Bicyclo hepta-2,4-dienes, valence isomerism

Bicyclic—monocyclic valence isomerization of a radical dianion in the bicyclo[3 1.0]hexanesemidione system has been demonstrated. Symmetrical 1,4-semldiones formally derived from cycloheptatriene and cyclobctatetraene perfer to exist in the bicyclic (4.1.0 and 4.2.0) structures. Bicyclic—monocyclic valence isomerization in the bicyclo[4.1.0]hept-3-ene-2,5-dione system occurs more readily for the dianion than for the radical anion. Several radical anions derived from the (CH)6 io annulenes are reported. In the case of the 1,2-oxygenated derivatives of (CH)io the dianions or radical anions are stable, but the diones undergo valence isomerization and under oxida-time conditions are converted to 4-hydroxynaphthalene-1,2-semiquinone. Enediol dianions in the bicyclo [2.2 l]hepta-2,5-dione and l-carbQalkoxybicyclo[3 2.1] octa-2,6-diene systems have been observed to undergo... 

Products. The valence tautomerization reaction (18), which gives rise to bicyclo [3.2.0] hepta-2,6-diene, was first reported by Dauben and Cargill. The reaction has a quantum yield of 0.01 or less in the vapor phase and is a minor mode of isomerization. In the condensed phase, this reaction accounts for much of the 1,3,5-cycloheptatriene that is photolyzed although the quantum yield is of the same order (0.03). A second isomerization process gives rise to toluene. The quantum yield... 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

The reaction of oxepin (228 R = H) was complicated by the simultaneous photochemical reaction of benzene oxide, the valence isomer of oxepin.26 The results varied with solvent, temperature, and wavelength.269 The reaction proceeded with high selectivity to 2-oxa-bicyclo[3.2.0]hepta-3,6-diene (229 R = H) upon irradiation (A >310 nm) at room temperature. In most other cases the reaction was attended with the formation of phenol, probably from benzene oxide via Dewar benzene oxide, as this compound is known to isomerize photo-... 

---