Silicon-carbon compounds silyl radicals

Compounds with two or more silicon atoms directly attached to one another, subdivided into sections based first on the number of silicon atoms and then on the carbon functionality attached to the silicon atoms. Frequently, but not exclusively, the main photochemical behavior involves homolysis of a silicon-silicon bond yielding silyl radicals, but in some cases silylenes result directly from the photochemistry. The resulting compounds are frequently the products of a molecular rearrangement. 

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. 

Many of the results reviewed here suggest that a replacement of the usual alkyl or aryl substituents by silyl substituents in unsaturated silicon and germanium compounds may be rewarding. As we noted, silyl substituents do tilt the properties of silylenes, silyl radicals, and sequential BDE trends toward those in carbon chemistry. They have already been shown to stabilize disilenes with respect to dissociation to two silylenes, and this may be crucial to the further development of digermene and distannene chemistry. 

The access to silyl radicals is different from that to similar carbon compounds. Hydrogen abstraction is energetically unfavourable, while the pyrolysis which organosilicon compounds undergo differs from that of hydrocarbons due to the inability of silicon to form pirpn bonds. Pyrolysis of organosilicon compounds proceeds usually via non-chain sequences from which the rate of initial bond rupture into silyl radicals may be obtained 91 ... 

If organosilicon compounds have an acetoxy group on the carbon adjacent to the silicon atom, a facile 1,2-shift of the acetoxy group from carbon to silicon has been observed1 s, in addition to capture of the initially formed silyl radical by abstraction of halogen from the CC14 solvent (equation 3). Similar behavior was shown by a related benzyldimethylsilyl... 

Thus most of the photochemistry of simple organosilicon compounds appears to involve radical cleavage to form silyl and carbon radicals. The role of radicals in silicon chemistry has been reviewed12,18. However, as will be seen below, when silicon-silicon bonds are present, a much richer and more complex chemistry is observed, since they are much more susceptible to cleavage by processes which lead to other reactive species in addition to silyl radicals. 

The prefix sila- designates replacement of carbon by silicon in replacement nomenclature. Prefix names for radicals are formed analogously to those for the corresponding carbon-containing compounds. Thus silyl is used for SiH3—, silyene for —SiH2—, silylidyne for —SiH<, as well as trily, tetrayl, and so on for free valences(s) on ring structures. 

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