A study of the thermolyses of polyfluoroaryl-prop-2-ynyl ethers in inert solvents and with alkenes

Abstract

In this research the thermal reactions of polyfluoroarylprop-2-ynyl ethers in inert solvents and with alkenes was investigated. Chapter 1 discusses the history of the Claisen and Cope rearrangements with particular reference to the scope, mechanism and stereochemistry of these reactions. Chapter 2 examines the behaviour of aryl prop-2-enyl and aryl prop-2-ynyl ethers when theortho, para or both positions in the aromatic ring are substituted and illustrates the formation of both internal Diels-Alder adducts and cyclisation products. Chapter 3 reports the behaviour of polyfluoroaryl -prop-2- ynyl ethers and includes the first period of the author's own work. The thermolysis of these ethers was undertaken in a non-vitreous environment in both n-decane and 1,1,2-trichlorotrifluoroethaneleading to large quantities of the cyclisation products: 2- fluoromethyl-4,5,6,7-tetrafluorobenzo[b]furan (117) from pentafluorophenyl prop-2-ynyl ether (115) and 2-fluoromethyl-4,5,6,7,8,9- hexafluoronaphtho[2,1-b]furan (126) from 1,3,4,5,6,7,8-heptafluoro- 2-naphthyl prop-2-ynyl ether (120). The final chapter describes the second period of the author's work in which the thermolyses of 1,3,4,5,6,7,8-heptafluoro-2- naphthyl prop-2-ynyl ether (120) and 2-fluoromethyl-4,5,6,7,8,9-hexafluoronaphtho[2,1-b]furan (126) in the presence of (Z)-but-2-ene(120 only), 2,3-dimethylbut-2-ene and 3,3-dimethylbut-1-ene respectively were examined. The formation of the following novel alkenes is reported: 2-(2-methylbut-l-enyl)-4,5,6,7,8,9-hexafluoronaphtho[ 2,1-b]furan (137), 2-(2,3,3-trimethylbut-l-enyl)- 4,5,6,7,8,9-hexafluoronaphtho[2,1-b]furan (138), 2-(2,2,3-trimethyl-but-3-enyl)-4,5,6,7,8,9-hexafluoronaphtho[2,l-b]furan (139) and 2-(2,2,3-trimethyl but-3-enyl)-4,5,6,7,8,9-hexafluoronaphtho[ l,2-b]furan (140). The latter•s formation was attributed to the presence of 2,3,4,5,6,7,8-heptaf]uoro-l-naphthyl prop-2-ynyl ether(142) in the starting material, ether (120). The Chapter is concluded with a discussion of the mechanistic implications of the formation of compounds (138) and (139) from the two c(_6) alkenes in reaction with the ether (120) and the 2-fluoromethyl derivative (126). The particular reactions of 3,3-dimethylbut-l-ene with (120) and with (126) are especially interesting. Three mechanistic routes have been submitted to account for the products formed in these reactions: (a) that the formation of (138) and (139) requires an initial anti-Markownikov addition of a carbocation, subsequent rearrangement and proton loss; (b) that the formation of (138) and (139) requires initial attack by F(^ ) followed by anti-Markownikov adduct formation, ionisation by loss of F(^-) yielding a primary carbocation which rearranges and then loses a proton; and (c) that the formation of (138) and (139) really involves 2,3-dimethylbut-2-ene - formed by prior isomerisation of the terminal alkene by H(^+).