Pyrolysis reactions of 4-methyl-tetrafluorophenyl and pentafluorophenyl prop-2-enyl ethers: isomeric tetrahydroinden-1-ones from both intra-molecular Diels–Alder adducts of the Claisen rearrangement reaction from the 4-Me derivative: Mechanistic implicati

Heating 4-methyl-tetrafluorophenyl prop-2-enyl ether 16 in vacuo at 170 °C gives a mixture of products which includes 3-methyl-2,4,5,7-tetrafluorotricyclo[3.3.1.02,7]non-3-ene-6-one 18, the product of one of the two possible intra-molecular Diels–Alder reactions of the Claisen rearrangement intermediate 17. The product of the second intra-molecular Diels–Alder reaction, 22, is proposed as the intermediate in the formation of 6-methyl-2,5β,7,7aβ-tetrafluoro-3aβ,4,5,7a-tetrahydroinden-1-one 23 in low yield (4%) in the flash vapour phase (FVP) pyrolysis of 16 at 410 °C; the major product is 7-methyl-2,5β,6,7aβ-tetrafluoro-3aβ,4,5,7a-tetrahydroinden-1-one 27 (38%) which is the same as 23 but with the 6-Me and 7-F substituents interchanged. The facile formation of this unpredictable product is rationalised as proceeding by a retro Diels-Alder reaction of 18 to the tethered 3-methyl-2,4,5-trifluoro-2,4-cyclohexadienylmethyl fluoroketene 24 which has a choice of two intra-molecular recyclisations (by another π4s+π2s Diels-Alder reaction to 25 skeletally identical with 22, and/or a π2s+π2a route to 26) both products reacting further to 27. Under FVP conditions at 400 °C, pentafluorophenyl prop-2-enyl ether 1 (RF) gives 1,2,3,4,6-pentafluorotricyclo[4.3.0.02,8]non-3-ene-5-one 33 (3%), the basic structure of which is found in 26.