Cascade Amination and Aza-6π-Annulation-Aromatization Strategy for the Synthesis of β-Pyrimidine-Fused Porphyrins

Novel nickel(II) and copper(II) complexes of 2-(N,N-dimethylformamidine)-3-formyl-5,10,15,20-tetraarylporphyrins have been synthesized for the first time from 2-aminoporphyrins under Vilsmeier-Haack conditions. These porphyrins are utilized as new building blocks to construct diverse β-pyrimidine-fused 5,10,15,20-tetraarylporphyrins in good yields via a cascade ammonia-mediated condensation and intramolecular aza-6π-annulation/aromatization in 1,2-dichloroethane at 80 °C. Furthermore, copper(II) β-pyrimidine-fused porphyrins underwent demetallation in the presence of conc. H2SO4 to afford free-base porphyrins, which on zinc insertion using Zn(OAc)2 in CHCl3-MeOH provided zinc(II) β-pyrimidine-fused porphyrins in appreciable yields. Notably, these newly synthesized π-extended porphyrins displayed a modest bathochromic shift in their electronic absorption and emission spectra as compared to the traditional meso-tetraarylporphyrins. However, the protonated porphyrins (2a) and (3g) displayed a significant red-shifted absorption.