Synthesis and Studies of Fused Benzo-Benzisapphyrins.

Abstract

A series of rare examples of fused benzo-benzisapphyrins were synthesized readily by (3 + 2) condensation of benzodipyrrole-derived diol and para-benzitripyrrane in the presence of 0.5 equiv of TFA in CH2Cl2 under inert atmosphere conditions accompanied by DDQ oxidation in open air. The crude compounds were separated by basic alumina column chromatography and afforded pure fused benzo-benzisapphyrins in 20-22% yields. The fused sapphyrins were characterized in detail by high-resolution mass spectrometry (HRMS) and one-dimensional (1D) and two-dimensional (2D) NMR spectroscopy. The 1H NMR spectra recorded at both 298 and at 233 K clearly exhibited the presence of a strong diatropic ring current in benzo-benzisapphyrins, and the macrocycles are of aromatic nature. The DFT-optimized structure of benzo-benzisapphyrin revealed that the macrocycle was planar to a great extent due to the rigid structure of the dibenzopyrrole moiety, and the NICS(0) value of -11.2 ppm supports the aromatic nature of macrocycles. The absorption spectra of benzo-benzisapphyrins showed three weak Q bands approximately in the region of 650-900 nm and a strong Soret band at 480 nm, along with a shoulder band at ∼510 nm. The diprotonated derivative generated by the addition of excess TFA to the benzo-benzisapphyrin macrocycle exhibited bathochromically shifted absorption bands compared to the free base macrocycle.