Facile aerial oxidation of a porphyrin. Part 5. Unsymmetrical meso-tetra-aryl porphyrins with 3,5-di-t-butyl-4-hydroxyphenyl and 3-hydroxyphenyl substituents

Abstract

The aerial oxidation in basified dichloromethane (DCM) solutions of porphyrins (2b)–(4b), unsymmetrically meso-substituted with 3,5-di-t-butyl-4-hydroxyphenyl (DtB4HP) and 3-hydroxyphenyl (3-HP) groups, has been investigated using -u.v.–visible spectroscopy and cyclic voltammetry. As in the analogous series (2a)–(4a), basification with 1 mol dm^–3 tetrabutylammonium hydroxide (tnBAH) in MeOH produces very little aerial oxidation. However, basification with 1 mol dm^–3 potassium hydroxide (KOH) in MeOH leads to aerial oxidation of (2b)–(4b), but to a lesser extent than in the series (2a)–(4a). Except for porphyrin (3b) this trend in lowered oxygen reactivity in base is not reflected in increased first half-wave oxidation potentials (E^⊖) from cyclic voltammetry, compared to the series (2a)–(4a). An explanation of these differences is offered in terms of the relative availability to oxygen of DtB4HP electron density in the two sets of porphyrins.