Ester to aldehyde

The reduction of esters to aldehydes is carried out with hydrides such as NaH2Al(0CH2CH20CH2)2, NaALH, or... 

McFADYEN - STEVENS Ester Reduction Reduction of esters to aldehyde via hydrazides. 

Addition of sulfonic acid esters to aldehydes or ketones... 

Specific enol equivalents will be needed for both synthons (61) and (66), Since (61) is to give a double bond but (66) is to give an alcohol, the logical choices are a Wittig reagent - actually (67) - for (61) and a Reformatsky reagent for (66). The ester to aldehyde conversion (65 63) Is easiest by over-reduction and re-... 

Hydrogenolysis of esters to aldehydes or alcohols needs high temperatures and high pressures. Moreover, it leads to the formation of acids, alcohols, and hydrocarbons. In contrast, bimetallic M-Sn alloys (M = Rh, Ru, Ni) supported on sihca are very selective for the hydrogenolysis of ethyl acetate into ethanol [181]. For example while the selectivity to ethanol is 12% with Ru/Si02, it increases up to 90% for a Ru-Sn/Si02 catalyst with a Sn/Ru ratio of 2.5 [182]. In addition, the reaction proceeds at lower temperatures than with the classical catalysts (550 K instead of temperatures higher than 700 K). The first step is the coordination of the ester to the alloy (Scheme 46), and most probably onto the tin atoms. After insertion into the M - H bond, the acetal intermediate decomposes into acetaldehyde and an ethoxide intermediate, which are both transformed into ethanol under H2. 

The DIBAL reduction of esters to aldehydes in the presence of phosphonate anions appears to solve problems of overreduction to alcohol and provides a good general method of 2-carbon homologation... 

Personal communication from Dr. J. V. Karabinos. Subsequent work on the reduction of steroid acids by Spero, McIntosh and Levin (ref. 12) has shown that Raney niokel partially deactivated by acetone is particularly suitable for the desulfurization of thiol esters to aldehydes. 

Hydrogenolysis of esters to aldehydes or alcohols is difficult to attain either by homogeneous or heterogeneous catalysis. In fact, high temperatures and high pressures are required to achieve the reaction, leading to a non-selective hydrogenolysis with formation of acids, alcohols, CO2, CO and hydrocarbons. 

Desulfurization by refluxing with Raney nickel in 70% ethanol for 6 hours converted thiol esters to aldehydes in 57-73% yields (exceptionally 22% yield) [1101] ( Procedure 6, p. 205). Desulfurization of a dithioester, methyl dithio-phenylacetate, by refluxing with Raney nickel in 80% ethanol for 1 hour afforded 65% yield of ethylbenzene [1102]. 

Sterically bulky reducing agents, e.g. diisohutylaluminium hydride (DIBAH), can selectively reduce esters to aldehydes. The reaction is carried out at low temperatures (—78 °C) in toluene. Diisohutylaluminium hydride has two bulky isobutyl groups, which make this reagent less reactive than LiAlH4. 

Preparation Essentially this reaction involves the preparation of a low-valent titanium reagent that then couples carbonyl groups, including esters to aldehydes/ketones. Generally, TiCLt is reduced with some reducing agent (LiAIH4, Zn, Mg). 

Fig. 17.61. Mechanism of the DIBAL reduction of carboxylic esters to aldehydes and further...

Fig. 14.53. Mechanism of the DIBAL reduction of carboxylic esters to aldehydes and further to alcohols. In nonpolar solvents the reaction stops with the formation of the tetrahedral intermediate A. During aqueous workup, A is converted into the aldehyde via the hemiacetal. In polar solvents, however, the tetrahedral intermediate A quickly decomposes forming the aldehyde via complex B. In the latter situation the aldehyde successfully competes with unreacted ester for the remaining DIBAL. The aldehyde is reduced preferentially, since the aldehyde is the stronger electrophile, and it is converted into the alcohol.

Acid chlorides are more reactive than other acid derivatives, and they are reduced to aldehydes by mild reducing agents such as lithium tri-ferf-butoxyaluminum hydride. Diisobutylaluminum hydride (DIBAL-H) reduces esters to aldehydes at low temperatures, and it also reduces nitriles to aldehydes. These reductions were covered in Sections 18-9,18-10, and 20-13. 

TABLE 10.3 Vapor-Phase Hydrogenation of Carboxylic Acids and Esters to Aldehydes over Modified Zr02 Catalyst0 ... 

Red-Al can reduce esters to aldehyde in the presence of N-methyl-2-pyrrolidinone (NMP), whereas without the presence of an amine only dicarbinol results. ... 

Originally devised as a method for the conversion of amino acids or amino acid esters to aldehydes. The Akabori reaction has been modihed for use in the determination of C-terminal amino acids by performing the reaction in the presence of hydrazine and for the production of derivatives useful for mass spectrometric identihcation. See Ambach, E. and Beck, W., Metal-complexes with biologically important ligands. 35. Nickel, cobalt, palladium, and platinum complexes with Schiff-bases of... 

There is no generally useful nonhydride method for the direct reduction of carboxylic acid esters to aldehydes. There are, however, procedures which are valuable under particular circumstances. An important example is the one-electron reduction of aldonolactones to aldoses. Two factors presumably contribute to the success of these reactions firstly the presence of electron-withdrawing substituents in the substrates, raising the reactivity of the carbonyl group, and secondly the ability of the products to form cyclic hemiacetals stable to further reduction. 

Dimethyl sulfoxide (DMSO), (CH3)2SO, is a versatile reagent for the oxidation of alcohols to carbonyl compounds under gentle conditions. In addition to the previously mentioned dehydrogenations, it is capable of other oxidations acetylenes to a-diketones [997], alkyl halides to aldehydes 998, 999], tosyl esters to aldehydes [1000], methylene groups adjacent to carbonyl groups to carbonyls [1001, 1002], a-halocarbonyl compounds to u-dicarbonyl compounds [1003,1004,1005], aldehydes to acids [1006], and phosphine sulfides and selenides to phosphine oxides [1007]. 

The reduction method applied is typically used for the selective reduction of carboxylic esters to aldehydes. The reaction is carried out in non-coordinating solvents such as toluene or dichloromethane. In the first step the addition of a hydride takes place, which results in the formation of stabilized tetragonal intermediates. These intermediates withstand further reductions, because the essential elimination of an aluminum alkoxide species is unfavored in non-coordinating solvents. Finally, the Al-0 bond is broken during aqueous workup, yielding a hemiacetal, which equilibrates with the aldehyde. The mode of action is depicted in the margin. ... 

The reduction of esters to aldehydes generally works best when alkoxy or amino functionality is in close proximity to the ester group, as in a- or -alkoxy esters. A neighboring alkoxy group will stabilize the tetrahedral intermediate through chelation and prevent overreduction. DIBAL-H-mediated mono-reduction of lactones delivers the corresponding lactols (hemiacetals). ... 

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