Butadiene-cyclobutene interconversion thermal

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. 

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. 

Cyclobutenes.— Theoretical studies of the butadiene-cyclobutene interconversion have been published and the photolytic intramolecular cycliza-tion of substituted 2-pyridones (164) is a related preparative method. The reaction may be reversed thermally and various chemical transformations of (165) have been effected. 9,10-Cyclobutenophenanthrene (167) has been synthesized starting from phenanthraquinone. The final stage is an in situ irradiation of the elimination product (166X which could be trapped by maleic anhydride but not isolated owing to its propensity for dimerization and polymerization. 

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. 

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. 

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. 

An electrocyclic reaction is as easy to analyze as that. Identify the HOMO, and then see whether conrotatory or disrotatory motion is demanded of the end carbons by the lobes of that molecular orbital. All electrocyclic reactions can be understood in this same simple way. The theory tells us that the thermal interconversion of cyclobutene and 1,3-butadiene must take place in a conrotatory way. For the cyclobutene studied by Vogel, conrotation requires the stereochemical relationship that he observed. The cis 3,4-disubstituted cyclobutene can only open in conrotatory fashion, and conrotation forces the formation of the cis,trans diene. Note that there are always two possible conrotatory modes (Fig. 20.12), either one giving the same product in this case. 

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