Silanes, redistribution

Silane redistribution 6 mg of (3-(methylsilyl)propyl) triphenylphosphonium hexafluorophosphate(V) (0.0121 mmol, 1 equivalent), 93 mg of Karstedt s catalyst (Pt 3.91% 0.000954 mmol, 0.8 equivalent), 6 mg of triphenylphosphine (0.0229 mmol, 1.9 equivalent) were dissolved in 20 mL of freshly distilled fluoroben-zene in a 50 mL Schlenk flask in an inert atmosphere glovebox. The flask was equipped with a septum, removed from the glovebox and installed with PEEK tubing leading to the mass spectrometer. The flask was pressurized with N2 and monitored by ESI-MS. Once a steady signal was obtained, 0.15 g of freshly distilled phenylsilane (1.39 mmol, 115 equivalents) was injected into the reaction flask. The reaction was initiated at room temperature and then was heated to reflux after 20 min to drive it to completion. 

Rate law and mechanism. The redistribution of alkyl groups on silanes in benzene is catalyzed by aluminum bromide. Suggest a scheme for it on the basis of the rate equation given. 

In many cases, these cyclic siloxanes have to be removed from the system by distillation or fractionation, in order to obtain pure products. On the other hand, cyclic siloxanes where n = 3 and n = 4 are the two most important monomers used in the commercial production of various siloxane polymers or oligomers via the so-called equilibration or redistribution reactions which will be discussed in detail in Sect. 2.4. Therefore, in modern silicone technology, aqueous hydrolysis of chloro-silanes is usually employed for the preparation of cyclic siloxane monomers 122> more than for the direct synthesis of the (Si—X) functional oligomers. Equilibration reactions are the method of choice for the synthesis of functionally terminated siloxane oligomers. 

The first step for the synthesis of a melt spinnable polysilane is the alkoxylation and distillation of the residue (Figure 1). 1,2-dimethyltetramethoxydisilane and 1,1,2-trimethyltrimethoxydisilane are mixed in a special ratio and a poly silane will be obtained by a catalytic redistribution reaction. Catalysts for this reaction are alkali alkoxides like sodium methoxylate. Phenylmethoxydisilanes [22] or phenylchloride are used as additives. A mixture of methyltrimethoxysilane and dimethyldimethoxy-silane distils off as a byproduct of the redistribution reaction. Figure 2 shows the mechanism of the catalytic redistribution. 

Another example of the data provided by our approach is shown in Fig. 4, which tracks the platinum-catalyzed redistribution of two silanes [16], one charge-tagged with a phosphonium group. This demonstrates the ability of the technique to monitor air-sensitive reactions in non-routine solvents in this case, fluorobenzene. The temperature of the reaction was increased from room temperature to reflux at 20 min the catalyst used was Karstedt s catalyst, Pt[(H2C = CHMe2Si)20]2(Fig.5). 

Fig. 5. Normalized intensity vs. time trace for charged silane (m/z 349) and appearance of the product of redistribution (m/z 425). The phosphonium silane has hexafluorophosphate as counterion the catalyst was added at t= 2 min and the solution heated to reflux at 20 min.

The authors suggested a mechanism via the intermediacy of a reactive pentacoordi-nated hydrosilyl anion,50c which is formed by the addition of hydride (H-) on the silanes, for the redistribution reactions. 

In addition to ruthenium, Tilley and coworkers also reported that cationic iridium silylenoid complexes were efficient olefin hydrosilation catalysts [reaction (7.6)].56 This silylene complex catalyzes the hydrosilation of unhindered mono- or disubsti-tuted olefins with primary silanes to produce secondary silanes with anti Markovni-kov selectivity. Iridium catalyst 32 exhibited reactivity patterns similar to those of ruthenium 30 only primary silanes were allowed as substrates. In contrast to 30, cationic iridium 32 catalyzed the redistribution of silanes. Exposing phenylsilane to 5 mol% of 32 in the absence of olefin produced diphenylsilane, phenylsilane, and silane. 

Redistribution of substituents on silanes that is closely related to the reaction of Eq. (6) also occurs when dihydrodisilanes are used instead of dihydrosilanes [Eq. (22)].26 In this reaction, a silyl(silylene) intermediate that is analogous to H shown in Scheme 4 is proposed to form via the oxidative addition of dihydrodisilane followed by migration of the terminal silyl group to the metal center. 

Although partial replacement of the SiH occurs in the more sterically hindered combinations, it is not possible to achieve partial substitution with the less hindered silanes, even using substoichiometric amounts of alcohol and low temperatures. The authors suggested that rapid redistribution of the hydroalkoxysilane intermediates may account for this behavior. This suggestion seems reasonable, since we have observed that the rates of dimethyltitanocene-catalyzed redistributions are strongly dependent on ste-ric encumbrance on the silicon.143... 

A similar treatment of the data of Ojima et al. shows that 0.66 mmol each of Me and Si and 3.3 mol of H are unaccounted for by the yields of products given in Eq. (32). These missing quantities correspond to a 26% yield each of MeSiH3 and H2. The higher yield of H2 in the Rh-cata-lyzed reaction vis-a-vis the Ir-catalyzed reaction is consistent with the formation of di- and trisilanes with the Rh catalyst. For each mole of Si—Si bonds formed, 1 mol of H2 equivalent must be evolved [Eq. (35)]. The H2 may enter into the general redistribution scheme before escaping from solution and hence may not appear exclusively as H2, but may also contribute to the hydrogen-rich silane fraction. 

As was done in the metal-catalyzed redistributions of the hydridodi-silanes, the active metal-silyl species can be intercepted by other substrates (e.g., acetylene) thus leading to synthetically useful reactions. For example, the cyclic disilanes react with acetylenes and dienes to give a variety of products, depending on the metal catalyst, the substrate, etc. (48-50) [Eqs. (77-78)]. 

In redistribution reactions catalyzed by transition metal complexes, di-or polysiloxanes are expected to share some characteristics of both monosilanes and di- or poly silanes. Similarities to the former are expected since the very reactive Si—Si bond is replaced by the less reactive Si—O bond, and to the latter because there are two or more exchangeable sites in the molecule. 

Even though much of the purported evidence for silylenoid species is weak or nonexistent upon close examination, such species may nevertheless be produced, especially if the possibility of dinuclear complexes is considered. In fact, there are several lines of evidence which do suggest that some sort of silylenoid species is generated from silanes and low-valent metal complexes. First, as Eq. (74) implies, the same dimethylsili-con metal complex is apparently generated from two diverse silicon compounds. Second, even early in the reaction, the most abundant product in the disproportionation of EE is ED E, which is the result of a SiO/Me exchange. If the redistribution were occurring solely by addition/elimina-tion [e.g. Eqs. (142-145)], then the observed SiO/Me exchange is the result of insertion into Si—O and Si—C bonds [Eqs. (148-149)]. Now,... 

The Si-H bond is particularly labile under Lewis acid conditions, and when stoichiometric quantities of hydrosilanes are used, the reaction can be hazardous. For example, the redistribution of the very stable phenylsilane generates silane, which reacts explosively with air. 

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