Hydrogenation of acids

The selective hydrogenation of acid chlorides to aldehydes is known as the Rosenmund reduction (49). 

Like aldehydes and ketones, the a-hydrogens of acid and acid derivatives are acidic and can be abstracted with base to generate the carban-ions, which can then react with various electrophiles such as halogens, aldehydes, ketones, unsaturated carbonyl compounds, and imines, to give the corresponding products. Many of these reactions can be performed in aqueous conditions. These have been covered in related chapters. 

Water does not dissolve many organic compounds but it can be used as a solvent, especially in hydrogenations of acids and their salts. It may have some deleterious effects for example it was found to enhance hydrogenolysis of vinylic halogens [66],... 

PLACE A STRIP OF ZINC IN A TEST TUBE. POUR A FEW ml HYDROCHLORIC ACID ON IT. ZINC DISSOLVES, SETTING THE HYDROGEN OF ACID FREE. 

The catalytic hydrogenation of acid chlorides allows the formation of aldehydes. 

Scheme 10.8 The cleavage of the linkages in the hydrogenation of acid amides.

Rosenmund reduction , i.e. catalytic hydrogenation, of acid chlorides with poisoned Pd-BaS04 (in the presence of sulfur and quinoline poison) afforded aldehyde. 

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. 

Selective hydrogenation of acids to aldehydes is very difficult under high pressure, because the product is, in general, more easily hydrogenated than the substrate over conventional catalysts. The key point of our research was how to fine-tune the properties of the catalyst in such a way that it becomes active and selective. 

Nevertheless only scare data is available in the recent literature on the application of Group VIII noble metal (M) or rhenium-based mono- and Re-M bimetallic catalysts, in the hydrogenolysis of esters or hydrogenation of acids to alcohols. Recently a few publications, - and patents. have been reported on the transformation of different carbonyl compounds (saturated and unsaturated esters, acids and carboxamides) over rhenium-containing catalysts. In the bimetallic catalysts used for the hydrogenation of carbonyl compounds the rhenium was combined with Pd, or Rh. In the case of catalysts used for the hydrogenation of unsaturated carbonyl compounds the rhenium is usually modified with tin. ... 

The data of Figs. 10-26, /, g, and h may be used to study the direct hydrogenation of acids to alcohols. For example. 

The reader will perceive that while I have given both the rival theories of the constitution of Acids and Salts, I have indicated a decided preference for the new one, that, namely, which considers all hpdrated oxygeit adds as really hydrogen adds, and thus unites all acids into one series, while salts are viewed as compounds in which the hydrogen of acids is replaced, wholly or partially, by its equivalent of metals. 

Hydrogenation of acid or methyl ester occurs at 250-300 bar and 250-300 °C in the presence of about 2% of copper chromite. Under these conditions double bonds are also reduced so that the products are mainly saturated alcohols. Special catalysts which do not reduce unsaturated centres have been described but these are not commonly employed (see also Section 10.1.2). 

Although phosphinite- and NHC-based iridium catalysts show very similar enantiose-lectivities in the hydrogenation of various olefins, replacement of the phosphinite group by an A-heterocyclic carbene (NHC) unit results in particularly effective catalysts which are much better suited for the hydrogenation of acid-sensitive substrates beeause of the lower acidity of iridium hydride intermediates produced. The new NHC-pyridine ligands are also likely to prove useful for other applications in asymmetric catalysis." ... 

The reduction of acid derivatives to aldehydes requires control because aldehydes are more easily reduced than the acid derivatives. The hydrogenation of acid chlorides in the presence of quinoline-5 poisoned palladium catalyst, which is a modification of the Rosenmund reduction, shows this selectivity (Eq. 6.19) [33]. 

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