The [3+2] annulation of formed trifluorodiazoethane with 4-nitroisoxazoles: access to trifluoromethylpyrazolo[3,4-]isoxazoles.

A general and practical KCO-promoted [3+2] annulation between trifluoroacetaldehyde -triftosylhydrazone (TFHZ-Tfs) and 4-nitroisoxazoles has been developed to access structurally diverse trifluoromethylpyrazolo[3,4-]isoxazoles. Mechanistically, CFCHN is formed by TFHZ-Tfs decomposition and involves a formal [3+2] dearomative cycloaddition with 4-nitroisoxazoles, followed by aromatization the elimination of the nitro group. Furthermore, this protocol is metal-free, scalable, mild, and operationally safe, with a broad substrate scope and high functional group compatibility.