The last important topic in this chapter is “simple ring formation.”

1. The last important topic in this chapter is “simple ring formation.” This involves a completely new type of reaction, developed by two Germans, Otto Diels and Kurt Alder, in the 1930’s. The reaction has come to be known as the Diels-Alder Reaction. WWU -- Chemistry

2. The Diels-Alder Reaction diene + dienophile cyclohexene This is an example of a cycloaddition. = gain of bond order NOTE: = loss of bond order WWU -- Chemistry

3. The Prototype Diels-Alder Reaction Because each of the reactants is a gas, the reaction must be conducted in a sealed tube at elevated pressure and temperature. WWU -- Chemistry

4. Another simple example: Notice that the product still retains the essential structure of a cyclohexene. WWU -- Chemistry

5. Whatever substituents may be attached to the diene or to the dienophile, the same substituents will appear in the product. WWU -- Chemistry

6. … for example: WWU -- Chemistry

7. WWU -- Chemistry

8. Notice the stereospecificity -- we will explore this shortly WWU -- Chemistry

9. This one comes from “PLKE-Micro-3” Notice the stereochemistry of addition -- we will explore that later WWU -- Chemistry

10. Classic Reaction -- The Cracking of Dicyclopentadiene This reaction is reversible -- by heating dicyclopentadiene to 180 - 200 °C, it can be “cracked” to 1,3-cyclopentadiene WWU -- Chemistry

11. OK, so what about the stereospecificity? We know that the diene must be in the s-cis conformation WWU -- Chemistry

12. This conformational requirement leads to a stereospecific interaction with the p-orbitals of the diene and p-orbitals of the dienophile. The interaction of these orbitals is shown on the next slide and through the use of molecular models. WWU -- Chemistry

13. WWU -- Chemistry

14. -Elimination(Section 9.13) A carbene is an example of divalent carbon. Any strong base, not just NaOH, can be used to remove the H and the X. WWU -- Chemistry

15. Example of -Elimination WWU -- Chemistry

16. Another Example C4H9Li is butyllithium(see Chapter 15). It is a very strongly basic substance. WWU -- Chemistry

17. Carbenes can also be generated by photochemical processes. WWU -- Chemistry

18. Now that we have carbenes, what can we do with them? Carbenes are able to add across the double bond of an alkene to form cyclopropanes (Section 14.12) WWU -- Chemistry

19. Example This is another example of a simple ring formation. WWU -- Chemistry

20. Example from “PLKE-Micro-3” Dichlorocarbene is never isolated; it reacts with cyclohexene as soon as it is formed. WWU -- Chemistry