Silanols reaction with

Scheme 4 Model silanol reaction with acryloxymethylacryloxydimethylsilane...

Two-part room temperature condensation cure. Silicone can be formulated into two-part systems [3,12,14,33] that prevent the reactive groups from coming into contact before they are needed. The reactions in these systems are based on the condensation of a silanol group with an alkoxy silane group, catalyzed by organo-tin compounds (Scheme 9). 

While structurally equivalent, they usually differ from a reactivity point of view. Firstly, the organometallic reagent, after reaction with the surface, readily extrudes extra ligands upon reaction with surface silanols, while they can give a stable adduct in solution. 

Photolytic generation of silene Me3Si(Me)C=SiMe2 in the presence of an excess of D20 or a trace of H20 affords disiloxanes 25, presumably via condensation of the silanol 26 in the case of D20, or by reaction of the silanol 27 with further silene in the case of H20 (Scheme 7) (138, 139). 

Many other methods have been used to prepare bonded phases these include esterification of the surface silanol groups with alco-Tiols, or conversion of the silanol groups to Si—Cl using thionyl chloride, followed by reaction with an organometallic compound. If you are interested, there are details in the textbooks by Knox or by Hamilton and Sewell. 

Transition metal alkyl compounds react with the -OH groups on the surface of silica in a manner similar to that described for the silanol [reaction (13)] and as with the latter more than one type of bonding is possible. Silica dried at 200°C reacts with Zr(allyl)4 to give two molecules of propene per metal atom and utilizing in the course of this process two -OH groups per metal atom. The chemistry of the process is accurately described by the equation... 

Fig. 4 (a) Chemical structure of tetra-ethyl-ortho-silicate (TEOS). (b) Removal of two oxygen atoms from TEOS. (c) Mechanism depicting how TEOS chemisorbs onto silanol groups (Si-OH) on surface, (d) Formation of Si-O-Si bridges by elimination reactions with neighboring molecules. (Figure adapted from [40])... 

On the basis of available experimental data, it is impossible to choose a definite pathway of elimination of silanol. However, study of silylation of methyl P -nitropropionate (411) with BSA in the presence of trapping agents rigorously proved that silyl nitronate D is initially formed. This compound can be detected in the [3 + 2]-cycloaddition reaction with methyl acrylate product (413). If silylation of AN (411) is performed in the presence of ethyl vinyl ether, a-nitrosoalkene E can be successfully trapped in as heterodiene a Diels-Alder reaction. Dihydroox-azine (414) is formed, and its silylation affords isolable product (415). 

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