Amidinium

Most of the qualitative relationships between color and structure of methine dyes based on the resonance theory were established independently during the 1940 s by Brooker and coworkers (16, 72-74) and by Kiprianov (75-78), and specific application to thiazolo dyes appeared later with the studies of Knott (79) and Rout (80-84). In this approach, the absorptions of dyes belonging to amidinium ionic system are conveyed by a group of contributing structures resulting from the different ways of localization of the 2n rr electrons on the 2n l atoms of the chromophoric cationic chain, rather than by a single formula ... 

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

The stabilizing influence in the hydrated cation is the amidinium resonance. If a solution of the cation is neutralized, a short-lived hydrated neutral molecule (4) (half-life 9 sec at pH 10) is obtained with an ultraviolet spectrum similar to that of the hydrated cation but shifted to longer wavelengths (5 m/ ). Supporting evidence can be derived from the anhydrous nature of the cation of 4-nitroiso-quinoline (pK 1.35), in which the nitro group has a similar electronic influence to that of the ring nitrogen atom N-I in quinazoline and where amidinium resonance is not possible. 

The parent substance, 3,4-dihydroquinazoline, is strongly basic (pKa 9.19) on account of the amidinium resonance present in the cation. Spectroscopic evidence favors this resonance and is illustrated... 

Tetrahydroquinazolines are cyclic methylene diamines and are, therefore, readily hydrolyzed by acid to the corresponding o-aminobenzy]amines, 1,2,3,4-Tetrahydroquinazoline is a strong base (pi a 7.65), though it is not as strong as 3,4-dihydroquinazoline because of the absence of amidinium resonance in the cation. The ultraviolet spectrum is only slightly altered on protonation clearly indicating that it takes place on N-3. ... 

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... 

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. 

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). 

Water-soluble azo compounds include 4,4 -azobis(4-cyanovaleric acid) (29) and the amidinium hydrochlorides (22 and 23). 

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

Amidines and carboxylic acids have been considered separately from other heterovinylene and heterovinylidene sets, because although the amidines, 26, and carboxylic acids, 28, themselves have a heterovinylene structure, the amidinium, 27, and carboxylate, 29, ions with which they are in equilibrium have structures which are neither heterovinylene nor heterovinylidene. We shall find it convenient to discuss separately the amidines and the carboxylic acids. 

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. 

It is obvious that N,0-bis(trimethylsilylated)acetamides or N,0-bis(trimethyl-silylated)formamides 22 or N,0-bis(trimethylsilyl)benzamide 296 should react likewise with the hydrochlorides or hydriodides of primary or secondary amines or with the free amines in the presence of equivalent amounts of e.g., NH4CI or NH4I to give the corresponding amidinium salts. 

Another family of coupling reagents frequently used with the Fmoc method is related to A-hydroxybenzotriazole and A-hydroxy 7-azabenzotriazole but also incorporates phosphonium or amidinium groups. The latter can exist in either the O-(uronium) or A-(guanidinium) forms.48 Both can effect coupling. The former are more reactive but isomerize to the latter. Which form is present depends on the protocol of preparation, including the amine used and the time before addition of the carboxylic acid 49 The... 

Investigations of hypervalent phosphorus compounds have tended to concentrate on the more stable bicyclic systems. Elegant studies of spirophosphoranes and a thorough investigation of permutational isomerisations within amidinium fluorphosphates are worth noting. 

Very similar reactions have been described using unsubstituted pyrazole-1 -carbox-amidinium chloride d83... 

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. 

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