Computer aided design of organic superbases: the role of intramolecular hydrogen bonding

Abstract

The role of intramolecular hydrogen bonding (IMHB) in determining the proton affinities and basicities of some bis(tetramethylguanidine)systems was examined. For this purpose a series of molecular backbone moieties serving as carriers of the bis(tetramethylguanidine)crowns were explored. It was found that the best backbones are provided by phenanthrene and 9,10-dihidrophenanthrene, giving rise to proton affinities as large as 268.2 and 266.8 kcal mol¹⁻, respectively. The corresponding pK_a values in acetonitrile are 29.0 and 28.8, implying that these two compounds [6(bs)and 5(bs)] are candidates for powerful superbases. Their intramolecular hydrogen bond strengths are ≈19 kcal mol¹⁻, which result inter alia from the partial protonation of the vis-à-vis guanidine group.