Silicon-containing silyl

The reactions of atoms or radicals with silicon hydrides, germanium hydrides, and tin hydrides are the key steps in formation of the metal-centered radicals [Eq. (1)]. Silyl radicals play a strategic role in diverse areas of science, from the production of silicon-containing ceramics to applications in polymers and organic synthesis.1 Tin hydrides have been widely applied in synthesis in radical chain reactions that were well established decades ago.2,3 Germanium hydrides have been less commonly employed but provide some attractive features for organic synthesis. 

The importance of carbon-centred radical cyclizations in organic chemistry has been documented in the large number of papers published each year and numerous reviews and books dealing with this subject. In Chapter 7 the reader can find a collection of such processes mediated by organosilanes. The silicon-centred radical cyclizations have instead received very little attention, although there has been a growing interest in silicon-containing compounds from a synthetic point of view, due to their versatility and applicability to material science. As we shall see, this area of research is very active and some recent examples show the potentiality of silyl radical cyclization in the construction of complex molecules. 

One section in this review will deal with silyl cations, another with hypervalent silicon compounds. The concept of hypervalent sihcon compounds belongs, strictly speaking, to the class of Lewis base catalysis. However, since a Lewis base forms in situ with a silicon containing reagent or SiCl an intermediate, which functions as a Lewis acid to activate substrates during the reaction, we would also present a few examples in this review. Since silicon is a semimetal we leave it up to the reader to decide whether silicon catalysts should be considered as organocatalysts. 

The size of a silyl group in silicon-containing reagents greatly influences their reactivity212 and selectivity. Some prominent examples are discussed in the following reactions of 12 classes. 

Allylsilanes 12 react with a-ketoesters 174 to give [3 + 2] cycloadducts 175 in 50-85% yields through 1,2-silyl migration (equation 73)59,228. Use of an allylic trimethylsilane produces an allyl alcohol by-product through a competing silyl elimination process. It can be circumvented by use of a bulkier silicon-containing reagent, such as the... 

The silyl-modified Prins reaction and the silyl-modified Sakurai reaction are common methods employed for dihydropyran (and tetrahydropyran) synthesis, and are in fact the same reaction. For the sake of clarity, the term silyl-Prins cyclization is adopted herein. A review of the use of silicon containing compounds in reactions of this type is available <1995CRV1375>. 

Silylative Coupling Condensation of Silicon-Containing Dienes.225... 

The nomenclature of organosilicon compounds is based on the derivatives of silane, disilane, and higher polysilanes. When the silicon group is used as a substituent (silicon-containing radical), silyl, disilanyl, trisilanyl, and polysilanyl are used ... 

Among silicon-containing acetylenes, 3-(trimethylsilyl)-l-octyne and its homologs are converted to high-molecular-weight polymers36 (Table 11). The MoCls-cocatalyst and Mo(CO)6—CC —hv systems afford the highest MW s. On the other hand, 1-silyl-acetylenes such as l-(trimethylsilyl)acetylene and l-(dimethylphenylsilyl)acetylene afford partly toluene-insoluble polymers 34,35), and Mn of the soluble fractions is no more than ca. 1 x 104. 

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