Applications of Phase Transfer Catalysis to Arenediazonium Cation Chemistry

Arenediazonium cations have been known for many years but the synthetic utility has been diminished by their lack of stability and by their proclivity for undergoing sundry side reactions. Arenediazonium BF₄ and PF₆ salts are generally stable solids, but are relatively insoluble in nonpolar organic media. Crown complexation and other forms of phase transfer catalysis increase solubility and allow numerous synthetic transformations to be conducted in good yield. The yield improvements result from a reduction in competing side reactions rather than any fundamental change in mechanism. The reactions to which the phase transfer technique is successfully applied are diazo coupling, reduction, halogenation, arylation and cyclization. In the latter case, the use of potassium superoxide is sometimes a more efficacious reagent than potassium acetate. The phase transfer approach can be used in the high yield synthesis of arenediazosulfones and arenediazocyanides. The latter undergo cycloaddition with many dienes to yield pyridazine derivatives.