310_23 - Organic Organic Lecture Series CH 310 N LECTURE 23...

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Unformatted text preview: Organic Organic Lecture Series CH 310 N LECTURE 23 Textbook Assignment: Chapter 24: Diels Alder 24.6 & 24.7 Homework (for credit): Problem Set – 8 Posted Today’s Topics: The Diels-Alder Rxn Notice & Announcements: EXAM 3- Monday 11/14 7-9 A-L:WEL 1.308, M-Z: WEL 1.316 1 Organic Lecture Series – the conformation of the diene must be s-cis 2 Organic Organic Lecture Series – (2Z,4Z)-2,4-hexadiene is unreactive in DielsAlder reactions because nonbonded interactions prevent it from assuming the planar s-cis conformation 3 Organic Lecture Series reaction is facilitated by a combination of electronwithdrawing substituents on the dienophile and electron-releasing substituents on the diene 200° C pressure + 1,3-Butadiene Ethylene Cyclohexene O O 140° C + 1,3-Butadiene 3-Buten-2-one O O + 2,3-Dimethyl- 3-Buten-2-one 1,3-butadiene 30° C 4 Organic Organic Lecture Series Use these to activate the diene Use these to activate the dienophile 5 Organic Lecture Series – the Diels-Alder reaction is shown in the following way: Cyclohexene products Diene Dienophile Adduct • the special value of D-A reactions are that they (1) form six-membered rings (2) form 2 new C-C bonds at the same time (3) are stereospecific and regioselective 6 Organic Organic Lecture Series Use of Diels-Alder Reaction to synthesize Bicyclic systems 7 Organic Lecture Series – the Diels-Alder reaction can be used to form bicyclic systems room H temperature + 170° C Diene Dienophile H Dicyclopentadiene (endo form) 8 Organic Organic Lecture Series – exo and endo are relative to the double bond derived from the diene the double bond derived from exo (outside) the diene relative to the double endo (inside) bond 9 Organic Lecture Series 10 The Endo Addition Rule Organic Organic Lecture Series – for a Diels-Alder reaction under kinetic control, endo orientation of the dienophile is favored 11 Organic Lecture Series A closer examination of the transition state: Major product: endo isomer 12 Organic Organic Lecture Series A closer examination of the cycloadduct: π bond formed New σ bonds formed 13 Organic Lecture Series Stereochemistry of the Dienophile the configuration of the dienophile is retained cis-alkene gives cis products 14 Organic Stereochemistry of the Diene Organic Lecture Series the configuration of the diene is retained: 15 Stereochemistry of the Diene Organic Lecture Series The diene’s substitutents are turned “inside out” 16 Organic Organic Lecture Series 17 Organic Lecture Series • Mechanism – no evidence for the participation of either radical or ionic intermediates – the Diels-Alder reaction is a pericyclic reaction • Pericyclic reaction: a reaction that takes place in a single step, without intermediates, and involves a cyclic redistribution of bonding electrons 18 Organic Organic Lecture Series Summary 19 Organic Lecture Series Summary 20 Organic Organic Lecture Series Aromatic Transition States • Hückel criteria for aromaticity: the presence of (4n + 2) π electrons in a ring that is planar and fully conjugated: 21 Organic Lecture Series Aromatic Transition States • Just as aromaticity imparts a special stability to certain types of molecules and ions, the presence of (4n + 2) electrons in a cyclic transition state imparts a special stability to these transition states 22 Organic Organic Lecture Series Aromatic Transition States decarboxylation of β-keto acids and β-dicarboxylic acids (Section 17.9) O H O O H C O (A cyclic six-membered transition state) O O + CO 2 O enol of a ketone 23 Organic Lecture Series Exam 3 Cut-Off 24 ...
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