Structures and rearrangements of bicyclo[1.1.0]butane radical cations: an ESR and ENDOR study

Radical cations were generated by γ-irradiation in Freon matrices from bicyclo[1.1.O]butane (1), 1,3-dimethylbicyclo[1.1.O]butane (2), 1-methylbicyclo[1.1.0]butane (3), tricyclo[3.1.0.0²˙⁶]hexane (4), and tricyclo-[4.1.0.0²˙⁷] heptane (5), as well as from some deuterio derivatives of 1, 3, and 5. Under these conditions, the bicyclic radical cations 1˙⁺, 2˙⁺, and 3˙⁺ were persistent and could be characterized by their hyperfine data with the use of ESR and ENDOR spectroscopy. A detailed analysis of these data indicates that a methyl group withdraws ca. 15% of the spin population from the substituted carbon atom 1 or 3 of bicyclobutane. In contrast to l˙⁺-3˙⁺, the tricyclic radical cations 4˙⁺ and 5˙⁺ are not sufficiently long-lived for ESR and ENDOR studies, as they readily rearrange to the radical cations of cyclohexa-1,3-diene (6) and cyclohepta-1,3-diene (8), respectively. The isomerization of 5˙⁺ to 8˙⁺ possibly proceeds via the radical cation of di-bicyclo[3.2.0]hept-6-ene (7) which undergoes ring opening to yield 8˙⁺. Deuterium labelings of 5˙⁺ and 8˙⁺ point out that the initial step in this isomerization is the cleavage of a lateral bond in the bicyclobutane moiety, and the same statement should hold for the rearrangement of 4˙⁺ to 6˙⁺. The present work emphasizes the importance of ENDOR spectroscopy for the full characterization of radical cations in Freon matrices. Not only can smaller coupling constants and differences in the larger ones be determined, but it is also possible to derive the absolute sign of these values from the anisotropic components of their ENDOR signals. Because of its lower resolving power, this information is not available with the use of ESR spectroscopy alone, in particular for radicals in rigid solutions