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Outline for Today Review MO construction for Conjugated Systems Discuss Diels-Alder Reaction Chapter 14 Aromaticity Tie in Aromaticity to Diels-Alder Further Reading: Structure and Mechanism in Organic Chemistry By Felix A. Carrol Chapter 4.1-2 MO theory/aromaticity Chapter 11.4 Cycloadditions Lectures available online at http://perceco2.chem.upenn.edu/~percec/classes.html http://perceco2.chem.upenn.edu/~percec/classes.html Until further notice. Slide 2 MOs for Ethene Slide 3 MOs for Allyl Systems Slide 4 MOs for Butadiene Slide 5 MOs for Hexatriene Slide 6 Diels-Alder Reaction: The Basics S-cis diene required, s-trans does not work Concerted reaction Bond made and broken simultaneously Slide 7 A Simple Example: 2+2 Cycloadditons Out of phaseIn Phase Slide 8 Suprafacial vs. Antarafacial Cycloaddition Slide 9 Molecular Orbitals of Diels Alder Cycloaddition Normal Electron Demand/Inverse-Electron Demand/ Thermal Diels-AlderPhotochemical Diels-Alder Slide 10 Normal versus Inverse Electron Demand NormalInverse Slide 11 Woodward Hoffman Rules for Cycloadditons A Reaction occurs when the bonding electrons of a product can be transferred, without a symmetry imposed barrier, to the bonding orbitals of the product. Last Chapter of The Conservation of Orbital Symmetry Exceptions? There are none. Slide 12 Overall Diels Alder Transition State Slide 13 Stereochemistry of Diels-Alder Reactions: Effect of Dienophile Structure Slide 14 Stereochemistry of Diels-Alder Reactions: Effect of Diene Structure Slide 15 Diels Alder Approaches: Regio and Stereochemistry Slide 16 Endo-Selectivity: Secondary Orbital Overlap Slide 17 Controlling Regioselectivity of Diels Alder Reactions Interacting Orbitals Asymmetrically Amplified to create regioselectivit Slide 18 Other types of Diels-Alder Reactions Slide 19 Lecture 2: Aromatic Compounds Chapter 14 in Solomons 9/e Slide 20 History of the Benzene Structure Slide 21 Example of Aromatic Compounds: Motivation Diels-Transition State Fullerenes Slide 22 Brief Note on Benzene Nomenclature Slide 23 Aromatic Stabilization: Resonance Stabilization Slide 24 Benzene Immune to Many Standard De-aromitizing Reactions Slide 25 MO Description of Benzene Note: There is an error in the diagram on page 605 of Solomons. E=- E=-2 E=+2 E=+ Overall stabilization=8, compared to 6 for 3 ethenes or 7 for hexatriene Slide 26 Hckels Rule/Frost Circles 4n+2 electrons = high stabilization due to ideal filling of bonding orbitals More stable than linear polyene equivalents, closed shell configuration Slide 27 4n electron = anti-aromatic 1,3-cyclobutadiene Less stable than linear butadiene open shell configuration Does not exist under non stabilized conditions Slide 28 Aromatic and Nonaromatic Annulenes: Application of 4n and 4n+2 rules Slide 29 Polycyclic Aromatics Slide 30 Benzene NMR Aromatic Ring Currents 1 H NMR for aromatic Hydrogens : 6.0-9.5 ppm 13 C NMR for aromatic Carbon :100-170 ppm Slide 31 The Allotropes of Carbons b,d,e,f,h all aromatic Slide 32 Cylcopentadienyl Cations and Anions Anti Aromatic Aromatic Slide 33 Heterocyclic Aromatics Slide 34 Slide 35 Protonation of Pyrolles and Pyridines Slide 36 Biochemically Relevant Aromatics Amino Acids Slide 37 Biologically Relevant Aromatics NADH NAD+ Nicotinamide adeine dinucleotide, the biolgical hydrogenator Slide 38 Diels-Alder and Hckel Theory Transition state has 6=4n+2 where n=1 electrons, therefore is aromatic and low in energy, despite high entropic cost. Note: If Diels-Alder Substrate is Aromatic to begin with will often not participate.