Cascade Radical Pathway-Enabled Nitrogen-Sulfur Coupling: Access to Isothiazolo[3,4-b]-meso-tetraarylporphyrins

A catalyst-free radical-mediated domino strategy for the construction of isothiazolo[3,4-b]-meso-tetraarylporphyrins was developed. During the course of the reaction, 2-benzothioylamino-3-thioformyl-meso-tetraarylporphyrins generated in situ after the addition of Lawesson's reagent to a solution of 2-benzoylamino-3-formyl-meso-tetraarylporphyrins in refluxing toluene underwent a homolytic cleavage to produce nitrogen-sulfur radicals. Subsequently, the formation of a new N-S bond through an intramolecular cascade radical coupling provided direct access to novel β-isothiazole-fused porphyrins.

A divergent one-pot thiol-Michael strategy to create β-thiophene-fused porphyrins

A divergent one-pot domino strategy for the synthesis of nickel(II) and copper(II) β-thiophene-fused 5,10,15,20-tetraarylporphyrins was developed through a thiol-Michael addition of thioglycolic/thiolactic acid to the corresponding 2-iminoporphyrins, formed in situ after the reaction of nickel(II) and copper(II) 2-formyl-5,10,15,20-tetraarylporphyrins with sterically hindered tert-butylamine in 1,2-dichloroethane at 80 °C. Interestingly, the reaction of 2-formylporphyrins with comparatively less sterically hindered primary amines and thioglycolic acid afforded a mixture of β-substituted porphyrinic thiazolidinones and β-thiophene-fused porphyrins. A similar one-pot thiol-Michael protocol was applied to construct a novel free-base thieno[2,3-b]-meso-tetrakis(4-methoxyphenyl)porphyrin, which underwent zinc insertion by using zinc acetate in a CHCl3-MeOH mixture and afforded zinc(II) β-thiophene-fused meso-tetrakis(4-methoxyphenyl)porphyrin in an appreciable isolated yield. On photophysical evaluation, these new porphyrins displayed a modest bathochromically shifted electronic absorption in contrast to meso-tetraarylporphyrin building blocks.

Unravelling a trichloroacetic acid-catalyzed cascade access to benzo[f]chromeno[2,3-h]quinoxalinoporphyrins

A facile one-pot four-component synthetic methodology is evolved to construct novel copper(II) benzo[f]chromeno[2,3-h]quinoxalinoporphyrins in good yields via a sequential reaction of copper(II) 2,3-diamino-5,10,15,20-tetraarylporphyrins, 2-hydroxynaphthalene-1,4-dione, aromatic aldehydes, and dimedone in the presence of a catalytic amount of trichloroacetic acid in chloroform at 65 °C. Further, the newly prepared copper(II) porphyrins were transformed to the corresponding free base and zinc(II) benzo[f]chromeno[2,3-h]quinoxalinoporphyrins under standard demetallation and zinc insertion conditions. The absorption and emission properties of the obtained porphyrins were investigated by using UV-visible and fluorescence spectroscopy. The preliminary photophysical results revealed a significant red-shift in their absorption and emission spectra as compared to the meso-tetrakis(4-methylphenyl)porphyrins due to the extended π-conjugation.