Silyl radicals synthesis

Developments in the synthesis and characterization of stable silylenes (RiSi ) open a new route for the generation of silyl radicals. For example, dialkylsilylene 2 is monomeric and stable at 0 °C, whereas N-heterocyclic silylene 3 is stable at room temperature under anaerobic conditions. The reactions of silylene 3 with a variety of free radicals have been studied by product characterization, EPR spectroscopy, and DFT calculations (Reaction 3). EPR studies have shown the formation of several radical adducts 4, which represent a new type of neutral silyl radicals stabilized by delocalization. The products obtained by addition of 2,2,6,6-tetramethyl-l-piperidinyloxy (TEMPO) to silylenes 2 and 3 has been studied in some detail. ... 

The chemistry of the heavy analogs of organic free radicals, that is, radicals centered on the Si, Ge, Sn, and Pb atoms, has been thoroughly reviewed several times, particularly that of the silyl radicals. Therefore, in the present review we will briefly bring together the most important discoveries in this field with particular attention paid to the most recent developments and progress, especially in the synthesis of the stable representatives of heavier group 14 elements centered radicals. ... 

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 tert-butoxyl radicals with trisubstituted silanes [Eqs. (6) and (7)] has been used extensively for the generation of silyl radicals in ESR studies, in time-resolved optical techniques, and in organic synthesis. Absolute rate constants for reaction (7) were measured directly by LFP techniques,56,62,63 whereas the gas phase kinetic values for reactions of Me3SiH were obtained by competition with decomposition of the tert-butoxyl radical.64,65... 

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 addition of trisubstituted silanes to carbonyl sulfide has been applied to the synthesis of the corresponding silanethiol derivatives (Reaction 5.40) [78]. In Scheme 5.12 the mechanism is depicted, starting from the silyl radical attack to the sulfur atom of 0=C=S and ejection of carbon monoxide. The resulting silanethiyl radical abstracts hydrogen from the starting silane, to give the silanethiol and to generate fresh silyl radical (see Section 3.4). 

Sequences (or cascades) of radical reactions involving the 5-endo-trig cyclization of silyl radical to an allyloxy-type substituent as the key step have been developed and applied to the synthesis of natural products [7 11]. The concept and the sequence of radical reactions is outlined in Scheme 6.7. A radical of... 

The radical-based strategy has invaded the field of A-containing heterocycles. Cyclizations mediated by silyl radicals have been introduced as the key step in the synthesis of alkaloids and pharmacologically active compounds, with many advantages both in terms of selectivity and bio-compatibility. Some of the most significant and innovative examples are described in this section. 

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