Cyclobutenes, interconversion

For the butadiene-cyclobutene interconversion, the transition states for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, i.e., the total set of 2p orbitals involved in the reaction process, not the individual MOs. Each of the orbitals is tc in character, and the phase difference is represented by shading. The tilt at C-1 and C-4 as the butadiene system rotates toward the transition state is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

In contrast to the prediction that the disrotatory product should be formed, the coiu otatoiy product is somewhat favored in each case. We will return to the mechanism of the butadiene-cyclobutene interconversion in Section 13.4. 

The 1,3-diene—cyclobutene interconversion can even be applied to benzene rings. For example,379 photolysis of l,2,4-tri-/-butylbenzene (92) gives l,2,5-tri-/-butyl[2.2.0]hexadiene (93, a Dewar benzene).380 The reaction owes its success to the fact that once 93 is formed,... 

One further example of selection rules for reactions is provided by the intramolecular conversion of an open-chain, conjugated polyene to a cyclic olefin with one less pair of n electrons. The simplest example is the butadiene-cyclobutene interconversion ... 

Fig. 18.4. The 1,3-diene-cyclobutene interconversion. The orbitals shown are not molecular orbitals, but a basis set of p-atomic orbitals, (a) Disrotatory ring closure gives zero sign inversion, (b) Conrotatory ring closure gives one sign inversion. We could have chosen to show any other basis set (e.g., another basis set would have two plus lobes above the plane and two below, etc.). This would change the number of sign inversion, but the disrotatory mode would stiU have an even number of sign inversions, and the conrotatory mode an odd number, whichever basis set was chosen.

The 1,3-diene-cyclobutene interconversion can even be applied to benzene... 

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. 

A mechanistic model based on minimization of steric repulsion in transition structures can rationalize results of these butadiene-cyclobutene interconversions, but that explanation cannot explain the results of analogous cyclohexadiene-hexatriene reactions. As shown in equation 11.5, frflMS,ds,trajis-2,4,6-octatriene (13) is converted only to 14, so this process must take place through rotation of the two methyl groups in different directions (analogous to paths (c) or (d) in Figure 11.2). On the other hand. 

For the butadiene-cyclobutene interconversion, this analysis involves drawing the array of interacting basis set orbitals, i.e., the atomic orbitals that make up the molecular orbitals of the reacting system. Notice that this approach is different from the two earlier approaches, in which the molecular orbitals were considered. 

In the case of butadiene to cyclobutene interconversion (4n 7u-electron system), a disrotatory mode of ring closure leads to a Hiickel array, which is antiaromatic with 4n 7c-electrons (Figure 2.9). Therefore, the disrotatory mode... 

FIGURE 2.11 Butadiene-cyclobutene interconversion on the basis of FMO approach. 

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. 

An electrocyclic reaction involves the conversion of a tt system with n electrons to a cyclic system with -2 -ir electrons and a (x bond, or the reverse. Eqs. 15.17 and 15.18 show two prototype reactions, the butadiene-cyclobutene interconversion and the hexatriene-cyclohex-adiene interconversion. Once again, the arrow pushing does not reflect the mechanism of the reaction. 

A. Orbital symmetry analysis of the butadiene-cyclobutene interconversion. B. The lines connecting the reactant and product orbitals for the conrotatory process are given in black, while the correlation for the disrotatory process is given in dashed color. 

Develop state correlation diagrams for the conrotatory and disrotatory butadiene-cyclobutene interconversions and discuss their implications. 

A simple example to predict feasibility of reaction conditions by this method is 1, 3-butadiene cyclobutene interconversion which is discussed below ... 

Fig. 3.2. Correlation diagram to predict feasibility of 1,3-batadiene-cyclobutene interconversion by conrotatory mode.

Prediction of electrocyclic reactions has been discussed in unit III. Prediction by correlation diagram method has been done through the example of butadiene cyclobutene interconversion. One important thing noted there is that energies of some molecular orbitals increase (upward slope), but those of... 

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. 

---