(27h) Nucleophilic Aromatic Substitution in High-Temperature Ionic Liquids

Nucleophilic aromatic substitution is an important step in many syntheses of industrial importance. Often, achieving viable reaction rates for a needed substitution requires fluoride as a leaving group as well as multiple activating groups that necessitate later reaction steps. Furthermore, health and environmental hazards are introduced by the employment of polar organic solvents such as DMSO and acetonitrile to enhance reaction rates. In this work, those solvents are replaced by perarlyphosphonium and perarylsulfonium triflate ionic liquids that exhibit stability at high temperatures. Thus, it is possible to thermally accelerate reaction rates, reducing the extent of activation required and eliminating the need for fluoride leaving groups.

The substitution of bromide in 4-bromonitrobenzene by phenoxide was investigated in four thermally stable ionic liquids at temperatures ranging from 130 to 200 °C. Reaction rates were such that equilibrium conversions were achieved in 1-3 hours, depending on temperature. Temperature and solvent effects on both reaction rates and equilibrium conversions were observed and evaluated. The solvent effects will be analyzed and explained with the support of free energy calculations of reaction pathways using density functional theory.