Olefins hydrosilation

Recent investigations have been concerned with the reactivities observed with secondary silanes R2SiH2. In these cases, a dehydrogenative coupling of silanes to disilanes is observed as a side reaction of the hydrosilation. However, the hydrosilation can be totally suppressed if the olefins are omitted. The key intermediate in the coupling reaction has been identified as a silylene complex (sect. 2.5.4). 

During the synthesis of functional disiloxanes via hydrosilation, the starting materials are usually either tetramethyldisiloxane or dimethylchlorosilane and a proper olefinic (mostly allyl type) compound having the desired functional end group. If dimethylchlorosilane is employed, the hydrosilation is usually followed by hydrolysis. As a specific example, the synt hesis of 1,3-bis(3-glycidoxypropyl)tetramethyldisiloxane is shown in Reaction Scheme IV. 

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Hydrosilation reactions have been one of the earlier techniques utilized in the preparation of siloxane containing block copolymers 22,23). A major application of this method has been in the synthesis of polysiloxane-poly(alkylene oxide) block copolymers 23), which find extensive applications as emulsifiers and stabilizers, especially in the urethane foam formulations 23-43). These types of reactions are conducted between silane (Si H) terminated siloxane oligomers and olefinically terminated poly-(alkylene oxide) oligomers. Consequently the resulting system contains (Si—C) linkages between different segments. Earlier developments in the field have been reviewed 22, 23,43> Recently hydrosilation reactions have been used effectively by Ringsdorf 255) and Finkelmann 256) for the synthesis of various novel thermoplastic liquid crystalline copolymers where siloxanes have been utilized as flexible spacers. Introduction of flexible siloxanes also improved the processibility of these materials. 

The first example of acid catalysis appeared in a 1934 patent in which it is claimed that surface catalysts, particularly hydrosilicates of large surface area , known at that time under the trade name Tonsil, Franconit, Granisol, etc. lead to a smooth addition of the olefine to the molecule of the primary aromatic amine . Aniline and cyclohexene were reacted over Tonsil at 230-240°C to give, inter alia, the hydroamination product, N-cyclohexylaniline [47]. 

Pesek, J., Matyska, T., and Hemphaelae, H., HPLC evaluation of mono-ol, butylphenyl and perfluorinated columns prepared via olefin hydrosilation on a silica hydride intermediate, Chromatographia, 43(1/2), 10, 1996. 

Titanocene catalysts do not catalyze the hydrosilation of most internal olefins, although they can attach active olefins such as styrene, or norbornene to the growing polymer chain ends. The zirconocene-based catalysts, on the other hand, can be powerful hydrosilation catalysts and the remarkable copolymer synthesis shown in Equation 3 can be easily achieved under mild conditions (V7). 

With cyclohexene, polymerization occurs more rapidly than hydrosilation. After polymerization has proceeded to completion, there is a slow hydrosilation to introduce cyclohexyl groups onto the polymer chain, to a maximum extent of about 50 per cent of the Si-H groups. With more reactive olefins, such as styrene, hydrosilation occurs more rapidly than polymerization and the polymerization reaction is suppressed. As in the polymerization reaction, the reactivity of primary silanes is much greater than... 

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 range of reactions which have been examined is wide (248) and includes hydrogenations (256), ammonia synthesis (257), polymerizations (257), and oxidations (258). Little activity has occurred in this area during the past few years. Recent reports of the effects of sonication on heterogeneous catalysis include the liquefaction of coal by hydrogenation with Cu/Zn (259), the hydrogenation of olefins by formic acid with Pd on carbon (260), and the hydrosilation of 1-alkenes by Pt on carbon (261). 

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). 

A very strong solution of chloroplatinic acid (about 50% in isopropanol) was stirred with an olefin to make a mixture containing the extremely high concentration of about 0.01 g-atom of Pt/mole of olefin. This is more than 100 times the amount one would use for hydrosilation. At room... 

Experiments of this type in any olefin, even one such as cyclohexene which is exceedingly difficult to hydrosilate, produced optically clear solutions with no detectable precipitation of Pt(0). Such solutions are usually yellow and stable indefinitely at room temperature. If they are warmed to about 80°-100°C they darken, turn brown, and become colloidal dispersions of Pt(0). Platinum in the brown dispersions precipitates as a fine powder, sometimes in a few days at room temperature, sometimes in a few weeks. Catalytic activity was obviously reduced or completely lost by such solutions after they turned brown. A clear solution... 

Chalk and Elarrod (11a) compared the above ethylene Pt(II) complex with chloroplatinic acid for hydrosilation, and found that each gave essentially the same results in terms of rate, yields, and products. Plati-num(II) complexes and rhodium(I) complexes were very much alike in their behavior. No system was found in which a palladium olefin complex brought about hydrosilation. In most systems the palladium complex was very rapidly reduced to the metal. 

The Pt(0) catalyst did not cause hydrosilation of internal olefins such as 2-hexene or cyclohexene, but it was effective with conjugated diolefins. With isoprene, even in excess, it produced considerable amounts of diadduct as well as 1,2-monoadduct. (This is anomalous, as will be discussed later in this review.) The principal products are shown in Eq. (8) ... 

Experiments with trichlorosilane-d, Cl3SiD, were most instructive about side reactions that can take place in the hypothetical catalytic olefin PtH(—S ) species during hydrosilation. Although trichlorosilane-d and methyldichlorosilane showed no exchange of deuterium and hydrogen at 100°C during many hours in the absence of a catalyst, traces... 

Studies of hydrosilation with trichlorosilane-d (2f) proved that exchange can also take place between SiD and C—H bonds in olefins during hydrosilation. Isobutylene was chosen as the olefin for this study because both it and isobutyltrichlorosilane have H NMR spectra that are easy to interpret, and because movement of the double bond can give rise to no detectable isomerization. Excess trichlorosilane-d with isobutylene and chloroplatinic acid was sealed into a Pyrex tube and kept near 25°C overnight. Deuterosilation was complete in less than 1 hour. Analysis of the product after about 16 hours indicated reactions that can be summed up as follows ... 

Excess MeCl2SiH added alowly to A at reflux gave 82% of compound I and 18% of II. One equivalent of MeCl2SiH was added to solutions of one equivalent of both A and B or A and C. In either case the recovered olefin was largely C. The relative yields of I and II from the competitive reactions would indicate relative reactivities of B/A/C of 1/0.07/0.02. B was apparently more reactive than A or C. This suggests that C formed from B and A after most of the II or I was made. If A and B formed C before hydrosilation, the three olefins should exhibit relative reactivities close to 1/1/1, and they should each have made the same mixture of adducts. 

In continuing the study of hydrosilation of alkylcyclohexenes, Benkeser et al. (13) studied in detail the addition of Cl3SiH (with H2PtCl6) to 1-, 3-, and 4-ethylcyclohexenes, as well as to ethylidene- and vinylcy-clohexenes. With an olefin/Cl3SiH ratio of 1/1.9 at reflux, vinylcyclo-hexane gave a 68% yield of adduct in 0.5 hours. The other olefins required much more time e.g., 96 hours was needed to achieve 83% yield from 3-ethylcyclohexene. Each olefin made the same adduct— C6HnCH2CH2SiCl3—and recovered olefins were mixtures of all possible... 

Hydrosilation of alkylcyclohexenes illustrates the ability of active catalytic complexes to isomerize olefins and the tendency chlorosilanes have to form primary alkyl adducts even if this requires a skeletal rearrange-... 

The recent discovery that a chiral phosphine ligand in a platinum(II) complex can give rise to a catalytic asymmetric hydrosilation of prochiral olefins seems to prove that a phosphine ligand can be included in the coordination sphere of platinum in an active catalytic species, but that when a phosphine ligand is so included, the activity of the species is reduced by several orders of magnitude. 

Methyldichlorosilane was by far the most reactive in hydrosilation of 1,1-disubstituted olefins. Trialkylsilanes did not add at all, even at 120°C. Trichlorosilane gave complicated results involving isomerization of olefins and dimerization of a-methylstyrene, and products were not optically active. 2-Methylbutene-2 and trichlorosilane gave two adducts, 2-meth-ylbutyltrichlorosilane and 3-methylbutyltrichlorosilane. The latter required isomerization of the olefin. 2,3-Dimethylbutene-l gave one adduct in 70% yield, and it was optically slightly active [0.8% (R) isomer]. 

Asymmetric hydrosilation of prochiral olefins other than 1,1-disubsti-tuted ones was unsuccessful because they were unreactive under the conditions employed. Styrene afforded a 20% yield of 1-phenylethyl- and 50% of 2-phenylethylmethyldichlorosilane, both inactive. The 1-phenylethyl isomer had a possibility of activity with its asymmetric center, PhMeC HSi=. 

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... 

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... 

---