Orbital symmetry requirements Diels-Alder reaction

The addition of alkenes to dienes is a very useful method for the formation of six-membered carbocyclic rings. The reaction is known as the Diels-Alder reaction. The concerted nature of the mechanism was generally agreed on and the stereospecificity of the reaction was firmly established even before the importance of orbital symmetry was recognized. In the terminology of orbital-symmetry classification, the Diels-Alder reaction is a [ 4,+ 2 ] cycloaddition, an allowed process. The stereochemistry of both the diene and the alkene (the alkene is often called the dienophile) is retained in the cyclization process. The transition state for addition requires the diene to adopt the s-cis conformation. The diene and alkene approach... 

The penultimate item in Table 6.5 is, of course, the Diels-Alder reaction, discussed earlier in Chapter 4. As pointed out there, the process is considered to be concerted and the stereochemistry in the diene and dienophile (here taken as the alkene) is retained. In expanding upon those themes, it is now worthwhile examining substituted dienes and dienophiles. However, in doing so, the simplifying assumption will be made that despite the perturbations introduced by substituents (e.g., overall symmetry may be destroyed), the principles requiring the conservation of orbital symmetry are maintaned and the analyses are similar. Consider the substitution of an electron acceptor (A) on the alkene and an electron donor (D) on the diene (Scheme 6.51). 

Most of the ene reactions are concerted and orbital symmetry allowed processes involving all suprafacial transition states of 6e (4jt and 2endo orientations of the electron withdrawing group as depicted in (Fig. 5.1). The addition of the ene to the enophile is stereospecific syn. The TS requires higher activation energy compared to that of Diels-Alder reaction because two cr-electrons of the allylic CT-bond are involved instead of a rr-bond of a diene. 

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