PARTIALLY FLUORINATED HETEROCYCLIC COMPOUNDS PART 28. THE HYDROLYSIS OF 1,3,4,5,6,7,8-HEPTAFLUORO-2-NAPHTHYLIDENERHODANINE TO FORM RING-FUSED THIOPHENES. A SIGNIFICANT PROPORTION OF A LINEAR NAPHTHOTHIOPHENE DERIVATIVE ACCOMPANYING AN ANGULAR NAPHTHOTHIOPHENE DERIVATIVE

Article abstract of DOI:10.1016/S0022-1139(00)80498-4

1,3,4,5,6,7,8-Heptafluoronaphthalene-2-carbaldehyde (7), prepared via a Grignard reaction from 2-bromo-1,3,4,5,6,7,8-heptafluoronaphthalene (9) with N-methylformanilide, reacted with rhodanine to give 1,3,4,5,6,7,8-heptafluoro-2-naphthylidenerhodanine (5).Hydrolysis of (5) with base gave a mixture of 4,5,6,7,8,9-hexafluoronaphthop<2,3-b>thiophene-2-carboxylic acid (11) and 4,5,6,7,8,9-hexafluoronaphtho<1,2-b>thiophene-2-carboxylic acid (12) resulting from the intermediate thiolate (6) displacing fluorine at sites 3 and 1 in the naphthalene ring in the ratio 22:78 respectively, which represents a significantly high portion of the linearly cyclised product accompanying the angularly cyclised product.Decarboxylation of the mixture of (11) and (12) gave 4,5,6,7,8,9-hexafluoronaphtho<2,3-b>thiophene (13) and 4,5,6,7,8,9-hexafluoronaphtho<1,2-b>thiophene (14) respectively, while treatment of the mixture (11) and (12) with diazomethane gave an inseparable mixture of the methyl 2-carboxylates (15) and (16) respectively.