Efficient synthesis of 4-fluoro-5-(perfluoroalkyl)pyrazoles from organofluorosilicon building blocks

A series of 4 -fluoro - 5 -(perfluoroalkyl)pyrazoles has been prepared by h eterocyclization of hemiperfluoroenones 3 or synthetic equivalents 1 and 2 with methylhydrazine. Compounds 1-3 were obtained by reaction of acylsilane s and perfluoroorganometallic reagents. The fluorinated pyrazoles could als o be prepared in a one-pot reaction from the starting acylsilanes. This met hod is very general and has been applied to aromatic, aliphatic and carbohy drate derivatives, as well as to bis(pyrazole) derivatives. The reaction is completely regiospecific and the regiochemistry has been determined by HMB C correlations, F-19-H-1 and H-1-H-1 NOE and by comparison of chemical shif ts and carbon-fluorine coupling constants of ring pyrazole carbon and fluor ine atoms.