Propeller, Linear, Cruciform and Stellate Spiro-bicarbazolium Salts

A general synthetic approach to halogenated tetraaryl-ammonium salts has been developed and illustrated crystallographically. Bromide ammonium salts used as common synthetic intermediates together with Suzuki coupling of these bromides to a family of boronic acids provided a simplified strategy for arylation. Resolution of the C-2 subset of spiro-bicarbazolium derivatives led to the first examples of enantiopure spiro-bicarbazoliums and the assignment of their absolute configuration by comparison of computational and experimental electronic circular dichroism (ECD) spectra. An ECD comparison with Prelog's spirobifluorenes is provided. The absolute configuration of the meta-bromide spirobicarbazolium salt was confirmed by anomalous scattering. Cruciform and stellate tetra-substituted salts provide a test of the limits of the methodology, and their structures suggest them to be candidates for MOF building blocks.