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.