Experiment_6 - Experiment 6: Carbonyl Reduction: Sodium...

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Experiment 6: Carbonyl Reduction: Sodium Borohydride Reduction of 4-tert-Butylcyclohexanone. Aldehydes and ketones play a central role in organic chemis- try and many reactions have been developed to modify these functional groups. Among these is reduction (addition of H 2 across a C=O bond) of a ketone. Many reagents effect this particular transformation, but metal-hydride reagents such as lithium aluminum hydride (LiAlH 4 ) and sodium borohydride (NaBH 4 ) are most commonly used. LiAlH 4 reduces a wide range of functional groups. Conversely, NaBH 4 is a much milder reducing agent and, in synthetic situations, can be re- lied on to selectively reduce a ldehydes and ketones in the presence of many other functional groups. This selectivity makes NaBH 4 the preferred reagent for reduction of aldehydes and ketones. Mechanisti- cally, metal-hydride reagents can be thought of as hydride (H - ) sources that add to the electrophillic carbonyl carbon of ketones and aldehydes. In actuality, metal-hydride reac- tions are a bit more complex. Kinetic examination has revealed that a molecule of the methanol solvent typically used in sodium borohydride reductions is involved in the transi- tion state (see figure in margin). Two diastereomeric secondary alcohol products are produced by reduction of 4- t- butylcyclohexanone. Because the t -butyl group locks the cyclohexyl ring into a single con- former, two products arise from addition of hydride [H ] to the C=O from the axial and equatorial faces. Attack of NaBH 4 from the axial direction gives the hydroxyl group ori- ented equatorially. Conversely, attack from the equatorial direction gives the hydroxyl group oriented axially. Small hydride reagents prefer axial addition and large hydride rea- gents prefer to add from the equatorial face. Because one stereoisomer predominates over the other (4:1), this reduction is described as being stereoselective.
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This note was uploaded on 09/28/2010 for the course SCI mcdb 1a & taught by Professor Bush during the Spring '10 term at UCSB.

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Experiment_6 - Experiment 6: Carbonyl Reduction: Sodium...

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