Hydrosilation SYNTHESIS

The hydrosilation of conjugated dienes is catalyzed by transition metal complexes34, and has been developed into a useful asymmetric synthesis of allylsilanes35,36. 

Recently, this work has been extended and further developed by Brown-Wensley into a preparative method for the synthesis of disilanes. The results of competitive reactions with several silanes allow insight into the reaction kinetics, in particular the relative rates of disilane formation versus hydrosilation (Table 5a, b) [61]. 

Although there are several possible methods for the synthesis of organofunctionally terminated disiloxanes, hydrosilation reactions have become the most popular and practical process. Hydrosilation is the term used for the addition of silicon hydrides... 

Synthesis ofa,[Pg.13]

During the synthesis of functional disiloxanes via hydrosilation, the starting materials are usually either tetramethyldisiloxane or dimethylchlorosilane and a proper olefinic (mostly allyl type) compound having the desired functional end group. If dimethylchlorosilane is employed, the hydrosilation is usually followed by hydrolysis. As a specific example, the synt hesis of 1,3-bis(3-glycidoxypropyl)tetramethyldisiloxane is shown in Reaction Scheme IV. 

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

Controlled synthesis of AB, ABA and (AB)n type PEO (A) and PDMS (B) copolymers by hydrosilation of mono- or diallyl-terminated PEO oligomers and telechelic (Si—H) terminated PDMS oligomers were reported by Haesslin 263). Hydrosilation reactions were conducted in bulk, at 90 °C, in the presence of hexachloroplatinic acid catalyst. Completion of the reactions were monitored by H-NMR, GPC and VPO. The molecular weights obtained were in good agreement with theoretical expecta-... 

Finkelmann et al. 256 274,2781 have also investigated the synthesis and the characteristics of siloxane based, crosslinked, liquid crystalline polymers. This new type of materials displays both liquid crystallinity and rubber elasticity. The synthesis of these networks is achieved by the hydrosilation of dimethylsiloxane-(hydrogen, methyl)siloxane copolymers and vinyl terminated mesogenic molecules in the presence of low molecular weight a,co-vinyl terminated dimethylsiloxane crosslinking agents156 ... 

Titanocene catalysts do not catalyze the hydrosilation of most internal olefins, although they can attach active olefins such as styrene, or norbornene to the growing polymer chain ends. The zirconocene-based catalysts, on the other hand, can be powerful hydrosilation catalysts and the remarkable copolymer synthesis shown in Equation 3 can be easily achieved under mild conditions (V7). 

The ubiquitous hydrosilation reaction, popular especially in silicone manufacturing,132 has been utilized by Houser and Keller for the synthesis of the networked polymers (104) (Fig. 64) from the reaction of the l,7-bis(vinyltetramethyldisiloxyl)-w-carborane monomer with the polymeric crosslinker, poly(methylhydrosiloxane).133 The reactions were catalyzed by the Speier s catalyst, H2PtCl6. Three samples were... 

The range of reactions which have been examined is wide (248) and includes hydrogenations (256), ammonia synthesis (257), polymerizations (257), and oxidations (258). Little activity has occurred in this area during the past few years. Recent reports of the effects of sonication on heterogeneous catalysis include the liquefaction of coal by hydrogenation with Cu/Zn (259), the hydrogenation of olefins by formic acid with Pd on carbon (260), and the hydrosilation of 1-alkenes by Pt on carbon (261). 

Pesek, J.J., Matyska, M.T., Oliva, M., and Evanchic, M., Synthesis and characterization of bonded phases made via hydrosilation of aUcynes on silica hydride surfaces, J. Chromatogr. A, 818, 145, 1998. 

We have previously reported on the synthesis of a series of mono- and bifunctional poly(DMS) having a variety of reactive end groups, such as silan (Si-H), vinyl, hydroxyalkyl, dimethylamino and carboxyllic acid groups.7 We have also described already on telechelic poly(DMS) having tosylate end group, lb and l b, where the hydrosilation reaction of poly(DMS) having silan end group was performed with allyl alcohol in the presence of Pt/C catalyst, followed by the tosylation reaction with tosyl chloride in the presence of dimethylaminopyridine.9... 

The novel spirosilane (48) can be elegantly prepared in a two-step synthesis which combines both of the general routes described above. Hydrosilation gives the y-chloropropylsilane which is then cyclized with magnesium (Scheme 62) (71DOK(198)112). 

The catalytic asymmetric hydrosilation of a prochiral ketone to the corresponding chiral silyl ether followed by a mild hydrolysis is in principle an attractive route for the preparation of chiral alcohols that has die advantage that it would not require high hydrogen pressure to effect the reduction (1-6). Despite the synthetic potential only limited application of this technique to the synthesis of complex organic molecules has been made (7-9). This is in part due to the relatively low optical... 

An important potential target of asymmetric hydrosilation catalysis is to the reduction of the cyclopenten-l,4-diones, 1, producing the 4-silyloxyeyclopentenones, 2, which are key intermediates in the synthesis of singleisomer prostaglandins (18-22). 

Boldyrev demonstrated that mechanochemical reactions of solids in the presence of water can be considered as hydrothermal ones. The products of mechanochemical synthesis for a series of reactions were found to be similar to the products of hydrothermal reactions. These reactions include the synthesis of CaO-FeO-2Si02 (gedenbergite) [31] and calcium hydrosilicates, eg., 5Ca0-6Si02-7H20 [32], as well as others. 

The preparahon of calcium hydrosilicates with Ca/Si = 2 by means of mechanical treatment was inveshgated in [29]. It is known that it is very difficult to obtain hydrosihcates characterized by the ratio Ca/Si = 2 by any method including hydrothermal synthesis. The mixture under activation was 2CaO+Si02 (sihca gel) + 2-fold excess of water (with respect to theoretical amount). Activation resulted in the formation of X-ray amorphous hydrosilicate differed from that prepared by hydrothermal synthesis it was a mixture of monomers and dimers. 

The possibility of mechanochemical reactions to occur via hydrothermal mechanism was demonstrated in [32] for the synthesis of calcinm hydrosilicates. Optimal water content values (humidity) of solid mixtures of correspondent hydrates were estimated for the achievement of local hydrothermal conditions (elevated temperature and pressure) in mechanochemical activators. It was shown experimentally that the main factor determining the kinetics and composition of the products was the molar fraction of the components, the same as for hydrothermal reactions, while the effect of temperatnre and pressnre was mnch weaker. 

While stndying the mechanism of mechanochemical synthesis of magnesinm silicate, it was shovra that the components are dissolved in the liqnid phase and interact with each other [33]. At first, the precipitation of hydrosilicates from the solntion in fine disperse and X-ray amorphons state occnrs. Then the growth of larger crystals takes place. Therefore, the prodncts are not detected directly after mechanical activation some exposnre is necessary for them to be crystallized. 

Asymmetric Synthesis by Homogeneous Catalysis Boron Hydrides Boron Organoboranes Hydrosilation Catalysis Rhodium Organometallic Chemistry. 

The synthesis of optically active amines by hydrosilation of imines was achieved in excellent e.e.s using a related catalyst system (7f,/f)-(EBTHI)TiH. This is conveniently generated... 

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