The reactions of organometallic compounds containing silicon. Part III. Reactions of trimethyl-, dimethylphenyl-, methyldiphenyl-, and triphenyl-silyl-lithium with fluorene

Abstract

Dimethylphenyl-, methyldiphenyl-, and triphenyl-silyl-lithium react very rapidly with fluorene in tetrahydrofuran. The reaction can be followed at low temperature with a stop-flow technique. The thermodynamic constants of activation at –30° for this reaction with Ph₃SiLi, MePh₂SiLi, and Me₂PhSiLi, respectively are: ΔG‡= 14·0, 13·6, 13·6 kcal. mole^–1; ΔH‡= 6·1, 6·6, 6·2 kcal. mole^–1; ΔS‡=–32·5, –28·8, –30·3 cal. mole^–1 deg.^–1. Addition of LiBPh₄ to these systems has negligible effect on the rate constants.

The fact that the replacement of phenyl by methyl produces so small a change has been discussed in relation to the constancy of λ_max. for R₃SiLi as phenyl is replaced by methyl.

For the reaction of MePh₂SiLi with fluorene at –30° in dimethoxyethane the thermodynamic constants of activation are: ΔG‡= 11·7 kcal. mole^–1; ΔH‡= 3·5 kcal. mole^–1; ΔS‡=–33·7 cal. mole^–1 deg^–1.

Hexamethyldisilane has been cleaved by lithium in tetrahydrofuran solution to give trimethyl-silyl-lithium, and the reaction of this with fluorene has been studied qualitatively.