Conrotation and disrotation

In order to convert cyclobutene into butadiene, the four MOs labelled ct, tt, it and cr must be converted into ij 1, v 2> 3 and v 4. There are two stereochemically distinct ways in which this might be accomplished conrotation and disrotation. As mentioned earlier, the symmetry element preserved during a disrotatory ring closing is the mirror plane (m) and for conrotation it is C2. 

FIGURE 9.5 Conrotation and disrotation of terminal methylene groups in a 5—1,3-butadiene. 

Excitation of 1,3—butadiene photochemicaUy causes excitation of an electron from the HOMO to the LUMO, which has different symmetry. Figure 9.7 shows the LUMO of 1,3—butadiene and how the orbital is affected by both conrotation and disrotation. 

FIGURE 9.6 Symmetry of the HOMO of 1,3-butadiene and the changes that occur during conrotation and disrotation. 

Fig. 14.21. Cyclization of the cis-butadiene. (a) Start from the top coniotation (leftward) and disrotation (rightward) lead in general to different products, but only one of these transformation is symmetry allowed, (b) The doubly occupied n orbitals of the butadiene (p (HOMO-1) and (HOMO), (c) The transformation of >P2 under the conrotation and disrotation. The conrotation is symmetry-allowed, and the disrotation is symmetry-forbidden.

We will now show that closed shell electrocyclization ractions are actual sigma-pi transfer reactions very much analogous to the Diels-Alder transfer reaction and that the conrotation and disrotation transition state complexes are bound electronically in a manner analogous to the antarafacial and suprafacial (or vice versa) transition states of 4N + 2 electron cycloadditions. 

Figure 7. Conrotation and disrotation CH2 = CH - CH complexes. In (a),reactants are converted to excited products, while,in (b),there is smooth conversion of reactants to products. Also compare Figure 7 with Figure 3.

Like Hoffmann s EH calculations, the reaction dynamics calculations predict that double rotation should be favored over single rotation. However, the dynamics calculations predict that double rotation in cyclopropane should consist of a mixture of both conrotation and disrotation, with the former being favored over the latter by a factor of only about two. Unfortunately, the experiments on cyclopropane-rf2 can distinguish single from double rotation but cannot distinguish conrotation from disrotation or from a mixture of these two modes of double rotation. 

In the boat conformations (a)-(c), the sigma overlap of the shaded jr-orbitais at C4 and C5 can only occur if concerted rotations about the C3, C4 and C5, C6 axes take place as indicated. Such rotations, which are of particular significance in discussing electrocyclic reactions, either occur in the same relative direction as in (c), or are opposed as in (a) and (b). These types of concerted rotational motion have been called conrotation and disrotation by Woodward and Hoffmann (1965, 1968, 1969) see Section 3.3(b). 

Electronic factors provide the dominant reasons for the choice between conrotation and disrotation, and we shall consider this topic in detail in Chapter 4. However, when the choice is between the two disrotatory modes (or the two conrotatory modes), the electronic factors can still exert a profound influence, but in a rather subtle manner. Consider, for example, the disrotatory scission of the cyclopropyl cation, a species which appears to have, at best, a very transitory existence. Indeed, in most cases it appears that ring scission occurs in concerted sequence with the departure of the leaving group from the cyclopropane ring. The allyl cation is then the primary product (Equation 3.19). The participation of cr-bonds in concerted ionization-rearrangement processes is well-known, as in the pinacol-pinacolone reaction and the Beckmann rearrangement and the present case is no exception. 

In the case of (i) we can predict, because of our previous findings, that the only distinguishable rotation modes are conrotation and disrotation. Each mode has two formal possibilities, and hence there is a maximum of four possible polyenic products when the substituents a are all different. In reaction (3.25), for example, it is clear that the extrusion of sulphur dioxide is accompanied by the outward disrotatory motion of the methyl groups so as to form tranj,tra s-hexa-2,4-diene. The alternative disrotatory process, in which the methyl groups rotate inward, is of higher energy and would have resulted in the c/s.cis-product. Conversely, the conrotatory motions each would have yielded d5,traHS-hexa-2,4-diene reaction (3.25) is therefore highly stereospecific. 

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. 

You will find a set of handheld models invaluable in helping to visualize the stereochemistry of the reactions in this chapter. Build a model of (2/s,4/s)-hexadiene and examine both conrotation to give trons-3,4-dimethylcyclobutene and disrotation to give cis-3,4-dimethylcyclobutene. 

Build a model of the following compound. Then replace the connector for the bond that is part of both rings with two separate connectors. Do a conrotation and a disrotation to see the strain that is incurred in each process. 

The crux of the puzzle was, however, that only one of the two possible transformations took place, as if one of the rotations was forbidden, but another one allowed. It was even pire, since the allowed rotation was sometimes conrotation and sometimes disrotation, depending which alkene was considered. 

The MOVB diagrams for the conrotation and disrotatlon transition states are shown in Figure 7 and it is evident that the difference between an axis of symmetry (conrotation) and a plane of symmetry (disrotation) is expressed only in the pi MO s of the core. It is easy to see that the former is entirely analogous to that for 4s + 2s and the latter is exactly analogous to that for 4s + 2a cycloadditions (compare Figures 7 and 3). Hence, allyl cation closes to cyclopropyl cation in a disrotatory fashion for the same reason that 1,3-butadiene adds to ethylene in a suprafacial manner. 

Primes with the AO basis of the product are used to denote the fact that the corresponding atomic orbitals x can differ from the AO basis of the reactant, for example because of different spatial orientation. This distinction between the AO bases of the reactant and the product is very important since it is just precisely from here that the possibility arises to exploit the formalism for the discrimination between the forbidden and the allowed mechanisms, i.e., in our case, between the conrotation and the disrotation. The basis of this discrimination are the so-called assigning tables, the physical meaning of which is just in providing the detailed specification of the mutual transformation between bases % and x. which is the necessary prerequisite for the calculation of the overlap integral S p. The same problem was encountered also by Trindle [33], and his mapping analysis failed to find broader use only because of considerable numerical complexity. On the other hand, the overlap determinant method solves this problem much more simply and its use is really a matter of seconds using only pen and paper. 

These were first concerted (pericyclic) reactions to be discovered by Woodward and Hoffmann in 1965 and in them ring opening and ring closure may take place by two modes viz conrotation or disrotation. If both the involved orbitals rotate in the same direction (either clockwise or anticlockwise) it is known as conrotation and when involved orbitals rotate in the opposite directions, i.e., one rotates clockwise and other anticlockwise it is known as disrotation. 

The conrotatory motion involves an antarafacial interaction between the termini, and disrotation involves a suprafacial interaction between these centres. There are two distinct possibilities for each mode of rotation, and therefore four possible products in aU. In many cases, however, the inherent mmetry of the system may not allow such distinction to be made. Even in cases where the two forms of one mode are distinguishable, it does not necessarily follow that both will occur the geometry of the system and the steric factors can be decisive. For example, the conrotatory ting-opening of trans-3,4-dimethylcyclobut-l-ene should, in principle, yield a 1 1 mixture of trans, /nms-hexa-2,4-diene and as, cis-hexa-2,4-diene (Equation 3.17). However, the inward conrotation of the two methyl groups is unfavourable because of the rapid increase in steric compression between these two substituents, and the 78... 

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. ... 

Use orbital drawings to show that conrotation of a triene is thermally forbidden and that disrotation is photochemically forbidden. 

Figure 8.14 Suprafacial disrotation) and antarafacial conrotation) modes in an electrocyclic reaction.

Thermally, it will proceed via a Mobius topology involving one antarafacial component. Both methyl groups will rotate in the same direction conrotation) leaving one endocyclic and one exocyclic. However, under photochemical conditions, a [4n] ir-reaction is predicted to proceed via a Hiickel topology with suprafacial (disrotation) bond formation. 

A worked problem follows and several examples of electrocyclic reactions occur in the problems at the end of the chapter. The important principle is that for 4n electrons, conrotation will give the favoured Mobius transition state, whereas for An+ 2 electrons, disrotation will give the favoured Htickel transition state. 

Let us now consider the interaction of two p orbitals in the construction of a a bond and let us also consider that these two p orbitals are, to begin with, parallel to each other. Two situations, say A and B, arise. In situation A, one p orbital must rotate clockwise and the other anticlockwise to place the lobes of similar signs in a coaxial manner to overlap and result in the desired orbitals rotate in mutually opposite directions and the latter rotation is known as conrotation for the two orbitals rotating in the same direction. Whereas mirror plane symmetry is maintained during disrotation, C2 symmetry is retained during conrotation maintains. Incidentally, a bonding bond orbital is symmetric to both the mirror plane and the C2 axis. 

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