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GSU CHEM 3410 - Introduction to Cumulated Dienes
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CHEM 3410 1st Edition Lecture 3Outline of Current LectureI. Introduction to Cumulated DienesII. Kinetic vs Thermodynamic Control PracticeIII. Diels-Alder Reactions or “Dies-Alder Cycloadditions”A.Definition B.Requirements of the DienophileC.Requirement of the Diene D. SummaryE. Example ReactionsOutline of Last LectureI. Electrophilic Addition to Conjugated Dienes II. Kinetic vs Thermodynamic ControlIII. Steric Hindrance RecapCurrent LectureI. Introduction to Cumulated DienesIn the cumulated diene, we have asp hybridized carbon joined to two sp2 carbons, forming a chiral carbon though it does not have a chiral center. That center carbon be displayed by a dot inmost representations. We have pi bonds, but they only stretch across three atoms. These are cumulated dienes/allenes.Also, the cumulated dienes and nonconjugated dienes are both less stable than the conjugated dienes. The conjugated dienes have the delocalization of electrons, whereas These 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.cumulated dienes and nonconjugated dienes have pi electron density located differently across the molecule.II. Kinetic vs Thermodynamic Control PracticeCOLD IS KINETIC CONDITIONS. HEATED IS THERMODYNAMIC CONDITIONSIn this example, notice the heated conditions. Using heat, we are providing the reaction with enough energy to cross the higher energy threshold and give us the more stable product; the thermodynamic product.1,2 is mono substituted, 1,4 is trisubsituited. The 1,4 product is morehighly substituted, so it is more stable.The 1,4 product is our thermodynamic product in this reaction, and it is our major product.Notice that this reaction is completed in cold conditions. By freezing the reaction, we have removed energy, so there is not enough energy for resonance and to cross that higher energy threshold. Your major product will be your kinetic, 1,2 product. It is the fastest to form because it requires no arrangement and can form with little effort. IN FREEZING CONDITIONS, YOUR MAJOR PRODUCT WILL ALWAYS BE YOUR 1,2 PRODUCT.III. Diels –Alder Reactions or Diels-Alder CycloadditionsIII.A. Definition:The concerted reaction between a conjugated diene and an alkene (or other dienophile) to turn them into a single product. This product will always contain a cyclohexane ring. They are addition reactions that end in a cyclic product.The conditions will most often be Toluene/ Heat or /Toluene, Heat.III.B. Requirements of the Dienophileo The dienophile can be an alkene or alkyne.o The dienophile must contain an electron withdrawing group.Neutral or dienophiles without electron withdrawing group will not react under Diels-Alder conditions. Anelectron withdrawing group causes a partial positive charge on the dienophile, allowing it to react. In this example, it is the carbonyl.Withdrawing group Examples:Not reactive:These have no partially positive areas, thus they have no electron withdrawing groups. They would not react in a Diels-Alder Reaction. Reactive:These reactive dienophiles can distribute partial positive charge in a resonance-like manner. The partial positive rests on the terminal alkene because of this distribution. If you forget where the partial positive charge should be, draw the resonance. The carbon- carbon bond on the terminal alkene of the dienophile is not itself charged, but it is in conjugated with a bond that is a polar bond. Therefore, the partial charge can be distributed across that framework.This is how the reaction would play out. There is a cyclic movement of electrons from the dienophile to the diene. In this reaction, we are turning pi bonds into the sigma bonds between the cyclic carbons. The longest arrows shown below show the formation of sigma bonds from the pi bonds.The figures have been labeled to show the locations of the attachments.Not that this is an addition reaction, and addition reactions remove degrees of unsaturation (pi bonds) and to remove turn two reactants into two products.Diene Dienophile Cyclic ProductIII.C. Requirements of the Dieneo The diene must be able to adopt the S-cis conformation. Recall the conformation refers to not a cis and trans double bond (that’s stereochemistry), it refers to the rotation around a single bond. A conformational change is the difference between a 180 degree dihedral and a 120 degree dihedral. The difference is in the rotation around a single bond. Recall the Newman projections of last semester.o Steric hindrance may prevent changes in conformationIf the S-trans molecule is able to convert to S-cis, it can react because this molecule has free rotation.Reactive:Furthermore, remember that Diels-Alder reactions are concerted. It must happen all at once. The reaction cannot occur all at once if the electrons must travel a considerably longer distance in order to react with the second carbon of the dienophile. That is why S-trans dienes cannot react in a Diels-Alder reaction. Some moleculeswill be unable to adopt an s-cis conformation because they are LOCKED into acyclic relationship.The end are basically stapled together, eliminating free rotation.Another issue that a molecule may face will be steric. Some molecules will not have free rotation because of steric repulsion. They will be unable to rotate to S-cis, thus they will be unable to participate in Diels-Alder Reactions.III.D. SummaryRequirements for Diel’s Alder Reactions:1. The dienophile must be an alkene or alkyne.2. The dienophile must contain an electron withdrawing group.3. The diene must be able to adopt the S-cis conformation.4. The diene will not be able to adopt the S-cis conformation if it is sterically hindered or locked into conformation by a ring.III.E. Example


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GSU CHEM 3410 - Introduction to Cumulated Dienes

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