Silyl radical Subject

I thank Keith U. Ingold for having introduced me to this subject. When I arrived in Ottawa at the National Research Council of Canada in 1979 for three years postdoctoral work with him, very little was known on the reactivity of silyl radicals. At that time, several papers dealing with kinetics of silyl radicals were published, which allowed the reactivity of silyl radical to be translated into a quantitative base. Special thanks go to David Griller for his collaboration on the initial work on hydrogen donor abilities of silicon hydrides during the late 1980s. 

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. 

Synthesis of Biaryls. When TBDPS protected 2-bromobenzyl alcohol is subjected to standard radical conditions, the initially formed aryl radical reacts with one phenyl ring of the TBDPS group at its ipso position, and the resulting intermediate further transforms into a silyl radical, which then reduced to the corresponding silane. After this phenyl migration, desilylation by methyl lithium eventually provides the biphenyl product (eq 10). Other phenyl silyl ethers of this type can also experience the same aryl migration and produce biaryls. 

This chapter will concentrate on the chemistry of metal-14-centered anions (Ge, Sn, Pb). These compounds and their silyl analogues are ionic or polarized alkaline and alkaline earth metal-14 compounds, as well as delocalized molecules such as metalloles. Ammonium metallates Mi4 R4N+ or metal-14-centered anion radicals are also considered. The subject was explored during the 1960s and 1970s and thoroughly reviewed in 1982 and 1995 in Comprehensive Organometallic Chemistry, Vols. I and and for silicon species in a previous volume of this series . By that time the main routes to metal-14 anions were known. Since then, the subject has been developed in the topics of particular syntheses, stabilization using steric hindrance, electronic effects and complexation, spectroscopic and structural analyses "... 

Scheme 14 Indanone (146) was converted to alcohol (147). The acetyl derivative of (147) after demethylation was made to react with glyoxal to obtain lactone (149), which reacted with dibromocarbene and diazomethane to yield (150). Its silyl derivative was subjected to ring expansion by radical pathway to obtain cycloheptatriene (151). Its formyl derivative on dehydration gave the natural product lettucenin A.

The silyl linkage can also be in the form of -O-Si-C-. or -C-Si-C-. One such example is shown in eq 19 Compound 36 was prepared by treating 34 with i-Pr2SiCl2, followed by enol 35. Subjection of 36 to radical cyclization gave product 37 in 50% yield. ... 

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