Hydrides reduction of ketones and aldehydes

The mechanism of lithium aluminum hydride reduction of aldehydes and ketones IS analogous to that of sodium borohydride except that the reduction and hydrolysis... 

Hydride reduction of aldehydes and ketones occurs via the general mechanism of nucleophilic addition— that is, nucleophilic attack followed by protonation. Mechanism 20.3 is shown using LiAlH4, but an analogous mechanism can be written for NaBH4. 

The mechanism of lithium aluminum hydride reduction of aldehydes and ketones is analogous to that of sodium borohydride except that the reduction and hydrolysis stages are independent operations. The reduction is carried out in diethyl ether, followed by a separate hydrolysis step when water is added to the reaction mixture. 

The mechanism is similar to the hydride reduction of aldehydes and ketones (eq. 9.33). 

A series of papers on the metal hydride reduction of aldehydes and ketones, - and some other functional groups, has demonstrated the importance of the metal ion for example, y " is indispensable in the lithium aluminium hydride reduction of many ketones in aprotic solvents, through transition state (46), and reaction is prevented by the specific lithium complexing cryptand (47). 

General Hydride Reductions of Aldehydes and Ketones to Alcohols... 

Examples of hydride reduction of aldehydes and ketones are shown in Figure 14.51. Notice in the first reaction that sodium borohydride selectively reduces the aldehyde in the presence of an ester nitro and nitrile groups are also tolerated. In the second reaction, the borohydride reduction of a ketone, the diastereoselectivity, reflects the steric hindrance to the approach of the methyl groups on the bridging carbon atom block the approach from the top face of the molecule. By contrast, little diastereoselectivity is observed in the reduction of 2-hydroxyclobutanone. The final example illustrates the reduction of an a,p-unsaturated ketone. 

For most laboratory scale reductions of aldehydes and ketones catalytic hydro genation has been replaced by methods based on metal hydride reducing agents The two most common reagents are sodium borohydride and lithium aluminum hydride... 

Lithium aluminum hydride, LiAIH4/ is another reducing agent often used for reduction of aldehydes and ketones. A grayish powder that is soluble in ether and tetrabydrofuran, LiAlH4 is much more reactive than NaBH4 but also more dangerous. It reacts violently with water and decomposes explosively when heated above 120 °C. 

Formation of an Alcohol The simplest reaction of a tetrahedral alkoxide intermediate is protonation to yield an alcohol. We ve already seen two examples of this kind of process during reduction of aldehydes and ketones with hydride reagents such as NaBH4 and LiAlH4 (Section 17.4) and during Grignard reactions (Section 17.5). During a reduction, the nucleophile that adds to the carbonyl... 

By reduction of aldehydes and ketones Aldehydes and ketones are reduced to the corresponding alcohols by addition of hydrogen in the presence of catalysts (catalytic hydrogenation). The usual catalyst is a finely divided metal such as platinum, palladium or nickel. It is also prepared by treating aldehydes and ketones with sodium borohydride (NaBH4) or lithium aluminium hydride (LLAIH4). Aldehydes yield primary alcohols whereas ketones give secondary alcohols. 

This procedure illustrates a general method for the reduction of aldehyde and ketone functions to methylene groups under very mild conditions. Since strong acids and bases are not employed, this procedure is of particular importance for the reduction of ketones possessing an adjacent chiral center. Moreover, the use of deuterated metal hydrides permits the preparation of labelled compounds. ... 

HYDRIDE AS A NUCLEOPHILE REDUCTION OF ALDEHYDES AND KETONES, LAH AND SODIUM BOROHYDRIDE... 

Hydride as a nucleophile reduction of aldehydes and ketones, lithium aluminium hydride and sodium borohydride... 

Solutions of low-valence titanium chloride (titanium dichloride) are prepared in situ by reduction of solutions of titanium trichloride in tetrahydrofuran or 1,2-dimethoxyethane with lithium aluminum hydride [204, 205], with lithium or potassium [206], with magnesium [207, 208] or with a zinc-copper couple [209,210]. Such solutions effect hydrogenolysis of halogens [208], deoxygenation of epoxides [204] and reduction of aldehydes and ketones to alkenes [205,... 

REDUCTION WITH LITHIUM ALUMINUM HYDRIDE Acidic Quenching. Reduction of Aldehydes and Ketones [ 5]... 

Hydride ion and its derivatives LiAlH4 and NaBH4. Reduction of aldehydes and ketones. 

Reduction of aldehydes and ketones. Earlier work on amine borane reagents was conducted mainly with tertiary amines and led to the conclusion that these borane complexes reduced carbonyl compounds very slowly, at least under neutral conditions, and that the yield of alcohols is low. Actually complexes of borane with primary amines, NHj or (CH3)3CNH2, reduce carbonyl compounds rapidly and with utilization of the three hydride equivalents. BH3 NH3 is less subject to steric effects than traditional complex hydrides. A particular advantage is that NH3 BH3 and (CH3)3CNH2 BH3 reduce aldehyde groups much more rapidly than keto groups, but cyclohexanone can be reduced selectively in the presence of aliphatic and aromatic acyclic ketones. 

From reduction of aldehydes and ketones Using aluminum-based reagents Diisobutylaluminum hydride-Tin(II) chloride-(S)-l-[l-Methyl-2-pyrrolidinyl] methylpiperidine, 116... 

This section covers the reduction of aldehydes and ketones, in complex molecules using hydride transfer reagents. Many of these are complex reagents designed specifically for particular reactions. LAH has been used for the reduction of cyclopropyl ketones199. Trialkyltin moieties in the cyclopropane ring cause diastereoselective reduction to occur (equation 51). 

Lithium borohydride is intermediate in activity as a reducing agent between lithium aluminium hydride and sodium borohydride. In addition to the reduction of aldehydes and ketones it will readily reduce esters to alcohols. It can be prepared in situ by the addition of an equivalent quantity of lithium chloride to a 1m solution of sodium borohydride in diglyme. Lithium borohydride should be handled with as much caution as lithium aluminum hydride. It may react rapidly and violently with water contact with skin and clothing should be avoided. 

Reduction of aldehydes and ketones The reactivity of tin hydrides for this reduction follows the order ... 

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