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Introduction to Complex Metal Hydrides

Further more of a hydride anion to an aldehyde or ketone would make an alkoxide anion, which on protonation should yield the subsequent alcohol. Aldehydes would give 1º-alcohols (as displayed in the equation) and ketones would give 2º-alcohols.

RCH=O   +   H:(-)  →    RCH2O(-)   +   H3O(-)   →   RCH2OH

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

Reagent

Preferred Solvents

Functions Reduced

Reaction Work-up

Sodium Borohydride
    NaBH4

ethanol; aqueous ethanol
15% NaOH; diglyme
avoid strong acids

aldehydes to 1º-alcohols
ketones to 2º-alcohols
inert to most other functions

1) simple neutralization 
2) extraction of product

Lithium Aluminum Hydride
    LiAlH4

ether; THF
avoid alcohols and amines
avoid halogenated compounds
avoid strong acids

aldehydes to 1º-alcohols
ketones to 2º-alcohols
carboxylic acids to 1º-alcohols
esters to alcohols
epoxides to alcohols
nitriles & amides to amines
halides & tosylates to alkanes
most functions react

1) careful addition of water
2) remove aluminum salts
3) extraction of product

Some illustrations of aldehyde and ketone reductions, employing the reagents explained above, are existed in the diagram below. The first three reactions depict that all four hydrogens of the complex metal hydrides may function as hydride anion equivalents that bond to the carbonyl carbon atom. The resulting alkoxide salts are insoluble and require to be hydrolyzed (with care) before the alcohol result can be isolated, in the LiAlH4 reduction. In the borohydride reduction the hydroxylic solvent system gets this hydrolysis automatically. The sodium,lithium, aluminum and boron end up like soluble inorganic salts. The last reaction depicts how an acetal derivative may be used to avoid reduction of a carbonyl function (in this case a ketone). Keep in mind, with the exception of epoxides, ethers are usually unreactive with strong bases or nucleophiles. The acid catalyzed hydrolysis of aluminum salts also influences the removal of the acetal. This equation is classic in not being balanced (that is it does not identify the Complex Metal Hydrides of the reagent).

1631_Complex Metal Hydrides Homework Help.jpg

Reduction of α,β-unsaturated ketones by metal hydride reagents occasionally leads to a saturated alcohol, particularly with sodium borohydride. This result is produced by an initial conjugate addition of hydride to the β-carbon atom that is followed by ketonization of the enol product and reduction of the resulting saturated ketone (equation no.1 below). If the saturated alcohol is the wanted result, catalytic hydrogenation prior to (or following) the hydride reduction might be essential. To prevent reduction of the double bond, the cerium(III) chloride is added to the reaction and it is generally carried out below 0 ºC, as displayed in equation no.2.

747_Complex Metal Hydrides Homework Help 1.jpg

Prior to leaving this topic it should be noticed that diborane, B2H6, a gas that was employed in ether solution to prepare alkyl boranes from alkenes, also reduces several carbonyl groups. Subsequently, selective reactions with substrates having both functional groups might not be possible. In difference to the metal hydride reagents, diborane is a comparatively electrophilic reagent, like witnessed by its ability to reduce alkenes. This variationss also effects the rate of reduction observed for the two aldehydes displayed below. The first, 2,2-dimethylpropanal is less electrophilic as compare to the second, that is activated by the electron withdrawing chlorine substituents.

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