Alkyl derivatives alkylsilane

Figure 3.29 Synthesis of a PMO derivative with the use of a siloxane-based oligomer that consists of an alkylsilane nucleus (white) and three branching trimethyoxysilyl groups (light grey) This precursor acts simultaneously both as organosilica source and surfactant. The alkyl chain length (n) determined the phase of the composite for n = 10a2Dhexagonalphaseisformed for n = 16, a lamellar phase is obtained. This approach can also be employed as co-condensation reaction together with TMOS.

As a general mle, unless an anion-stabilizing group, such as phenyl, or a heteroatom such as sulfur is present, the alkylsilane is not readily deprotonated. The a-halosilane can be deprotonated but, unlike the readily available chloromethyltrimethylsilane, there are few general methods to this approach. Al-kyllithium reagents add to vinylsilanes ( ) to produce the carbanion (287). Silyl derivatives with heteroatoms, such as sulfur, selenium, silicon or tin, in the a-position (288) may be transmetallated (Scheme 41). Besides the difficulty in synthesizing the anion, alkene formation lacks specificity for simple di- and tri-alkyl-substituted alkenes. As a result, the Peterson reaction of an a-silyl carbanion with a carbonyl has found the greatest utility in the synthesis of methylene derivatives, (as discussed in Section 3.1.3), heterosubstituted alkenes and a,p-unsaturated esters, aldehydes and nitriles. 

Following this oxidative addition, insertion of the olefin or alkyne occurs into the platinum-hydride bond to form an alkyl or vinyl hydride complex. The regioselectivity of the insertion step and the chemistry of the alkyl silyl complexes control the overall regioselectivity of the hydrosilylation of olefins. Recall that these platinum catalysts form terminal alkylsilane products. This regioselectivity indicates that insertions of terminal olefins occur in order to generate a linear alkylplatinum intermediate. Apparently, the insertions of styrene and acrylic acid derivatives into the platinum hydride in these catalysts occur with the same regiochemistry. 

HP-HIC sorbents have been described, including those derived from polyethersi-lanes containing w-alkyl groups of different ligand chain length and hydropho-bicity [73] or from Af-acetyl-amino-w-alkylsilanes [274]. 

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