Koreeda - Chap17-1 - F- "r cl { .. A'.9-H I...

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Unformatted text preview: F- "r cl { .. A'.9-H I enol filr. "-") -G;; oo-) l, .{o- ^ro' enolate .X: (1) Generation of enolates A /-:8" n_H (base) \1^/*rra- (a-) fi / nyarogens A .aV (o /^'v (o ,)[^4" vs ).":t oj-H ", ) j-t H-C:H nucleophilic site (o: ? J!':.e - / -c-H carbanion \1q I alpha- (o-) carbon ambident nucleophile . 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 a-hydrogen is the presence of a C=O group. Therefore, the following mechanism for the deprotonation of the cr-hydrogen by a base is recommended: A / o a-:8" >l<l-H > o\-n H o o I z\r_H \ H + H_B Chem 215 F10 Notes - Dr. Masato Koreeda - Chapter 17: Enols and Enolate Anions I. Nucleophiles Enols: under acidic conditions Enolates: under basic conditions Page 1 of20. Date: October 29,2010 as Nucleophiles Electrophiles 1. H-X, H3O+ acids, (H3C)3Si 2. Halogens (Cl2; Br2; 12) 3.A-orO A, aldehydes/ketones 4.R lO I z= cl. oR' Z 5. R-x alkyl, allyl, a benzyl halides with enolates react at O o with enolates react at cabanion C o o A H_B ?Orenucleophilic site A"__n enolate t I (i) H3C-MgBr higher AEa for deprotonnation not much deprotonation + Note: A carbanion addition reaction to a C=O is irreversible. Kinetically, addition to the C=O carbon favored. lower AEa for addition B.,Mso !J *."b -h BrMg o ""/?aX H ?<x H q HaC + h" HsCMgBr OMgBr Chem 215 F10 Notes - Dr. Masato Koreeda - Page 2 of 20. Enolate formation vs carbanion addition (cont'd). (ii) When (H3C)3C-Li or HTCCHTCHLi(CH,) is used, enolate increased steric energy for the addition. Date: October 29,2010 formation is greatly favored due to the (iii) Commonly used non-nucleophilic bases: Lithium diisopropylamide = LDA Deprotonation (or enolate formation) favored. One of the most frequently used, strong base! so se)- Li-N /- Preparation: pKa = 36 H, e9 {J\_ H3c_c.c16coti H2 H2 -@fri LierJ /- H. H3c-",c16"@zf! + 2Li H2 H2 Enolate formation'. i@a r'r )\T-H *\-* H .'+ tetrahydrofuran (solvent)-78'C pKa - 50 1l\ tt2 u + HgC-C,C-az! H2 H2 ^\- Lie:N: /- or Li- Rl=- ? _N: /- tetrahydrofuran (solvent)-78'C CHO S@ ,."-1"' -r"ljsd:. n.t'i"--fb Hz lert-butyllithium sec-butyllithium r$A Na- H - (sodium hydride); Just a base; not a nucleophilic hydride source (i.e., does not reduce a ketone nor aldehyde). 6)(:) t< - -o - c(cH.). (potass ium t ert-butoxi de) o@ 6o Li - C (C H 3)s Q er t-butylli thi um) o@ 6o Li-NtCH(CH3)212 (lithium diisopropylamide; LDA) )- N: /- oo li@-(-* \ H o l.-g tio ' ]-H H Chem 215 F10 Notes - Dr. Masato Koreeda II. Reactions of Enols and Enolates (1) Reactions of enolates R e action s with " slower" - re acti ng electro ph i I es....
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Koreeda - Chap17-1 - F- "r cl { .. A'.9-H I...

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