How delocalised are resonance-stabilised 1-[4-(N-methyl-N-alkylamino)phenyl]-2,2,2-trifluoroethyl carbocations?

Abstract

An N-trifluoroethyl for N-methyl substitution causes a ≥ 10^4.1-fold increase in the equilibrium constant for the reaction of 4-Me₂NArCH(CF₃)⁺ with 4-nitrobenzoate anion in 20% acetonitrile in water. The N-trifluoroethyl for N-methyl substitution causes a 10^5.0-fold increase in the dissociation constant for loss of a proton from nitrogen at 4-Me₂NH⁺ArCH(CF₃)OH, a reaction in which there is a full unit decrease in charge at nitrogen. These data show that the reaction of 4-Me₂NArCH(CF₃)⁺ with 4-nitrobenzoate anion behaves as if there were a decrease in effective charge at nitrogen of ≥ 0.80 units on moving from the carbocation to the neutral nucleophile adduct 4-Me(R)NArCH(CF₃)(4-nitrobenzoate). The results suggest that the positive charge at these carbocations is largely localised at nitrogen, so that they more closely resemble iminium ions than benzyl carbocations.