Rearrangements of Free Radicals, XIII [1] . Thermal and Photochemical Rearrangements of Cyclic C 8 H 9 Radicals in Adamantane Matrix

The ESR spectra observed after X‐irradiation of bicyclo‐[5.1.0]octa‐2,5‐diene (homotropylidene) ( 12 ) in a [D 16 ]adamantane matrix at 210 K have 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 in a [D 16 ]adamantane matrix. In all cases, UV irradiation of the matrix caused an irreversible transformation of radical 8 (and probably 7 ) into the bicyclo[3.3.0]octa‐2,6‐diene‐4‐yl radical ( 17 ). Tricyclo[3.3.0.0 2,4 ]oct‐6‐en‐7‐yl radical ( 19 ), generated by X‐irradiation of tricyclo[3.3.0.0 2,4 ]oct‐2‐ene ( 20 ) in adamantane, is thermally stable up to 370 K, but also undergoes a facile rearrangement to the radical 17 on UV irradiation. This process is reasonably explained to occur stepwise via radicals 7 and 8 . The postulated reaction paths and the spectral assignments are supported by semiempirical (AM1, PM3), abinitio (UHF/3‐21G * ), and molecular mechanics (MM2ERW) calculations, which are in accord with the finding that 8 is energetically more stable than 7 . The quantum mechanical calculations predict that a degenerate sigmatropic circumambulation of the cyclopropane ring in radical 7 should favorably compete with its ring opening.