Norcaradiene valence isomers

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... 

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 above sequence of events also takes place with other cyclophanes [14, 47, 49], as illustrated in Scheme 2-4 with the benzofuranophane 54 as just one additional example. Here the primary adduct 55 may either stabilize itself to the orrAt4/7Bra-cyclophane 56 or ring-open to the tropylidenophanes 57 and 59. We assume that the latter interconvert thermally by 1,5-hydrogen shifts since the necessary intermediate, 58 (not isolated), can be trapped via its norcaradiene valence isomer 61 to the 3 1 adduct 62, a process which is accompanied by the formation of an isomer of 56, the j an7-dinitrile 60 ... 

Thermal and photolytic sigmatropic shifts within 7-spironorcaradienes have been reported. The norcaradiene valence isomer of the enol of eucar-vone (33Q has been proposed as an intermediate in the reaction of (336) with TCNE to give (337). ... 

X and Y, Electron withdrawing 7r-substituents (e.g. CN) appear to be essential for the stabilization of the norcaradiene valence isomer. Valence isomerization in the azocine series is also structure dependent. Thus (24) is wholly converted into (25) in the cases fi = 3 or 4, whereas with = 5 the equilibrium is only established at 100°, and when n = 6 only (24) is present. 

Cycloheptatriene represents a challenging substrate for cycloaddition. It can react either in a [2 + 4 ]- or [2 + 6]-cycloaddition to yield isomeric endoperoxides 53 and 54, respectively (Scheme 9.8). Moreover, it may give a 1,2-dioxetane (55) in a [2 + 2] process. Norcaradiene, its valence isomer, may also yield a [2 + 4] adduct (56). All four products were detected in the transformation of the nonsubstituted molecule.421,422... 

One-electron reduction of a norcaradiene derivative produces the corresponding anion radical. The conditions of the odd-electron delocalization in this anion radical are less favorable than in its valence isomer. According to calculations, the unpaired electron incorporation in the nonatetraenyl Ti-syslcm lowers the energy content by 0.62(3. However, the anion radical initially formed is less stable than the benzotropylidene anion radical. The latter is the end product of the isomerization (Gerson et al. 1978) (Scheme 8-27). 

Fluorenylidene attacks benzene to give a mixture of the valence isomers norcaradiene 185 and cyclo-heptatriene 186 (Scheme 35). ... 

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.). 

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. 

Photo-oxygenation of 1,3,5-cycloheptatrienes 39 which can undergo valence tautomerization to norcaradiene isomers, leads to the bicyclic endoperoxides 40 and/or tricyclic derivatives 41 depending on the steric and electronic nature of the substituents (Sch. 21) [33]. 

---