Strained DEWAR benzene derivatives

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. 

Scheme 12.2 S5mthesis of the highly strained [l.l]paracyclophane 12.1a from the bis(Dewar benzene) derivative 12.3.

A number of semibullvalenes and some barbaralanes, which undergo a facile Cope rearrangement via bicyclooctadienyl diradicals, produce various cyclic peroxides on exposure to triplet oxygen ". Thus, cyclopentano semibullvalene 82a is peroxidated with air to give a mixture of 5-membered 83a and 6-membered endoperoxide 84a in a 1 2 ratio, whereas the cyclohexano derivative 82b yields only the 6-membered cyclic peroxide 84b (Scheme 15). The energy in the highly strained mono(Dewar benzene) isomer of... 

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... 

The strained double bonds in Dewar benzene and thiophene make them good dipolarophiles in 1,3-cycloadditions. The hexamethyl,148 1,3,5-trimethyl, and perfluoro-1,3-dimethyl122 Dewar benzenes all yield a monoadduct by reaction with phenyl azide. The hexafluoro derivative, however, gives, depending on the conditions, either a monoadduct (19) or a mixture of mono-and bisadducts accompanied by an aziridine resulting from thermolysis of the triazoline ring system (Scheme 19).146... 

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)... 

In derivatives of Dewar benzene, the bond length of the bridging C-C bond is found to lie in the range 156-159 pm. The elongation of bond length is mainly due to the strain caused by the fusion of two four-membered rings. 

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... 

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. 

Prismane is the most highly strained valence-bond isomer of a benzene. Therefore, it isomerizes to the benzene isomer possibly via its Dewar isomer. However, since the activation energy of the isomerization of the Dewar isomer to the benzene is small, the Dewar isomer is rarely observed. Many photochemical isomerizations of benzene derivatives are assumed to proceed via Dewar benzenes and prismanes but in most of cases, such isomers have not been observed. 

A detailed examination of the preparation and photochemical activity of strained paracyclophanes has been reported. The starting materials are the Dewar benzenes (94). Irradiation of these at 254 nm at 77 K results in ring opening to the corresponding paracyclophane. Interestingly the unsubstituted derivative (94, R = H) breaks down to yield (95) and ethene on prolonged irradiation. This reactivity is not observed for the substituted compounds. The work was extended to cover systems such as (96) and (97), which both follow the... 

No general pattern can be recognized in the reaction of azides with cycloalkenes. Angle strain can greatly enhance the rate of azide addition to cycloalkenes, as shown by work on norbornene and its derivatives (Huisgen et al., 1965), on hexa-methylbicyclo[2.2.0]hexa-2,5-diene (Dewar benzene) (Paquette et al., 1972) and related compounds (review Lwowski, 1984, p. 579ff). Normally, dihydrotriazoles are primary products, followed by a dediazoniation to aziridines. Diazo compounds are formed only in rare cases, e.g., with trifluoromethylated Dewar thiophenes (Kobayashi et al., 1977, 1980). 

Skeletal rearrangements of some strained compounds, such as bulvalene to bicyclo[4.2.2]deca-2,4,7,9-tetraene, cubane to cuneane, hexamethyl Dewar benzene to hexamethylbenzene (eq 52), and quadricyclane to norbornadiene (eq 53), are catalyzed by derivatives of PdCl2. 

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