Silylative coupling alkenes with vinylsilanes

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

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. 

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. 

Evidence for the migratory insertion of ethylene [46] and vinylsilane [47] into the Ru-Si bond yielding vinylsilane and two bis(silyl)ethene regioisomers [E-l,2-bis(silyl)ethene and l,l-bis(silyl)ethene],respectively,has proved that in the reaction referred to as the metathesis of vinylsilanes and their cometathesis with olefins, instead of the C=C bond cleavage formally characterizing alkene metathesis (Eq. 24a), a new type of olefin conversion that is a silylative coupling of olefins with vinylsilanes occurs (Eq. 24b). 

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]. 

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]. 

While vinylsilanes undergo productive cross-metathesis (Mo and Ru carbenes) with allyl-substituted functionalized alkenes, their effective transformation with derivatives containing a fimctionalized group attached directly to a carbon -carbon double bond can be achieved only via silylative coupling catalyzed by metal complexes containing (or generating) M-H and/or M-Si bonds (M = Ru, Rh, Ir). 

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. 

---