Silicon-substituted terminal

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

Butylation of phenol with tert-butylalcohol (TBA) is reported on medium pore alumino-phosphates AIPO4 -11, -31 and -41. Their activity is compared with their silicon substituted versions SAPO-11, -31 and -41. AIPO4 frameworks without silicon substitution were found to be active. Terminal hydroxyls in AlP04 s are active for butylation of phenol. It was found that a temperature as moderate as HO C is sufficient for considerable butylation of phenol. However, selectivities of the industrially important 4-TBP and 2,4-DTBP are enhanced at higher temperatures. An equimolar feed concentration of phenol and TBA is optimum to yield 4-TBP and 2,4-DTBP selectively and also a lower space velocity of about Ihr favours this reaction. 

The acidic properties of cobalt and silicon-substituted AlPO-5, -11, and -44 have been characterized by JSnchen et al. [111,276] by adsorption calorimetry of acetonitrile at 303 K, after activation at 720 K. Adsorption calorimetric measurements indicated that the adsorption potential of the samples for acetonitrile was enhanced upon cobalt incorporation. The heat curves exhibited at least two steps indicating the existence of acid sites of different strengths. The heats of adsorption indicated the formation of strong acid sites, due to the cobalt incorporation, as well as the presence of weaker acid sites, probably terminal P - OH groups. 

The l,3,2-diazasilacyclopent-5-ene ring is formed by the action of bulky substituted amino-trifluorosilanes (207) on JV-lithiated acetone /-butylhydrazone (208). The intermediate (209) is cyclized by lithiation of a terminal Me group by (208) (Scheme 22) <84ZAAC(514)49>. Fluoro-silylhydrazone (209) (made also with R R SiFz) may be prepared at a low temperature and cyclized by Bu Li at reflux bulky substituents at nitrogen and silicon atoms in (209) are essential to ensure fair yields <79JOM(178)409>. 

Examination of the absorption spectra of the new polysilane materials reveals a number of interesting features (14). As shown in Table III, simple alkyl substituted polymers show absorption maxima around 300-310 nm. Aryl substitution directly on the silicon backbone, however, results in a strong bathochromic shift to 335-345 nm. It is noteworthy that 4, which has a pendant aromatic side group that is buffered from the backbone by a saturated spacer atom, absorbs in the same region as the peralkyl derivatives. This red shift for the silane polymers with aromatic substituents directly bonded to the backbone is reminiscent of a similar observation for phenyl substituted and terminate silicon catenates relative to the corresponding permethyl derivatives... 

However, the directing influence of silicon can be overcome if the vinylsilane contains another substituent that can stabilize a carbocation more strongly than silicon. For example, when the silyl group is attached to C-2 of a terminal alkene, reaction occurs to give the more substituted carbocation 82 (equation 44)107. Similarly, if the silicon is bound to the same carbon atom as a phenyl group, reaction occurs via the benzyl cation to give the product shown in equation 45108. 

The assembly of monolayers of alkylbromide-functionalized Co nanoparticles onto amino-terminated silicon surfaces through direct nucleophilic substitution was reported by Kim et al.17 The nanoparticle density on the surface can be controlled by changing the immersion time of the silicon surface in the nanoparticle solution. Directed assembly of nanoparticles was observed on a chemically patterned surface. 

In view of the preference of the tetrasilabuta-1,3-diene 139 for the s-cis form, it seemed worthwhile to examine its behavior in [4 + 2] cycloadditions of the Diels-Alder type. Since 139, like many disilenes, should behave as an electron-rich diene, we attempted to react it with various electron-poor and also with some electron-rich olefins. No reaction was detected in any case. Only in the presence of water did 139 react with quinones to furnish the unsymmetrically substituted disilenes 36 and 37 (see Section III.A). The effective shielding of the double bonds by the bulky aryl groups and, above all, the 1, 4-separation of the terminal silicon atoms of about 5.40 A appear to be responsible for these failures. Thus, it was surprising that treatment of 139 with the heavier chalcogens afforded five-membered ring compounds in a formal [4 + 1] cycloaddition (see below). 

Silicone polymers commonly have cross-linked network structures created by reacting a reactive prepolymer with a cross-linking agent. For example, a hydroxy-terminated polydimethylsiloxane is cross-linked with a methoxy-substituted silane and a catalyst as follows ... 

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