By the Rosenmund reduction

Commercially available 2-ethylhexanal (Eastman practical grade) is purified by fractional distillation b.p. 163-163.2°/atm., no 1.4133. Other aldehydes are conveniently prepared by the Rosenmund reduction.2 If the aldehyde is relatively unstable toward autoxidation,3 a catalytic amount (0.5-1.0 g.) of hydroquinone is added with the aldehyde-bromo ester solution. 

Cyclobutanecarboxaldehyde has been prepared in very low yield by the Rosenmund reduction procedure.6 A 46% yield of the 2,4-dinitrophenylhydrazone derivative has also been reported, with the aldehyde formed as an intermediate, in the reaction of the acid chloride and lithium tri-Mmtoxyaluminum hydride at —78° in diglyme.7... 

Thiophenealdehyde has been prepared by the decarboxylation of 2-thienylglyoxalic acid, by the action of 2-thienylmagne-sium iodide on ethyl orthoformate followed by hydrolysis of tbe acetal, in small yields by the Rosenmund reduction of 2-thio-phenecarboxylic acid chloride, in small yields by the action of hydrogen cyanide, hydrogen chloride, and aluminum chloride on thiophene using benzene as a solvent, by a series of reactions from l-chloro-2,3-diketocyclopentane, by the hydrolysis of 2-thienylmethylhexamethylenetetrammonium chloride in neutral solution, and by the action of N-mcthylformanilidc on thiophene in the presence of phosphorus oxychloride. ... 

Acid chlorides can be reduced to aldehydes either by catalytic hydrogenation or by reaction with a metal hydride. In both cases, the reagent and the reaction conditions are selected to avoid the further reduction of the aldehyde. The conversion of an acid chloride to an aldehyde can be carried out by the Rosenmund reduction, which uses hydrogen gas and a modified palladium catalyst. The palladium catalyst is altered to prevent further reduction of the aldehyde. To prepare the catalyst, the palladium is treated with quinoline, an aromatic heterocyclic amine, and is heated with sulfhr. 

The Pd-catalyzed hydrogenoiysis of acyl chlorides with hydrogen to give aldehydes is called the Rosenmund reduction. Rosenmund reduction catalyzed by supported Pd is explained by the formation of an acylpalladium complex and its hydrogenolysis[744]. Aldehydes can be obtained using other hydrides. For example, the Pd-catalyzed reaction of acyl halides with tin hydride gives aldehydes[745]. This is the tin Form of Rosenmund reduction. Aldehydes are i ormed by the reaction of the thio esters 873 with hydrosilanes[746,747]. 

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],... 

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... 

The procedure described is a modification of the general procedure of Angyal2 for the preparation of aldehydes from benzylamines by the Sommelet reaction. Isophthalaldehyde has been prepared from w-xylene by preparation of the tetrachloro derivative and hydrolysis,3 from isophthaloyl chloride by the Rosenmund reaction,4 from a,a -dibromo-m-xylene by the Sommelet reaction,5 and from isophthaloyl chloride by reduction with lithium tri-Cbutoxyaluminumhydride.6... 

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). 

A variation of the Rosenmund reduction is heating of an acyl chloride at 50° with an equivalent of triethylsilane in the presence of 10% palladium on charcoal. Yields of aldehydes obtained by this method ranged from 45% to 75% [80]. 

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. 

The catalytic reduction of carboxylic acid chlorides by the Rosenmund procedure may be used for the preparation of aliphatic aldehydes but its application is mainly for the synthesis of aromatic aldehydes (e.g. Expt 6.120). Alternative procedures for the chemical reduction of acid chlorides include reduction with... 

Reduction of acid chlorides to aldehydes One of the most useful synthetic transformations in organic synthesis is the conversion of an acid chloride to the corresponding aldehyde without over-reduction to the alcohol. Until recently, this type of selective reduction was difficult to accomplish and was most frequently effected by catalytic hydrogenation (the Rosenmund reduction section 6.4.1). However, in the past few years, several novel reducing agents have been developed to accomplish the desired transformation. Among the reagents that are available for the partial reduction of acyl chlorides to aldehydes are bis(triphenylphosphine)cuprous borohydride , sodium or lithium tri-terf-butoxyaluminium hydride, complex copper cyanotrihydridoborate salts °, anionic iron carbonyl complexes and tri-n-butyltin hydride in the presence of tetrakis(triphenylphosphine)palladium(0). ... 

The synthesis of aldehydes by the Sommclet reaction, the Rosenmund reduction and the Stephens reaction all involve the conversion of a group already present in the molecule. The Rosenmund reduction (Scheme 6.7) is the catalytic hydrogenation of a benzoyl chloride in the presence of a catalyst poison, quinoline/sulfur, which prevents over-reduction to the alcohol. In the Stephens reaction (Scheme 6.7), a nitrile group is reduced by tin(II) chloride and hydrochloric acid to an imine salt, which is then hydrolysed. 

Acyl halides are sensitive to the same groups of reducing agents that are used with aldehydes and ketones. However, it is possible, by using a catalyst poison such as barium sulfate, to stop the reduction at the stage at which only the halogen atom has been replaced by hydrogen, as in the Rosenmund reduction (Scheme 3.65). 

However, in 1944 it was reported by Sakurai and Tanabe that with acetone or ethyl acetate as solvent, Pd-BaSOa as catalyst (no regulator) and in the presence of A, A -dimethylaniline as an HCl acceptor, the Rosenmund reduction occurs cleanly and rapidly at room temperature. More recently, three other groups have reported mild and effective variants of the Rosenmund. In each case the key element appears to be the use of a basic additive to remove the HCl produced by the reduction and possibly also to regulate the catalyst activity. All the methods involve the use of closed hydrogenation apparatus and thus have a clear safety advantage over the classical procedure. 

The mechanism of the Rosenmund reduction has attracted occasional attention. The discovery of Tsuji et al that palladium metal can catalyze the conversion of acyl chlorides (11) into alkenes (13), carbon monoxide and HCl, presumably via intermediates of the form (12 Scheme 5), suggests that (12) may also be an intermediate in the Rosenmund reaction. Consistent with this suggestion is the observation that the unsaturated acyl chloride (14) is converted to phenol by a palladium catalyst.22... 

Tsuji, J., Ono, K., Kajimoto, T. Organic syntheses with noble metal compounds. XX. Decarbonylation of acyl chloride and aldehyde catalyzed by palladium and its relation with the Rosenmund reduction. Tetrahedron Lett. 1965, 4565-4568. 

The first synthesis, that of eri/fftro-tetrodialdose, was made by Wohl and Mylo, starting from acetylene. The basis of the method employed (both here and in the further syntheses of this dialdose) is the hydroxylation of malealdehyde or a derivative of the latter. The next method introduced was Uenzelmann s application of catalytic reduction by the Rosenmund procedure to tetra-O-acetylmucyl dichloride he thus obtained the corresponding tetraacetate of gaZacto-hexodialdose. By this procedure, the only known heptodialdose has also been prepared. Another reduction procedure, controlled reduction of D-glucurono-6,3-lactone with borohydride or sodium amalgam, was used by MacDonald and H. O. L. Fischer and by F. G. Fischer and Schmidt. ... 

The use of both types of modifier to influence the selectivity of heterogeneous catalysts is not new. It has long been known, for example, that modifiers can have a powerful selectivity-enhancing effect in catalytic hydrogenation the Rosenmund reduction of acid chlorides to aldehydes is an early example of this. Another well-known modifier effect is rate and selectivity enhancement by bismuth in precious metal-catalyzed oxidations (Section 9.3). We feel, however, that the enormous po-... 

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