Huckel and Mobius Expanded para-Benziporphyrins: Synthesis and Aromaticity Switching

The four expanded p-benziporphyrins A,C-di-p-benzi[24]pentaphyrin(1.1.1.1.1), N-fused A-p-benzi[24] pentaphyrin, A,D-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1), and A,C-di-p-benzi[28]hexaphyrin(1.1.1.1.1.1) were obtained in threecomponent Lindsey-type macrocyclizations. These compounds were explored as macrocyclic ligands and as potential aromaticity switches. A BODIPY-like difluoroboron complex was obtained from the A,C-di-p-benzi[24] pentaphyrin, whereas A,C-di-p-benzi[28]hexaphyrin yielded a Mobius-aromatic Pd-II complex containing fused pyrrole and phenylene subunits. Conformational behavior, tautomerism, and acid-base chemistry of the new macrocycles were characterized by means of NMR spectroscopy and DFT calculations. Free base N-fused A-p-benzi[24]pentaphyrin showed temperature-dependent Huckel-Mobius aromaticity switching, whereas the A,C-di-p-benzi[28]hexaphyrin formed a Mobius-aromatic dication.