REDUCTION REACTIONS OF ALDEHYDES AND KETONES

Aldehydes and ketones can be reduced to alcohols using reducing agents such as lithium aluminum hydride (LiAlH ), and sodium borohydride (NaBH ). 

Sodium borohydride (NaBH ) can reduce aldehydes and ketones when reacted in aqueous or alcohol solutions. Carboxylic acids and esters are not reactive to sodium borohydride reduction. Lithium aluminum hydride (LiAlH ), a more powerful reducing agent, can reduce aldehydes, ketones, esters, and carboxylic acids. 

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Table 17 2 summarizes the reactions of aldehydes and ketones that you ve seen m ear her chapters All are valuable tools to the synthetic chemist Carbonyl groups provide access to hydrocarbons by Clemmensen or Wolff-Kishner reduction (Section 12 8) to alcohols by reduction (Section 15 2) or by reaction with Grignard or organolithmm reagents (Sections 14 6 and 14 7)... 

Besides direct reduction, a one-pot reductive amination of aldehydes and ketones with a-picoline-borane in methanol, in water, and in neat conditions gives the corresponding amine products (Scheme 8.2).40 The synthesis of primary amines can be performed via the reductive amination of the corresponding carbonyl compounds with aqueous ammonia with soluble Rh-catalyst (Eq. 8.17).41 Up to an 86% yield and a 97% selectivity for benzylamines were obtained for the reaction of various benzaldehydes. The use of a bimetallic catalyst based on Rh/Ir is preferable for aliphatic aldehydes. 

The Barhier-type reaction of aldehydes and ketones with allyl halides (485) in the presence of Sml2, leading to homoallyl alcohols (486), has received recent interest as a one-step alternative to the Grignard reaction. However, the reactions require the use of stoichiometric amounts of the reducing Sm(III) species. Recently, the electroreductive Barhier-type allylation of carbonyl compounds in an SmH-mediated reaction has been developed [569]. The electrolysis of (485) is carried out in a DMF-SmCl3-(Mg/Ni) system in an undivided cell to give the adduct (486) in 50 85% yields (Scheme 168) [569]. Electrosynthesis of y-butyrolactones has been achieved by the reductive coupling of ethyl 3-chloropropionate with carbonyl compounds in the presence of a catalytic amount of SmCfi [570]. 

In general, any reaction which results in the addition of two hydrogen atoms across a jt bond of any type are reductions. Conversions of aldehydes and ketones... 

A typical reaction of aldehydes and ketones is addition to the C—O ji bond. Examples of addition reagents are H2 (resulting in reduction to the corresponding alcohol), ROH (to give a hemiacetal or hemiketal) and RM (yielding a metal alkoxide). Only the hydrogenation reaction produces an addition product for which there is any useful quantity of thermochemical data, however. Equation 34 represents an overall reaction of the carbonyl compound with a (hypothetical) reagent XY, an equation which includes any reaction, subsequent to an initial addition reaction, to form products for which there are sufficient data. 

Reactions of Aldehydes and Ketones Aldehydes and ketones are central players in organic synthesis because of the wide variety of reactions they undergo. Likewise, their chemistry is important to biological systems. We will consider just two of the reactions involving these compounds oxidation/reduction and formation of acetals and ketals. 

In contrast with the common activity of oxo-rhenium compounds in oxidation catalysis (see above), the Re -dioxo-complex [Re02l(PPh3)2] catalyzes the reductive hydrosilylation of aldehydes and ketones to give silyl ethers (reaction... 

Nucleophilic addition reactions of aldehydes and ketones (a) Addition of hydride reduction (Section 19.8)... 

Introduction. The reactions of aldehydes and ketones can be conveniently divided into three groups (1) reactions involving an addition to the carbonyl group, (2) reactions involving oxidation-reduction of the carbonyl group, and (3) condensation and polymerization reactions. 

Cha, J. S., Park, J. H. Reaction of aldehydes and ketones with boron triisopropoxide. The Meerwein-Ponndorf-Verley type reduction of boron aikoxides. 1. Bull. Korean Chem. Soc. 2002, 23, 1051-1052. 

Several theoretical studies have considered LiH addition as a model for the more computationally difficult L1A1H4 and NaBH4 [45]. However, reaction of aldehydes and ketones with LiH seldom if ever leads to reduction [46]. AIH3, while less commonly used than the complex boron and aluminium hydrides, is useful for reducing carbonyls [47] and therefore is a suitable model for computational study. Calculations [2, 5] show that gas-phase reduction of formaldehyde by AIH3 occurs by formation of complex 1, which rearranges via a four-centre transition state to form an aluminium methoxide product. Two conformational isomers of... 

Several contributions have appeared in which intermolecular reductive couplings between ketones or aldehydes and electron-deficient alkenes have been described [30h, 53]. Reactions of aldehydes and ketones with unsaturated esters result in the formation of lactones. Diastereoselectivities are normally modest in these cases, but can be quite high for certain substitution patterns (Eq. 45). 

An important additional amine synthesis in ibis section is reductive aminalion of aldehydes and ketones. In particular, reductive aminalion of a ketone is a belter way to make an amine attached to a 2° alkyl group than is Sn2 reaction with a 2° haloalkane. 

Dihydroisocoumarins are conveniently obtained from the reaction of aldehydes and ketones with the dianions derived from o-toluic acids, though the products are not always stereochemically pure (94SC779). However, enantiomerically pure dihydroisocoumarins result from the stereoselective reduction of (arene)-tricarbonylchromium cmnplexes of homobenzylic ketones by DIB ALH, followed by oxidation of the resulting lactols (94CCS01). 

Catal)rtic reactions often give surprising results. An interesting result is the reductive amination of aldehydes and ketones. The keto compounds react with primary or secondary amines or ammonia in the presence of H2 and a suitable catalyst (e.g., Pd) to give a new amine. Thus, as expected, 2-methylcyclohexanone is converted to 2-methylcyclohexylamine (Eq. 8-16). 

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