darxn - Diels-Alder Reaction (a very important reaction)...

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Reaction between a conjugated diene and an alkene (dienophile) to give a cyclohexene Diene Dienophile cyclohexene The Diels-Alder reaction is favored by electron withdrawing groups on the dienophile and electron donating groups on the diene. Good Dienophiles: O O O O O H O R O OR O H H H H ethylene (unreactive) conjugated carbonyls (aldehydes, ketones and esters) C N CO 2 R Mechanism: Pericyclic Reaction- proceeds in a single step via an "aromatic" transition state. Diels-Alder Transition State = Benzene The diene must adopt an s-cis conformation to be reactive: s-cis (reactive conformation) s-trans (unreactive conformation) Endo vs. Exo Transition State: Generally, the endo transition state is favored. H H H H exo endo minor major Stereochemistry: In pericyclic reactions, the stereochemistry of the reactants is preserved in the product. Recall the cylcopropanation of alkenes by carbenes which is also a pericyclic reaction.
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darxn - Diels-Alder Reaction (a very important reaction)...

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