A-silyl radicals

The carbon-centered radical R, resulting from the initial atom (or group) removal by a silyl radical or by addition of a silyl radical to an unsaturated bond, can be designed to undergo a number of consecutive reactions prior to H-atom transfer. The key step in these consecutive reactions generally involves the intra-or inter-molecular addition of R to a multiple-bonded carbon acceptor. As an example, the propagation steps for the reductive alkylation of alkenes by (TMSfsSiH are shown in Scheme 6. 

The reduction of Cl3SiC6H2-2,4,6-Ph3 with lithium metal in the presence of trimethylsilyl chloride led to the isolation of a silafluorene compound that was formed by an intramolecular ring closure.100 Although the mechanism is unknown, it is believed that reduction affords a silyl radical, which may... 

Extensive mechanistic investigation of the ring expansion 33 —> 34 was performed in order to differentiate between a ring-opening reaction to give a silyl radical 39 (path a), followed by the 6-endo cyclization, or a pentavalent silicon transition state 40 (path b). It was clearly demonstrated that the ring expansion proceeds via a pentavalent silicon transition state (Scheme 6.10) [16]. 

P. Mayon and Y. Chapleur, Exclusive 6-endo radical cyclizations of a-silyl radicals derived from carbohydrate allylic silylethers, Tetrahedron Lett. 35 3703 (1994). 

The mechanism in Scheme 5.1 can be elaborated further, as shown in Scheme 5.2.17 The reaction of chlorosilanes with sodium probably proceeds by an initial single electron transfer to form an anion radical, which loses chloride rapidly to form a silyl radical. 

The second channel for the reaction exists that leads to the same products. This is the direct abstraction of an oxygen atom from N20 by a silyl radical. Figure 7.2f shows the structure of the transition state (TS-4) corresponding to this channel. According to the calculations, this channel is characterized by high activation energy (10.6kcal/mol). 

However, because a reverse reaction of oxygen addition to a silyl radical proceeds with a high rate, one can state that the formation of the main products of transformation is related to the first process. 

PhSeSiPh2(f-Bu) afforded a silyl radical that can abstract a bromine atom from 152 to give the radical 153. Cyclization affords radical 154, which react with diphenyl diselenide to give 155. Diphenyl diselenide results from the oxidation of the phenylselenolate anion presumably by the oxygen present in the solvent. 

In the presence of a radical initiator like peroxide or AIBN, or under UV or y-ray irradiation, the Si—H bond is cleaved homolytically to afford a silyl radical (3 Scheme 1), which adds across a C=C or bond. The regioselectivity of the addition is governed mainly by the stability of the newly generated radical center. The resulting radical (4) picks up hydrogen from another molecule of hydrosilane to complete the catalytic cycle and regenerate the active silyl radical (3). ... 

Disilanaphthalenes can be prepared by the thermolysis of a disilacyclopropane in deuteriated benzene the reaction may be monitored by NMR spectroscopy <83JA7776>. A mechanism of formation is proposed which involves Si—Si bond rupture followed by C—Si bond formation via a silyl radical. Thermolysis of 2-(mesityl)-2-(phenylethynyl)hexamethyltrisilane also results in two isomeric 1,3-disilanaphthalenes <860M1518> but better yields are achieved if the reaction (either thermally or photochemically) is catalyzed by a nickel complex <86JA7417, 890M2050>. Nickel-... 

The facts that thiols are good H-atom donors toward alkyl radicals and that silyl radicals are among the most reactive known species for abstraction and addition reactions suggest that any class of compounds which allows the transformation of a thiyl to a silyl radical via a fast intramolecular rearrangement will potentially be a good radical-based reducing agent. The silanethiols 11 and 12 are found to have this property [84, 85]. The reductions of bromides, iodides and isocyanides by thiol 12 are demonstrated to follow the expected mechanism [85]. 

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