Silylative coupling process

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. 

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. 

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. 

Although the silylative coupling cyclization is accompanied by linear oligomeric products and the process required a longer reaction time (3-4 weeks), this particular method opens up a new route to synthesize silicon-containing e ro-methylenes, which cannot be prepared via ring-closing diene metathesis. 

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. 

The occurrence of two competing pathways was also observed in the reaction of some enol silyl ethers with diphenyliodonium fluoride (19), which afforded the mono- or the di-phenylated products in moderate to good yields. This C-phenylation reaction is likely to occur by a ligand coupling process. However, with some hindered silyl enol ethers, for example (20), diketone dimers (21) were also isolated, indicative of a free-radical component in the overall mechanistic picture. ... 

Hydrolysis of the trialkyl silyl groups gives poly(vinyl alcohol). Block copolymers in which one block is poly( vinyl alcohol) can be synthesized if a telechehc with an aldehyde terminal group is used to initiate the aldol GTP structures such as polyfsty-rene-h/oc -vinyl alcohol) can be prepared in this way. Alternatively, as silyl ketene acetals can react with aldehydes, block structures can be formed by a coupling process. 

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]. 

Although for the catalytic transformations of organosilicon compounds only hydrosilylation is well known as industrially important process, in the last 20 years other reactions of silicon compounds catalyzed by transition metal complexes have been discovered and developed. They include double (bis)silylation of alkenes and alkynes, silylative coupling of alkenes and alkynes with vinylsi-lanes, dehydrocoupling of hydrosilanes, silylformylation and silylcarbonylation of unsaturated compounds, and dehydrogenative silylation of alkenes and alkynes with hydrosilanes. Only the latter, as related to hydrosilylation (and very often its side reaction), has been discussed here (13). 

The Cadiot-Chodkiewicz coupling typically proceeds under conditions which are considerably milder than Castro-Stephens reactions. Triethylsilylacetylene 74 rapidly undergoes Cadiot-Chodkiewicz coupling with alkynyl bromide 75 to generate the unsymmetrical bisalkyne 76 in nearly quantitative yield when those two reactants are treated with catalytic cuprous chloride and catalytic ammonium hydroxide in -butylamine solution. This coupling process affords one of the best entries into compounds such as 76 and is permissive of TES and larger silylated copper acetylide species because of the lower reaction temperature. ... 

Optically active (Z)-l-substituted-2-alkenylsilanes are also available by asymmetric cross coupling, and similarly react with aldehydes in the presence of titanium(IV) chloride by an SE process in which the electrophile attacks the allylsilane double bond unit with respect to the leaving silyl group to form ( )-s)vr-products. However the enantiomeric excesses of these (Z)-allylsilanes tend to be lower than those of their ( )-isomers, and their reactions with aldehydes tend to be less stereoselective with more of the (E)-anti products being obtained74. 

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