Bicyclo hepta-2,4-dienes, valence

A direct entry to the quadricyclene series has been gained through photoisomerization of bicyclo [2.2.1 ]heptadienes. Bicyclo [2.2.1 ]hepta-dien-2,3-dicarboxylic acid (Formula 194), undergoes light-induced valence tautomerization to Formula 195 (75). The parent quadricyclene 196 has been prepared by direct irradiation of bicyclo [2.2. l]heptadiene... 

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... 

Thermal decomposition of 2-diazohexafluoropropane, or 2-diazo-3,3,3-trifluoro-propanenitrile in excess benzene (150-200 "C, autoclave) resulted in efficient cyclopropanation of an aromatic C-C double bond. In solution, the 7-trifluoromethylnorcaradiene 9 (-bicyclo[4.1.0]hepta-2,4-diene) so formed is in rapid valence equilibrium with the corresponding cycloheptatriene 10 however, this equilibrium is shifted predominantly or completely to the cycloheptatriene side. As a byproduct in the synthesis, the product of carbene insertion into an aromatic C-H bond is obtained. An experimental procedure for the synthesis of 10b can be found in Houben-Weyl, Vol. 19 b, p 1031. 

Cyclopropanation of monocyclic aromatic rings (benzene and its derivatives) yields 7-acyl-bicyclo[4.1.0]hepta-2,5-dienes that are subject to the norcaradiene/cycloheptatriene valence equilibrium (see Houben-Weyl, Vol.4/3, p509 and Section 1.2.I.2.4.2.6.2.). Whereas 7-mono-... 

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). 

By thermolysis or photolysis of appropriate diazo compounds, various substituted cy-anocarbenes can be generated. The addition of these carbenes to benzene gave 7-substituted bicyclo[4.1,0]hepta-2,4-diene-7-carbonitrile/cyclohepta-l, 3,5-triene-7-earbonitrile 9/10, valence-tautomeric isomers. ... 

Irradiation in 5% aqueous sulfuric acid of tropylium ion (tetrafluoro-borate salt) 50 34> was found to give rise to bicyclo[3.2.0]hepta-3,6-dien-2-ol (52) and the corresponding ether 53. The authors proposed that irradiation of 50 produces the valence bond isomer 51, a highly reactive... 

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-... 

The valence photoisomerization of tropolone derivatives (118) to give optically active bicyclo[3.2.0]-hepta-3,6-dien-2-ones (Scheme 26), was improved in CD complexes. This sigmatropic rearrangement occurs in complexes dispersed in both protic and aprotic solvents with higher rates and better yields than in the homogeneous solutions. The amplitudes of the optical rotation of the photoproducts obtained from fi-CD complexes were larger than those obtained from a-CD complexes [307]. 

Heptatriene 18 having electron-withdrawing substituents, undergoes rapid conversion into bicyclo[4.1.0]-hepta-2,4-diene 19 and both the isomers remain in equilibrium because of low activation energy Ea < 10 kcal/mol. This transformation is known as valence tautomerism [18]. 

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... 

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