Silylative Coupling of Alkenes with Vinylsilanes

Evidence for the migratory insertion of ethylene [121], vinylsilane [122], and styrene [123] into the Ru-Si bond (yielding vinylsilane and two (1,2- and 1,1-bis(silyl)ethene) regioisomers, respectively) showed that in the reaction first reported in 1984 as the metathesis (disproportionation) of vinylsilanes and their co-metathesis with olefins [124], instead of the C=C bond cleavage (formally characterizing alkene metathesis (eq. (8)), a new type of olefin conversion was revealed - silylative coupling of olefins with vinylsilanes. 

This mechanism of catalysis was also proved by the stoichiometric reactions as well as above all by a new diagnostic tool in this type of the reaction, i. e., an MS study of the product of the deuterated styrene with vinylsilane) 123, 124] or deu-terated vinylsilane with vinyl alkyl ethers [127]. Moreover, similarly to the case 

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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. 

Subsequent extensive synthetic and catalytic studies have shown Aat silylative coupling of alkenes with vinyl-substituted silicon compounds proceeds (similarly to the hydrosilylation and dehydrogenative silylation reactions) via active intermediates containing M-Si (silicometallics) and M-H bonds (where M = Ru, Rh, Ir, Co, Fe). The insertion of alkene into M-Si bonds and vinylsilanes into M-H bonds, followed by elimination of vinylsilane and ethene, respectively, are the key steps in this new process [9]. 

Silylative Coupling (trans-Silylation) of Alkenes with Vinylsilanes. 207... 

Catalytic activity of synthesised Rh(I) siloxide complexes has been demonstrated in some reactions, i.e. in the hydrosilylation of alkenes [17] and allyl alkyl ethers [14, 18, 19] and in the silylative coupling of vinylsilanes with alkenes [20]. 

Contrary to expectations based on the efficient silylative coupling of the heteroatom (O, N, Si and B), functionalized alkenes with vinylsilanes catalyzed by Ru-H and/or Ru-Si-containing complexes do not undergo this transformation, which has been explained by formation of an Ru-S complex into which no insertion of vinylsilanes (a step necessary in the catalytic cycle of SC) was observed. 

Vinylsilanes undergo productive cross-metathesis (CM) and silylative coupling (SC) with allyl-substituted (N, B)functionalized alkenes to yield l-silyl-3,Ar, -substituted propenes with preference (for V-derivatives) and exclusive formation (for boronates) of the f-isomer. 

Vinylsilanes are important alkene derivatives that have been widely used as synthetic intermediates, monomers for copolymer plastics, and coupling agents for hybrid silicon materials (1). Transition-metal-catalyzed hydrosilylation and bis-silylation of alkynes represent the most straightforward and atom-economic routes to vinylsilanes (2). The original reports on palladium-catalyzed bis-silylation of alkynes with disilanes were published by Kumada and Sakurai (3). 

The main advantage of using silyl ethers in cross<oupling reactions is the ability to incorporate them into molecules by a number of methods. Cyclic silyl ethers, as a class, nicely illustrate this attribute. The well-known hydrosilylation of alkynes to form vinylsilanes can easily be rendered intramolecular by attachment of the silane as, for example, a homopropargyl silyl ether to form an oxasilacyclopentane 108 (Scheme 7.28) [53]. In this stmcture, the double-bond geometry is defined by the stereochemical course of hydrosilylation and the ether tether defines the location of the silicon atom with respect to the alkene. Thus, the siHcon-oxygen bond in this molecule serves to direct the hydrosilylation, as well as to activate the siHcon for cross-coupling. 

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