NMR spectra of porphyrins. 26—conjugation versus steric repulsions in a planar chiral meso‐ Aminoporphyrin

Abstract

AbstractThe proton NMR spectra of the free base porphyrins γ‐meso‐dimethylaminopyrroetioporphyrin‐XV (1) and the corresponding γ‐diethylamino compound 2 show no temperature dependence, and are interpreted as being due to an essentially orthogonal conformation of the γ‐NR2 substituents. However, both of the corresponding dications 3 and 4, respectively, in CDCl3–TFA solution, give temperature‐dependent proton NMR spectra. Nuclear Overhauser enhancement difference spectra allow complete assignment of the spectrum of 3, and hence the interpretation of the temperature dependence. The spectra arise from a skew conformation of the NR2 groups in which the barrier to rotation through the orthogonal conformation is ca 13.6 kcal mol (56.8 kJ mol−1), and the barrier to rotation through the planar conformation is greater than this.