Amidinium ion

Fig. 1. Chromophoric systems. Note that (A) = amidinium-ion system (a cyanine), (B) = carboxyl-ion system (an oxonol), and (C)...

Carboxyl-related and acyl substituents. Included here are cyano, protonated amidinium ion, thionoacyl, acyl (Ar—CO, H—CO, Alkyl—CO), carboxamido, carboaryloxy, carboalkoxy, carboxy (unionized), amidino (unionized), and carboxylate anion, listed approximately in order of decreasing electron attraction or activation. The relative activation by some of these groups (e.g., ketone, aldehyde, nitrile) will change upon reversible interaction with the nucleophile, which will vary with the group and with the nucleophile (e.g., MeO , N3, NCS ). Irreversible interaction will be obvious when the reaction products in kinetic studies are characterized. 

Alkyl A-2-selenazolines, preparation. 259 Amidinium ion, ionic system of, 68 2-Amino-4-aryl-5-acetic acid selenazoles. 

Values of pKa for substituted amidines were correlated with the Hammett equation by Charton (190). Data are available for one set of substituted amidinium ions and one set of C-substituted-N-phenylamidinium ions. The sets studied are shown in Table XXX. Results of the correlations are set forth in Table XXXI and values of pj in Table XXXII. Significant correlations were obtained for both sets. In the case of the ionization of N-phenyl amidinium ions, the possibility of tautomerism exists, Charton (170) has presented arguments which suggest that the predominant tautomer is 30. The localized effect is predominant in both of the sets of amidinium ions studied. 

Reactions of nitriles may be promoted by the presence of divalent Co. The conversion of sterically unhindered nitriles RCN (R = Me, Et, n-Pr, w-Bu) into the corresponding amidines RC(NH2) = NH occurs in the presence of Co(N03)2.6H20 and the ketoxime Me2C=NOH. The metal free amidinium ions were crystallized as their nitrate salts.358... 

The charges of the molecule are important for binding. When the amidine group accepts a proton and becomes an amidinium ion, noformycin and amidinomycin become biscationic and bind strongly to AT sequences. These two molecules have large pKa values at both basic sites and, as such, are biscationic at physiological pH. 

Moving closer to a cationic transition state mimic, Hasserodt et al. (1996) used the amidinium ion [75] as a TSA for cyclization of the arenesulfonate ester [761- One antibody raised to this hapten, 17G8, catalysed the conversion of substrate [76] into a mixture of the 1,6-dimethylcyclohexene [77] and 2 methylene-1-methylcyclohexane [78] (Fig. 28) (Appendix entry 15.3). By contrast the uncatalysed cyclization of [76] formed a mixture of 1,2-... 

It is perhaps worth mentioning that substitution of other groups for the C-methyl group in amidinium ions has an effect on their pA a-values, which is best correlated with Hammett s -constants (p = —1T98), and the same is true of C-substituted N-phenylami-dinium ions (p = —12-08) (Charton, 1965). This is in line with the suggestion made on page 286, that -constants are a measure of the inductive and mesomeric interactions of groups with a carbon in the sp state of hybridization. 

Conjugated systems analogous to the amidinium ion arise in the protonation of 2-aminopyridines, which are known to be protonated on the ring nitrogen, giving the resonance stabilized ion [57] (Albert... 

As a true testament to the potential long-term impact of H-bonding activation, a number of ureas, thioureas, and acid catalysts are now finding broad application in a large number of classical and modem carbon-carbon bond-forming processes. On one hand, Johnston s chiral amidinium ion 28 was elegantly applied to the asymmetric aza-Henry reactions (Scheme 11.12d). On the other hand, chiral phosphoric acids (e.g., 29 and 30), initially developed by Akiyama and Terada, have been successfully employed in Mannich reactions, hydrophosphonylation reac-tions, aza-Friedel-Crafts alkylations (Scheme 11.12e), and in the first example... 

Most of the sensitizing dyes contain the amidinium ion or amidic chromophore systems (principally cyanines and mero-cyanines). A few contain the carboxyl ion system. In practice, the dyes are adsorbed by the silver halide and sensitize to radiation absorbed by the adsorbed dye. 

Confirming the above results, heating the carboxylic acid salts 384 and 385 (X=amidinium ion) for 1 min at their melting points (165° and 195°C respectively) gave a high yield of the 5-E, 9-E isomeric macrolide 386. Note again that, in the dioxolenium intermediate 387, the o — COO bond becomes anti-periplanar to the C9 - 0 bond and consequently the decarboxylation produces an E double-bond. 

Table 3. Calculated model reactions of the amidinium ions 115 and 116 with H2 in the absence and presence ofa base ((BLYP/6-311G(2d, 2p) level).

The amidine formation can take place when polymers with a degree of hydrolysis in the order of 20-80% are used. Furthermore acidic surface groups can also induce the amidinium ion formation. 

These reactions usually are slow at room temperature, and the adhesives must be cured with heat. Tertiary amines or amidines can be used to catalyze the reaction whereby the car-boxylate anion is the nucleophile and the ammonium or amidinium ion act as hydrogen-bond donors. Dusek et al. have shown that transesterification only occurs after essentially complete consumption of the carboxylic acid in compositions initially having equivalent amounts of acid and epoxy groups.40 41 The transesterification leads to an equilibrium sol-gel composition. 

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