Mechanism of decarbonylation

CIDNP studies have proven to be a valuable tool in investigating the mechanisms of decarbonylation and disproportionation reactions in micelles27 29). Since the mechamisms involve the formation of triplet radical pairs, nuclear polarization of the protons near the radical centers occurs and results in the observation of emission or enhanced absorption in the NMR spectra of products of the radical pairs. For example, the photolysis of di-t-butyl ketone (11) in HDTCI yields both decarbonylation and disproportionation products (Scheme VII)27,29). The CIDNP spectra (Fig. 12) taken at various concentrations of copper chloride (free radical scavenger) illustrates that the intramicellar product is isobutylene (72), while 2,2,4,4-tetramethylbutane (13) and 2-methyl-propane (14) are the extramicellar products. 

The results discussed above lead to the conclusion that the mechanism of decarbonylation of aldehydes is very similar to that postulated for the decarbonylation of acid chlorides. However, kinetic studies of the reaction show that a different rate -limiting step is operative with aldehydes. With acid chlorides, the rate-limiting step is thought to be migration or reductive elimination, depending on the R-group. " A detailed kinetic study on the stoichiometric decarbonylation of aldehydes with RhCl(PPh3)3 has... 

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... 

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. 

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

At the HF/6-31G level, ketenyl carbenes (1) were calculated to be intermediates in the decarbonylation of 1,2-bisketenes (2) to form cyclopropenones." At the MP2/6-31G and B3LYP levels, however, decarbonylation was predicted to form the cyclopropenones directly. The anfi-ketenyl carbenes were found to be 2.2-5.4kcalmoP higher in energy than the syn isomers (1). The mechanism of reaction of [l.l.ljpropellane with singlet dihalocarbene has been reported. ... 

Further theoretical study of the mechanism of decomposition of /3-propiolactone and jS-butyrolactone, to form CO2 and ethene or propene, respectively, has concluded that the process can best be described as asynchronous and concerted. Calculations also suggest that concerted processes are preferred for both decarbonylation and decarboxylation of jj-thiobutyrolactone. ... 

Figure 27. Mechanism of radiation induced chain scission in PMMA. Homolysis of the mainchain-carbonyl carbon bond is indicated as the initial step. Acylcarbon-oxygen, sigma bond homolysis also occurs but rapid decarbonylation ultimately leads to the same indicated products.

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. 

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. 

It should be noted, however, that the relation between branching of R and decarbonylation cannot alone be taken as evidence in favor of the mechanism of Eqs. (49) and (50). Such a relation could equally well fit other mechanistic explanations. Thus a similar relationship exists for the decarbonylation of acyl radicals (151). 

We further examined this process in S. crassipalpis and five other insect species. In our laboratory, microsomes from all six species required NADPH and 02 for hydrocarbon production from aldehydes and produced hydrocarbon and C02, but not CO (Mpuru el al., 1996). Thus, the mechanism of hydrocarbon formation remains controversial, with evidence obtained favoring both a decarbonylation and a decarboxylation mechanism. The resolution of the problem awaits cloning, expressing and assaying the enzymes involved. 

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... 

Early studies in a termite (Chu and Blomquist, 1980a), a cockroach (Major and Blomquist, 1978) and the housefly (Tillman-Wall et al., 1992) showed that tritium-labeled fatty acids were converted in vivo to hydrocarbons one carbon shorter. The mechanism of how this occurs has been controversial. Kolattukudy and co-workers have proposed a mechanism in which a fatty acyl-CoA is reduced to the aldehyde and, in the absence of cofactors, is decarbonylated to the hydrocarbon one carbon shorter and carbon monoxide. This has been demonstrated in plants, algae, vertebrates (Bognar et al, 1984 Cheesbrough and Kolattukudy, 1984, 1988 Dennis and Kolattukudy, 1991) and the flesh fly Sarcophaga crassipalpis (Yoder et al., 1992). 

A study of the influence of the solvent, stoichiometry, temperature and kinetics has been carried out31 to optimize the decarbonylation of 2-[r8F]fluoro-4-methoxybenzaldehyde. The highest decarbonylation yield achieved at 150 °C was 85 + 5%. No detailed study of the mechanism of this reaction has been presented and the lack of 100% yield has not been explained as was done in the case of decarbonylation of lactic acid33. 

The mechanism of the decarbonylation reaction is not yet known. Vandor35 suggested that a vapor-phase Cannizzaro reaction takes place when the reaction is carried out with an excess of steam. 

That alkenes are formed from those alkyl groups containing a jS-hydrogen atom strongly implies that the mechanism of alkene formation involves a /3-hydride abstraction step. There is a very pronounced kinetic isotope effect when C6D5CD2CH2COCI is decarbonylated, which indicates that not only does a jS-deuteride abstraction take place but that it is also rate determining. Further evidence for the participation of a /3-hydride abstraction comes from the decarbonylation of erythro- or t/ reo-2,3-diphenylbutanoyl chloride, where the former yields the (ii)-alkene and the latter the (Z)-isomer. 

From the above results it is evident that a lower energy content of the decomposing molecule favours the formation of the unsaturated aldehyde at the expense of decarbonylation. The explanation of this fact has been attempted on the basis of both the concerted and the biradical mechanisms. 

However, this reaction is always in competition with the other reactions that are taking place (i.e., decarbonylation, condensation, decomposition). The mechanism of a particular fire retardant is the summed effect of all simultaneous reactions. This summed effect is especially evident in the synergism of some compounds the effect of two compounds together is greater than the summed effect of each individual one alone (9, 51, 71-73). 

---