Aminium ions

As in the case of diazotization by N203 (Sec. 3.1), either the formation of XNO or the nitrosation of the amine (or of the aminium ion) may be rate-limiting. Under most experimental conditions the second alternative applies. If a steady-state concentration of XNO exists (which is however, not always the case) the reaction system of Schemes 3-26 and 3-27 yields the rate equation shown in Scheme 3-29 if it is the amine base (ArNH2) that is nitrosated. Xa is the acidity constant of the conjugate acid (ArNH3). 

The trends in IpK in passing from water to the less polar alcohols can be explained in part by considering the equilibria for carboxylic acid and aminium ion dissociation shown in Equations (4) and (5). Carboxylic acid dissociation creates two opposite charges, while aminium ion dissociation simply relocates (H+) from the amine to the solvent as ROH, which may be... 

Table 2 pA), values of various monobasic acids and aminium ions as determined by Hyperquad analysis of titration data at 1 x 10 3 M in anhydrous ethanol, T = 25.0 °C... 

Oxidation of PBN by tris(4-bromophenyl)aminium ion (TBPA +) in the presence of nucleophiles 110... 

OXIDATION OF PBN BY TRIS(4-BROMOPHENYL)AMINIUM ION (TBPA +) ([10P) IN THE PRESENCE OF NUCLEOPHILES... 

The same CIMS approach has been used for investigating the self-recognition processes in proton-bound tartrate trimers. The trimer chirality effect is consistent with the heterochiral trimers as more stable than the homochiral ones. The reverse is true when the proton in the proton-bound trimers is replaced by hydronium, ammonium ion, or primary aminium ions. " This changeover is... 

Oxidation to aminium ions, aminyl radicals and hytbazines... 

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... 

One-electron oxidants convert tertiary amines into aminium ions. Chlorine dioxide is an example of such a reagent this converts triethylamine and other tertiary alkylamines into aminium ions, which normally react further by the loss of hydrogen from an a-carbon atom. An example of a synthetic (plication of this reaction sequence is shown in Scheme 24, the inteimediate iminium ion being intercepted intramolecularly." Nitrosonium tetrafluoroborate and dioxygenyl hexafluoroantimonate, 02 SbF6, are also good one-electron oxidants which can be used at low temperature." ... 

Reagents for amination, nitrosation and nitration of tertiary alkylamines are discussed in the appropriate reviews listed in Sections 6.1.3.1.4 and 6.I.3.I.5. Tertiary amines can be nitrosated with dealkylation by dinitrogen tetroxide for example, 1-methylpiperidine gave 1-nitrosopiperidine (80%). lliis reaction probably starts by one-electron oxidation of the amine, the aminium ion then undergoing dealkylation. Other oxidative d ylations and dealkylations include the formation of iV-nitrosodibenzylamine in high yield from the acid chloride (PhCH2)2NCOCl and sodium nitrite and the conversion of the amine (43) into the nitramine (44) with nitric acid. ... 

Asymmetric Michael Reactions. Asymmetric induction has been observed in Michael-type addition reactions that are catalyzed by chiral amines. The Ai-benzyl fluoride salt of quinine has been particularly successful since the fluoride ion serves as a base and the aminium ion as a source of chirality. Drastic improvements in optical purity (1-23%) have resulted by changing from quinine to the N-benzyl fluoride salt (eq 11). ... 

The electronic absorption maxima of the few other aminium ions that have been investigated are given in Table 3 [22, 56],... 

The acidities of aminium ions derived from primary and secondary amines have also been measured a few representative examples are listed in Table 4, with the acidities of the parent compounds. 

The pATa of the aminium ions is generally lower than those of the corresponding amine. The greater acidity of the aminium ion can be attributed to the greater amoimt of s character in the lone-pair orbital of the amine radical, (CH3)2N , which is sp -hybridized compared, with the more basic lone pair orbital in (CH3)2NH, which is sp -hybridized. In contrast, the pA a of the anilinium radical is much higher than that of the protonated aniline. The basicity of the anilinium radical is comparable with that of (CH3)2NH+ , reflecting the sp -hybrid character of the N-H bonds in the two species. The low basicity of the parent aniline results from... 

The prototype cyclodimerization of 1,3-cyclohexadiene catalyzed by tris(4-bromo-phenyl)aminium ion (Ar3N ) proceeds smoothly in CH2CI2 at room temperature or below to a 70% yield of the Diels-Alder dimer [Eq. (51)] [116]. The direct anodic oxidation of 1,3-cyclohexadiene results in formation of the Diels-Alder product only in low yield, with the major product being a polymer linked through the 1,4-positions [119]. 

Goodin, 1970). Thus, there is no point in searching for the esr spectrum of the pyridine cation radical. Probably the same situation prevails in reactions of other amines with TCNE. For example, a variety of arylamines react with TCNE and form, finally, tricyano-vinyl derivatives (Foster, 1974). The reaction is thought to involve complex formation first (Isaacs, 1966). The TCNE - is produced over a period of minutes, while the other anticipated radical, presumed to be an aminium ion, is not observed. Its absence is attributed to dimerization and proton loss, although the anticipated products (hydrazobenzene and its isomers) are not found. In these reactions, therefore, there can be little doubt that the TCNE" - arises from reactions of TCNE with hydroxide ion and/or cyanide ion (formed in the tricyanovinylation reaction). Thus, in these cases the formation of an anion radical is not linked to electron transfer from an organic donor (Isaacs, 1966). 

Wieland, 1907 Wieland and Wecker, 1910). Oxidation was carried out in an inert solvent such as benzene, from which the salt precipitated. The easier and more reliable preparation of perchlorate salts by the iodine-silver perchlorate method (Weitz and Schwechten, 1926, 1927), to which we have referred earlier (p. 168), allowed a much clearer understanding of the nature of triarylaminium ions to be obtained. Isolation of perchlorates permitted chemical studies, and easy reduction to the triarylamine by iodide ion, ferrous ion, etc., was consistent with the cation-radical view that was developed. The name aminium ion was coined by Weitz. Other salts were prepared such as tritolylaminium picrate (by oxidation of the amine with lead dioxide in the presence of picric acid), and it was also recognized that conjugate anions in salts obtained by oxidation with antimony pentachloride, phosphorus pentachloride and ferric chloride had to be complex anions rather than simple anionic radicals. This is a particularly pertinent point in antimony pentachloride oxidations (p. 165). 

Tri-/ -tolylaminium perchlorate is reduced by propanol and butanol in reactions which are first order in aminium ion and second order in alcohol (Belozerov et al., 1972), and it is proposed that a complex of cation radical and solvent breaks down as shown in... 

Diquinane synthesis. 2-(4-Alkenyl)cyclopropyl sulfides are Uansformed into diquinanes on oxidation with tris(4-bromophenyl)aminium ions in dichloromethane at room temperature. 

The mechanism in Scheme 8 was proposed for the oxidation reaction. In the first step, the Cu(II) salt, which is formed in the autooxidation of cuprous chloride, forms a complex with the amine. This is followed by a rate-determining electron transfer from the amine to the Cu(II) species giving Cu(I) and an aminium radical. The subsequent steps were considered to be fast. The authors accounted for the secondary hydrogen-deuterium kinetic isotope effect by suggesting that there was hyperconjugative electron release to the aminium ion nitrogen that forms in the slow step of the reaction. 

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