Copper- or Thermally Induced Divergent Outcomes: Synthesis of 4-Methyl 2H-Chromenes and Spiro-4H-Pyrazoles.

An unprecedented divergent outcome transformation of alkyne tethered N-sulfonyl hydrazones is reported, which provides a direct and effective access to 4-methyl 2H-chromene derivatives and spiro-4H-pyrazoles in the presence of copper catalyst or under thermal conditions, respectively. The notable features of this process include readily available starting materials, an inexpensive copper catalyst, mild reaction conditions, broad substrate scope, diverse transformations and potential applications of these generated products. Mechanistic studies indicate that the 3H-pyrazole, which is generated via direct [3+2] cycloaddition, is the communal key intermediate of these two divergent transformations. To the best of our knowledge, this is the only example of sulfonation reaction that goes through a cascade process involving 3H-pyrazole, which was isolated and confirmed by single crystal X-ray diffraction analysis for the first time.