MECHANISM OF AZO COUPLING REACTIONS .36. THE RELATIONSHIP BETWEEN THESTRUCTURE AND THE CATALYTIC ACTIVITY OF ANIONIC PHASE-TRANSFER CATALYSTS IN AZO COUPLING REACTIONS

A series of 15 sodium arenesulfonates containing alkyl, hydroxyl or me thoxyl 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 dec omposition of the diazonium ion 1 were measured. The influence of coun ter anions on the partition coefficient (alpha) for the diazonium ion 1 in a dichloromethane/water system, and the effects of the solvent po larity and structure of counter anions on the reaction rates of the az o coupling of the diazonium ion 1 with N,N-dimethylaniline (4) and 1,3 ,5-trimethoxybenzene (2) in organic solvents and in dichloromethane/wa ter system, were investigated. The symmetrical structure of the tetrak is[3,5-bis(trifluoromethyl)phenyl]borate (TFPB) anion was proved to be an essential factor regarding the remarkably high alpha for the diazo nium ion 1 ion-paired with TFPB. The looseness and tightness of ion pa irs of highly desolvated diazonium ion 1 in a nonpolar organic media i s 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, howe ver, the amount of saturated water and/or additives in the dichloromet hane phase, which serves as a base for the rate-limiting deprotonation , becomes the most influential factor for an effective azo coupling re action under phase-transfer catalysis conditions.