Study of Sustainability and Scalability in the Cp∗Rh(III)-Catalyzed Direct C-H Amidation with 1,4,2-Dioxazol-5-ones

The practical aspects of Cp∗Rh(III)-catalyzed direct C-H amidation with 1,4,2-dioxazol-5-ones were investigated on the operational safety, use of green solvent, and scalability. Differential scanning calorimeter (DSC) measurement showed that 3-phenyl-1,4,2-dioxazol-5-one is thermally stable while benzoyl azide, a conventionally employed precursor of acyl nitrene, rapidly decomposes to isocyanate. It was confirmed that the replacement of acyl azide with 1,4,2-dioxazol-5-one brings not only high reactivity but also improvement in safety. In respect to a green process development, functional group tolerant Cp∗Rh(III) catalyst exhibited high reactivity in ethyl acetate, successfully replacing 1,2-dichloroethane solvent used in the original report. Upon the validation on safety and environmental concerns, scalability was also tested. Two different types of arenes bearing pyridyl and oxime directing groups showed excellent conversions on tens of gram scale reactions, and single recrystallization gave desired products with high yields and purity.