Efficient Synthesis of Multiply Seven‐Membered‐Ring Fused Porphyrins by Rhodium‐Catalyzed [5+2] Annulation

Rhodium‐catalyzed reaction of meso‐pyrrol‐2‐yl Ni II porphyrins with internal alkynes proceeded smoothly to give singly, doubly, and quadruply seven‐membered‐ring fused Ni II porphyrins as the first example of [5+2] annulation reaction for porphyrin substrates, where the meso‐appended pyrrole unit serves as a directing group. These reactions are operationally quite simple and easy. A plausible reaction mechanism was proposed on the basis of the isolation of a key intermediate. The structures of the representative products have been revealed to be considerably bent owing to the implemented seven‐membered rings. Doubly fused Zn II and Ni II porphyrins possessing an unsubstituted meso‐position were dimerized to meso‐meso linked dimers by aerobic oxidation with Zn(OAc) 2 ⋅2 H 2 O and electrochemical oxidation, respectively. The optical and electrochemical highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps, as well as the aromatic characters of the fused porphyrins decrease as the fusion parts increases.