Thermal Effect in the Microwave-assisted Aminolysis of Benzoates and Amines

Background: Microwave selective heating thermal effect is obvious in unimolecular organic reactions. However, it is unclear whether it exists in bimolecular organic reactions under strictly controlled reaction temperature conditions. Objective: To determine whether microwave selective heating effect exists in the microwave-assisted bimolecular reactions. Methods: Hammett linear relationships in “one-pot” aminolyses of mixed 4-nitrophenyl substituted benzoates with benzylamine and 4-nitrophenyl benzoate with mixed substituted anilines were selected as molecular level probes to explore the thermal effect in the microwave-assisted bimolecular reactions. Results: In less polar solvent, there is an obvious “hot spots” effect. “One-pot” aminolyses of mixed 4-nitrophenyl substituted benzoates with benzylamine and 4-nitrophenyl benzoate with mixed substituted anilines were performed in less polar solvent toluene under oil-bath and microwave heating conditions. Generally, slopes of Hammett plots or effect of substituents on reaction rates decrease along with temperature increases under oil-bath heating conditions. Under microwave irradiation conditions, slopes of Hammett plots or effect of substituents on reaction rates decrease in comparison with those under oil-bath heating conditions at the same setting temperature, revealing that higher temperature regions (“hot spots”) still exist in intermolecular organic reactions. Conclusion: Microwave selective heating thermal effect still exists in bimolecular organic reactions under strictly controlled reaction temperature conditions, revealing that higher temperature regions (“hot spots”) do exist in intermolecular organic reactions.