Investigating different strategies towards the preparation of chiral and achiral bis-picket fence corroles

Abstract

Three different strategies have been designed and tested for the preparation of sterically hindered chiral and achiral bis-picket fence corroles. All attempts to prepare an A3 corrole, with six pivaloylamido pickets in the ortho-positions of the meso-aryl substituents, failed. Mass spectrometrical investigations of the reaction mixtures prove the very slow formation of tetrapyrrolic condensation products from strongly hindered benzaldehydes. For sterical reasons, however, these potential precursors seem to be forced into a stretched conformation incapable of oxidative macrocyclization. For A2B-type bis-picket fence corroles, the formation of trace amounts of four-fold pivaloylamido substituted species with a sterically innocent aryl group at the 10-position can be shown. A reverse order of steps, with the palladium-catalyzed amidations of o-bromophenyl groups at the 5- and 15-positions of the completed corrole macrocycle as the last step, has finally been found as the key to a successful and versatile synthesis of such chiral and achiral sterically encumbered corrole ligands.