Radical aminyl

The effects of uv radiation on V/-nitroso compounds depend on the pH and the medium. Under neutral conditions and ia the absence of radical scavengers, these compounds often appear chemically stable, although the E—Z equiUbrium, with respect to rotation around the N—N bond, can be affected (70). This apparent stabiUty is due to rapid recombination of aminyl radicals and nitric oxide [10102-43-9] formed duting photolysis. In the presence of radical scavengers nitrosamines decay rapidly (71). At lower pH, a variety of photoproducts are formed, including compounds attributed to photoelimination, photoreduction, and photo-oxidation (69). Low concentrations of most nitrosamines, even at neutral pH, can be eliminated by prolonged kradiation at 366 nm. This technique is used ki the identification of /V-nitrosamines that are present ki low concentrations ki complex mixtures (72). 

Eor antioxidant activity, the reaction of aminyl radicals with peroxy radicals is very beneficial. The nitroxyl radicals formed in this reaction are extremely effective oxidation inhibitors. Nitroxides function by trapping chain-propagating alkyl radicals to give hydroxylamine ethers. These ethers, in turn, quench chain propagating peroxy radicals and in the process regenerate the original nitroxides. The cycHc nature of this process accounts for the superlative antioxidant activity of nitroxides (see Antioxidants). Thus, antioxidant activity improves with an increase in stabiUty of the aminyl and nitroxyl radicals. Consequendy, commercial DPA antioxidants are alkylated in the ortho and para positions to prevent undesirable coupling reactions. 

Although aminyl radicals are stable towards oxygen, they can oxidi2e other aromatic amines, phenols and thiols (10), and regenerate the diarylamine. Thus, mixtures of phenols and diarylamines frequendy show better antioxidant activity than either one alone. This is called synergism. 

Due to the radical nature of the Hofmann-Lbffler-Freytag reaction, a deviation was observed when there was a pendant terminal olefin on the substrate. When the aminyl radical from N-chloroamine 31 had a choice between addition to the double bond... 

In conclusion, the Hofmann-Loffler-Freytag reaction tends to give moderate and sometimes poor yields for the preparation pyrrolidines under the classic conditions. Nonetheless, the utility of this reaction to functionalize molecules via the aminyl radical mechanism plays an unique role in the tool box for the organic chemist, enabling transformations not easily achievable using other means. Furthermore, milder conditions and better yields can be achieved by taking advantages of the newer developments such as the Suarez modification. 

Other side reactions that have been reported are cleavage of the carbon-nitrogen bond to form 24 and an aminyl radical 25 or scission of the tliiocarbonyl-sulfur bond to form a thiyl radical 26 and 27 (Scheme 9.I0). U 6 4 Thiocarbonyl-sulfur bond cleavage may be a preferred pathway in the case of primary dithioearbamates. 

The attacking radical need not always be at carbon. Amidyl radical are known and give cyclization reactions. Aminyl radical cyclizations have been reported. Oxygen radical can be generated under photochemical conditions, and they add to alkenes in a normal manner. ... 

The reductive couphng of imines can follow different pathways, depending on the nature of the one-electron reducing agent (cathode, metal, low-valent metal salt), the presence of a protic or electrophihc reagent, and the experimental conditions (Scheme 2). Starting from the imine 7, the one-electron reduction is facihtated by the preliminary formation of the iminiiim ion 8 by protonation or reaction with an electrophile, e.g., trimethylsilyl (TMS) chloride. Alternatively, the radical anion 9 is first formed by direct reduction of the imine 7, followed by protonation or reaction with the electrophile, so giving the same intermediate a-amino radical 10. The 1,2-diamine 11 can be formed from the radical 10 by dimerization (and subsequent removal of the electrophile) or addition to the iminium ion 8, followed by one-electron reduction of the so formed aminyl radical. In certain cases/conditions the radical 9 can be further reduced to the carbanion 12, which then attacks the... 

One-electron oxidation of the adenine moiety of DNA and 2 -deoxyadenos-ine (dAdo) (45) gives rise to related purine radical cations 46 that may undergo either hydration to generate 8-hydroxy-7,8-dihydroadenyl radicals (47) or deprotonation to give rise to the 6-aminyl radicals 50. The formation of 8-oxo-7,8-dihydro-2 -deoxyadenosine (8-oxodAdo) (48) and 4,6-diamino-5-formamidopyrimidine (FapyAde) (49) is likely explained in terms of oxidation and reduction of 8-hydroxy-7,8-dihydroadenyl precursor radicals 47, respectively [90]. Another modified nucleoside that was found to be generated upon type I mediated one-electron oxidation of 45 by photoexcited riboflavin and menadione is 2 -deoxyinosine (51) [29]. The latter nucleoside is likely to arise from deamination of 6-aminyl radicals (50). Overall, the yield of formation of 8-oxodAdo 48 and FapyAde 49 upon one-electron oxidation of DNA is about 10-fold-lower than that of 8-oxodGuo 44 and FapyGua 43, similar to OH radical mediated reactions [91]. 

Molecular Orbital Description of Arylaminyl Radicals. Aryl-aminyl radicals, as previously discussed, are intermediates in both the photolysis of alkyl N-arylcarbamates (7, 8) and the photolysis of arylamines (18). A simplified mechanism for photolysis of arylamines and alkyl N-arylcarbamates is illustrated in Scheme IV for the general case. An indication of the reactivity of the... 

An efficient methodology for the construction of pyrrolizidines and other polycyclic nitrogen heterocycles using a radical domino sequence has been revealed by Bowman and coworkers [46]. These authors employed sulfenamides as substrates, which easily form aminyl radicals by treatment with tributyltin hydride and AIBN. For instance, 3-101 smoothly underwent a twofold 5-exo-trig cyclization to give the tetracyclic pyrrolizidine product 3-105 in 90% yield (Scheme 3.26). As intermediates, the radicals 3-102 to 3-104 can be assumed. 

Scheme 3.26. Synthesis of pyrrozidine 3-105 by a domino radical cyclization via an aminyl radical.

New cyclizations via photochemically generated aminyl radicals have been reported, including further examples of the Hofmann-Loeffler-Freytag reaction.313 Intramolecular addition of an aminyl radical, generated by photochemically induced nitrogen chlorine bond homoysis, is also accompanied by cyclization as illustrated by the conversion of the unsaturated N-chloroamide 378 to the pyrrolidine 379.314 Piperidine formation can also... 

Reactions of Phenoxyl and Aminyl Radicals with Hydrocarbons, Hydroperoxides, and Dioxygen... 

Cyclic Chain Termination by Aromatic Amines and Aminyl Radicals... 

Mechanism X Inhibitor Reacts with R02 and Radical In Reacts with Dioxygen Inhibitors such as diatomic phenols (hydroquinone, pyrocatechol), aminophenols, and aromatic diamines produce phenoxyl and aminyl radicals, which are efficient hydrogen donors rapidly reacting with dioxygen [56], for example,... 

The experimental values of rate constants of R02 reactions with aromatic amines (AmH) are given in Database [52], The experimental measurement of the rate constant /c7 for aromatic amines from kinetics of oxidation faced with great difficulties. These difficulties arise due to the extremely high activity of aminyl radicals toward hydroperoxide [53-56], The reaction... 

Aminyl radicals react with peroxyl radicals by two ways with the formation of either N—O or C—O bonds [63], The decomposition of the resulting unstable peroxides gives rise to the nitroxyl radical and iminoquinone, respectively. 

Vardanyan [65,66] discovered the phenomenon of CL in the reaction of peroxyl radicals with the aminyl radical. In the process of liquid-phase oxidation, CL results from the disproportionation reactions of primary and secondary peroxyl radicals, giving rise to trip-let-excited carbonyl compounds (see Chapter 2). The addition of an inhibitor reduces the concentration of peroxyl radicals and, hence, the rate of R02 disproportionation and the intensity of CL. As the inhibitor is consumed in the oxidized hydrocarbon the initial level of CL is recovered. On the other hand, the addition of primary and secondary aromatic amines to chlorobenzene containing some amounts of alcohols, esters, ethers, or water enhances the CL by 1.5 to 7 times [66]. This effect is probably due to the reaction of peroxyl radicals with the aminyl radical, since the addition of phenol to the reaction mixture under these conditions must extinguish CL. Indeed, the fast exchange reaction... 

Nitroxyl radicals produced in the reactions of R02 with aminyl radicals react with peroxyl radicals. The latter reaction is considerably slower than the reaction of peroxyl with aminyl radicals, which can be seen from the following data derived by flash photolysis (the photolysis of bis(l,l-dimethylethyl)peroxide in toluene was performed in the presence of... 

The most rapid reaction is N—N-dimerization (the rates of reactions A, B, C are related as 1 0.15 0.02 [94], Naphthylaminyl radicals recombine with the formation of N—C-dimers only [95], probably because voluminous naphthalene rings sterically hinder N—N-dimerization. A correlation between the rate constant of hyperfine splitting on the nitrogen atom of the aminyl radical and the rate constant of recombination of substituted ( (YC6H4)2N ) diphenyl-aminyl radicals was observed [95],... 

The aminyl radicals of /)-phenylenediarylamines readily undergo disproportionation. 

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