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

Abstract

A series of porphyrins (2a)–(5a), unsymmetrically substituted in their meso-positions with 3,5-di-tbutyl-4-hydroxyphenyl(DtB4HP) and 4-hydroxyphenyl (4HP) groups, has been synthesised and characterised. The two di-DtB4HP di-4HP isomeric porphyrins, (3a) and (4a)(called ‘cis’ and ‘trans’, respectively), in particular are distinguishable by ¹H n.m.r. spectroscopy. In basified dichloromethane (DCM) solutions, the porphyrins undergo drastic colour and u.v.–visible changes which, in 1 mol dm^–3 methanolic potassium hydroxide, is indicative of irreversible aerial oxidation that is slower and less extensive than for the symmetrically tetrakis-DtB4H P substituted porphyrin (1). Cyclic voltammetry supports this, indicating that in basified DCM solutions, one-electron oxidation of porphyrins (2a)–(5a) becomes increasingly more difficult than for (1), as the DtB4HP groups are replaced by 4HP. An explanation of the relative redox activity of these porphyrins compared with (1), is offered in terms of the different electron-releasing properties of the two types of phenolic substituent.