Outline for Today

1. 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 Until further notice.

2. MO’s for Ethene

3. MO’s for Allyl Systems

4. MO’s for Butadiene

5. MO’s for Hexatriene

6. Diels-Alder Reaction:The Basics S-cis diene required, s-trans does not work Concerted reaction Bond made and broken simultaneously

7. A Simple Example: 2+2 Cycloadditons Out of phase In Phase

8. Suprafacial vs. AntarafacialCycloaddition

9. Molecular Orbitals of Diels Alder Cycloaddition Normal Electron Demand/ Inverse-Electron Demand/ Thermal Diels-Alder Photochemical Diels-Alder

10. Normal versus Inverse Electron Demand Normal Inverse

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.”

12. Overall Diels Alder Transition State

13. Stereochemistry of Diels-Alder Reactions: Effect of Dienophile Structure

14. Stereochemistry of Diels-Alder Reactions: Effect of Diene Structure

15. Diels Alder Approaches: Regio and Stereochemistry

16. Endo-Selectivity: Secondary Orbital Overlap

17. Controlling Regioselectivity of Diels Alder Reactions Interacting Orbitals Asymmetrically Amplified to create regioselectivit

18. Other types of Diels-Alder Reactions

19. Lecture 2: Aromatic Compounds Chapter 14 in Solomons 9/e

20. History of the Benzene Structure

21. Example of Aromatic Compounds: Motivation Diels-Transition State Fullerenes

22. Brief Note on Benzene Nomenclature

23. Aromatic Stabilization: Resonance Stabilization

24. Benzene Immune to Many Standard De-aromitizing Reactions

25. MO Description of Benzene Note: There is an error in the diagram on page 605 of Solomons. E=α+2β E=α+β E=α-β E=α-2β Overall stabilization=8β, compared to 6β for 3 ethenes or 7β for hexatriene

26. Hückel’s Rule/Frost Circles 4n+2 π – electrons = high stabilization due to ideal filling of bonding orbitals More stable than linear polyene equivalents, closed shell configuration

27. 4n π electron = anti-aromatic 1,3-cyclobutadiene Less stable than linear butadiene – open shell configuration Does not exist under non stabilized conditions

28. Aromatic and Nonaromatic Annulenes: Application of 4n and 4n+2 rules

29. Polycyclic Aromatics

30. Benzene NMR Aromatic Ring Currents 1H NMR for aromatic Hydrogens δ: 6.0-9.5 ppm 13C NMR for aromatic Carbon δ:100-170 ppm

31. The Allotropes of Carbons b,d,e,f,h all aromatic

32. Cylcopentadienyl Cations and Anions Anti Aromatic Aromatic

33. Heterocyclic Aromatics

34. Heterocyclic Aromatics

35. Protonation of Pyrolles and Pyridines

36. Biochemically Relevant Aromatics Amino Acids

37. Biologically Relevant Aromatics Nicotinamide adeine dinucleotide, the biolgical hydrogenator NADH NAD+

38. Diels-Alder and Hückel 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.