Regioselective syntheses and structural characterizations of 2,3-dibromo-and 2,3,7,8,12,13-hexabromo-5,10,15,20-tetraphenylporphyrins

The title dibromoporphyrins 4/5 and hexabromoporphyrins 11/12 are prepared from H-2(2-NO2TPP) 1 and Cu(2-NO2TPP) 8, respectively. The beta-nitro group confines the 18-pi-annulene system of a tetraphenylporphyrin to its N22H-N 24H aromatic delocalization pathway which induces the localization of an an tipodal double bond on the porphyrin periphery and enhances its susceptibil ity to electrophilic attack. Dibromination of H-2(2-NO2TPP) 1 occurs regios electively affording the 12,13-dibromo-2-nitroporphyrin 2 which, upon Micha el addition of NaBH4 and re-aromatization of the resulting nitrochlorin 3, provides an entry to 2,3-dibromoTPP 4/5 as well as an improved route to 2,3 -dicyanoporphyrins 6/7. Perbromination of Cu(2-NO2TPP) 8 and denitration of 9 gave, after demetalation, 2,3,7,8,12,13-hexabromoTPP 12. Both 4 and 12 a re structurally characterized by X-ray crystallography. (C) 1999 Elsevier S cience Ltd. AU rights reserved.