Electrocyclic reactions rotational modes

Electrocyclic reactions of 1,3,5-trienes lead to 1,3-cyclohexadienes. These ring closures also exhibit a high degree of stereospecificity. The ring closure is normally the favored reaction in this case, because the cyclic compound, which has six a bonds and two IT bonds, is thermodynamically more stable than the triene, which has five a and three ir bonds. The stereospecificity is illustrated with octatrienes 3 and 4. ,Z, -2,4,6-Octatriene (3) cyclizes only to cw-5,6-dimethyl-l,3-cyclohexadiene, whereas the , Z,Z-2,4,6-octa-triene (4) leads exclusively to the trans cyclohexadiene isomer. A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode... 

It would be a good point here to remind you that, although all electrocyclic reactions are allowed both thermally and photochemically pro viding the rotation is right, the steric requirements for con- or disrotatory cyclization or ring opening may make one or both modes impossible. 

Disrotatory and conrotatory rotation The concerted rotation around two bonds in the same direction, either clockwise or counterclockwise, is described as conrotatory. In the electrocyclic ring opening, the terminal p-orbitals rotate (or twist, roughly 90°) in the same direction known as conrotation (comparable to antarafacial) to form a new ct-bond. In disrotatory cyclization (comparable to suprafacial) the terminal p-orbitals rotate in opposite directions. These two modes of the electrocyclic reaction are shown in Fig. 8.37. 

These electrocyclic reactions and their conrotatory or disrotatory nature can be readily understood on the basis of aromaticity in the transition state (Figure 7.16). For the conrotatory mode, the rotations of the breaking cr orbitals bring about a phase change in the cyclic transition state continuous red-to-red overlap cannot be maintained. Thus the... 

Electrocyclic reactions can also be analyzed on the basis of the idea that transition states can be classified as aromatic or antiaromatic, just as is the case for ground state molecules. A stabilized aromatic TS results in a low activation energy, i.e., an allowed reaction. An antiaromatic TS has a high energy barrier and corresponds to a forbidden process. The analysis of electrocyclizations by this process consists of examining the array of basis set orbitals that is present in the transition structure and classifying the system as aromatic or antiaromatic. For the butadiene-cyclobutene interconversion, the TSs for conrotatory and disrotatory interconversion are shown below. The array of orbitals represents the basis set orbitals, that is, the complete set of 2p orbitals involved in the reaction process, not the individual molecular orbitals. The tilt at C(l) and C(4) as the butadiene system rotates toward the TS is different for the disrotatory and conrotatory modes. The dashed line represents the a bond that is being broken (or formed). 

A point of particular importance regarding the stereochemistry of this reaction is that the groups at the termini of the triene system rotate in the opposite sense during the cyclization process. This mode of electrocyclic reaction is called disrotatory,... 

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