Diels-Alder reactions suprafacial overlap

Likewise with the dienophile the maleate and fumarate esters 2.90 and 2,92 react with butadiene to give diastereoisomeric adducts 2.91 and 2.93, in which the substituents retain, as a consequence of the suprafacial nature of the developing overlap, the cis and trans relationships they had in the dienophiles. Diels-Alder reactions are much used in organic synthesis, not only because two new C-C bonds are made in one step, but also because the relative stereochemistry of up to four new stereogenic centres is predictable. 

In all of the above discussion we have assumed that a given molecule forms both the new ct bonds from the same face of the n system. This manner of bond formation, called suprafacial, is certainly most reasonable and almost always takes place. The subscript s is used to designate this geometry, and a normal Diels-Alder reaction would be called a [ 2s + 4J-cycloaddition (the subscript 71 indicates that n electrons are involved in the cycloaddition). However, we can conceive of another approach in which the newly forming bonds of the diene lie on opposite faces of the n system, that is, they point in opposite directions. This type of orientation of the newly formed bonds is called antarafacial, and the reaction would be a [ 2 + 4a]-cycloaddition (a stands for antarafacial). We can easily show by the frontier-orbital method that this reaction (and consequently the reverse ring-opening reactions) are thermally forbidden and photoche-mically allowed. Thus in order for a [fZs + -reaction to proceed, overlap between the highest occupied n orbital of the alkene and the lowest unoccupied 71 orbital of the diene would have to occur as shown in Fig. 15.10, with a + lobe... 

Some cycloadditions proceed thermally, whereas others require hv. The dependence of certain cycloadditions on the presence of light can be explained by examining interactions between the MOs of the two reacting components. Frontier MO theory suggests that the rate of cycloadditions is determined by the strength of the interaction of the HOMO of one component with the LUMO of the other. In normal electron-demand Diels-Alder reactions, HOMOdiene ( Ai) interacts with LUMOdienophiie (< >i ) There is positive overlap between the orbitals where the two cr bonds form when both components of the reaction react from the same face of the tt system (suprafacially). 

Frontier orbital analysis of a [4 -I- 2] cycloaddition reaction shows that overlap of in-phase orbitals to form the two new a bonds requires suprafacial orbital overlap (Figure 29.5). This is tme whether we use the LUMO of the dienophile (a system with one TT bond Figure 29.1) and the HOMO of the diene (a system with two conjugated rr bonds Figure 29.2) or the HOMO of the dienophile and the LUMO of the diene. Now we can understand why Diels-Alder reactions occur with relative ease (Section 8.8). 

If butadiene and an appropriately substituted ethylene approach and begin to overlap as in Equation 5.2, there is a favorable phase relationship using the HOMO of the diene and the LUMO of ethylene (the frontier molecular orbitals) for a face-to-face joining. This is, of course, the familiar Diels-Alder reaction, and it is thermally allowed. With respect to both components of the reaction, the reaction occurs from the same face, termed suprafacial addition. Because there are four % electrons in butadiene and two % electrons in ethylene, the Diels-Alder reaction is named as a [ 4 -i- 2 ] reaction. The stereochemical consequences of this approach are further illustrated in Section 8.6. 

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