Alkynes metal-catalyzed hydrosilylation

Pyridyldimethylsilane 24 is a reagent for the metal-catalyzed hydrosilylation of alkynes and alkenes <2004EROS1>. 4-(Trifluoromethyl)pyridine 25 has been used as a building block in heterocycle synthesis <2005EROS1>. 

Metal-catalyzed hydrosilylation of alkynes constitutes the most efficient and atom-economical access to highly valuable vinylsilanes [27,28]. This transformation can afford three different isomers a,p-( ) and p-(Z) (Scheme 5.11). Their ratio depends upon the metal, its ligands, the alkyne, the silane and the reaction conditions. The modification of one of these parameters often leads to impressive effects on the outcome of the reaction. 

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). 

A variety of transition metal complexes catalyze hydrosilylation of alkynes. Catalysis of hydrosilylation by rhodium gives T -alkenylsilanes from 1-alkynes.74... 

Another important contribution of transition metal-catalyzed alkyne hydrosilylation continues to be the mechanistic analysis of catalysis. As a relatively simple addition process, hydrosilylation has lent itself to extensive and thorough mechanistic analysis yet, numerous reaction pathways have now been postulated, and it is clear that many paths are possible. Importantly, principles from hydrosilylation reactions often are of use in the understanding of related, more complex transformations. Despite past achievements, much current thinking is based as much on speculation as on solid data. It is likely that continued, detailed exploration of the mechanistic underpinnings of hydrosilylation reactions will lead to new understanding and better reactions. 

These critical aspects of the classical fluorous biphasic catalysis led in recent works to the development of protocols for the conversions with modified catalyst systems in non-fluorinated hydrocarbons as solvents. As part of the BMBE lighthouse project, Gladyzs and coworkers appHed this concept to C - C coupHng reactions (Suzuki reaction) and other metal-catalyzed addition reactions (hydrosilylation, selective alcoholysis of alkynes), which have direct relevance for the synthesis of fine chemicals and specialties [74]. 

The most notable point of this reaction is that the internal sp-c xhon is selectively carbonylated to form (Z)-14a predominantly, although the ZjE ratio is likely to depend on reaction temperature, time, and catalyst precursor. It is revealed that the stereochemistry of the transition metal-catalyzed addition to alkynes is intrinsically cis. Isomerization from (Z)-14a to ( )-14a proceeds as an independent event from silylformylation. This feature sharply contrasts to the results observed in hydrosilylation of 13 with Me2PhSiH (Equation (3)). ... 

Herein, we report on a novel process for the synthesis of organomodlfied polydimethylsiloxanes employing ionic liquids for the heterogenization and/or immobilization of the precious metal catalyst [13]. The advantage of this novel hydrosilylation process is that standard hydrosilylation catalysts can be used without the need for prior modification to prevent catalyst leaching. To the best of our knowledge, this is the first example of a hydrosilylation of olefmic compounds using ionic liquids (Scheme 1). However, a method for the transition metal-catalyzed hydroboration and hydrosilylation of alkynes in ionic liquids has recently been described [14]. 

In reactions that are formally analogous to hydrosilylation, transition-metal complexes catalyze the insertion of unsaturated hydrocarbons into other Si-X (X = C, Si, Sn, etc.) bonds. Palladium catalysts seem to work best in many of these reactions. The work of Kumada and coworkers has already been referred to in connection with metal-catalyzed silylene transfer to alkynes (see equation 47)95"97. Sakurai s group has shown that the cyclic disilane 55 will add to alkynes in the presence of a palladium catalyst (equation 110, see also equation 80). The unstrained disilane Me3SiSiMe3 undergoes a similar reaction... 

This trend was rather surprising, since it had been generally accepted that a transition metal or metal complex catalyzed hydrosilylation of alkynes proceeded through exclusive ds-addition2. The stereoselectivity observed in this reaction resembles that obtained in the peroxide-promoted reaction of trichlorosilane with alkynes63,37. 

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. 

Organolanthanide catalysts provide a new alternative to traditional, late transition metal-based catalysts for selective hydrosilylation of alkenes and alkynes (for recent reviews, see (105)). Mechanistically, the transformation is characteristic of early transition metal-catalyzed reaction. However, it is worth... 

Several independent protocols using a combination of transition metal-catalyzed stereoselective hydrosilylation, such as palladium-catalyzed crosscoupling sequence leading to stereodefined r-conjugated alkene derivatives, have been successfully developed in the last decade (4). Alkenylsilanes or siloxanes, prepared via platinum or rhodium complex-catalyzed intermolecular hydrosilylation of terminal alkynes have been highly stereospecifically cross-coupled with aryl and alkenyl halides to give unsymmetrical stilbenes, alkenylbenzenes, and conjugated dienes (Scheme 24) (4). 

It has been shown that the stereochemistry of the hydrosilylation of 1-aUcynes giving 1-silyl-l-alkenes depends on the catalysts or promoters used. For example, the reactions under radical conditions give the cis-product predominantly via trans-addition , while the platinum-catalyzed reactions afford the trans-product via exclusive cts-addition. In the reactions catalyzed by rhodium complexes, thermodynamically unfavorable c/s-1-silyl-l-alkenes are formed via apparent trans-addition as the major or almost exclusive product. Since the trans-addition of HSiEts to 1-alkynes catalyz by RhCl(PPh3)3 was first reported in 1974 , there have been controversy and dispute on the mechanism of this mysterious trans-addition that is vray rare in transition-metal-catalyzed addition reactions to aUtynes. Recently, iridium 4i6 mthenium complexes were also found to give the ds-product with extremely high selectivity (vide supra). 

Reaction with Lithium Alnminum Hydride. Treatment of the reagent with LiAIELj followed by acetylation of the resulting metal alkoxide with acetyl chloride in situ leads to dimethyl[2-(2-acetoxymethyl)phenyl]silane (eq 5). The hydrosilane thus obtained undergoes the platinum-catalyzed hydrosilylation of alkynes to give alkenyl[2-(hydroxymethyl)phenyl]dimethyl-silanes, which can be used for the above transformations, upon deacetylation under basic conditions. 

Other computational studies involving NHC-Cu species considered the formation of phenylisocyanates from nitrobenzene, and the development of [3+2] cycloaddition reactions for the formation of 1,2,3-triazoles. In the latter case the use of NHCs allowed the direct use of copper(i) catalysts, whereas copper(ii) precursors were predominant before. With [(NHC)CuBr] the reaction could be run on water and was successful even for internal alkynes, an unusual observation because the intermediacy of Cu-acetylides had previously been assumed. Calculations showed that the [(SIMes)Cu] fragment was ideally set up to bind internal alkynes in an i] -fashion and hence activate them towards cycloaddition. With terminal alkynes the acetylide route may still be operative. Other computational studies on the catalytic activity of [(NHC)Cu] complexes in which the NHC has no particular role but to stabilize the metal by strong o-donation and offer steric protection have been reported, including the activation of CO2 by [(NHC)Cu(EPh3)] (E = Si, Ge, Sn) and the carboxylation of the C-H bond of heteroarenes. The barriers of the two steps of the catalytic cycle of the [(NHC)Cu ]-catalyzed hydrosilylation of ketones have been computed, yet it was shown that the nature of the NHC was not a controlling factor. While the barrier of the hydrocupration step is determined by the nature of the ketone, that of the o-bond metathesis step occurs mainly under electronic control. 

Organosilicon compounds are widely used in our daily life as oil, grease, rubbers, cosmetics, medicinal chemicals, etc. However, these compounds are not naturally occurring substances but artificially produced ones (for reviews of organosilicon chemistry, see [59-64]). Hydrosilylation reactions catalyzed by a transition-metal catalyst are one of the most powerful tools for the synthesis of organosilicon compounds. Reaction of an unsaturated C-C bond such as alkynes or alkenes with hydrosilane affords a vinyl- or alkylsilane, respectively (Scheme 16). 

The very first example of the catalytic reductive cyclization of an acetylenic aldehyde involves the use of a late transition metal catalyst. Exposure of alkynal 78a to a catalytic amount of Rh2Co2(CO)12 in the presence of Et3SiH induces highly stereoselective hydrosilylation-cyclization to provide the allylic alcohol 78b.1 8 This rhodium-based catalytic system is applicable to the cyclization of terminal alkynes to form five-membered rings, thus complementing the scope of the titanocene-catalyzed reaction (Scheme 54). 

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