Cycloheptatrienes-norcaradiene

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

In 80, where 77 is fused to two benzene rings in such a way that no canonical form can be written in which both benzene rings have six electrons, the aromaticity is reduced by annellation, as shown by the fact that the molecule rapidly converts to the more stable 81, in which both benzene rings can be fully aromatic (this is similar to the cycloheptatriene-norcaradiene conversions discussed on p. 1448). 

An interesting example of l,3-cyclohexadiene-13 5-triene interconversion is the reaction of norcaradienes to give cycloheptatrienes. Norcaradienes give this reaction so readily (because they are cw-1,2-divinylcyclopropanes, see p. 1445)... 

Further experimental and theoretical studies on the rotational barriers of the metal fragment in (cycloheptatriene)Cr(CO)3 complexes195 suggest that (cycloheptatriene)-Cr(CO)3 complexes in general are in equilibrium with their norcaradiene valence isomers and their ground state conformation is controlled by the same electronic factors which effect the cycloheptatriene-norcaradiene equilibrium195. 

The valence isomeric relationship in the cycloheptatriene/norcaradiene system has been known for over ninety years. ... 

However, it has been shown that substituents can change the position of the cycloheptatriene/ norcaradiene equilibrium. Thus n-electron-withdrawing substituents in 1 (X = Y = CN X = COjH, Y = H ) are able to stabilize the norcaradiene structure by shortening the distal (C2-C3) bond and lengthening the vicinal (C1-C2, C1-C3) bonds. With 7r-electron-do-nating substituents the situation is more complex. Only strong n-donors (e.g. 0 , CHj) effect stabilization by lengthening the cyclopropane bonds."... 

Scheme 2.12. Pressure-induced Diels-Alder reaction of cycloheptatriene norcaradiene with Ceo as dienophile.

The effects of substituents at the 1-, 2-, 3-, and 7-positions of 7-methylcyclohepta-triene on the cycloheptatriene-norcaradiene equilibrimn have been examined. Where the C-7 substituent is a carbenium ion stabilized by heteroatoms, e.g. the bis(methylamino) substituted cation (255), the equilibrium is towards cycloheptatriene ... 

Syndiesis of Cyclopentatrienyl Derivatives of Boron. Analysis of the Eqnilibriunt Cycloheptatriene-Norcaradiene... 

Paramagentic toalogues to the Cycloheptatriene-Norcaradiene Equilibrium, ifie 1 4-semidione (t prepared as shbvm In Scheme 3 unquestionably exists in the bicyclic structure ( ). This is easily ascertained... 

Cycloheptatriene gives (181) on reaction with bis(trifluoromethyl)thioketen, via an ene reaction, a cycloheptatriene-norcaradiene valence tautomerism, and an intramolecular Diels-Alder reaction. Cyclo-octa-1,5-diene gives (182) and (183). 

The position of the disrotatory cycloheptatriene-norcaradiene equilibrium (cf. Equation 6.24 = CXY) is very dependent on the C7 substituents... 

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