Conjugated dienes hydrosilylation

For the selective 1,4-hydrosilylation to 133 giving the allenic product 134, the presence and orientation of the two -SiMe3 groups at the 1- and 4-positions in 133 play important roles. Reaction of the hydrosilanes with the closely related conjugated enynes 136 and 137 in the presence of the Pt or Rh catalyst proceeded in a 1,2-addi-tion manner to produce conjugated dienes instead of allenes [108],... 

Dienes. Hydrosilylation of conjugated dienes often leads to a complex mixture of products composed of regio- and stereoisomers of 1 1 addition, 1 2 addition products, and oligomers. Dihydrosilanes may form silacycloalkanes as well. In contrast... 

Stereoselective hydrosilylation of conjugated dienes gives chiral monoaddition products, sometimes as a mixture of regioisomers. The yield and enantiomeric excess of the products depend on the nature of the chiral organometallic catalyst used. 

Other Rh catalysts were also employed for hydrosilation of a,p-unsaturated carbonyl compounds and unsaturated nitriles. Thus, Rh(acac)2 and a tetrakis( jL-acetato)dirhodium cluster were used as catalysts in the hydrosilation of a,P-unsaturated aldehydes. These reactions, however, are not chemoselective, as alkynes, conjugated dienes and alkenes are also hydrosilylated, and allylic heterosubstituents are reduc-tively cleaved. 

Conjugated dienes like 1,3-dienes give 1 1 adducts of various types usually 1,2- and 1,4-addition products, and the regio- and stereo-isomers of each type, but also 1 2 adducts, and dimerization and reduction products. The product distribution depends on the kinds of dienes, silanes and particularly on the kind of catalyst. Of these, chromium hexacarbonyl is specific for the formation of (Z)-crotylsilanes from butadienes. Similar (Z)-selective hydrosilylation is observed with HSiCb and [Pd(PPh3)4] catalyst. The reaction pattern of 1,3-butadiene is summarized in Scheme 8. 

Early studies on the palladium-catalyzed asymmetric hydrosilylation of cyclic conjugated dienes employing menthyl- and neomenthyldiphenylphosphine and ferrocenylamino-phosphine ligands gave low enantiomeric excesses of the corresponding allylsilane [13]. 

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... 

The Ziegler system ML -A1(C2H5)3 (M=Ni, Co or Fe n=2, 3 L=acac or Cl) has been used mostly in the addition of trialkyl and triphenylsilanes to conjugated dienes and trienes (especially Ni(acac)2-A1(C2H5)3) as well as in hydrosilylation of olefins with functional groups (the three-component system with additional PPh3) [2, 5]. 

The hydrosilylation of conjugated dienes such as butadiene and isoprene using HjPtClg as the catalyst proceeds only at elevated temperatures giving a mixture of products. The hydrosilylation of butadiene with HSiMeCl2 affords a 1 1 mixture of monoadduct and diadduct (combined yield 50%), the former including but-2-enylsilane as major (85-90%) and but-3-enylsilane as minor products (15-10%) ... 

Nickel complexes also exhibit a high catalytic activity toward the hydrosilylation of conjugated dienes under mild conditions. The reaction usually occurs in the manner of a 1,4-addition, but the regio- and stereoselectivity is rather low compared with that achieved by palladium catalysts. 

The hydrosilylation of fully conjugated diene esters followed by hydrolysis on silica gives the corresponding 0,y-unsaturated esters through 1,6-addition (equation 73)193. When a y,<5-double bond is a part of a cyclic structure, viz., the double bond is... 

Hydroboration and hydrosilylation. A mixture of EtjSiH and BCI3 is used for hydroboration of alkenes, leading to alkyldichloroboranes. Conjugated dienes give (Zj-alkenyltriethylsilanes on reaction with EtjSiH photochemically in the presence of Cr(CO),. 

Cyclodextrins are the most used host compounds so far. Native cyclodextrins are effective inverse phase-transfer catalysts for the deoxygenation of allylic alcohols [3], epoxidation [4], oxidation [5], or hydrosilylation [6] of olefins, reduction of a,y9-unsaturated acids [7], a-keto ester [8], conjugated dienes [9] or aryl alkyl ketones [10]. Interestingly, chemically modified cyclodextrins like the partially O-methylated y0-cyclodextrin (RAME-y9-CD) show a better catalytic activity than native cyclodextrins in numerous reactions such as the Wacker oxidation [11], hydrogenation of... 

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