Gas-phase basicity of N1,N1-dimethyl-N2-alkylformamidines: substituent polarizability effects

Abstract

Gas-phase basicities (GBs) for a series of 14 N 1 ,N 1 -dimethyl-N 2 -alkylformamidines are obtained from proton-transfer equilibra, using Fourier transform ion cyclotron resonance mass spectrometry. The imino nitrogen atom appears to be the preferred protonation site. Relative GBs are linearly correlated to the alkyl substituent polarizability. Compared to other nitrogen bases (amines and nitriles), the sensitivity to this effect is strongly reduced by charge delocalization. Larger than expected GBs for long-chain alkyl derivatives are attributed to a coiling effect. An electron-withdrawing effect of the cyclopropyl linked to the electron-rich imino nitrogen atom is proposed as an explanation of the relatively weak GB measured for the corresponding compound