Molecular Orbital Analysis of the Diels-Alder Reaction

14 Molecular Orbital Analysis of the DIels-Alder Reaction 

The conventional curved-arrow description of the Diels-Alder reaction shows a one-step mechanism involving a cyclic reorganization of six electrons—four from the diene plus two from the dienophile—in the transition state. 

Such a concerted process is more consistent with the stereochemical observations than one in which the two new a bonds are formed in separate steps. Substituents that are cis in the dienophile remain cis in the product those that are trans in the dienophile remain trans in the product. 

Contrast the Diels-Alder reaction with one that looks similar, the cycloaddition of one ethylene molecule to another to give cyclobutane. 

The LUMO of ethylene and the HOMO of 1,3-butadiene have the proper symmetry to allow cycloaddition. The two molecules approach each other in parallel planes, and electrons flow from the HOMO of 1,3-butadiene to the LUMO of ethylene, a Bonds form when orbitals of the same symmetry (blue-to-blue and red-to-red) overlap. 

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A TT MOLECULAR ORBITAL ANALYSIS OF THE DIELS-ALDER REACTION... 

To derive the frontier molecular orbital analysis for any [3,3]-shift, we ll use 1,5-hexatriene as the model, just as we used butadiene and ethylene as models for the frontier molecular orbital analysis of all Diels-Alder reactions. As always, in a frontier molecular orbital analysis, we first identify a proposed geometry for the reaction. Let s propose a chairlike transition state in which the carbons on the ends of the chain react from the top of one ir bond and the bottom of the other. 

The frontier molecular orbital analysis led to the conclusion that butadiene will react with ethylene to give cyclohexene if both butadiene and ethylene collide in a suprafacial manner. However, ethylene will not react with ethylene in an analogous manner. We can extend these conclusions to the reaction of any conjugated diene with any alkene and to any alkene with another alkene. Presented below are several examples and practical considerations of the allowed 4 + 2 reaction (the Diels-Alder reaction). The suprafacial approach of both reactants has important consequences on the stereochemistry of the Diels-Alder reaction. 

PROBLEM 12.59 Here are two reactions of allyl systems that are related to the Diels-Alder reaction. Use a HOMO-LUMO molecular orbital analysis to show why reaction a succeeds whereas reaction b fails. 

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