Partially fluorinated heterocyclic compounds. Part 21. Isomerisations of pentafluorophenyl and 1,3,4,5,6,7,8-heptafluoro-2-naphthyl prop-2-ynyl sulphides: differing courses of reactions of the naphthyl sulphides and ethers in glass and nickel apparatus. Considerations of mechanism

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Abstract
Pentafluorophenyl prop-2-ynyl sulphide (1) has been isomerised to 4,5,6,7-tetrafluoro-2-fluoromethyl-benzo[b]thiophene (3) in a liquid phase reaction while 1,3,4,5,6,7,8-heptafluoro-2-naphthyl prop-2-ynyl sulphide (2) gave 4,5,6,7,8,9-hexafluoro-2-fluoromethylnaphtho[2,1 -b]thiophene (4) in both liquid and vapour phase reactions. Compound (1) reacted with p-xylene at 180 °C to give 2-(2,5-dimethylbenzyl)-4,5,6,7-tetrafluorobenzo[b]thiophene (5), which was also formed from (3), p-xylene, and BF3–diethyl ether at 25 °C. The sulphide (2) reacted with benzene and with p-xylene, at 160 °C, to give the 2-benzyl- and 2-(2,5-dimethylbenzyl)-4,5,6,7,8,9-hexafluoronaphtho[2,1 -b]thiophene compounds (7) and (8) respectively; (7) and (8) were also formed from (4), with benzene and p-xylene respectively and BF3–diethyl ether. At 160 °C in nickel apparatus, the 2-fluoromethyl compound (4) was shown to be an intermediate in the conversion of (2) into (8) with p-xylene. At 140 °C in nickel apparatus, (2) and p-xylene gave (4) as the major product whereas in glass apparatus only (8) was formed. The glass surface of the reaction vessel acted as a Lewis acid catalyst at elevated temperatures, an effect which was also observed in the reactions of 1,3,4,5,6,7,8-heptafluoro-2-naphthyl prop-2-ynyl ether (9) with p-xylene and isopropylbenzene at 140 °C; the aromatic substitution products (10) and (14) respectively were formed, whereas in nickel apparatus, the 2-fluoromethyl compound (11) was the main product in both reactions. Compound (11) reacted with p-xylene in glass apparatus at 140 °C to give (10) exclusively; in N,N-diethylaniline at 140 °C, (11) was recovered unchanged. Charge-separated species (19) and (25) are proposed as intermediates in the reactions of the ether (9) and sulphide (2)via the heterolytic fission of the sp3 C–F bond in the Claisen rearrangement intermediates (13) and (24) respectively. The isomerisations of (2) and (9) therefore, proceed via reactions which include ′1,4′-fluorine shifts which occur via ionic mechanisms.
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