Diels Alder Rxn.docx - Bryanna Tanase John Torres Geoffrey...

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Bryanna Tanase John Torres Geoffrey Gray Diels Alder Reaction: Synthesis of 9,10 dianthroceno-9,10-endo-α,β-succinic anhydride
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Introduction A Diels Alder reaction is a 4+2 addition reaction of a conjugated diene to an alkene (dienophile) to produce a cyclohexene (six membered ring with a double bond) 2 . A conjugated diene is a compound that is made of carbons and hydrogens and has alternating single and double bonds 2 . A simple example of this reaction is the reaction of 1,3 butadiene with ethene to form cyclohexene as shown below. The reaction occurs with the addition of heat 1 . Figure 1: Diels Alder Reaction of 1,3 butadiene with ethene 2 These reactions can also be referred to as cycloaddition because they form a cyclic product by means of a cyclic transition state 2 , and the reaction is concerted, meaning it happens in one step without a carbocation intermediate 2 . A simple mechanism for the reaction is shown below. Figure 2: Simple Diels Alder Mechanism 2 The reaction has a high degree of regio and stereo selectivity because of the concerted mechanism and is used in synthetic organic chemistry 2 to make products such as insecticides and textile finishing agents 5 . Diels Alder reactions are usually thermodynamically favorable due to the formation of one new sigma bond and two new pi bonds 2 . The conditions for this reaction are most favorable when the dienophile (alkene) contains electron withdrawing groups such as a carbonyl or cyano
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group, and the diene (conjugated system) contains electron donating groups such as an alkyl or amino group 1 . The dienophile is acting as the electrophile because it needs electrons, and the diene is the nucleophile because it wants to give its electrons away and be closer to the positively charged nucleus of an atom 2 . Examples of dienes and dienophiles are shown in Figure 3. Figure 3: Examples of Dienes and Dienophiles 2 The stereochemistry required for this reaction to occur is syn with respect to the diene and dienophile because the two new sigma bonds form on the same face of the cyclohexene ring 2 . Syn orientation will allow for the product with the maximum overlap of pi electrons in the transition state to form 1 . An example of syn addition is shown in Figure 4. Figure 4: Example of syn addition 2 Because the substituents on the dienophile are trans (on opposite sides of the double bond) the substituents add to the same side of the cyclohexene ring and are oriented in opposite ways 2 , but if the substituents on the dienophile were cis (on the same side of the double bond) the substituents would add on the same side of the ring and be oriented in the same way 2 .
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Diels Alder Reactions can give stereoisomers based on whether the dienophile lies under or away from the diene in the transition state 2 . When the diene and dienophile are aligned directly over each other, the endo product is formed 2 . In the endo product, the dienophile lies under the diene, and this is the favored stereochemistry for the product because there is a greater
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