A novel efficient tandem reaction of hydrazones and alpha-bromo ketones is reported to prepare 1,3,5-trisubstituted pyrazoles by visible light catalysis. In this system, the monosubstituted hydrazones show wonderful reaction activity with alkyl radicals, generated from alpha-bromo ketones. A radical addition followed by intramolecular cyclization affords the important pyrazole skeleton in good to excellent yields. This efficient strategy with mild condition and wide group tolerance provides a potential approach to the 1,3,5-trisubstituted pyrazoles.
J. Org. Chem., 2016