Nonplanar Porphyrins by N-Substitution: A Neglected Pathway

Abstract

N‐substitution of porphyrins has been a neglected route towards nonplanar porphyrins for the past decades. Previously, they featured in a host of potential medicinal and biochemical applications. However, tailored syntheses of N‐methylated porphyrins, improvements of synthetic methodology, or full conformational analyses were lacking since the initial studies. Here we investigated and optimized synthetic pathways to generate specific N‐methylated porphyrins exclusively and in good yields. Full characterizations of the spectroscopy and structural properties associated with the insertion of different numbers of methyl into the porphyrin core of 5,10,15,20‐tetrasubstituted A4‐ and 5,15‐disubstituted A2‐type porphyrins was carried out by using UV/Vis, NMR, and X‐ray crystallographic techniques. The latter, in conjunction with detailed normal structural decomposition analyses, identified the structural consequences of number and isomeric pattern of N‐methylation in terms of macrocycle nonplanarity and the underlying out‐of‐plane and in‐plane distortion modes.