Silylative coupling reactions

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). 

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]. 

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]. 

The silylative coupling reaction of l,2-bis(dimethyIvinylsiloxy)ethane was effectively catalyzed by 1 mol% of ruthenium hydride catalyst, and the divinyl compound was completely consumed within 1 h at 80 °C. The reaction successfully proceeded without the solvent under air, but toluene could also be employed without affecting either the activity of the catalyst or the selectivity of this process. Application of this catalytic system for silylative coupling cyclization of l,2-bis(dimethyl-vinylsiloxy)ethane gave exclusively a cyclic product (Isolated yield 85%) with the exo-methylene bond between two silicon atoms in the molecule (2,2,4,4-tetramethyl-3-methylene-l,5-dioxa-2,4-dlsilacycloheptane) accompanied only by trace amounts of oligomers. 

Addition reactions are not the sole processes of the transformation of the vinylsilane group to another functional group. A silylative coupling reaction recently discovered by Marciniec et al. [12], may be used for the introduction of functional groups to polysiloxane, too. 

Whereas the vinyl groups of Da are accessible for functionalization by hydroboration or hydrosilylation, they are inert to functionalization by cross-metathesis. Alternatively, formal metathesis products can be obtained by the ruthenium-catalyzed silylative coupling reaction. This method involves the combination of a vinyl silane and an olefin in the presence of a ruthenium catalyst, to provide an alkenylsilane (see eq 7). The application of this reaction to Da provides substitution at each of the four vinyl groups, resulting in a cyclic tetraalkenyltetramethylcyclote-trasiloxane. The silylative coupling reaction of both Da and has been demonstrated with styrenes and enol ethers. ... 

Alkenylsilanes generally do not participate in traditional crossmetathesis reactions. Alternatively, the ruthenium-catalyzed silylative coupling reaction of vinylsilanes and olefins provides formal metathesis products (eq 7). Although the products are the same, the reactions proceed through a distinct mechanism, converting vinylsilanes into substituted alkenylsilanes. 

The synthesis of vinyl iodides and bromides starting from unactivated styrenes has been achieved using a silylative coupling reaction, followed by trapping with NXS (Scheme 7.125) [188]. The authors focused on the preparation of vinyl iodides and bromides due to their popularity in transition metal-catalyzed cross-coupling reactions. The first step in the synthesis was a ruthenium-catalyzed silylative coupling using the vinyl silane. The intermediate vinyl... 

Monosubstitution of acetylene itself is not easy. Therefore, trimethylsilyl-acetylene (297)[ 202-206] is used as a protected acetylene. The coupling reaction of trimethylsilylacetylene (297) proceeds most efficiently in piperidine as a solvent[207]. After the coupling, the silyl group is removed by treatment with fluoride anion. Hexabromobenzene undergoes complete hexasubstitution with trimethylsilylacetylene to form hexaethynylbenzene (298) after desilylation in total yield of 28% for the six reactions[208,209]. The product was converted into tris(benzocyclobutadieno)benzene (299). Similarly, hexabutadiynylben-zene was prepared[210j. 

It is considered that the stannyl or silyl radical and the alkyl radical are reactive intermediates in these reactions. In contrast to the selective formation of the arylchalcogenosilanes in the above radical reactions, the cross-coupling reaction of a hydrosilane with alkyl(aryl)sulfides catalyzed by palladium nanoparticles results in the selective formation of the corresponding alkylthiosilanes.42... 

Classical C,C-coupling reactions of AN anions (Henry, Michael, and Mannich) involve complex systems of equilibria and, consequently, generally not performed in protic solvents. The introduction of the silyl protecting group allows one to perform these reactions in an aprotic medium to prepare or retain products unstable in the presence of active protons. In addition, the use of nucleophiles which are specifically active toward silicon (e.g., the fluoride anion) enables one to design a process in which the effective concentration of a-nitro carbanions is maintained low. 

Steric hindrance in the silyl groups of cation (349) and nucleophile (352) has virtually no effect on the rate constant of the C,C-coupling reaction. Hence, it can be concluded that, at least for silyl-containing nucleophiles (352), elimination of the trialkylsilyl group from cationic intermediate A is not the rate-determining step of the reaction sequence (Scheme 3.207). 

Both silyl- and alkyl esters derived from primary AN are readily involved in C,C-coupling reactions with silyl ketene acetal (Scheme 3.208, Eq. 1) (484). 

Two procedures were developed for C,C-coupling reactions of silyl esters of primary AN. One approach involves two steps and the synthesis of intermediate SENAs according to standard procedures. Another procedure is based on the one-pot reaction of AN with the DBU/TBSOTf system in a ratio of 1 1.1 followed by the addition of silyl ketene acetal and a catalytic amount of TBSOTf. 

SENAs derived from primary AN can undergo C,C-coupling reactions with other silyl-containing nucleophiles (Eq. 2), which was exemplified by the reaction of SENA derived from nitroethane. It should be noted that high diastereoselectiv-ity can sometimes be achieved by introducing a substituent at the reaction center of the nucleophile. 

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