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

The gas‐phase acidity (GA) of a series of 1,1‐bis(trifluoromethanesulfonyl)propane derivatives, Tf₂CHCH₂CH(R¹)R², 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 R¹ and R² or Ar groups on the acidity were examined. In Tf₂CHCH₂CH(R¹)R², 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 R¹ or R² group and between the hydrogen atom of the CHR¹R² moiety and the SO₂CF₃ group were correlated in terms of an equation, GA = −17.0Σσ_I + 3.4Σσ_α + 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.