Benzene valence bond isomers

UV irradiafion of perfluoro o, m-, and p-xylenes in the gas phase gives a mixture of all possible Dewar benzene isomers Prismane valence bond isomers are proposed to be intermediates [148]... 

The parent structure, oxepin (7), exists in a state of spontaneous equilibration with the valence bond isomer benzene oxide at ambient temperature. 

Advances in photochemistry stimulated studies on valence-bond isomers of benzene. The first valence-bond isomer of a substituted benzene was isolated by van Tamelen and a co-worker in 19621 (Eq. 1), and since then many valence-bond isomers have been isolated or postulated as intermediates in photoreactions. 

After a brief survey of the history of valence-bond isomers of aromatic compounds, new syntheses and the reactions of these isomers reported in the last decade are reviewed. In the second chapter, the valence-bond isomers of homoaromatic compounds, especially benzene derivatives, are described and in the third chapter those of heterocyclic compounds. Photoreactions of perfluoroalkylated aromatic compounds afford valence-bond isomers in high yields. These isomers are very stable and useful for the synthesis of highly strained compounds. Therefore, the emphasis is put on the chemistry of trifluoromethylated benzvalenes, Dewar thiophenes, and Dewar pyrroles. 

However, the progress of photochemistry suggested that some valence-bond isomers of benzene could play an important role in the isomerization of a substituted benzene. Some attempts were made to isolate such isomers in the 1960 s. The first success was the isolation of Dewar benzene stabilized with tert-butyl groups by van Tamelen. This was the start of the isolation of many valence-bond isomers of aromatic compounds in the 1960 s. Most of these isomers produced in the photoreaction are substituted by large substituents like a tert-butyl group. 

In 1970, unsubstituted valence-bond isomers of benzene were synthesized on a preparative scale and their properties were fully investigated. Since the end of 1960, the chemistry of fluorine compounds was rapidly developed and the photolysis of perfluoro aromatic compound afforded valence-bond isomers in much higher yields than the corresponding hydrocarbon counterparts. These isomers were much more stable than the hydrocarbon analogs and could be used as starting materials for the synthesis of strained compounds. 

Valence-Bond Isomers of Homoaromatic Compounds 2.1 Syntheses of Dewar Benzenes... 

The synthesis of Dewar benzenes from hydrocarbons has been briefly surveyed. These examples show that photoreactions of strained benzenes give Dewar isomers, whereas other benzenes do not give Dewar isomers in preparative quantities so that the latter must be synthesized by cyclization reactions. In contrast to these hydrocarbons, the photoreaction of perfluorinated aromatic compounds gives the corresponding valence-bond isomers in good yields. Three of the most typical examples are described by Eqs. (11) to (13). 

Prismane is the most highly strained valence-bond isomer of a benzene. Therefore, it isomerizes to the benzene isomer possibly via its Dewar isomer. However, since the activation energy of the isomerization of the Dewar isomer to the benzene is small, the Dewar isomer is rarely observed. Many photochemical isomerizations of benzene derivatives are assumed to proceed via Dewar benzenes and prismanes but in most of cases, such isomers have not been observed. 

Valence-bond isomers having a bicyclobutane part have been synthesized only when they are fused with a benzene ring (162, 163) 178,179). However, due to the stabilizing effect of trifluoromethyl groups non-fused compounds have been isolated. 

Phototransposition reactions of substituted benzenes and heteroarenes by way of valence-bond isomers and their diradical precursors, have been studied for a considerable number of years and are the subject of several reports in the review period. Such reactions within the six isomers of dimethylbenzotrifluoride are reported to be efficient, with each isomer giving rise to the others. The major product isomers observed in each case, however, allow the starting isomers to be divided into the two triads of 2,6-, 2,3- and 3,4-dimethyl- and 3,5-, 2,4- and... 

Valence-bond isomers of aromatic compounds (both 6-membered and 5-mem-bered) that are stabilized by trifluoromethyl groups are reviewed, and it is concluded that both steric and electronic effects contribute to the stabilizing influence of the CF3 group. A fascinating example is provided of a substituted Dewar benzene (1) that is more stable thermodynamically than the isomeric... 

The formation of benzvalene from the irradiation of benzene was first reported in 1967,8 but the process continues to receive further study. The quantum yield (0.18) for formation of this valence-bond isomer from 254 nm radiation of oxygen-free solutions of benzene in hexane is reported to be independent of temperature within the range 9—50 °C. The benzvalene is estimated as 5,6-di-bromobicyclo[2,l,l]hex-2-ene and the results are discussed in terms of a kinetic model which incorporates the known step of triplet-benzene-sensitized disappearance of the benzvalene. Light-induced transposition of ring substituents in... 

Bicyclopropenyl Systems. Hexakis(trifluoromethyl)bicyclopropenyl has remarkable thermal stability (r > 2 h at 360°Q and it is worthy of mention that this thermodynamically least stable benzene isomer is kinetically the most stable compound. On photolysis, however, a mixture of all five valence-bond isomers is obtained. 

The valence-bond isomers of hexakis(trifluoromethyl)benzene (Vol. 1, p. 263) are among the most stable such isomers known. Kinetic and thermodynamic parameters for their thermal rearrangement (Scheme 25) have been determined by differential scanning calorimetry and by conventional techniques... 

The Photochemical synthesis of alkyl substituted benzene takes advantage of various pathways to provide one or more of the valence bond isomers or a positional isomer of the starting material is illustrated for 1,2,4-tiibutyl benzene, shown below. The formation of the 1,3,5-isomer results from cycloreversion of the prismane depicted or arises via the benzvalene to deld the least sterically congested product. By an analogus sequence o-xylene can gives nv and p-isomer. 

Barlow, M. G., Haszeldine, R. N., and Hubbard, R., Valence-bond isomer chemistry. Part 1. The valence-bond isomers of hexakis(trifluoromethyl)- and hexakis(pentafluoroethyl)-benzenes, /. Chem. Soc. (C), 1232-1237, 1970. 

The chemical properties of this hypothetical benzene would be just those expected for the valence-bond structures I and II, and, indeed, the substance would be correctly described as a mixture of these two isomers or tautomers. 

Quinone methides are strikingly different from the 1,2- and 1,4-isomers, because there is no direct orbital interaction between the meta-oxygen and carbon substituents at the benzene ring. Consequently, the neutral valence bond resonance form for the 1,3-quinone methide is a triplet biradical (Scheme 1). These 1,3-quinone methides are chemically more unstable and difficult to generate than their 1,2- and 1,4-isomers, which exist as stable neutral molecules.8... 

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 three Dewar structures 5, 11 and 12 (Dewar benzene) are also considered to contribute to the resonance hybrid (according to valence bond theory, approximately 20% in total) and to the extra stability. Dewar benzene has now been prepared. It is a bent, non-planar molecule and is not aromatic. It gradually reverts to benzene at room temperature. The Ladenburg structure, prismane i6), is an explosive liquid. Dewar benzene and prismane are valence isomers of benzene. 

Many polyfluorobenzenes undergo photochemical valence-bond isomerization to give, in most cases, one or more para-bonded isomers. Hexafluoro-benzene gives up to 60% of hexafluorobicyclo[2,2,0]hexa-2,5-diene (111)... 

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... 

Such properties are also reflected in the relative stability of the MgHg valence isomers (Table 2). It is well known that benzene (CgHg) is very stable due to cyclic delocalization of its six re electrons (aromatic stabilization), and it is much more stable than other strained valence isomers — Dewar benzene, benzvalene and prismane13,14. However, the tendency is completely reversed in the case of heavier atoms the isomers with a smaller number of double bonds are more favorable. As a result, the prismane structure becomes much more stable than the benzene structure on going from carbon to tin atoms10,15. 

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