Carboxyl radicals generation

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 ... 

Intramolecular addition of trialkylboranes to imines and related compounds have been reported and the main results are part of review articles [94, 95]. Addition of ethyl radicals generated from Et3B to aldimines affords the desired addition product in fair to good yield but low diaster control (Scheme 40, Eq. 40a) [96]. Similar reactions with aldoxime ethers [97], aldehyde hydrazones [97], and N-sulfonylaldimines [98] are reported. Radical addition to ketimines has been recently reported (Eq. 40b) [99]. Addition of triethylborane to 2H-azirine-3-carboxylate derivatives is reported [100]. Very recently, Somfai has extended this reaction to the addition of different alkyl radicals generated from trialkylboranes to a chiral ester of 2ff-azirine-3-carboxylate under Lewis acid activation with CuCl (Eq. 40c) [101]. 

Thus, the observed aromatic carboxylic acids at chain ends [11, 25] would be accounted for by the hydrogen abstraction by the carboxyl radical (Scheme 18.1, path A). This, of course, generates another radical species, R, (not shown) capable of carrying on further degradative reactions. 

Scheme 24 Reaction of radicals generated by anodic decarboxylation of carboxylic acids.

Unsaturated carboxylic acids can be de-carboxylated to alkyl radicals that undergo an intramolecular addition. The S-exo-trig-cyclization of fi-allyloxy radicals, generated from an appropriate carboxylic acid, combined with a final heterocoupling has been applied to synthesize a precursor of prostaglandine PGF2q (Fig. 47) [246] and a branched carbohydrate (ratio of diastereo-mers 1.8 1) (Fig. 48) [247]. A radical tandem cyclization of a doubly unsaturated monocyclic carbocyclic acid provides a... 

Bromine-atom atomic resonance absorption spectrometry (ARAS) has been applied to measure the thermal decomposition rate constants of CF3Br in Kr over the temperature range 1222-1624 K. The results were found to be consistent with recently published theory. The formation of cyclopent[a]indene and acenaphthylene from alkyl esters of biphenyl-mono- and -di-carboxylic acids has been observed in flash vacuum pyrolyses at 1000-1100 °C. The kinetics and mechanisms of free-radical generation in the ternary system containing styrene epoxide, / -TsOH, and i-PrOH have been examined in both the presence and absence of O2. ... 

Experimental evidence indicates that the alkyl radical intermediates from the anodic oxidation of carboxylates are generated in free solution and have no memory of the configuration of the carboxyl group that was eliminated. Where the carboxylic acid function is attached to an asymmetric carbon atom as in 15, the Kolbe coupling reaction leads to complete racemization [67]. Anodic oxidation of (+)-2-... 

Carboxylate radicals can be generated in several ways. One is the thermal decomposition of diacyl peroxides, which are compounds with rather weak 0-0 bonds ... 

Manevich Y, Held KD, Biaglow JE (1997) Coumarin-3-carboxylic acid as a detector for hydroxyl radicals generated chemically and by gamma radiation. Radiat Res 148 580-591 Maples KR, Johnson NF (1992) Fiber-induced hydroxyl radical formation correlation with mesothelioma induction on rats and humans. Carcinogenesis 13 2035-2039 MarkG, Korth H-G, Schuchmann H-P, von Sonntag C (1996) The photochemistry of aqueous nitrate revisited. J Photochem Photobiol A Chem 101 89-103 Maskos Z, Rush JD, Koppenol WH (1990) The hydroxylation of the salicylate anion by a Fenton reaction and v-radiolysis a consideration of the respective mechanisms. Free Rad Biol Med 8 153-162... 

Diacyl peroxides are another important source of free-radicals and, consequently, are also commonly used as initiators of free-radical reactions. There is a vast amount of data pertaining to the kinetics and mechanism of decomposition of these compounds in conventional solvents there are a number of side reactions, both radical and ionic in nature, that complicate the kinetics of their decomposition. Generally, these compounds decompose by initial 0-0 bond cleavage that generates carboxyl radicals (RC02 ), which subsequently decarboxylate yielding R (Scheme 4.7)... 

Iwasawa, N. Funahashi, M. Hayakawa, S. Ikeno, T. Narasaka, K. Synthesis of mediumsized bicyclic compounds by intramolecular cyclization of cyclic /i-keto radicals generated from cyclopropanols using Mn(III) tris(pyri-dine-2-carboxylate) and its application to total synthesis of 10-isothiocyanatoguaia-6-ene. 

In an approach to direct C-functionalization of triazolo[4,5-c]pyridines, shown in Scheme 3, 1-methyl (or phenyl)[l,2,3]triazolo[4,5-c]pyridines (26,33) are alkylated exclusively at C-4 by radicals generated by decarboxylation of carboxylic acids (ammonium persulfate-sulfuric acid-silver nitrate) <90ZOB683>. However, with /-butanol various products are obtained depending on the catalyst employed. For example, with ammonium persulfate-sulfuric acid-silver nitrate, exclusive C(4)-methylation (34) was observed, while ammonium persulfate-sulfuric acid gave exclusively C(4)-/ -hydroxy-/ ,/ -dimethylethylation (cf. (36)). The /-butyl analogue (35) was obtained by decarboxylation of pivalic acid. 

GENERATION OF CARBOXYL RADICALS FUNCTIONAL GROUP COMPATABILITY... 

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