Conrotatory ring closing

In contrast, in conrotatory ring-closing 4.34 —> 4.35, one of the outer substituents and one of the inner substituents, both labelled R, rise to become cis, so that the bottom lobe of the p-orbital at one end forms a o-bond by overlap with the top lobe of the p-orbital at the other end. The rotations are now in the same sense, either both clockwise or both anticlockwise. It follows that the outer substituents become trans to each other on cyclization. In the ring-opening, 4.35 — 4.34, the two substituents that are cis to each other move in the same direction, one to an outer position and the other to an inner position by clockwise rotations, as drawn here. Alternatively, of course, they could both move by anticlockwise rotations. The rules for which stereochemistry is followed by which system are these ... 

Thus, conrotatory ring closing of butadiene to cyclobutene is symmetry allowed under thermal conditions. However, under photochemical conditions the LUMO becomes HOMO and the disrotatory ring opening is symmetry allowed (Fig. 8.48). 

FIGURE 18.3 Thermal ring closing of a 1,3-diene. Conrotatory motion is required for two + lobes to overlap. 

We may also look at this reaction from the opposite direction (ring closing). For this direction the rule is that those lobes of orbitals that overlap (in the HOMO) must be of the same sign. For thermal cyclization of butadienes, this requires conrotatory motion (Fig. 18.3). In the photochemical process the HOMO is the %3 orbital, so that disrotatory motion is required for lobes of the same sign to overlap. 

For the 1,2-dimethylenecyclobutane rearrangement, Gajewski and Shih 53> have demonstrated preferred conrotatory ring opening and closing. For the allene-allene cycloaddition, then, the sequence would be disrotatory motion as two allenes approach to form the perpendicular biallylene intermediate, followed by conrotatory closure of that species. 

However, the HOMO of butadiene is 2 (in the ground state), in which the orbital lobes (terminal) that overlap to make the new a-bond have the opposite phase (sign of the wave function). Thus, in this case, the new a-bond between these two terminal orbital lobes cannot be formed by the disrotation. Thus, disrotatory ring closing of a diene to cyclobutene is thermally forbidden. If the terminal orbital lobes of the HOMO of butadiene were to close, it would be in a conrotatory fashion (Fig. 8.47). 

As we saw on p. 1210, in this method we choose a basis set of p orbitals and look for sign inversions in the transition state. Figure 18.4 shows a basis set for a 1,3-diene. It is seen that disrotatory ring closing (Fig. 18.4a) results in overlap of plus lobes only, while in conrotatory closing (Fig. 18.4h) there is one overlap of a plus... 

Under photochemical conditions, the electrocyclic ring closing of butadienes always proceeds by the disrotatory pathway, which is the opposite of the result under thermal conditions. The FMOs make the stereochemical result easy to understand. Under photochemical conditions, an electron is promoted from the HOMO i]ii to the LUMO 1//7, so i//i becomes the HOMO. Molecular orbital i//i has an antibonding interaction between the termini of the 77 system in the conrotatory TS but a bonding interaction between the termini of the 77 system in the disrotatory TS, so the reaction proceeds in a disrotatory fashion. 

Therefore, the application of the Woodward-Hoffmann rules26 indicates the possibility of thermal reaction in a disrotatory mode as well as photochemical reaction in a conrotatory mode. Experimentally, no thermal ring-closing reaction was observed for these derivatives. 

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