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Hydrosilylation of Cyclic Olefins

The asymmetric hydrosilylation of a-methylstyrene with methyldi-chlorosilane has been catalyzed by (/ )-benzylmethylphenylphosphine complexes of platinum(II) 302) or nickel(II) 304) to give a 5 or 17.6% excess of one enantiomer in the addition product, 2-phenylpropyl-methyldichlorosilane. The corresponding palladium(II) complexes were, however, only slightly useful for asymmetric synthesis in hydrosilylation of olefins. Nevertheless, palladium(II) complexes of methyldiphenyl-phosphine or epimeric neomethyldiphenylphosphine, where the dissymmetry is remote from the phosphorus, are especially useful for the induction of asymmetry in the hydrosilylation of styrene and some cyclic conjugated dienes 199). A similar procedure has been used for... [Pg.319]

The hydrosilylation of alkenes produces terminal alkylsilane products. Several examples of these reactions described in Speier s original paper are shown in Equations 16.18-16.22. These examples first show that the terminal anti-Markovnikov products are formed from a-olefins (Equation 16.18). These results also show that linear products are formed from the hydrosilylation of a,S-unsaturated esters with Speier s catalyst (Equation 16.19). Reactions of internal olefins are more complex. Reactions of imsubstituted cyclic alkenes form a single symmetrical product (Equation 16.20). However, as shown in Equations 16.21a and 16.21b, reactions of internal olefins form the same major product as reactions of terminal olefins. This result was corifusing at the time, but the now weU-known isomerization of secondary alkyl complexes to primary alkyl complexes accounts for this result. More details about this isomerization process are given in Section 16.3.5 that covers the mechanism of hydrosilylation. Finally, the silane can affect regioselectivity of the hydrosilylation of alkenes catalyzed by Speier s catalyst. Reaction of dichlorosilane with 2-hexene formed the 2- and 3-alkylsilanes without formation of the terminal alkylsilane (Equation 16.22). ... [Pg.679]

Catalyst 70 is very effective for the reaction of terminal alkenes, however 1,1-disubstituted olefins provide hydrosilylation products presumably, this is due to steric hindrance [45]. When a catalyst with an open geometry (78 or 79) is employed, 1,1-disubstituted alkenes are inserted into C-Y bonds to give quaternary carbon centers with high diastereoselectivities (Scheme 18). As before, initial insertion into the less hindered alkene is followed by cyclic insertion into the more hindered alkene (entry 1) [45]. Catalyst 79 is more active than is 78, operating with shorter reaction times (entries 2 and 3) and reduced temperatures. Transannular cyclization was possible in moderate yield (entry 4), as was formation of spirocyclic or propellane products... [Pg.233]

Whereas the vinyl groups of Da are accessible for functionalization by hydroboration or hydrosilylation, they are inert to functionalization by cross-metathesis. Alternatively, formal metathesis products can be obtained by the ruthenium-catalyzed silylative coupling reaction. This method involves the combination of a vinyl silane and an olefin in the presence of a ruthenium catalyst, to provide an alkenylsilane (see eq 7). The application of this reaction to Da provides substitution at each of the four vinyl groups, resulting in a cyclic tetraalkenyltetramethylcyclote-trasiloxane. The silylative coupling reaction of both Da and has been demonstrated with styrenes and enol ethers. ... [Pg.485]

Alkyne RCM reactions provide not only a useful macrocyc-Uzation technique but also a solution to avoid the issue on the unpredictable alkene geometry by olefin RCM macro-cycUzations. Thus, the Lindlar reduction of the formed cyclic alkynes leads to cyclic Z-alkenes, and the methodology has been widely employed in natural product syntheses.The hydrosilylation-protodesilylation of the cyclic alkynes " is the secured access to cyclic E-alkenes. As described below, the alkyne RCM-based strategies with the subsequent transformations indicated their usefiilness in target-oriented syntheses. [Pg.720]


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