Silanes, addition

The coordinated cyclohexenones react from half-chair conformations A and B in order to show a maximum of tr-overlap. The 4-methyl-2-cyclohexenone prefers, for stereoelectronic factors, the half-chair A, which leads to the ci.s-product on (2-propenyl)silane addition (path a), even if this is not the sterically least hindered approach. 

Alkenes with two reactive carbon-carbon double bonds per molecule like 1,5-hexadiene or diallyl ether are used in the synthesis of silicone compounds which can be later crosslinked by hydrosilylation. A sufficiently high excess of double bonds helps to prevent the dienes from taking part in silane addition across both olefmic ends, but trouble comes from double bond isomerization (Eq. 2). 

In the hydrosilylation of alkynes the product distribution depends on the reaction conditions.2 With [RhCl(/7 -cod)(NHC)] and [RhCl(PPh3)2(NHC)] silane addition results in mixtures of the cis and trans isomers. Irradiation increases the reaction rates for these reactions. However, no details about the fate of the NHC ligand under these conditions are reported. 

Modified Parfait Protocol. Teflon adsorbents were cleaned by 24-h extraction with acetone in a Soxhlet apparatus and stored in acetone. Cleaned Chromosorb T was used to fill a 150-mL silanized addition funnel equipped with a 24-40 male joint on the bottom. A plug of silanized glass wool supported the bed just above the stopcock. The Teflon bed was compressed slightly with a large stirring rod to keep the top of the bed in place while the bed was washed with at least 6 bed volumes (900 mL) of high-purity water. The washed Teflon was kept covered with water and used immediately. 

The acid-base properties of polymers, fillers and silane additives, as described by Fowkes [14] can be used to predict the effect of silanes on the dispersion of fillers in polymer, and viscosity of the mix. In general, opposites attract (give good dispersion) while like materials repel (poor dispersion) [15]. The effect of cationic silane (Z-6032) on the dispersion of silica (acidic filler) in this particular unsaturated polyester resin (acidic polymer) is shown in Table 6. Addition of Z-6032 in increments to a silica-filled polyester resin lowered the viscosity of the mixture to a minimum at about 0.4% silane based on the filler. 

Effect of silane addition level on the compressive properties of an anhydride cured epoxide silica microballoon syntactic foam (APES, cured at 100 C for 4 h, nominal density 0.35 g/cm )... 

Figure 7. The effect of S wt% silane addition on the stress/strain behaviour of an MNA/Epikote 828/glass microballoon syntactic foam.

Step b Two acid-catalyzed cyclizations ally silane addition to an iminium ion followed by intramolecular imine alkylation and loss of MeOH. 

The reaction temperature was found to have a marked effect on the regioselectivity of silane addition, namely 1,2-addition of trisubstituted silanes is... 

The reaction temperature had a dramatic impact on the enantioselectivity of the silane addition. Using acetophenone, diphenylsilane and the (S-Binap)Rh(PPh3)2 catalyst as a model system, the initial reaction temperature was varied from -40°C to 55°C. When carried out under the standard conditions (vide supra), the reduction gave an optical yield of 26% (5-isomer). Lowering the initial temperature to 0°C caused a decrease in the e.e. to 12% and decreasing the temperature further still to -40°C resulted in an optical yield of only 7%. Increasing the initial temperature, on the other hand, to 55°C (1,2-dichloroethane solvent) raised the optical yield to 55%. 

Silane addition to Rh1 alkyls may be accompanied by alkane elimination to give rare examples of Rh1 silyl species 75... 

Double silane addition to Rh1 leads to Rhv products, e.g., (21-XVI).74 Similarly, reaction of HSiCl3 with (arene)Cr(CO)3 under photolytic conditions gives the CrIV complex (21-XVII).76... 

SnCU is effective in promoting the addition of nucleophiles to simple aldehydes. Among the most synthetically useful additions are allylstannane and -silane additions. The product distribution in the stannane reactions can be influenced by the order of addition, stoichiometry, and reaction temperature. The anti geometry of the tin-aldehyde complex is favored because of steric interactions. Furthermore, the six-coordinate 2 1 complex is most probably the reactive intermediate in these systems. The use of crotylstaimanes provides evidence for competing transmetalation pathways (Eq. 35) [60]. TiCU results in superior selectivity. 

Nucleophilic addition to less reactive ketone carbonyls by Lewis acid activation is also possible. Evans and co-workers have reported enol silane addition to pyruvate esters mediated by chiral copper Lewis acids (Sch. 36) [72]. The aldol reactions proceed with high facial selectivity to provide the tertiary alcohol products 153. The chemical efficiency is, however, reduced when a bulky alkyl group is present at the ketone carbonyl. Addition of more functionalized enol silanes (155) to keto esters enables the establishment of two contiguous chiral centers, a substitution pattern present in a variety of natural products. The stereochemistry of the major product is syn, irrespective of the enol silane geometry. Once again, bidentate coordination of the substrate to the Lewis acid was essential for obtaining high selectivity. 

SILANES addition of silanes to alkene 15-19 SULFONIC ACIDS ... 

R. A. Benkeser Silane addition reactions— their synthetic utility and mechanism, pp. 133-140 (20). 

Special methods of incorporation carbon fiber was added at the end of mixing to limit breakage " silane addition stage was varied to improve properties of silica-filled composite ... 

Fig.4. The Seebach model as applied to enol silane additions to aldehydes emphasizes the incipient steric interactions that result from Burgi-Dimitz constraints...

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