Selective, Ambient-Temperature C‑4 Deuteration of Pyrazole Derivatives by D₂O

A simple, inexpensive, and highly efficient procedure for the selective deuteration (up to >99% D atom %) of the C-4 position of pyrazole substrates activated by NH₂ or OH groups at the C-3(5) position is reported. The deuteration reaction is carried out by simply dissolving the substrate in D₂O or other deuterated protic solvents, either in the absence of a catalyst with heating, or under acidic catalysis at ambient temperature; the products are obtained by simple evaporation of the solvent.