Diels-Alder reaction suprafacial geometry

Fora [4 + 2 -7r-electron cycloaddition (Diels-Aldei reaction), let s arbitrarily select the diene LUMO and the alkene HOMO. The symmetries of the two ground-slate orbitals are such that bonding of the terminal lobes can occur with suprafacial geometry (Figure 30.9), so the Diels-Alder reaction takes place readily under thermal conditions. Note that, as with electrocyclic reactions, we need be concerned only with the terminal lobes. For purposes of prediction, interactions among the interior lobes need not be considered. 

In all of the above discussion, we have assumed that a given molecule forms both the new s bonds from the same face of the 7t 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 [7t2s+,i4s] cycloaddition (the subscript n 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. 

The Diels-Alder reaction is a thermal [4 + 2] cycloaddition, which occurs with suprafacial geometry. The stereochemistry of the diene is maintained in the product. 

The Diels-Alder reaction is stereospecific. The diene and dienophile approach suprafacially with respect to both reactants to form the new a bonds, while the original geometry is minimally shifted (Section 5.3.1). Because of this, groups that are cis in the dienophile remain cis in the cyclohexene. The stereochemical relationships in the diene are maintained as well, which limits the number of possible regio- and stereoisomers. The endo/exo ratio can he controlled, often by the use of a catalyst. An example is shown in Equation 8.28 [39]. CWoroaluminate A-l-butylpyridinium chloride (AlCljiBPC) is an ionic liquid which has been used as a substitute for water in these reactions. 

For the Diels-Alder reaction, we will select the HOMO of the diene and the LUMO of the alkene. Therefore, in the diene supplies the electrons. To accommodate the electron pair, the alkene uses ti. Both orbitals are antisymmetric, and the bonding of the terminal lobes occurs if the two molecules are arranged with suprafacial geometry (Figure 25.9). Note that we have to consider the signs of the wave functions only at the terminal lobes. 

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