Cyclobutene disrotatory opening

The concerted disrotatory opening of the cyclobutene is much easier in the aldehyde than in the starting alcohol. Thus, while the alcohol could be easily isolated, its oxidation to aldehyde leads to a cyclobutene that could not be isolated, because it suffers a quick opening of the ring to the final product. 

The thermal ring opening of bicyclo[3.2.0]hepta-3,6-dien-2-one A to give tropolone B is forbidden (disrotatory opening of a cyclobutene). So, in spite of the strain in the starting material, it only occurs at 350 °C. 

As we saw in Section 2.B, the interconversion of butadiene and cyclobutene can occur through either a disrotatory or a conrotatory process. As the following diagram shows, the disrotatory opening is symmetrical with respect to a O plane, whereas conrotatory opening is symmetrical with respect to the C axis. 

Finally, calculations revealed that a possible reaction intermediate, namely cycloheptatetraene, which might be formed by a necessarily disrotatory cyclobutene ring opening in the bicyclic diene, would be higher in energy than the observed activation energy. 

The conversion of bicyclo[3.2.0]hepta-2,6-diene to cycloheptatriene proceeds with log k = 14.0 - 39 500/23RT. The reaction would appear to be a forbidden disrotatory cyclobutene ring opening which occurs with reasonable facility due to the stabilization of developing radicals by an adjacent double bond (Scheme 8.13). 

The experimental free energy of activation for the ring opening is 35 kcal/mol which could easily be due to of a symmetry forbidden, non-concerted disrotatory cyclobutene ring opening. 

A disrotatory, non-concerted cyclobutene ring opening to give a biradical was proposed for the reaction pathway. 

Note that the four carbon atoms do not lie in the same plane in the calculated trai ition structure for the thermal conrotatory opening of cyclobutene. Lee, P. S. Sakai, S. Horstermatm, P. Roth, W. R. Kallel, E. A. Houk, K. N. /. Am. Chem. Soc. 2003,325,5839 found that the energy gap between the forbidden disrotatory pathway and the allowed conrotatory pathway decreases as the transition structure for cyclobutene ring-opening becomes more planar. 

During the disrotatory opening of cyclobutene to butadiene, a plane of S3mi-metry is maintained at every stage of the reaction ... 

It is easy to analyze an electrocyclic reaction by constructing a correlation diagram, which is simply a diagram showing the possible transformation of reactant orbitals to product orbitals. Let us first analyze a disrotatory opening of cyclobutene in which a mirror plane symmetry (m) is maintained (Figure 2.4). 

Q 14. While benzo analogues II—IV undergo cycloreversion of the central cyclohexane ring by all disrotatory opening, the triscyclobutenocyclohexane I does so by stepwise conrotatory cyclobutene rupture. How do you explain these observations ... 

The bicyclo[4,2,0]octa-2,4,7-trienyl iron carbonyl complexes (285) and (286) are stable, even in refluxing benzene, in marked contrast to the rapid valence isomerization of free bicyclo[4,2,0]octa-2,4,7-triene to cyclo-octatetraene. This stability is due to the loss of the inert gas configuration on rearrangement to a diTiapto-cyclo-octatetra-ene-Fe(CO)3 complex. The inert gas structure could be retained if the cyclobutene ring of the complexes opened via a forbidden disrotatory opening, but this does not occur because the metal is not in a position to help sterically. ... 

The ring opening of the codimer (221) of cyclobutadiene and benzocyclo-butadiene rearranges on heating to benzocyclooctatetraene (224). This reaction inevitably involves, as indicated, the successive disrotatory opening of two cyclobutene rings via antiaromatic transition states. The reaction is... 

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