THERMAL REARRANGEMENTS OF 9,10-BIS(TRIFLUOROVINYL)PHENANTHRENE

Results from a study of the thermal unimolecular rearrangement of 9,10 -bis(trifluorovinyl)-phenanthrene are reported. Kinetic and product st udies indicate that it is unexpectedly resistant to rearrangement, tha t its expected 6pi electrocyclic rearrangement plays but a minor role, and that the major rearrangement process was a virtually unprecedente d thermal reaction for 1,3,5-trienes, that of conversion to a bicyclo[ 3.1.0]hex-2-ene system (7). Activation parameters are provided for the minor electrocyclic process (DELTAH(double dagger) = 29.9 kcal/mol; D ELTAS(double dagger) = -19.6 eu) and for the conversion to 7 (DELTAH(d ouble dagger) = 34.4 kcal/mol; DELTAS(double dagger) = -6.6 eu) as wel l as for the secondary conversions of 7 to 5-tetrafluoro-1,2-(9,10-phe nanthro)cyclopent-1-ene (8) (DELTAH(double dagger) = 31.3 kcal/mol; DE LTAS(double dagger) = -12.7 eu) and to -trifluoro-4-(trifluoromethyl)- 1,3-cyclopentadiene (9) (DLETAH(double dagger) = 31.4 kcal/mol; DELTAS (double dagger) = -13.4 eu). A rarely encountered fluorine steric effe ct deriving from the stringent steric demands of the reaction's boat-l ike transition state is invoked to explain the inhibition of 1's elect rocyclic process, while equally rare thermal 1,2-fluorine atom shifts are proposed to explain the rearrangement of 7.