Acyl halides, decarbonylation

Coupling of aromatic acyl halides, with decarbonylation... 

Palladium chloride and metallic palladium are useful for carbonylating olefinic and acetylenic compounds. Further, palladium is active for decarbonylation of aldehydes and acyl halides. Homogeneous decarbonylation of aldehydes and acyl halides and carbonylation of alkyl halides were carried out smoothly using rhodium complexes. An acyl-rhodium complex, thought to be an intermediate in decarbonylation, was isolated by the oxidative addition of acyl halide to chlorotris(triphenylphosphine)rhodium. The mechanisms of these carbonylation and decarbonylation reactions are discussed. 

Acyl halides were decarbonylated homogeneously to olefins when they were heated to 200°C. in the presence of a catalytic amount of chlorocarbonylbis(triphenylphosphine)rhodium (XII). It is possible to... 

Recently Blum reported that chlorotris(triphenylphosphine) rhodium (XI) is an active catalyst for the decarbonylation of aroyl halides and showed several examples (2). But in this case too, the real catalyst seems to be chlorocarbonylbis(triphenylphosphine)rhodium (XII), which is formed in situ from XI by the stoichiometric reaction with acyl halides. Formation of alkyl halides by decarbonylation of acyl halides can be carried out by the Hunsdiecker reaction, but the reaction is unsatisfactory when applied to aroyl halides. Therefore, the decarbonylation reaction of aroyl halides by the rhodium complex is a new and useful means of introducing halogen onto the aromatic ring. 

The facile decarbonylation [Eq. (5)] of the acyl derivatives permits the preparation of many alkyl and aryl derivatives of manganese which are not obtainable by the direct reaction of Na[Mn(CO)j] with alkyl halides. This decarbonylation process can be thermally or photolytically effected. Chemical decarbonylation is also possible with reagents such as trimeth-ylamine N-oxide (41). 

Acyl halides are decarbonylated under mild conditions, but halides containing -hydrogens also undergo dehydrohalogenation and alkenes rather than alkanes are obtained. ° Early reports suggested that aromatic acyl chlorides, bromides and fluorides could be decarbonylated, but more recent work indicates that the reaction is not as simple as first thought and variable results have been obtained. °°... 

The acylpalladium halide complex 84 is an intermediate of catalytic decarbony-lation of aroyl halides 83 [53]. The decarbonylation of 84 generates the aryl-palladium intermediate 85 at higher temperature which undergoes facile alkene insertion. Therefore, similar to aryl halides, acyl halides can be used for the alkene insertion. The reaction is carried out with a phosphine-free Pd catalyst in the presence of tertiary amines [54]. Higher yields were obtained by using a mixture of K2CO3 and benzyltrioctylammonium chloride [55]. 

Acyi halides are reactive compounds and react with nucleophiles without a catalyst, but they are activated further by forming the acylpalladium intermediates, which undergo insertion and further transformations. The decarbonyla-tive reaction of acyl chlorides as pseudo-halides to form the aryipalladium is treated in Section 1,1.1.1. The reaction without decarbonylation is treated in this section. 

Various organotin reagents react with acyl and aroyl halides under mild conditions without decarbonylation to give carbonyl compounds[390,39l]. Alkyl- or alkenyltin reagents react with acyl and aroyl chlorides to give ketones[548.733,734]. One example is the preparation of the a,/3-dnsaturated 7-keto esters 860 and 861, carried out under a CO atmosphere[735]. The reaction has been applied intramolecularly to the synthesis of the macrocyclic keto... 

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 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],... 

Wilkinson s catalyst has also been reported to decarbonylate aromatic acyl halides at 180°C (ArCOX ArX). This reaction has been carried out with acyl iodides, bromides, and chlorides. Aliphatic acyl halides that lack an a hydrogen also give this reaction, but if an a hydrogen is present, elimination takes place instead (17-16). Aromatic acyl cyanides give aryl cyanides (ArCOCN—> ArCN). Aromatic acyl chlorides and cyanides can also be decarbonylated with palladium catalysts. °... 

It is possible to decarbonylate acyl halides in another way, to give alkanes (RCOCl —> RH). This is done by heating the substrate with tripropylsilane PrsSiH in... 

Rhodium(I) complexes are effective reagents and/or catalysts for the decarbonylation of acyl halides and aldehydes 9 11,34,195,230,231,236). The compound Rh(PPh3)3Cl, especially, has received considerable attention. The first step in such reactions involves oxidative addition to Rh(I) of the organic molecule, exemplified by the following ... 

An intermediate acylnickel halide is first formed by oxidative addition of acyl halides to zero-valent nickel. This intermediate can attack unsaturated ligands with subsequent proton attack from water. It can give rise to benzyl- or benzoin-type coupling products, partially decarbonylate to give ketones, or react with organic halides to give ketones as well. Protonation of certain complexes can give aldehydes. Nickel chloride also acts as catalyst for Friedel-Crafts-type reactions. 

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). 

Decarbonylation of Aldehydes and Acyl Halides Carbonyl-extrusion... 

It is possible to decarbonylate acyl halides in another way, to give alkanes (RCOC1 - RH). This is done by heating the substrate with tripropylsilane Pr,SiH in the presence of t-butyl peroxide.464 Yields are good for R = primary or secondary alkyl and poor for R = tertiary alkyl or benzylic. There is no reaction when R = aryl. (See also the decarbonylation ArCOCl — ArAr mentioned in 4-38.)... 

Decarbonylation of Acyl Halides Hydro-chloroformyl-elimination... 

Decarbonylation of acyl halides 7-20 Cleavage of Michael adducts 7-21 Deoxygenation of vic-diols 7-22 Cleavage of cyclic thionocarbonates 7-23 Deoxidation of epoxides 7-24 Desulfurization of episulfides 7-25 Reaction of a-halo sulfones with bases (Ramberg-Backlund)... 

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). 

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)... 

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