Structural fluxionality in the tricyclo[3.3.1.0]nona-3,6-dienyl and bicyclo[3.2.2]nona-2,6,8-trienyl radicals

E.s.r. observation of radicals derived from 9-bromotricyclo[3.3.1.0]nona-3,6-diene, and related compounds, showed that they rearrange by β-scission to bicyclo[3.2.2]nona-2,6,8-trienyl radicals extremely rapidly; the latter radicals have hyperfine splittings similar to those of allyl radicals. 9-Deuterio- and 2-deuterio-9-bromotricyclo [3.3.1.0]nona-3,6-diene were reduced with tributyltin hydride. The pattern of deuterium scrambling in the bicyclo[3.2.2]nona-2,6,8-triene and tricyclo[3.3.1.0]nona-3,6-diene products showed that the intermediate radicals take part in a degenerate rearrangement sequence which makes them fully fluxional in three dimensions. The two radicals are in equilibrium at ca. 375 K, but the bicyclotrienyl species is more important by a factor of ca. 10². Neither the experimental results nor MNDO semiempirical calculations provided any evidence of additional thermodynamic stabilisation in the radical pair beyond that expected for allyl delocalisation.