ISSN:
0018-019X
Schlagwort(e):
Chemistry
;
Organic Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
The radical anions of [2.2]paracyclophane-1,9-diene (2) and its 1,10,12,13,15,16-hexadeuterio derivative 2-D6, as well as those of 4,5,7,8-tetramethyl[2.2]paracyclophane-1,9-diene (3) and its 12,13,15,16-tetradeuterio derivative 3-D4, have been studied by ESR spectroscopy. The coupling constants for 2-· at 178 K are 0.422 mT for four equivalent olefinic protons and 0.046 and 0.020 mT, each for a set of four equivalent aromatic protons. This hyperfine pattern is consistent with either benzene ring bearing two pairs of equivalent protons and it points to a lowering of the anticipated D2h symmetry. The ESR spectra of 2-· are strongly temperature dependent, due to modulation of the two coupling constants of 0.046 and 0.020 mT; these have opposite signs and average to 0.013 mT at 273 K. The experimental findings are interpreted in terms of a transition state of D2h symmetry, 33 kJ/mol above two interconverting equivalent conformations of lower symmetry. Several pieces of evidence suggest that this symmetry is D2, i.e., the benzene rings in 2-· are twisted in opposite directions about the vertical axis. Temperature dependence of the ESR spectra, resulting from modulation of the hyperfine interactions with the aromatic protons, is also observed for 2-D6-· and 3-·. In the case of 3-·, the olefinic protons are, as expected, only equivalent in pairs, the pertinent coupling constants being 0.560 and 0.325 mT. Upon standing at low temperatures, 2-· and 3-· gradually convert into the radical anions of [2.2]paracyclophane (1) and its 4,5,7,8-tetramethyl derivative, respectively. At higher temperatures, cleavage of one bridging chain in 2-· also occurs, with the formation of the radical anion of (E)-4,4′-dimethylstilbene (7). Both reactions of 2-· must involve the transient radical anion of [2.2]paracyclophane-1-ene (4) as proved by the observation of the spectra of 1-· and 7-· with 4 as the starting material.
Zusätzliches Material:
10 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/hlca.19850680212
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