Ch_322b_13.09

Ch_322b_13.09 - Chapter 13.9 lecture note

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The Diels-Alder Reaction A 1,4-Cycloaddition Reaction of Dienes A very useful cycloaddition addition reaction of 1,3-dienes and alkenes to produce cyclohexenes was discovered in 1928 by Otto Diels and Kurt Alder. They received the Nobel Prize in Chemistry in 1950 for this discovery. C C H H CH 2 CH 2 1,3-butadiene (s- cis conformation) + C C H H C = O C O maleic anhydride O C C H H CH 2 CH 2 C C H H C O C O cycloaddition product a cycloalkene "adduct" O
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In shorthand structures: A 4 + 2 ! -Electron Cycloaddition The Diels-Alder reaction involves a conjugated diene (a 4 ! -electron system) and an alkene (a 2 ! -electron system). The latter is called a dienophile . During the reaction, the two ! -bonds in the diene and the ! -bond of the dienophile are transformed into two new "# bonds and one new ! -bond in the adduct. O + O a diene in s- cis conformation an alkene an "adduct" (a cyclohexene) O O O O + three ! bonds ! ! ! ! " " one new ! bond two new " bonds
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Factors Favoring the Diels-Alder Reaction For many years, it has been known that this cycloaddition reaction is promoted by electron-withdrawing groups in the dienophile and electron-releasing groups in the diene . CH 3 CH 3 + O = H 2,3-dimethyl- 1,3-butadiene propenal 30 o C O H C CH 3 CH 3 (100%) The Diels-Alder reaction is enhanced by high temperature and pressure. + 1,3-butadiene ethene 200 o C pressure cyclohexene
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The Diels-Alder Reaction is enhanced by the presence of Lewis Acids. CH 3 OCH 3 + O = O OH OH AlCl 3 ethyl ether, 25 o C CH 3 O O CH 3 OH OH (80%)
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Many Diels-Alder reactions involve unsymmetrical reactants that could lead to constitutional isomers by way of different orientations of the reactants in the cycloaddition. OEt C Observed 2-Ethoxy-3-cyclohexenecarbaldehyde is produced.
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This note was uploaded on 10/20/2009 for the course CHEM 322BL at USC.

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Ch_322b_13.09 - Chapter 13.9 lecture note

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