1.4- Hydrosilation

Hydrosilation denotes the process of addition of one or more Si —H bonds to any other reagent. Most commonly utilized hydrosilating agents are SiR3H, SiHX3, and SiHR X3 (X = halogen, OR), and silated substrates are olefins, acetylenes, and ketones. 

The mechanism of hydrosilation is similar to the previously discussed mechanisms of hydrogenation or hydroformylation reactions see equations (13.159) and (13.160). 

Sometimes, during hydrosilation of olefins, alkenylsilanes are also formed in addition to alkylsilanes.  

SiEt3(C6Hi3) + ( )-Et3SiCH = CHC4H9 + (E )-Et3SiCH2CH = CHC3H7 

This reaction is catalyzed by rhodium complexes such as [Rh2(pmcp)2X4] (X = C1, I), [Rh(pmcp)H2(SiEt3)2], [Rh(pmcp)(C2H4)2], [Rh2Cl2(CO)4], [RhCl(PPh3)3], and Rh(acac)3. The yield of alkenylsilanes increases as the ratio of SiEt3H/olefin decreases. 

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The most common oxidation states and corresponding electronic configurations of platiaum are +2 which is square planar, and +4 which is octahedral. Compounds in oxidation states between 0 and +6 [t) exist. Platiaum hydrosilation catalysts are used in the manufacture of siHcone polymers. Several platiaum coordination compounds are important chemotherapeutic agents used for the treatment of cancer. 

Catalysis. Platinum-catalyzed hydrosilation is used for cross-linking siUcone polymers and for the preparation of functionally substituted silane monomers (250). The most widely used catalyst is chloroplatinic acid (Spier s catalyst), H2PtCl3. Other compounds that catalyze the reaction include Pt(Il)... 

A wide variety of organosiUcone resins containing a combination of M, D, T, and/or Q groups have been prepared and many are commercially manufactured. In addition, resins containing hydrosilation-reactive SiH and SiVi groups or other functionahties, including OH and phenyl groups, are known. Two classes of sihcone resins are most widely used in the sihcone industry MQ and TD resins. 

Both the ethyl and sodium siUcate processes can be modified by substituting SiH- or SiVi-functional chlorosilanes or combiaations of chlorosilanes to produce hydrosilation-reactive MQ resias (396,397). 

Silicon—Ca.rbon Thermoset. The Sycar resins of Hercules are sihcon—carbon thermosets cured through the hydrosilation of sihcon hydride and sihcon vinyl groups with a trace amount of platinum catalyst. The material is a fast-cure system (<15 min at 180°C) and shows low moisture absorption that outperforms conventional thermosets such as polyimides and epoxies. Furthermore, the Sycar material provides excellent mechanical and physical properties used in printed wiring board (PWB) laminates and encapsulants such as flow coatable or glob-top coating of chip-on-board type apphcations. 

Platinum compounds Hydrosilation cross-linking of silicone polymers Hydrogenation, isomerization and hydroformylation of alkenes Automobile exhaust catalyst Sensitization dermatitis... 

Newer silicone adhesives having solids levels up to 97% are also commercially available [109]. Instead of using silanol condensation reactions, they rely on addition chemistry between vinyl functional silicone oligomers and silicon hydride terminated silicones. This addition reaction is typically facilitated with platinum derived catalysts. This hydrosilation process can be run at reduced oven temperatures, but the finished products typically do not yield the same balance of properties as seen for condensation cure materials. 

Hydrosilation silicones or addition cure systems utilize a hydride functional crosslinker with a vinyl functional base polymer and a noble metal catalyst. While the cure can be initiated with UV [48,49], thermal cure versions dominate the commercial market [23,50]. In thermal cure systems, inhibitors are necessary for processing and anchorage additives are common. 

Advantages of the hydrosilation system (Fig. 3) include the elimination of solvent, improved cure speed, and potential for UV or thermal cure. Drawbacks to the system include more expensive multiroll coating methods, potential poisoning of the Pt catalyst (with Sn, S, Cr, amines, etc.), poor anchorage to some films, and a need to carefully balance the hydride to vinyl ratio employed for cure to avoid detrimental interactions with acid containing adhesives [23,53]. 

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

Useful stereoselective syntheses of ( )- and (Z)-2-butenyl(trifluoro)silanes have been reported. The (Z)-isomer is available by hydrosilation of butadiene, the (E)-compound being prepared by displacement of chloride from ( )-l-chloro-2-butene37. 

Optically active 3-(trimethylsilyl)cyclopentene of moderate enantiomeric excess is available by asymmetric hydrosilation (see Section 1.3.3.3.5.1.5.) and reacts with aldehydes with reasonable stereoselectivity in the presence of titanium(IV) chloride36. 

Efficient asymmetric transfer is also observed for 1,2-benzenediol induced reactions of a-sub-stituted (Z)- and ( ,)-allyl(trialkoxy)silanes prepared by hydrosilation of conjugated dienes, although in this case the electrophile attacks the C-C double bond of the allylsilane syn with... 

With the stable donor adducts of silylene complexes, valuable model compounds are now available for reactive intermediates which otherwise cannot be observed directly. For example, a side reaction occurring in the hydrosilation process [61 -63], is the dehydrogenative coupling of silanes to disilanes. This reaction could be explained in terms of a silylene transfer reaction with a coordinated silylene as the key intermediate. 

An important example in this context is the hydrosilation reaction [121]. Hydrosilation is the formal addition of a silane to an alkene in presence of a hydrosilation catalyst. This reaction has numerous applications e.g., with a suitable catalyst, an enantioselective hydrosilation is possible [122] and also hydrosilation and double hydrosilation of alkines [123] are known. 

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

The Formation of SiSi Bonds from Silanes in the Presence of a Hydrosilation... 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

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]. 

Table 5b. Relative rates of disilane formation and hydrosilation process in the presence of various catalysts...

C and 200°C. Catalysts such as aluminum hydrosilicate, phosphoric acid, sulfuric acid, BF3, tertiary amine, or sodium alkyl sulfate are used to increase the reaction rate see Eq. (64). 

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. 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Most of the recent developments in hydrosilation reactions are disclosed in the patent literature. Relatively few mechanistic studies are available in the open literature and much more fundamental research is needed. 

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. 

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