Thermal Rearrangements of 9,10-Bis(trifluorovinyl)phenanthrene

Article abstract of DOI:10.1021/jo00077a027

Results from a study of the thermal unimolecular rearrangement of 9,10-bis(trifluorovinyl)phenanthrene are reported.Kinetic and product studies indicate that it is unexpectedly resistant to rearrangement, that its expected 6? electrocyclic rearrangement plays but a minor role, and that the major rearrangement process was a virtually unprecedented 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 (ΔH(excit.) = 29.9 kcal/mol; ΔS(excit.) = -19.6 eu) and for the conversion to 7 (ΔH(excit.) = 34.4 kcal/mol; ΔS(excit.) = -6.6 eu) as well as for the secondary conversions of 7 to 4-(difluoromethylidene)-3,3,5,5-tetrafluoro-1,2-(9,10-phenanthro)cyclopent-1-ene (8) (ΔH(excit.) = 31.3 kcal/mol; ΔS(excit.) = -12.7 eu) and to 1,2-(9,10-phenanthro)-3,5,5-trifluoro-4-(trifluoromethyl)-1,3-cyclopentadiene (9) (ΔH(excit.) = 31.4 kcal/mol; ΔS(excit.) = -13.4 eu).A rarely encountered fluorine steric effect deriving from the stringent steric demands of the reaction's boat-like transition state is invoked to explain the inhibition of 1's electrocyclic process, while equally rare thermal 1,2-fluorine atom shifts are proposed to explain the rearrangement of 7.