Terminal alkenes hydrosilylations

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... 

Wilkinson s catalyst brings about the hydrosilylation of a range of terminal alkenes (1-octene, trimethylvinylsilane) by 2-dimethylsilylpyridine with good regioselectivity for the anti-Markovnikoff product. Both 3-dimethylsilylpyridine and dimethylphenylsilane are less reactive sources of Si-H. In contrast, these two substrates are far more reactive than 2-dimethylsilylpyridine for the hydrosilylation of alkynes by [Pt(CH2 = CHSiMe2)20]/PR3 (R = Ph, Bu ). This difference was explained to be due to the operation of the two different pathways for Si-H addition—the standard Chalk-Harrod pathway with platinum and the modified Chalk-Harrod pathway with rhodium.108... 

Table 7 Regio- and enantioselectivity for hydrosilylation of terminal alkenes with...

Another example of the palladium-catalyzed asymmetric hydrosilylation of simple terminal alkene, 1-hexene, was reported recently where rfl-phosphoramidite 21d gave 35% yield of (i )-2-hexanol with 68% ee.45... 

The hydrosilylation of alkenes with trialkylsilanes in the presence of Lewis acid catalysts under mild conditions gives the corresponding (trialkylsilyl)alkanes [Eq. (22)]. Reaction with terminal alkenes such as 1-hexene and 1-dodecene at room temperature gives hydrosilylation products in 57 and 58% yields, respectively. Reactions with activated styrene derivatives such as styrene, / -chlorostyrene, and a-methylstyrene at —20°C afford hydrosilylated products in 55-61% yields. ... 

Many of the characteristic features of hydroalanation of alkenes (reactivities, selectivities) are very similar to those of hydrosilylation. Terminal alkenes react readily in hydroalumination, whereas internal alkenes are much less reactive. Aluminum usually adds selectively to the terminal carbon. Hydroalumination of styrene, however, leads to a mixture of regioisomers.392 When hydroalumination of alkenes is followed by protolysis, saturated hydrocarbons are formed that is, net hydrogenation of the carbon-carbon double bond may be achieved. The difference in reactivity of different double bonds allows selective hydroalumination of the less hindered bond in dienes 393... 

Alkene isomerization is common during hydrosilylation. At partial conversion in the reaction of ethylcyclohexenes, the recovered olefin contained all possible isomers except vinylcyclohexane.426 Even in the transformation of the most reactive terminal alkenes, the recovered unreacted olefin is a mixture of isomers. Isomerization thus may prevent complete conversion of 1-alkenes. Similar doublebond migration is not observed in free-radical addition 428... 

Hydroboration of a variety of alkenes and terminal alkynes with catecholborane in the fluorous solvent perfluoromethylcyclohexane was performed using fluorous analogs of the Wilkinson catalyst.135 136 Recycling of a rhodium-based alkene hydrosilylation catalyst was also successful.137 Activated aromatics and naphthalene showed satisfactory reactivity in Friedel-Crafts acylation with acid anhydrides in the presence of Yb tris(perfluoroalkanesulfonyl)methide catalysts.138... 

The C—Si bond formed by the hydrosilation of alkene is a stable bond. Although it is difficult to convert the C—Si bond to other functional groups, it can be converted to alcohols by oxidation with MCPBA or H2O2. This reaction enhances the usefulness of hydrosilylation of alkenes [219], Combination of intramolecular hydrosilylation of allylic or homoallylic alcohols and the oxidation offers regio- and stereoselective preparation of diols [220], Internal alkenes are difficult to hydrosilylate without isomerization to terminal alkenes. However, intramolecular hydrosilation of internal alkenes can be carried out without isomerization. Intramolecular hydrosilylation of the silyl ether 572 of the homoallylic alcohol 571 afforded 573 regio- and stereoselectively, and the Prelog-Djerassi lactone 574 was prepared by applying this method. 

Hie highest with substrates containing two such groups. The reaction of configuration at the oxidized carbon atom, used for a one-pot conversion of a terminal alkene into the anti-ty a hydrosilylation-oxidation sequence (equation II). ... 

Hydrosilylation of terminal alkenes using the air-stable silane (EtO)2MeSiH in the presence of either HzPtCb or (PhsPbRhCl results in the introduction of silicon exclusively at the terminal carbon atom. When coupled widi oxidative cleavage, this protocol provides a simple one-pot synthesis of anti-Mar-kovnikov alcohols firom terminal alkenes (Scheme 7). 

In principle, two regioisomers are produced by the reaction of terminal alkenes and hydrosilanes one where the terminal carbon forms a bond to the silyl group (normal product) and the other which has the silyl group at the inner carbon (branched product). Platinum and rhodium catalysts yield in general the normal products, whereas palladium catalysts sometimes prefer the reaction pathway leading to the branched products. Ruthenium and rhodium afford additional by-products such as alkenylsilanes and alkanes. A typical example is the hydrosilylation of 1-octene (equation 30). ... 

Asymmetric hydrosilylations of terminal alkenes, 1-arylalkenes, norbomenes and dihydrofurans with HSiCIj have been successfully performed by Hayashi and coworkers [914, 915, 916, 1340, 1341]. These reactions take place at 40°C when catalyzed by chiral palladium complexes, and the most efficient ligand is monophosphine 3.51 (R = Me) (Figure 7.19). The regioselectivity of the hydrosilylation of terminal olefins is opposite to that usually observed after treatment with H2O2/KF, secondary alcohols are obtained as major products [752, 855, 1340], The regioisomeric primary alcohols are typically formed in only about 10% yield in these reactions. 

Carbonylative hydrosilylation.1 lrCl(CO)j (or lr4(CO)i2) catalyzes this reaction with terminal alkenes to form the cnol silyl ethers of acylsilancs. Acetal, epoxide, and cyano groups arc not affected. 

For hydrosilylation of alkenes, the reaction rate increases with temperature and hence many of these reaetions have been performed at 100 °C. Higher reaction rates are obtained for silanes with very electronegative substituents and low steric requirements (e.g. HSi(OEt)3 > HSi(i-Pr)3). Terminal alkenes usually are hydrosilylated in an anti-Markovnikov sense to give terminal silanes. Internal alkenes tend not to react (e.g. cyclohexene), or iso-merize to the terminal alkene which is then hydrosilylated (eq 10). Conversely, terminal alkenes may be partially isomerized to un-reactive internal alkenes before the addition of silane can occur. 1,4-Additions to dienes are frequently observed, and the product distributions are extremely sensitive to the silane used (eq 11). 

Liquid-crystalline (LC) silsesquioxanes having various mesogenic moieties were synthesized by the hydrosilylation reaction of octa(hydridosilsesquioxane) and terminal alkenes with mesogenic groups using hexachloroplatinic acid as a catalyst... 

Stereoselective intramolecular alkene hydrosilylation followed by Si-C cleavage is a valuable route to diols both relative - and absolute stereochemistries may be controlled. The rates of the fundamental steps in the [Rh(diphosphine)] catalysed reactions are controlled to some extent by the nature of the diphosphine. From deuterium-labelling studies a silyl insertion mechanism becomes apparent. Whether such mechanisms are applicable to C=0 hydrosilylation versions of this reaction is not yet known. Other highly enantioselective (83-96% ee) C=0 silylations use 13,162 but attempts to use readily available (-)-sparteine as a rhodium ligand are much less successful (5-8% ee).i Further details of the spectacularly effective MOP ligand 14 have appeared.i Optical purities of 93-96% ee are realised using 0.01 mol% Pd2( x-Cl)2( n-C3H5)2 and 0.02 mol% 14 for terminal alkenes and norbomene. 

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