GAS-PHASE BASICITY OF N-1,N-1-DIMETHYLFORMAMIDINES - SUBSTITUENT POLARIZABILITY AND FIELD EFFECTS AND COMPARISON WITH BRONSTED BASICITY IN SOLUTION

Gas-phase basicities (GBs) for a series of 11 N1,N1-dimethylformamidines (15-25) bearing substituents with heteroatoms and/or pi-bonds linked directly to the N2 atom or separated by a (CH2)n group are obtained from proton transfer equilibria measurements using FT-ICR mass spectrometry. Comparison of the GBs obtained here with those reported previously for formamidines with alkyl groups (1-14) (J. Org. Chem. 1991,56,3669-3673) confirms the earlier conclusion that for all derivatives, even for compounds with a second potentially basic site in the substituent, the N2 atom is the favored site of protonation in the gas phase. Relative GBs of formamidines 1-25 are linearly correlated to the substituent polarizability and field effects. Deviations are observed for bifunctional compounds containing the OMe and NMe2 groups, for which the additional lone pair enhances the basicity. The sensitivity of the amidine group to the global substituent effect (polarizability, field and "two electron pairs") as compared to the amino group in amines appears to be reduced by a factor of about 1.6 because of charge delocalization. A comparison of the gas-phase substituent effects with the substituent effects in a hydroxylic solvent (95.6% ethanol-water mixture) shows that the field effect in the gas phase is twice as large as that in solution. An electron-withdrawing field effect of the cyclopropyl group is confirmed.