Photoreactions of aromatic compounds—XXXV : Nucleophilic photosubstitution of methoxy substituted aromatic compounds. Monophotonic ionization of the triplet

Abstract

Methoxy groups exert an activating and ortho/para directing influence in light induced nucleophilic substitution reactions (cyanation, hydroxylation, etc) of aromatic compounds in aqueous media. The first chemical step in these processes is monophotonic ionization of the aromatic compound in its lowest triplet state, followed by reaction of the radical cation with the nucleophile Quantum yields of photocyanation of 4-fluoro- and 4-chloroanisole indicate that in 99% (mole fraction) water virtually all triplets formed undergo electron ejection. This hypothesis is in agreement with the results of charge density calculations for the radical cations. It is directly supported by the similarity of the product composition of these photochemical substitutions with that of anodic substitutions, where the intermediacy of an aromatic cation is generally accepted. The presence of an oxidizing agent (oxygen, or persulphate) is required only when a hydrogen is replaced. The nucleophilic photosubstitution at electron rich aromatic systems in solvents as water can therefore be classified as an S r + n 1 ( 3Ar *) process.