DEPENDENCE OF KINETIC ISOTOPE EFFECT ON STRUCTURE OF ORGANOLITHIUM METALATING REAGENT IN METALATION OF TRIPHENYLMETHANE

Measurements have been made of kH/kD kinetic isotope effects associated with the metalation of a series of triarylmethanes (α-H and α-D) by six organolithium reagents (RLi) in tetrahydrofuran at 22°. The substantial variation in kH/kD value observed with benzyllithium, allyllithium, phenyllithium, vinyllithium, n-butyl-lithium and methyllithium, is attributed to different transition state geometries, dependent on the structures of the organolithium compounds. Hammet σ ρ{variant} plots for each RLi metalating reagent yield ρ{variant} values within the range 2.2 to 3.0, suggesting appreciable negative charge occurs on the triarylmethyl voiety in the transition states. It is inferred from kH/kD values measured with (p-methylphenyl)diphenylmethane, (p-methoxyphenyl)diphenylmethane, and triphenylmethane, that the effective basicity of organolithiums towards these substrates increases in the seriesn-butyllithium 〈vinyllithium〈methyllithium〈benzyllithium〈phenyllithium. © 1969.