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enols and enolate anions

enols and enolate anions - Chem 215 F07 Notes(for 10/17-29...

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Chem 215 F07 Notes (for 10/17-29)– Dr. Masato Koreeda - Page 1 of 20. Date: October 17, 2007 Chapter 17: Enols and Enolate Anions as Nucleophiles Enols: under acidic conditions 1. H-X, H 3 O+ acids, (H 3 C) 3 Si-X: with enolates react at O Enolates: under basic conditions 2. Halogens (Cl 2 ; Br 2 ; I 2 ) 3. O O H enol enolate I. Nucleophiles Electrophiles Rx on O Rx on C R R' O aldehydes/ketones 4. R Z O Z = Cl, OR' 5. R X alkyl, allyl, & benzyl halides with enolates react at cabanion C Rx on C (1) Generation of enolates C H O H H B C O H H (base) C O H H H B carbanion enolate nucleophilic site nucleophilic site ambident nucleophile alpha - ( ! -) carbon alpha - ( ! -) hydrogens Halogens and most of the carbon electrophiles react at the carbanion center. The solution structures of carbanions closely resemble the structures of enolates. In addition, the only reason for the acidity of the α -hydrogen is the presence of a C=O group. Therefore, the following mechanism for the deprotonation of the α -hydrogen by a base is recommended: C H O H H B ! O H H H B + C H O H H C ! C H O H H C vs C H O H H H 3 C BrMg C H O H H H 3 C BrMg not much deprotonation Kinetically, addition to the C=O carbon favored. E O + H 3 CMgBr higher " Ea for deprotonnation lower " Ea for addition OMgBr H 3 C OMgBr + CH 4 (i) H 3 C-MgBr Note: A carbanion addition reaction to a C=O is irreversible .
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Chem 215 F07 Notes (for 10/17-29) – Dr. Masato Koreeda - Page 2 of 20. Date: October 17, 2007 Enolate formation vs carbanion addition (cont’d). (ii) When (H 3 C) 3 C-Li or H 3 CCH 2 CHLi(CH 3 ) is used, enolate formation is greatly favored due to the increased steric energy for the addition. H 3 C C Li H 3 C C H 2 C CH 3 CH 3 H 3 C ! ! tert -butyllithium H Li ! ! sec -butyllithium Deprotonation (or enolate formation) favored. (iii) Commonly used non-nucleophilic bases: Na H (sodium hydride); O C(CH 3 ) 3 (potassium tert -butoxide) Li C(CH 3 ) 3 ( tert -butyllithium) Li N[CH(CH 3 ) 2 ] ( l ithium d iisopropyl a mide; LDA) K Just a base; not a nucleophilic hydride source (i.e., does not reduce a ketone nor aldehyde). ! ! ! ! C H O H H ! O H H + " " " " L ithium d iisopropyl a mide = LDA N Li One of the most frequently used, strong base! N H H 3 C C H 2 H 2 C C H 2 Li " " H 3 C C H 2 H 2 C C H 2 Br " " pKa = 36 P r e p a r a t i o n : 2 Li + LiBr N Li N Li or O tetrahydrofuran (solvent) -78 °C + H 3 C C H 2 H 2 C C H 2 H pKa ~ 50 E n o l a t e f o r m a t i o n : N Li Li N H O tetrahydrofuran (solvent) -78 °C O H H Li
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Chem 215 F07 Notes (for 10/17-29) – Dr. Masato Koreeda - Page 3 of 20. Date: October 17, 2007 II. Reactions of Enols and Enolates (1) Reactions of enolates O H H M O H H M O H H El O H H El O H H El O H H El O H H El Reactions with "slower"- reacting electrophiles. Reactions with "fast"- reacting electrophiles. "transition state" resembles the structure of the starting enolate "transition state" resembles the structure of the more stable, C=O group containing product The transition states for the reactions of enolates with highly reactive electrophiles, e.g., H-X, H 3 O * , and (H 3 C) 3 Si-Cl, closely resemble the structures of the starting enolates. In contrast, the reactions with slower- reacting carbon electrophiles, e.g., R-X and R-C(=O)-Z, go through the transitions states that have an
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