Dewar benzene, stability

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. 

The compound was reasonably stable, reverting to the aromatic starting material only on heating. Part of the stability of this particular Dewar beii2ene derivative can be attributed to steric factors. The r-butyl groups are farther apart in the Dewar benzene stracture than in the aromatic structure. The unsubstituted Dewar benzene was successfully prepared in 1963. 

This compound is less stable than 5 and reverts to benzene with a half-life of about 2 days at 25°C, with AH = 23 kcal/mol. The observed kinetic stability of Dewar benzene is surprisingly high when one considers that its conversion to benzene is exothermic by 71 kcal/mol. The stability of Dewar benzene is intimately related to the orbital symmetry requirements for concerted electrocyclic transformations. The concerted thermal pathway should be conrotatory, since the reaction is the ring opening of a cyclobutene and therefore leads not to benzene, but to a highly strained Z,Z, -cyclohexatriene. A disrotatory process, which would lead directly to benzene, is forbidden. ... 

The Dewar benzene of hexafluorobenzene formed an adduct with pheny-lazide that gave a polyfluoro-l//-azepine on pyrolysis. R=C02Et (47) was obtained when ethylazidoformate was decomposed in C6F6 [82JCS(P1)2101]. Photolysis of (47) yielded a 2-aza-bicyclo(3.2.0.)hepta-3,6-diene, which, in contrast to its nonfluorinated analogue, showed excellent thermal stability (3 h, 200°C, 88% recovered) [82JCS(P1 )2105]. 

Ab initio (3-21G( )//STO-3G) calculations by Chandrasekhar and Schleyer163 on 1,4-disilabenzene 58, its Dewar benzene isomer 59, and a silylene isomer 60 showed that all three species exhibited approximately similar stabilities, the silylene 60 being 9.9 kcal mol-1 more stable than the planar aromatic form 58, which was 5.9 kcal mol-1 more stable than the Dewar benzene form 59. 

The stabilization of iminoboranes can yield five different tj ies of products cyclodimers (1,3,2,4-diazadiboretidines, Di), cyclotrimers (borazines, Tr), bicyclotrimers (Dewar borazines, Tr ), cyclotetramers (octahydro-l,3,5,7-tetraza-2,4,6,8-tetraborocines, Te), and polymers (polyiminoboranes, Po) these substances are isoelectronic with cyclobutadienes, benzenes, Dewar benzenes, cyclooctatetraenes, and polyalkynes, respectively, which are all known to be products of the thermodynamic stabilization of alkynes. 

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 ... 

While the desired goal, to reverse the thermochemical stabilities of benzene and Dewar benzene, has not been achieved, the calculations have clearly shown their value as a viable alternative to experiment to rapidly explore the limits of what is possible. 

A key step in one route to the synthesis of hexamethyl Dewar benzene is the cycloaddition of 2-butyne to tetramethylcyclobutadiene (stabilized by A1 cation). Using the parent compounds (no methyls), develop a Woodward-Hoffmann orbital correlation diagram for the reaction and determine whether the reaction is thermally allowed. 

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. 

In essence, there are only two really important themes in chemistry structure and reactivity. In structural problems, we usually compare the relative stabilities of two isomers (1 and 2) or conformers (3 and 4). Their energy differences are of the order of a few percent. Thus, benzene (1) is more stable than Dewar benzene (2) by 60 kcal mol-1, about 5% of its molecular energy (-1230 kcal mol-1) 3 Similarly, frans-butadiene (3) is more stable than ds-butadiene (4) by 2.7 kcal mol-1, or 3% of its energy of formation. 

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. 

As chemists learned more about the effects of structure on the stability of organic compounds, it became apparent that Dewar benzene is much less stable than benzene. Not only does it have a considerable amount of angle strain, but it also has none of the stabilization due to aromaticity that benzene has. Because of these factors, Dewar benzene is 71 kcal/mol (297 kJ/mol) less stable than benzene. Because the conversion of Dewar benzene to benzene is so exothermic and involves an apparently simple electron reorganization, many chemists believed that the isolation of this strained isomer would prove to be impossible. They thought that if it were prepared, it would rapidly convert to benzene. In support of tills view, numerous attempts to synthesize Dewar benzene met with failure. 

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]. 

Paracyclophane, which exists as the minor component of an equilibrium with its valence-isomeric Dewar benzene, is the smallest member of this series to exhibit sufficient stability to be studied directly [3]. In order to further stabilize the [5]paracyclophane unit, Bickelhaupt [4] recently synthesized a benzannulated version, namely [5](l,4)naphthalenophane (2, Scheme 1). Even though this compound also exists in equilibrium with the Dewar naphthalene 1 from which it was produced, the proportion of the cy-clophane (35 % of the mixture) is higher than that in any previously reported case. A particularly interesting feature of this system is that there are two observable bridge conformers of 2 in solution in a ratio of 95 5. Unfortunately, despite considerable effort, assignment of the two conformers was not possible. 

The simplest member of the class is the [l]-ladderane, a small and familiar strained molecule known as cyclobutane. The strain in a [n]-ladderane increases with the number of fused rings and the introduction of multiple bonds." The [2]-ladderane derivative, Dewar benzene, is highly unstable and converts readily to the conjugation-stabilized counterpart, benzene. Depending on the stereochemistry of bridgehead atoms in the fiised-ring system, [njladderanes... 

These observations indicate the existence of high energy barriers that literally lock the Dewar benzene inside the energy well and prevent its immediate conversion to benzene. Kinetic stability makes the synthesis of Dewar benzene feasible. The methods employed for its preparation are sufficiently mild so the opportunity for the concurrent reaction (the formation of the more stable isomer benzene 3) was safely excluded. 

For reasons of symmetry, the bridged Si-Si bond interacts with tts and TTs MOs, and thus the former is destabilized and the latter is stabilized (Fig. 16). The splittings of ita/tts and Tr lTr s can be estimated to be 0.25 and 1.22 eV, respectively. Due to the reason of the symmetry, the photochemical excitation of the hexasila-Dewar benzene 29 readily occurs to give the hexasilaprismane 25 (Scheme 20). 

The synthesis of unsubstituted Dewar benzene was accomplished by van Tamelen (3) 4). Thus, dihydrophthalic anhydride was photochemically isomerized to a [2.2.0]-ring system which was decarboxylated oxidatively to Dewar benzene. This compound has a higher stability than expected from the high strain of its ring system (t1/2 = 37.2 h at 24.3°). This stability was later explained by the rule of the conservation of the orbital symmetry by Woodward and Hoffmann. 

After the isolation of a Dewar benzene substituted by tert-butyl groups, van Tamelen tried to isolate a Dewar furan stabilized by terr-butyl groups. However, the photolysis of di- or tri-fe/7-butylfuran did not give any Dewar compound but only cyclopropenyl ketones and its ring-opened products I04>. The reaction of di-fert-butylfuran is described by the following equation (104). 

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 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. 

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