The 1,3,5-Trinitrobenzene–Phenoxide σ-Complex. Ambident Phenoxide Ion in SNAr Systems

1,3,5-Trinitrobenzene reacts with potassium phenoxide in dimethyl sulfoxide solution to yield a σ-complex in which bonding has occurred between the para carbon of the phenoxy moiety and an unsubstituted carbon of the nitroaromatic substrate. A minor proportion of ortho adduct is also obtained. When the reaction is performed in DMSO–MeOH (80:20, v/v) one obtains initially the methoxide adduct of TNB which is then smoothly transformed into the phenoxide adduct. The latter is stable to dilute mineral acid, unlike the former; formation of a σ-complex involving bonding between the phenoxy oxygen and a carbon of TNB is thereby excluded. The reaction represents the first instance of phenoxide ion acting as an ambident nucleophile towards aromatic carbon. The implication of this finding on the course of related SNAr reactions is discussed on the basis of kinetic and thermodynamic control of product formation.