Decarbonylation of acyl

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 same is true for decarbonylation of acyl radicals. The rates of decarbonylation have been measured over a very wide range of structural types. There is a very strong dependence of the rate on the stability of the radical that results from decarbonylation. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s . ... 

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 ... 

As data for the rates of spin-trapping reactions are accumulated, so it becomes possible to use the competition experiment in reverse , i.e. to determine rates of rearrangement, fragmentation, atom transfer, etc. which can compete with spin trapping. An attempt to estimate rates of decarbonylation of acyl radicals depended on this approach (Perkins and Roberts, 1973). Although the results obtained were intuitively reasonable, they depended on the assumption that the rate of scavenging of acyl radicals by MNP would be no different from that measured for the butoxycarbonyl radical. This still awaits experimental verification. Another application, reported recently, was to the rates of rearrangement (23) of a series of (o-(alkoxycarbonyl)-alkyl radicals... 

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 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)... 

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)... 

Decarbonylation of acyl cyanides.9 Aromatic acyl cyanides, which are easily obtained by oxidation of cyanohydrins with /-butyl hydroperoxide catalyzed by RuC12[P(C6H5)3]3, undergo decarbonylation to nitriles in high yield when heated in the presence of Pd(0). 

Cleavage of ethers Organoaluminum reagents, 202 Decarbonylation of acyl halides Tetrakis(triphenylphosphine)palla-dium(0), 289 Dehalogenation... 

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. 

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). 

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],... 

Hunsdiecker reaction does not work so well in aromatic carboxylic acids [75, 76]. The rate constants for decarbonylation of acyl radicals are lowered as shown in Table 1.18. 

Decarbonylation of acyl radicals is another kind of fragmentation reaction ... 

Reaction of all of the radical pairs within their initial cages does not preclude one or both of the radicals from undergoing a structural change (N. B., step [6] in Scheme 13.1). In the case of photo-Fries reactions, the most commonly encountered structural change is loss of carbon monoxide (CO) from the acyl radical, leading to formation of an alkyl radical. The rates of decarbonylation of acyl radicals have been measured for a wide variety of acyl structures as a function of the medium viscosity... 

The decarbonylation of acyl chlorides or fluorides in the presence of potassium iodide is a simple one-step reaction for the preparation of fluoroalkyl iodides, e.g, formation of 2. ... 

The treittment of polyfluoroacyl chlorides with stoichiometric amounts of tctrucarbonyl-nickcl(O) at ca. 150 C resulted in decarbonylation of acyl radicals. When such reactions were carried out in an aromatic hydrocarbon medium, polyfluoroalkyl-subslitutcd aromatic compounds. e.g, 4. were obtained. The aromatic compounds used were benzene, toluene or bro-mobenzene. ... 

However, in other instances, postelimination reactions can change the nature of the products. The main difficulty arises from the readdition of the alkene initially produced to the hydrido complex. This can result in isotopic scrambling or isomerization of the alkene upon reelimination. The formation of internal alkene products from the [Pd(PPh3)4]-catalyzed decarbonylation of acyl cyanides (equation 2) may also arise from postelimination reactions between the alkene and [PdH(CN)(CO)(PPh3) ] initially formed. The importance of this reaction in isomerizing the anti-Bredt alkenes formed by the catalytic decarbonylation of 3-chlorocarbonyltricyclo[5.3.1.0 ]iindecane is shown in Scheme 3. 

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