Immonium cation intermediate

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.

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],... 

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.

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... 

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... 

As predicted in 1965, some hydantoin derivatives are precursors of carbonium-immonium ions, which are powerful electrophiles.326 These include the 4-hydroxy-2-imidazolidinones (Section IV,C). 5-Alkoxyhyd-antoins (135) undergo successful inter- and intramolecular amidoalkyla-tions through cation intermediates (I36).327... 

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. 

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. 

Synthetic work in this area includes a new, stereospecific synthesis of sparteine and a synthesis of isosophoramine. The reduction of quinolizidone (16) by means of lithium aluminium hydride affords, as noted previously, a dimeric product (17), presumably via condensation of the initially formed enamine (18) with the corresponding immonium cation (19). With di-isobutyl aluminium hydride the yield of (17) can be increased to 85%. Analogous reduction of the dilactam (20), prepared by base-catalysed condensation of the bromo-lactam (21) with a-piperidone, gave an intermediate (22), in which the enamine and immonium ion units are present within the same molecule. Intramolecular cyclization of (22), via a transition state derived from the conformation (23), spontaneously gave the... 

Cationic polymerization is the conversion of low molecular weight monomeric molecules into high molecular weight polymeric ones, via a mechanisn involving stepwise growth of a carbonium (RjC ), carbox-onium (RO=CR2), oxonium (RsO" ), sulphonium (R3S ) or immonium (R2N=CR2) ion. Both vinyl and cyclic monomers are susceptible to polymerization by such intermediates, e.g. 

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