Week_6_PurpleBook2007

Week_6_PurpleBook2007 - 141 Acidity of Carbonyl Compounds...

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R OR O O H H RO OR O O H H R CH 2 R O RO 2 R O NC CH 2 R Acidity of Carbonyl Compounds 1. All of the following compounds have acdic C—H bonds. The pKa of each compound is indicated: Compound pKa 11 13 20 25 31 Conjugate Base O O With each of the above compounds: • identify the most acidic proton of each compound • draw each conjugate base • use resonance structures to show stabilization of the conjugate base • rationalize the acidity order shown • predict and explain the order of nucleophilicity of the conjugate bases 3. The following molecules are both ketones, and are structurally similar. However, they display strikingly different basicity. Explain this observation. pKa = 20 pKa = 40 2. Provide reasonable curved-arrow mechanisms for keto-enol tautomerization in acid and in base: O OH O H 3 O + OH Week 6 141
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O O H O O OCH 3 H O O OCH 3 Controlling Enolate Formation 1. Each of the following enolates can be formed from a specific carbonyl compound. Write the structure of the carbonyl precursor which could be used, and give specific conditions to form the enolate. Week 6 142
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TsO OTs CN Simple Enolate Mechanisms 1. Provide a curved-arrow mechanism for each of the following transformations. O O (hint: CN is also acting as a base) O O Br Br 2 trace H + NaCN Week 6 143
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Simple Enolate Reactions 1. 2. 1. Each of the following transformations can be carried out in one or two steps. Fill in the reagents required for each step. Be specific! If a second step is not needed, please put an "X" through the second box. 2. 2. O O O O CH 3 O 3 Br OCH 3 O Br OCH 3 O 3 O C N C N 2. H OH 2. O Week 6 144
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Stereochemistry of Enolate Formation and Alkylation 1. Provide a mechanism which explains why only one H is replaced with a D. O Me Me NaOD, D 2 O O Me Me D 2. Explain why only one of the three possible products below is observed: O 1. LDA (1.1 eq.) 2. MeI O O O yes no no Me H Me H Me Week 6 145
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O H H O O Basic Aldol and Claisen Mechanisms H 2 CO + H 2 O work-up MeO O 1. Predict the product, and provide a curved-arrow mechanism for each of the following transformations. O OMe LDA (0.9 equiv.) –78°C NaOMe NaOMe Week 6 146
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