[3+2]Cyclization of Difluoroacetohydrazonoyl Bromides withβ-(N,N-Dimethylamino)enones/Enoates/Enamides
A[3+2]cyclization of difluoromethyl nitrile imines generated in situ from difluoroacetohydrazonoyl bromides with β-(N,N-dimethylamino)enones/enoates/enamides in the presence of base is described.The cycloaddition reaction pro-duced the pyrazoline intermediates,from which 3,4-disubstituted-3-difluoromethylpyrazoles were obtained in moderate to good yields via spontaneous elimination of the Me2NH molecule.The reaction is fully regioseclective by the interaction of lowest unoccupied molecular orbital(LUMO)of the 1,3-dipoles and highest occupied molecular orbital(HOMO)of electron-rich alkenes(inverse-electron-demand).This approach expands the types of[3+2]cyclization involved difluoro-methyl nitrile imines,which provides a simple,efficient and novel method for the synthesis of 3-difluoromethylpyrazoles.
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