An unusual [4 + 2] fusion strategy to forge meso-N/O-heteroarene-fused (quinoidal) porphyrins with intense near-infrared Q-bands

摘要

Here we present a divergent synthesis of brand-new types of meso-N / O -heteroarene-fused (quinoidal) porphyrins through Rh-catalyzed β-C–H activation/annulation of 5,15-dioxoporphyrins and dioxime derivatives with alkynes, in which the synthetic disconnections are difficult to access through the commonly used intramolecular cyclization strategy. Using the O -methyl oxime as a traceless oxidizing directing group, the meso-N -embedded pyridine-fused anti -quinoidal porphyrin 3 and pyridinium-fused cation 4 are formed with controllable chemoselectivity and complete anti -selectivity. Replacing the exocyclic oxime with a carbonyl group delivers the pyran-fused porphyrin 5 , achieving structural conversion from a quinoidal conformation to a stable porphyrin macrocycle. Further oxidation of the expanded dimer 5ea gives the oxonium 6 , which exhibits intense near-infrared (NIR) Q-bands up to 1300 nm. Theoretical studies demonstrate that the incorporation of a heteroatom at the meso -position enables more effective π-extension, resulting in a 22π aromatic ( vs. 18π aromatic) character of pyran-fused porphyrins ( syn / anti - 5aa ). Compared with the commercially available methylene blue ( MB ), syn - 5al exhibits a better ability ( Φ _(Δ) = 0.61) to sensitize singlet oxygen ( ~(1) O _(2) ) when irradiated with a 680 nm laser beam, and has potential as a photodynamic therapy (PDT) photosensitizer in the body's therapeutic window (650–900 nm).