Amidinium cations

Peganine has two nitrogen atoms and its monoacidity is due to the formation of a cyclic amidinium cation. The Schopf-Oechler scheme for the synthesis of peganine from o-aminobenzaldehyde and... 

When 1-methyl-, 1,2- and 1,3-dimethyl-indoles were oxidized on a platinum electrode in methanolic ammonium bromide solution, in addition to the oxidation products, products of nuclear bromination at the 3-and 5-positions were observed. 1,2- Dimethylindole (20) gave 3-bromo-1,2-dimethylindole (81CCC3278) [bromine in chloroform gave the same product (85CHE786)]. In acidic conditions the amidinium cation formed from 20 was brominated in the 5-position (Scheme 14). Acylated 2-aminoindoles reacted similarly in neutral media to give 3-bromo derivatives and when protonated to give 5-bromo products. Bromine in chloroform transformed l-methyl-2-dimethylaminoindole (21) into the 3-bromo derivative (85CHE782) (Scheme 15). 

Ciloslowski, J., Boche, G. (1997) Geometry-tunahle Lewis acidity of amidinium cations and its relevance to redox reactions of the Thauer metal-free hydrogenase - a theoretical study. Angeiv. Chem. Int. Ed. Engl. 36, 107-9. 

Increasing numbers of nitrogen atoms increase not only the kinetic susceptibility toward attack but also the thermodynamic stability of the adducts. Reversible covalent hydration of C = N bonds has been observed in a number of heterocyclic compounds (76AHC(20)117). Pyrimidines with electron-withdrawing groups and most quinazolines show this phenomenon of covalent hydration . Thus, in aqueous solution the cation of 5-nitropyrimidine exists as (164) and quinazoline cation largely as (165). These cations possess amidinium cation resonance. The neutral pteridine molecule is covalently hydrated in aqueous solution. Solvent isotope effects on the equilibria of mono- (166) and dihydration (167) of neutral pteridine as followed by NMR are near unity (83JOC2280). The cation of 1,4,5,8-tetraazanaphthalene exists as a bis-covalent hydrate (168). 

Bis(amidinium) dicarboxylates form tape structures in which each cation possesses two DD faces, and each anion two AA faces (Fig. 19) [151-153]. These tapes can be linked into sheets via the addition of extra dicarboxylic acid or by introduction of further hydrogen bonding groups into the cations [154] or the anions [155]. The same bis(amidinium) cations form sheet structures with [Fe(CN)6]3, ... 

The three amino-pyridines are all more basic than pyridine itself and form crystalline salts by protonation at the ring nitrogen. The a- and y-isomers are monobasic only, because charge delocalisation over both nitrogen atoms, in the manner of an amidinium cation, prevents the addition of a second proton. The effect of the delocalisation is strongest in 4-aminopyridine 9.1) and much weaker in 2-aminopyridine (p/ aH 7.2). Delocalisation is not possible for the P-isomer, which thus can form a di-cation in strong acid (p/faHS 6.6 and -1.5). " ... 

Amidinium cations are interesting functional groups for the purpose of crystal engineering. Hosseini et al. have applied amidinium 37 in the design of tapes using the fumarate dianion 38 as a spacer [67]. In the crystal structure the tapes are linked together by fumaric acid spacers therefore propagating into sheets (Fig. 27). 

Amidinium cations are used as an alternative to guanidinium cation and as they are more easily accessible in organic synthesis. Gramlich and co-... 

In a reaction mixture where the initiators are strong acids, the strongest nucleophiles are the monomers. Acylations of the monomers with the amidinium cations result in formations of... 

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