Silylene anion radical

Several examples of carbenoid ion-radicals are discussed within this book. A silylene anion-radical preparation and properties is exemplified here. Scheme 2.5 shows the path to this species. Tetrakis(di-tert-butytmethylsilyl)disilylene was reduced by lithium or sodium salt of naphthalene anion-radical in THF at 78°C and then 12-crown-4 was added to the resulting reaction mixture. The silylene anion-radical was obtained as the corresponding alkali salt. Red crystals of the salt were isolated and characterized by ESR spectroscopy and x-ray crystallography (Inoue et al. 2007). 

The authors proposed the following picture of the silylene anion-radical formation. Treatment of the starting material by the naphthalene anion-radical salt with lithium or sodium (the metals are denoted here as M) results in two-electron reduction of >Si=Si< bond with the formation of >SiM—MSi< intermediate. The existence of this intermediate was experimentally proven. The crown ether removes the alkali cation, leaving behind the >Si - Si< counterpart. This sharply increases electrostatic repulsion within the silicon-silicon bond and generates the driving force for its dissociation. In a control experiment, with the alkali cation inserted into the crown ether, >Si — Si< species does dissociate into two [>Si ] particles. 

Silylene Anion Radical [Bis(1,4,7,10-tetraoxacyclododecane) lithium(1 )]-1,1,3,3-tetra- butyl-1,3-dimethyltrisilane-2-yl-2-ylide 30 Synthesis of silylene anion radical [bisfl,4,7,10-tetraoxacyclododecane) lithiumi 1 ]-1,1,3,3-tetra-tbutyl- 1,3-dimethyltrisilane-2-yl-2-ylide 30... 

Tetrakis di-fbutylmethylsilyl)disilene has proved to be a versatile precursor for the isolation of remarkable radical species (6,7). The disilene bearing four fBu2MeSi groups can be successfully synthesized by two routes (7). The treatment of tetrakis di-fbutyl-methylsilyl)disilene with 2.2 equiv of lithium naphthalenide in THF at — 78°C afforded the silylene anion radical in 56% yield. The reaction rnkture was first slowly warmed to room temperature. The dark blue color of disilene completely disappeared and a red solution was produced during the reaction. The eventual addition of 4.3 equiv of 12-crown-4 to the resulting reaction mixture led to the isolation of the silylene anion radical as air-and moisture-sensitive red crystals (8). 

Scheme 6.1.1.1 Synthesis of Silylene Anion Radical [Bis(1,4,7,10-tetraoxacyclododecane) lithium 1)1-1,1A3-tetra-tbutyl-1,3-dimethyltrisilane-2-yl-2-ylide.

A lithium naphthalenide solution can be prepared by stirring equimolar quantities of naphthalene and lithium in THF. This solution of lithium naphthalenide (0.34 mmol) in THF was added to tetrakis di-fbutylmethylsilyl)disilene (112 mg, 0.16 mmol) in THF 2.5 mL) at -78°C with the help of a syringe. Afterward the reaction mixture was allowed to warm up to room temperature over 2 h. There was a rapid color change resulting in the formation of the 1,2-dianion of the disilene (6). This dianion was further treated with 12-crown-4 (110 irL, 0.69 mmol, 4.3 equiv) at room temperature. Afterward THF was removed in vacuo and hexane was added to the reaction mixture. The hexane solution was filtered and overnight standing at -30°C led to the isolation of silylene anion radical as air- and moisture-sensitive red crystals (128 mg, 56%). Mp 138-139°C (Scheme 6.1.1.1). 

West et al. 4, 5) have suggested a number of possible reaction intermediates, which include anions, radical anions, radicals, and diradicals. Zeig-ler (6, 7) has proposed, on the basis of some radical-trapping experiments, that the intermediate is, at least at some point, a radical. He showed that the diradical silylene was not an intermediate, and he also stressed the importance of bulk solvent composition on the course of the reaction. The bulk solvent composition determines the expansion of the polymer coil as it interacts with the sodium surface. Miller et al. (8) initially suggested that the reaction, which is promoted by the addition of diethylene glycol dimethyl ether, proceeds by an anionic process, although they later (9) accepted Zeigler s bulk-solvent model. 

Organosilicon chemistry has expanded its scope considerably in the last two decades. One of the most remarkable achievements is the progress which was made in the elucidation of the mechanisms in silicon chemistry, which now become comparable to those in carbon chemistry. The behavior of reactive intermediates such as silylenes, silyl radicals and silyl anions are well explored, although the chemistry of silyl cations is still controversial. Doubly-bonded silicon species are now well understood " but triply-bonded silicon is still elusive. 

Anion radicals of silylenes are a fascinating class of reactive intermediates that have been intensively investigated (1). However, their isolation and characterization have been achieved very recently (2—5). 

The first step in the polymerization is the electron transfer from sodium to dichlorosilane and the formation of the corresponding radical anion. The latter upon elimination of the chloride anion is transformed to the silyl radical. To fit the chain growth mechanism, the reactivities of the macromolecular radicals must be higher than the reactivities of the monomeric radicals. The latter after electron transfer and elimination of chloride anion could be transformed to the reactive silylenes. Thus, in principle, two or more mechanisms of chain growth are possible ... 

Carbene type radical anions have been often postulated as reactive intermediates. An tetratrimethylsilyl substituted silylene (16) was reduced with several alkali metals in dimethoxyethane to yield a persistent silylene radical anion.169... 

From these findings a new mechanism can be derived the reaction is initiated by a fast single electron transfer, followed by the migration of a chloride anion leading to the radical pair 6 + CCI3. This radical pair can recombine under formation of the insertion reaction product 2 or can include the reaction with a second silylene molecule 1 yielding the final reaction product 3. 

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