Silicon radicals synthesis

Scheme 18. Silicon-directed radical cyclization in Myers s synthesis of (+)-tunicamycin V (97).

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

The all-silicon version of the cyclobutenyl radical, cyclotetrasilenyl radical 40, was recently reported by Sekiguchi et al. Its synthesis was accomplished by the one-electron reduction of the precursor cationic species, cyclotetrasilenylium ion 20 TPFPB with either t-Bu3SiNa or KCg in Et20 (Scheme 2.33). The four-membered Si4 ring of 40 is nearly planar with the Sil-Si2 and Si2-Si3 bonds... 

This strategy was later refined by Stork and co-workers142 who used silicon (e.g. 164) as the tethering atom [Scheme 55a)]. The latter strategy was adapted by Sinay and co-workers143 for the synthesis of C-disaccharides (e.g. 169) by employing 8- and 9-endo radical cyclizations with the readily prepared silaketal connectors [Scheme 55b)]. 

Skrydstrup, Beau and co-workers122 have adapted Stork s method to the SmI2-reduction of glycosyl pyridyl sulfones bearing a silicon-tethered unsaturated group at HO-C(2). An example is shown with the synthesis of methyl a-C-zso-maltoside 172 from alkyne 170 via the 5-exo-dig radical cyclization of 171 (Scheme 56).144... 

Pyun, J. Xia, J. Matyjaszewski, K. Organic-Inorganic Hybrid Materials from Polysiloxanes and Polysilsesquioxanes Using Controlled/ Living Radical Polymerization. In Synthesis and Properties of Silicones and Silicone-Modified Materials Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds. ACS Symposium Series 838 American Chemical Society Washington, DC, 2003 pp 273—284. 

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 reaction of thermally and photochemically generated tcrt-butoxyl radicals with silicon hydrides (Reactions 3.13 and 3.14) has been extensively used for the generation of silyl radicals in EPR studies, time-resolved optical techniques, and organic synthesis. 

The use of free-radical reactions in organic synthesis started with the reduction of functional groups. The purpose of this chapter is to give an overview of the relevance of silanes as efficient and effective sources for facile hydrogen atom transfer by radical chain processes. A number of reviews [1-7] have described some specific areas in detail. Reaction (4.1) represents the reduction of a functional group by silicon hydride which, in order to be a radical chain process, has to be associated with initiation, propagation and termination steps of the radical species. Scheme 4.1 illustrates the insertion of Reaction (4.1) in a radical chain process. 

The same methodological approach has been extended to the synthesis of biaryls [24]. Reaction (6.12) shows a few examples in which the transfers of functionalized aryl groups from silicon to aryl radicals are successful. A mechanistic scheme similar to that reported in Reaction (6.10) has been proposed. 

TMS)3SiH has also been used as the mediator of C—C bond formation between an acyl radical and an a, p-unsaturated lactam ester (Reaction 7.9). The resulting ketone can be envisaged as potentially useful for the synthesis of 2-acylindole alkaloids [17]. Here, the effects of both H-donating ability and steric hindrance given by the silicon hydride can be seen. 

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