215F07-notes-9-21-07

215F07-notes-9-21-07 - Chem 215 F07 Notes Dr. Masato...

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Unformatted text preview: Chem 215 F07 Notes Dr. Masato Koreeda - Page 1 of 4. Date: September 21, 2007 Chapter 13. Alcohols, Diols, and Ethers: V. Reactions of alcohols: Alcohols to alkyl halides VIII. Epoxides (or Oxiranes): b. Ring-opening reactions of epoxides: Epoxides are highly strained and easily undergo ring-opening reactions under both acidic and basic conditions. Acidic conditions: ring-opening reactions proceed rapidly at low temperatures (usually at room temp or below); elongated C-O bonds of the protonated, highly strained epoxides believed to be the origin of high reactivity. Acyclic epoxides H Br O H H3C H CH3 HBr 0 C H3C H Br OH H CH3 + HO H H3C CH3 H Br meso (2S,3S) racemic mixture H (2R,3R) H O SN2-like inversion of stereochemistry here! Cyclic-fused epoxides O H CH3OH O H H3C Br H CH3 H H3C H CH3 Br SN2-like inversion of stereochemistry here! O H O H O H O H CH3 O CH3 O H transproduct O CH3 + enantiomer inversion of stereochemistry H CH3 or simply B Note: The mechanism for the ring-opening of epxodies under acidic conditions is quite similar to that of bromination of alkenes. Br Br Br bromonium ion intermediate Br Br Br transproduct inversion of stereochemistry + enantiomer Stereochemical/regiochemical issues: O H3C D3C H CH3 H3O18 0 C HO18 H3C D3C OH H CH3 regiochemically, stereochemically clean formation of this diol. stereochemical inversion observed here! retention Specific incorporation of HO18 here! Chem 215 F07 Notes Dr. Masato Koreeda - Page 2 of 4. Date: September 21, 2007 Mechanistic interpretation on the stereochemical/regiochemical outcome under acidic conditions: more elongated C--OH bond; -charge delocalized because this more substituted C can better stalilize the postive charge H O O H CH3 H3C D3C H CH3 H3C D3C H H H O18 H O H3C D3C H CH3 In the transition state for the attack of H2O (H2O18 in this case), the nucleophile attacks preferentially the carbon center that can better stabilize a positive character (i.e., the more substituted carbon). Can accommodate charge better here! O H3C D3C H2O18 H H CH3 H3C D3C HO18 OH H CH3 stereochemical inversion observed here! retention Specific incorporation of HO18 here! an SN2-like stereochemical inverstion at a more substituted carbon center Epoxide-ring opening under basic conditions A straight SN2 process at the less-substituted carbon with a stereochemical inversion. O H3C H3C H D O H H3C D H3C H3CCH2O Na OCH2CH3 + Na Na O H3C H3C H OCH2CH3 H H3CCH2O Na H3CCH2OH D OCH2CH3 HO H3C H3C H less-substituted C; SN2 reaction here! D OCH2CH3 Summary of epoxide-ring opening reactions: Acidic conditions: SN2 like interm of the stereochemical inversion, but at the more substituted C. Ring opening at the more substituted C with the inversion of stereochemistry. Basic conditions: pure SN2 Ring opening at the less substituted C with the inversion of stereochemistry. Chem 215 F07 Notes Dr. Masato Koreeda - Page 3 of 4. Date: September 21, 2007 1,2-Diaxial opening of cyclohexene oxide O H3O OH OH OH HO more stable conformer H O O H OH axial-like axial OH axial axial-like H2O only 1,2-diaxial transition state feasible for an SN2 like process to occur thermodynamically less favored conformer This is the form produced first, then inverts to the top conformer. Problems: Show the structure of the expected major product for each of the following two epoxide ring-opening reactions. CH3OH, H+ O H3CO Na CH3OH ...
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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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