Immonium cation

Finally, quinonoid phenylogous cyclopropenium immonium cations 133 have been obtained from ethoxy cation 75 and tertiary aromatic amines61,76 ... 

Similiar cyanine types were obtained from the immonium cation 138 and diphenyl cyclopropenone106, t07 and from 2,3- or 3,4-dimethylindolizinium cations and 75 in the presence of base10S i08 ... 

The synthesis of cyclopropenone imines 3 has been accomplished by several methods. Thus aromatic amines, e.g. p-nitraniline, can be reacted either with diphenyl cyclopropenone in HCl/ethanol or with the ethoxy cation 75 forming the immonium cation 150, which is deprotonated by tertiary bases to the N-(p-nitro-phenyl)-imine /5/llsl ... 

The highly stable cyclopropenium immonium cations 153 are formed from the ethoxy cyclopropenium cation 75 with primary and secondary aliphatic or aromatic amines42-119> ... 

Cyclopropenium-immonium cations 231 and 232 have been found to behave in analogy to triafulvenes according to investigations by Krebs45 ... 

The AG values for the rotation around the C3-N bond were obtained by the coalescence method and kinetic measurements of the equilibration of isolated and configurationally established cis-trans isomers of type 232. The barrier of rotation is considerably lower than in ordinary immonium cations, e.g. 233 AG is decreased by electron-withdrawing substituents at the nitrogen atom, whilst at the three-ring the opposite effect is observed. 

The log k and AG values of immonium cations of type 232 and the log K (equilibrium) values can be correlated (see diagram below) satisfactorily with the a+ constants of Brown. 

The products from the cyclopropenium immonium cations 153 and primary and secondary amines88 vary with amine structure and basicity of the amino function... 

Figure 4.24 Two amine-containing molecules can be crosslinked by formaldehyde through formation of a quaternary ammonium salt with subsequent dehydration to an immonium cation intermediate. This active species then can react with a second amine compound to form stable secondary amine bonds.

Figure 28.4 Formaldehyde can be used to capture protein interactions if it is used at low concentrations. The reaction proceeds through modification of a protein to create an intermediate immonium cation, which then goes on to react with a neighboring protein to form the crosslinked product via secondary amine bonds.

Syntheses of a,(3-unsaturated thioketones, based on the hydrothiolysis of conjugated unsaturated immonium cation, have been reviewed by Timokhina and Voronkov.71... 

Electrochemical oxidation of aliphatic tertiary amines in acetonitrile together with compounds having weakly acidic hydrogens, such as dimethyl malonate, leads to addition of the malonate anion to the immonium cation intermediate. 2,4,6-Collidine is added to combine with protons, which are released during reaction [90],... 

The enamine-immonium cation system of 9,10-dimethoxy-l,2,3,4-6,7-hexahydrobenzo[a]quinolizinium salt [166]. 

The Vilsmeier-Haack formylation procedure (Scheme 24) provides the most effective synthesis of formylpyrroles and indoles. Reaction of the heterocycles with the immonium cation (72), derived from DMF or (V-methylformanilide with an acid chloride, such as phosphorus oxychloride, thionyl chloride, phosgene, oxalyl chloride, benzoyl chloride or bromotriphenylphosphonium bromide, yields the intermediate heteroarylimmonium salt (73). Under suitable reaction conditions, this salt may be isolated from the reaction involving phosphorus oxychloride as an impure chlorophosphate (78TH30500) or precipitated from the reaction system as the thermally unstable perchlorate by the addition of sodium... 

Iminium salts (182) were starting materials in reactions with enamines (93CB133 94CB1437), which proceed by two different pathways with the formation of bicyclic ketones (183) and with the formation of substituted pyridines. The authors assume that the reaction takes place by a double electrophilic attack of the salt (182) in the /3-positions of the enamine and the resulting immonium cations undergo a retro-Mannich type of reaction with the opening of one of the piperidine rings. 

A secondary reaction pathway also is possible in formaldehyde-facilitated conjugations. Formaldehyde may react with a primary amine to form a quaternary ammonium salt. This intermediate spontaneously reacts to create a highly active immonium cation with loss of one molecule of water (Ji, 1983 Blass et al, 1965). The immonium cation... 

The intermediate immonium cation has been intercepted by an enoxysilane to yield a (3-amino ketone and by thioacetamide, phenacetyl cyanide and methyl propiolate to yield functionalized derivatives.219... 

Immonium salts have been used as model compounds for the study of the dimerization reaction in aprotic media, partly because no protons are involved in the reaction. It was shown that the electrodimerization process for immonium cations was purely radical [39,40],... 

Substituted cyclic immonium salts are also electrochemically hydrogenated in aprop-tic solvents to give stereoselectively the isomeric cylic amines [188] The stereoselectivity is influenced by solvent and supporting electrolyte. Immonium cations can be hydrodimer-ized with a certain class of bromo compounds [189] with some stereoselectivity. 

Like sulfur mustard, the nitrogen mustards combine predominantly with the thiol group of molecules and are excreted as conjugated cysteinyl derivatives. Both nitrogen and sulfur mustards have structural similarities and have common chemical reactions. A key reaction is the intramolecular cyclization in a polar solvent such as water to form a cyclic onium cation and a free anion. Nitrogen mustards form the immonium cation, while the sulfur mustard forms the sulfonium cation. The cyclized form is responsible for the varied effects of mustards, which are similar. In nitrogen mustard the sulfur is replaced by nitrogen. 

Path 2 is observed if either a good leaving group (e.g., R =benzyl) is present or the resulting immonium cation is stabilized. This path should be examined in detail in each reaction since a concerted E-2 mechanism gives rise to the same ion pair without the intermediate existence of a free ylid. The extent of C-H vs. N+-C bond breaking in the transition state must be considered to distinguish between these possibilities. 

In the case of phenol, with the free para position, due to the interaction between the phenolic group (acidic) and the aminic nitrogen (basic) of the amino alcohol, the ortho position is occupied first [9]. After the synthesis of Mannich bases, the water resulting from the reaction and the water from the aqueous solution of formaldehyde is distilled under vacuum, at 90-125 °C (preferably in the range 90-100 °C). A low range of distillation temperatures is preferred in order to avoid the tendency of the Mannich base to condensate to superior oligomers (with 2-3 aromatic nuclei), which increase substantially the viscosity of Mannich base and, of course, of final Mannich polyol. The mechanism of the Mannich reaction is considered to be a two-step mechanism. In the first step the reaction between formaldehyde and the primary or secondary amine (reaction 15.2) takes place, with the formation of an immonium cation [7-9, 22, 23]. 

In the second step, the reactive immonium cation formed reacts with the tautomeric forms of the phenolate anions, having negative charges, in the ortho and para positions (reaction 15.3). Finally, by a tautomerisation reaction, the reformation of the aromatic... 

The open chain form has exactly the structure of the immonium cation, the classic intermediate of Mannich reaction. The mechanism of reaction with phenols is presented in reaction 15.20. 

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