Valence-bond Isomers of Aromatic Compounds

Allylpentafluorobenzene photoisomerizes to the Dewar-benzene (123), but pentailuorostyrenes yield benzocyclobutenes, e.g. (124), by fluorine migration.  

Full details of the reaction of hexahuoro-Dewar-benzene with the 1,3-dipoles, phenyl azide and diazomethane (see Vol. 1, p. 268) are available, and further examples of reactions with 2,2,2-trifluorodiazoethane, 2,4,6-trimethylbenzonitrile oxide, and benzonitrile JV-phenylimide have been described. 2-Methoxypenta-fluorobicyclo[2,2,0]hexa-2,5-diene yields an adduct with phenyl azide by addition to the CF= 1F bond, and cji o-5,6-dibromohexafluorobicyclo[2,2,0]hex-2-ene, but not the corresponding tra/u-dibromo-compound, yields adduct (125). 

Pentakis(pentafluoroethyl)pyridine yields very stable pant-hooded and prismane isomers (see Vol. 1, p. 268), and these have now been described in fuU. No comparable isomers are formed from peifluoro-2,4,5-triethyl- and -2,3,4,6-tetraethyl-pyridines. The poro-bonded pyridine (12S), formed photochemically from its parent pyridine, formed complexes with palladiumfn) and platinum(u) chlorides.  

Reagents , furan or tetramethylfuran in CHCia, room temperature ii, PhjP iii, u.v., MeCN. 

Diverse products may be obtained from the reaction of perfluoroiodoalkanes with olefins of the type Rp-CH CHa in the presence of copper (see p. 63). With copper powder in DMF, reductive coupling and dehydrofluorination may result in the formation of dienes of the type RpCF CH CH CFRp. 2 Kolbe electrolysis of 2/T-hexailuoroisobutyric acid at a platinum electrode yields the hexane (CFs)2CH-CH(CF3)2 (67%), together with (CFs)2CH2 and CFg-CHiCFg, and this hexane, when treated with potassium hydroxide in di-n-butyl ether, yields perfluoro- 

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Valence Bond Isomers of Aromatic Compounds Stabilized by Trifluoromethyl Groups Kohayashi Y Kumadaki, I Acc Chem Res 14 76-82 46... 

Kobayashi, Y., and Kumadaki, I. Valence-Bond Isomer of Aromatic Compounds. 123, 103-150... 

Valence Bond Isomers of Aromatic Compounds Stabilized by Trifluoromethyl Groups ... 

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. 

So far, we have shown synthesis and reactions of valence bond isomers of aromatic compounds, and much less of theoretical parts are discussed. However, these compounds have extensively been studied by means of the molecular orbital theory. Some MO calculations are used for the interpretation of photoelectron spectra. 

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