Valence isomers of benzene

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

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. 

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-... 

Among the radical cations derived from strained ring hydrocarbons those of composition (CH)6, the valence isomers of benzene (128) (Scheme 11), have attracted considerable interest. In addition to the one-electron oxidation products of Dewar benzene (126) [360-368), bicylopropenyl (129) [369], benzvalene (130)... 

The X-ray crystal structure of benzene, proving the equivalence of the six C-C bonds, appeared in 1929 and 1932, and Pauling reported its electron-diffraction data in 1931. Note that several of the structures proposed in the 19th century, such as Dewar benzene (non-planar) and Ladenburg s prismane, which are valence isomers of benzene, have now actually been prepared from benzene derivatives photochemically. They are kinetically stabilized, since they do not spontaneously revert to benzene or its derivatives [17-20]. 

Things become complex indeed when we consider all possible combinations 6C + 6H featuring covalent bonds four per carbon and one per hydrogen. There are 217 of these, becoming 328 if diastereomers and enantiomers are taken into account, nearly all unknown and likely highly unstable. We list the four valence isomers of benzene (see for example Ref. 85), all of which appear fairly prominently in the literature (note Figure 2) ... 

In the table below are given the valence isomers of benzene (for definition and explanation see the article, benzene and its isomers, resonance, vol. 6, no. 5, pp. 74-78, 2001). A few properties that are listed here give an idea of how different and interesting such isomers could be. These structures can be found among the 217 structures given in this poster. ... 

Figure 2 The valence isomers of benzene, as taken from the poster associated with Ref. 54. Courtesy of the Indian Academy of Sciences.

CarbenoU synthesis of hydrocarbons. Katz et al. - reported a simple synthesis of the previously rare valence isomer of benzene, benzvalene (3), from cyclopentadiene (1). The diene is first converted into the cyclopentadienyl anion (2) by treatment with methyllithium in dimethyl ether then a solution of methylene chloride and methyl-lithium in diethyl ether is added at —45°. Benzvalene (3) was obtained in 24, yield together with a trace (6.4%) of benzene. 

Compound 19, which yields the valence isomers of benzene on direct irradiation, and diazacyclooctatraene (20) as the major product upon triplet sensitization, is an interesting example of the different reactivity of the S, and Ti states of azo compounds (Turro et al., 1977a) ... 

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. 

The unidentified peak in the chromatogram (Figure 3) gives a negative test for an acetylenic hydrogen (ammoniacal cuprous chloride solution) and does not disappear after the sample is subjected to free radical catalysis for several hours. Thus, the open chain acetylenic isomers of benzene, hexa-l,3-diene-5-yne, 1,4-hexadiyne, 1,5-hexadiyne, and 1,5-hexadien-3-yne do not appear to account for this peak. The possibility of valence isomers of benzene such as bicyclo (2.2.0) hexa-2, 5-diene,... 

There has been considerable interest in benzene during the year. The re-aromatization of valence-isomers of benzene has been recently discussed in detail separately by Dewar and Dorko, and their co-workers. Stein and his co-workers have confirmed that benzvalene arises from vibrationally excited Sj benzene, and report that the population of non-totally symmetrical vibrational... 

These principles serve to explain many phenomena in organic chemistry that would otherwise be difficult to understand. The bicyclo[2.2.0]hexa-2,5-diene ring system is a valence isomer of benzene that is known as Dewar benzene. While many attempts were made to prepare Dewar benzene, there was a widespread feeling that all efforts would be fruitless, and that Dewar benzene would not be isolable, but would directly revert to benzene. The situation changed abruptly in 1962, when van Tamelen and Pappas reported the first stable Dewar benzene which they prepared by photolysis of 1,2,4-tri-tert-butylbenzene ... 

An intriguing and famous application of the orbital symmetry rules concerns Dewar benzene (Eq. 15.19). This valence isomer of benzene is highly strained and much less stable than benzene. It would appear that a very simple process of just cleaving the central bond would relieve a great deal of strain and produce a highly stable, aromatic product. Nevertheless, Dewar benzene is a relatively persistent molecule. 

Perfluoropentaethylmethylbenzene isomerizes photochemically to a mixture of Dewar-benzene isomers. Further irradiation converts these into the prismanes. These valence isomers of benzene are remarkably stable, and appear to owe their stability, in part, to destabilization of the parent benzene by non-bonded interactions. 

Gimarc and co-workers have studied P and As clusters using AE methods, (a) D. S. IXhrren and B. M. Gimarc, /. Am. Chem. Soc., 114, 5378 (1992). Valence Isomers of Benzene and Their Relationship to the Isoelectronic Isomers of P4. (b) D. S. Warren, B. M. Gimarc, and M. Zhao, Inorg. Chem., 33,710 (1994). Valence Isomers of Benzene and Their Relationship to the Isoelectronic Isomers of Asg. 

An interesting comparison is offered by the prismane molecule, a polycyclic hydrocarbon with formula whose carbon and hydrogen atoms are arranged in the shape of a six-atom triangular prism. The magnetic symmetry is the same as cyclopropane s D-jhiCih)- It is a valence isomer of benzene, far less stable than the archetypal aromatic molecule. A spatial ring current model for this molecule has been reported [115]. 

Benzvalene, a valence isomer of benzene, has been polymerized by ring-opening metathesis polymerization in the presence of tungsten-carbene complexes to form polybenzvalene [Eq. 83)]. 

Reviews have appeared on aspects of benzenoid reactivity. These include reports on the Hofmann-Loeffler-Freytag reaction on benzene rings, valence isomers of benzene, and the organic photochemistry of benzene. ... 

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