Lecture Note 2 - Oxidation Reactions Dale L Boger IV...

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Oxidation Reactions Dale L. Boger 41 Comprehensive Org. Syn. ; Vol. 1, 819; Vol. 7, pp. 357 and 389 (asymmetric). A. Epoxidation Reactions: Oxidation of Carbon–Carbon Double Bonds RO O H O + CC H O + O Rate increases: R = CH 3 < C 6 H 5 < m -ClC 6 H 4 < H < p -NO 2 C 6 H 4 < CO 2 H < CF 3 p K a of acid (RCO 2 H): 4.8 4.2 3.9 3.8 3.4 2.9 0 The lower the p a , the greater the reactivity (i.e., the better the leaving group). 1. Peracid Reactivity IV. Oxidation Reactions 2. Mechanism R O O H O Butterfly mechanism R O O H O + 3. Stereochemistry a. Stereochemistry of olefin is maintained: diastereospecific. b. Reaction rate is insensitive to solvent polarity implying concerted mechanism without intermediacy of ionic intermediates. c. Less hindered face of olefin is epoxidized. R R R R R R O O + m- CPBA CH 2 Cl 2 R = H 20 min, 25 °C 99% 1% R = CH 3 24 h, 25 °C < 10% 90% (usual representation) R O O O H Bartlett Rec. Chem. Prog. 1950 , 11 , 47. Refined representation: trans antiperiplanar arrangement of O–O bond and reacting alkene, n- π * stabilization by reacting lone pair in plane. The synchronicity of epoxide C–O bond formation and an overall transition state structure postulated using ab initio calculations and experimental kinetic isotope effects. Singleton, Houk J. Am. Chem. Soc. 1997 , 119 , 3385. Brown 1970 , 92 , 6914.
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Modern Organic Chemistry The Scripps Research Institute 42 4. Chemoselectivity _ Electrophilic reagent: most nucleophilic C=C reacts fastest. RO > R > EWG >>> > m -CPBA –10 °C, 1 h O cis : trans 1 : 1 C 6 H 5 CO 3 H CHCl 3 , 10 min 0 °C O H H OH O HO 2 C C 6 H 5 CO 3 H C 6 H 6 –dioxane 25 °C, 24 h H H OH O HO 2 C O 80% H OH H O CO 2 H H 5. Diastereoselectivity a. Endocyclic Olefins Rickborn J. Org. Chem. 1965 , 30 , 2212. Me Me H H m- CPBA 25 °C, Et 2 O Me Me H H Me Me H H O O + 87 : 13 Me H H Me - Examples Concave face hindered toward peracid attack Convex face open to peracid attack Hückel Chem. Ber. 1955 , 88 , 346. Woodward Tetrahedron 1958 , 2 , 1. Tamm Helv. Chim. Acta 1975 , 58 , 1162. Destabilizing steric interaction between reagent and axial Me Attack principally from this face
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Oxidation Reactions Dale L. Boger 43 Me H Me H Me H O O O O H O R O O H O R vs. ∆∆ G ∆∆ Small difference for products: but larger difference for reagent approach in transition state. H b. Exocyclic Olefins more hindered face less hindered face RCO 3 H + less stable product _ Solvent dependent _ The effective size of the reagent increases with increasing solvent polarity, i.e. the solvation shell of _ Small reagent preference: axial attack and 1,3-diaxial interactions vary with size of the reagent _ Large reagent preference: equatorial attack and 1,2-interactions (torsional strain) are RCO 3 H + 41 59 : Me Me Me Me Me Me O Me Me Me O CCl 4 C 6 H 6 CH 2 Cl 2 or CHCl 3 75% 80% 83% 25% 20% 17% H H H H H H H H H H H H H H O O the reagent increases in size. relatively invariant with the size of the reagent Me Me Me Henbest J. Chem. Soc., Chem. Commun.
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This note was uploaded on 03/14/2012 for the course CHEMISTRY 101 taught by Professor Louie during the Spring '11 term at Chemnitz University of Technology.

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Lecture Note 2 - Oxidation Reactions Dale L Boger IV...

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