1 Chapter 15: Conjugation and Reactions of Dienes I. Introduction to Conjugation There are several possible arrangements for a molecule which contains two double bonds (diene): 1. Isolated: (two or more single bonds between them) 2. Conjugated: (one single bond between them) 3. Cumulated: (zero single bonds between them. These are also known as allenes.) We now need to determine which type of diene is the most stable II. Stabilities of Alkenes 1) Molecules with just one double bond: Recall that heat of hydrogenation data showed us that di-substituted double bonds are more stable than mono-substituted double bonds. 2) Molecules with multiple double bonds: If we have two isolated double bonds the heat of hydrogenation is essentially to the sum of the values for the individual double bonds. However, conjugated dienes have heats of hydrogenation that are than the sum of the individual double bonds. The question we need to answer is why this is true.
2 Allenes, which have cumulated double bonds are found to be less stable than isolated double bonds. Increasing Stability Order (least to most stable)
3 III. MO Picture of Conjugated Systems A. Orbital Diagram of 1,3-butadiene. B. MO s of Ethene Let s revisit ethylene (aka ethene) first
4 C. What about 1,3-butadiene? We will assume that the molecule is linear for MO treatment (easier to draw!) D. Interconversion of the Diene (s = single bond isomer) IV. Allylic Resonance and Stability Allyl Structure:
5 A. Allylic Radicals Allylic radicals (radicals on an allylic position) are stabilized by resonance: Because of resonance stabilization, allylic radicals are more stable than 3 o radicals (as shown by the trend in bond dissociation energies):
6 B. Allylic Carbocations Allylic carbocations are also resonance stabilized: General allylic stability of carbocations Stability of 1 allylic 2 carbocation. Stability of 2 allylic 3 carbocation. V. Reactions A. Selective Bromination at Allylic Positions (Allylic Bromination) Observed reaction: Mechanism: Free radical chain mechanism (see chapter 4 for review)
7 NBS provides a low concentration of Br 2, doesn t allow reaction with alkenes. Reaction and Regioselectivity: both carbons of a resonance delocalized allylic radical can react with Br 2, leading to regioisomers: Regiochemistry: pi bonds will be preferred when there is a choice of locations for forming a pi bond:
8 B. 1,2 and 1,4 Addition to Conjugated Dienes Addition to conjugated dienes can produce more than one product: Which product from above is more stable?
9 Energy Diagrams of 1,4 and 1,2 Additions
10 C. The Diels-Alder Reaction This is one of the most powerful reactions in organic chemistry, and is especially useful for creating 6 membered rings. The Diels-Alder reaction is also called a [4+2] cycloaddition. 1. Mechanism: The mechanism of a Diels-Alder is a simultaneous cyclic movement of six electrons. The reaction is concerted, with bonds breaking and forming at the same time. The geometry required for the concerted process must allow the p-orbitals at the ends of the diene to overlap with the p-orbitals of the dienophile.
11 2. Stereochemistry Rule 1: The s-cis conformation is required so that the end p-orbitals can still overlap with the p-orbitals on the dienophile. As expected, molecules with groups that disfavor the s-cis conformation will react slower, and dienes unable to attain the s-cis conformation will not undergo Diels Alder reactions at all. Cyclopentadiene, which has the diene fixed in the s-cis formation, is a very reactive diene for DA reactions (we will use this in our 226 lab!). Rule 2: The dienophile adds to one face of the diene, and the concerted reaction mechanism does not allow for any substituents to change their stereochemistry. (cis = cis, and trans = trans)
12 Rule 3: Normally the largest substituent goes into the exo position, because that is the sterically least demanding position. NOT THE CASE HERE: when a dienophile has a bond as a substituent on the dienophilic multiple bond, there is secondary orbital overlap, which results in the endo addition preferred (~90%). Position of the diene/ dienophile This is because the p orbitals in the electron withdrawing group interact with the p orbitals on the central C2 and C3 p orbitals of the diene. This secondary overlap lowers the energy of the TS and so is favored. Example 1:
13 Example2: Dienophile with cis-substituents that are the same. Example 3: Dienophile with trans-substituents that are the same.
14 What if the diene has electron donating substituents? Then we have more to consider. UGHHHHHH!!!!! Often it is observed that single (mixture of enantiomers) products result from DA reactions even with unsymmetrically substituted starting materials. This can be explained by the hypothetical visualization of the reaction proceeding step wise as opposed to synchronously. (This not true there are no charged intermediates). Case 1:
15 Case 2: