2.4.6- Octatriene electrocyclization

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... 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

For the photochemical electrocyclic reaction of dienes, irradiation promotes one electron from tt3 to nf. For 2,4,6-octatriene, the conrota-tory mode of reaction gives the trans isomer (Scheme 8.6). 

Exercise 21-23 Show how one can predict the stereochemistry of the electrocyclic rearrangement of frans,c/ s,frans-2,4,6-octatriene to 5,6-dimethyM, 3-cyclohexadiene by a favorable concerted thermal mechanism. 

The example just cited provides a verification of the prediction that the excited-state reactions should be conrotatory for six-electron systems. The prototype octatriene-cyclohexadiene interconversion (Equation 12.64) shows the same pattern.117 The network of photochemical and thermal electrocyclic reactions connected with the formation of vitamin D provide several further examples.118... 

With two more electrons in the conjugated system, making eight in all, the octatetraenes 4.51 and 4.54 show conrotatory closure giving the cyclo-octatrienes 4.52 and 4.55, However, the reaction can only just be stopped at this stage, for the products undergo a second electrocyclic reaction giving the bicyclic dienes 4.53 and 4.56 as a result of the allowed disrotatory reaction of the all-m hexatriene units. 

Now it works In fact, extension of this reasoning to other electrocyclic reactions tells you that they are all allowed—provided you choose to make the conjugated system react with itself supra-facially for (An + 2) K systems and antarafaciaWy for (4n) n systems. This may not seem particularly informative, since how you draw the dotted line has no effect on the reaction product in these cases. But it can make a difference. Here is the electrocyclic ring closure of an octatriene, showing the product from (a) suprafacial reaction and ( ) antarafacial reaction. 

Stereochemistry Electrocyclic reactions, like all pericyclic processes, exhibit great stereospecificity. The stereospecificity of such reactions is demonstrated by thermal closure of fransjds,trans-2,4,6-octatriene (8.12) to ds-5,6-dimethyl-1,3-cyclohexadiene (8.13) and of the isomeric frans,cis,czs-octatriene (8.14) to frans-5,6-dimethyl-1,3-cyclohexadiene (8.15). 

Alkynes undergo cycloaddition on irradiation with benzene or naphthalene derivatives or with other aromatic compounds. With a benzene derivative the product is usually a cyclo-octatetraene which results from thermal electrocyclic ring-opening of the bicyclo-octatriene formed initially by 1,2-addition of the alkyne to the benzene ring (equation 61) . The intermediate can be trapped using a dienophile such as tetracyanoethylene (equation 62) ". The first step of the photoaddition process involves excitation of the alkyne , and orbital symmetry considerations suggest that concerted 1,2-addition is allowed if the alkyne is excited but not if the benzene is excited ... 

Electrocyclic interconversions of 2,4.6-octatriene isomers and 5,6-dimethyl-1,3-cyclohexadiene isomers. 

An electrocyclic reaction is an intramolecular reaction in which the rearrangement of tt electrons leads to a cyclic product that has one fewer tt bond than the reactant. An electrocyclic reaction is completely stereoselective—it preferentially forms one stereoisomer. For example, when (2 , 4Z,6 )-octatriene undergoes an electrocyclic reaction under thermal conditions, only the cis product is formed when (2 , 4 6Z)-octatriene undergoes an electrocyclic reaction under thermal conditions, only the trans product is formed. Recall that E means the high-priority groups are on opposite sides of the double bond, and Z means the high-priority groups are on the same side of the double bond (Section 3.5). 

A4) of a compound with three tt bonds is asymmetric (Figure 29.3). Therefore, under Lm 3u LergJngring°doVu photochemical conditions, (2 ,4Z,6Z)-octatriene undergoes conrotatory ring closure, controls the stereochemical outcome so both methyl groups are pushed down (or up) and the cis product is formed. of an electrocyclic reaction. 

Let s first look at the electrocyclic reaction of (2 ,4 6 )-octatriene. The ground-state HOMO (i/ s) of a compound with three conjugated bonds is symmetric (Figure 28.3). This means that ring closure under thermal conditions is disrotatory. In disrotatory ring closure of (2 , 4. 6 )-octatriene, the methyl groups are both pushed up (or down), which results in formation of the cis product. 

Now let s look at the electrocyclic reaction of (2i ,4Z,6Z)-octatriene. Because its ground-state HOMO is also symmetric, it, too, undergoes disrotatory ring closure. In disrotatory ring closure, one methyl group is pushed up and the other is pushed down, which results in formation of the trans product. The enantiomer is obtained by reversing the groups that are pushed up and down. 

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