Compounds of the formula I ##STR1## wherein X is chlorine or fluorine, are suitable for producing 2,2-dimethyl-3-(2'-chloro-3',3',3'-trifluoroprop-1'-en-1'-yl)- and 2,2-dimethyl-3-(2',3'-dichloro-3',3'-difluoroprop-1'-en-1'-yl)-cyclopropanecarboxylic acids and insecticidally effective esters thereof (pyrethroids). The cyclobutanones (I) can be produced by the addition reaction of 2,4,4,5-tetrachloro-5,5-difluoro- or 2,4,4-trichloro-5,5,5-trifluoropentane-1-carboxylic acid chloride with isobutylene, and rearrangement of the resulting 2-chloro-2-(2',2',3'-trichloro-3',3'-difluoropropyl)- or 2-chloro-2-(2',2'-dichloro-3',3',3'-trifluoropropyl)-3,3-dimethylcyclobutan-1-one in the presence of catalysts.