Cooperative intramolecular hydrogen bonding effect and basicity -: An ab initio and DFT study of the superbasic properties of N-[(dimethylamino)alkyl]-2,3-diaminocycloprop-2-ene-1-imines

The MP2(fc)/6-311+G**//HF/6-31G* and B3LYP/6311+G**//HF/6-31G* calculations show that N-[3-(dimethylamino)propyl] and N-[4-(dimethylamino)butyl] derivatives of 2,3-diamino-cycloprop-2-ene-1-imine (CPI) possess high gas phase proton affinities (PA) and pronounced basicities in acetonitrile (MeCN). Tris[N-dimethylaminopropyl(-butyl)]CPI derivatives achieve superbasic PA and pK, values. The reason behind these remarkable features is the stability of the resulting conjugate acids. They are stabilized by aromatization of the three-membered ring accompanied by the cationic resonance between the cyclopropyl ring and amino nitrogen atoms of the side chains, and, last but not least, by the cooperative multiple intramolecular hydrogen bonding effect. The latter is particularly enhanced in tris-substituted CPIs. It was found that the N-[4-(dimethylamino)butyl] side chains contribute more to the stability of the conjugate acids than those of the corresponding compound with a propyl chain, because they undergo a somewhat larger relaxation upon protonation. The studied compounds might be helpful in building up a dense ladder of strong bases and superbases, which could contribute to the development of the chemistry of superacids and superbases. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).