Benzene rings electrocyclic reaction

There seems to be no great difference in the free energy between acyclic triene and the cyclic diene. This is because of smaller strain in the six-membered ring as compared with the four-membered one. On the other hand in 6n electron system in electrocyclic process there is more efficient absorption in the near regions of u.v. spectrum. This is why under both thermal and photochemical conditions, the (1, 6) electrocyclic reactions are reversible. Side reactions are more frequent in reversible. Side reactions are more frequent in reversible transformations of trienes than in dienes. The dehydrogenation of cyclic dienes to aromatic compounds may also occur in the thermal process. On heating cyclohexadiene yields benzene and hydrogen. 

The spontaneous oxepin-benzene oxide isomerization proceeds in accordance with the Woodward-Hoffmann rules of orbital symmetry control and may thus be classified as an allowed thermal disrotatory electrocyclic reaction. A considerable amount of structural information about both oxepin and benzene oxide has been obtained from theoretical calculations using ab initio SCF and semiempirical (MINDO/3) MO calculations (80JA1255). Thus the oxepin ring was predicted to be either a flattened boat structure (MINDO/3) or a planar ring (SCF), indicative of a very low barrier to interconversion between boat conformations. Both methods of calculation indicated that the benzene oxide tautomer... 

The thermal ring closure reaction of a 1,3,5-triene to a 1,3-cyclohexadiene occurs by a concerted disrotatory electrocyclic mechanism. An example of the latter is the oxepin-benzene oxide equilibrium (7) which favors the oxepin tautomer at higher temperatures (Section 5.17.1.2). Oxepin (7) was found to rearrange to phenol during attempted distillation at normal pressure (67AG(E)385>. This aromatization reaction may be considered as a spontaneous rearrangement of the oxirane ring to the dienone isomer followed by enolization (equation 7). 

There are now two possible routes to the final product. Reductive elimination would insert the new alkyne into one of the old C-Co bonds and form a seven-membered ring heterocycle. This could close in an electrocyclic reaction to give the new six-membered ring with the cobalt fused on one side and hence the cobalt complex of the new benzene. 

Many arene oxides are in dynamic equilibrium with their oxepin forms. The parent molecules, benzene oxide la and oxepin lb, are related as valence tautomers that interconvert by an allowed disrotatory electrocyclic reaction. Structural identification of la and lb was based initially upon spectroscopic evidence and chemical transformation to stable products of known structure. Thus the arene-oxide structure was inferred from its typical dienoid (4 + 27t cycloaddition) and epoxide (ring-opening, aromatization) reactions, while the oxepin structure was deduced by catalytic hydrogenation of the triene oxepin to form oxepane. 

The cyclobutene opens in a four-electron conrotatory electrocyclic reaction. The cyanide can go either way and it will prefer to go out. The diene so produced is unstable and is extremely reactive in 3 cloadditions as it regenerates the benzene ring that way. The regiochemistry is predictably ortho md we should probably use the HOMO of the diene and the LUMO of the unsaturated ester. 

The reaction with excess bromoketone starts in the same way but the oxyallyl cation is capture by one of the benzene rings in a four-electron (and therefore conrotatory) electrocyclic closure give the new five-membered ring. 

The cyclobutene ring first opens in an electrocyclic reaction 152. This must be conrotatory as it is a four electron process but there is no stereochemistry at this stage. Then an intramolecular Diels-Alder cycloaddition 153 closes the new six-membered ring. This is a particularly favourable reaction as the formation of the alkene completes a benzene ring. It would not be possible to prepare such an unstable diene so a tandem process is necessary. 

A mixture of a tetrasubstituted thiophene and the ruby-red 1,2-dithiin (59) is formed when thiobenzophenone reacts with tetracyanoethylene in refluxing benzene. It is considered that initial [2-i-2]-cycloadditions involving the C=S and nitrile groups are followed by electrocyclic ring opening and ring closure reactions <97LA1677>. 

The thiophene ring is less aromatic than benzene this allows for a rich potential of chemistry in electrocyclic reactions. Thiophene derivatives may be induced to react as monoene (o) or diene (6) components in these reactions by proper control of functionalization and activation (Scheme 7). Photochemical [2 + 2] cycloadditions are also possible if ethylene is sufficiently activated by... 

You met electrocyclic reactions the double bonds of benzene. The product is not stable, but immediately undergoes electrolike this in Chapter 35. cyclic ring opening. 

Cyclocarbonylation of 3-phenyl-2-methylallyl acetate 386 afforded 1-naphthyl acetate 388 under harsh conditions (160 C, 70 atm) in the presence of acetic anhydride and EtaN. This interesting reaction may be explained by electrophilic attack of the acylpalladium group in the intermediate 387 on the benzene ring. Another possibility is the formation of the ketene 389 and its electrocyclization [151]. 

AN ELECTROCYCLIC REACTION OF THE BENZENE RING THE CLAISEN REARRANGEMENT... 

Electrocyclic reactions have been used to prepare four-membered rings. Irradiation of 1,2-dihydrophthalic acid anhydride gave the bicyclo[2,2,0]hexene (76) which was oxidatively decarboxylated to Dewar-benzene (77) (Scheme 25). ... 

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. ... 

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