Vinyl silicon effect

Grayson and Whitham have found the regiochemistry of addition of ISCN (from LiSCN and I2) to vinyl silanes to be consistent with the intermediacy of an iodonium ion which is particularly stabilized at one of the carbons due to the -silicon effect.129 The products were a thiocyanate in one case and an isothiocyanate in the other (Scheme 65).129... 

Stone and coworkers determined the /3-silicon effect in w-alkyl- and aryl-substituted carbenium ions20 and vinyl cations21 by measuring in a high-pressure mass spectrometer the thermodynamic data for the association of various alkenes and alkynes with trimethylsilylium ion. From their measured thermodynamic data they calculated, by using equations 13 and 14, the /S-silyl stabilization energies listed in Table 1. 

Kresge and Tobin80 investigated the /1-silicon effect on the hydrolysis of vinyl ethers (equation 29) and found a rate acceleration on the hydrolysis of 175 compared with 176, and hence a stabilizing effect of the /1-silyl group on the intermediate -ethoxy carbocation 177 compared with 178. The acceleration is small the rate factor (175) (176) of 129 is equivalent to a free energy of activation difference AAG of 2.9 kcalmol-1,... 

However, it must be taken into account that the a-phenylvinyl cation 185 is already highly stabilized by the phenyl substituent, leading consequently to a smaller -silicon effect in the vinyl cation 183. Ab initio calculations by Buzek predicted for 184 an additional stabilization of 10 kcalmol-1 by the silyl group7. The thermodynamic stabilization of 183 compared with 185, experimentally determined by Stone and coworkers in the gas phase, is 9 kcalmol 121. Thus, the kinetically determined stabilization of the transition state is only about 6 kcalmol-1 smaller than the /J-silyl effect for stabilization of the ground state carbocation. 

The association of Me3Si+ with alkynes is complicated by gas phase polymerization in HPMS and only few representative alkynes could be studied134. Comparatively, AH° and —AS0 are larger for the alkenes with respect to the alkynes and the derived /1-silicon effect is considerably smaller for vinyl cations. This conclusion is in some disagreement... 

Figure 6,46 (a) Positive hyperconjugative stabilization of carbocations in the silicon effect and relative stabilization energies provided by C-H, C-C, and C-Si donors, (b) Selectivity observed in vinyl- and allylsilane electrophilic... 

Vinylchlorosilanes are prepared most conveniently from ethylchlorosilanes by a modification of the chlorination-dehydrochlorination reactions in which the dehydrohalo-genation is effected by a high-boiling tertiary amine. Although the experimental procedure herein described is based on the preparation of vinyltrichlorosilane, the method can be adapted to the synthesis of many other vinyl silicon compounds. 

The mechanism of catalysis involves the insertion of the vinyl-silicon dienes into M-H and M-Si bond, followed by p-Si and P-H elimination, to yield ethene and two isomeric trans and gem- bis(vinylsilyl)ethenes respectively. In the presence of [RuCl2(CO)3] as a catalyst, rraw5-E-CH2=CH[Si]CH=CH[Si]CH=CH2 is exclusively formed [23]. Under optimum conditions the well-defined trans-iaciic (A) silylene (siloxylene)-vinylene (M = 6350-8650 M /Mn = 1.12-1.16) [23] and silazanylene-vinylene (Mw = 813) [18] polymers can be prepared. On the other hand, gem- dimeric products can be effectively formed with (B) when the reaction is catalyzed by [(cod)RuX]2 (where X = Cl, OSiMes) but finally intramolecular ring closure takes place to yield respective cyclotetrasiloxane, cyclotetrasilazane and cyclohexacarbosilanes [22]. 

Modified alkyd resins In this group one finds styrenated alkyds, vinyl toluenated alkyds, oil-modified vinyl resins, acrylic alkyds, silicone alkyds and polyurethane alkyds. The modifying component usually has a number of effects. It always increases the molecular weight of the alkyd polymer, and may impart hardness, durability, or chemical resistance. It also affects the solubility of the polymer in solvents. 

This section will describe the Friedel-Crafts alkylation reactions of aromatic hydrocarbons with alkenylchlorosilanes containing short chain alkenyl groups such as allyl and vinyl. The reaction will be discussed in terms of the substituent effect on silicon and the arene rings. 

The disilanickela compound 21 is not effective in the nickel-catalyzed double silylation reaction with styrene. However, the stoichiometric reaction of 21 with styrene afforded 4,5-carboranylene-l,l,3,3-tetramethyl-2-phenylmethylene-1,3-disilacyclopentane 29. A key feature in the 111 NMR spectmm of 29 includes a singlet at 7.71 ppm assigned to the vinyl proton. A characteristic low-frequency 13C NMR resonance at 139.75 ppm provides evidence for a tethered carbon atom of the two silicon moieties. Unambiguous confirmation was provided by X-ray crystallographic analysis of 29. 

BXL Plastics ERP Division is to introduce new grades of antistatic and conductive closed-cell crosslinked PE and ethylene copolymer foam at the Internepcon Exhibition to meet a growing demand from the electronics industry, for ways of minimising the effect of static electricity on circuits embodying static-sensitive devices, particularly using metal oxide/silicone technology. Very brief details are noted of Evazote C conductive closedcell, crosslinked ethylene-vinyl acetate copolymer foam. 

The hydroxide anion catalyzed rate constants for the scries of alkyl tris-(2-methoxyethoxy)silanes obtained by Fohl were used to define a modified Taft equation, log(/cHO//cHO ) = 2.48cr +1.67 Es [42], A good correlation was obtained, except for vinyl and phenyl substituents. The anomalous behavior observed for phenyl and vinyl tris-(2-methoxyethoxy)silanes may have resulted because the steric parameter or the polar parameter may be influenced by the carbon-carbon double bond. The steric parameter for a,yS-unsaturated substituents may include an appreciable resonance effect. The polar parameter values may be influenced by the ability of silicon to back-bond through d orbitals with the a,/J-unsaturated system [37,49]. 

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