Kinetic and thermodynamic effects in the thermal electrocyclic ring-openings of 3-fluorocyclobutene, 3,3-difluorocyclobutene, and 3-(trifluoromethyl)cyclobutene

Article abstract of DOI:10.1021/ja00157a055

The synthesis and thermal electrocyclic ring-opening of 3-fluorocylobutane, 4, 3,3-difluorocyclobutane, 5, and 3-(trifluoromethyl)cyclobutane, 6, are reported. Activation energies for their ring-openings were found to be 28.1, 45.0, and 36.3 kcal/mol, respectively. 6 was found to form both the (E)- and the (Z)-5,5,5-trifluoro-1,3,butadienes, in a 95:5 ratio. Thermal equilibrations of the diene products from 4 and 6 were also carried out. The results demonstrate that a CF₃ group exhibits only a slight preference for outward rotation (ΔE(a) = 1.2 kcal/mol), while a fluorine substituent gives rise to a much more dramatic outward rotational preference (ΔE(a) = 13.8 kcal/mol). These results were consistent with those previously reported for perfluorinated systems and with theoretical expectations.