Carbenoid Ion-Radicals

This class of ion-radicals is characterized by the localization of an unpaired electron at the atom bearing a free (valence) electron pair. Although their applicability in organic synthesis remains an open question, the preparative methods and electron structure of carbene ion-radicals attract some attention of the researchers. Probably, it is an initial step to a new chapter in organic ion-radical chemistry. 

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. 

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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). 

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