REARRANGEMENTS OF FREE-RADICALS .13. THERMAL AND PHOTOCHEMICAL REARRANGEMENTS OF CYCLIC C8H9 RADICALS IN ADAMANTANE MATRIX

The ESR spectra observed after X-irradiation of bicyclo-[5.1.0]octa-2, 5-diene (homotropylidene) (12) in a [D16]adamantane matrix at 210 K ha ve been identified to be mainly due to the cyclooctatrienyl radical (8 ), formed by thermal ring opening of the initial bicyclo[5.1.0]octa-2, 5-diene-4-yl (homotropylidenyl)radical (7). The same spectrum has also been observed in X-irradiation of bromocyclooctatriene and a mixture of bromocyclooctatriene and 7-bromobicyclo[4.2.0]cycloocta-2,4-diene i n a [D16]adamantane matrix. In all cases, UV irradiation of the matrix caused an irreversible transformation of radical 8 (and probably 7) i nto the bicyclo[3.3.0]octa-2,6-diene-4-yl radical (17). Tricyclo[3.0.0 (2,4)]oct-6-en-7-yl radical (19), generated by X-irradiation of tricyc lo[3.3.0.0(2,4)]oct-2-ene (20) in adamantane, is thermally stable up t o 370 K, but also undergoes a facile rearrangement to the radical 17 o n UV irradiation. This process is reasonably explained to occur stepwi se via radicals 7 and 8. The postulated reaction paths and the spectra l assignments are supported by semiempirical (AM1, PM3), ab-initio (UH F/13-21G), and molecular mechanics (MM2ERW) calculations, which are i n accord with the finding that 8 is energetically more stable than 7. The quantum mechanical calculations predict that a degenerate sigmatro pic circumambulation of the cyclopropane ring in radical 7 should favo rably compete with its ring opening.