Copper- or Thermally Induced Divergent Outcomes: Synthesis of 4-Methyl 2H-Chromenes and Spiro-4H-Pyrazoles
Yang Zheng[a], Lihua Qiu*[a]（邱丽华）, Kemiao Hong[a], Shanliang Dong[a], and Xinfang Xu*[a, b]（徐新芳）
[a] Key Laboratory of Organic Synthesis of Jiangsu Province, College of
Chemistry, Chemical Engineering and Materials Science，Soochow University, Suzhou 215123 (P.R. China)
[b] State Key Laboratory of Elemento-organic Chemistry，Nankai University, Tianjin 300071 (P.R. China)
Chem. Eur. J. 2018, 24, 6705 --6711
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.