Silanetriols preparation

R. Murugavel, A. Voigt, M. G. Walawalkar, H. W. Roesky, Silanetriols Preparation and Their Reactions, in Organosilicon Chemistry III From Molecules to Materials (N. Auner, J. Weis), VCH, Weinheim, 1998, p. 376. 

It is unlikely that T2R2 compounds can be prepared by the usual hydrolytic routes to siloxanes, but it might be possible to prepare one by dehydration of a very sterically hindered silanetriol, RSi(OH)3 which can readily be isolated, ... 

As mentioned above, hydrolysis of bulky RSiCls precursors tends to give a variety of silanols, these may be isolated and used for the preparation of Tg derivatives. Thus, simple silanetriols RSifOHls or disiloxanetetraols [RSi(OH)2l20 (R = fBu, fHexyl, or 2,4,6-iPr3C6H2) may be dehydrated and... 

Stable silanetriols such as Me3CSi(OH)3 or 2,4,6-tris(tert-butyl)phenylsilanetriol have also been prepared and reviewed [49]. 

The increasing tendency of silanols to condense as the number of OH groups on a particular silicon increases means that relatively few silanetriols have been isolated and even fewer structurally characterized. A wide range of substituted arylsilanetriols have been prepared (102), but none have been studied by x-ray crystallography. The first silanetriol to be structurally characterized was c-C6HuSi(OH)3, which has a sheet structure comprising double layers of molecules joined in a... 

Individual examples of monomeric silanols such as triethylsilanol [10] and phenylsilanetriol [II] have been prepared under regimens different from those used for surface treatments and they exhibit extended stability. To date, no monomeric silanetriols have been isolated from aqueous hydrolysates of alkoxysilanes. The kinetics of silanetriol condensation have been studied [12]. The conditions which promote the hydrolysis of alkoxysilanes also promote condensation of silanols the persistence of monomeric silanetriols for more than a few hours in typical solutions is unlikely. However, the persistence of silanols in reaction mixtures containing condensed structures have been observed empirically [ 13] and by JVSi-NMR [ 14,15],... 

Silanetriols eluded all attempts at their isolation until a few years ago when Tyler (88) prepared phenylsilanetriol from phenyltrimethoxysilane and dilute acetic acid at about 10°. More recently Takiguchi (82) obtained this substance from phenyltrichloiosilane in the presence of aniline. Andrianov, Zhdanov and Morganova (7) obtained dichloro-phenylsilanetriol by hydrolyzing the triacetoxysilane. The interesting tetrol, bis-(dichlorophenyl)-disiloxanetetrol, was similarly obtained from bis-(dichlorophenyl)-tetraacetoxydisiloxane. 

The synthesis of hexakis(cyclohexylsilsesquioxane) in 10% yield by the treatment of cyclohexyltrichlorosilane with water was first reported in 1965108. The X-ray structure of hexakis(cyclohexylsilsesquioxane) was determined by Molloy and coworkers in 1994109. The hexasilsesquioxanes (80) substituted by /-butyl or 1,1,2-trimethylpropyl groups are prepared by condensation of the corresponding silanetriol (78) or tetrahydroxysiloxane (79) using dicyclohexylcarbodiimide (DCC) as a dehydrating reagent in DMSO or DMF (Scheme 20)no. 

Novel silanetriols containing a nitrogen substituent at silicon can be prepared by hydrolysis if the substituents are large enough to prevent hydrolysis of the Si-N bonds present. This has been elegantly demonstrated by Roesky and coworkers27-30 who have shown that the stable silanetriols 1 can be prepared in 70-88% yields according to equation 3. 

Silanetriols have been useful building blocks for the preparation of three-dimensional metallasiloxanes. The presence of a metal in the metallasi-loxane framework not only makes these compoimds thermally stable but also improves their catalytic properties. Similarly, silica surfaces act as hosts for numerous transition metal complexes, which are known to catalyze a variety of organic transformations (43). 

The use of large, nitrogen-centered substituents at a silanetriol center does allow the isolation and characterization of stable sUanetriols as shown in equation (24). These compounds can be used to prepare transition metal sUoxide complexes (Scheme 34), still containing potentially moisture-sensitive Si N bonds that may be precursors to metal-containing ceramics for reviews of these compounds, see References 193-195. 

Towards this end, we have been recently successful in synthesizing a series of silanetriols with diverse substituents and studying their chemistry in detail. The various sections of this article summarize the following aspects of silanol chemistry (a) various synthetic routes available for the preparation of soluble and stable silanetriols starting from commonly available starting materials, (b) common spectroscopic and structural properties of silanetriols, (c) reactions of silanetriols with various metal precursors leading to the preparation of novel polyhedral metallasiloxanes, (d) structures of metallasiloxanes, and (e) possible applications. 

The preparation of stable silanetriols of the type RSi(OH)3 has proven to be difficult, compared to the other types of silanols, in view of their well known tendency to self-condense and result in siloxane rings and polymers [9]. However, in recent years, the use of very bulky R groups on silicon and carefully chosen experimental conditions has led to the isolation of many stable silanetriols. While silanetriols such as [Pg.377]

In 1955, Tyler reported the preparation of the first silanetriol, PhSi(OH)3 (1) starting from phenyltrimethoxysilane [10]. The acid assisted hydrolysis of PhSi(OMe)3 at 10 °C resulted in the isolation of phenylsilanetriol in 75% yield (Scheme 1). This compound was found to be very unstable and decomposes on heating. Acid or base impurities also lead to the decomposition of this compound. Later, this synthetic methodology was used by other workers [11,12] to prepare a series of ortho-, meta-, and para-substituted phenylsilanetriols (2) starting from the respective aryltrimethoxysilanes (Scheme 1). Similarly, the hydrolysis of cyclohexyltrimethoxysilane in the presence of acetic acid leads to the isolation of (c-C6Hn)Si(OH)3 (3) [13] (Scheme 1) whose crystal structure has been later determined by X-ray diffraction studies [14]. 

Silanetriols can also be prepared by amine-assisted hydrolysis of the respective trichlorosilanes. Phenylsilanetriol was thus prepared by Takiguchi by the hydrolysis of PhSiCb with stoichiometric amounts of water in the presence of aniline as the HCl acceptor (Scheme 1) [15]. This mild synthetic procedure later proved to be very useful in the preparation of the tri-/-butoxysiloxysilanetriol (fBuO)3SiOSi(OH)3 (4) [16]. 

We have also prepared the 0-bonded silanetriol (2,4,6-rBu3C6H2)OSi(OH)3 (18) starting from 2,4,6-tri-r-butylphenol (Scheme 4) [27]. However, the synthesis of other O-bonded silanetriols proved to be difficult due to the ready cleavage of the Si-O(aryl) bond... 

Hydroformylation reactions are important from the industrial point of view and the two commonly used hydroformylation catalysts are either Rh or Co based. We thought it would be interesting to anchor a SiOs unit on a cobalt cluster via hydrosilylation. This would be a close model to a silica-supported cobalt cluster. Secondly, since the reactions of silanetriols have been demonstrated to afford three-dimensional metallasiloxanes, we anticipated that this silanetriol would react with substrates such as trialkylaluminums, affording cobalt carbonyl cluster anchored aluminosiloxanes. Such compounds would resemble a modified zeolite having on its surface catalytically active cobalt carbonyl moieties and might inspire the preparation of actual zeolite systems with these modifications. 

All the compounds i, ii, and in were isolated during the years 1912- 1914, the last two being the first cyclic silicon polymers to be characterized. The first dialkylsilanediol was prepared in 1946 by the hydrolysis, by weak alkali, of (C2Hs)2SiCl2 it is a stable crystalline solid. Later, (CH3)2Si(OH)2 was prepared by the hydrolysis of (CH3)2Si(OCH3)2 as a crystalline solid extremely sensitive to acid or alkali, which cause condensation to polysiloxanes, mostly the cyclic tri- and tetra-compounds. Other diols, prepared include the n- and iso-propyl and butyl, and tert-butyl compounds, in addition to tetramethyldisiloxane-l,3-diol (iv). No silanetriol RSi(0H)3 has been isolated. 

In fact, these cyclic trimers may be incorporated into silicone resins of the type employed in surface coatings. In such products, a silanetriol is incorporated to cross-link the resin (this cross-linking reaction has been previously described) and to allow the presence of excess hydroxyl groups after resin preparation. These materials containing excess hydroxyl groups are then applied to a substrate as a surface coating and subsequently cured via a thermal cycle of up to 60 min at 500 °F. When catalyzed, their cure cycle may be reduced to 30-60 min at 400 °F. Some typical catalysts and the levels at which they are usually employed are listed in Table II. 

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