Redistribution of groups on silicon

Although kis expected to be very large, occasionally the coordinatively unsaturated species, L M, will be intercepted by another substrate (e.g., a Si—C bond) and give rise to some net reaction, such as redistribution of groups on silicon. Stated another way, one function of the Si—H bond is to generate an active catalyst from some catalyst precursor [e.g., Eq. (23)] (19). A fuller discussion of mechanistic implications is reserved for Section III. 

For group IV metals, the ease of redistribution parallels the size of the central atom. Thus for systems in which M = Sn and X and Y are alkyl, aryl, hydrogen, or electronegative groups such as halogens or alkoxy groups, equilibration can often be reached under very mild conditions, that is, <200 °C in the absence of a catalyst. The redistribution of groups on silicon, however, is a feasible process only in the presence of a catalyst, normally a Lewis acid such as aluminum chloride, and at elevated temperatures. 

At the outset of this discussion, it is perhaps worth noting that there is a considerable difference between the lability of groups on silicon toward redistribution catalyzed by acids or bases (see Section I) and transition metal complexes. Thus, oxy substituents are classified as "labile and hydrogen as semi-labile toward acid- or base-catalyzed redistribution, whereas the reverse is usually the case with transition metal-catalyzed redistributions. Thus, these two sets of catalysts are complementary in their capacity to labilize the various substituents on silicon. 

It is apparent from the examples in this section that the lability of the groups on silicon is greatly dependent on the catalyst. With conventional acid or base catalysts, SiO— is classed as a labile ligand, SiH as semi-labile, and Si—R (R = alkyl or aryl) as nonlabile (/). However, with low-valent complexes of the group VIII metals, SiH is the most labile ligand, and SiO— and Si—R appear to have comparable reactivities. Hence, these two sets of catalysts types are complementary in their capacity to redistribute ligands on silicon. 

Because of the commercial importance of halosilanes, particularly the dihalo and trihalo derivatives, in the production of silicone polymers, the reaction in which M = Si has received considerable attention. For example, the direct process produces MegSiCl and MeSiClg as part of a complex mixture including the valuable Me2SiCl2, which is diflScult to separate because of similar boiling points. Fortunately, the equilibration of alkyl groups on silicon is not statistical, a fact that permits a very useful application of the redistribution reaction ... 

As stated earlier by Chojnowski and Cypryk [13], a functional disiloxane can also be introduced in the DCM, in addition to triflic add, to generate a,(o-telechelic PDMS for use in further grafting reactions, such as divinyltetramethyldisiloxane V2 [74] or tetramethyldisiloxane M [75]. Another team also functionalized polystyrene chains (prepared by radical polymerization) on both ends with pentameth-yldisUoxane groups to produce, through triflic add redistribution with D4, a triblock copolymer this technique was quoted by the authors to be easier than the conventional approach, which employs the sequential anionic polymerization of both monomers [76]. Another example worthy of mention here is that reported by Cai and Weber, who started with a tetrakis Si—H-functionalized precursor (tetrakis (dimethylsiloxy)silane) to generate silicone stars [77]. 

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