Decarbonylation of acyl radicals

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 . ... 

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... 

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). 

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 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. ... 

The products were obtained in excellent yields after simple hexane extraction. The removal of bromine and iodine proceeded smoothly. The removal of PhSe afforded methylcyclohexanone, indicating that the decarbonylation of acyl radicals takes place. The efficiency of deoxygenation of alcohols (Barton-McCombie reaction) is independent of the type of thiocarbonyl derivative (i.e. 0-arylthio-carbonate, O-thioxocarbamate, thiocarbonylimidazole or xanthate), as previously reported for (TMS)3SiH in organic solvents.25... 

Several acyl radical clocks have been calibrated, and these are collected in a recent excellent review of the general subject [44]. Examples of the two types of unim-olecular clock reactions, decarbonylations and cyclizations, are shown in Fig. 7, with rate constants for reactions at ambient temperature. Decarbonylations of acyl radicals, as shown for radical 16 [45], and the related decarboxylations of alkox-ycarbonyl radicals such as 17 [2] have log A terms of about 13 for cases where alkyl radical products are formed [46, 47]. The decarbonylation reactions involve a reduction in charge separation in the transition states, and the kinetics are sensitive to solvent polarity with decreases in rates as polarity increases [45]. Cyclization reactions, such as that shown for radical 18, are complicated. The 5-exo products shown are the predominant first-formed products, but they further rearrange to the thermodynamically favored 6-endo products by addition of the radical center to the carbonyl group to give a cyclopropyloxyl radical followed by ring opening [48]. 

The photodissociative pathway was confirmed by Meyer and Hammond, who foimd that essentially no o- or p-hydroxyacetophenone was formed in the photolysis of phenyl acetate in the gas phase. Instead, all products could be rationalized by recombination of phenoxy and methyl radicals (formed from decarbonylation of acyl radicals). Similarly, a photodissociative mechanism for the photo-Claisen reaction was supported by observation of products expected from the recombination of radicals produced by photodissociation of 3-methyl-l-phenoxybut-2-ene (113, Table 12.6). In addition to phenol, products of the reaction are the rearranged ether 114, the two y,y-dimethylallyl phenols 115 and 116, and the two rearranged allyl phenols... 

Tsentalovich, Y. P. and Fischer, H., Solvent effect on the decarbonylation of acyl radicals studied by laser flash photolysis, /. Chem. Soc., Perkin Trans. 2, 729,1994. 

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