Endo and exo addition in a Diels-Alder reaction

A question of regioselectivity arises when both the diene and the alkene are unsymmetrically substituted. Generally, there is a preference for the ortho and para orientations, respectively, as in the examples shown.  

This preference can be rationalized by qualitative molecular orbital considerations that assume that the favored transition state will be that in which there is the strongest interaction between the HOMO of the diene and the LUMO of the 

Diels-Alder cycloadditions are sensitive to steric effects of two major types. Bulky substituents on the dienophile or on the termini of the diene can hinder approach of the two components to each other and decrease the rate of reaction. This can be seen in the relative reactivity of 1-substituted butadienes toward maleic anhydride  

Substitution of hydrogen by methyl results in a slight rate increase, probably as a result of an electronic effect, while a 1-ferf-butyl substituent produces a significant rate decrease. Apparently, any steric retardation to approach of the dienophile by a methyl substituent is insignificant compared to its electronic effect. With the larger rerr-butyl group, the steric effect is dominant. 

The other steric effect has to do with intramolecular van der Wa als repulsions between substituents in the diene. Adoption of the s-cis conformation of the diene in the transition state may be accompanied by an unfavorable repulsion between substituents that do not interact strongly in the ground state. Toward tetracyanoethylene (a very reactive dienophile), rran5-l,3-pentadiene is 10 times more reactive than 4-methyl-1,3-pentadiene because of the unfavorable interaction between the additional methyl substituent and the hydrogen at C-1 in the s-cis conformation. Relatively small substituents at C-2 and C-3 of the diene exert little 

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Figure 7.2. Endo and exo addition in a Diels-Alder reaction.

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