Carboxylic acids radical formation from

Following this mechanism, luciferase-bound reduced FMN (FMNH intermediate I) reacts with oxygen to form the 4a-hydroperoxy-4a,5-dihydroFMN intermediate II (HF-OCT). The addition of a long-chain aliphatic aldehyde generates the 4a-peroxyhemiacetal-4a,5-dihydroFMN intermediate III (HF-OOCH(OH)R), which is subsequently converted to a radical pair of 4a-hydroxy-4a,5-dihydroFMN radical cation (IV+ HF-OH+ ) and a carboxylic acid radical anion RC(0H)0" A 1-e transfer from RC(0H)0- to IV+ produces the excited 4a-hydoxy-4a,5-dihydroFMN intermediate IV. Relaxation of IV to the ground state produces bio luminescence ( max 490 nm) with a quantum yield of about 0.16. Finally, IV decays to form FMN and water. The present report addresses several key issues of this mechanism with respect to the identity of the proposed excited emitter HF-OH, the energetics of its formation, and the requirement of a hydrophobic luciferase active site for a high quantum yield of the emitter. 

Thus, it has been proposed that the homolytic decomposition of hydroperoxides can be induced by sulfenic acid (12,13). There is evidence that various carboxylic acids can promote radical formation from hydroperoxides at elevated temperatures (II, 14). The intermediate thiosul-furous acid (Reaction 7) itself may function as the source of radicals, since sulfinic acid is known to initiate the radical polymerization of vinyl monomers at 20°C (15). Based on the AIBN-initiated oxidation of cumene, Koelewijn and Berger (16) proposed that pro-oxidant effects arise from catalysis of the radical decomposition of hydroperoxides by intermediate compound formation between the hydroperoxide and sulfoxide. However, under our conditions hydroperoxide was stable in the presence of sulfoxide alone. 

Hydroperoxides decompose relatively slowly at ambient temperatures in the dark, but in light they are readily photolysed to free radicals, (Scheme 1.1, reaction d). Consequently, the rate of photo-oxidation of the hydrocarbon polymers is orders of magnitude higher than thermal oxidation. In addition, small amounts of transition metal compounds, notably iron, cobalt, manganese and copper, have a powerful catalytic effect on radical formation from hydroperoxides [14], leading to rapid molecular weight reduction by breakdown of the intermediate alkoxyl radical and the formation of carboxylic acids and esters as oxidation end products (see Scheme 1.2) [15]. 

Alkanes are formed when the radical intermediate abstracts hydrogen from solvent faster than it is oxidized to the carbocation. This reductive step is promoted by good hydrogen donor solvents. It is also more prevalent for primary alkyl radicals because of the higher activation energy associated with formation of primary carbocations. The most favorable conditions for alkane formation involve photochemical decomposition of the carboxylic acid in chloroform, which is a relatively good hydrogen donor. 

Figure4.14 Schematic representation ofthe proposed mechanism ofthe photocatalytic reforming of glucose on Pt-Ti02 involving the formation of various radicals, aldehydes, and carboxylic acids. Adapted from [170] (2008) with permission from Elsevier.

A radical tandem cyclization, consisting of two radical carbocyclizations and a heterocoupling reaction, has been achieved by electrolysis of unsaturated carboxylic acids with different coacids. This provides a short synthetic sequence to tricyclic products, for example, triquinanes, starting from carboxylic acids which are accessible in few steps (Scheme 6) [123]. The selectivity for the formation of the tricyclic, bi-cyclic, and monocyclic product depending on the current density could be predicted by applying a mathematical simulation based on the proposed mechanism. 

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. ... 

NaClO, or else in the two-phase system but with a quaternary ammonium (viz. AUquat) ion as a phase-transfer catalyst, overoxidation to the corresponding carboxylic acid is obtained (entry 4). Therefore, by proper choice of the experimental conditions, a synthetically useful distinction in products formation can be made for the oxidation of primary alcohols, even though we are far from a satisfactory understanding of the reason behind this different behaviour. In fact TEMPO, as a well-known inhibitor of free-radical processes is allegedly responsible for the lack of overoxidation of an aldehyde to carboxylic acid (entry 3) this notwithstanding, TEMPO is also present under those conditions where the overoxidation does occur (eutry 4). Moreover, a commou teuet is that the formation of the hydrated form of an aldehyde (in water solution) prevents further oxidation to the carboxylic acid however, both entries 3 and 4 refer to water-organic solutions, and their... 

The unexpected formation of the blue crystalline radical cation (97) from the reaction of triazinium salt (98) with tetracyanoethylene has been reported and the product identified by its EPR spectrum and by X-ray crystallography (Scheme 42).199 Carboxylic acids react with the photochemically produced excited state of N-t-a-phenynitrone (PBN) to furnish acyloxy spin adducts RCOOPBN. The reaction was assumed to proceed via ET oxidation of PBN to give the PBN radical cation followed by reaction with RCO2H.200 The mechanism of the protodiazoniation of 4-nitrobenzenediazonium fluoroborate to nitrobenzene in DMF has been studied.201 Trapping experiments were consistent with kinetic isotope effects calculated for the DMF radical cation. The effect of the coupling of radicals with different sulfur radical cations in diazadithiafulvalenes has been investigated.202... 

Following the study of the simple coupling of radicals derived from the salt of a single carboxylic acid, it was found that the electrolysis of a mixture of carboxylate anions or of the salts of half esters of dicarboxylic acids increased the synthetic value of the method. This arises from the possibility of the formation of symmetrical and unsymmetrical coupled products of the derived radicals. These anodic syntheses are illustrated in the synthesis of hexacosane (Expt 5.11), sebacic acid (decanedioic acid), octadecanedioic acid and myristic acid (tetra-decanoic acid), in Expt 5.131. 

Interaction of C02 with organic anion radicals leads, as a rule, to carboxylic acids C02 anion radicals are not formed. Even such a one-electron reductant as 02 in aprotic medium simply adds to C02. The addition product, in turn, accepts one electron from 02 (Sawyer Gibian 1979). The total result consists of the formation of an aprotic equivalent of peroxycarboxyic acid according to Scheme 1-84 ... 

The atmospheric chemical kinetics of linear perfluorinated aldehyde hydrates, Cx-F2x+iCH(OH)2, have been measured for x = 1,3, and 4, focusing on formation (from aldehyde, by hydration), dehydration, and chlorine atom- and hydroxyl radical-initiated oxidation.211 The latter reaction is implicated as a significant source of perfluorinated carboxylic acids in the environment. 

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