Tunable Aerobic Oxidative Hydroxylation/Dehydrogenative Homocoupling of Pyrazol-5-ones under Transition-Metal-Free Conditions.

A practical and tunable transition-metal-free aerobic oxidation of pyrazol-5-ones preparing either 4-hydroxypyrazoles (via C-H hydroxylation) or bispyrazoles (via dehydrogenative homocoupling) is described. The K2CO3/dioxane reagent system predominately promoted hydroxylation to deliver the α-hydroxylated pyrazoles. In contrast, the formation of bispyrazoles was overwhelmingly preferred with CH3CN as the reaction medium without any additives.