3-CamphorRedHLSS08 - Stereoselective Reduction of Camphor1...

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Stereoselective Reduction of Camphor 1 Reading Assignment: a) Loudon, G.M. Organic Chemistry , 4 th ed.; Oxford University Press: New York, 2002, pgs 861-865 b) Fessenden, R.J., Fessenden, J.S., Feist, P. Organic Laboratory Techniques , 3 rd ed.; Brooks/Cole: Pacific Grove, 2001, pgs 133-138 (TLC). Introduction : Lithium aluminum hydride (LiAlH 4 ) and sodium borohydride (NaBH 4 ) are widely used for the reduction of carbonyl groups (C=O). Both reagents are convenient sources of hydride ion (H: ). The hydride ion is a very strong base and, when associated with aluminum or boron, a great nucleophile. Lithium aluminum hydride is extremely reactive and reduces the carbonyl group in aldehydes, ketones, carboxylic acids, esters, and amides. LiAlH 4 is an extremely strong base and reacts violently with water and alcohol solvents to produce hydrogen gas. Hence, it can only be used in anhydrous non- protic solvents such as tetrahydrofuran (THF) or diethyl ether. Sodium borohydride is a much milder reducing agent and reduces only aldehydes and ketones to the corresponding alcohols. NaBH 4 reductions are performed in methanol, ethanol or water and if the two reducing agents give the same selectivity NaBH 4 is the reagent of choice because it is easy to store and handle. In today’s lab you will reduce a ketone to a secondary alcohol (you learned the reverse of this reaction in 151). The general reactivity of an aldehyde or ketone carbonyl can be understood by considering the resonance structure shown below in which the pi electrons are delocalized onto the oxygen. The result is that the oxygen has partial negative charge buildup and is therefore slightly Lewis basic/nucleophilic. The carbon has positive charge buildup and therefore it acts like a Lewis acid/electrophile. R C O R' aldehyde: R = H ketone: R = alkyl, aryl, alkenyl R C O R' THEREFORE: R C O R' ! + ! - Electrophilic site Nucleophilic site Figure 1. General carbonyl reactivity. The general mechanism for the sodium borohydride reduction of a simple ketone, acetone, is shown below (figure 2). H 3 C CH 3 O H B H H H H O CH 3 Na + H 3 C CH 3 H O H + OCH 3 B H H H Na + Figure 2. General mechanism of sodium borohydride reduction.
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The nucleophilic hydride donated by NaBH 4 attacks the electrophilic carbonyl carbon to form a new carbon/hydrogen bond. This step is irreversible (why?) The pi electrons of the carbonyl abstract a proton from the alcohol solvent and the Lewis basic alcohol oxygen coordinates with the boron atom. The exact timing of the bond making and breaking is uncertain. Notice that after the borohydride reduces a single carbonyl three B-H bonds remain. Therefore, one equivalent of borohydride can reduce four equivalents of ketone.
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This note was uploaded on 05/13/2010 for the course CHEM 152L taught by Professor Baldwin during the Summer '09 term at Duke.

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3-CamphorRedHLSS08 - Stereoselective Reduction of Camphor1...

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