Butadiene-cyclobutene

C4H6)t 1, 3-Butadiene, cyclobutene, 3-methylcyclopropene, 1-butyne 2-butyne, 1, 2-butadiene 6-10 g, m, p 725... 

Obviously, while the disrotatory ring closure with conservation of mirror plane symmetry will take place thermally, the conrotatory ring closure with conservation of C2 axis of symmetry will be photochemical. Whether indeed there is such a predicted mode of ring closure, and it certainly is, can be deduced easily from product s stereochemistry. It must be noted that there is a switch in the mode of ring closure in moving from transformation 1,3-butadiene > cyclobutene to the transformation... 

There have been a number of computational studies of the 1,3-butadiene-cyclobutene electrocyclization9 The approaches usually involve location of the minimum energy TS (as described in Section 3.2.2.3) and evaluation of its characteristics. These computational approaches confirm the preference for the conrotatory process, and DFT and CI-MO calculations can provide good estimates of The aromaticity of the TS structures can also be evaluated computationally. The criteria are the same as for ground state molecules, namely energy, bond lengths, and magnetic properties. 

Certain polyenes and cyclic compounds can be interconverted through a pericyclic process known as an electrocyclic reaction. Examples include the 1,3-butadiene-cyclobutene and 1,3-cyclohexadiene-l,3,5-hexatriene interconversions (Figs. 20.5 and 20.16). 

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

Fig. 3.3. Correlation diagram to predict feasibility of 1,3-butadiene-cyclobutene interconversion by disrotatory mode.

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. 

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

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. 

Figure 9.16. Disrotatory state diagram for butadiene-cyclobutene.

Construct a state correlation diagram for the conrotatory butadiene-cyclobutene system. 

The simple picture given above agrees remarkably well with the observed experimental facts but it has raised certain problems. For example, let us consider butadiene-cyclobutene ring closure... 

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 ... 

R. Wallace, Chem. Phys., 37, 285 (1979). Vibronic State Symmetry, Selection Rules and Transition Probabilities for a Molecular Rearrangement Process. The Butadiene-Cyclobutene Rearrangement. 

An intriguing point is to be seen, however. Thus the Oosterhoff state correlation, in which St did not interact with the S0 and S2 configurations, results not only from Brillouin s theorem precluding interaction of S with So but also from the exact symmetry of the cyclobutadiene — cyclobutene reacting system which imparts different symmetries to the St and S2 configurations. With different symmetries, S and S2 also cannot interact. More generally, a reacting system will not have perfect symmetry. Even in the butadiene — cyclobutene case, one can expect molecular deformations to break up this symmetry. 

Figure 7-19. Correlation diagram for the conrotatory ring closure in the butadiene-cyclobutene isomerization. Adaptation of Figure 10.12 from reference [82] with permission.

The important role of avoided crossings and the resulting pericyclic minima for the mechanisms of photochemical reactions was first pointed out on the example of the butadiene-cyclobutene conversion (van der Lugt and Oosterhoff, 1969). 

In contrast to cycloadditions, which almost invariably take place with a total of (4 2) electrons, there are many examples of electrocyclic reactions taking place when the total number of electrons is a (An) number. However, those electrocyclic reactions with (An) electrons, like the butadiene-cyclobutene equilibrium, 6.50 6.51, differ strikingly in their stereochemistry from those reactions mobilising (An+2) electrons, like the hexatriene-cyclohexadiene equilibrium, 6.52 —> 6.53. This is only revealed when the parent systems are... 

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