Dewar benzene structures

Perfluoro(3,5-dialkvlpyridazines) 35 rearrange photolytieally to the 2,6-disubstituted pyra/ines 36.40 The mechanism that best explains these results involves rearrangement of an intermediate Dewar benzene structure 37 to 38, followed by rearrangement to the pyrazine structure (Scheme 12).40... 

Benzene still provides fruitful labour for the dedicated photochemist. A three photon excitation study of the So(A, ) - SjfBju) transition in the neat liquid shows that under this condition internal states exist which make the experimentally observed 3 photon allowed pathway. A photochemical study of both benzene and pyridine shows that Dewar benzene structure formation involves S1-S2 state mixing by irradiation at 253.7 nm. For pyridine in a xenon matrix all photochemical reactions are quenched and evidence for the formation triplet state of this molecule is produced for the first time. The photophysical significance of PE surface geometries of the low... 

A particularly interesting case involves the bicyclo[2.2.0]hexa-2,5-diene system. This ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. Attempts prior to 1960 to prepare Dewar benzene derivatives failed, and the pessimistic opinion was that such efforts would be fruitless because Dewar benzene would be so unstable as to immediately revert to benzene. Then in 1962, van Tamelen and Pappas isolated a stable Dewar benzene derivative 9 by photolysis of l,2,4-tri-(/-butyl)benzene. The compound was reasonably stable, reverting to the aromatic starting material only on heating. Part of the stability of this particular derivative can be attributed to steric factors. The /-butyl groups are farther apart in the Dewar benzene structure than in the aromatic structure. 

By a careful compilation of the number of isomers of the known isomers of various aromatic compounds at that time, Kekuld postulated the benzene ring. Several other structures of benzene have been proposed, e.g., the Dewar benzene structure, theLadenburg benzene structure. 

In the Dewar benzene structure, the bulky tert-butyl groups are farther apart and interact less with each other than they do in the aromatic structure and, in conjunction with other factors, render the Dewar form stable enough to be isolable. Reversion to the aromatic form occurs only on heating. The parent compound was reported by van Tamelen and Pappas in 1963 ... 

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

The 2-I-2-cycloaddition of l,l-difluoro-2,2-bis(dimethylamino)ethene with ethyl propiolate yields a cyclobutene which readily undergoes electrocyclic ring opening to form a diene. The 1,4-diacetal-bridged cycloadduct (7), produced in four steps from the 2 -I- 2-photocycloaddition of DMAD with l,4-dichlorobut-2-ene, reacts with DMAD to provide a synthesis of the Dewar-benzene structure (8) (Scheme 3). ... 

DEPT-NMR spectrum. 6-methyl-5-hepten-2-ol, 451 Detergent, structure of, 1065 Deuterium isotope effect, 386-387 El reaction and, 392 E2 reaction and, 386-387 Dewar benzene. 1201 Dextromethorphan, structure of, 294 Dextrorotatory, 295 Dextrose, structure of. 973 Dialkylamine, pKa of, 852 Diastereomers, 302-303 kinds of, 310-311 Diastereotopic (NMR), 456... 

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. 

The calculations above did not give satisfactory results concerning the structure of even the parent hydrocarbons (cyclobutane and bicydo[2.2.0] hexane), but highly strained cyclobutene, methylene-cyclobutene, Dewar benzene, and so on, were shown to be handled well by MM2 (83a). 

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 results obtained in the photolysis of perfluoroxylenes do not require intervention of benzvalenc isomers, as is favored for the hydrocarbon case, nor are structures 12 14 necessarily involved. A previous study21 reported that photolysis of ofyi-xylcnc 8 leads to Dewar benzene isomers 11 and 14. 

The product shown in Equation 13.66 is a derivative of Dewar benzene the unsubstituted analog, in a planar conformation, was originally suggested by Dewar as the structure of benzene.102 Once formed, such highly strained molecules cannot undergo photochemical reversion because they lack a low-lying... 

For benzene several isomers are possible, most of them suggested in the last century as possible structures for benzene before the Kekule model was proposed and accepted. Prismane, Dewar benzene, and benzvalene (see Fig. 5) all might be formed from excited benzene molecules provided proper modes of vibration were... 

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. 

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. 

Before continuing with the discussion, it is instructive to relate these VB states to those that arise from the MO picture of benzene. Benzene possesses degenerate pairs of HOMOs and LUMOs, and hence the excitations from the two HOMOs to the two LUMOs give rise to four singlet excited states of the following symmetries B2u, E2g, and Blu. As can be seen from Fig. 7.4a—c, the B2u is the covalent state made from the Kekule structures, while E2g is the covalent state made from the Dewar structures. The Blu state is predominantly ionic, made from the monoionic VB structures (see those in Chapter 5) (5). The excited Alg state made from the Dewar VB structures (Fig. 7.4c) corresponds to higher rank MO excitation. 

The Dewar benzene derivative l,8, 3,5-naphtho[5.2.2]propella-3,8,10-triene, Ci.sHu. also has Z = 4, and a crystallographic mirror plane passes through the central naphthalene C-C bond and the midpoints of the bonds C(8)-C(8 ), C(9)-C(9 ) and C(10)-C(10/). The measured bond lengths in the strained ring system and crystal structure are shown in Fig. 9.6.11. 

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. 

The 7t-system is described by all five Rumer structures, which is the complete spin-space (i.e. Fig. 3 and Fig. 4). This allows a smooth transition from benzene, where the 2 Kekule structures are most important, to the highly bent Dewar benzene, where only one of the Dewar structures (Fig 4) is important. All the orbitals, doubly occupied and singly occupied are fully optimised. For each bent structure, the orbitals from the preceding less bent structure were used as initial guess. This and the choice of wavefunction ensure that an aromatic 7i-system can be identified, even when no symmetry separation exists. All orbitals were completely optimised so we have a wavefunction of the spin-coupled type. This is the type of wavefunction used by Cooper et al. [52] in their study of benzene. 

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