Chapter30 - Pericyclic Reactions What are they? 30....

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1 30. Orbitals and Organic Chemistry: Pericyclic Reactions Based on McMurry’s Organic Chemistry , 6 th edition ©2003 Ronald Kluger Department of Chemistry University of Toronto Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003 2 Pericyclic Reactions – What are they? ± Involves several simultaneous bond-making breaking process with a cyclic transition state involving delocalized electrons ± The combination of steps is called a concerted process where intermediates are skipped Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003 3 30.1 Molecular Orbitals of Conjugated π Systems ± A conjugated diene or polyene has alternating double and single bonds ± Bonding MOs are lower in energy than the isolated p atomic orbitals and have the fewest nodes ± Antibonding MOs are higher in energy ± See Figure 30.1 for a diagram Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003 4 Molecular orbital principles ± Based on wave mechanics ± Whole molecule has orbitals but π orbitals are highest energy ± Transitions from HOMO to LUMO occur upon absorption of light of appropriate energy ± Energy levels of Molecular Orbitals are below and above atomic orbitals ± Conjugation gives extended orbitals
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2 Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003 5 Cyclic conjugated systems ± Energy levels coincide with Frost’s Circle ± Atomic level is α = nonbonding orbital, Coulomb integral ± Resonance integral provides stabilization ± Circle radius = 2 β ± Draw with vertex down Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003 6 1,3,5-Hexatriene ± Three double bonds and six π MOs ± Only bonding orbitals, ψ 1 , ψ 2 , and ψ 3 , are filled in the ground state ± On irradiation with ultraviolet light an electron is promoted from ψ 3 to the lowest-energy unfilled orbital ( ψ 4 *) ± This is the first (lowest energy) excited state ± See the diagram in Figure 30.2 Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003 7 30.2 Molecular Orbitals and Pericyclic Reactions ± If the symmetries of both reactant and product orbitals match the reaction is said to be symmetry allowed under the Woodward-Hoffmann Rules (these relate the electronic configuration of reactants to the type of pericyclic reaction and its stereochemical imperatives) ± If the symmetries of reactant and product orbitals do not correlate, the reaction is symmetry-disallowed and there no low energy concerted paths ± Fukui’s approach: we need to consider only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), called the frontier orbitals Based on McMurry, Organic Chemistry, Chapter 30, 6th edition, (c) 2003 8 30.3 Electrocyclic Reactions ± These are pericyclic processes that involves the cyclization of a conjugated polyene ± One π bond is broken, the other π bonds change position, a new σ bond is formed, and a cyclic
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This note was uploaded on 04/18/2011 for the course CHM 151 taught by Professor Dong during the Winter '08 term at University of Toronto.

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Chapter30 - Pericyclic Reactions What are they? 30....

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