Synthesis and switching the aromatic character of oxatriphyrins(2.1.1)

Article abstract of DOI:10.1002/anie.201310129

Triangularly shaped, contracted porphyrinoids belong to a group of molecules where the geometry significantly modifies the observed electronic properties. The need for a controllable, effective, and widely applicable approach to triphyrins drives extensive research towards macrocyclic materials that act as potential controlling motifs by switching their aromaticity. Two isomeric thiophene-fused triphyrins(2.1.1) were synthesized by applying an innovative approach. Spectroscopic techniques (NMR, UV/Vis) show that both macrocycles are aromatic and quantitatively convert into anti-aromatic structures after reduction with a zinc amalgam. The reduced forms were stabilized through boron(III) coordination, thereby allowing the observation of anti-aromatic 16 π delocalization within a contracted porphyrin. Going for a spin: The precisely crafted thiophene-fused oxatriphyrins(2.1.1) show an aromatic character consistent with 14 π or 18 π delocalization pathways. The aromaticity is strongly dependent on a thiophene incorporation mode and can be switched to anti-aromatic (16 π electrons) or non-aromatic by reduction with a zinc amalgam. Both reduced structures are stable as boron(III) complexes.