Aldehyde, decarbonylation

Morimoto, Kakiuchi, and co-workers were the first to show that aldehydes are a useful source of CO in the catalytic PKR [68]. Based on 13C-labeling experiments, it was proposed that after decarbonylation of the aldehyde, an active metal catalyst is formed. This was proven by the absence of free carbon monoxide. As a consequence CO, which is directly generated by previous aldehyde decarbonylation, is incorporated in situ into the carbonylative coupling. The best results were obtained using C5F5CHO and cinnamaldehyde as CO source in combination with [RhCl(cod)]2/dppp as the catalyst system. In the presence of an excess of aldehyde the corresponding products were isolated in the range of 52-97%. 

For reviews of free-radical aldehyde decarbonylations, see Vinogradov Nikishin Russ. Chem. Rev. 1971, 40, 916-932 Schubert Kintner. in Patai, Ref. 189, pp. 711-735. 

An interesting means of improving the selectivity of Pd for the conversion of unconjugated dienes, such as 1,4-cyclooctadiene to the monoene is to add phenylacetaldehyde to the mixture undergoing reaction (ref. 36). The mechanism of action is not established but it may involve aldehyde decarbonylation to form adsorbed CO but the addition of small amounts of CO to the reactants does not reproduce the effect of the aldehyde (ref. 37). Means to modify the metal suface in other ways can prove effective, the studies of Ni catalysts by Okamoto et al. afford an interesting example of an attempt to reach a more fundamental understanding of catalyst selectivity. 

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). 

The above data which are presented in Tables I-IV show that the diphosphine complexes are effective catalysts for the aldehyde decarbonylation. Although these reactions tolerate various functional groups such as carboxylic acids, ethers, ketones, olefins, and aryl chlorides,... 

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 ... 

Acetaldehyde decomposition, reaction pathway control, 14-15 Acetylene, continuous catalytic conversion over metal-modified shape-selective zeolite catalyst, 355-370 Acid-catalyzed shape selectivity in zeolites primary shape selectivity, 209-211 secondary shape selectivity, 211-213 Acid molecular sieves, reactions of m-diisopropylbenzene, 222-230 Activation of C-H, C-C, and C-0 bonds of oxygenates on Rh(l 11) bond-activation sequences, 350-353 divergence of alcohol and aldehyde decarbonylation pathways, 347-351 experimental procedure, 347 Additives, selectivity, 7,8r Adsorption of benzene on NaX and NaY zeolites, homogeneous, See Homogeneous adsorption of benzene on NaX and NaY zeolites... 

Alcohol and aldehyde decarbonylation on Rh(l 11), activation of C-H, C-C, and C-0 bonds, 345-353 Alkane dehydroeyelization with Pt-Sn-alumina catalysts aromatic formation, 120 preparation condition effect, 119... 

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. ... 

With respect to carbonylation chemistry Sakakura and Tanaka have shown that irridiation of rhodium complex RhCl(CO)(PMe3)2 in pentane as solvent under a CO atmosphere (1 bar) at ambient temperature gives rise to carbonylated products. Selectivity for linear aldehyde is > 98 % (eq. (11)) [45]. This insertion reaction is photochemically driven, since it is known that aldehyde decarbonylation is a thermodynamically favorable process. Other photochemial C-H activation reactions have been investigated [46]. When toluene is reacted in the presence of CO and the same Rh complex, phenyl acetaldehyde is obtained as the major product (eq. (12)) [47]. 

Bernard, K. A., Atwood, J. D. Evidence for carbon-oxygen bond formation, aldehyde decarbonylation, and dimerization by reaction of formaldehyde and acetaldehyde with trans-ROIr(CO)(PPh3)2. Organometallics 1988, 7, 235-236. 

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). ° ... 

Aldehyde decarbonylation (2, 451 3, 327-329 4, 559). Suggs has isolated the acyirhodium hydride 2 in 95% yield from the reaction of 8-quinoUnecarboxy-uldehyde (1) with Wilkinson s catalyst in CH2CI2. The hydride on refluxing in benzene is converted quantitatively into quinoline and RhCl(CO)[P(C6H5)3]2. 

A considerable amount of work has been performed over the last 15 years to determine the mechanism of acid chloride decarbonylation with RhCl(PPh3)3/ " Although the discovery of aldehyde decarbonylation preceded that of acid chlorides/ much more time has been spent on the acid chloride system because it is more easily studied. Many intermediates have been isolated and characterized (see Table 1). Even though the mechanism of the catalytic reaction is not well understood, the mechanism for the stoichiometric decarbonylation of acid chlorides has been proposed. However, the generally accepted mechanism has recently been challenged/ In this section, we will first review the stoichiometric decarbonylation mechanism for acid chlorides, followed by the stoichiometric decarbonylation of aldehydes. Finally, the mechanism of catalytic decarbonlyation of acid chlorides and aldehydes will be discussed. 

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 stereochemistry of aldehyde decarbonylation has received much attention. Walborski and Allen have shown that the decarbonylation of optically active aldehydes proceeds with 93% retention of configuration,as shown in Equation 11 ... 

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