Dewar benzenes ring opening

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

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. ... 

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 . ... 

Irradiation of the substituted cyclohexadiene resulted in the formation of the Dewar benzene skeleton by a disrotatory ring closure. Reaction with lead tetraacetate (a reaction that is not covered in this book) was used to remove the anhydride group and introduce the final double bond of Dewar benzene. Again, because of the forbidden nature of the conrotatory opening to benzene, Dewar benzene has an appreciable lifetime. At 25°C the half-life for its conversion to benzene is 2 days, and at 90°C its half-life is 30 min. 

Fused cyclobutene rings are easily opened. Hexam-ethyl(Dewar)benzene (HMDB) is thermolyzedto hexamethyl-benzene at 100-140 °C, but [RhCl(HMDB)]2 catalyzes the conversion at <70 °C. Since the catalyst itself is stable at 100 °C, it has been suggested that the catalyst is cleaved by excess substrate and that the rate-determining step is the elimination of hexamethylbenzene from [RhCl(HMDB)2]. 

The Marcus equation provides a nice conceptual tool for understanding trends in reactivity. Consider for example the degenerate Cope rearrangement of 1,5-hexadiene and the ring-opening of Dewar benzene (bicyclo-[2,2,0]hexa-2,5-diene) to benzene. Figure 15.29. The experimentally observed activation energies are 34 kcahmol and 23 kcal/mol, respectively. The Cope reaction is an example of a Woodward-Hoffmann... 

Two reports of transition metal-catalyzed disrotatoiy cyclobutene ring opening have appeared. Volger and Hogeveen described the valence isomerization of hexamethyl-Dewar-benzene (IV) to hexamethyl-benzene (V) catalyzed by /t-dichlorodi(hexamethyl-Dewar-benzene)-dirhodium in the temperature range 60-70° (37). 

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). 

Johnson, R. P., Daoust, K. J., Electrocyclic Ring Opening Modes of Dewar Benzenes Ab Initio Predictions for Mobius Benzene and trans Dewar Benzene as New C6H6 Isomers, J. Am. Chem. Soc. 1996, 118, 7381 7385. 

Cyclooctatrienes undergo both ring opening and cydization reactions. The cydization reactions produce both bicyclic and tricyclic derivative. Similarly, benzene derivatives cyclize to bicyclic derivatives (Dewar benzenes) or tricyclic analogues (benzvalenes). Tropolones undergo cydization in one of three directions depending upon the substituents present. 

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... 

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