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. Bu^s or Bu^t) 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.