Acid halide decarbonylation

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]. 

Carboxylic acid chlorides and chloroformate esters add to tetrakis(triphenylphosphine)palladium(0) to form acylpalladium derivatives (equation 42).102 On heating, the acylpalladium complexes can lose carbon monoxide (reversibly). Attempts to employ acid halides in vinylic acylations, therefore, often result in obtaining decarbonylated products (see below). However, there are some exceptions. Acylation may occur when the alkenes are highly reactive and/or in cases where the acylpalladium complexes are resistant to decarbonylation and in situations where intramolecular reactions can form five-membered rings. 

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). ... 

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. ... 

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... 

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. 

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]. 

The mechanism of aldehyde decarbonylation is thought to follow the established mechanism for acyl halide decarbonylation discussed in the previous section (Equation 7, where A = H). Several observations support this idea, even though intermediates are much more labile than those of the acid chloride system. 

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. " ... 

Wilkinson s catalyst and chlorocarbonylbis(triphenylphosphine)rhodium [or iridium] can catalyze the thermal decarbonylation of aromatic acid halides to aryl halides. An intermediate Rh(III) hydride is involved in the reaction (Suggs, 1978). For aliphatic acid halides, subsequent elimination of HX frequently occurs from the generated alkyl halide (Ohno and Tsuji 1968 Blum et al., 1%7, 1971 Strohmeier and Pfohler, 1976). 

Other sources for cross-coupling reactions are aldehydes and carboxylic acids after decarbonylation and decarboxylation, respectively, which can be reacted with aryl halides to form biaryls. The following Experimental Procedure illustrates the potential of this quite atom-economic reaction. In this case, a copper co-catalyst promotes both the decarboxylation and the cross-coupling. 

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],... 

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). 

Many carboxylic acids lose carbon dioxide on either direct or sensitized irradiation, and in some cases (4.10 the evidence points to the operation of an initial electron-transfer mechanism rather than primary a-deavage. Cleavage occurs readily with acyl halides, and this can [ead to overall decarbonylation (4.11). Aldehydes also cleave readily, since the (0=)C—H bond is more prone to homolysis than the (0= C-C bond. This offers a convenient method for replacing the aldehydic hydrogen by deuterium in aromatic aldehydes (4.12. and a similar initial reaction step accounts for the production of chain-Iengtheped amides when formamide is irradiated in the presence of a terminal alkene (4.13). 

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