Selective octabromination of tetraarylporphyrins based on meso-substituent identity: Structural and electrochemical studies

Synthesis and isolation of selectively brominated tetraarylporphyrin derivatives is reported. Treatment with bromine of meso-5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl) porphyrin (1) or meso-5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl) porphinatocopper(II) (2-Cu) yields products octabrominated at the 2,6-positions of meso-aryl substituents [5,10,15,20-tetrakis(2,6-dibromo-3,4,5-trimethoxyphenyl) porphyrin, (2,6)Br(8)1] or macrocyclic beta-positions. The latter of these (beta-brominated) was identified as the oxoporphyrinogen 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(3,5-di-t-butyl-4-oxo-cyclohexa-2,5-dienylidene)porphyrinogen 3 obtained due to the adventitious oxidation and demetalation of 2-Cu, which could be alkylated at its macrocyclic nitrogen atoms yielding N-21,N-22,N-23,N-24-tetrakis(4-bromobenzyl)-2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(3,5-di-t-butyl-4-oxo-cyclo hexa-2,5-dienylidene) porphyrinogen 4. The former compound (2,6)Br(8)1 was complexed with Zn(II) ((2,6)Br(8)1-Zn) or Cu(II) ((2,6)Br(8)1-Cu) and could also be subjected to further bromination yielding 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(2,6-dibromo-3,4,5-trimethoxyphenyl) porphyrin, Br(16)1. The effect on the electrochemical properties of the 2,6-bromophenyl-substituted porphyrin compounds over the more highly brominated products was assessed.