Benzene valence isomers

Figure 12.8 Energy profile for the interconversion of benzene valence isomer derivatives.

Energies of benzene valence isomers relative to benzene (kcal/mol)... 

In summary, the radical cations of the benzene valence isomers show several interesting structures. Although most of their structural features can be rationalized by considering the HOMOs of the precursor molecules, some of the species show substantial changes in individual bond lengths. Accordingly, species such as the 2B2 and 2A1 Dewar benzene radical cations, the 2Bj and 2At benzvalene radical cations, or the 2Bt prismane radical cation cannot be expected to qualify as Koopmans radical cations. To date, most of the information available in this series is based on CIDNP results and ab initio calculations. It is safe to predict increasing involvement of ESR spectroscopy in this area. 

The light-induced isomerization of bicyclo[2.2.0]hexa-2,5-dienes to prismanes (see Houben-Weyl, Vol. 4/5 a, pp232 and 472) has, until recently, been of little preparative use as the starting compounds were usually obtained by irradiation of a suitable benzene precursor, and only photoequilibria were observed between the different benzene valence isomers. Recent examples of successful light-induced syntheses of prismanes involved fast photochemical isomerization of a sterically crowded Dewar benzene 1 to less sterically crowded 2, which then underwent a (slower) ring closure to the corresponding prismane 3. ... 

Dewar benzene is a valence isomer of benzene, to which it reverts on heating. 

The bicyclo[2.2.0]hexa-2,5-diene ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. After many attempts to prepare Dewar benzene derivatives failed, a pessimistic opinion existed that all such efforts would be finitless because Dewar benzene would be so unstable as to immediately revert to benzene. Then, in 1962, van Tamelen and Pappas isolated a stable Dewar benzene derivative by photolysis of 1,2,4-tri(/-butyl)benzene. ... 

Finally, there is a series of publications by Prinzbach and co-workers on 4,7-dihydro-l//-l,4,7-triazonine 42b (80CB3127, 80CB3161, 88CB757, 89AG1386). Tliis compound, obtained by a 3valence isomer) is a nonaromatic, nonplanar 127r-electron system, better represented as 42c. 

Since both oxepin and its valence isomer benzene oxide contain a x-tb-diene structure they are prone to Diels-Alder addition reactions. The dienophiles 4-phenyl- and 4-methyl-4//-l,2,4-triazole-3,5-dione react with substituted oxepins at room temperature to give the 1 1 adducts 7 formed by addition to the diene structure of the respective benzene oxide.149 190,222... 

Bicyclo[2.2.0]hexadienes and prismanes are valence isomers of benzenes. These compiounds actually have the structures that were proposed for benzenes in the nineteenth century. Prismanes have the Ladenburg formula, and bicyclo[2.2.0]-hexadienes have the Dewar formula. Because of this bicyclo[2.2.0]hexadiene is often called Dewar benzene. On page 32 it was mentioned that Dewar formulas are canonical forms (though not very important) of benzenes. Yet, they also exist as separate compounds in which the positions of the nuclei are different from those of benzenes. 

Secondly, in view of the generally accepted mechanism for the sulfur extrusion reaction, which involves valence isomerization of the thiepin ring to its corresponding thianorcaradiene (benzene episulfide) isomer followed by irreversible loss of sulfur,... 

Two rings linked by sharing the same bond instead of the same atom lead to annulated bicyclic or tricyclic compounds, the propellanes. In the case of poly-unsaturated molecules, an interesting case is represented by the bicyclo[2.2.0] type. The parent compound Dewar benzene (bicyclo[2.2.0]hexa-2,5-diene) (DEW) is the smallest bicyclic diene which is an often discussed valence isomer of aromatic benzene Unsubstituted DEW is a very... 

Dioxocines 218 are in turn paratropic and exist in equilibrium with their 2o — 2ti valence isomers sjn-benzene dioxides 217 (Scheme 80).259 Their chemistry and behavior in magnetic fields have been evaluated.260 261... 

Oxidation of 4-methyl-2-tert-butylphenol (225, Scheme 57) by 2,3-di-chloro-5,6-dicyano-l,4-benzoquinone in methanol affords the dibenzofuran 228. The intermediate oxepinobenzofuran 226, as its valence isomer 227, undergoes nucleophilic attack by methanol and subsequent dienone-benzene rearrangement with the migration of the methoxy group. 

What is the effect of the three bulky tert-butyl groups in altering the relative stabilities of benzene and its valence isomer, Dewar benzene Is it sufficient to overcome what must be the considerable difference in stabilities of the parent compounds If not, can even more crowded systems be envisioned which would overcome this difference ... 

The formation of benzene and pyridine presumably occurs via the bicyclo[4.2.0] valence isomers (175) and (176), although adducts of these structures with dienophiles were not obtained on heating or during irradiation. The difference in products in thermal and photochemical reactions could be explained either by interconversion of (173) and (174) and rate-limiting isomerizations of (173) to (176) and (174) to (175), or alternatively, rate-limiting isomerization of (173) to (174) and symmetry-allowed 4ir photocyclization of (173) to (175) (79JOC1264). 

For reviews of valence isomers of benzene and some related compounds, see Kobayashi Kumadaki Top. Cure. Chem. 1964,123, 103-150 Bickelhaupt de Wolf Red. Trav. Chim. Pays-Bos 1988, 107, 459-478-... 

Support for these mechanistic proposals was boosted by the successful isolation of related valence isomers from benzenes or heteroaromatic compounds carrying bulky (t-butyl) or fluorinated groups. 1, 3,5-Tri-t-butvl benzene gives rise to a benzvalene 13.26), but fluorinated substituents or more severe steric crowding (e.g. 3.271 lead to a preference for bicyclo[2.2.0]hexadiene formation Ithese latter isomers are sometimes called Dewar benzenes I. as does the presence of nitrogen in the ring (3.28. ... 

Valence isomer formation is a feature also of the photochemistry of naphthalenes (3.33) and anthracenes for naphthalenes, as for benzenes, the extent ol steric crowding helps to determine which type of valence isomer predominates, since there is more severe interaction in the bicyclohexadiene products than in the benzvalene products. Amongst five-membered heteroaromalic compounds there are many known ring photoisomerizations that involve conversion of a 2-substituled to a 3-substituted system (e.g. 3.34). In some cases non-aromatic products can be isolated, such as bicyclo[2.1.0]pentene analogues from thiophenes 13.35). or acylcycfopropenes from furans (3.36) related species may be... 

Photoadditions that arise by initial excitation of the aromatic compound are not common. Benzvalenes are readily attacked by hydroxylic compounds, and so irradiation of benzene in aqueous solutions of acetic acid, for example, results in the formation of a bicydic product (and an isomer derived from it by subsequent photoisomerizationl as a result of addition to the initially formed valence isomer (3.38). A different kind of photoaddition occurs when benzenes react photochemically with amines cyclohexa-T, 4-dienes are the major products (3.39), accompanied by cyclohexa-1.3-dienes, and unlike many of the photochemical reactions of benzene this does not suffer loss of efficiency in scaling-up. 

The equilibrium position is strongly affected by substitution (see CHEC 5.17.1.2) and the nature of heteroatom. For instance, in the case of thiepine its potential valence isomer, benzene episulfide, is too unstable to be detected. 

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