Decarbonylation mechanism

The rate of decomposition of (C2H5)4N+ 25 was found to be first order and independent of added phosphite. At 63°C, the following activation parameters were obtained A//1 = 29.0 1.5 kcal/mol A5 = 7.9 6.1 eu. These data suggest that (provided the reaction is analogous to well-established metal acyl decarbonylation mechanisms) (4, 5) loss of phosphite is followed by rapid hydride migration to the metal, as shown in Eq. (31). None of the formyl could be detected to be in equilibrium with (CO)4FeH , even in the presence of excess (ArO)3P. 

Yoder J. A., Denlinger D. L., Dennis M. W. and Kolattukudy P. E. (1992) Enhancement of diapausing flesh fly puparia with additional hydrocarbons and evidence for alkane biosynthesis by a decarbonylation mechanism. Insect Biochem. Molec. Biol. 22, 237-243. 

This decarbonylation mechanism is involved in the following reactions ... 

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. 

Madsen et al. performed a mechanistical investigation [47] on their previously published reaction shown in Scheme 8.2 [48]. In this study, both benzaldehyde-and phenylacetaldehyde analogs were used, but for reasons of brevity, we will focus on the results concerning the benzaldehyde model substrate. The goal of the study was to elucidate the details of the decarbonylation mechanism and confirm these details with experimental evidence. 

From these facts, a mechanism of the Rosenmund reduction has been proposed, in which the formation of the acylpalladium species 893 is the first step of the aldehyde formation and also the decarbonylation, although the Rosenmund reduction proceeds under heterogeneous conditions[744]. 

Analogously, the tetrabromotetrapropylporphycene 9 furnishes the corresponding isocorroie-carbaldehyde 10 in good yields, but in this case no decarbonylated product was observed. The mechanism of these interesting ring contractions of porphycenes into isocorroles still needs to be determined. 

This was proved by showing that the reciprocal of the life-time of the pivaloyl ion, t5co+> is independent of the concentration of t-butyl ion. A mechanism based on equation (7) would have resulted in t5,co+ being proportional to [t-C4H ]. The rate constant of decarbonylation of t-C4H9CO+ in HF—SbFg (equimolar) and in FHSO3—SbFs (equimolar) was determined to be sec. This... 

The decarbonylation of aromatic aldehydes with sulfuric acid" is the reverse of the Gatterman-Koch reaction (11-16). It has been carried out with trialkyl- and trialkoxybenzaldehydes. The reaction takes place by the ordinary arenium ion mechanism the attacking species is H and the leaving group is HCO, which can lose a proton to give CO or combine with OH from the water solvent to give formic acid." Aromatic aldehydes have also been decarbonylated with basic catalysts." When basic catalysts are used, the mechanism is probably similar to the SeI process of 11-38. See also 14-39. 

The insertion of a carbonyl group into a metal-alkyl or metal-aryl bond, and the reverse reaction involving decarbonylation of an acyl complex, have been studied from both the synthetic and mechanistic points of view. The mechanism proposed for this type of reaction seems well established and is... 

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. 

The decarbonylations, which do not appear to be affected by light, are reasonably selective with aromatic aldehydes, yielding the expected product however, significant amounts of other products are obtained with non-aromatic substrates (e.g. cyclohexane-aldehyde gives methylcyclopentane and small amounts of n-hexane, as well as the expected cyclohexane and cyclohexen-4-al gives both cyclohexene and cyclohexane). Indeed, the unexpected products perhaps provided a major clue to an understanding of the reaction mechanism(s) involved. 

However, the decarbonylation reaction can be suppressed by the use of specially tailored chelating groups. Intermolecular processes involving dienes and salicylaldehydes are now known, and are thought to proceed via a double chelation mechanism, akin to the Jun-type system. Rhodium-catalyzed reactions lead to hydroacylated products, under relatively mild conditions (Equation (134)).117... 

The mechanism for this process is presumed to involve activation of the formyl C-H bond, following pyridine coordination, by the Ru catalyst, which may be mononuclear or in a cluster form. Such chelation suppresses a decarbonylation route (Scheme 26).119... 

The Dotz reaction mechanism has received further support from kinetic and theoretical studies. An early kinetic investigation [37] and the observation that the reaction of the metal carbene with the alkyne is supressed in the presence of external carbon monoxide [38] indicated that the rate-determining step is a reversible decarbonylation of the original carbene complex. Additional evidence for the Dotz mechanistic hyphotesis has been provided by extended Hiickel molecular orbital [23, 24] and quantum chemical calculations [25],... 

In Scheme 2 the general mechanism of this transformation is shown. Clearly, the in situ formation of CO by a decarbonylation process takes advantage of the formation of metal carbonyls, which are known to be the key intermediates in the PKR. 

The greater lability of complex 146.C (compared to 145.c), as evinced by the much shorter reaction time, is typical of those that bear a carbomethoxy or acetyl substituent at the central carbon of an i73-allylic ligand. The temperature required for complete decarbonylation of complexes of type 146 and 148 increases with the size of the R-substituent, which suggests a mechanism involving hydride transfer.111 This would also explain the observed activating effect of the centrally located carbomethoxy group in 146.C, which would clearly labilize the methyl proton shown explicitly in 146. 

Alexander, J.J. Mechanism of Photochemical Decarbonylation of Acetyldicar-bonyl-r -Cyclopentadienyliron. J. Am. Chem. Soc. 1975, 97, 1729-1732. 

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