Conrotation

Table 11.1 Directive Effect of Substituents on Inward/Outward Conrotation in Cyclobutene Ring Opening"...

It has already been pointed out that if a thermal reaction is disrotatoiy, then the photochemical reaction would be conrotating and vice-versa. This has been utilized in synthetic organic chemistry, because by choosing the appropriate conditions, the stereochemistry of the product can be selected. 

The origin of this preference is illustrated by Eq. 13.3. When the allene is terminally substituted, as it is in 6, it is axially chiral. Consequently, there are two reaction pathways that the allenyl vinyl ketone (10 in Eq. 13.3) can follow. The major reaction pathway for 10 involves counterclockwise conrotation, leading to 11, whereas the... 

An alternative approach for generating the pentadienyl carbocation that is needed for the Nazarov cyclization has been demonstrated by de Lera and co-workers [20, 21] (Eq. 13.18). Vinylallene acetal 56 is converted to a ca 1 1 mixture of cyclopentenes 57 and 58 upon exposure to toluenesulfonic acid in acetone at room temperature. The reaction presumably involves initial generation of carbocation 59 that undergoes conrotation to give 60. Intramolecular trapping of the carbocation by the pendant hydroxyl group leads to the observed product. Depending on whether the conrotation in 59 takes place clockwise or counterclockwise, E- (57) or Z-(58) products are formed. 

Alkyl substituents are not just stereochemical markers. At Cl and C3 of 8 they can hyperconjugatively donate electrons to both the symmetric and antisymmetric combinations of 2p-n AOs at these carbons. As a result, hyperconjugative destabilization of both these combinations of 2p-n AOs by electron donation from alkyl substituents at Cl and C3 should reduce the preference for the configuration in which the antisymmetric combination of 2p-n AOs is doubly occupied. In fact, our (2/2)CASSCF calculations on 8a and on its 1,3-dimethyl derivative indicated that the methyl substituents should wipe out almost completely the weak preference for conrotation over monorotation that we computed for the stereomutation of unsubstituted 7. ... 

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. ... 

Conjugated dienes, 11 Conjugation, 11, 368 Conrotation, 345 Coordination polymerisation, 322 Copolymerisation, 322 Cope reaction, 268 Cope rearrangement, 354 Cracking, 112, 305 Cram s rule, 235 Cross-linking, 323... 

We can see how this can occur if, as the ring opens, the ends of the diene twist in the same direction ( / or /, conrotatory) as indicated in the equation. You will notice that with this particular case, if conrotation occurs to the left, rather than the right, the same final product results ... 

When a cyclobutene is so constituted that conrotation cannot occur for steric reasons, then the concerted reaction cannot occur easily. Substances that otherwise might be predicted to be highly unstable often turn out to be relatively stable. An example is bicyclo[2.1.0]-2-pentene, which at first sight might seem incapable of isolation because of the possibility of immediate rearrangement to 1,3-cyclopentadjene. This rearrangement does occur, but not so fast as to preclude isolation of the substance ... 

In contrast, the Mobius transition state (one having an odd number of nodes) is formed by conrotation and is favorable with four An) electrons ... 

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

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