Octavinyloctasilsesquioxane: Synthesis and Derivatives
Octavinyloctasilsesquioxane is among the most versatile and widely studied polyhedral oligomeric silsesquioxane (POSS) platforms in hybrid materials chemistry. Its eight peripheral vinyl groups, arranged symmetrically about the cubic silsesquioxane core, consistent with the T8 cage classification, provide an exceptionally rich surface for chemical functionalization through reactions characteristic of terminal alkenes, enabling the rational synthesis of dendrimers, polymer networks, glycoclusters, and metal-coordinating ligands. This article reviews how octavinyloctasilsesquioxane (octavinyl POSS, OVS) is prepared and surveys the principal routes to its derivatives — thiol-ene and radical phosphine addition, epoxidation, platinum-catalysed hydrosilylation, palladium-catalysed Heck coupling and ruthenium-catalysed olefin metathesis — together with the architectures each of them delivers.
Synthesis and Historical Development
Octavinyloctasilsesquioxane was first prepared in 1978 by K. A. Andrianov and co-workers through the hydrolytic polycondensation of vinyltrichlorosilane, albeit with a yield of less than 10%.1 Subsequent efforts by numerous research groups sought to improve the synthetic methodology.2,3,4 In 1997, P. G. Harrison and C. Hall reported the preparation of (CH2=CHSiO1.5)8 in 30% yield through the hydrolysis of CH2=CHSiCl3 in ethanol.5 The use of an ion-exchange resin as a catalyst for the hydrolysis and condensation of CH2=CHSiCl3 further improved the reaction yield to 40%.6 A substantially higher yield of 80% was subsequently achieved by employing CH2=CHSi(OEt)3 as the silicon precursor in the presence of tetramethylammonium hydroxide as a phase-transfer catalyst.7
Functionalization via Alkene Reactivity
The chemical versatility of octavinyloctasilsesquioxane derives directly from the reactivity of its eight terminal vinyl groups toward a broad range of transformations applicable to alkene substrates. Thiol-ene functionalization with reagents such as thiophenol or cyclohexanethiol can be accomplished using azobisisobutyronitrile (AIBN) as a radical initiator or through irradiation with ultraviolet light, affording thioether-functionalized POSS derivatives bearing sulfur-containing peripheral arms.8,9,10,11 Radical addition of phosphines and phosphonates, including diethylphosphine or diethyl phosphate, to the vinyl periphery produces phosphorus-substituted silsesquioxanes that have been employed as coordinating ligands for transition metals, most notably rhodium.12 Oxidation of the vinyl groups with meta-chloroperoxybenzoic acid (m-CPBA) converts the terminal alkenes to epoxide moieties, and the resulting epoxy-functionalized silsesquioxane is capable of undergoing Lewis acid-initiated ring-opening polymerization to yield organic-inorganic hybrid composite materials. The sections that follow examine in more detail the three transformations that have proved most productive for building functional POSS architectures: hydrosilylation, Heck coupling and olefin metathesis.
Figure 1. Selected functionalization reactions of octavinyloctasilsesquioxane. AIBN: azobisisobutyronitrile; m-CPBA: meta-chloroperoxybenzoic acid.
Hydrosilylation of the Vinyl Periphery
Hydrosilylation reverses the logic of the two coupling reactions above: instead of extending the arms with carbon fragments, it adds silicon. The Si–H bond of a hydrosilane adds across the C=C bonds anchored at the corners of the cage, giving branched derivatives that carry additional silicon atoms in every side arm, with yields typically in the range of 70–80% (Figure 2). Such silicon-rich cubes are attractive precursors to silicone resins, crosslinkers for addition-cure elastomers, and intermediates on the way to higher-generation POSS dendrimers of the type discussed above.17
Figure 2. Hydrosilylation of octavinyloctasilsesquioxane with hydrosilanes. Conditions: Karstedt’s catalyst, Et2O.17
One practical caveat applies to every hydrosilylation of this type. The addition can proceed in two directions, giving either the linear α (anti-Markovnikov) product or the branched β (Markovnikov) product, and in silsesquioxane chemistry the outcome is usually a mixture of both, so that purification of the product is required. The same regiochemical question arises in the complementary, and synthetically equivalent, approach in which the Si–H groups sit on the cage — as in octakis(dimethylsiloxy)octasilsesquioxane — and the alkene is the external partner (Figure 3). Platinum catalysts, most often Karstedt’s complex, Pt(dvs), or hexachloroplatinic acid, are used in both directions because of their high activity and selectivity.18,19
Figure 3. Platinum-catalysed hydrosilylation of octakis(dimethylsiloxy)octasilsesquioxane with styrene, illustrating the α/β regiochemistry characteristic of silsesquioxane hydrosilylation. Conditions: Pt(dvs), toluene, 60 °C.19
Dendrimer Synthesis via Hydrosilylation
Platinum-catalyzed hydrosilylation of octavinyloctasilsesquioxane with di- and trichlorosilanes produces chlorosilyl-functionalized silsesquioxane building blocks suitable for the construction of dendrimeric architectures, an approach related to the solid-state polymerization of functionalized POSS monomers. Subsequent treatment of the chlorosilyl-functionalized intermediate with an appropriate Grignard reagent affords a POSS-based dendrimer bearing 24 terminal vinyl groups per cubic silsesquioxane core (Figure 4), representing a significant increase in peripheral functionality relative to the parent compound. This stepwise approach to POSS-cored dendrimers illustrates the capacity of the octavinyl scaffold to serve as a platform for the hierarchical construction of well-defined macromolecular structures with precisely controlled peripheral chemistry.12
Figure 4. Synthesis of a POSS-cored dendrimer bearing 24 terminal vinyl groups via platinum-catalyzed hydrosilylation and Grignard functionalization of octavinyloctasilsesquioxane.5
Heck Coupling with Aryl Halides
Palladium-catalysed Mizoroki–Heck coupling provides a direct route from octavinyloctasilsesquioxane to aryl-substituted cages without disturbing the siloxane core. Reaction with aryl bromides or iodides proceeds at the terminal vinyl groups and delivers mono- and disubstituted products, the distribution of which is governed by the stoichiometry of the haloaromatic: approximately eight equivalents of Ar–X favour the monosubstituted, stilbene-like derivative, whereas a sixteen-equivalent excess drives the system toward the disubstituted cage (Figure 5). Because the coupling tolerates extended aromatic systems — fluorene, naphthalene, pyrene and anthracene among them — it has become the method of choice for installing chromophores on the POSS periphery. The resulting organic–inorganic hybrids are photoluminescent and solution-processable, and have been applied as emissive and charge-transport layers in organic light-emitting diodes and related electroluminescent devices.13,14,15
Figure 5. Heck coupling of octavinyloctasilsesquioxane with aryl halides (X = Br, I), giving mono- or disubstituted derivatives depending on the number of equivalents of Ar–X.
Olefin Metathesis with Functionalised Alkenes
Ruthenium-catalysed cross-metathesis offers a complementary entry to the same family of chromophore-bearing cages. Coupling octavinyloctasilsesquioxane with styrenes and related alkenes carrying aromatic substituents affords products closely analogous to those obtained by Heck coupling, with the carbon–carbon double bond retained in the side arm and ethylene released as the only by-product (Figure 6). Substituted styrenes bearing halide or trifluoromethyl groups, as well as biphenyl, thiophene, naphthalene and anthracene units, have all been installed in this way under mild conditions and with high selectivity for the E isomer. The vinyl-functionalised cubic silsesquioxanes obtained have been proposed by their authors as building blocks for optoelectronic materials.16
Figure 6. Ruthenium-catalysed cross-metathesis of octavinyloctasilsesquioxane with functionalised alkenes (X = Cl, Br, CF3).
References
(1) Andrianov, K. A.; Petrovnin, N. M.; Vasil’eva, T. V.; Skhlover, V. E.; D’yanchenko, B. I. Hydrolytic Polycondensation of Higher Alkyltriethoxysilanes. Zh Obshch Khim 1978, 48, 2692–2695.
(2) Voronkov, M. G.; Martynova, T. N.; Mirskov, R. G.; Belyi, V. I. Octavinylsilsesquioxane. Russian Journal of Organic Chemistry 1979, 49, 1522–1525.
(3) Kovrigin, V. M.; Lavrent’ev, V. I. Chromatographic-Mass-Spectroscopic Study of the Mechanism of Formation of Pervinyloctasilsesquioxane in Polycondensation of Vinyltrichlorosilane in Butanol. Russian Journal of Organic Chemistry 1989, 59, 377–383.
(4) Bonhomme, C.; Tolédano, P.; Maquet, J.; Livage, J.; Bonhomme-Coury, L. Studies of Octameric Vinylsilasesquioxane by Carbon-13 and Silicon-29 Cross Polarization Magic Angle Spinning and Inversion Recovery Crosspolarization Nuclear Magnetic Resonance Spectroscopy. J. Chem. Soc., Dalton Trans. 1997, 9, 1617–1626.
(5) Harrison, P. G.; Hall, C. Preparation and Characterization of Octasilsesquioxane Cage Monomers. Main Group Metal Chemistry 1997, 20, 515–529.
(6) Dare, E. O.; Liu, L.-K.; Peng, J. Modified Procedure for Improved Synthesis of Some Octameric Silsesquioxanes via Hydrolytic Polycondensation in the Presence of Amberlite Ion-Exchange Resins. Dalton Trans. 2006, 30, 3668–3671.
(7) Gao, J.; Wang, S.; Zhang, X.; Run, M. Synthesis of Cage Octa(Vinyl)Silsesquioxane. Youjigui Cailiao 2005, 19, 5–7.
(8) König, H. J.; Marsmann, H. C.; Letzel, M. C. Thioether Functionalized Octasilsesquioxanes. In Organosilicon Chemistry V; Auner, N., Weis, J., Eds.; Wiley-VCH Verlag GmbH, 2003; pp 425–428.
(9) Gao, Y.; Eguchi, A.; Kakehi, K.; Lee, Y. C. Efficient Preparation of Glycoclusters from Silsesquioxanes. Org. Lett. 2004, 6, 3457–3460.
(10) Lücke, S.; Stoppek-Langner, K.; Kuchinke, J.; Krebs, B. Octakis-(Dimethylphosphanoethyl)-Octasilsesquioxane: Synthesis, Characterization and Reactivity. Journal of Organometallic Chemistry 1999, 584, 11–15.
(11) Piorecka, K.; Radzikowska, E.; Kurjata, J.; Rozga-Wijas, K.; Stanczyk, W. A.; Wielgus, E. Synthesis of the First POSS Cage-Anthracycline Conjugates via Amide Bonds. New J. Chem. 2016, 40, 5997–6000.
(12) Ropartz, L.; Morris, R. E.; Schwarz, G. P.; Foster, D. F.; Cole-Hamilton, D. J. Dendrimer-Bound Tertiary Phosphines for Alkene Hydroformylation. Inorganic Chemistry Communications 2000, 3, 714–717.
(13) Lo, M. Y.; Ueno, K.; Tanabe, H.; Sellinger, A. Silsesquioxane-Based Nanocomposite Dendrimers with Photo-Luminescent and Charge Transport Properties. Chem. Rec. 2006, 6 (3), 157–168.
(14) Lo, M. Y.; Zhen, C.; Lauters, M.; Jabbour, G. E.; Sellinger, A. Organic−Inorganic Hybrids Based on Pyrene Functionalized Octavinylsilsesquioxane Cores for Application in OLEDs. J. Am. Chem. Soc. 2007, 129 (18), 5808–5809.
(15) Sellinger, A.; Tamaki, R.; Laine, R. M.; Ueno, K.; Tanabe, H.; Williams, E.; Jabbour, G. E. Heck Coupling of Haloaromatics with Octavinylsilsesquioxane: Solution Processable Nanocomposites for Application in Electroluminescent Devices. Chem. Commun. 2005, No. 29, 3700–3702.
(16) Żak, P.; Marciniec, B.; Majchrzak, M.; Pietraszuk, C. Highly Effective Synthesis of Vinylfunctionalised Cubic Silsesquioxanes. J. Organomet. Chem. 2011, 696 (4), 887–891.
(17) Cheng, G.; Vautravers, N. R.; Morris, R. E.; Cole-Hamilton, D. J. Synthesis of Functional Cubes from Octavinylsilsesquioxane (OVS). Org. Biomol. Chem. 2008, 6 (24), 4662–4667.
(18) Sommer, L. H.; Pietrusza, E. W.; Whitmore, F. C. Peroxide-Catalyzed Addition of Trichlorosilane to 1-Octene. J. Am. Chem. Soc. 1947, 69 (1), 188.
(19) Sheen, Y.-C.; Lu, C.-H.; Huang, C.-F.; Kuo, S.-W.; Chang, F.-C. Synthesis and Characterization of Amorphous Octakis-Functionalized Polyhedral Oligomeric Silsesquioxanes for Polymer Nanocomposites. Polymer 2008, 49 (18), 4017–4024.
DOI: 10.1039/a700700k
Full text: Dalton Transactions → RSC Publishing
Octavinyloctasilsesquioxane is compared with the other cage building blocks, from the hydride to the metal-bearing derivatives, on the page overview of selected POSS compounds.
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Polyhedral Oligomeric Silsesquioxane (POSS) Chemistry. (2023). Octavinyloctasilsesquioxane: Synthesis and Derivatives. https://doi.org/10.59350/51kjv-fvt95
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