BaS04-poisoned palladium catalyst

Hydrogenation reduction of acid chloride to aldehyde using BaS04-poisoned palladium catalyst. Without poison, the resulting aldehyde may be further reduced to alcohol. 

For example, reduction of acid chlorides (RCOCl), prepared from carboxylic acids by reaction between the acid and, for example, thionyl chloride (vide infra), with hydrogen over a barium sulfate (BaS04) poisoned palladium (Pd) catalyst (the Rosenmund reduction), can often be used to produce the corresponding aldehyde (RCHO). The same product can more easily be obtained from the same starting material by using commercially available lithium aluminum tri-r-butoxy hydride (LiAlH[OC(CH3)3]3) in an ether solvent,such as bis(2-methoxyethyl)ether [diglyme, (CH30CH2CH2)20], at -78°C (Scheme 9.106). 

The use of Raney nickel and of noble metals (palladium) may be singled out from the catalytic processes available in this field. For instance, Wessely and Sinwel434 hydrogenated substituted benzyl halides with a 20% Pd-BaS04 catalyst in alcohol in presence of dimethylaniline. Addition of a base is essential to bind the acid formed which would otherwise poison the catalyst. 

In the Rosenmund reaction (Eq. 5-84), acid chlorides are hydrogenated to aldehydes. The catalyst is a supported palladium catalyst (5 % Pd/BaS04) poisoned by sulfur compounds such as quinoline, tiourea, or thiophene to prevent further reduction of the aldehyde. 

A catalyst known as Lindlar s catalyst (which you will meet again in Chapter 27) is used to reduce alkynes to alkenes, but does not easily reduce alkenes to alkanes. This requires rather subtle chemoselectivity alkenes are usually hydrogenated at least as fast as alkynes, so we need to be sure the reaction stops once the alkene has been formed. The Lindlar catalyst is a palladium catalyst (Pd/CaC03) deliberately poisoned with lead. The lead lessens the activity of the catalyst and makes further reduction of the alkene product slow most palladium catalysts would reduce alkynes all the way to alkanes. Best selectivities are obtained if quinoline is also added to the reaction, and alkyne to alkene reductions work with Pd/ BaS04 -I- quinoline too. Even so, Lindlar reactions often have to be monitored carefully to make sure that over-reduction is not taking place. 

Addition of 1 mol of hydrogen to the carbon-carbon triple bond can be accomplished stereospecifically. Catalytic reduction leads to the cis isomer. This is most often carried out using Lindlar catalyst, a lead-poisoned palladium-on-calcium carbonate preparation. Palladium on BaS04 is an alternative. Some examples are recorded in Scheme 3.10. Numerous other catalyst systems have been employed to effect the same reduction. Many specific cases are cited in reviews of catalytic hydrogenations. If the trans alkene is desired, the usual method is a dissolving-metal reduction in ammonia. This reaction is believed to involve two successive series of reduction by sodium and protonation ... 

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