Butadiene-cyclobutene interconversion photochemical

Predictions for 1, 3, 5-Hexatriene cyclohexadiene interconversions These predictions can be made on the similar grounds as for 1, 3-butadiene cyclobutene interconversion. Photochemical reaction is feasible by conrotatory mode whereas thermal reaction follow disrotatory mode of ring closure as is explainable by Fig. 4.1. and Fig. 4.2, respectively. 

The case of butadiene-cyclobutene interconversion, which one might expect to provide a straightforward example illustrating the stereoselectivity of photochemical electrocyclization, is actually quite complex, especially when substituted systems are involved. We first consider experimental outcomes from the photolysis of butadiene and substituted derivatives, as well as the reverse reaction, the photochemical ringopening reactions of cyclobutenes. We then examine the 1,3,5-hexatriene system in the same way. 

Q 5. In the three reactions shown below, the compounds readily undergo symmetry-allowed disrotatory butadiene—cyclobutene interconversion under photochemical conditions. In the first two reactions, the bicyclic isomer cannot revert thermally to the starting compound. However, in the third reaction, the bicyclic isomer reverts thermally to the starting compound, i.e., diazepinone. Explain. 

However, the appropriate symmetry is present if mirror symmetry is conserved. That is, the mirror symmetry for excited cyclobutene is a, symmetric n, symmetric 71, antisymmetric while for 1,3-butadiene the corresponding states are Tq, symmetric 7t2, antisymmetric ti i, symmetric. Thus, in this excited state cyclobutene has two symmetric and one antisymmetric mirror relationships as does 1,3-butadiene. The interconversion of these species in these states conserves symmetry. Since mirror symmetry requires that disrotatory processes occur, we can confidently predict that the photochemical process will generate cis,trans- or (ZT0-2,4-hexadiene from trans- or ( )-3,4-dime thy Icy clobutene and trans,trans- or ( , )-2,4-hexadiene from cis- or (Z)-3,4-dimethylcyclobutene with no cross over since only a disrotatory process will conserve orbital symmetry. 

Draw correlation diagram for disrotatoiy interconversion of 1, 3-butadiene cyclobutene. Also decide if reaction is thermally allowed or photochemically feasible. 

A silicon version of the well-studied electrocyclic interconversion of C4H6 (cyclobutene, bicyclobutane, butadiene, etc.) has been investigated both experimentally23,101 and theoretically.102,103 Irradiation of 51 in 3-methylpentane (X >420nm) gives a 1 9 mixture of 51 and bicyclo[1.1.0]tetrasilane 88 at the photostationary state as determined by UV-vis spectroscopy.23 When the mixture is left for 12 h in the dark at rt, 51 is produced quantitatively [Eq. (58)]. The photochemical conversion of 51 to 88 and the thermal reversion can be repeated more than 10 times without any appreciable side reactions. 

This reasoning was used by the author in 1961 to rationalize the ubiquitous photochemical cyclization of butadienes to cyclobutenes here it was noted that the excited state has a high 1,4-bond order. The same reasoning was applied 6,12) to understanding the key step of cyclohexadienone rearrangements (vide infra). Still another example is the decreased central bond order in the excited state of stilbene which, as Daudel has noted 13), is in accord with photochemical cis-trans interconversion. 

After studying the correlation diagrams for both conrotatory and disrotatory processes, it is concluded that the interconversion of butadiene to cyclobutene thermally proceeds in a conrotatory fashion while photochemically it proceeds in a disrotatory fashion. 

Remarkably, it was discovered that the stereochemical outcome of the reaction was dependent on the energy source. Heat gave one stereochemical result and light another. Figure 20.7 sums up the results of the thermal and photochemical interconversions of cyclobutenes and butadienes. In the thermal reaction, a cis 3,4-disubstituted cyclobutene yields the cis,trans diene. By contrast, in the photochemical process the same cis cyclobutene yields a pair of molecules, the cis,cis diene and the trans,trans diene. 

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