Selective Conversion of Planar trans-Chlorins into Highly Twisted Doubly Fused Porphyrins or Chlorins via Oxidative Fusion

Abstract

β-to-o-phenyl doubly fused porphyrins (DFPs) or chlorins (DFCs) were selectively synthesized by facile oxidative fusion of trans-chlorins using 2,3-dichloro-5,6-dicyano-1,4- benzoquinone (DDQ) in good-to-excellent yields (70–92%) under mild reaction conditions with high atom economy. The selectivity in product formation (difused porphyrin or chlorin) was controlled by the presence or absence of a Ni(II) ion in the macrocyclic core. Notably, nickel(II) trans-chlorins selectively yielded DFPs, whereas free-base trans-chlorins afforded only DFCs. The synthesized fused porphyrinoids exhibited significantly red-shifted electronic spectral features (Δλmax = 16–53 nm) of the Soret band due to the extended π conjugation and highly twisted macrocyclic conformation (twist angle ∼20–34°). Inner-core NHs of fused chlorins exhibited a tremendous downfield shift (Δδ = 1.71–2.02 ppm) compared to their precursors. The overall protonation constants for indanedione-substituted free-base-difused chlorins (4–6) were profoundly higher (∼20–50-fold) compared to dicyanomethyl-appended free-base-difused chlorins (10–12) because of the combined effect of the electronic nature of the β-substituents and nonplanarity of the macrocyclic core. The first oxidation potential of H2DFC(MN)2Ph2 (12) was 0.54 V cathodically shifted with respect to H2DFC(MN)2 (10) because of the electron-donating nature of the β-phenyl groups, which resulted in extensive destabilization of the highest occupied molecular orbital.