This invention relates to fluorine-substituted benzofurans, and more particularly to tetrafluoro benzofurans, their synthesis, and use of the tetrafluoro benzofurans as intermediates for the preparation of certain pyrazoline insecticides described in the application referred to above.  The tetrafluorobenzofurans of this invention are more precisely designated "2,2,3,3-tetrafluoro-2,3-dihydrobenzofurans" and it will be understood that references throughout this application to "tetrafluorobenzofuran" are abbreviations of the more specific nomenclature. 
The furanyl ring cannot be selectively fluorinated in an effective manner, particularly when substituted other than by fluorine.  It is necessary, therefore, to develop, and it is an object of the present invention to provide, a method of indirectly incorporating fluorine into the furanyl ring in order to provide intermediates for synthesis of the aforementioned pyrazoline insecticides.  The latter synthesis is summarized hereinafter. 
The coupling of halogenated aromatic and aliphatic compounds to form fluoroalkyl-substituted aromatic compounds is known (British Pat.  Nos.  1416181 and 1420211).  However, the coupling does not involve ring closure of a fluorinated ether group on an aromatic nucleus to form a fluorinated benzofuran as in the present invention.  2,2,3,3-Tetrafluorobenzofuran is known (Tetrahedron Letters, No.  11, 869-870, 1975) but not when carrying other substituents.