Reduction reaction aldehydes/ketones

In this section primarily reductions of aldehydes, ketones, and esters with sodium, lithium, and potassium in the presence of TCS 14 are discussed closely related reductions with metals such as Zn, Mg, Mn, Sm, Ti, etc., in the presence of TCS 14 are described in Section 13.2. Treatment of ethyl isobutyrate with sodium in the presence of TCS 14 in toluene affords the O-silylated Riihlmann-acyloin-condensation product 1915, which can be readily desilylated to the free acyloin 1916 [119]. Further reactions of methyl or ethyl 1,2- or 1,4-dicarboxylates are discussed elsewhere [120-122]. The same reaction with trimethylsilyl isobutyrate affords the C,0-silylated alcohol 1917, in 72% yield, which is desilylated to 1918 [123] (Scheme 12.34). Likewise, reduction of the diesters 1919 affords the cyclized O-silylated acyloin products 1920 in high yields, which give on saponification the acyloins 1921 [119]. Whereas electroreduction on a Mg-electrode in the presence of MesSiCl 14 converts esters such as ethyl cyclohexane-carboxylate via 1922 and subsequent saponification into acyloins such as 1923 [124], electroreduction of esters such as ethyl cyclohexylcarboxylate using a Mg-electrode without Me3SiCl 14 yields 1,2-ketones such as 1924 [125] (Scheme 12.34). 

In organic chemistry, reduction is defined as a reaction in which a carbon atom forms fewer bonds to oxygen, O, or more bonds to hydrogen, H. Often, a C=0 bond or C=C bond is reduced to a single bond by reduction. A reduction that transforms double C=C or C=0 bonds to single bonds may also be classified as an addition reaction. Aldehydes, ketones, and carboxylic acids can be reduced to become alcohols. Alkenes and alkynes can be reduced by the addition of H2 to become alkanes. 

The reduction of double bond-containing functionalities, especially alkenes and carbonyl compounds, is an important methodology used in synthetic elaboration. In this section the stereocontrolled reduction of aldehydes, ketones and C=N-containing compounds and the catalytic hydrogenation of alkenes are covered, among other reductions. The emphasis here is placed on stereocontrolled reactions. 

A one-pot reaction has been developed for the reduction of aldehydes, ketones, and primary, secondary and tertiary alcohols into their corresponding alkyl function using either diethylsilane or n-butylsilane as the reducing agent in the presence of the Lewis acid catalyst tris(pentafluorophenyl)borane carbon-carbon double bonds remain unaffected.366 Aliphatic and aromatic polycarboxylic acids are also conveniently reduced to their corresponding alkanes using the same reagents and catalyst.367... 

Meerwein-Pondorf-Verley reduction is the hydrogenation in which alcohols are used as a source of hydrogen, and one of the hydrogen transfer reactions. M-P-V reduction of aldehydes, ketones and esters are efficiently catalyzed by hydrous Zr02 catalyst[18]. In these reactions, 2-propanol is the best for hydrogen source. 

Bp3-OEt2 followed hy DiisobutyUduminum Hydride is used for the 1,2-reduction of y-aimno-Q, -unsaturated esters to give unsaturated amino alcohols, which are chiral building blocks for a -amino acids. Q , -Unsaturated nitroalkenes can be reduced to hydroxylamines by Sodium Borohydride and BF3-OEt2 in THF extended reaction times result in the reduction of the hydroxylamines to alkylamines. Diphenylamine-borane is prepared from sodium borohydride, BF3-OEt2, and diphenylamine in THF at 0 °C. This solid is more stable in air than BF3-THF and is almost as reactive in the reduction of aldehydes, ketones, carboxyhc acids, esters, and anhydrides, as well as in the hydrob-oration of alkenes. 

Redox reactions catalyzed by alcohol dehydrogenases (e.g., from horse liver, HLADH) may be performed in organic solvents in both the reduction and oxidation mode, if the recycling system is appropriately modified (Sect. 2.2.1). Reduction of aldehydes/ketones and oxidation of alcohols is effected by NADH- or NAD" -recycling, using ethanol or wobutyraldehyde respectively. 

Benzene was introduced in Chapter 5 (Section 5.10). Chapter 21 will discuss many benzene derivatives, along with the chemical reactions that are characteristic of these compounds. In the context of dissolving metal reductions of aldehydes, ketones, and alkynes, however, one reaction of benzene must be introduced. When benzene (65) is treated with sodium metal in a mixture of liquid ammonia and ethanol, the product is 1,4-cyclohexadiene 66. Note that the nonconjugated diene is formed. The reaction follows a similar mechanism to that presented for alkynes. Initial electron transfer from sodium metal to benzene leads to radical anion 67. Resonance delocalization as shown shordd favor the resonance contributor 67B due to charge separation. 

This is the case with NAD(P)H-dependent dehydrogenases, where enzymes find applications in several synthetic processes (comprising the reduction of aldehydes, ketones, carboxylic acids, double, and triple carbon-carbon bonds), aimed at the preparation of chiral enantiopure bioactive compounds and of building blocks for fine chemicals and pharmaceutical products. Moreover, dehydrogenase-catalyzed oxidation reactions are gaining increasing interest as an environmentally friendly alternative to chemical oxidation processes, especially in those cases where a defined selectivity (either stereo-, regio-, or chemoselectivity) is required as well [1]. 

Full details of the reductive power of 9-borabicyclo[3,3,l]nonane have appeared and the susceptibility to reduction of aldehyde, ketone, and a number of other functional groups has been reviewed. Since tetra-n-butylammonium borohydride in dichloromethane reduces organic compounds with the same selectivity exhibited by sodium borohydride in aqueous or alcoholic media, the former system might prove the reagent of choice in those cases in which the use of protic solvents would lead to undesirable side-reactions. ... 

The condensation of aldehydes and ketones with succinic esters in the presence of sodium ethoxide is known as the Stobbe condensation. The reaction with sodium ethoxide is comparatively slow and a httlo reduction of the ketonic compound to the carbinol usually occurs a shorter reaction time and a better yield is generally obtained with the more powerful condensing agent potassium ieri.-butoxide or with sodium hydride. Thus benzophenone condenses with diethyl succinate in the presence of potassium [Pg.919]

The aromatic primary and secondary stibines are readily oxidized by air, but they are considerably more stable than their aHphatic counterparts. Diphenylstibine is a powerful reducing agent, reacting with many acids to Hberate hydrogen (79). It has also been used for the selective reduction of aldehydes and ketones to the corresponding alcohols (80). At low temperatures, diphenylstibine undergoes an addition reaction with ketene (81) ... 

Alcohols are among the most versatile of all organic compounds. They occur widely in nature, are important industrial 7, and have an unusually rich chemistry. The most widely used methods of alcohol synthesis start with carbonyl compounds. Aldehydes, ketones, esters, and carboxylic acids are reduced by reaction with LiAlH4. Aldehydes, esters, and carboxylic acids yield primary alcohols (RCH2OH) on reduction ketones yield secondary alcohols (R2CHOH). 

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... 

Treatment of an aldose or ketose with NaBH4 reduces it to a polyalcohol called an alditol. The reduction occurs by reaction of the open-chain form present in the aldehyde/ketone hemiacetal equilibrium. Although only a small amount of the open-chain form is present at any given time, that small amount is reduced, more is produced by opening of the pyranose form, that additional amount is reduced, and so on, until the entire sample has undergone reaction. 

Aldehydes and ketones can be converted to ethers by treatment with an alcohol and triethylsilane in the presence of a strong acid or by hydrogenation in alcoholic acid in the presence of platinum oxide. The process can formally be regarded as addition of ROH to give a hemiacetal RR C(OH)OR", followed by reduction of the OH. In this respect, it is similar to 16-14. In a similar reaction, ketones can be converted to carboxylic esters (reductive acylation of ketones) by treatment with an acyl chloride and triphenyltin hydride. " ... 

Diols (pinacols) can be synthesized by reduction of aldehydes and ketones with active metals such as sodium, magnesium, or aluminum. Aromatic ketones give better yields than aliphatic ones. The use of a Mg—Mgl2 mixture has been called the Gomberg-Bachmann pinacol synthesis. As with a number of other reactions involving sodium, there is a direct electron transfer here, converting the ketone or aldehyde to a ketyl, which dimerizes. 

PROBLEMS Identify the starting ketone or aldehyde you would use to prepare each of the following alcohols through reduction reactions ... 

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