Free-radical reactions hydrosilylation

Hydrosilylations of fluorine-containing alkenes are free radical reactions initiated by UV light or organic peroxides The direction of addition is the same as with fluonnated alkyl halides However, the reaction between hydrosilanes and fluorine-containing olefins catalyzed by platinum group metal complexes may result in bidirectional addition and/or formation of a vinylic silane, the latter by de hydrogenative silylation[/] The natures of both the silane and the catalyst affect the outcome of the reaction[7] A random selection of some typical new reactions of silanes are shown in Table 1 [1, 2, 3 4]... 

SiHcone mbber has a three-dimensional network stmeture caused by cross-linking of polydimethjlsiloxane chains. Three reaction types are predominantly employed for the formation of siHcone networks (155) peroxide-induced free-radical processes, hydrosilylation addition cure, and condensation cure. SiHcones have also been cross-Hnked using radiation to produce free radicals or to induce photoinitiated reactions. 

Free-Radical-lnitiated Hydrosilylation. In early reports, it was suggested that addition of hydrosilanes to multiple bonds could proceed as free-radical chain process because of a relatively low energy of the Si—H bond in comparison with that of the C—H bond. For this reason, many synthetic and mechanistic studies have been devoted to the hydrosilylation of carbon-carbon (C=C, CM])) bond initiated by free radicals generated in the reaction mixture (3,6,10,16). Free-radical addition of hydrosilanes resembles the addition of hydrogen bromide to alkenes and always occurs according to the anti-Markownikov rule (3). 

In 1993, Linford firstly reported a quite useful method to prepare monolayers of alkyl chains by thermal hydros-ilylation of hydrogen-terminated silicon surfaces [25]. Alkyl chains are covalently bound to Si surface by Si-C bonds. This thermal hydrosilylation could be attributed to a free-radical process with 1-alkene. First, a diacyl peroxide initiator was used to produce free radicals. However, at higher temperature, only hydrogen-terminated silicon and a neat solution of 1-alkene or 1-alkyne can form Si-C linkages [26]. Furthermore, lately it is found that such Si-C covalent links can be observed even in dilute solutions of 1-alkenes [27]. In that case, the density of monolayer packing strongly depends on the reaction temperature. 

PhSeSiRs reacts with BusSnH under free radical conditions and affords the corresponding silicon hydride (Reaction 1.8) [19,20]. This method of generating RsSi radicals has been successfully applied to hydrosilylation of carbonyl groups, which is generally a sluggish reaction (see Chapter 5). 

Alkene isomerization is common during hydrosilylation. At partial conversion in the reaction of ethylcyclohexenes, the recovered olefin contained all possible isomers except vinylcyclohexane.426 Even in the transformation of the most reactive terminal alkenes, the recovered unreacted olefin is a mixture of isomers. Isomerization thus may prevent complete conversion of 1-alkenes. Similar doublebond migration is not observed in free-radical addition 428... 

TMS)sSiH adds across the C=C and C=0 double bonds of a variety of compounds under free-radical conditions. The propagation steps for these hydrosilylation processes are reported in equations 26 and 27. The available rate constants for the reaction of (TMS Si radicals with some ketones and alkenes (equation 26) are collected in Table 3. In the ketone series, the rate constants decrease in the series of quinone > diaryl ketone > dialkyl ketone50. On the other hand, the rate constants for the addition of (TMS)3Si radical to activated alkenes21 are close to 108 M 1 s 1. 

The addition of (TMS SiH across carbon-carbon multiple bonds under free-radical conditions is well documented82. Although no recent reports of such hydrosilylation processes are reported, the addition of (TMS Si radical to multiple bonds, followed by other radical reactions, were investigated (vide infra). The hydrosilylation of ketones and aldehydes is also well known83. In this respect Brook and coworkers have recently shown that the (TMS)3Si group can be used for the protection of primary and secondary alcohols84. 

The ability of these macromonomers to participate in free-radical copolymerization was not tested. They were used28b) for the synthesis of tri- and multiblock copolymers by means of hydrosilylation, i.e. an addition reaction between vinylsilyl groups and the —Si — H functions at the chain end of poly(dimethylsiloxane)mole-cules. 

Hydrosilylation can also be initiated by a free-radical mechanism (227—229). A photochemical route uses photosensitizers such as peresters to generate radicals in the system. Unfortunately, the reaction is quite sluggish. In several apphcations, radiation is used in combination with platinum and an inhibitor to cure via hydro silylation (230—232). The inhibitor is either destroyed or deactivated by uv radiation. 

On the other hand, the results of the hydrosilylation reactions using different styrene-silane ratios for a reaction model indicate that the most adequate proportion was two mol of styrene for 1.5 mol of silane, and it was selected for ulterior experiments. The use of lower ratios than the above yields large quantities of disiloxane which is probably produced by a free-radical, non metal-catalysed, reaction pathway. 

Kim et al. [46,47] reported the synthesis of fluorosilicone block copolymers of poly(perfluoroalkylethyl acrylate)-fc-poly(3-[m s(trimethylsilyloxy)-silyl] propyl methacrylates) (PFA-i>-PSiMAs) by a three-step synthetic approach. In the first step, a PFA macromonomer (PFAM) was made by free radical polymerization. Thereafter, a condensation reaction was applied to prepare the PFAM initiator (PFAMI). Finally, the PFAMI and SiMA were reacted to prepare the PFA-i>-PSiMAs block copolymers. In early studies, synthesis of fluorosilicone block copolymers was reported by Boutevin et al. [48-50]. However, two-step hydrosilylation was carried out to prepare the photo-cross-linkahle fluorinated PDMS as reported by Boutevin et al. [48]. In another study, Luo et al. [51] prepared poly(dimethylsiloxane)- -poly(2,2,3,3, 4,4,4-heptafluorobutyl methacrylate- -poly(styrene)... 

---