Electrocyclic reactions disrotation

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. 

Explain whether each of these electrocyclic reactions proceeds by conrotation or disrotation. 

Disrotation (Section 22.1) Rotation of orbitals in opposite directions in an electrocyclic reaction. 

In electrocyclic reactions, the suprafacial mode involves each component of the new CT-bond being formed from the same face of the reactant Tr-system (this is equivalent to disrotation). The antarafacial mode involves twisting of the orbitals so that the two components of the new a-bond form from the opposite face of the reactant TT-system (this is equivalent to conrotation) (Fig. 8.14). 

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

The easiest way to rationalize the stereospecificity in electrocyclic reactions is by examining the symmetry of the HOMO of the open (non-cyclic) molecule, regardless of whether it is the reactant or the product. For example, the HOMO of hexatriene is 3, in which orbital lobes (terminal) that overlap to make the new a-bond have the same phase (sign of the wave function). Thus, in this case, the new cr-bond between these two terminal orbital lobes can be formed only by the disrotation suprafacial overlap) (Fig. 8.45). If the terminal orbital lobes of HOMO of hexatriene were to close in a conrotatory antarafacial overlap) fashion, an antibonding interaction would result. 

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. 

The interconversion B C is 10-electron electrocyclic reaction thermal disrotation). 

This is 6-electron electrocyclic reaction which proceeds by disrotatory motion under thermal conditions. Geometry of II presents no steric hinderance in disrotation. 

Electrocyclization reactions are equilibria, and it is important to bear in mind that the selection rules do not allow a prediction to be made of the position of the equilibrium. Where the substrate is chiral, there will be two possibilities either for conrotation or for disrotation, each of which is allowed by the selection mles. Whether one mode of electrocyclization will be favored over the other will depend on the difference in activation barriers for the two processes. 

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). 

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. 

The former now appear to be well characterized, but the [8 + 2] reactions are comparatively rare and the known examples are ambiguous. An [S + 2] cyclo-addition is possibly involved in reaction (6.72). The initial adduct, shown to be present at lower temperatures, underwent a further electrocyclic transformation with disrotation. Again, however, the available evidence does not rule out a 1,4-dipolar intermediate (cf. S3). 

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