BIS(TRIFLUOROMETHYL)BENZENES AND OLIGO(TRIFLUOROMETHYL)BENZENES - HYDROGEN METAL EXCHANGE-RATES AND GAS-PHASE ACIDITIES/

The proton mobilities (kinetic acidities) of bis- and tris(trifluorome thyl)benzene are dictated to a large extent by steric factors; the tri fluoromethyl group is a fairly bulky substituent that can seriously im pede 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, Th e 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 t rifluoromethyl group lowers it by 13 kcal mol(-1) when located ortho w ith respect to the carbanion, and by 10 kcal mol(-1) when located in a meta or para position.