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lec_15_16_17_06 - Woodward-Hoffmann Rules for...

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15 16 17 Massachusetts Institute of Technology Organic Chemistry 5.13 October 16, 18, and 20, 2006 Prof. Timothy F. Jamison Notes for Lectures 15, 16, and 17 Cycloaddition Reactions Molecule of the Day cantharidin Cantharidin is a powerful irritant isolated from Cantharis vesicatoria , a.k.a. the "blistering beetle" or "Spanish fly", and is found in the eponymous, alleged aphrodisiac. Several chemistry research groups have prepared cantharidin by total synthesis, and in many of these a Diels-Alder reaction was a key step. O Me Me O O O Cycloaddition: A pericyclic reaction in which 2 separate conjugated, overlapping arrays of orbitals combine . Cycloadditions proceed by way of a cyclic transition state , and 2 sigma bonds are formed during the course of the reaction. A suprafacial process ("s" in the table below) is one in which the bonds made or broken lie on the same face of the orbital array undergoing reaction. In an antarafacial process ("a"), the newly formed or broken bonds lie on opposite faces
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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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