Cyclohexadienes electrocyclic ring closure

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

J. Rimmelin and G. Jenner, Tetrahedron, 30, 3081 (1974). A recent measurement of the pressure and temperature dependence of die electrocyclic ring-closure of Z-l,3,5-hexatriene to 1,3-cyclohexadiene in the range of 200 to 2500 bar and 100 to 125 °C does not show a significant temperature dependence of die activation volume (M. K. Diedrich and F. -G. Klarner, unpublished results). 

Electrocyclic ring closures are particularly important in the formation of six-membered rings many are hetero analogues of the hexatriene-cyclohexadiene transformation 4—>5. As discussed in Section 3.2.1.6.1, they are frequently involved in ring interconversions initiated by nucleophilic attack on a six-membered ring. Further examples are discussed in Sections 4.2.3.6 (preparation of seven-membered rings) and 4.4.8.2.2.2 (formation of bicyclic 6,6 ring systems). 

Figure 3 Comparison of the orbital correlation diagrams for the thennal dis (a) and con (b) 6e electrocyclic ring closures. Note that the electronic configuration is shown in each case only for the ground state of the hexatriene revening to the cyclohexadiene via the dis and con modes.

The proposal that the wavelength dependence of the product yields from irradiation of Z-14 is due largely to selective excitation of specific conformers is supported by the observation that the UV spectrum of the compound can be simulated accurately by a 9 1 combination of the spectra of Z-2-rcrr-butyl- (Z-131) and Z-2,5-di-fert-butyl-l,3,5-hexatriene (Z-15), which serve as model compounds for the cZi and cZc conformers of Z-14, respectively . Both Z-15 and the perfluorinated ,Z, -4,5-dimethyl-2,4,6-octatri-ene derivative (135) are thought to adopt preferred helical cZc conformations, and undergo highly selective electrocyclic ring closure to the corresponding cyclohexadienes 132 and 136, respectively upon irradiation (equations 49 the reaction of Z-15 the... 

Similar arguments hold for the transition state of the electrocyclic ring closure of hexatriene forming 1,3-cyclohexadiene (Figure 13.22). There is a strong diatropic ring current in the transition structure, which is in agreement with the aromaticity concept of pericyclic transition states. 

FIGURE 20.56 The electrocyclic ring closure of c j-l,3,S-hexatriene to 1,3-cyclohexadiene is similar to a Cope rearrangement. 

Theoretical studies have been carried out on the conrotatory and disrotatory reaction pathways of hexa-l,3,5-triene to cyclohexadiene, and the effect of solvent and salt effects on the rates of the electrocyclic ring closure of (IZ, 3Z, 5 )-l,2,6-triphenylhexa-l,3,5-triene has been determined. An ab initio study of the electro-cyclization of (Z)-hexa-l,3,5-triene and its hetero-substituted analogues has been undertaken.The involvement of a lone pair on the nitrogen or oxygen atom appears to facilitate the interaction between the terminal atoms that bond to each other to close the ring. It has been reported that the intramoiecular aza-Diels-Alder reaction of an a./S-unsaturated hydrazones to a quinone, as in (197), is followed by an unprecedented rearrangement in which the aminoaryi moiety formally undergoes a 1,2-shift to yield benzo- or pyrido-[fc]acridine-6,11-dienes (198). 

In Summary Conjugated dienes and hexatrienes are capable of (reversible) electrocyclic ring closures to cyclobutenes and 1,3-cyclohexadienes, respectively. The diene-cyclobutene system prefers thermal conrotatory and photochemical disrotatory modes. The triene-cyclohexadiene system reacts in the opposite way, proceeding through thermal disrotatory and photochemical comotatory rearrangements. The stereochemistry of such electrocychc reactions is governed by the Woodward-Hoffmann rules. 

The higher analogs in the series of electrocyclic reactions, the electrocyclic ring closures of 1,3,5-hexatriene and 1,3,5,7-octatetraene, have been studied using HF, MP, and DFT methods. Figure 2 shows the calculated transition structure for the disrotatory ring closure to 1,3-cyclohexadiene, and selected results for this reaction are summarized in Table 2. 

Figure 2 Transition structure for the electrocyclic ring closure of 1,3,5-hexatriene to 1,3-cyclohexadiene...

The above are in class of cascade of reactions, that involve two successive transformations Ru-catalyzed coupling of 1,6-diynes with acyclic alkenes forming bicyclic cyclohexadiene through electrocyclic ring closure and the migration of the double bond. Under the same reaction conditions, the [2-b2-b2] cycloaddition with cyclic olefins results in tricyclic cyclohexadienes with another arrangement of conjugated double bonds. 

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). 

Figure 2 Classification of the six-electron (6e ) electrocyclizations is based on the structure of the resulting cyclohexadiene derivatives (Types I-XVU) irrespective of the method of ring closure, whether thermally, photochemically or otherwise induced...

Another, recently developed method to synthesize a broad spectrum of oligo-and polycyclic aromatics and heteroaromatics in a surprisingly simple manner is likewise based on an electrocyclic - but thermal - hexatriene-cyclohexadiene ring-closure combined with an elimination reaction. This new synthetic method and its scope will be the topic of the following report. 

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