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Unformatted text preview: Woodward-Hoffmann Rules for Cycloadditions Stereochemical Course # Electrons Thermal Mode Photochemical Mode 4n + 2 [s + s] [s + a] 4n [s + a] [s + s] 15-2 Coefficients of Frontier Molecular Orbitals FMO Analysis of the "Ortho-Para Rule" and the "Alder Endo Rule" For an in-depth discussion, see I. Fleming, "Frontier Molecular Orbitals and Organic Chemical Reactions," Wiley, 1976, pp. 121-181. OMe CO 2 Me OMe OMe CO 2 Me HOMO HOMO LUMO CO 2 Me LUMO CO 2 Me OMe OMe R ignoring OMe considering OMe considering CO 2 Me ignoring CO vs. OMe overlap maximized OMe OMe etc. O OMe O OMe R MeO • Regioselectivity? "ortho-para rule" • Diastereoselectivity? "Alder endo rule" OMe OMe vs. O OMe secondary orbital interaction Note: changes to coefficients of anti-bonding orbitals are opposite those seen in bonding orbitals. consider orbitals of CO LUMO LUMO resonance hybrid description...
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