Rearrangements of Free Radicals, XIII[1]. Thermal and Photochemical Rearrangements of Cyclic C8H9 Radicals in Adamantane Matrix

Abstract

AbstractThe 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 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 [D16]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[3.3.0.02,4]oct‐6‐en‐7‐yl radical (19), generated by X‐irradiation of tricyclo[3.3.0.02,4]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.