Aldehydes from carboxylic acid reduction

Until about 1950, reduction of carboxylic acids and their derivatives to aldehydes was not straightforward, and even one of the best methods, the Rosenmund hydrogenation of acid chlorides, required very careful control of both the reaction conditions and preparation of catalyst. The advent of aluminum and boron hydrides and their ready commercial availability transformeKl the situation to such an extent that the formation of aldehydes from carboxylic acids, acid chlorides, esters, amides, nitriles and similar groups in the presence of other reducible functional groups has become a relatively easy operation on both small and large scale. 

One of the most useful applications of the alkoxy reagents is in the preparation of aldehydes from carboxylic acids by partial reduction of the acid chlorides or dialkylamides. Acid chlorides are readily reduced with lithium aluminium hydride or with sodium borohydride to the corresponding alcohols, but with one equivalent of lithium tri-t-butoxyaluminium hydride, high yields of the aldehyde can be obtained, even in the presence of other functional groups (7.74). 

Aldehydes and Ketones from Carboxylic Acids. Reduction of the derived acylimidazole (2) with Lithium Aluminum Hydride achieves conversion of an aliphatic or aromatic carboxylic acid to an aldehyde (eq 6). DiisobutyUduminum Hydride has also been used, allowing preparation of a-acylamino aldehydes fromiV-protected amino acids. Similarly, reaction of... 

Palladium-carbon sodium acetate Aldehydes from carboxylic acid chlorides Modified Rosenmund-Zetzscfae reduction... 

Reductions with diisobutylaluminum hydride Amines and aldehydes from carboxylic acid amides... 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

A related convenient and mild method for the preparation of aliphatic and aromatic aldehydes in high yield from carboxylic acids is the reductive cleavage of l-acyl-3-methylimidazole-2-thiones by tributylstannane. 

The mixture of the reaction products can be shifted from oxidation to reduction products depending on the method chosen to work up the reaction mixture, and can include chain scission products such as alcohols, aldehydes, ketones, carboxylic acids, and diacids. 

Ozone, while somewhat inconvenient to use, is way qiecific in its reactions with alkenes. It is widely employed for selective synthesis, for qualitative and quantitative analysis of unsaturated compounds, and for studying the position of double bonds in macromolecules. The nature of the products obtained from ozonolysis reactions is determitted by the way in which the reaction is carried out Different workup procedures (hydrolytic, reductive or oxidative) can be used to produce alcohols, aldehydes, ketones, carboxylic acids or esters. 

Hydrocarbon oxidations are also possible at Pt electrodes at elevated temperatures, for example, 250°C in phosphoric acid (92). For aliphatic hydrocarbons it is of some special interest that electrochemical oxidations all the way to CO2 and H2O or H can be achieved at Pt (61). Oxidation of olefins is also possible, but under some conditions, for example, at Pd, aldehydes are a product (62, 93). The fact that aliphatic hydrocarbons can be oxidized largely to CO2 plus H2O indicates that the intermediate stages in such multielectron oxidations must proceed successively on the electrode surface with a series of intermediates remaining chemisorbed, as otherwise aldehydes or carboxylic acids would appear in solution, which is not normally observed. Interesting attempts were made by Bruckenstein (94) to identify some of the intermediates by reductive desorption from porous electrodes into a mass spectrometer. 

The synthesis and some reactions of the aldehyde (371) from the alkene (370) have been described, e.g. oxidation to the carboxylic acid, reduction to the 4-methyl compound, and Grignard and Wittig reactions. When the dioxaphos-phorinan (372), in which all methyl groups are equatorial, was oxidized with peroxide, a mixture of the stereoisomers (373) was obtained. 

Reduction of aldehydes and ketones allylic alcohols from a, 3-unsaturated aldehydes and ketones alcohols from carboxylic acid chlorides amines from aliphatic azides.21 ... 

Aldehydes are formed by oxidizing a primary alcohol. In the laboratory potassium dichromate(VI) is used in sulfuric acid. They can be further oxidized to carboxylic acids. Reduction (using a catalyst or nascent hydrogen from sodium amalgam in water) produces the parent alcohol. 

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