Silicones systemic reactions

Titanium Silicides. The titanium—silicon system includes Ti Si, Ti Si, TiSi, and TiSi (154). Physical properties are summarized in Table 18. Direct synthesis by heating the elements in vacuo or in a protective atmosphere is possible. In the latter case, it is convenient to use titanium hydride instead of titanium metal. Other preparative methods include high temperature electrolysis of molten salt baths containing titanium dioxide and alkalifluorosiUcate (155) reaction of TiCl, SiCl, and H2 at ca 1150°C, using appropriate reactant quantities for both TiSi and TiSi2 (156) and, for Ti Si, reaction between titanium dioxide and calcium siUcide at ca 1200°C, followed by dissolution of excess lime and calcium siUcate in acetic acid. 

The relation between the potential-composition data for these two systems under equilibrium conditions is shown in Fig. 13. It is seen that the phase Li2hSn (Lil3Sn5) is stable over a potential range that includes the upper two-phase reconstitution reaction plateau in the lithium-silicon system. Therefore, lithium can react with Si to form the phase Li, 7 S i... 

Another successful method for the highly diastereoselective silicon allylation reaction is the allyltrifluorosilane- cesium fluoride system discovered by Sakurai et al. in 1987s (Scheme 3.2e). After a mixture of aldehyde,the allylic trifluorosi-lane 4, and cesium fluoride in a ratio of 1 2 23 was stirred in THF, the reaction mixture was quenched with a solution of HC1 in MeOH to afford the products desired (5) in excellent yield and exceptionally high diastereoselectivity. In addition, the reaction is highly regioselective in that the carbon-carbon bond formation occurs exclusively at the ycarbon of allylic silanes. 

Systems which contain cr(E-E)-7r conjugated systems (E = Si or Ge) have been the subject of many photochemical investigations. For the most part investigations have focussed on silicon systems, that first identified carbon-silicon double-bonded species as photoproducts . Later studies also indicated that arylsilanes undergo 1,3-sigmatropic shift reactions as well as extrusion processes yielding silylenes . ESR studies have... 

A synthetically useful example of this process is the conversion of 117 to 120, which involves a 1,2-alkyl shift, and was part of Hwu s synthesis of (-)-solavetivone. 38 jhe alkyl fragment is actually part of the bicyclic ling system, one arm of the bicyclo[4.4.0]decane ring system. Reaction of the OH unit with the Lewis acid resulted in formation of the tertiary cation 118, which was followed by a 1,2-alkyls shift to give 119, where the new cation is stabilized by the adjacent silicon of the trimethylsilyl group. 39 Loss of the trimethylsilyl group from 119 gives spiran (120). 

Unusual compounds can be formed more readily in multivalent recoil atom systems than in monovalent recoil atom systems. In the latter case, unusual compounds are rarely formed during the primary abstraction and substitution processes. The only likely >vay for their formation to occur is through the reactions of reactive intermediates formed via secondary decomposition processes. Typical examples are the formation of CTF and fluorocyclopropanes described previously. On the other hand, multivalent recoil atoms and the reactive intermediates resulting from their abstraction or insertion reactions can frequently react to give unusual products that wrould not be easily formed by other means. Therefore, it is not surprising that various unusual compoimds are detected in recoil carbon and recofi silicon systems. 

Figure 4. Oxidant - Silicon Dioxide Film - Silicon System with Diffusive Flux, Fp and Reaction Flux, F2 ...

SCHEME 21 Attempted boron and silicon aldol reactions with our bicyclic system. 

Cab-O-Sil EH-5 Cab-O-SII HS-5 Cab-0-SII M-5 Cab-O-SII PTG Cab-a Sll TS-720 Siloxanes and silicones, dimethyl, reaction prods, with silica rheology control agent, liq. systems Tullanox HM-250 rheology control agent, low PVC Nopco DSX 3000 theology control agent, low-solids systems... 

Silane-terminated poiysiioxanes are a new class of high reactive rtv-1 silicone systems. The synthesis of these polymers is shown in Eq. 1. In a first step a hydroxy-terminated polydimethylsiloxane is converted to the corresponding aminopropyl-polydimethylsiloxane by reaction with stoichiometric amounts of 3-[(2,2-dimethyl-l,2-a2asiiolidin-l-yl)dimethylsilyl]-l -propylamine at room temperature. The silane termination reaction in a second step is carried out under the same conditions without adding further catalysts, due to the fast reaction of NCO groups with the primary amino groups. 

Systemic reactions have been reported following the introduction of silicone into the body. In one instance, a severe systemic reaction consisting of a febrile illness, acute renal insufficiency, respiratory compromise. 

One-part silicone systems are ready to use, require no mixing, present no pot-life problem, and are generally the least expensive. Conventional one-part adhesive/sealants are available with two different types of cure systems acid and non-acid cure. Both require moisture from the atmosphere to cure. The acid-curing type has the greatest unprimed adhesion and the longest shelf life. The non-acid-curing type is effective when the acetic acid released by the cure reaction may cause corrosion, or be otherwise objectionable. 

The solution-metal-silicon system constitutes a microscopic electrochemical cell that induces anodic silicon etch. For the most commonly employed oxidants H2O2 and HNO3, the proposed cathodic reactions provide free positive carriers to be transferred to the sihcon (Table 1). 

Denmark and co-workers reported a good example of torquoselection in the silicon-directed Nazarov cyclization (see Section 3.4.5.1). They demonstrated that cyclohexenyl-derived divinyl ketones 26 cyclize to give the relative stereoisomer 27 as the major product (see Section 3.4.5.1 for the mechanism of the silicon-directed reaction). The use of bulky alkyl groups (such as /-butyl) and/or bulky silicon substituents gave the best selectivity, at the expense of the chemical yield. It is interesting that the corresponding cyclopentenyl-derived systems gave only poor torque-selectivity. 

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