Pyrolyses of pentafluorophenyl prop-2-enyl and [2,3,3-2H3]prop-2-enyl ethers. Formation of 1-fluorovinyl 2,3,4-trifluorophenyl ketones via internal Diels–Alder reactions

Abstract

The formation of 1-fluorovinyl 2,3,4-trifluorophenyl ketone (XIV) by pyrolysis of pentafluorophenyl prop-2-enyl ether (I) at 440 °C in the vapour phase is rationalised in terms of a reaction involving one of two possible internal Diels–Alder adducts (V) of the intermediate 2,3,4,5,6-pentafluoro-2-(prop-2-enyl)cyclohexa-3,5-dienone (II). The same adduct is an intermediate in a reaction which enables an ortho–ortho rearrangement of the allyl group in (II) to take place before conversion of (II) into (XIV), a reaction which has been observed by using pentafluorophenyl [2,3,3-²H₃]prop-2-enyl ether (VI). This gives not only 1-fluoro[2,2-²H₂]vinyl 2,3,4-trifluoro[5-²H₁]-phenyl ketone (XVIII)(1 part), the product expected from 2-([1,1,2-²H₃]prop-2-enyl)cyclohexa-3,5-dienone (IX), but also 1-fluorovinyl 2,3,4-trifluoro[5,6-²H₂]phenyl ketone (XIX)(1 part), from 2-([2,3,3-²H₃]prop-2-enyl)cyclohexa-3,5-dienone (X), the relative proportions of the products indicating complete equilibration of (IX) and (X) before further reaction.