Hydrosilation, of alkynes

Alkenylsilanes can be made by Lewis acid-catalyzed hydrosilation of alkynes. Both AlClj and C2H5A1C12 are effective catalysts.73 The reaction proceeds by net anti addition, giving the Z-alkenylsilane. The reaction is regioselective for silylation of the terminal carbon. 

Subsequently, cationic rhodium catalysts are also found to be effective for the regio- and stereoselective hydrosilation of alkynes in aqueous media. Recently, Oshima et al. reported a rhodium-catalyzed hydrosilylation of alkynes in an aqueous micellar system. A combination of [RhCl(nbd)]2 and bis-(diphenylphosphi no)propanc (dppp) were shown to be effective for the ( >selective hydrosilation in the presence of sodium dodecylsulfate (SDS), an anionic surfactant, in water.86 An anionic surfactant is essential for this ( )-selective hydrosilation, possibly because anionic micelles are helpful for the formation of a cationic rhodium species via dissociation of the Rh-Cl bond. For example, Triton X-100, a neutral surfactant, gave nonstereoselective hydrosilation whereas methyltrioctylammonium chloride, a cationic surfactant, resulted in none of the hydrosilation products. It was also found that the selectivity can be switched from E to Z in the presence of sodium iodide (Eq. 4.47). 

A recent investigation of the hydrosilation of alkynes by Et3SiH in the presence of the bridged derivative (PhCH2)Me2SiPt[P(cyclo-hexyl)3](p.-H) 2, analogous to (LVIII), has shown that the rate increases as the 7r-acceptor power of the alkyne increases, consistent with the formation of Si-Pt-alkyne complexes as intermediates 416). This complements earlier kinetic studies on the hydrosilation of alkenes catalyzed by Co, Rh, and Pt compounds, in which similar Si-metal-alkene intermediates were postulated 85, 134). 

A similar frani -selectivity was also found for the aluminum trichloride catalyzed hydrosilation of alkynes. Z-Vinylsilanes can be obtained by this method as the exclusive products. 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

B.i.c. Palladium-Catalyzed Hydrosilation ofAlkynes. Relatively little has been reported on the Pd-catalyzed hydrosilation of alkynes. This is in sharp contrast with the corresponding hydrostannation of alkynes discussed later in Sect. C. The reaction of terminal alkynes with HSiEts in the presence of polyborane-containing Pd and Pt complexes yielded the results summarized in Scheme 10, which indicate that Pd and Pt catalysts... 

In addition to these methods, platinum-catalysed hydrosilation of alkynes can be an effective method for the formation of vinyl silanes. Coupling of a vinyl silane 2346, made by hydrosilylation, with a crowded aryl iodide 2347 was a key step in a synthesis of an HMG-CoA reductase inhibitor, NK-104 (Scheme 2.109) 2.349. ... 

Takahashi s group has also reported hydrosilation of a number of alkynes with various silanes, using the [Cp2TiCl2/2 n-BuLi] combination, which leads to products with excellent regio- and -selectivity [Eq. (13)]. It is of note that known zircon-ocene and hafnocene species do not catalyze the hydrosilation of alkynes. 

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

Finally, recent catalytic studies in which silicon-metal intermediates have been implicated include the photocatalysis of hydrosilation (Si-Fe, 452 Si-Co, 448 Si-Rh, 441) and the double silylation of alkynes with disilanes (Si-Pd, 457). 

Other late transition metals used in special cases of hydrosilation include cobalt, iron, ruthenium (vide infra for reactions with alkynes), osmium, chromium, molybdenum, tungsten and copper. Metallocenes (see Metallocene Complexes) of early transition metals and lanthanides have also been found to catalyze the hydrosilation of a number of unsaturated compounds including alkenes and esters (vide infra). 

Alkynes can be advantageonsly hydrosilated to give vinylsUanes. The reaction with the triple bond is much faster than with an olefin, therefore a snbsequent hydrosilation step to the disilylalkane is nsnaUy not encountered. This different reactivity can be exploited in the hydrosilation of vinylacetylene, where 1-sUyl-l,3-butadiene is formed chemoselectively (eqnation 4). ... 

The hydrosilation of 1-alkynes can give either Z-or E-isomers (see (E) (Z) Isomers) of the linear vinylsilane or the Markownikow product (equation 5). Reaction of 1-hexyne with trichlorosilane under platinum catalysis conditions gives a mixture of 1- and 2-silyl-l-hexene (78 22). From the E-stereochemistry of the 1-silyl isomer, a yyn-addition mechanism can be assumed. 

The indenyl complexes are also efficient precatalysts for the hydrosUylation see Hydrosilation) of olefins, alkynes, and ketones. This reaction is believed to involve a hydrido intermediate, which can be generated initially via different routes (Scheme 9). 

Oxidative addition of the H—Si bond is the most studied of the group IVB elements with the exception of carbon because of its relation to the industrially important catalytic hydrosilation of alkenes, alkynes and ketones. Compounds containing the Si—M bond are stable. While they are also synthesized by routes other than oxidative addition of the H—Si bond T H—Si, H—Ge and H—Sn add to transition metals ... 

Other Rh catalysts were also employed for hydrosilation of a,p-unsaturated carbonyl compounds and unsaturated nitriles. Thus, Rh(acac)2 and a tetrakis( jL-acetato)dirhodium cluster were used as catalysts in the hydrosilation of a,P-unsaturated aldehydes. These reactions, however, are not chemoselective, as alkynes, conjugated dienes and alkenes are also hydrosilylated, and allylic heterosubstituents are reduc-tively cleaved. 

The hydrosilation of alkenes and alkynes is catalyzed by transition metal complexes, including several platinum species, and oxidative addition of the Si—H bond is a fundamental step in the process. 

Additions of Si-H bonds to alkynes occur under similar conditions and with the same catalysts as hydrosilation of alkenes. Free radical addition to terminal alkynes gives cis products by a stereospecific terminal trans addition . Supported platinum catalysts give trans products by a terminal cis addition . Chloroplatinic acid catalyzed additions to terminal alkynes give mixtures of trans-1-alkenylsilanes and trans-2-alkenylsilanes in a ratio ranging from 1 1 to 1 5 depending on the substituents on silicon . Addition of SiH2Cl2 to CH2=CHC(CH3)3 gives trans-1-alkenyl- and bis(trans-l-alkenyl)silane products, but no (2-alkenyl)silane °. Internal alkynes react more slowly than terminal alkynes, and even reactions catalyzed by chloroplatinic acid require heat. 

Hydrosilations of dienes and alkynes have been studied, and in both cases single and double silylations are possible. For 1,3-dienes, both 1,2 and 1,4 addition products can be observed8,195 213 271-272. A Ziegler-type catalyst composed of nickel(II) pentadienoate and triethylaluminum is active in hydrosilylation of terminal alkynes, but this reaction is accompanied by oxidative dimerization of the alkyne (equation 101). The major product in most cases was the head-to-head dimer273. 

Co2Rh2(CO)i2] and [Co3Rh(CO)i2] catalyze the hydrosilation of isoprene, cyclohexanone, cyclohexenone, and 1-alkynes, the regioseleetive inter- or intramolecular (Eq. 5) silylformylation of alkynes and 1-alkynals, and the silylcarbo-cyclization of enynes, diynes, and alkynals. These synthetically important reactions have been reviewed recently. 

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