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I need help explaing this mechanism  Diels-Alder Reaction between anthracene and Maleic Anhydride

I need help explaing this mechanism  Diels-Alder Reaction between anthracene and Maleic Anhydride

Chemistry
Chemistry
10th Edition
ISBN:9781305957404
Author:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Publisher:Steven S. Zumdahl, Susan A. Zumdahl, Donald J. DeCoste
Chapter1: Chemical Foundations
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I need help explaing this mechanism 

Diels-Alder Reaction between anthracene and Maleic Anhydride

Diels-Alder Reaction Between Anthracene and Maleic Anhydride AICI3 or heat Anthracene Maleic Anhyrdide Diels-Alder Adduct Background. The Diels-Alder reaction is one of the most useful reactions in all of organic chemistry. Usually, an electron-rich diene is reacted with an electron-poor dienophile resulting in the formation of a new six-membered ring. There are no ionic or radical intermediates in this reaction. Instead, bond-breaking and bond-making occurs at the same time; the reaction is said to be concerted. Electrons flow from the Highest Occupied Molecular Orbital (HOMO) of either reactant to the Lowest Unoccupied Molecular Orbital (LUMO) of the other reactant. The smaller the energy gap between the HOMO and LUMO, the less energy is needed to move the electrons from the HOMO to the LUMO, and the faster is the reaction. In fact, the role of electron-withdrawing substituents on the dienophile is to lower the energy of the LUMO, thus decreasing the HOMO-LUMO energy gap, and making the Diels-Alder reaction kinetically feasible. Benzene and related aromatic compounds do not undergo Diels-Alder reactions. However, anthracene, which consists of three fused benzene rings, does undergo Diels- Alder reactions, although the kinetic barrier is quite high. One of the reasons for the high kinetic barrier is the fact that as a result of the pi-bond of one of the anthracene rings reacting, this ring will no longer be aromatic. It has been estimated that at room- temperature, it would take 4,800 hours to react anthracene with the excellent dienophile (low-lying LUMO) maleic anhydride! There are two ways of accelerating such slow Diels-Alder reactions: One possibility is to simply reflux the solution in a solvent with a high boiling point. This is in fact what we will be doing in our laboratory experiment. Another method that has become very important was first discovered by Prof. Eaton (then at Harvard) and coworkers.' A Lewis acid such as AlCl, is added to the reaction mixture. It forms a complex with the dienophile making it much more electron-deficient and lowering the energy of its LUMO, thereby lowering the overall activation energy needed for the reaction. The first Lewis-acid catalyzed Diels-Alder reaction studied was in fact the reaction of anthracene with maleic anhydride. Eaton reported that addition of one to two equivalents of AlCl, led to a complete reaction within minutes at room-temperature instead of the 4,800 hours needed without the catalyst. Since then, many different Lewis- acid catalysts have been used to accelerate Diels-Alder reactions. Much research is still being undertaken with chiral Lewis acids as this may lead to the selective formation of only one stereoisomer.?
Transcribed Image Text:Diels-Alder Reaction Between Anthracene and Maleic Anhydride AICI3 or heat Anthracene Maleic Anhyrdide Diels-Alder Adduct Background. The Diels-Alder reaction is one of the most useful reactions in all of organic chemistry. Usually, an electron-rich diene is reacted with an electron-poor dienophile resulting in the formation of a new six-membered ring. There are no ionic or radical intermediates in this reaction. Instead, bond-breaking and bond-making occurs at the same time; the reaction is said to be concerted. Electrons flow from the Highest Occupied Molecular Orbital (HOMO) of either reactant to the Lowest Unoccupied Molecular Orbital (LUMO) of the other reactant. The smaller the energy gap between the HOMO and LUMO, the less energy is needed to move the electrons from the HOMO to the LUMO, and the faster is the reaction. In fact, the role of electron-withdrawing substituents on the dienophile is to lower the energy of the LUMO, thus decreasing the HOMO-LUMO energy gap, and making the Diels-Alder reaction kinetically feasible. Benzene and related aromatic compounds do not undergo Diels-Alder reactions. However, anthracene, which consists of three fused benzene rings, does undergo Diels- Alder reactions, although the kinetic barrier is quite high. One of the reasons for the high kinetic barrier is the fact that as a result of the pi-bond of one of the anthracene rings reacting, this ring will no longer be aromatic. It has been estimated that at room- temperature, it would take 4,800 hours to react anthracene with the excellent dienophile (low-lying LUMO) maleic anhydride! There are two ways of accelerating such slow Diels-Alder reactions: One possibility is to simply reflux the solution in a solvent with a high boiling point. This is in fact what we will be doing in our laboratory experiment. Another method that has become very important was first discovered by Prof. Eaton (then at Harvard) and coworkers.' A Lewis acid such as AlCl, is added to the reaction mixture. It forms a complex with the dienophile making it much more electron-deficient and lowering the energy of its LUMO, thereby lowering the overall activation energy needed for the reaction. The first Lewis-acid catalyzed Diels-Alder reaction studied was in fact the reaction of anthracene with maleic anhydride. Eaton reported that addition of one to two equivalents of AlCl, led to a complete reaction within minutes at room-temperature instead of the 4,800 hours needed without the catalyst. Since then, many different Lewis- acid catalysts have been used to accelerate Diels-Alder reactions. Much research is still being undertaken with chiral Lewis acids as this may lead to the selective formation of only one stereoisomer.?
Anthracene Maleic Anhydride Xylene 4,10 - di hydro anthracene - 9,10 – «,B - 5uccinic anhydxide
Transcribed Image Text:Anthracene Maleic Anhydride Xylene 4,10 - di hydro anthracene - 9,10 – «,B - 5uccinic anhydxide
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