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

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.