Mechanism of Azo Coupling Reactions: Part XXXVI. The Relationship between the Structure and the Catalytic Activity of Anionic Phase-Transfer Catalysts in Azo Coupling Reactions

Abstract

Abstract A series of 15 sodium arenesulfonates containing alkyl, hydroxyl or methoxyl groups was investigated for the phase-transfer catalysis of the azo coupling reaction of the 4-methoxybenzenediazonium ion (1) with 1,3,5-trimethoxybenzene (2) in a dichloromethane/water two-phase system, in which the yields, initial reaction rates and influence of the decomposition of the diazonium ion 1 were measured. The influence of counter anions on the partition coefficient (α) for the diazonium ion 1 in a dichloromethane/water system, and the effects of the solvent polarity and structure of counter anions on the reaction rates of the azo coupling of the diazonium ion 1 with N,N-dimethylaniline (4) and 1,3,5-trimethoxybenzene (2) in organic solvents and in dichloromethane/water system, were investigated. The symmetrical structure of the tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB) anion was proved to be an essential factor regarding the remarkably high α for the diazonium ion 1 ion-paired with TFPB. The looseness and tightness of ion pairs of highly desolvated diazonium ion 1 in a nonpolar organic media is one of the most significant factors concerning the reactivity of the diazonium ion 1 in a rate-limiting azo coupling with 4. In a reaction of the diazonium ion 1 with 2 in a dichloromethane/water system, however, the amount of saturated water and/or additives in the dichloromethane phase, which serves as a base for the rate-limiting deprotonation, becomes the most influential factor for an effective azo coupling reaction under phase-transfer catalysis conditions.