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 implications of a thermal retro π4s+π2s reaction of one of the adducts and recyclisations by π4s+π2s and/or π2s+π2a routes

Heating 4-methyl-tetrafluorophenyl prop-2-enyl ether 16 in vacuo at 170 degreesC gives a mixture of products which includes 3-methyl-2,4,5,7-tetrafluorotricyclo[3.3. 1.0(2,7)]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-2,5beta,7,7abeta-tetrafluoro-3abeta,4,5,7a-tetrahydroinden-1-one 23 in low yield (4%) in the flash vapour phase (FVP) pyrolysis of 16 at 410 degreesC; the major product is 7-methyl-2,5beta,6,7abeta-tetrafluoro-3abeta,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 pi4s + pi2s Diels-Alder reaction to 25 skeletally identical with 22, and/or a pi2s + pi2a route to 26) both products reacting further to 27. Under FVP conditions at 400 degreesC, pentafluorophenyl prop-2-enyl ether 1 (R=F) gives 1,2,3,4,6-pentafluorotricyclo[4.3.0.0(2,8)] non-3-ene-5 -one 33 (3%), the basic structure of which is found in 26. (C) 2002 Elsevier Science B.V. All rights reserved.