Decarbonylation, of acid halide

Delgado, F., Cabrera, A., Gomez-Lara, J. Steric and electronic influences on the reaction mechanism of the catalytic decarbonylation of acid halides in homogeneous phase using rhodium carbonyl complexes. J. Moi. Catai. 1983, 22, 83-87. 

Reductive elimination of RCl from [R(X)RhCl(CO)L2] (which occurs during the decarbonylation of acid halides using rhodium catalysts) has been assumed to involve a trans stereochemistry the fact that the yields of chlorobenzene and bromobenzene from benzoyl halides rise as the cone angle of the phosphine, L, is decreased, confirms this point of view. " ... 

Acyl halides are intermediates of the carbonylations of alkenes and organic-halides. Decarbonylation of acyl halides as a reversible process of the carbo-nylation is possible with Pd catalyst. The decarbonylation of aliphatic acid chlorides proceeds with Pd(0) catalyst, such as Pd on carbon or PdC, at around 200 °C[109,753]. The product is a mixture of isomeric internal alkenes. For example, when decanoyl chloride is heated with PdCF at 200 C in a distillation flask, rapid evolution of CO and HCl stops after I h, during which time a mixture of nonene isomers was distilled off in a high yield. The decarbonylation of phenylpropionyl chloride (883) affords styrene (53%). In addition, l,5-diphenyl-l-penten-3-one (884) is obtained as a byproduct (10%). formed by the insertion of styrene into the acyl chloride. Formation of the latter supports the formation of acylpalladium species as an intermediate of the decarbonylation. Decarbonylation of the benzoyl chloride 885 can be carried out in good yields at 360 with Pd on carbon as a catalyst, yielding the aryl chloride 886[754]. 

The four-coordinate alkyl complex, LNiI(C0)CH3, may coordinate with carbon monoxide to regenerate the five coordinate alkyl species, and this leads to insertion to form Ni-acyl complex. This complex, LNil (CO)(COCH3), can be cleaved either by water yielding acetic acid or by methanol to give methyl acetate. However, in the presence of high iodide concentration formation of acetyl iodide may predominate (29). This step is reversible and can lead to decarbonylation under low carbon monoxide partial pressure. Similar decarbonylations of acyl halides by nickel complexes are known (34). 

While most of the above carbonylations are carried out at pressures greater than 40 atm (isocyanate and acetic acid formations are exceptions), decarbonylations are low pressure reactions. Decarbonylation of acyl halides catalyzed by (3P)2RhCOCl leads either to halides (65) (Reaction 19)... 

Generally, the products formed by the catalytic decarbony-lations of acid halides are easily predicted. Some of the few instances where this is not the case are the decarbonylations of pent-3-enoyl or 2-methylbut-3-enoyl chlorides. Here the formation of ry - y complexes after decarbonylation results in several products being formed. ... 

One of the standard methods for the preparation of aldehydes involves the reduction of acid halides. A variety of stoichiometric reducing systems are available for this transfomiation, which include NaAlH(OBu-r)3, LiAlHfOBu-O.i, NaBHfOMe). Catalytic hydrogenation with H2 and Pd on carbon is also a popular method. In contrast, methods based on the radical reduction of acyl halides are synthetically less important. Radical reduction methods involve generation and subsequent hydrogen abstraction as key steps, which is complicated by decarbonylation of the intermediate acyl radicals. The first example in Scheme 4-1 shows that this competitive reaction is temperature dependent, where an acyl radical is generated from an acyl phenyl selenide via the abstraction of a phenylseleno group by tributyltin radical [5]. 

B. PALLADIUM-CATALYZED DECARBONYLATION OF ACYL HALIDES AND ACID ANHYDRIDES... 

Ynones can be generated either by carbonylative Sonogashira coupling of halides, by coupling of acid halides or by decarbonylative reaction of a-ketoacid chlorides. ... 

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],... 

Decarbonylation of aldehydes and acid halides is an important synthetic reaction (i, 2) and using various transition-metal complexes as stoichiometric or catalytic reagents for this process has... 

Heating fluorocarboxylic acid halides to 550-650 C gives the corresponding alkyl halides as the main products." " When the reaction is carried out with an acyl fluoride in the liquid phase in the presence of antimony(V) fluoride, the decarbonylation takes place at a temperature of ca. 25 to 250 C." e.g. 7H-perfluoroheptanoyl fluoride gives under these conditions l//-perfluorohexane (1). ... 

Carboxylic acid halides present interesting alternative processes (Scheme 5). Oxidative addition of acid chlorides to Pd occurs under mild conditions, and the resulting acyl-Pd complex will undergo decarbonylation upon heating. The decarbonylated product, an organo-Pd species, can undergo... 

This alternative to the Friedel-Crafts reaction, extensively developed by Stille and coworkers, is particularly important, since the reaction conditions are essentially neutral, and so provides a method for acylation of compounds containing an acid-sensitive functionality which would preclude the use of the Friedel-Crafts reaction. Reaction temperatures are often below 100 C, and high (1000-fold) turnovers of the catalyst have been achieved. Solvents employed include chloroform, toluene, and, on occasions, HMPA. Some reactions have been carried out under an atmosphere of carbon monoxide to prevent excessive decarbonylation of the acyl palladium intermediate. Indeed, carbonylative coupling of alkenylstannanes with allyl halides in the presence of carbon monoxide ca. 3 atm or greater 1 atm =101 kPa) offers an alternative to the Friedel-Crafts acylation, ketones being formed by the reaction of the stannane with the acyl species formed by carbon monoxide insertion into the allyl palladium intermediate. ... 

The a>bromo carboxylic acid required for preparation of the a>alkoxy acid is often obtained by a malonic ester synthesis the malonic ester is treated with an alkyl halide, the alkylmalonic acid obtained by hydrolysis of the product is brominated by elemental bromine at a low temperature, and the resulting a>bromo dicarboxylic acid is decarboxylated.54 This has led to a particularly important synthesis of aldehydes which utilizes the decarbonylation of -hydroxy or a-alkoxy acids for conversion of an alkyl halide RX into the aldehyde RCHO containing one more carbon atom ... 

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