Regioselective reaction of 5,15-disubstituted porphyrins with organolithium reagents—synthetic access to 5,10,15-trisubstituted porphyrins and directly meso-meso-linked bisporphyrins

Abstract

The reaction of organolithium reagents with 5,15-disubstituted porphyrins, followed by hydrolysis of the intermediate anion to give a porphodimethene and oxidation with DDQ allows the facile preparation of 5,10,15-tri- and 5,10,15,20-tetrasubstituted porphyrins with mixed meso substituent types. Easily available 5,15-disubstituted porphyrins react almost quantitatively with sterically undemanding organolithium reagents (PhLi, BuLi) to give the respective 5,10,15-trisubstituted porphyrins (A2B-type). When bulkier reagents are used (e.g. Bus or But) the number and yield of side products increases considerably. Thus, the regioselectivity of the nucleophilic attack (meso versus β) depends on the steric hindrance of the LiR reagents. The 5,10,15-trisubstituted porphyrins can in turn be used for another cycle of nucleophilic attack, hydrolysis, and oxidation giving rise to 5,10,15,20-tetrasubstituted porphyrins of the A2B2- or A2BC-types. In contrast, reaction of 5,15-disubstituted porphyrins with LiR and direct oxidation of the intermediate anion without a hydrolysis step gives directly linked (5,5′) bisporphyrins allowing the preparation of bisporphyrins with mixed meso substituent pattern. In addition, the crystal structures of two 5,10,15-trisubstituted porphyrins (25, 28) are reported. Both compounds exhibit moderately nonplanar macrocycles as a result of the sterically hindered meso substituents.