Oxepin, resonance energy

NMR analysis does not provide an accurate estimate of the proportions of oxepin and benzene oxide present at ambient temperature due to a fast rate of isomerization (on the NMR timescale). The oxepin-benzene oxide ratio was found to be dependent upon solvent, temperature, substituents (electronic and steric effects) and the resonance energy of the molecule. 

Similarly, the fusion of an aromatic ring to the oxepin-benzene oxide system was found to drive the equilibrium toward extremes in either direction. The calculated resonance energies for oxepins (26), (27) and (28) were 4.81, 78.46 and 81.72 kJ mol-1 respectively (70T4269). These calculated values concur with experimental observations since oxepins (27) and (28) have been synthesized and are relatively stable compounds. The formation of 2-benzoxepin (26) from naphthalene 1,2-oxide would involve a considerable loss in resonance energy to the system and has not been detected spectroscopically (67AG(E)385). 

HMO calculations, based on localized polyenes instead of isolated alkenes, can account for the heats of atomization of furan (41.64 observed, 41.69 eV) and of dibenzofuran (109.09 observed, 108.92 eV). For resonance energies (quoted as resonance energy per electron, REPE) they give furan, 0.007 oxepin, -0.006 benzo[6]furan, 0.036 benzo[c]furan, 0.002 and dibenzofuran, 0.047 /3 (72T3657). That furan emerges as hardly more aromatic than a diene while pyrrole (REPE 0.039/3) is clearly aromatic is in line with other results, including those from MINDO/3 and topological methods (see Section... 

In the cases of 170, 171,75-79 and 45, the resonance energy of the benzene moiety ( 39 kcal) trips the stability order irretrievably on the side of oxepin in the first two cases and in favor of arene oxide in the case of 45. Tautomerism, which ordinarily is manifested with systems having small energy differences, is therefore ruled out. However, in the case of the 9,10-oxide this limitation does not exist, but even so, the compound exists only in the oxepin form (70),8 possibly because of a very high strain on the oxirane ring in the epoxy form. 

Enantiopure epoxides (3/ ,4Y)-dibenz[ 7, ]anthracene 3,4-oxide and (3iJ,4Y)-phenanthrene 3,4-oxide were synthesized via involved routes and were observed to spontaneously racemize. This racemization of arene oxides is in accordance with perturbation molecular orbital predictions based on resonance energy considerations, and presumably occurs via an electrocyclic rearrangement to the corresponding (undetected) oxepine tautomer (Scheme 17) <2001J(P1)1091>. 

A useful reaction for the synthesis of unsaturated seven-membered heterocycles is the (2 + 2)-cycloaddition of heteroaromatic compounds, e.g., 1 //-pyrrole, furan, or thiophene derivatives, with acetylenes. In combination with a subsequent intramolecular (2 + 2)-cycloreversion (Section IV,B,2) of the annulated cyclobutene moiety, ring enlargement with two carbon atoms can be achieved. 1-Heterocycloheptatrienes, such as benzol6]azepines,26,27,65,66 benzo[fc]oxepins,67,68 benzo[6]-thiepins,69,70 and thiepins,18,71 have been successfully prepared in this way other routes are either nonexistent or laborious.72 In these compounds the reacting carbon-carbon double bond constitutes part of a (4n + 2)7r-electron system and in the (2 + 2)-cycloaddition the resonance energy of the aromatic nucleus is lost. Just like the nonaromatic heterocycles, heteroaromatic compounds have been reported to undergo (2 + 2)-cycloaddition reactions both with electron-deficient and with electron-rich acetylenes. 

The nature and position of substitution has a profound influence upon the oxepin-arene oxide equilibrium position. The effect of substituents on the relative energies of each valence tautomer has been calculated (80JA1255) and these theoretical results are in accord with the limited experimental data which are available. In general terms, oxepins substituted at the 3-position are less favored than the corresponding arene oxides, while the reverse obtained for 2- and 4-substituted oxepins. This substituent effect has been rationalized in terms of a preference for the maximum number of alternative resonance contributors. The influence of both 7r donating and v withdrawing substituents oh the oxepin contribution is summarized in Scheme 2. This latter effect may be considered as an electronic substituent effect. 

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