CHEM 3332 1st Edition Lecture 9Outline of Last Lecture Types of Dienes:Nomenclature: review alkenes – diene (cis/trans/EZ)Preparation of compound diene: elimination of HX from allylic halideStability of Conjugated Dienes is slightly more stable1.2.3.4.Explanation of Stability:AllylCation:C+ Stability:How does conjugation affect reactivity?Mechanism of addition of HBRRatio of products: At Low temperature: less stable alkeneAt High temperature: more stable alkeneThe process above is an example of Rate (Kinetic) versus Equilibriu, (Thermodynamic) Control. Chart:Chart:Outline of Current Lecture Allylic BrominationConsider allylic bromination in an asymmetric alkeneSN2 Displacement Reactions of Allylic Halides (and Tosylates)A.B.A.B.Example:The Diels-Alder Reaction: a [4+2] cycloaddition and a Nobel Prize winning reactionMechanism:The dienophile:1. Must have at least one good electron withdrawing group to be reactive (electron withdrawal by resonance is best)Example:Other dienophiles:2. Stereochemistry of the dienophile is maintainedThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.a. Exampleb. Example 3. The Endo Rule: The electron withdrawing substituent of the dienophile prefers to occupy the endo position on the new ring systemExample:a. Exampleb. ExampleExplanation One more “endo” example:The Diene1. Electron donating groups enhance reactivity, but are not required. Examples – alkyl groups (RO-)2. Conformtion:3. Translating the stereochemistry of the diene to the product:Diels-Alder Reactions Using the Unsymmetrical ReagentsTwo possible orientation for reagents in the transition state:Example 1:Example 2:Explantion: Stability of imaginary intermediate that results fro, the imaginary electron flow from donating to withdrawing group or the push = pull mechanism.1.2.3.Current
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