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29: Orbitals and Organic Chemistry - Pericyclic Reactions
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29.1: Molecular Orbitals of Conjugated Pi Systems
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HOMO and LUMO are often referred to as frontier orbitals and their energy difference is termed the HOMO–LUMO gap. One common way of thinking about reactions in this way is through the concept of frontier orbitals. This idea says that if one species is going to donate electrons to another in order to form a new bond, then the donated electrons are most likely going to come from the highest occupied energy level.
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29.2: Electrocyclic Reactions
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An electrocyclic reaction is the concerted cyclization of a conjugated π-electron system by converting one π-bond to a ring forming σ-bond. The key sigma bond must be formed at the terminus of a pi system. These reactions classified by the number of pi electrons involved.
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29.3: Stereochemistry of Thermal Electrocyclic Reactions
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Frontier orbital theory can predict the stereochemistry of electrocyclic reactions. Electrons in the HOMO are the highest energy and therefore the most easily moved during a reaction. A molecular orbital diagram can be used to determine the orbital symmetry of a conjugated polyene's HOMO. Thermal reactions utilize the HOMO from the ground-state electron configuration of the molecular orbital diagram while photochemical reactions utilize the HOMO in the excited-state electron configuration.
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29.4: Photochemical Electrocyclic Reactions
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Electron excitation changes the symmetry of the new HOMO which has a corresponding effect on the reaction stereochemistry. Under photochemical reaction conditions conjugated dienes undergo disrotatory cyclization whereas under thermal conditions they underwent conrotatory cyclization. Likewise, conjugated triene undergo conrotatory photochemical cyclization while undergoing disrotatory thermal cyclization.
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29.5: Cycloaddition Reactions
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A concerted combination of two π-electron systems to form a ring of atoms having two new σ bonds and two fewer π bonds is called a cycloaddition reaction. The number of participating π-electrons in each component is given in brackets preceding the name of the reaction. The Diels-Alder reaction is the most useful cycloaddition reaction due to the ubiquity of 6-membered rings and its ability to reliably control stereochemistry in the product.
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29.6: Stereochemistry of Cycloadditions
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Frontier orbital theory can be used to predict if a given cycloaddition will occur with suprafacial or with antarafacial geometry. In a standard Diels-Alder reaction, bonding interactions are created when the electron containing HOMO of the diene donates electrons to the electron vacant LUMO of the other the dienophile. The dienophile has one pi bond, so it will use the pi MOs for a 2 atom system.
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29.7: Sigmatropic Rearrangements
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Molecular rearrangements in which a σ-bonded atom or group, flanked by one or more π-electron systems, shifts to a new location with a corresponding reorganization of the π-bonds are called sigmatropic reactions. The reactant and product have the same number and type of bonds, just different bond locations.
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29.8: Some Examples of Sigmatropic Rearrangements
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29.9: A Summary of Rules for Pericyclic Reactions
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