Hydrosilylation regioselective

In the presence of HjPtClg, hydrosilanes add to various 1-alkenes to give the corresponding terminal adducts in high yields ". Halogenated olefins such as 3,4-dichlo-robut-l-ene , 3,3,3-trifluoropropene , perfluoroethylene , and 1,1-difluoroethene are also readily hydrosilylated. 1,1-Difluoroethene is hydrosilylated regioselectively to give... 

A higher degree of regioselectivity can be attained by the use of lower temperatures, as exemplified by the hydrosilylation of hex-l-yne with trichlorosilane (J). 

The hydrosilylation of alkynes has also been studied using as catalysts Pt, Rh, Ir and Ni complexes. The improvement of the regioselectivity of the catalyst and the understanding of stereoelectronic factors that control it have been major incentives for the ongoing research. From numerous studies involving non-NHC catalysts, it has been established that there is a complex dependence of the product ratio on the type of metal, the aUcyne, the metal coordination sphere, the charge (cationic versus neutral) of the catalytic complex and the reaction conditions. In the Speier s and Karstedt s systems, mixtures of the thermodynamically more stable a- and -E-isomers are observed. Bulky phosphine ligands have been used on many occasions in order to obtain selectively P-f -isomers. 

A variety of other addition reactions occurring regioselectively are also known. These include hydrocyanation, hydroalumination, hydrosilylation, and hydrozirconation. 

Palladium-catalyzed hydrosilylation of styrene derivatives usually proceeds with high regioselectivity to produce benzylic silanes, 1-aryl-1-silyle thanes, because of the... 

A Ni(0)-catalyzed l,oo-hydrosilylation across the two dienyl moieties of 1,3,8,10-undecatetraene 9 proceeds regioselectively and stereoselectively and provides vie-trans-divinyl cyclopentane products 10 in modest yield (Eq. 3) [13]. The reaction shows an interesting stereoselectivity with respect to the substituent geometry both of the vinyl groups of 10a and 10b are stereoisomeric to each other, and one of the two double bonds is cis and the other is trans. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

These results clearly demonstrate a remarkable regioselectivity in the hydrosilylation of enyne compounds and that this approach works well for syntheses of siloxane polymers with reactive olefinic groups. 

Wilkinson s catalyst brings about the hydrosilylation of a range of terminal alkenes (1-octene, trimethylvinylsilane) by 2-dimethylsilylpyridine with good regioselectivity for the anti-Markovnikoff product. Both 3-dimethylsilylpyridine and dimethylphenylsilane are less reactive sources of Si-H. In contrast, these two substrates are far more reactive than 2-dimethylsilylpyridine for the hydrosilylation of alkynes by [Pt(CH2 = CHSiMe2)20]/PR3 (R = Ph, Bu ). This difference was explained to be due to the operation of the two different pathways for Si-H addition—the standard Chalk-Harrod pathway with platinum and the modified Chalk-Harrod pathway with rhodium.108... 

The hydrosilylation of butadiene proceeds with palladium compounds even in the absence of phosphines. Other ligands, such as glyoxime, benzonitrile, and 1,5-cyclooctadiene, can be used as effective ligands for the hydrosilylation of butadiene (65, 67). The reaction of trichlorosilane and dichlorosilane with isoprene proceeded regioselectively and stereo-selectively to give Z-l-trichlorosilyl-2-methyl-2-butene (67) (65, 66, 68). No reaction of trimethylsilane with isoprene took place, and this shows the lower reactivity of trialkylsilane. 

The rhodium-catalyzed cyclization/hydrosilylation of internal diyne proceeds efficiently with high stereoselectivity (Scheme 106). However, terminal diynes show low reactivity to rhodium cationic complexes. Tolerance of functionalities seems to be equivalent between the rhodium and platinum catalysts. The bulkiness of the hydrosilane used is very important for the regioselectivity of the rhodium-catalyzed cyclization/hydrosilylation. For example, less-hindered dimethylethylsilane gives disilylated diene without cyclization (resulting in the double hydrosilylation of the two alkynes), and /-butyldimethylsilane leads to the formation of cyclotrimerization compound. 

The asymmetric hydrosilylation that has been most extensively studied so far is the palladium-catalyzed hydrosilylation of styrene derivatives with trichlorosilane. This is mainly due to the easy manipulation of this reaction, which usually proceeds with perfect regioselectivity in giving benzylic silanes, 1-aryl-1-silylethanes. This regioselectivity is ascribed to the formation of stable 7t-benzylpalladium intermediates (Scheme 3).1,S Sa It is known that bisphosphine-palladium complexes are catalytically much less active than monophosphine-palladium complexes, and, hence, asymmetric synthesis has been attempted by use of chiral monodentate phosphine ligands. In the first report published in 1972, menthyldiphenylphosphine 4a and neomenthyldiphenylphosphine 4b have been used for the palladium-catalyzed reaction of styrene 1 with trichlorosilane. The reactions gave l-(trichlorosilyl)-l-phenylethane 2 with 34% and 22% ee, respectively (entries 1 and 2 in Table l).22 23... 

A chiral bis(oxazolinyl)phenylrhodium complex was found to catalyze the asymmetric hydrosilylation of styrenes with hydro(alkoxy)silanes such as HSiMe(OEt)2 (Scheme 7).47 Although the regioselectivity in forming branched product 27 is modest, the enantiomeric purity of the branched product 27 is excellent for styrene and its derivatives substituted on the phenyl group. The hydrosilylation products were readily converted into the corresponding benzylic alcohols 29 (up to 95% ee) by the Tamao oxidation. 

Hydrosilylation of alkyl-substituted 1,3-dienes 46g-46j in the presence of a ferrocenylmonophosphine-palladium catalyst also proceeded with high regioselectivity to give the corresponding 1,4-addition products with moderate enantioselectivity (entries 13-16).52 62 Enantioselectivity was improved by using ligands 37f and 37g (entries 17 and 18).57a... 

It is well documented that hydrosilylation of alkyl-substituted terminal olefins catalyzed by transition metal complexes proceeds with high regioselectivity in giving linear hydrosilylation products which do not possess a stereogenic carbon center.2 It follows that the asymmetric synthesis by use of the hydrosilylation of alkyl-substituted... 

The fact that isomers 8a, 8b and 8c are the lowest energy styrene coordinated complexes have potentially important ramifications that concern the modified-Chalk-Harrod mechanism and the regioselectivity observed in the hydrosilylation. With the modified-Chalk-Harrod mechanism olefin insertion into the M-Si bond follows styrene coordination. However, in all three isomers depicted in Figure 8, the coordinated hydride lies between the... 

If we were to assume that the reaction followed the Chalk-Harrod mechanism, then insertion of the olefin into the Pd-hydride bond in all three isomers 8a-c would lead to the correct regioisomer product. Thus, to some degree the regioselectivity of the hydrosilylation in this catalyst system is determined in the styrene coordination step. We will discuss the origin of the regioselectivity in more detail in Section 4. 

The formation of the highly stabilized V-allyl complexes have important implications to the regioselectivity of the hydrosilylation. The reason for this is that the p3-allyl complex cannot form if the hydride is transferred to the 3-carbon of the styrene which leads to the unbranched regioisomer of the product. We will discuss the regioselectivity in more detail in Section 4. 

A detailed study of the mechanism of the enantioselective palladium catalyzed hydrosilylation of styrene with trichlorosilane was carried out with combined QM/MM ab initio molecular dynamics simulations. A number of fundamental mechanistic questions have been addressed, including the main features of the catalytic cycle, as well as the specific nature of the regioselectivity and enatioselectivity. 

While hydrosilylation of 1-alkenes and HSiCl3 with platinum catalysts provides linear products (1-trichlorosilylalkanes), palladium chloride modified with phosphines gives products carrying the trichlorosilyl group at the secondary carbon. This is highly remarkable because all other metal complexes studied so far lead to 1-substituted products. This regioselectivity leads to the possibility to carry out asymmetric hydrosilylation. 

Hydrosilylation of monosubstituted and. em-disubstituted olefins (Reactions 5.3 and 5.4) are efficient processes and have been shown to occur with high regioselectivity (awti-Markovnikov) in the case of both electron-rich and electron-poor olefins [25]. For cis or trans disubstituted double bonds, hydrosilylation is still an efficient process, although it requires slightly longer reaction times and an activating substituent (Reaction 5.5) [25]. Any hydrosilylation product has been observed with 1,2-dialkyl-and 1,2-diaryl-substituted olefins, due to the predominant reversible addition of (TMS)3Si radical to the double bond [19]. 

A moderate pressure (>5 atm.) of CO in the reaction system leads to the selective formation of 29, while alkynes undergo rhodium-catalyzed hydrosilylation with a hydrosilane to afford vinylsilanes in the absence of CO. The presence of the rhodium complex is crucial for the smooth progression of siiyiformyiation, regardless of the presence of mononuclear or polynuclear complexes. This generalization is supported by the studies of many others [15]. The most important feature of this reaction is the excellent regioselectivity, which favors the formylation of the internal sp-carbon of the acetylenic bond of terminal... 

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