Disrotation

SD(Q)-CI/6-31G //(2/2)CASSCF/6-31G calculations confirmed the qualitative expectation that coupled disrotation should be preferred to both conrotation and monorotation in the stereomutation of 9a. In addition, unlike the case in 8a, the calculations found that the preference for coupled rotation was enhanced by the addition of methyl groups to the terminal carbons of 10a. Hyperconjugative electron donation from the methyl groups in 10b and 10c enhances electron donation from the symmetric combination of 2p-n AOs on the terminal carbons into the antibonding orbitals of the C—F bonds at C2. 

For the cis isomer, the ratio of racemization to epimerization was found to be 107 at 274.5 °C, and the cis isomer was found to undergo racemization 16.2 times faster than the trans isomer. The ratio of racemization to epimerization for the latter was found to be only 6.6 at 274.5 °C. Thus, these experiments confirmed the computational predictions that in 9b and 9c, double methylene rotations are faster than any process that leads to net single methylene rotation, and the preferred mode of double rotation is disrotation, rather than conrotation. ... 

Important mechanistic consequences may also derive from geometrical isomerism. m-3.4-Dimethylcyclobutene may ring-open to form either cis, trans- or qa/ .fra/i,s -2,4-hexadiene. The methyl groups may rotate away from each other (disrotation, Eq. (3)) to form the trans, trans isomer. Alternatively, they... 

In this case, the disrotation of the groups toward one another would lead to the cis,cis,cis product. Because this product is not formed, it seems likely that rotation of the methyl groups toward each other must be sterically unfavorable ... 

The same reasoning can be extended to electrocyclic reactions of 1,3,5-trienes and 1,3-cyclohexadienes, which involve An + 2 electrons and consequently favor Hiickel transition states attained by disrotation. 

Mobius transition state (achieved by conrotation), whereas the photochemical reaction is most favorable with a Hiickel transition state (achieved by disrotation). 

Figure 4.3. The direction of disrotation in ring expansion reactions.

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