Exceptional Superbasicity of Bis(guanidine) Proton Sponges Imposed by the Bis(secododecahedrane) Molecular Scaffold: A Computational Study

Abstract

AbstractThe exceptional superbasicity of a series of bis(guanidines) with a bis(secododecahedrane) molecular scaffold was found in the course of a computational study. The 3‐syn,13‐syn‐disubstituted bis(secododecahedrane) skeleton ensures that nitrogen lone pairs of electrons are in close proximity, and the rigid framework of the polycyclic cage guarantees acid/base properties similar to those of proton sponges. Amine functionalization with alkyl substituents, imines, and guanidines leads to high basicity. Based on DFT computations, bis(guanidine)–bis(secododecahedrane) proton sponges with a (1,3‐dimethylimidazolidin‐2‐ylidene)amino moiety are predicted to have very high basicities, both in the gas phase and acetonitrile. This skeleton provides the highest proton affinity(PA) value among all the aliphatic proton sponges reported (288.7 kcal mol–1, whereas the gas‐phase basicity is 284.3 kcal mol–1, at the B3LYP/6‐311+G**//B3LYP/6‐31G* level). In acetonitrile the calculated PA is 319.4 kcal mol–1 and the estimated pKa is 38.5. Substitution of bis(secododecahedrane) with the intrinsically more basic nitrogen functionalities increases the PA to 316.3 kcal mol–1 and the pKa to 46.