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Lithiation of Diphenyl(triorganosilyl)methanes

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journal contribution
posted on 2001-12-31, 00:00 authored by Michael S. Hill, Peter B. Hitchcock
The lithiation of several triorganosilyl-substituted diphenylmethanes, (R3Si)Ph2CH, in THF has been studied. The structures of two of these compounds have been determined by X-ray diffraction studies and shown to consist of solvent-separated ion pairs [Li(THF)4][Li{C(SiR3)Ph2}2] (4, R3 = Me3; 5, R3 = Me2Ph), in which the anions are lithates, each containing lithium bound to two η2-coordinated, silylated diphenylmethyl subunits. Despite contact with lithium the organic residues are almost planar (sum of angles around C(1) for both structures, 356.7°), indicating a high degree of anionic charge delocalization. This is further reflected by the collated multinuclear NMR data. Reaction of [Li(THF)4][Li{C(SiMe3)Ph2}2] with TMEDA in hexane was shown by an X-ray diffraction study to have resulted in complete removal of lithium from the anion coordination sphere to give [Li(TMEDA)2][(Me3Si)Ph2C]. This consists of discrete planar [(Me3Si)Ph2C]- carbanions and [Li(TMEDA)2]+ countercations. The behavior of the lithiated diphenylmethanes was rationalized to be a result of both electronic (by extensive charge delocalization) and kinetic (by the bulky R3Si substituent) stabilization.