Abstract:The proton mobilities (kinetic acidities) of bis-and tris(trifluoromethyl)benzene are dictated to a large extent by steric factors; the trifluoromethyl group is a fairly bulky substituent that can seriously impede the approach of the metalating reagent. Most satisfactory results in terms of yields and selectivities have been achieved with lithium 2,2,6,6-tetramethylpiperidide or with methyllithium in the presence of potassium tert-butoxide, a slim version of the standard superbase. The rates of deprotonation under irreversible conditions do not parallel the thermodynamic (equilibrium) acidities. Substituent effects on the deprotonation energies in the gas phase appear to be additive: each trifluoromethyl group lowers it by 13 kcal mol À1 when located ortho with respect to the carbanion, and by 10 kcal mol À1 when located in a meta or para position.

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Optional site selectivity can be established in numerous neighboring group-assisted metalation reactions of arenes by mechanism-based substrateheagent matching" p31.