Rosenmund catalysts

The reduction of acyl halides with hydrogen to form aldehydes using Pd catalyst is well known as the Rosenmund reduction[756]. Some acyl chlorides give decarbonyiation products rather than aldehydes under Rosenmund conditions. The diene 890 was obtained by decarbonyiation in an attempted Rosenmund reduction of acetyloleanolic acid chloride (889)[757], Rosenmund reduction of sterically hindered acyl chlorides such as diphenyl- and tnpheny-lacetyl chloride (891) gives the decarbonylated products 892[758],... 

The catalyst commonly used in this method is 5 wt % palladium supported on barium sulfate inhibited with quinoline—sulfur, thiourea, or thiophene to prevent reduction of the product aldehyde. A procedure is found in the Hterature (57). Suitable solvents are toluene, benzene, and xylene used under reflux conditions. Interestingly, it is now thought that Rosenmund s method (59) originally was successful because of the presence of sulfur compounds in the xylene used, since the need for an inhibitor to reduce catalyst activity was not described until three years later (60). 

Rosenmund reaction is the action between acid chloride and hydrogen in the presence of palladium catalyst to produce aldehydes... 

As catalyst for the Rosenmund reaction palladium on a support, e.g. palladium on barium sulfate, is most often used. The palladium has to be made less active in order to avoid further reduction of the aldehyde to the corresponding alcohol. Such a poisoned catalyst is obtained for example by the addition of quinoline and sulfur. Recent reports state that the reactivity of the catalyst is determined by the morphology of the palladium surface." ... 

A number of side-reactions may be observed with the Rosenmund reduction, which however can be avoided by proper reaction conditions. A poorly deactivated catalyst will lead to reduction of aldehyde 2 to the alcohol 4, or even to... 

In certain sensitive compounds, such as triphenylacetyl chloride, decar-bonylation may be the major reaction (59). Palladium, the preferred catalyst for the Rosenmund reduction, is also an excellent catalyst for decarbonylation of aldehydes (27,28,65), and decarbonylation may occur after aldehyde formation. 

Carboxylic acid chlorides can be converted to aldehydes by hydrogenolysis on a poisoned Pd/BaS04 catalyst. This is the classic Rosenmund reduction. Over more active catalysts, further hydrogenation gives the corresponding alcohols. Bases are used to react with the HCl. For example, 2,5-dimethylpyridine was used as base in a Rosenmund reduction.457... 

Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called Rosenmund reduction. 

Palladium catalysts are more often modified for special selectivities than platinum catalysts. Palladium prepared by reduction of palladium chloride with sodium borohydride Procedure 4, p. 205) is suitable for the reduction of unsaturated aldehydes to saturated aldehydes [i7]. Palladimn on barium sulfate deactivated with sulfur compounds, most frequently the so-called quinoline-5 obtained by boiling quinoline with sulfur [34], is suitable for the Rosenmund reduction [i5] (p. 144). Palladium on calcium carbonate deactivated by lead acetate Lindlar s catalyst) is used for partial hydrogenation of acetylenes to cw-alkenes [36] (p. 44). 

Acid chlorides, R(Ar)COCl, are reduced to R(Ar)CHO by Hj/Pd(S), a moderate catalyst that does not reduce RCHO to RCHjOH (Rosenmund reduction). Acid chlorides, esters (R(Ar)COOR), and nitriles (RC N) are reduced with lithium tri-t-butoxyaluminum hydride, LiAlH[OC(CH3)3]j, at very low temperatures, followed by HjO. The net reaction is a displacement of X by H",... 

Notes Used for the conversion of an acid chloride to an aldehyde. Quinoline is often added to further reduce catalyst activity. See Rosenmund Reduction. 

This preparation illustrates Rosenmund s method of synthesizing aldehydes without use of a poisoned catalyst. See p. 84 for the more general form of the method. 

For use in the Rosenmund reduction (Expt 6.120) the catalyst is moderated by the addition of the appropriate quantity of a quinoline-sulphur poison prepared in the following manner. Heat under reflux 1 g of sulphur with 6g of quinoline for 5 hours and dilute the resulting brown liquid to 70 ml with xylene which has been purified by distillation over anhydrous aluminium chloride. Thiourea (about 20% by weight of the palladium-barium sulphate catalyst) may also be used as a catalyst poison. 

Acid chlorides can be selectively hydrogenated in the presence of a catalyst (palladium deposited on a carrier, which is usually barium sulphate but is occasionally charcoal). The reaction, which involves the hydrogenolysis of the carbon-halogen bond, is known as the Rosenmund reduction and has been widely used for the synthesis of aromatic and heterocyclic aldehydes. 

In general, the Rosenmund reaction is carried out by heating one mole of trisodium or tripotassium arsenite with one mole of aromatic halide in boiling aqueous ethanol solution. Certain minor modifications of this procedure have been employed. If the aromatic halide contains an addic group (e.g., o-bromophenylarsonic add), no ethanol is necessary.82 Copper powder, as well as cuprous chloride, is sometimes employed as catalyst, but the value of either is uncertain.82- 86 Reactions of bromo-benzene and p-br omo acetophepo ne must be run in sealed tubes at temperatures of 150-2000.88 87 82... 

Pd-BaS04 catalyst has often been used for selective hydrogenations such as the Rosenmund reduction (Chapter 13) and the hydrogenation of acetylenic to ethylenic compounds (see, e.g., Chapter 4, eqs. 4.8-4.10). 

---