An Efficient and Concise Synthesis of Heteroaryldipyrromethanes, Tetrapyrazolylporphyrins and Metalloporphyrins

ABSTRACT

One pot synthesis of terpyrromethanes (3a) and [di(1H-pyrrol-2-yl)methyl]heteroarenes (3b-m) has been delineated by Amberlyst 15 catalyzed condensation of pyrrole and heterocyclic aldehydes (2), not reported earlier. The compounds 3a and 3b have been formylated to 5-[di(5-formyl-1H-pyrrol-2-yl)methyl]-1H-pyrrole-2-carbaldehyde (4a) and 5-[(5-formyl-1H-pyrrol-2-yl)(3-pyridinyl)methyl]-1H-pyrrole-2-carbaldehyde (4b), by using a mixture of POCl3 and DMF as formylating agent. Similarly, 5-alkyl-1H-pyrrole-2-carbaldehyde (8) has also been prepared from 2-alkyl-1H-pyrroles (6), obtainable by acylation followed by Wolf-Kishner reduction. Amberlyst 15 / TFA catalyzed the condensation of 8 with pyrrole which gave 3a instead of expected product 10, due to an unusual cleavage of alkyl substituent at position 5 in 8. CS₂ addition to 6 followed by methylation provided methyl 5-alkyl-1H-pyrrole-2-carbodithioates (7, which on reaction with hydrazine hydrate yielded 5-alkyl-1H-pyrrole-2-carbothiohydrazide (9). Tetrapyrazolylporphyrins (12) have been synthesized directly by condensation of pyrrole with 3-aryl-4-formyl-1-phenylpyrazole (11) in propionic acid. These porphyrins were transformed to metalloporphyrins (13) from reaction with zinc and copper acetate separately.