Intermediate aminyl

Nucleophilic carbon radicals can C-alkylate imines, a process which is found to be substantially facilitated by an o-phenolic substituent as in e.g. (32).85 The hydroxyl is presumed to stabilize an intermediate aminyl radical. An enantioselective version of the reaction is also reported. 

Miyabe et al. developed a tandem addition/cycUzation reaction featuring an unprecedented addition of alkoxycarbonyl-stabihzed radicals on oxime ethers [117], and leading to the diastereoselective formation of /1-amino-y-lactone derivatives [118,119]. The reaction proceeds smoothly in the absence of toxic tin hydride and heavy metals via a route involving a triethylborane-mediated iodine atom-transfer process (Scheme 37). Decisive points for the success of this reaction are (1) the differentiation of the two electrophilic radical acceptors (the acrylate and the aldoxime ether moieties) towards the nucleophilic alkyl radical and (2) the high reactivity of triethylborane as a trapping reagent toward a key intermediate aminyl radical 125. The presence of the bulky substituent R proved to be important not only for the... 

Treatment of oxaziridine 294 with copper(l) triphenylphosphine chloride afforded cleanly lactam 295 in 82% yield (Scheme 8) <1998TL5853, 2006S1981>. The lactam presumably results from the closure of an intermediate aminyl... 

It seems obvious that all these different ways of formation of aminyloxides lead through intermediate aminyl radicals. But this is not generally true. Reaction of secondary amines with hydrogen peroxide/tungstate is assumed to proceed through hydroxylamine which is oxidized in a subsequent reaction step to aminyloxide36. ... 

On the other hand it seems certain that oxidation of hydrazines and tetrazenes occurs through intermediate aminyl radicals. Unambiguous evidence for direct reaction of aminyl radical and oxygen could be obtained only for reaction of 2,2,6,6-tetramethylpiperidyl. This radical is at once converted to corresponding 170 labeled aminyloxide 3 when 170-oxygen is allowed to diffuse into the cooled solution whereas under the same conditions in absence of oxygen it is relatively stable8. ... 

Evidence for the formation of the intermediate aminyl radical [114] is obtained from the oxidation of A//-octachlorodiphenylamine, which gives isolable perchlorodiphenylaminyl radical (PDA ) (Bandres, 1975). The formation of heptachloroaniline from tetrachloroaniline indicates that chlorine is captured by intermediate aminyl radicals, a process that is favoured over dimerization. Analogously PDA- reacts with chlorine to give perchloro-7V-phenylcyclohexa-2,5-dienimine, which, for steric and electronic reasons, does not isomerize to undecachlorodiphenylamine (113). 

The ability of the intermediate aminyl radicals to rearrange was exploited by several authors. For instance, macrocyclic lactams are obtained in excellent yields via a ring expansion process developed by Kim (Scheme 8.41, top)." Preparation of cychc imines via a tin hydride promoted radical cyclization onto an hydrazone is also possible (Scheme... 

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... 

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. 

A new cyclic mechanism of chain termination by nitroxyl radicals, including the formation of aminyl radicals as intermediate species, has been proposed by Korcek and coworkers [42,43]. It was shown that the addition of 4,4 -dioctyldiphenylnitroxyl radical to the hexa-decane that is oxidized (T = 433 K) leads to the formation of the corresponding diphenyl-amine as an intermediate compound during its transformations. The following cyclic mechanism of chain termination was suggested ... 

Naumov P, Sakurai K, Tanaka M, Kara H (2007) Direct observation of aminyl radical intermediate during single-crystal to single-crystal photoinduced Orton rearrangement. J Phys Chem B 111 10373-10378... 

Flash photolysis and laser flash photolysis are probably the most versatile of the methods in the above list they have been particularly useful in identifying very short-lived intermediates such as radicals, radical cations and anions, triplet states, carbenium ions and carbanions. They provide a wealth of structural, kinetic and thermodynamic information, and a simplified generic experimental arrangement of a system suitable for studying very fast and ultrafast processes is shown in Fig. 3.8. Examples of applications include the keton-isation of acetophenone enol in aqueous buffer solutions [35], kinetic and thermodynamic characterisation of the aminium radical cation and aminyl radical derived from N-phenyl-glycine [36] and phenylureas [37], and the first direct observation of a radical cation derived from an enol ether [38],... 

An amidyl radical is intermediate in reactivity between a neutral aminyl and an aminium cation radical due to the electron withdrawing ability of the carbonyl group. An intuitive advantage of amidyl radicals over aminium cation radicals is that reactions can be carried out under strictly neutral conditions, and, by reduction or hydrolysis of the amide, amidyl radical reactions become equivalent to reactions of neutral dialkyl or monoalkyl aminyl radicals. Preparation of AMraloamides and their rearrangements have been reviewed (71S1). 

Neutral aminyl radicals generated from tin hydride-mediated reactions of sulfenamides (Section II,F) have been shown to undergo cyclizations when energetically favored by addition to a strained alkene or by formation of a stabilized intermediate benzylic radical. In both cases, the reverse reaction, cleavage of the /3-amino radical, apparently did not occur (92TL4993). 

In the literature, the simultaneous formation of the major NMMO degradation products N-methylmorpholine, morpholine and formaldehyde [20] is always attributed to the disproportionation of the primary aminyl radical 3, as a - not further defined - redox process between two molecules of 3, in which one is reduced to N-methylmorpholine (2) and the other oxidized to N-(methylene)morpholinium cation (17), which upon addition of water, forms morpholine and formaldehyde. Also this disproportionation can be rationalized as a radical coupling reaction which proceeds through recombination of N-centered 3 and C-centered 4, via the ammonium aminal intermediate 16 as the actual recombination product (Scheme 6). The intermediacy of this species was indeed confirmed by isolation from the reaction mixture and full characterization [17]. Compound 16 is quite labile and immediately de-... 

The one-electron oxidation of a secondary amine results in the formation of a secondary aminium ion which on deprotonation gives an aminyl radical (Scheme 1). The nature of the final products derived from these intermediates dqiends very much on the structure of the substrate and the reaction conditions. If the amine has a hydrogen atom on the a-carbon atom the major products usually result from deprotonation at this a-position. With aromatic secondary amines, products can result from coupling of the delocalized radicals at a ring carbon atom. The formal dimerization of aminyl radicals shown in Scheme 21 is therefore not often a useful method of preparation of hydrazines. Nickel peroxide has been used to oxidize diphenylamine to tetraphenylhydrazine in moderate yield, and other secondary arylamines also give... 

Photoaddition of amines has also been reported and is often the result of initial electron-transfer from the amino-group. Photoadducts of triethylamine with styrene and a-methylstyrene have been obtained carbon dioxide reacts with the styrene radical anion intermediates to give carboxylic acids. In the photoreaction of arenecarbonitriles with aliphatic amines, both aminyl and... 

Photoinduced oxidation of amines leads to the formation of amine radical cations. These intermediates display both radical and cationic character with the N-H protons of the primary and secondary amine cation radicals being acidic. As a result, an efficient deprotonation occurs at the carbon a to the nitrogen radical cation giving aminyl radicals (Eq. (3)) [100]. 

Substituted 3-anilino-l,5-diphenylpyrazoles were oxidized with lead(iv) tetraacetate in benzene or methylene chloride solution <1997M261>. ESR measurements confirmed the formation of aminyl radicals from /irzra-CFIs-substituted pyrazoles. The radical intermediates from unsubstituted pyrazoles were recognized by their transformations to triarylaminium cation radicals. These were generated by consecutive oxidation of the dimeric products the structures of which were proved by NMR spectroscopy and labeling. 

The anions 13 of the N.O-substituted hydroxylamines 12 are in equilibrium with the rearranged anions 14. Oxidation by oxygen or electrolytically yields the corresponding aminyloxides 16. Abstraction of hydrogen from 12 also yields aminyl-oxides 16 which must be formed by rearrangement of the intermediate radical 1530 ... 

On the other hand oxidation above the nitrone stage 171 is possible in aminyl-oxides like 173. In these cases the intermediate nitrones 174 being potential tautomers of corresponding hydroxylamines (p. 69) are further dehydrogenated giving aminyloxides 175. Thus aminyloxides 176 and 178 containing amino groups in the... 

Again, no reaction was observed in the absence of mercaptoethanol. The mechanistic steps for the transformation were elucidated by the Chatgilialoglu et ah and are represented in Scheme 4.13, in analogy with the pathway reported for the radical reduction of aromatic azides with triethylsilane in toluene, with the addition of silyl radicals to the azide function, liberation of nitrogen and formation of silyl-substituted aminyl radical. The thiol is the hydrogen atom donor to this intermediate and it can be regenerated by its interaction with the silane, thus propagating the chain. The hydrolysis of the silylamine occurred... 

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