Olefin complexes dehydrogenative silylation

The dehydrogenative silylation of olefins, closely related to hydrosiiyiation, is promoted by a ruthenium carbonyl complex, RujCCOlij. The product vinylsilane is always the frans-isomer ... 

The dehydrogenative silylation of olefins, which is closely related to hydrosilylation, is effectively promoted by a ruthenium carbonyl cluster complex, Ru3(CO)12. The produced vinylsilane was the trans-isomer in every case examined76 (equation 26). 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

The hydrosilylation of alkenes (Equation 16.12) and alkynes (Equation 16.13), alternatively termed hydrosilation, is the addition of a silicon-hydrogen bond across the C-C TT-bond to form a new alkylsilane or vinylsilane. This reaction has been catalyzed by complexes containing many different metals, but is most commonly conducted with complexes of platinum, rhodium, and palladium. The hydrosilylation of alkenes t3q>ically forms terminal alkylsilanes as the major regioisomer, and the hydrosilylation of vinylarenes often generates the chiral branched alkylsilane. The hydrosilylation of alkynes has also been developed. As shown generally in Equation 16.13, these reactions can occur by either cis or trans addition, depending on the catalyst. In some cases, the reactions of silanes with olefins form vinylsilanes (called dehydrogenative silylation. Equation 16.14). The addition of an Si-Si bond of a disilane across an olefin has also been reported (Equation 16.15), and this reaction is called disilation of olefins. 

Basic methods for their production involve the hydrosilylation of alkylacetylenes catalyzed by platinum complexes [8] and the dehydrogenative silylation of olefins, e.g. styrene [9], 1-hexene [10,11], are catalyzed by rhodium [10], ruthenium [9,12, 13] and iridium [11] complexes and photocatalyzed by iron and cobalt [14,15] carbonyls. 

The latter reaction was revealed as a side-reaction of the hydrosilylation occurring especially in the presence of Fe and Co-triad complexes and this made the basis for an alternative to the Chalk and Harrod concept of the hydrosilylation known as the Seitz and Wrighton mechanism [8,15]. The key step of this mechanism involves insertion of an alkene into a metal-silicon bond (equation 2). Concurrent insertion of olefin into the M-H and M-Si bonds can potentially lead to a complex containing a-alkyl and a-silylalkyl ligands. Competitive P-H transfer from the two ligands to the metal is a decisive step for alternative hydrosilylation and dehydrogenative silylation [16]... 

Contrary to the previously reported reactions with the M-H and M-Si initial complexes the proposed mechanism of catalysis by [(cod)M(OSiMe3)]2 (where M= Rh, Ir) does not involve highly activated migratory insertion of olefin into the Rh-Si bond (the associative mechanism) since the final step of the product formation occurs via a lower activated step of reductive elimination of product (the dissociative mechanism) (Scheme 4). The reaction under study is conceptually related to dehydrogenative silylation since the basic reaction involves the silylation of a substrate such as styrene by vinylsilane instead in the hydrosilane, equations 17a and 17b. by hydrosilanes... 

Effective disproportionation and co-disproportionation of vinylsilane with ruthenium complexes containing the Ru-H, Ru-Si bond, called subsequently silylative coupling or trans-si y aiion of olefins with vinylsubstituted silanes, was revealed in 1984 as a new synthetic route to substituted vinylsilanes and are commonly used as organic reagents. Subsequent extensive synthetic and catalytic study has shown that silylative coupling of olefins with vinylsubstituted silicon compounds occurs (similarly to the hydrosilylation and dehydrogenative silylation reactions) via active intermediates containing the M-Si (silicometallics) and the M-H bond (where M = Ru, Rh, Ir, Co, Fe). The insertion of olefin into M-Si bond and vinylsilanes into M-H followed by elimination of vinylsilane and ethane respectively, are the key steps in this new process. 

The reaction occurs via the formation of a potential complex containing a-alkyl and <7-silylalkyl ligands. The j8-H-transfer from the two ligands to the metal proceeding concurrently is a decisive step for two alternative reactions, that is, hydrosilylation and/or dehydrogenative silylation of olefins that have been recently reviewed in the catalytic and synthetic aspects (8,13,44). 

In most cases, group R (Scheme 4) depicts an electronegative substituent in such olefins as styrene, substituted styrenes, trifluoropropene, and vinyl trisub-stituted silanes. Complexes of iron and cobalt triads have appeared extremely favorable catalysts of the dehydrogenative silylation, but Ni, Pd, and Pt complexes have also recently been reported as active catalysts of these olefins conversions [for reviews see References (3,11,13,18). 

Over the last two decades, Wilkinson complex and related phosphine complexes of rhodium(I) have been used in numerous reactions for synthetic purposes, such as in the hydrosilylation of styrene and vinylcyclo-propene to yield ring-opening products of vinylamines. The [ (dippe)Rh 2(/u.-H)2] complex [where dippe = l,2-bis(diisopropylphosphino)ethane] is active in the hydrosilylation of olefins by diphenylsilane (4). Rhodium complexes were extremely favorable catalysts for dehydrogenative silylation of alkenes and divinyldiorganosilanes (4,13). 

Iron Triad Complexes. Most Fe and Ru (and also Os) complexes including carbonyls used in the hydrosilylation of olefins give either hydrosilylation prod-nets accompanied by unsaturated vinylsilyl derivatives as products of dehydrogenative silylation or exclusively the latter (6,11-13). 

Athene Hydrogenations. Treatment of olefins with Zr catalysts effected their saturation in 50% yield along with 50% dehydrogenative silylation as calculated by GC (eq 20). Ru car-bene complexes also afforded the reduced product in the presence of PhsSiH. Reduction of an olefin via radical-chain reductive car-boxyalkylation proceeded with PhsSiH and a homochiral thiol catalyst upon TBHN initiation (eq 21). The olefin of vinylstan-nanes could be reduced without any protodestannylated product produced (eq 22). ... 

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