Diels-Alder cycloaddition reaction orbital symmetry control

According to frontier molecular orbital theory (FMO), the reactivity, regio-chemistry and stereochemistry of the Diels-Alder reaction are controlled by the suprafacial in phase interaction of the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied molecular orbital (LUMO) of the other. [17e, 41-43, 64] These orbitals are the closest in energy Scheme 1.14 illustrates the two dominant orbital interactions of a symmetry-allowed Diels-Alder cycloaddition. 

Cycloaddition reactions are also orbital symmetry-controlled, pericyclic reactions. We have seen one example already, the Diels-v lder reaction, and we will use it as our prototype. We found the Diels-Alder cycloaddition to be a thermal process that takes place in a concerted (one-step) fashion, passing over a cyclic transition state. Several stereochemical labeling experiments were described in Chapter 12 (p. 549), all of which showed that the reaction involved neither diradical nor polar intermediates. This stereospecificity is important because orbital symmetry considerations apply only to concerted reactions. Of course, all reactions can be subdivided into series of single-step, single-barrier processes, and each of these steps could be... 

Although most of this section will deal with rearrangements of neutral species, we may see a few anions and cations—the key common feature of this group of reactions is that, like Diels Alder and other cycloaddition processes, they are controlled by orbital symmetry. 

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