Benzvalene, stability

The stability of benzvalene may be discussed by dividing the molecule into two parts, the tetramethine part and the dimethine part, as illustrated below. 

Proof for the existence of benzene isomers in irradiated benzene has been obtained in several ways. These will not be discussed in detail, but they may be classified broadly as physical and chemical. Nuclear magnetic resonance has been used by Wilzbach and Kaplan to identify benzvalene.39 Prismane has also been identified by NMR and by vapor-phase chromatography. The Dewar form has been synthesized in several steps which start with ris-1,2-dihydrophthalic anhydride. Photochemically this compound yields bicyclo(2,2,0)hexa-5-ene-2,3-dicarboxylic aqid anhydride. This was followed by catalytic reduction and oxidative decarboxylation to give the Dewar form of benzene.39 The method of synthesis alone provides some basis for structure assignment but several other bits of supporting evidence were also adduced. Dewar benzene has a half-life of about 48 hr at room temperature in pyridine solution and its stability decreases rapidly as the temperature is raised. 

The triazoline adducts from benzvalene (Scheme 21)162 and diphospha-benzvalene (Scheme 22) photolyze to yield novel tetracyclic aziridine ring systems165 that are valence isomers of azepines,162 whereas that from De-war thiophene (Scheme 20) gives a novel tricyclic aziridine that desulfurizes with triphenylphosphine to yield the trifluoromethylated Dewar pyrrole (Scheme 153).15 9,160 The stabilization of these strained molecules is attributed to the perfluoroalkyl effect.159... 

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. 

So far, valence bond isomers of oxygen-, sulfur- and nitrogen-containing heteroaromatic compounds were discussed where no benzvalene-type isomers had been isolated. The authors of this article synthesized tetrakis(trifluoromethyl)-l,4-diphos-phabenzene m> and obtained a diphosphabenzvalene by the photoreaction 172). The trifluoromethyl groups stabilize diphosphabenzene which is an electronically unstable compound (157). Thus, tetrakis(trifluoromethyl)-1,4-diphosphabenzene can be isolated. 

Table 6.4 shows the principal photoreactions of aromatic compounds that we discuss in this chapter. Upon irradiation, aromatic compounds, such as benzenes, naphthalenes and some of their heterocyclic analogues, undergo remarkable rearrangements that lead to some non-aromatic highly strained products, such as benzvalene and Dewar benzene (entry 1), which can be isolated under specific conditions. Quantum and chemical reaction yields are usually low however, photochemistry may still represent the most convenient way for their preparation. While bulky ring substituents usually enhance the stability of those products, aromatic hydrocarbons substituted with less sterically demanding substituents exhibit ring isomerization (phototransposition) (entry 2). 

The photoisomerization of benzene into Dewar benzene, benzvalene and fulvene is a well-known process, and in 1995 the interconversion between the [4] paracyclophane (3) and the 1,4-bridged Dewar isomer (4) in a matrix at 77 K was described. The analogous Dewar isomers (5) and (6) have been synthesized by conventional means in order to investigate the properties of the derived substituted [4]paracyclophanes and the kinetic stabilization of this skeleton by substituents which sterically hinder reactions at the bridgehead sites. Irradiation under matrix isolation conditions of (5) and (6) yields the corresponding cyclophanes which have widely differing half-lives (1 min at — 90°C to 0.5h at — 20°C) for their cycloreversions the authors also discuss in detail the aromaticity of the extremely bent benzene ring in the [4]paracyclophanes. 

The question of stmcture, stability, and properties of benzene has held a central position in organic chemistry. The Kekule s hypothesis for the stability of cyclohexatriene prompted development of theory whose extensions to transition states has had as much impact as on ground states. However, there are five known isomers of (CH)6 benzene, Dewar benzene, benzvalene, prismane, and biscyclo-propenyl (Scheme 7.4). The highly strained quadricyclo[2.2.0.0 ,0 ] hexane isomer is also of interest. 

Z-diene and the second cyclization to (259). The formation of oxepin from bicyclo[2,2,0]hex-2-ene does not involve benzvalene oxide as an intermediate. Norcaradienes. The formation and stability of the norcaradienes (111), (128)/ and (207) have been discussed. Thermolysis of the bisnorcaradienes(260) causes disrotatory ring cleavage to (261) and (262), each of which undergoes aromatization by a [1,5] hydrogen shift (A T 15— 20 kcalmol ). For(260 R R and/or =... 

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