MHz): d ˆ 6.06 (m, 2H; CH2 ), 5.89
ˆ
ˆ
(m, 1H; CH ), 4.62(m, 1H; C H(CH3)2), 1.77 (br m, 30H; CH2-C6H11), 1.27
(m, 36H; CH2-C6H11 and CH3-OiPr), 0.81 (m, 6H; CH-C6H11); 13C NMR
(CDCl3): d ˆ 135.69 (s, CH2-vinyl), 130.03 (s, CH-vinyl), 79.51 (s,
CH(CH3)2), 27.52, 26.88, 26.83, 26.75, 26.70, 26.66 (s, CH2), 25.65 (s, CH3,
OiPr), 23.22, 23.19, 23.14, 23.09 (s, CH, 1:1:2:2 ratio); 29Si NMR (CDCl3):
d ˆ À67.20, À68.60, À69.36, À69.53 (s, Si-CH, 2:1:2:1 ratio), À78.96 (s, Si-
vinyl); elemental analysis calcd for C41H76O13Si7Ti: C 48.21, H 7.50; found:
C 49.05, H 7.82.
[7] H. C. L. Abbenhuis, S. Krijnen, R. A. van Santen, Chem. Commun.
1997, 331.
[8] K. I. Alder, D. C. Sherrington, J. Mater. Chem. 2000, 10, 1103.
[9] F. J. Feher, R. Terroba, J. W. Ziller, Chem. Commun. 1999, 2153.
[10] I. E. Buys, T. W. Hambley, D. J. Houlton, T. Maschmeyer, A. F.
Masters, A. K. Smith, J. Mol. Catal. 1994, 86, 309.
Immobilization of 2a and 2b in polysiloxane (3): Compound 1a (726.9 mg)
dissolved in toluene (3 mL) was added to methylhydrosiloxane dime-
thylsiloxane copolymer (3.0 g; HMS-301, Gelest) diluted with toluene
(4 mL) and platinum divinyltetramethyldisiloxane complex in xylene
(20 mg; Gelest). The mixture was stirred at room temperature for 0.5 h,
and then the Pt catalyst (20 mg) and vinyl-terminated polydimethylsiloxane
DMS-V05 (2.40 g) dliuted in hexane (3 mL) was added. The reaction
[11] K. Wada, M. Bundo, D. Nakabayashi, N. Itayama, T. Kondo, T.
Mitsudo, Chem. Lett. 2000, 628.
¬
[12] a) S. Krijnen, P. Sanchez, B. T. F. Jakobs, J. H. C. van Hooff, Micro-
porous Mesoporous Mater. 1999, 31, 163; b) M. G. Clerici, P. Ingallina,
J. Catal. 1993, 140, 71.
[13] J. P. Collman, Science 1993, 261, 1404.
Angew. Chem. Int. Ed. 2002, 41, No. 4
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639