Pyrolyses of pentafluorophenyl prop-2-enyl and [2,3,3-2H3]prop-2-enyl ethers. Reactions proceeding via internal Diels–Alder reactions

Abstract

The formation of 2,5β,6,7,7aβ-pentafluoro-3aβ,4,5,7a-tetrahydroinden-1-one (V) by the pyrolysis of pentafluorophenyl prop-2-enyl ether (I) at 480° in the vapour phase is rationalised in terms of a reaction involving one of the two possible internal Diels–Alder adducts of the intermediate 2,3,4,5,6-pentafluoro-2-(prop-2-enyl)cyclohexa-3,5-dienone (II). It is proposed that the second Diels–Alder 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 (V). This reaction has been observed using pentafluorophenyl [2,3,3-²H₃]prop-2-enyl ether (XII), which on pyrolysis not only gives [3,3aβ,5α-²H₃]-2,5,6,7,7aβ-pentafluoro-3a,4,5,7a-tetrahydroinden-1-one (XV)(9 parts), the product expected from the intermediate 2-([1,1,2-²H₃]prop-2-enyl)cyclohexa-3,5-dienone (XIII) but also [3aβ,4,4-²H₃]-2,5β,6,7,7aβ-pentafluoro-3a,4,5,7a-tetrahydroinden-1-one (XVIII)(10 parts) from 2-([2,3,3-²H₃]prop-2-enyl)cyclohexa-3,5-dienone (XIX) formed by the prior ortho–ortho rearrangement of (XIII).