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Chapter 8 Reduction Reactions: Overview

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1 Chapter 8 Reduction Reactions: Overview
Organic Chemistry of Fine Chemicals Spring 2006 Chapter 8 Reduction Reactions: Overview Addition of H/e- or removal of O/e-negative groups H atom donors: H2 & [Pd/Pt], H-Y-H [H-] donors: R2BH, NaBHR3, R2AlH, LiAlHR3, R3Si/C-H [H•] donors: R3SnH, R3SiH e- donors: Lio, Nao, Zno  M+ + n e- reductive removal: C-X/C=O  C-H/C-H2 OCFC-Chap 8 Chapter 8

2 Chapter 8 Reduction Reactions (I)
Organic Chemistry of Fine Chemicals Spring 2006 Chapter 8 Reduction Reactions (I) catalytic hydrogenation:  160 Table 8.1 heterogeneous catalysts: easy separation & recovery low-pressure: 1~10 atm of H2 at 0~100 oC high-pressure: up to 300 atm of H2 & 300 oC catalyst used: Raney Ni, Pt (PtO2), Pd, Rh, CuCr2O4 support: carbon > BaSO4 > CaCO3; solvent: AcOH > hexane OCFC-Chap 8 Chapter 8

3 Chapter 8 Reduction Reactions (II)
Organic Chemistry of Fine Chemicals Spring 2006 Chapter 8 Reduction Reactions (II) catalytic hydrogenation (continued) homogeneous catalysts: selective, predictable & mild the Wilkinson’s catalyst;  161 top (inseparable) transfer hydrogenation: use of hydrogen donors cyclohexene, isopropanol, formic acid, hydrazine hazardous H2 gas avoided OCFC-Chap 8 Chapter 8

4 Chapter 8 Reduction Reactions (III)
Organic Chemistry of Fine Chemicals Spring 2006 Chapter 8 Reduction Reactions (III) metal hydride reduction: MHn  H- ;  161 Table 8.2 reactivity with functional groups:  162 Table 8.3 chemoselectivity of metal hydrides:  162 middle double bonds vs carbonyls:  165 top reaction conditions: solvent (NaBH4 in diglyme; aldehydes over ketones), pH (NaBH3CN; 1o halides at pH = 7, aldehydes & ketones at pH = 3~4) OCFC-Chap 8 Chapter 8

5 Chapter 8 Reduction Reactions (IV)
Organic Chemistry of Fine Chemicals Spring 2006 Chapter 8 Reduction Reactions (IV) electron transfer: dissolving metals; Mo M+ + e- metals used: Li, Na, K, Mg, Ca, Zn, Sn, Fe / SmI2 reduction types of carbonyls: via radical anion,  163 (8.2) OCFC-Chap 8 Chapter 8

6 Organic Chemistry of Fine Chemicals
Spring 2006 Reduction of Alkenes Heterogeneous hydrogenation cis addition:  164; (6) (8) isomerization : Pd vs Pt,  164; (6) (7) addition to a less hindered face:  164 bottom Homogeneous catalyst: more selective,  165 top Hydrogen transfer: diimide (milder),  165 middle selective reduction of non-polar double bonds in the presence of polar a,b-unsaturated carbonyl double bonds OCFC-Chap 8 Chapter 8

7 Organic Chemistry of Fine Chemicals
Spring 2006 Reduction of Alkynes Z-alkenes Lindlar catalyst:  165 bottom DIBAL-H:  166 top spongy Ni cathode:  166 bottommost E-alkenes LAH of propargylic alcohols:  166 top electrochemical reduction:  166 bottom dissolving metal:  166 middle OCFC-Chap 8 Chapter 8

8 Reduction of Aldehydes & Ketones (I)
Organic Chemistry of Fine Chemicals Spring 2006 Reduction of Aldehydes & Ketones (I) Alcohols: [H] Meerwein-Pondorf-Verley reduction: selective,  167 top advantages:  167 middle & bottom stereochemistry: kinetic vs steric,  168, Table 8.4 & 8.5 small hydride: kinetically favored attack; axial large hydride: sterically less hindered attack ; equatorial OCFC-Chap 8 Chapter 8

9 Reduction of Aldehydes & Ketones (II)
Organic Chemistry of Fine Chemicals Spring 2006 Reduction of Aldehydes & Ketones (II) dimeric coupling: diols;  169 top & bottom saturated methylene: Clemmensen reduction: acidic conditions,  170 middle mechanism: three-phase reaction [toluene-aqueous-Zn(Hg)]  170 top (8.6); minimum bimolecular coupling Wolff-Kishner reduction: basic conditions,  170 bottom reductive coupling: ketyl radicals;  171 OCFC-Chap 8 Chapter 8

10 Reduction of Carboxylic Acids & Derivatives
Organic Chemistry of Fine Chemicals Spring 2006 Reduction of Carboxylic Acids & Derivatives acids, esters & amides to alcohols & amines catalytic hydrogenation: inert at low pressure metal hydrides:  162 Table 8.3, 172 top acids, esters & amides to aldehydes:  172– 173 via imidazole amides, DIBAL-H & LiAl(OEt)3 acid chlorides to aldehydes catalytic: Rosenmund reduction,  172 middle metal hydride: LiAlH(OtBu)3,  172 middle McFadyen-Stevens reaction:  173 middle OCFC-Chap 8 Chapter 8

11 Reduction of Acid Derivatives to Amines
Organic Chemistry of Fine Chemicals Spring 2006 Reduction of Acid Derivatives to Amines catalytic & hydride reduction of nitriles:  173 bot R-CN R-CH=NH R-CH2-NH2 aldehyde by hydrolysis of imines:  173 bot – 174 top reductive amination of imines & oximes catalytic hydrogenation of imines in-situ:  174 middle hydride reduction: NaBH3CN at pH 5~6,  174 bottom LAH reduction of aryl ketoximes:  175 top OCFC-Chap 8 Chapter 8

12 Hydrogenolysis of R-Y bonds
Organic Chemistry of Fine Chemicals Spring 2006 Hydrogenolysis of R-Y bonds C-X bonds: I > Br > Cl >> F; allyl & benzyl > alkyl catalysts: Pd vs Ni;  175 top metal hydrides: chemoselectivities,  176 top electron transfer: radical;  176 middle SmI2: a-haloketone;  177 bottom PhCH2-O / Bn-N: deprotection;  177 bot – 178 top C-S: Raney Ni,  148 (4) C-OSO2R:  177 middle OCFC-Chap 8 Chapter 8

13 Hydrogenolysis of Epoxides
Organic Chemistry of Fine Chemicals Spring 2006 Hydrogenolysis of Epoxides acidic conditions: at the more substituted carbon Pt:  178 top / Raney Ni:  178 middle (neutral solution) metal hydrides: HAlCl2;  178 bottom basic conditions: at the less substituted carbon Pd/C:  178 middle / Raney Ni (basic) metal hydrides: LiAlH4;  178 bottom SmI2: reductive opening;  179 top OCFC-Chap 8 Chapter 8

14 a,b-Unsaturated Carbonyl Compounds
Organic Chemistry of Fine Chemicals Spring 2006 a,b-Unsaturated Carbonyl Compounds chemoselectivity: C=C vs C=O or both catalytic:  179 bottom  Li in NH3:  180 middle metal hydrides: DIBAL-H;  180 top reduction to methylene: C=O to CH2 Raney Ni: thioketals;  180 middle side reactions of Wolff-Kishner:  180 middle OCFC-Chap 8 Chapter 8

15 Dissolving Metal Reductions
Organic Chemistry of Fine Chemicals Spring 2006 Dissolving Metal Reductions conjugated dienes: Z intermediate of 1,4-addition trapping experiment: radical anion;  180 bot – 181 top Birch reduction: (hetero)aromatic compounds mechanism: regioselectivity;  181 bottom examples:  181 bottom – 182 top OCFC-Chap 8 Chapter 8


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