Acyl aldehydes, decarbonylation

The acyl radicals obtained by hydrogen abstraction from aldehydes easily attack protonated heteroaromatic bases. With secondary and tertiary acyl radicals decarbonylation competes with the aromatic acylation [Eq. (12)]. 

Equations 2c and 2d show the acyl-alkyl migration and reductive elimination steps, respectively. There is good evidence that this same mechanistic scheme applies to the decarbonylation of aldehydes (see Equation set 2, X = H), although in this case reaction intermediates have not been isolated (3, 5, 9, 18). Additionally, evidence exists that the rate-determining step is oxidative addition for aldehyde decarbonylation (see Equation 2b, X = H) (3, 9, 18). Several recent reports have shown that for some special aldehydes, oxidative addition of the carbonyl-hydrogen bond indeed does occur using rhodium(I) complexes (8,19). In these studies a stable chelate was formed after oxidative addition that enabled isolation and characterization of the products (8, 19). 

While adsorbed primary alcohols on the Pd( 111) surface dehydrogenate sequentially to form the corresponding adsorbed aldehyde and acyl species prior to their decarbonylation (23), we have found no evidence for aldehyde formation from primary alcohols on Rh(l 11) (2124). Instead, alcohol and aldehyde decarbonylation pathways on Rh(lll) appear to be non-intersecting. This surprising divergence of reaction pathways for such closely related molecules is demonstrated by two critical observations ... 

Considerable information about the course of aldehyde decarbonylations has been gleaned from the decarbonylations of alk-4-enals. Pent-4-enals form cyclopentanones in high yield in decarbonylations catalyzed by [RhCl(PPh3)3], The major product from the decarbonylation of hex-4-enal is 2-methylcyclopentanone. As shown in Scheme 5, the cyclization reaction requires a vacant site on rhodium. The other products result from decarbonylation of the unsaturated acyl before cyclization can take place. In these cases, there is competition between addition of deuterium to C-1 of the alkenyl ligand or its addition to the alkene bond and the formation of an unstable metallocycle. ... 

The rhodium complex [RhCl(PPh3)3] readily brings about stoichiometric decarbonylation of aldehydes, acyl halides and diketones. A typical aldehyde decarbonylation is illustrated by equation (69). a,3-Unsaturated aldehydes are decarbonylated stereospecifically (equation 70), while with chiral aldehydes the stereochemistry is largely retained (equation 71). ° ... 

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. 

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],... 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

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 ... 

Hydrocarbon formation involves the removal of one carbon from an acyl-CoA to produce a one carbon shorter hydrocarbon. The mechanism behind this transformation is controversial. It has been suggested that it is either a decarbonylation or a decarboxylation reaction. The decarbonylation reaction involves reduction to an aldehyde intermediate and then decarbonylation to the hydrocarbon and releasing carbon monoxide without the requirement of oxygen or other cofactors [88,89]. In contrast, other work has shown that acyl-CoA is reduced to an aldehyde intermediate and then decarboxylated to the hydrocarbon, releasing carbon dioxide [90]. This reaction requires oxygen and NADPH and is apparently catalyzed by a cytochrome P450 [91]. Whether or not a decarbonylation reaction or a decarboxylation reaction produces hydrocarbons in insects awaits further research on the specific enzymes involved. 

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. 

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

CO into a metal-hydrogen bond, apparently analogous to the common insertion of CO into a metal-alkyl bond (6). Step (c) is the reductive elimination of an acyl group and a hydride, observed in catalytic decarbonylation of aldehydes (7,8). Steps (d-f) correspond to catalytic hydrogenation of an organic carbonyl compound to an alcohol that can be achieved by several mononuclear complexes (9JO). Schemes similar to this one have been proposed for the mechanism of CO reduction by heterogeneous catalysts, the latter considered to consist of effectively separate, one-metal atom centers (11,12). As noted earlier, however, this may not be a reasonable model. 

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). 

Upon acylation of some benzyl carbonyl compounds (25, R = H, Me 51, R = OH) dibenzo[a,tropylium salts 65 have been isolated in low yields (5-15 %) along with the major products, 2-benzopyrylium salts. Veratryl acetone 25 (R = Me) as well as homoveratric aldehyde 25 (R = H) (or carboxonium ions 31 which are formed from them) may undergo an oxidative a-cleavage, resulting in the benzyl cation 64. The formation of the same cation from homoveratric acid 51 is the result of decarbonylation of the acylium ion 63. Further interaction of the benzyl cation 64 with the substrate, followed by cyclization and oxidation, results in the polycyclic tropylium salts 65 (82ZOR589). 

Phenyl selenoesters have been reported to undergo reduction to the corresponding aldehydes and/or alkanes in the presence of (TMS)3SiH under free-radical conditions16. The decrease of aldehyde formation through the primary, secondary and tertiary substituted series, under the same conditions, indicated that a decarbonylation of acyl radicals takes place. Equation 11 shows an example of a tertiary substituted substrate. 

The decarbonylation of acyl halides and aldehydes proceeds under mild conditions to give aryl halides ArX and arenes ArH with a stoichiometric amount of RhCl(Ph3P)3. At the same time, RhCl(CO)(Ph3P)2 is formed which is inactive at moderate temperatures, and the reaction is stoichiometric [245-248],... 

The biosynthesis of hydrocarbons occurs by the microsomal elongation of straight chain, methyl-branched and unsaturated fatty acids to produce very long-chain fatty acyl-CoAs (Figure 11.1). The very long chain fatty acids are then reduced to aldehydes and converted to hydrocarbon by loss of the carboxyl carbon. The mechanism of hydrocarbon formation has been controversial. Kolattukudy and coworkers have reported that for a plant, an algae, a vertebrate and an insect, the aliphatic aldehyde is decarbonylated to the hydrocarbon and carbon monoxide, and that this process does not require cofactors (Cheesbrough and Kolattukudy, 1984 1988 Dennis and Kolattukudy, 1991,1992 Yoder et al., 1992). In contrast, the Blomquist laboratory has presented evidence that the aldehyde is converted to hydrocarbon and carbon dioxide in a process that... 

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