Cyclohexadiene-trienes

The cyclohexadiene-triene photoisomerization as well as cis-trans isomerization have been studied in impressive detail for compounds related to vitamin D (Havinga, 1962). No evidence could be obtained for the intermediacy of triplet states in the observed transformations and for this reason all the mechanisms considered do not include triplet states. 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

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... 

FIGURE 11.2 Heats of hydrogenation of cyclohexene, 1,3-cyclohexadiene, a hypothetical 1,3,5-cyclohexa-triene, and benzene. All heats of hydrogenation are in kilojoules per mole. 

Both reactions are reversible, and the position of the equilibrium depends on the specific case. In general, the triene cyclohexadiene equilibrium favors the cyclic product, whereas the diene cyclobutene equilibrium favors the unstrained open-chain product. 

Figure 30.3 Electrocyclic interconversions of 2,4,6-octa-triene isomers and 5,6-dimethyi-1,3-cyclohexadiene isomers.

An interesting example of l,3-cyclohexadiene-13 5-triene interconversion is the reaction of norcaradienes to give cycloheptatrienes. Norcaradienes give this reaction so readily (because they are cw-1,2-divinylcyclopropanes, see p. 1445)... 

Both the frontier-orbital and the Mobius-Hiickel methods can also be applied to the cyclohexadiene 1,3,5-triene reaction in either case the predicted result is that for the thermal process, only the disrotatory pathway is allowed, and for the... 

Indeed, cw-l,2-divinylcyclopropanes give this rearrangement so rapidly that they generally cannot be isolated at room temperature,though exceptions are known. When heated, 1,5-diynes are converted to 3,4-dimethylenecyclobu-tenes. A rate-determining Cope rearrangement is followed by a very rapid electro-cyclic (18-27) reaction. The interconversion of 1,3,5-trienes and cyclohexadienes... 

Trauner and colleagues [39] recently found a striking contrast in the thermal and catalyzed reactions of a triene. Thermal reaction of a trienolate readily underwent disrotatory electrocyclization to afford cyclohexadiene (delocalization band in Scheme 8) in accordance with the Woodward-Hoffmann rule. Surprisingly, treatment of the trienolate with Lewis acid did not result in the formation of the cyclohexadiene but rather gave bicyclo[3.1.0]hexene in a [4n +2nJ manner (pseudoexcitation band in Scheme 8). The catalyzed reaction is similar to the photochemical reaction in the delocalization band. 

The reaction of benzyne with cyclohexadiene has been known for some time 4>, but although a number of steroidal cis-dienes are readily available no reactions with arynes had been reported prior to our beginning such investigations 145>. This was somewhat surprising in view of the number of reports concerning the modification of steroids by means of reactions with carbenes 146 i49) and the known Diels-Alder reactions of steroidal dienes and trienes iso.isi). 

In an interesting illustration of these reactions, the two disymmetric trienes (+)-33a and (—)-33b were found to preserve their chirality upon photolysis at 193 K and provide cyclohexadienes 34a and 34b, respectively (Scheme 8)18. Upon warming above 205 K, however, they lose their chiral integrity by competitive disrotatory cyclization to the achiral dienes 35a and 35b. The thermal disrotatory closure to the cis-fused ring isomer is generally found to be extremely facile in these systems. 

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. 

For a 1,3,5-triene the symmetry of the HOMO is /3. Therefore a thermal cleavage of cyclohexadiene should be such that the positive lobes must lie on the same side of the plane which requires a disrotatory motion. 

Cyclohexadiene itself undergoes smooth photochemical ring opening to Z-l,3,5-hexatriene in both the gas phase (d> = 0.13)176 and in solution (d> = 0.41)71,177. As is almost always the case, extended irradiation in solution leads to the formation of a variety of isomeric products due to secondary irradiation of the Z-triene and its E-isomer (vide infra)11. 

Dauben and coworkers produced a number of lovely examples of the reaction in the course of their studies of the photochemistry of large-ring (Cs-Cn) cyclic trienes, many of which were produced by photochemical electrocycloreversion of the isomeric annulated cyclohexadiene derivatives (cf Reference 172 and references cited therein). Two examples... 

There is a very significant difference between the rate of aromatization of trans- and c/i-hexatriene (Table III), which shows that geometrical isomerization prior to cyclization may be rate limiting. Since this occurs via half-hydrogenated species (60), it is promoted by the presence of hydrogen, and so is benzene formation. It should be noted that cyelohexane and cyclohexene are produced from cw-triene. The hydrogenation of cyclohexadiene may explain their formation here and in other cases of stepwise Cg dehydro-cyclization. 

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). 

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