Gas-phase acidity of 1,1-bis(trifluoromethanesulfonyl)propane derivatives and related compounds: experimental and theoretical studies

摘要

The gas-phase acidity (GA) of a series of 1,1-bis(trifluoromethanesulfonyl)propane derivatives, Tf2CHCH2CH(R-1)R-2, and Tf2CHCH2Ar (Ar=phenol derivatives) was determined by measuring proton-transfer equilibria using a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer and by computing the free energy of the deprotonated carbanions and the corresponding neutrals. The effects of R-1 and R-2 or Ar groups on the acidity were examined. In Tf2CHCH2CH(R-1)R-2, the GA values calculated using the conformers of neutral molecules that are free from the intramolecular hydrogen-bonding interaction between the acidic hydrogen atom of the Tf2CH moiety and R-1 or R-2 group and between the hydrogen atom of the (CHRR2)-R-1 moiety and the SO2CF3 group were correlated in terms of an equation, GA=-17.0 sigma sigma(I)+3.4 sigma sigma+299.5. On the basis of this correlation, it was elucidated that the intramolecular hydrogen bonding or dipole-dipole interaction in the neutral molecule weakens significantly the acidity. In Tf2CHCH2Ar (5, 6 and 7), the GA was strengthened by the strong hydrogen-bonding interaction between the phenolic hydrogen in the aromatic moiety and the SO2CF3 group in the conjugate anion compared with that in the neutral molecule. Copyright (c) 2014 John Wiley & Sons, Ltd.