7- norcaradiene

The thermal rearrangements of methyl-substituted cycloheptatrienes have been proposed to proceed by sigmatropic migration of the norcaradiene valence tautomer. The first step is an electrocyclization analogous to those discussed in Section 11.1. 

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.). 

Photolysis in acetonitrile gives ortho- (10) andpara-phenylbenzaldehyde (11), as well as cis- (12) and fmws-diphenylstilbene (13). The mechanism for product formation is not clear, but the norcaradiene valence isomer is thought to be the most likely precursor, arising either via a secondary... 

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)... 

The molecules taking part in a valence tautomerization need not be equivalent. Thus, NMR spectra indicate that a true valence tautomerization exists at room temperature between the cycloheptatriene 110 and the norcaradiene (111). In this case one isomer (111) has the cw-l,2-divinylcyclopropane structure, while the other does not. In an analogous interconversion, benzene oxide and oxepin exist in a tautomeric equilibrium at room temperature. 

Typical carbene reactions, such as norcaradiene formation with solvent (20) must be accepted, as must the occasional cyclopropanation of the trapping reagents. 

Depending on the 7-substituent, the norcaradiene endoperoxide can be the major cycloadduct with singlet oxygen. For example, whereas the parent system (R = H) affords only about 4% of the norcaradiene endoperoxide 30a), the 7-cyano derivative... 

Only one example is known so far, namely the parent compound 22. As already mentioned, cycloheptatriene affords the norcaradiene endoperoxide (Eq. 13) and the tropilidene (2 + 4)-endoperoxide (Eq. 15) however, another major product is the tropilidene (2 + 6)-endoperoxide (Eq. 17)... 

Copper-catalyzed cyclopropanation of benzene and its derivatives by a diazoacetic ester yields a norcaradiene 230 which undergoes spontaneous ring opening to cyclo-heptariene 231. At the temperatures needed for successful cyclopropanation, sigma-tropic H-shifts leading to conjugated isomers of cycloheptatriene carboxylates cannot be avoided. The situation is complicated by the formation of regioisomers upon cyclopropanation of substituted benzenes, and separation of the cycloheptatriene isomers may became tedious if not impossible. 

In some cases the C /C2 double bond in methylene cyclopropenes and calicenes was found to show dienophilic functionality towards diene components. Thus, di-ethylamino butadiene combines with 497 to give the Diels-Alder adduct 507, whose proton-catalyzed elimination of amine interestingly did not lead to the dibenzo heptafulvalene 508, but to the methylene norcaradiene derivative 509293 ... 

Electrocyclic reactions have been performed with three of the bridged [ 11 Jan-nulenones. Both 11 and 13, which both contain a cycloheptatriene unit, undergo Diels-Alder additions with dienophiles via their norcaradiene valence tautomers 41 and 43 and yield adducts of the type 42 and 44. Annulenone 13 was found to react only with 4-phenyl-l,2,4-triazoline-3,5-dione whilst 11 underwent reaction with a variety of dienophiles. 3,8-Methano[ 11 Jannulenone 12 contains a tetraene system and undergoes addition reactions, apparently of the (8 + 2)-type, at the termini of the tetraene system. Thus with maleic anhydride the adduct 46, the valence tautomer of the initial adduct 45, was isolated. 

Photochemical decomposition of diazomethane yields methylene, which reacts with benzene to form toluene and cycloheptatriene (via norcaradiene) (Scheme 5).65 66... 

TABLE 4. 13C chemical shifts of cycloheptatriene and norcaradiene compounds... 

Norcaradiencarbonsauremethylester nimmt mit Pd/BaS04 zwei Mole Wasserstoff auf und liefert mit Maleinsaureanhydrid ein Addukt. Bei der photosensibilisierten Autoxydation absorbieren die Norcaradien-derivate rasch ein Mol Sauerstoff und bilden verhaltnismaBig stabile Endoperoxyde (192). Sixma und Detilleux (201) fiihrten Norcaradien-carbonsauremethyleSter, den sie nach der Vorschrift von Schenck ge-wannen, in das kristallisierte Dibromid tiber und regenerierten daraus mit Zink sehr reinen Ester. 

Roh- produkt Amid Norcaradien- verbindung Roh- produkt Amid MA- Addukt Norcaradien- verbindung1... 

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