Hydrosilylations alkenes

A 1990 patent reported the quantitative cydization of 1,5-hexadiene 66 to 67 with Cp 2LuCH3 (Cp = C5Mc5 anion) in the presence of PhSiH3 (Scheme 14) [39]. This result was followed by reports that Cp 2NdCH(SiMe3)2 reductively cyclized both 66 and 1,6-heptadiene (68) under similar conditions [40] and that Cp 2SmCH(SiMe3)2 also served as a precatalyst for the cydization of 1,5-hexadiene [41]. In the case of the neodymium and samarium catalysts competitive alkene hydrosilylation was observed. 

Metal complexes of lanthanides beyond lanthanocenes were used to catalyze the reductive coupling reaction of dienes. La[N(TMS)2h was found to effect the cyclization of 1,5-hexadiene in the presence of PhSiH3 (Eq. 13) [50]. Cyclized products 88 and 89 were isolated in a combined yield of 95% (88 89 = 4 1). It was suggested that the silacycloheptane 89 resulted from competitive alkene hydrosilylation followed by intramolecular hydrosilylation. 

Scheme 7.7 Mechanism of homogenous catalysis of alkene hydrosilylation by [ Rh( i-OSiMe3)(cod) 2].

Studies on the immobilization of Pt-based hydrosilylation catalysts have resulted in the development of polymer-supported Pt catalysts that exhibit high hydrosilylation and low isomerization activity, high selectivity, and stability in solventless alkene hydrosilylation at room temperature.627 Results with Rh(I) and Pt(II) complexes supported on polyamides628 and Mn-based carbonyl complexes immobilized on aminated poly(siloxane) have also been published.629 A supported Pt-Pd bimetallic colloid containing Pd as the core metal with Pt on the surface showed a remarkable shift in activity in the hydrosilylation of 1-octene.630... 

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... 

D. Other Reactions Associated with Alkene Hydrosilylation. 1714... 

When the hydrosilylation reactions were reviewed in this series and published in 19893, the Chalk-Harrod mechanism 69 (Scheme 5) was apparently the most widely accepted mechanism for the alkene hydrosilylation, with minor exceptions of photocatalyzed... 

Alkenylsilanes, mainly vinyl silanes and allyl silanes or related compounds, being widely used intermediates for organic synthesis can be efficiently prepared by several reactions catalyzed by transition-metal complexes, such as dehy-drogenative silylation of alkenes, hydrosilylation of alkynes, alkene metathesis, silylative coupling of alkenes with vinylsilanes, and coupling of alkynes with vinylsilanes [1-7]. Ruthenium complexes have been used for chemoselective, regioselective and stereoselective syntheses of unsaturated products. 

Transition metal carbonyls such as Co2(CO)8 and CoH(CO)4, formed in the reaction of R3SiH with dimer (but also Fe(CO)5 and M3(CO)i2 (M = Fe, Ru, Os)) have been found to be active catalysts for the hydrosilylation of olefins, dienes, unsaturated nitriles, and esters as well as for hydrosilylation C=0 and C=N bonds [56]. Hydrosilylation of phenylthioacetylenes in the presence of this catalyst is extremely regioselective [57]. Cobalt(I) complexes, e. g., CoH(X)2L3 (X = H, N), could be prospective candidates for investigation of the effectiveness of alkene hydrosilylation by trialkoxysilanes as well as dehydro-genative silylation [58]. Direct evidence for the silyl migration mechanism operative in a catalytic hydrosilylation pathway was presented by Brookhart and Grant [59] using the electrophilic Co cationic complex. 

The term hydrosilylation is used to describe addition reactions of R3SiH compounds to unsaturated reagents. The industrial importance of alkene hydrosilylation has led to a rapid development in this area. The synthetic scope of these reactions has been reviewed (179,185,187) so, again, we discuss here only the stereochemical and mechanistic aspects. 

Representative alkene hydrosilylation reactions carried out with the MOP ligands are identified in the following schemes. The low catalyst loading in many cases is noteworthy. Norbornene (2.145), dihydrofuran (2.146), styrene (2.147) and cy-clopentadiene (2.148) are all converted into the hydrosilylated derivatives with good to excellent enantioselectivity. In most cases, the trichlorosilanes were deriva-tised into the corresponding alcohols, although the trichloroallylsilane (2.152) was reacted with benzaldehyde to give the addition product (2.156). 

Much work has also been conducted on the hydrosilylation of ketones and imines (Equation 16.16). The products from these reactions are silyl ethers and sdylamines. These additions of silanes across C-X ir-bonds have been conducted predominantly for the purpose of generating optically active alcohols and amines after hydrolysis. Because the mechanism of these reactions is less defined than the mechanism of alkene hydrosilylation, and this chemistry lies outside the theme of this chapter, the hydrosilylation of ketones and imines is presented only briefly. Instead, this chapter provides an overview of the scope and motivation for the hydrosilylation of alkenes and alkynes and provides details on the mechanisms of these reactions catalyzed by complexes of various metals. Several comprehensive reviews of the scope of these reactions have been published. ... 

It is now accepted that alkene hydrosilylation catalyzed by several systems occur by the modified Chalk-Harrod medTanism. Wrighton s studies on tlie insertion of ethylene into the M-Si bond of (CO) CoSiMe3 led to die conclusion that this photocatalytic process occurs by the mechanism in Scheme 16.8. Conversion of the p-silylalkyl product of insertion to the free organosilane is thought to occur by a sequence of oxidative addition of silane... 

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