Dewar benzenes, and

Perfluorotetramethylthiadiphosphanorbornadiene and bis(trifluoromethyl) thiadiphosphole can be prepared by thermolysis of an adduct of methanol and hexakis(trifluoromethyl)-l,4-diphosphabarrelene with sulfur [113] (equation 23) Pyrolysis of the adduct of hexafluorinated Dewar benzene and phenyl azide results in ring expansion giving azepine, which photochemically yields an intramolecular 2-1-2 adduct, a good dienophile for the Diels-Alder reaction [114, //5] (equation 24) Thermolysis of fluonnated derivatives of 1,5-diazabicyclo-... 

This procedure is illustrative of the synthetic use of cyelobuta-dieneiron tricarbonyl2 as a source of highly reactive cyclobutadiene. Cyclobutadiene has been employed, for example, in the synthesis of oubane, Dewar benzenes, and a variety of other systems.3,1... 

The dichlororuthenium arene dimers are conveniently prepared by refluxing ethanolic ruthenium trichloride in the appropriate cyclohexadiene [19]. The di-chloro(pentamethylcyclopentadienyl) rhodium dimer is prepared by refluxing Dewar benzene and rhodium trichloride, whilst the dichloro(pentamethylcyclo-pentadienyl)iridium dimer is prepared by reaction of the cyclopentadiene with iridium trichloride [20]. Alternatively, the complexes can be purchased from most precious-metal suppliers. It should be noted that these ruthenium, rhodium and iridium arenes are all fine, dusty, solids and are potential respiratory sensitizers. Hence, the materials should be handled with great care, especially when weighing or charging operations are being carried out. Appropriate protective clothing and air extraction facilities should be used at all times. 

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). 

The special case of perfluorinated Dewar benzenes and heteroaromatic analogs is discussed in Section 5.3.4., together with other valence isomerizations of six-membered and larger rings. 

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... 

The primary adducts from the reaction of tetrachlorocyclopropene and azides yield chlorinated azabutadienes.170b The triazoline derived from Dewar benzene and phenyl azide, when heated in the presence of aluminum chloride, yields unique ring-enlarged heterocycles (Scheme 168).484... 

FIGURE 9. Energy diagram of Dewar benzene and of hexasila-Dewar benzene calculated at the HF/6-31G level... 

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 the above reaction, the product benzene is of D6h symmetry, but only C2v symmetry elements are common for both Dewar benzene and benzene. We can perform therefore the symmetry analysis of the VB structures within the C2v point group framework. There are 6 essential valence electrons involved in the reaction, numbered from 1 to 6. These six valence electrons form three localized bonds (two ji bonds and one o bonds) in Dewar benzene, and a n delocalized bond in benzene, which can be described by five independent BTs within the OEO formalism (see Eq.(65)). Their corresponding BTs are... 

Anodic and cathodic elimination is simply the reverse of cathodic [eqn (16)] and anodic [eqn (15)] addition, respectively. Important cases are anodic bisdecarboxylation, either in the 1,2- (Corey and Casanova, 1963 Radlick et al., 1968 Westberg and Dauben, 1968) or 1,3-fashion (Vellturo and Griffin, 1966), with the preparation of Dewar benzene and dimethyl bicyclobutane-2,4-dicarboxylate as the more prominent cases [eqns (25) and (26)], and cathodic dehalo-genation of dihalides with the halogens in the 1,2- (Zavada et al., 1963), 1,3- (Casanova, 1974 Gerdil, 1970 Rifi, 1967, 1969), 1,4-(Casanova and Rogers, 1974 Wiberg et al., 1974) and 1,6- (Covitz, 1967) positions. The synthesis of bicyclobutanes (27) and [2,2,2]propellane (28) bear witness to the usefulness of this reaction type. 

T. J. Katz, N. Acton, Synthesis of Prismane. J. Am. Chem. Soc. 1973, 95, 2738-2739 V. Ramamurthy, T. J. Katz, Energy-Storage and Release -Direct and Sensitized Photoreactions of Dewar Benzene and Prismane. Nouv. J. Chim. 1977, 1, 363-365. 

In recent years there has been a sharp decline in the number of reports dealing with photoisomerizations of benzenes into fulvenes, benzvalenes, bicyclo[2.2.0]hexadienes (Dewar-benzenes), and prismanes. There is now... 

On the other hand, shock waves generate high pressures as well as high temperatures, and, consequently, some fector in addition to heat must be involved in the shock reactioa Drickamer [145], for example, has suggested a close relationship between photochemistry and liigh-pressure chemistry. He experimentally showed that high-pressure conditions promoted the formation of pentacene dimers with cross-linked structure, the formation of which usually occurred in the photochemical reaction. If the shock reaction is a type of some reactions in excited states such as a photochemical reaction, many valence isomers such as Dewar benzene and benzvalene would be generated from benzene by shock waves, and the interaction between these isomers would produce various com-poimds such as derivatives of fiilvene. Such valence isomers are imstable and would not have been detected in our study. 

A careful reinvestigation of the photoreaction of tri-rert-butylbenzene showed that benzvalene is formed besides the corresponding Dewar benzene and prismane. This benzvalene is fairly stable (38) 30). 

To investigate the stnicture of the transition state in the isomerization, the activation volumes of the isomerization of Dewar benzene and hexamethyl Dewar benzene (42,43) were measured 44) ... 

Non-sensitized isomerization of unsubstituted Dewar benzene and benzvalene was found to proceed in high quantum yield (45, 46) 48). 

Lemal studied the isomerization of hexakis(trifluoromethyl)Dewar benzene and found that the half-life of the compound is much longer (4 h at 80°) than that of hexamethyl benzene (48 h at room temp.)so>. 

The photolysis of hexafluorobenzene in the presence of oxygen yields the corresponding Dewar benzene and its epoxide the latter may act as a dienophile and thermally rearrange to a perfluoro-l,3-cyclohexadienone (67)66). This reaction is in marked contrast to the reaction of hexamethyl Dewar benzene (see Eq. 56). 

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. 

Thermolysis of the three cyclopropenes 1 -3 leads to xylenes in differing ratios. The meso- and dl-forms interconvert at rates competitive with aromatization. The data are most consistent with initial cleavage of one cyclopropene to give a diradical, followed by expansion of the other ring, closure to a Dewar benzene and finally aromatization. ... 

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. 

Trimeriziation of alkynes [I. 33. before references]. 2-Butyne (1) is bicyclo-trimcrized by about 10 mole percent anhydrous aluminum chloride (or alkylaluminum dichloride) in benzene or methylene dichloride at 25-35° to a hydrocarbon (3) identified with accuracy and described by the picturesque name hexamethyl Dewar benzene and defined by the systematic name hexamethyltricyclo [2.2.0] hexa-2.5-... 

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