Pyrolysis reactions of 4-phenyl-2,3,5,6-tetrafluorophenyl prop-2-enyl ether and 4-trifluoromethyl-2,3,5,6-tetrafluorophenyl prop-2-enyl ether: remarkable rearrangement reactions of intramolecular Diels-Alder products. Mechanistic implications of a new thermal retro-Diels-Alder reaction to cyclohexa-2,4-dienylmethyl fluoroketenes and recyclisations by (pi)4(s)+(pi)2(s) and/or (pi)2(s)+(pi)2(a) routes

The pyrolyses of 4-phenyl-2,3,5,6-tetrafluorophenyl prop-2-enyl ether (26) under flash vapour phase (FVP) conditions at 350 degreesC and of 4-trifluor omethyl-2,3,5,6-tetrafluorophenyl prop-2-enyl ether (27) on heating in vacu o at 169 degreesC give mixtures of products which include 3-phenyl-2,4,5,7- tetrafluorotricyclo[3.3.1.0(2.7)]non-3-ene-6-one (28), and 3-trifluoromethy l-2,4,5,7-tetrafluorotricyclo[3.3.1.0(2.7)]non-3-ene-6-one (31), respective ly, the products of one of the two possible intramolecular Diels-Alder reac tions of the Claisen rearrangement intermediates 2. FVP of 26 at 430 degree sC and 27 at 450 degreesC give the bicyclic compounds 7-phenyl-2,5 beta ,6, 7a beta -tetrafluoro-3a beta ,4,5,7a-tetrahydroinden-1-one (34) and 7-trifl uoromethyl-2,5 beta ,6,7a beta -tetrafluoro-3a beta ,4,5,7a-tetrahydroinden -1-one (37). respectively. It is proposed that these latter two compounds a re formed via two possible intermediates produced by recyclisations of the tethered 2,4,5-trifluoro-3-(substituent)-2,4-cyclohexadienylmethyl fluoroke tene (23), itself formed via new retro-cyclisation reactions of 28 and 31, respectively. Also formed from 27 via 31 is 1,2,4 beta ,5,7-tetrafluoro-3-( trifluoromethyl)bicyclo[3.3.1]nona-2,6-dien-8-one (39). (C) 2001 Elsevier S cience B.V. All rights reserved.