Ring opening of Dewar benzene

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... 

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. 

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. 

Fig. 6 presents the necessary analysis. The elements of the C2v point group are conserved in a concerted ring opening reaction. The sum and difference of the two n bonds of Dewar benzene are of ai+bi symmetry. The long carbon-carbon occupied orbitals of benzene are of 2t and species in the De point gronp of the molecule. However we are interested only in their s)unmetry in Ca . As shown in Fig. 6 the three occupied MO s of n type are i, 62 and 61. Correlation with the bonds of Dewar benzene is not possible. 

The formation of Dewar benzene from 3,3 -bicyclopropenyl most reasonably occurs via opening of the cyclopropene single bond to a biradical (see Chapter 4, Section 1) which undergoes an expansion of the second ring to give a biradical which can close to the product (Scheme 1.6) ... 

In case of trans-isomer conrotatory ring opening is symmetry allowed under thermal conditions giving rise to stable benzene. For this reason derivatives of trans-isomer of Dewar-benzene is not known even if they spontaneously undergo conrotatory ring-opening to give benzenoid system of all cis-double bond. 

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]. 

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