Diels-Alder reaction motion

In general, cycloadditions catalyzed by Lewis acids proceed at significantly lower temperatures and with higher selectivities than their uncatalyzed counterparts. Factors that contribute to the increased selectivity of the catalyzed reactions include lower temperatures and more organized transition states. For enthalpy-controlled reactions, lowering temperatures increases selectivity (recall Section 1.4, equation 1.5). Coordination of a Lewis acid to the enone carbonyl not only activates the enone by electron withdrawal, it also restricts conformational motion and thereby reduces the number of competing transition states. Figure 6.12 illustrates several chiral auxiliaries for dienophile modification that have been used in the Diels-Alder reaction. 

Section 24.3.1 noted that the Alder endo rule is applied to acyclic dienes as well as cyclic dienes, but it is more difficult to see the product in the former case. In order to form 21, an endo approach (22) of acrylonitrile to the diene is required. An exo approach used transition state 23, which leads to the other diastereomer, 20. There is disrotatory motion for both endo and exo approaches. When the alkene has a substituent containing a n-bond—particularly, a C=0 unit—endo approach is usually favored. An endo approach of the alkene combined with a disrotatory motion of the groups on the diene leads to formation of one diastereomer as the major product. The Diels-Alder reaction is diastereose-lective. Because the alkene may approach from either the bottom (see Figure 24.4) or the top, the product will he racemic. 

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