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Rearrangements of Free Radicals, XIII [1] . Thermal and Photochemical Rearrangements of Cyclic C 8 H 9 Radicals in Adamantane Matrix
Author(s) -
Korth HansGert,
Sustmann Reiner,
Sicking Willi,
Klärner FrankGerrit,
Tashtoush Hasan I.
Publication year - 1993
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19931260824
Subject(s) - chemistry , radical , adamantane , bicyclic molecule , photochemistry , cyclobutadiene , diradical , cyclopropane , matrix isolation , ring (chemistry) , singlet state , medicinal chemistry , molecule , organic chemistry , excited state , physics , nuclear physics
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.