Vinylsilanes dehydrogenative silylations

Computational and catalytic studies of the hydrosilylation of terminal alkynes have been very recently reported, with the use of [ Ir( r-Cl)(Cl)(Cp ) 2] catalyst to afford highly stereoselectively P-Z-vinylsilanes with high yields (>90%) [35]. B-isomers can be also found among the products, due to subsequent Z —> E isomerization under the conditions employed. The catalytic cycle is based on an lr(lll)-lr(V) oxidahve addition and direct reductive elimination of the P-Z-vinylsilane. Other iridium complexes have been found to be active in the hydrosilylation of phenylacetylene and 1-alkynes for example, when phenylacetylene is used as a substrate, dehydrogenative silylation products are also formed (see Scheme 14.5 and Table 14.3). 

Whereas, plahnum complexes are used predominantly as efficient catalysts in the hydrosilylation of carbon-carbon mulhple bonds, cobalt and iron triad complexes play a cmcial role in the catalysis of other processes, such as the hydrosi-lylahon of C=0 and C=N, dehydrogenative silylation, sUylcarbonylahon, and silylation with vinylsilanes and disilanes. 

An interesting variation of the dehydrogenative silylation system involves the platinum complex-catalyzed reaction of 1-alkenes with disilanes to produce vinylsilanes.40 In this system, one H atom and a silyl group are released by the reactants to yield the alkenylsilane product, rather than the two hydrogens released in reactions of hydrosilanes [Eq. (6)]. 

Vinylsilanes and allylsilanes are prepared by dehydrogenative silylation of alkenes catalysed by Rh and other complexes [221]. A particularly effective catalyst for alkenylsilanes is Ru3(CO)12 [222], Using excess 1-hexene, 1-silyl-1-hexene 575 was... 

A highly selective dehydrogenative silylation of ethylene proceeds in the presence of RuH2(H2)2(PCy3)2, where Cy is cyclohexyl, as a catalyst precursor to yield vinylsilane under very mild conditions (Eq. 11). The formation of vinyl-silane is promoted by high olefin-to-silane ratios [22]. 

The number of examples of highly selective dehydrogenative silylation is still limited. The most convincing examples are Ru3(CO)i2- and Fe3(CO)i2-cata-lyzed reactions of styrene [106, 114] and vinylsilane [115] with HSiEts, RuH2(H2)2PCy3)2-catalyzed reaction of ethylene with HSiEt3 [116], and cationic rhodium complex-catalyzed dehydrogenative silylation, e.g., [117], as well as the nickel equivalent of the Karstedt catalyst [105]. 

The dehydrogenative silylation of olefins, closely related to hydrosiiyiation, is promoted by a ruthenium carbonyl complex, RujCCOlij. The product vinylsilane is always the frans-isomer ... 

The dehydrogenative silylation of olefins, which is closely related to hydrosilylation, is effectively promoted by a ruthenium carbonyl cluster complex, Ru3(CO)12. The produced vinylsilane was the trans-isomer in every case examined76 (equation 26). 

For instance, the reaction of EtaSiH and 2 equiv. of p-methoxystyrene in toluene with 1.0 mol% of 16a afforded at 100°C within 6 h the dehydrogenative silylation product ( )-l-(p-methoxystyryl)-2-(triethyl-silyl)ethylene in 95% yield. The reaction is of high selectivity that neither (Z)-isomers, nor branched dehydrogenative silylation products were seen. Less hydridic silanes, such as triphenylsilane, were less efficient than for instance EtsSiH. Other substituted styrenes such as p-methyl, p-chloro-, and p-fluorostyrene also afforded the corresponding tran -vinylsilanes in high yields and selectivities (up to 98%). In the case of aliphatic alkenes, such as -octene, allyltriethoxysilane, vinylcyclohexane, and ethylene, dehydrogenative silylations were still preferred, but showed less E/Z selectivity. Cyclic olefins, such as cyclooctene, furnished low conversions under the same reaction crmditions. The results are summarized in Scheme 19. 

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]. 

The hydrosilylation of alkenes (Equation 16.12) and alkynes (Equation 16.13), alternatively termed hydrosilation, is the addition of a silicon-hydrogen bond across the C-C TT-bond to form a new alkylsilane or vinylsilane. This reaction has been catalyzed by complexes containing many different metals, but is most commonly conducted with complexes of platinum, rhodium, and palladium. The hydrosilylation of alkenes t3q>ically forms terminal alkylsilanes as the major regioisomer, and the hydrosilylation of vinylarenes often generates the chiral branched alkylsilane. The hydrosilylation of alkynes has also been developed. As shown generally in Equation 16.13, these reactions can occur by either cis or trans addition, depending on the catalyst. In some cases, the reactions of silanes with olefins form vinylsilanes (called dehydrogenative silylation. Equation 16.14). The addition of an Si-Si bond of a disilane across an olefin has also been reported (Equation 16.15), and this reaction is called disilation of olefins. 

Contrary to the previously reported reactions with the M-H and M-Si initial complexes the proposed mechanism of catalysis by [(cod)M(OSiMe3)]2 (where M= Rh, Ir) does not involve highly activated migratory insertion of olefin into the Rh-Si bond (the associative mechanism) since the final step of the product formation occurs via a lower activated step of reductive elimination of product (the dissociative mechanism) (Scheme 4). The reaction under study is conceptually related to dehydrogenative silylation since the basic reaction involves the silylation of a substrate such as styrene by vinylsilane instead in the hydrosilane, equations 17a and 17b. by hydrosilanes... 

Contrary to the Pt-catalyzed hydrosilylation, the complexes of iron (Fe, Ru, and Os) and cobalt (Co, Rh, and Ir) triads catalyze dehydrogenative silylation (formation of vinylsilanes) competitively with the hydrosilylation (see Scheme 4). 

Although a lot of data have been reported on such competitive reactions, the number of examples of selective dehydrogenative silylation remains limited. Over the past two decades, the research efforts have been focused on the search for new selective catalysts of dehydrogenative silylation ensuring efficient generation of vinylsilanes and other vinylsilicon compounds as well as on mechanistic implications of transition metal complexes as real intermediates of these complicated processes. Dehydrogenative silylation has become a useful method for synthesis... 

A family of neutral cationic and zwitterionic Rh(I) and Ir(I) complexes with P,N-, P,0-, P(S),0-, and P(S),N-substituted indene ligands have been tested in the hydrosilylation and the dehydrogenative silylation of st3rrene. Cationic rhodium complexes, such as [Rh(cod)2]+BF4, have recently appeared as regio- and stereoselective catalysts for hydrosilylation of alkynes and exclusive formation of vinylsilanes instead of alkylsilanes in the hydrosilylation of alkenes (73). 

The complex [RuHCl(CO)(Pi-Pr3)2] was found to be a very active and highly selective catalyst for the addition of triethylsilane and [RuHCUCOlCPPhsls] was used in nonselective reaction of l-(trimethylsilyl)-l-buten-3-yne with triorganosi-lanes. On the other hand, [RuH2(H2)2(PCys)2] is a highly effective precursor for the selective dehydrogenative silylation of ethylene to vinylsilane (13). 

As in hydroformylation, both linear and branched products can be obtained from RCH=CH2.The dehydrogenative silylation product, RCH=CHSiR3, is often present and can even predominate under some conditions (Eq. 9.21). The dehydrogenative path can only be explained on the modified Chalk-Harrod mechanism of Eq. 9.23, in which the alkene first inserts into the M-Si bond. 3 elimination of the intermediate alkyl now leads directly to the vinylsilane, the two H atoms thus released go on to hydrogenate the substrate leading to coproduction of alkane. As in hydrogenation, syn addition is generally observed. Apparent anti addition is due to isomerization of the intermediate metal vinyl, as we saw in Eq. 7.21, also a reaction in which initial insertion of alkyne into the M-Si bond must... 

The main side reaction of the hydrosilation reaction is the dehydrogenating silation reaction. Under certain conditions this reaction can be the main or even the exclnsive reaction. The reaction can occnr, not only with alkenes, bnt also with almost all known substrates. It achieves vinylsilanes from alkenes, silylalkynes from alkynes, and silyl enol ethers from ketones. ... 

Systems and conditions that proceed cleanly by route c (Scheme 6.62) are efficient for catalytic dehydrogenative silation. A M-SiRs source is necessary and this can be a silane, with concomitant reduction of the alkene to give an alkane (Scheme 6.62, c). /l-SiRa elimination has been artfully used to produce a M-SiRs moiety from vinylsilanes or allylsilanes. Scheme 6.63 depicts the use of allylsilanes described by Murai et ai. to produce silyl substituted alkenes and propene as byproduct [194b]. 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

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