215F07-notes-10-17thru29-07

215F07-notes-10-17thru29-07 - Chem 215 F07 Notes (for...

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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 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 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 l f m : 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"- "transition state" resembles the
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This note was uploaded on 04/01/2008 for the course CHEM 215 taught by Professor Koreeda during the Fall '07 term at University of Michigan.

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215F07-notes-10-17thru29-07 - Chem 215 F07 Notes (for...

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