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Unformatted text preview: Experiment 7: Carbonyl Reduction: Sodium Borohydride Reduction of 4-tert-Butylcyclohexanone. Mohrig, Chapter 18. See slides and Mohrig Chapters for a discussion of MS, IR & NMR 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 relied on to reduce selectively aldehydes and ketones in the pres- ence of many other functional groups. This selectivity makes NaBH 4 the preferred reagent for reduction of aldehydes and ketones. Mechanistically, 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 reactions are a bit more complex. Kinetic examination has revealed that a molecule of the methanol solvent typically used in sodium borohydride reductions is in- volved in the transition state. 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, the 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 oriented 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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- Winter '07
- Organic chemistry