Syntheses of symmetrically substituted 5-alkyl- and 5-aryl-dihydrodipyrrins and of porphyrins and bisporphyrins therefrom

Acid-catalysed treatment of 2-unsubstituted pyrroles with alkyl or aryl acetals affords good yields of the corresponding symmetrical 5-substituted dihydrodipyrrins (e.g. 7, 19, 38, 51, 52). Using MacDonald ‘2 + 2’ methodology, such dihydrodipyrrins are transformed in several steps into 5,15-disubstituted porphyrins (e.g. 5, 23, 24, 26, 36), wherein the 5- and 15-substituents can be identical or different. Further functionalization of 5 affords nickel(II) 5-vinyl-15-substituted porphyrin 4 which, after Vilsmeier formylation (POCl3–DMF) of the nickel(II) complex, yields the corresponding nickel(II) 5-(formylvinyl)porphyrin 3; cyclization under acidic conditions then gives the nickel 15-phenylbenzochlorin 1. Nickel(0)-catalysed reductive dimerization of the nickel(II) 15-(p-chloromethylphenyl)porphyrin 48 yields the dihydrostilbene dimer 53, which can also be obtained in lower yield by reductive dimerization of 5-(p-chloromethylphenyl)dihydrodipyrrin 57, followed by MacDonald-type cyclization and metallation of the free-base dimer 54.