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 = −17.0ΣσI + 3.4Σσα + 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 © 2014 John Wiley & Sons, Ltd.