Dewar-benzene ring

Figure 15.29 The Cope rearrangement and Dewar benzene ring-opening reaction...

Benzene from Dewar-benzene ring. Dimethyl tetramethyl-Dewar-phthalate heated briefly at 130° -> dimethyl tetramethylphthalate. Y ca. 100%. R. Criegee and F. Zanker, Ang. Ch. 76, 716 (1964) dihydrobenzene ring s. D. Seebach, B. 97,2953 (1964). 

The bicyclo[2.2.0]hexa-2,5-diene ring system is a valence isomer of the benzene ring and is often referred to as Dewar benzene. After many attempts to prepare Dewar benzene derivatives failed, a pessimistic opinion existed that all such efforts would be finitless 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 by photolysis of 1,2,4-tri(/-butyl)benzene. ... 

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. ... 

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-... 

The Dewar structure (IV) has never been seriously considered since it suffers from the same defects, in an accentuated form, as the Kekule structure. Its advantage over the other structures is largely that it explains the intimate relationship that usually obtains between the para positions in the benzene ring. 

Various Dewar benzenes of type 27 are forma depending upon the size of the alicyclic ring. 

Two rings linked by sharing the same bond instead of the same atom lead to annulated bicyclic or tricyclic compounds, the propellanes. In the case of poly-unsaturated molecules, an interesting case is represented by the bicyclo[2.2.0] type. The parent compound Dewar benzene (bicyclo[2.2.0]hexa-2,5-diene) (DEW) is the smallest bicyclic diene which is an often discussed valence isomer of aromatic benzene Unsubstituted DEW is a very... 

Unsaturation is also important in the metal-catalysed disrotatory ring opening of XXXII ( hexamethyl-Dewar-benzene , HMDB) to hexamethylbenzene. This formally forbidden process is catalysed by monomeric HMDBRhCl (the reaction being of order 1/2 in [HMDBRhCl]2 and order 1 in substrate) Closely related is the conversion of XXXIII(a) to XXXIV, presumably by way of an unsaturated intermediate XXXIII ( ), since free CO or added alkene ligands inhibit the process . ... 

The 1,3-diene—cyclobutene interconversion can even be applied to benzene rings. For example,379 photolysis of l,2,4-tri-/-butylbenzene (92) gives l,2,5-tri-/-butyl[2.2.0]hexadiene (93, a Dewar benzene).380 The reaction owes its success to the fact that once 93 is formed,... 

Support for these mechanistic proposals was boosted by the successful isolation of related valence isomers from benzenes or heteroaromatic compounds carrying bulky (t-butyl) or fluorinated groups. 1, 3,5-Tri-t-butvl benzene gives rise to a benzvalene 13.26), but fluorinated substituents or more severe steric crowding (e.g. 3.271 lead to a preference for bicyclo[2.2.0]hexadiene formation Ithese latter isomers are sometimes called Dewar benzenes I. as does the presence of nitrogen in the ring (3.28. ... 

As will be seen in a later section, substituted benzenes rearrange photochemically. Thus o-xylene isomerizes to m-xylene and 1,3,5-tri-isobutyl benzene isomerizes to the 1,2,4- and the 1,2,3-triisobutyl benzenes.410 Such isomerizations conceivably could proceed through free-radical intermediates but Wilzbach and Kaplan and their coworkers have shown that the ring carbon to which the moving substituent is attached also changes position with the substituent. These authors offer the very reasonable explanation that the formation of benzvalene followed by rupture of bonds other than the new ones just formed could lead to rearrangements of the type in question. It should be noted that both benzvalene and prismane could serve as intermediates in this way but that Dewar benzene could not. 

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... 

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