ISSN:
0009-2940
Keywords:
Free radicals
;
Rearrangement
;
ESR spectroscopy
;
Adamantane matrix
;
Molecular orbital calculations
;
Chemistry
;
Inorganic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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 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.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/cber.19931260824
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