Acyloxy radicals decarboxylation

The extent of decarboxylation primarily depends on temperature, pressure, and the stabihty of the incipient R- radical. The more stable the R- radical, the faster and more extensive the decarboxylation. With many diacyl peroxides, decarboxylation and oxygen—oxygen bond scission occur simultaneously in the transition state. Acyloxy radicals are known to form initially only from diacetyl peroxide and from dibenzoyl peroxides (because of the relative instabihties of the corresponding methyl and phenyl radicals formed upon decarboxylation). Diacyl peroxides derived from non-a-branched carboxyhc acids, eg, dilauroyl peroxide, may also initially form acyloxy radical pairs however, these acyloxy radicals decarboxylate very rapidly and the initiating radicals are expected to be alkyl radicals. Diacyl peroxides are also susceptible to induced decompositions ... 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

Radicals react at the sulfur, and decomposition generating an acyloxy radical ensues. The acyloxy radical undergoes decarboxylation. Usually, the radieal then gives produet and another radical which can continue a chain reaction. The process can be illustrated by the reactions with tri-w-butylstannane and bromotrichloromethane. 

Earlier sections have already provided several examples of radical fragmentation reactions, although this terminology was not explicitly used. The facile decarboxylation of acyloxy radicals is an example. 

One-electron oxidation of carboxylate ions generates acyloxy radicals, which undergo decarboxylation. Such electron-transfer reactions can be effected by strong one-electron oxidants, such as Mn(HI), Ag(II), Ce(IV), and Pb(IV) These metal ions are also capable of oxidizing the radical intermediate, so the products are those expected from carbocations. The oxidative decarboxylation by Pb(IV) in the presence of halide salts leads to alkyl halides. For example, oxidation of pentanoic acid with lead tetraacetate in the presence of lithium chloride gives 1-chlorobutane in 71% yield ... 

A classic reaction involving electron transfer and decarboxylation of acyloxy radicals is the Kolbe electrolysis, in which an electron is abstracted from a carboxylate ion at the anode of an electrolysis system. This reaction gives products derived from coupling of the decarboxylated radicals. 

The high rate of decarboxylation of aliphatic acyloxy radicals is also the prime reason behind low initiator efficiencies (see 3.3.2.1.3). Decarboxylation occurs within the solvent cage and recombination gives alkane or ester byproducts. Cage return for LPO is 18-35% at 80 °C in -octane as compared to only 4% for BPO under similar conditions.144... 

Aliphatic acyloxy radicals undergo facile fragmentation with loss of carbon dioxide (Scheme 3,69) and, with few exceptions,428 do not have sufficient lifetime to enable direct reaction with monomers or other substrates. The rate constants for decarboxylation of aliphatic acyloxy radicals are in the range l 10xl09 M 1 s at 20 °C.429 lister end groups in polymers produced with aliphatic diacyl peroxides as initiators most likely arise by transfer to initiator (see 3.3.2.1,4). The chemistry of the carbon-centered radicals formed by (3-scission of acyloxy radicals is discussed above (see 3.4.1). 

The current-potential relationship indicates that the rate determining step for the Kolbe reaction in aqueous solution is most probably an irreversible 1 e-transfer to the carboxylate with simultaneous bond breaking leading to the alkyl radical and carbon dioxide [8]. However, also other rate determining steps have been proposed [10]. When the acyloxy radical is assumed as intermediate it would be very shortlived and decompose with a half life of t 10" to carbon dioxide and an alkyl radical [89]. From the thermochemical data it has been concluded that the rate of carbon dioxide elimination effects the product distribution. Olefin formation is assumed to be due to reaction of the carboxylate radical with the alkyl radical and the higher olefin ratio for propionate and butyrate is argued to be the result of the slower decarboxylation of these carboxylates [90]. 

Thne-of-flight (TOF) mass spectrometric analysis of the pyrolysis fragments of di-t-butyl peroxide suggests t-BuCO as the primary product, followed by decomposition of this radical into CHj.253 Elsewhere, the kinetics of the pyrolysis of dimethyl, diethyl, and di-t-butyl peroxides in a modified adiabatic bomb calorimeter have been investigated.254 The lifetime of acyloxy radicals, generated by the photolysis or thermolysis of acetyl propionyl peroxide, have been studied. Chemical nuclear polarization has been used to determine the rate constant for the decarboxylation of these radical intermediates.255... 

Grollmann U, Schnabel W (1980) On the kinetics of polymer degradation in solution, 9. Pulse radiolysis of polyethylene oxide). Makromol Chem 181 1215-1226 Hamer DH (1986) Metallothionein. In Richardson CC, Boyer PD, Dawid IB, Meister A (eds) Annual review of biochemistry. Annual Reviews, Palo Alto, pp 913-951 Held KD, Harrop HA, Michael BD (1985) Pulse radiolysis studies of the interactions of the sulfhydryl compound dithiothreitol and sugars. Radiat Res 103 171-185 Hilborn JW, PincockJA (1991) Rates of decarboxylation of acyloxy radicals formed in the photocleavage of substituted 1-naphthylmethyl alkanoates. J Am Chem Soc 113 2683-2686 Hiller K-O, Asmus K-D (1983) Formation and reduction reactions of a-amino radicals derived from methionine and its derivatives in aqueous solutions. J Phys Chem 87 3682-3688 Hiller K-O, Masloch B, Gobi M, Asmus K-D (1981) Mechanism of the OH radical induced oxidation of methionine in aqueous solution. J Am Chem Soc 103 2734-2743 Hoffman MZ, Hayon E (1972) One-electron reduction of the disulfide linkage in aqueous solution. Formation, protonation and decay kinetics of the RSSR radical. J Am Chem Soc 94 7950-7957... 

The use of hypervalent iodine reagents in carbon-carbon bond forming reactions is summarized with particular emphasis on applications in organic synthesis. The most important recent methods involve the radical decarboxylative alkylation of organic substrates with [bis(acyloxy)iodo]arenes, spirocyclization of para- and ortho-substituted phenols, the intramolecular oxidative coupling of phenol ethers, and the reactions of iodonium salts and ylides. A significant recent research activity is centered in the area of the transition metal-mediated coupling reactions of the alkenyl-, aryl-, and alkynyliodonium salts. 

Radical Decarboxylative Alkylation with [Bis(acyloxy)iodo]arenes 101... 

Radical decarboxylative alkylation of heteroaromatic bases mediated by [bis(acyloxy)iodo]arenes... 

Aqueous solutions of minium perchlorates were irradiated in the presence of carboxylate anions and alkylated adducts were formed [184, 185]. This was shown to occur through electron transfer quenching of the iminium excited singlet, followed by an efficient decarboxylation of the resulting acyloxy radical. When a-hydroxy carboxylates are used, a reduction product is also formed and probably linked to the formation of a ketyl radical whose reducing properties are known. 

As was indicated previously, primary alkyl diacyl peroxides undergo decomposition to give two acyloxy radicals, which subsequently undergo rapid decarboxylation. The rate coefficient for decarboxylation of the acetoxy radical is calculated to be 1.6 X 10 sec at 60 °C in n-octane . The corresponding activation parameters were = 6.6 kcal.mole and A = 3.5 x 10 sec An earlier report estimated the activation energy to be about 5 kcal.mole . It was pointed out by... 

Acetic acid gives the methyl radical as well as CHg. COgH, and malonic acid gives only 0112.00211. These decarboxylation products possibly arise from abstraction of carboxylic hydrogen followed by, or concerted with, decarboxylation of the acyloxy radical, e.g. 

The benzyl radical might be formed by abstraction of hydrogen from the carboxyl group and decarboxylation of the acyloxy radical (PhCHs.COaH- PhOHz.COa- PhCHs. + COa), but the effect of pH on the observed spectra, considered in the light of the results already discussed for the behaviour of glycol with the titanous-peroxide system, reveals a more likely mechanism namely, that addition of hydroxyl to an aromatic carbon atom is followed, in sufficiently acidic media, by the elimination of hydroxide ion from the ring concerted with the loss of carbon dioxide and a proton. A convenient representation, in the case of an initial reaction at the para position, is as follows ... 

The photochemical cleavage of naphthylmethyl alkanoates in methanol is reported to proceed by homolytic cleavage to naphthylmethyl radical and acyloxy radical,the latter decarboxylates in competition with electron transfer to give naphthylmethyl cation and carboxylate anion. Using known rates of electron transfer as a clock the rate constants for decarboxylation of the acyloxy radicals has been estimated.The light induced homolysis of 1-chloromethyl-naphthalene has also been studied using chemically induced dynamic electron polarisation (CIDEP) spectroscopy to detect the naphthyl-... 

Photolysis of the 1-naphthyhnethyl ester of phenylacetic acid (327) in methanol, for example, affords the 1-naphthylmethyl cation carboxylate anion pair in addition to 1-naphthylmethyl radical acyloxy radical pair intermediates, which, after decarboxylation, form an adduct with methanol (328, formed along with 329) or an in-cage radical coupling product 330 (Scheme 6.146).1030 The competition between the radical and ionic pathways was found to be very dependent upon the substituents on the naphthalene ring. 

Benzylic esters have been studied in considerable detail often as a continuation of the pioneering work by Zimmerman and co-workers (Scheme 2) in 1963 [44]. There are several reasons for this. First, the synthesis of compounds with the structural variables required to probe specific mechanistic questions is often straightforward. Second, products are usually formed from both ion pairs and radical pairs and, therefore, the structural variables that control this partitioning can be systematically studied. Third, the radical pair (ARCH2-O CO)— R) incorporates a built-in radical clock, the decarboxylation of the acyloxy radical, which serves as a useful probe for the reactivity of the radical pair. If the carbon of the acyloxy radical is sp hybridized, this decarboxylation rate is on the 1- to 1000-ps time scale, depending on R, so that decarboxylation will often occur within the solvent cage before diffusional escape. The topic of benzylic ester photochemistry has been recently reviewed twice by Pincock [5,98] and therefore only a brief summary will be given here. 

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