lecture 30 - 30 Orbitals and Organic Chemistry Pericyclic...

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30. Orbitals and Organic Chemistry: Pericyclic Reactions Based on McMurry’s Organic Chemistry , 7 th edition
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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 Why this chapter? To gain a better understanding of pericyclic reactions Understanding biological pathways where these reactions do occur
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3 30.1 Molecular Orbitals and Pericyclic Reactions of Conjugated 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
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4 1,3,5-Hexatriene Three double bonds and six MOs Only bonding orbitals, 1 , 2 , and 3 , are filled in the ground state On irradiation with ultraviolet light an electron is promoted from 3 to the lowest-energy unfilled orbital ( 4 *) This is the first (lowest energy) excited state See the diagram in Figure 30.2
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5 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 are 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
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