Hydrosilation of Acetylenes

Pukhnarevich et al. (55) showed that the structure of silanes, R3SiH (in which R3 was 15 combinations of R = Me, Et, n-Pr, i-Pr, i-Bu, Me3SiO—, C1CH2, EtO, i-PrO, and Ph) had very little effect on the ratio of isomeric adducts that form by their addition to 1-hexyne with chlo-roplatinic acid according to Eq. (60). 

The /3-adduct B was favored in every instance. The ratio of A/B varied from 1/1 for Me(Me3SiO)2SiH to 1/5 for Me2Me3CCH2SiH, but no identifiable trend could be discerned for electronic or steric effects on the A/ B ratios. 

The effect of solvents on the kinetics of addition to hexyne-1 was studied (56) with Et3SiH. Solvents with dielectric constants from 1.6 to 20.7 had scarcely any measureable effects on the rates of addition. Electron-donating solvents such as pyridine or dimethyl sulfoxide even in very small amounts stopped the reaction entirely. 

The effect of R in RC CH on rate of reaction was found (54) to be slight despite large differences in the inductive, steric, and conjugative characteristics of about ten widely differing R groups. 

Benkeser et al. (55) showed that a mole ratio of 2Cl3SiH to 1-hexyne would react completely in a long time. In 24 hours, a 98% yield of monoadducts had formed. After 111 hours, the same mixture yielded 20% 1,6-and 18% 1,2-bistrichlorosilylhexane. By increasing the concentration of catalyst, the yield of these two diadducts was increased to 44% and 40%, respectively. In this work, 1-trichlorosilyl-1-hexene behaved very much like 1-phenyl-l-hexene, shown in Table VI to form 1,6-diadducts. 

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Other substituent groups are usually introduced via Hydrosilation reactions. In this important reaction, Si-H bonds add across C=C or C=C bonds in 1,2 fashion to give products. Many transition metals and their complexes will catalyze hydrosilation, but the usual catalyst is platinum see Hydrosilation Catalysis). Thus CF3CH2CH2Si(Me)Cl2 is made by hydrosilation of 3,3,3-trifluoropropene with methyldichlorosilane (equation 12). n-Alkylmethyldichlorosilanes can be made similarly, by hydrosilation of l-aUcenes. Vinylmethyldichlorosilane may be obtained in analogous fashion by hydrosilation of acetylene, as shown in equation (13). An alternate route to the same compound is hydrosilation of vinyl chloride, followed by dehydrochlorination (equations 14 and 15). 

With dienes and acetylenes, only M = Fe, Co or Ni showed activity. The Fe and Co systems gave many side products. Therefore, the Ni catalyst was studied in greater detail. Bis-(7r-cycloocta-l,5-diene)nickel(0) was equivalent to the Ziegler catalyst in hydrosilation of penta-1,3-diene. 

Recently, various rhodium carbene complexes were investigated as catalysts for hydrosilation of olefins, acetylenes, and dienes to see whether carbene ligands modify catalytic activity. All reactions were... 

A few osmium complexes are active catalysts for the hydrosilation of phenyl-acetylene with Et3SiH. Whereas [RuHCl(CO)(PPr3)2] is stereospecific in providing Z-selectivity as well as quantitative yield, the corresponding Os complex yields high Z-adduct or /s-adduct depending on whether a stoichiometric amount or excess silane, respectively, is used. Some other examples of Os complexes have also been reported for the hydrosilation of phenylacetylene with EtsSiH. ... 

The use of Ni(0) catalysts for the hydrosilation of phenyl acetylene with PhsSiH and of various internal alkynes and conjugated diynes with Ph2SiH2, PhMc2SiH and EtsSiH has also been reported.It is of interest to note that although the Ni equivalent of Karstedt s catalyst has been prepared, dehydrogenative silylation of alkenes leading to unsaturated products is the dominant reaction and hydrosilation a secondary pathway with this Ni catalyst. 

The use of Lewis acids as catalysts for the hydrosilation of unsaturated compounds, hydrocarbons in particular, has seen a surge of interest and activity over the last dozen years. This is a refreshing and bold development since Lewis acids are also excellent catalysts for polymerization of olefins and acetylenes. 

Unlike the asymmetric hydrosilation of alkenes and acetylenes where Pd catalysts seem to dominate activity, for enantioselective hydrosilation of ketones a preponderance of Rh-based catalysts have been used, although a few other metals including Pd have been examined and some ligand templates such as binaphthyl are in common. 

Our interest in silicon chemistry quite naturally led to a study of the hydrosilation reaction, the addition of the Si-H group across an olefin or an acetylene. This reaction is one of the most useful methods of making silicon-carbon bonds and is an important industrial process. Typically, homogeneous catalysts based on platinum, rhodium or ruthenium are used, and while very efficient, they are not recoverable(46). 

The activity of Ziegler-type systems such as M(acac) -AlEt3 (M = Cr, Mn, Fe, Co, or Ni acac = acetylacetonate) was examined with 1-olefins and triethyl- or triethoxysilanes (55). Systems with nickel or cobalt showed low activity for hydrosilation but isomerized the olefin and were reduced to the metal. The study was extended to dienes and acetylenes (56). Isoprene gave the same products with these catalysts as are made with chloroplatinic acid. Penta-1,3-diene with Pt gave l-methylbut-2-en-ylsilanes. The Ziegler catalysts gave mainly penta-2-enylsilanes... 

The [2a + Itt] addition reaction ofdisilanes and acetylenes is closely related to hydrosilation. The prototype of the reaction is shown in the next example (equation 27). The cis addition is established similar to hydrosilation. 

The catalyzed additions of substituted germanes to terminal acetylenes, CHCCH2CI and acetylenic glycols resemble the corresponding hydrosilations . Chlorogermanes add spontaneously to CHCCH2CI, and catalysts tend to promote substitution of chlorine by hydrogen rather than addition to the triple bond. 

The addition of Si-H bonds to organic unsaturations such as olefins, acetylenes, and ketones is known as the hydrosilation reaction. A majority of the hydrosilation reactions are catalyzed by soluble transition-metal complexes and are known as homogeneous hydrosilation reactions [43]. Catalytic hydrosilation reactions are known to be very complex reactions. However, a few generalizations have been drawn about the mechanism of such reactions (Figure 15.4). Homogeneous olefin hydrosilation are presumed to start with an obligatory oxidative addition of the Si-H bond in a cis fashion to the catalytic metal [44]. This process is followed... 

Hydrosilation denotes the process of addition of one or more Si —H bonds to any other reagent. Most commonly utilized hydrosilating agents are SiR3H, SiHX3, and SiHR X3 (X = halogen, OR), and silated substrates are olefins, acetylenes, and ketones. 

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