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20.3: Chemoselective Reactions

The most common sources of the hydride Nucleophile are lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4). Note! The hydride anion is not present during this reaction; rather, these reagents serve as a source of hydride due to the presence of a polar metal-hydrogen bond. Because aluminum is less electronegative than boron, the Al-H bond in LiAlH4 is more polar, thereby, making LiAlH4 a stronger reducing agent.

Addition of a hydride anion (H:-) to an aldehyde or ketone gives an alkoxide anion, which on protonation yields the corresponding alcohol. Aldehydes produce 1º-alcohols and ketones produce 2º-alcohols.

          

In metal hydrides reductions the resulting alkoxide salts are insoluble and need to be hydrolyzed (with care) before the alcohol product can be isolated. In the sodium borohydride reduction the methanol solvent system achieves this hydrolysis automatically. In the lithium aluminum hydride reduction water is usually added in a second step. The lithium, sodium, boron and aluminum end up as soluble inorganic salts at the end of either reaction.  Note! LiAlH4 and NaBH4 are both capable of reducing aldehydes and ketones to the corresponding alcohol.

Example \(\PageIndex{1}\):

 

Mechanism

This mechanism is for a LiAlH4 reduction.  The mechanism for a NaBH4 reduction is the same except methanol is the proton source used in the second step.

1) Nucleopilic attack by the hydride anion

Fix1.jpg

2) The alkoxide is protonated

 

Fix2.jpg

 

Going from Reactants to Products Simplified

 

Properties of Hydride Sources

Two practical sources of hydride-like reactivity are the complex metal hydrides lithium aluminum hydride (LiAlH4) and sodium borohydride (NaBH4). These are both white (or near white) solids, which are prepared from lithium or sodium hydrides by reaction with aluminum or boron halides and esters. Lithium aluminum hydride is by far the most reactive of the two compounds, reacting violently with water, alcohols and other acidic groups with the evolution of hydrogen gas. The following table summarizes some important characteristics of these useful reagents.

 

Problems

1) Please draw the products of the following reactions:

 

 

2) Please draw the structure of the molecule which must be reacted to produce the product.

 

3) Deuterium oxide (D2O) is a form of water where the hydrogens have been replaced by deuteriums. For the following LiAlH4 reduction the water typically used has been replaced by deuterium oxide.  Please draw the product of the reaction and place the deuterium in the proper location. Hint! Look at the mechanism of the reaction.

 

Answers

 

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