Hydrosilylation of double bonds

The hydrosilylation/methanolysis reaction sequence. One of the most convenient methods is to use a sequence of hydrosilylation/methanolysis reactions as shown in equation 4 for the formation of 39. This direct synthetic pathway was reported in cases of the alkyl family of precursors 1290. This sequence is also used for the preparation of dendrimers and arborols121,123. Alternatively, hydrosilylation with HSi(OEt)3 can be performed advantageously in the case of dendrimers125,126 but it is of more interest in the case of carbonate precursors 26111. Indeed, hydrosilylation of double bonds can be achieved selectively in the presence of a carbonyl group with hexachloroplatinic acid or a rhenium catalyst130. 

Several workers have reported that low-valent palladium complexes are effective catalysts for the hydrosilylation of double bonds. The reaction of hexene-1 with trichlorosilane to give 1-trichlorosilylhexane has been catalyzed by a range of compounds (Table IV). It is particularly... 

Hydrosilylation of Double Bonds. Tris(trimethylsilyl)silane is capable of radical hydrosilylation of dialkyl ketones, alkenes, and alkynes. Hydrosilylation of alkenes yields the anti-Markovnikov products with high regio- and good diastereos-electivity (eq 5). By using a chiral alkene, complete stereocontrol can be achieved (eq 6) The silyl group can be converted to a hydroxyl group by Tamao oxidation. 

Alkenes with two reactive carbon-carbon double bonds per molecule like 1,5-hexadiene or diallyl ether are used in the synthesis of silicone compounds which can be later crosslinked by hydrosilylation. A sufficiently high excess of double bonds helps to prevent the dienes from taking part in silane addition across both olefmic ends, but trouble comes from double bond isomerization (Eq. 2). 

Acetylenic acrylates have been used to reduce side reactions in the preparation of acrylic sil(ox)anes by hydrosilylation [13,14], Allylic acrylates are known to result in addition products with both types of double bonds. Elimination of propene under loss of the allylic group is a major concern, because this path yields acryloxy silicone compounds with SiOC linkages of low hydrolytic stability. 

In order to better characterize the system, a further kinetic study was carried out on these two catalysts. Hydrosilylation mechanism has been thoroughly studied in the literature. This is a complex system, since the mechanism depends altogether on the catalyst, the reactants and the experimental conditions. This also explains why for each new reactant, a whole new experimental set-up has to be developed. In most cases already described, the limiting step is the insertion of platinum in the Si-H bond, leading to an apparent rate of reaction independent of double bond concentration ... 

Thus, the researches on reactions of thermal solid-phase hydrosilylation (without a catalyst and a solvent) in a surface layer of hydridesilica I and II made it possible to reveal the distinctions in the reactivity both of 1-olefins and hydridesilicas. It has been established that the activity of 1-olefins is symbate to the electron density on a carbon atom of double bonds, and the reactivity of hydridesilicas increases with the electron-accepting ability of substituents at silicon atoms. 

The asymmetric catalytic reduction of ketones (R2C=0) and imines (R2C=NR) with certain organohydrosilanes and transition-metal catalysts is named hydrosilylation and has been recognized as a versatile method providing optically active secondary alcohols and primary or secondary amines (Scheme 1) [1]. In this decade, high enantioselectivity over 90% has been realized by several catalytic systems [2,3]. Therefore the hydrosilylation can achieve a sufficient level to be a preparative method for the asymmetric reduction of double bond substrates. In addition, the manipulative feasibility of the catalytic hydrosilylation has played a major role as a probe reaction of asymmetric catalysis, so that the potential of newly designed chiral ligands and catalysts can be continuously scrutinized. 

Organosilicon compounds are largely produced by the hydrosilylation of unsaturated organic substrates [47]. Various transition metal catalysts have been used to obtain alkyl-SiR products from the reaction of H-SiRj with an alkene. Alkene insertion into an M-Si bond is recognized as a fairly common process which plays a key role in catalytic hydrosilylation processes. The reaction of 1,3-butadiene (3-6) with triethylsilane in the presence of [Cr(CO)g] under photochemical condition yields exclusively the ds-1,4-adduct, ds-l-(triethylsilyl)-2-butene (7) (Scheme 10.7) [48]. In all cases, 1-4 addition products form in major, however, in some cases 1-2 addition product (9) also forms in minor yield. Formation of product 12 can be rationalized in terms of double bond migration subsequent to the initial hydrosilylation (Scheme 10.7). 

Hydrosilylation of multiple bonds has not often been catalyzed by cyclopenta-dienylmetal complexes. Nevertheless, several methods concerning hydrosilylation of terminal and internal alkynes catalyzed by the ruthenium complex 53 and [(/] -C5Me5)Ru(MeCN)3] PF6 70 have been recently reported [36]. Of special interest is the complex 70, which has a high preference for the formation of the branched 71 over the linear product 72 (Scheme 28). The reaction of the Si-H bond with the triple bond is stereoselectively frans-addition. In the case of alkynyl silanes the reaction proceeds through endo-dig hydrosil tion to the silacycles 73 with endocyclic double bond (Scheme 29) [36b]. 

Abstract The use of A-heterocyclic carbene (NHC) complexes as homogeneous catalysts in addition reactions across carbon-carbon double and triple bonds and carbon-heteroatom double bonds is described. The discussion is focused on the description of the catalytic systems, their current mechanistic understanding and occasionally the relevant organometallic chemistry. The reaction types covered include hydrogenation, transfer hydrogenation, hydrosilylation, hydroboration and diboration, hydroamination, hydrothiolation, hydration, hydroarylation, allylic substitution, addition, chloroesterification and chloroacylation. 

In all of these cases, paUadium-catalyzed hydrosilylation proceeds via hydropalla-dation followed by reductive elimination of alkyl- and silyl group from the palladium. In the reaction of o-aUylstyrene (24) with trichlorosilane, which gives hydrosilylation products on the styrene double bond 25 and cycUzed product 26, the hy-dropalladation process is supported by the absence of side products which would result from the intermediate of the silylpaUadation process (Scheme 3-11) [37]. 

The discussion of the activation of bonds containing a group 15 element is continued in chapter five. D.K. Wicht and D.S. Glueck discuss the addition of phosphines, R2P-H, phosphites, (R0)2P(=0)H, and phosphine oxides R2P(=0)H to unsaturated substrates. Although the addition of P-H bonds can be sometimes achieved directly, the transition metal-catalyzed reaction is usually faster and may proceed with a different stereochemistry. As in hydrosilylations, palladium and platinum complexes are frequently employed as catalyst precursors for P-H additions to unsaturated hydrocarbons, but (chiral) lanthanide complexes were used with great success for the (enantioselective) addition to heteropolar double bond systems, such as aldehydes and imines whereby pharmaceutically valuable a-hydroxy or a-amino phosphonates were obtained efficiently. 

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. 

It has been shown that hydrosilylation may not perform as ideally as is required when preparing co-olefinic silicone compounds from organic a,co-dienes and hydrosil(ox)anes isomerization is a concern and the chemical equivalence of the double bonds requires a large excess of the diene compound to achieve essentially monohydrosilylation. Further side reactions are discussed by Torres et al [9],... 

The low induction for the acetoacetates was attributed to a transfer hydrogenation process within an enol form of the substrate, coordinated through the carbon-carbon double bond, CH3C(OH)=CH—C02R, rather than hydrosilylation of the carbonyl moiety (285). 

In conclusion, catalytic asymmetric hydrosilylation has been developed as one of the most efficient methods of asymmetric functionalization of carbon-carbon double bonds. The asymmetric hydrosilylation is reaching very high level in terms of both catalytic activity and enantioselectivity, and it is expected to be applied to industrial production of useful chiral molecules in the near future. 

Preparation of enantiomerically pare secondary amines by catalytic asymmetric hydrogenation or hydrosilylation of imines is as important as the preparation of alcohols from ketones. However, asymmetric hydrogenation of prochiral ON double bonds has received relatively less attention despite the obvious preparative potential of this process.98... 

Several successful results have been obtained in the asymmetric hydrogenation and asymmetric hydrosilylation of imines.101 An efficient enantioselective hydrogenation of the ON double bond was developed by Burk and Feast-er,101a who used [ R h (CO D) (D u P h o s) ] C Fi SO3 in the hydrogenation of N-aroylhydrazone 98. 

Intramolecular hydrosilylation.1 Hydrosilylation of internal double bonds requires drastic conditions and results in concomitant isomerization to the terminal position. However, an intramolecular hydrosilylation is possible with allylic or homoallylic alcohols under mild conditions by reaction with 1 at 25° to give a hydrosilyl ether (a), which then forms a cyclic ether (2) in the presence of H2PtCl6-6H20 at 60°. Oxidative cleavage of the C—Si bond results in a 1,3-diol (3). 

Hydrosilylation can be applied to alkenes, alkynes, and aldehydes or ketones. A wide range of metal compounds can be used as a catalyst. The most common and active ones for alkenes and alkynes are undoubtedly based on platinum. Hydrosilylation of C-0 double bonds gives silyl ethers, which are subsequently hydrolysed to their alcohols. The reaction is of interest in its enantioselective version in organic synthesis for making chiral alcohols, as the achiral hydrogenation of aldehydes or ketones does not justify the use of expensive silanes as a reagent. 

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]. 

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