Chem 216.experiment3-Diels-Alder

Chem 216.experiment3-Diels-Alder - Experiment 3 Endo- vs....

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Experiment 3 Endo- vs. Exo- Stereochemistry in the Diels-Alder Reaction Adapted from J. H. Cooley and R. V. Williams, " endo- and exo -Stereochemistry in the Diels- Alder Reaction: Kinetic versus Thermodynamic Control," J. Chem. Ed. 1997 , 74 , 582-584. The Diels-Alder Reaction The Diels-Alder is named for German chemists Otto Diels and Kurt Alder, who discovered the reaction of alkenes with 1,3-dienes in 1928. In the Diels-Alder reaction the alkene (dienophile) reacts with a 1,3-diene to give a cyclohexene ring. The reaction has become a very powerful method to prepare 6-membered rings and bicyclic molecules. Diels and Alder were awarded the Nobel Prize in 1950 for their pioneering work on this reaction. 1,3-diene dienophile heat The Diels-Alder reaction is an example of a pericyclic reaction, which is a concerted reaction that occurs through a cyclic transition state. There are three major classes of pericyclic reactions: sigmatropic rearrangements, electrocyclic reactions, and cycloaddition reactions. The Diels- Alder is an example of a cycloaddition reaction. Because pericyclic reactions involve cyclic arrays of electrons, there is a similar preference for numbers of electrons to that seen in conjugated cyclic molecules. Therefore, reactions that involve 4n+2 (i.e. an "aromatic" transition state) electrons in the transition state will occur under thermal (heating) conditions. In contrast, reactions involving 4n electrons, cannot be promoted by heating. Often these reactions can be promoted with light, however. O sigmatripoc rearrangemnt O electrocyclic reaction cycloaddition Reaction Because these reactions are concerted, they occur with retention of stereochemistry. For example, the reaction of fumaric acid with 1,3-butadiene would give trans -cyclohex-4-ene 1,2- dicarboxylic acid, while the reaction with maleic acid would give the cis -stereoisomer. Therefore, the alkene stereochemistry is retained in the product. HO 2 C CO 2 H Heat CO 2 H CO 2 H CO 2 H CO 2 H Heat CO 2 H CO 2 H
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Diels-Alder Experiment: 2 of 7 Prepared by Kevin H. Shaughnessy The University of Alabama The stereochemistry becomes even more complex if the 1,3-diene is part of a cyclic molecule. In that case, there are two ways by which the dienophile can approach the diene. Approach with the dienophile substituents under the diene is referred to as the endo -transition state, which leads to the endo -product. If the alkene substituents point away from the diene, then the exo -product is formed. Often there is a preference for either the endo - or exo -product depending on the reaction conditions, steric interactions between the molecules, and electronic factors. H CO 2 H CO 2 H CO 2 H CO 2 H H CO 2 H HO 2 C HO 2 C H H CO 2 H exo -transition state -product endo -transition state -product In this experiment we will study the preference for endo or exo -product formation in the Diels-Alder reaction of furan and N -phenylmaleimide as a function of temperature. Often reactions can show difference preferences as a function of temperature when the products represent a kinetically preferred product and a thermodynamically preferred product.
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Chem 216.experiment3-Diels-Alder - Experiment 3 Endo- vs....

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