Molecules cyclic silanes

An important property of the alkyl poly silanes is their electronic absorption at relatively low energy. This was particularly surprising because such molecules lack tt, d ot lone pair electrons they were the first substances containing only bonding cr-electrons to show such long-wavelength absorptions. The unusual electronic properties of the polysilanes have sparked considerable interest in the chemophysical properties of these molecules. The UV spectra of cyclic polysUanes follow a quite different pattern . 

Host 24 also stabilises highly reactive cyclic silanol oligomers by a similar in situ synthesis [66]. Phenyltrimethoxysilane, 26, rapidly forms polymeric material in aqueous solutions. The presence of 24 prevents this from occurring and isolates the cyclic trimer intermediate, 27. Several molecules of the silane starting... 

By carbosilanes we mean compounds with alternate carbon and silicon atoms in the molecule these may be linear (e.g., Cl3SbCH2-SiCl2-CH2-SiCl3) or cyclic [e.g., (SiCh-CfB ], the latter being termed cyclocarbo-silanes (23). Compounds which consist mainly of carbon atoms, between which a silicon atom is inserted as a hetero-atom, are thus excluded from consideration here. So, too, are polymeric carbosilanes, which have recently been reviewed elsewhere (59a). 

A niimber of B-N-Si cyclic systems have been prepared for the first time, by a variety of routes, chiefly (i) the reaction of N-lithio-aminoboranes with bis-(chlorosilylamines), (ii) transamination of (aminoborylamino)silanes, and (iii) condensation of Me2Si(NHMe)2 and alkylbis(dimethylamino)boranes. H, and n.m.r. data suggest that the silaborazine ring in, c.g., (39) is non-planar/ " The crystal structure of (40) shows that the molecule possesses Da symmetry. 

Only a few papers on silyl derivatives of heteroaromatics, e.g. indoles and pyrroles, have been published [1-7], After the synthesis of mono-, bis-, tris-, and tetralds(indol-l-yl)silanes and (pyrrol-l-yl)silanes, this chemistry is extended to furan compounds [7, 8]. A further interest is to couple the heterocycles with bridging silyl groups to obtain chains and cyclic molecules. 

The structure of the physisorbed silane molecules tends to be cyclic oligomers, and their molecular weight strongly depends on the pH of the filler (22). The pH of the filler is conveniently measured by the sliury pH method where a certain amount of filler is boiled in water for a specified time and the pH of the liquid measured. This method provides composite information about both the solubility of the selective surface components as well as the concentration of the siuface acid/alkaline centers. This sluny pH directly relates to the molecular weight of the conden silane rather than the concentration of the surface acid centers alone, since the solubility of the surface species and the concentration of acid centers both contribute to the pH of the silane solution that is on the filler surface. 

The main advantage of using silyl ethers in cross<oupling reactions is the ability to incorporate them into molecules by a number of methods. Cyclic silyl ethers, as a class, nicely illustrate this attribute. The well-known hydrosilylation of alkynes to form vinylsilanes can easily be rendered intramolecular by attachment of the silane as, for example, a homopropargyl silyl ether to form an oxasilacyclopentane 108 (Scheme 7.28) [53]. In this stmcture, the double-bond geometry is defined by the stereochemical course of hydrosilylation and the ether tether defines the location of the silicon atom with respect to the alkene. Thus, the siHcon-oxygen bond in this molecule serves to direct the hydrosilylation, as well as to activate the siHcon for cross-coupling. 

Silane-substituted vinyl aluminum reagents can also be added to five- or six-membered cyclic enones. The silane moiety in these molecules presents an excellent opportunity for further functionalization to synthesize useful chiral molecules. 

A cyclic trimer 409 (Scheme 5.1) has been obtained in [1] by condensation of Irialkoxy silane guest molecules encapsulated within the nanosized cavity of a self-assembled coordination capsule 488. If the free trimer 4W is a kinetic and short-lived product rapidly converting to the corresponding thermodynamically favored cyclic tetramer and other condensed products, its caged molecule is remarkably stable and remains intact over a long time at room temperature in both neutral aqueous solutions and acidic media (pH <1) [1]. 

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