2,2'-Bipyrrole-Based Porphyrinoids

A large number of porphyrinoids containing 2,2'-bipyrrole subunits have appeared since they were originally found as a component of sapphyrin and corrole, and it was found that the bipyrrole subunit endowed macrocycles with specific geometric features and electronic properties. Synthetic methods for bipyrrole-containing precursors for porphyrinoid are summarized in this review; these include coupling reactions of pyrrole rings, pyrrole ring-forming reactions leading directly to bipyrrole units, and synthetic reactions for oligopyrrolic compounds. Some hybrid oligopyrroles having nonpyrrole (hetero)aromatic ring(s) are also included. This review also describes porphyrinoids composed of bipyrrole subunits. Interesting electronic properties derived from strong cyclo-π-conjugation are highlighted in the bipyrrole-based porphyrinoids with or without meso-like carbons. Anion-binding chemistry is one of the main topics for bipyrrole-based macrocycles with less efficient or deficient cyclo-π-conjugation, such as those linked with electronically localized aromatic ring(s), with sp³ carbon(s), and with amido or imine connection(s). The principal concern in this review is porphyrinoids of relatively large ring size, composed of more than five units of pyrroles and (hetero)aromatic substitutes in total, and so bipyrrole-based porphyrinoids up to five pyrrolic units, such as corroles, porphycenes, sapphyrins, and smaragdyrin, will not be covered here except for some special cases.