Silylative coupling

The cross metathesis of vinylsilanes is catalyzed by the first-generation ruthenium catalyst 9. This transformation has been extensively investigated from both preparative and mechanistic points of view by Marciniec et al. [86]. Interestingly, the same vinylsilanes obtained from cross metathesis may also result from a ruthenium-hydride-catalyzed silylative coupling and there might be some interference of metathesis and nonmetathesis mechanisms [87]. 

Table 12 Compounds TsRs obtained by cross- -metathesis or silylative coupling of T8[CH = CH2]8 w ith alkenes ... 

Platinum complexes have been mainly used in the hydrosilylation of carbon-carbon bonds, and ruthenium complexes in the metathesis and silylative coupling of olefins with vinylsilanes. Most of these processes (except for olefin metathesis) may also proceed efficiently in the presence of rhodium and iridium complexes. 

During the past two decades, within the series of our studies, we have developed a silylative coupling reaction of olefins with vinylsubstituted siHcon compounds which takes place in the presence of transition-metal complexes (e.g. mthenium and rhodium) that initially contain or generate M—H and M—Si bonds (for reviews, see Refs [5] and [6]). The reaction involves activation of the =C—H bond of olefins and cleavage of the =C—Si bond of vinylsilane. The reaction, which is catalyzed by complexes of the type [ M( x-OSiMe3)(cod) 2] (where M = Rh, Ir) occurs according to Equation 14.12 [71, 72). 

However, in the presence of iridium siloxide and bulky substituents at the silicon in vinylsilanes instead of silylative coupling, the codimerization (hydrovi-nylation) of styrene and vinylsilanes occurs according to Equation 14.13 ... 

Another approach to silyl-coupled cyclodisilazanes starts with a fluorosilyl-substituted cyclotrisilazane and half an equivalent of BuLi. In this case, only half of... 

According to Scheme 2 an equilibrium exists between the anions of cyclotri- and cyclodisilazanes. Based on that equilibrium many isomeric ring compounds, silyl coupled four-membered rings and bicyclic Si—N compounds 111-115 are prepared (equation 31)96,97. The equilibrium of the lithium derivatives depends very strongly on the reaction conditions, for example on the temperature of the reaction 112 and 116 are formed at different temperatures (equation 32). 

The lithium compound has a polymeric lattice structure in the crystal, formed via Li F contacts. The LiNSiF part of the compound forms an eight-membered ring. The lithium is three-coordinated (sum of the angles = 359.3°)96,97. In the reactions of the lithium derivative with fluorosilanes, exocyclic substitution occurs, e.g., to give 119 (equation 33). The lithium compounds are stable up to about 100 °C. At higher temperatures Li—F elimination occurs and silyl coupled cyclodisilazanes (120) are obtained (equation 34)96,97. 

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

Silylative Coupling Condensation of Silicon-Containing Dienes.225... 

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. 

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). 

Many ruthenium complexes have been tested in the silylative coupling reaction. In the synthetic procedure the absence of by-products of the homocoupling of vinylsilanes is required so an excess of the olefin has usually been used. However, the screening tests performed at the 1 1 ratio of styrene and phenyldimethylvinylsilane with a variety of ruthenium catalysts have shown that pentacoordinated monocarbonyl bisphosphine complexes appear to be the most active and selective catalysts of which RuHCl(CO)(PCy3)2 has shown high catalytic activity under conditions of catalyst loadings as low as 0.05 mol % [55]. Cuprous salts (chloride, bromide) have recently been reported to be very successful co-catalysts of ruthenium phosphine complexes, markedly increasing the rate and selectivities of all ruthenium phosphine complexes [54]. 

Organosubstituted octasilsesquioxanes (Fig. lb) have also been prepared by cross-metathesis (CM) and silylative coupling of vinylsilsesquioxane with olefins in the presence of the ruthenium carbene complex Cl2(PCy3)2Ru(=CHPh) (Grubbs catalyst) and Ru-H (Ru-Si) complexes, for example, RuHCl(CO)(PCy3)2, respectively [57]. 

In the silylative coupling reactions of olefins and dienes with vinylsubsti-tuted silanes, ruthenium catalysts, containing initially or generating in situ Ru-H/Ru-Si bonds, catalyze polycondensation of divinylsubstituted silicon compounds to yield unsaturated silylene (siloxylene, silazanylene)-vinyl-ene-alkenylene (arylene) products (Eq. 112). For recent results see Refs. [177, 178] and for reviews see Refs. [6,7,117,118]. 

In the presence of a ruthenium complex, divinylsilicon compounds can also undergo co-polycondensation with dienes, for example, 1,4-divinylbenzene [178,179]. For recent reviews on the silylative coupling (co)polycondensation see Refs. [6,7,117,118]. 

The R1 values obtained for such phenylethynyl substituted siloxanes are higher then that reported for traditional aromatic-based systems [9] or the phenol modified ones (1.50-1.53) [10]. The synthesis of high refractive index (methyl)(diphenyle thenyl)-dichlorosilane via hydrosilylation was also described [1]. Such monomer was later hydrolyzed and condensed into silicone fluid. Similar process was also presented, applying silylative coupling process in the synthesis of an analogous (methyl)(phenylethenyl)diethoxysilane [11], so the two reactions shall be discussed in the following section. 

Comparison of effectiveness of the two processes, carried out in the presence of metallacarbene (metathesis) and non-metallacarbene catalysts (silylative coupling) is shown in Table 2. 

Table 2 Comparison of effectivness of metathesis and silylative coupling catalysts...

Coupling at a Susceptible Position Via Silanes De-silyl-coupling... 

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]. 

Keywords cross-metathesis, silylative coupling, tmn.s-silylation, ruthenium catalysts, vinylsilicon compound... 

Summary Two catalytic reactions, i.e. silylative coupling (mms-silylation) (SC) catalyzed by complexes containing or generating Ru-H and/or Ru-Si bonds (I, II, V, VI) and cross-metathesis (CM) catalyzed by mthenium-carbene (i.e. 1st and 2nd generation mthenium Grubbs catalyst (ID, IV)) of vinyl and allyl-substituted hetero(N,S,B)organic compounds with conunercially available vinyltrisubstituted silanes, siloxanes, and silsesquioxane have been overviewed. They provide a universal route toward the synthesis of well-defined molecular compounds with vinylsilicon functionality. 

In the last 15 years we have developed two new catalytic reactions between the same parent substances, i.e. silylative coupling (SC) (also called tmns-silylation or silyl groiq> transfer) and cross-metathesis (CM) of alkenes, which have provided an universal route for the synthesis of well-defined molecular compounds with vinylsilicon functionality. While the cross-metathesis is catalyzed by well-defined Ru and Mo carbenes, the silylative coupling is catalyzed by complexes initialing or generating M-H or M-Si bonds (where M = Ru, Rh, Ir). For recent reviews see Refs. [4-6],... 

Vinylamides such as V-vinylpyrrolidone, V-vinylphtalimide and V-vinylformamide undergo, in the presence of (I), stereoselective silylative coupling with various vinylsilanes up to ElZ = 99/1 [17] as well as with octavinylsilsesquioxane [15] (E Z= 95/5). Extended heating of the reaction mixture allowed exclusive isolation of the -isomer because of Z isomerization of the products. In contrast to the cross-metathesis with vinyl ethers, no reaction of iV-vinylpyrrolidinone... 

It has been assumed that the reaction of vinylsilanes with vinyl alkyl sulfides would also occur in the same manner as the reaction with vinyl ether [9] or vinyl pyrrolidinone [17], exhibiting active silylative coupling and inactive cross-metathesis. Nevertheless, we have observed the reverse. 

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