Protonation-triggered conformational modulation of an N,N′-dialkylbispidine: first observation of the elusive boat–boat conformer

Abstract

Modulation of the solution conformations of N,N′-bis(benzhydryl)bispidine has been achieved by protonation. Conformers have been characterized by NMR spectroscopy using nuclear Overhauser effects and residual dipolar couplings. In contrast to the preference for the chair–chair conformation for the free base and the monoprotonated species, the diprotonated bispidine is revealed to exist as a mixture of chair–boat and boat–boat conformers. While boat–boat conformers of bispidines have previously not been detected, they are here observed to constitute up to 70% of the bispidine population.