Diels-Alder reactions symmetry element

The rac-isomers have a twofold axis and therefore C2-symmetry. The meso-isomer has a mirror plane as the symmetry element and therefore Cs-symmetry. For polymerisation reactions the racemic mixture can be used since the two chains produced by the two enantiomers are identical when begin- and end-groups are not considered. Note When catalysts of this type are to be used for asymmetric synthesis, e.g. as Lewis acids in Diels-Alder reactions, separation of the enantiomers is a prerequisite [25],... 

Step 3. Identify any symmetry elements maintained throughout the course of the reaction. There may be more than one. For a Diels-Alder reaction, which we know to be suprafacial on both components 3.11, there is only the one, a plane of symmetry bisecting the bond between C-2 and C-3 of the diene and the Jt-bond of the dienophile. 

The essential features of the Diels-Alder reaction are a four-electron n system and a two-electron it system which interact by a HOMO-LUMO interaction. The Diels-Alder reaction uses a conjugated diene as the four-electron n system and a it bond between two elements as the two-electron component. However, other four-electron it systems could potentially interact widi olefins in a similar fashion to give cycloaddition products. For example, an allyl anion is a four-electron it system whose orbital diagram is shown below. The symmetry of the allyl anion nonbonding HOMO matches that of the olefin LUMO (as does the olefin HOMO and the allyl anion LUMO) thus effective overlap is possible and cycloaddition is allowed. The HOMO-LUMO energy gap determines the rate of reaction, which happens to be relatively slow in this case. 

The Woodward-Hoffmann rules also allow the prediction of the stereochemistry of pericyclic reactions. The Diels-Alder reaction is an example of (re4s + re2s) cycloaddition. The subscript s, meaning suprafacial, indicates that both elements of the addition take place on the same side of the re-system. Addition to opposite sides is termed antarafacial. The Woodward-Hoffmann rules apply only to concerted reactions and are derived from the symmetry properties of the orbitals involved in the transition state. These rules may be summarised as shown in Table 7.1. 

Let us go through the same steps for a symmetry-forbidden reaction, the ln s+A] cycloaddition 6.141. We first draw the reaction and put in the curly arrows—the orbitals are evidently the n and n of each of the n bonds. There are two symmetry elements maintained this time—a plane like that in the Diels-Alder reaction, bisecting the n bonds, but also another between the two reagents, which reflect each other through that plane. 

For these, several chiral Lewis acid catalysts, which have the C-2 symmetry element, were designed and tested for various asymmetric syntheses, and in 1985 we reported a zinc reagent and in 1988 a bulky aluminum reagent (Scheme 8) [15, 16]. The zinc reagent was used for asymmetric cyclization of unsaturated aldehyde and the aluminum catalyst for asymmetric hetero-Diels-Alder reaction with Danishefsky diene. Both catalysts effectively discriminate the enantioface of aldehydes for reactions. 

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