Olefin hydrosilylation

Ethylene coordinates too strongly to the ruthenium center to act as a hydrogen acceptor in this system, but slightly larger olefins do show activity. However, reaction selectivity is reduced compared with f-butyl ethylene. Thus, the use of cis and trans 2-pentene as hydrogen acceptor leads to products consisting of 90% carbosilane and 10% hydrosilylated olefin, and 1-hexene yields a carbosilane/hydrosilylation ratio of 4 6. In comparison, no hydrosilylation products are observed for dehydrocoupling in the presence of cyclohexene, but carbosilane formation is accompanied by disproportionation to benzene and cyclohexane as a side reaction. 

In recent years the solid-phase hydrosilylation reaction was successfully employed for synthesis of hydrolytically stable surface chemical compounds with Si-C bonds. Of special interest is application of this method for attachment of functional olefins, in particular of acrolein and some chiral ligands. Such matrices can be used for subsequent immobilization of a wide range of amine-containing organic reagents and in chiral chromatography. 

To date, numerous radical-induced hydrosilylations of terminal olefins or acetylenes have been reported for the H-terminated Si(l 11) surfaces. These reactions are mainly performed by using thermal conditions, UV irradiation, or electrochemistry. More recently, a very mild method was developed for the attachment of high-quality organic monolayers on crystalline silicon surfaces. 

The proposed mechanism for the hydrosilylation of olefins catalyzed by chloroplatinic acid is outlined in Fig. 6. Catalysis by square-planar or trigonal bipyramidal rf complexes can be similarly described (54, 55, 105). 

The mechanism for hydrosilylation in Figs. 6 and 7 clearly has much in common with suggestions regarding homogeneous transition metal catalysis for other processes involving olefins, such as hydrogenation, isomerization, the oxo reaction, and oligo- and polymerization. 

By analogy with hydroformylation, dicobalt octacarbonyl has been examined as a hydrosilylation catalyst. Various silanes and a-olefins react, often exothermically. Thermal deactivation occurs above 60° C hence, large exotherms and high temperatures must be avoided (56, 57,130). Isomerization is more pronounced than for the bridged olefin complexes of Pt(II) and Rh(I) (see below) it even occurs with trialkoxysilanes (57). Though isomerization is faster than hydrosilylation, little variation in the relative rates of these two processes with the nature of the silane is observed this is in marked contrast to the bridged systems (55). 

Excess of the olefin is desirable so that the formation of Si—Co complexes [Eq. (112) and Section II,B,7] is suppressed 54, 57). These silyl complexes are not hydrosilylation catalysts. 

Consistent with this mechanism, HCo(CO)4 is an effective hydrosilylation catalyst 13, 14, 57). In the absence of olefin, RjSiCo(CO)4 is formed (Section II,B,4). 

With alkoxysilanes, rapid hydrosilylation of the terminal olefin occurs, without isomerization, to give high yield of adduct. 

The most common behavior, especially for substrates with Si—Cl bonds, is the extensive isomerization of excess of olefin during rapid hydrosilylation. 

It is probable that during hydrosilylations these Ni(II) complexes are reduced to 7r-olefin Ni(0) species which then undergo an oxidative addition in an identical manner to that already discussed for the chloroplatinic acid case. There is current interest in such oxidations (83), and the platinum analog (Ph3P)2Pt(olefin) has been shown in one case (olefin = C2H4) to be an excellent hydrosilylation catalyst (240). In this system, intermediate low oxidation state Pt species have been isolated their nature is dependent on the electronegativity of the other groups attached to silicon. 

Very recently, the yttrium hydride [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -di-methylbiphenyl]YH(THF) 2 (36), conveniently generated in situ from [2,2 -bis(tert-butyldimethylsilylamido)-6,6 -dimethylbiphenyl]YMe(THF)2 (35) demonstrated its high catalytic activity in olefin hydrosilylation. This system represents the first use of a d° metal complex with non-Cp ligands for the catalytic hydrosilylation of olefins. Hydrosilylation of norbornene with PhSiHs gave the corresponding product (37) of 90% ee (Scheme 3-15) [43]. 

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... 

The reductive coupling of of dienes containing amine groups in the backbones allows for the production of alkaloid skeletons in relatively few steps [36,46,47]. Epilupinine 80 was formed in 51% yield after oxidation by treatment of the tertiary amine 81 with PhMeSiEh in the presence of catalytic 70 [46]. Notably, none of the trans isomer was observed in the product mixture (Eq. 11). The Cp fuMcTIIF was found to catalyze cyclization of unsubstituted allyl amine 82 to provide 83. This reaction proceeded in shorter time and with increased yield relative to the same reaction with 70 (Eq. 12) [47]. Substitution of either alkene prevented cyclization, possibly due to competitive intramolecular stabilization of the metal by nitrogen preventing coordination of the substituted olefin, and resulted in hydrosilylation of the less substituted olefin. 

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