Construction of coumarin-fused pyrido[2,3-b]porphyrins through a trichloroacetic acid-accelerated domino approach.

The first synthesis to construct coumarin-fused pyrido[2,3-b]porphyrins has been accomplished via a cascade reaction of 2-amino-meso-tetraphenylporphyrins, aromatic aldehydes and 4-hydroxycoumarin in o-dichlorobenzene containing a stoichiometric amount of trichloroacetic acid under reflux conditions. The methodology presented herein provides a direct access to a new series of π-extended nickel(ii) and copper(ii) porphyrin analogues in 58-69% isolated yields under one-pot operation. Furthermore, coumarin-fused copper(ii) pyrido[2,3-b]porphyrin underwent demetalation under the influence of concentrated sulfuric acid to produce free-base porphyrin which on zinc metal insertion using zinc acetate in a chloroform-methanol mixture afforded coumarin-fused zinc(ii) pyrido[2,3-b]porphyrin in good yield. The structures of the newly prepared coumarin-fused pyrido[2,3-b]porphyrins were established on the basis of spectral data analysis. In comparison with meso-tetraphenylporphyrin precursors, these porphyrinoids demonstrated a significant red-shift in their electronic absorption and emission spectra.