Basicity of trifluoromethylsulfonylformamidines. DFT and FTIR study and NBO analysis

The effect of the electron–acceptor substituent CF3SO2 at the imine nitrogen atom on the basicity and the electron distribution in N,N‐alkylformamidines (1, 2, 3, 4, 5) was studied experimentally by the FTIR spectroscopy and theoretically at the DFT (B3LYP/6‐311+G(d,p)) level of theory, including the natural bond orbital (NBO) analysis. The calculated proton affinities of the imine nitrogen atom and the sulfonyl oxygen (PAN′ and PAO) depend on the atomic charges, the CN′ and N′―S bond polarity and on the energy of interaction of the amine nitrogen and the oxygen lone pairs with antibonding π* and σ*‐orbitals. The basicity of the imine nitrogen atom is increased with the increase of the electron‐donating power of the substituent at the amine nitrogen atom due to stronger interaction nN → π*CN′, but is decreased for the electron‐withdrawing groups MeSO2 and CF3SO2 at the imine nitrogen atom in spite of the increase of this conjugation. Protonation of (1, 2, 3, 4, 5) in CH2Cl2 solution in the presence of CF3SO3H occurs at the imine nitrogen atom, while the formation of hydrogen bonds with 4‐fluorophenol takes place at the sulfonyl oxygen atom, whose basicity is lower than that of N,N′‐dimethylmethanesulfonamide but higher than of N,N′‐dimethyltrifluoromethanesulfonamide. Copyright © 2015 John Wiley & Sons, Ltd. The electron‐donating substituents at the amine nitrogen in trifluoromethylsulfonylamidines expectedly increase the proton affinity of the imine nitrogen because of the enhanced orbital interaction nN→π*C=N′, whereas the electron‐withdrawing groups MeSO2 and CF3SO2 at the imine nitrogen decrease these proton affinity values in spite of the increased conjugation, apparently, because of the strong inductive effect.