Pyridinium salts structure

In view of the fact that benzylic cations are responsible for the initiation, graft copolymerization is feasible when the polymer supported pyridinium salts are employed. Endo and co-workers [41] prepared vinyl monomers having pyridinium salt structure. In this case, p-chloromethyl styrene was used as the benzyl halide and the general synthetic strategy was followed. The polymerizable salt was then homo and copolymerized with styrene (Scheme 7). 

After its isolation, the structure of alkaloid deplancheine (7) was unambiguously proved by several total syntheses. In one of the first approaches (14), 1,4-dihydropyridine derivative 161, obtained by sodium dithionite reduction of A-[2-(indol-3-yl)ethyl]pyridinium salt 160, was cyclized in acidic medium to yield quinolizidine derivative 162. Upon refluxing 162 with hydrochloric acid, hydrolysis and decarboxylation took place. In the final step of the synthesis, the conjugated iminium salt 163 was selectively reduced to racemic deplancheine. 

Using a different set of hydrogen bonding fragments Kruger and Martin have reported the formation and structural characterization of the double helicate 71 [93]. This helical structure can be prepared by assembling 72 diammonium-bis-pyridinium salt around two chloride anions (see Scheme 36). 

When mechanical vibration of bis(pyridinium) salts (see Scheme 5.5) was conducted with a stainless steel ball in a stainless steel blender at room temperature under strict anaerobic conditions, the powdery white snrface of the dicationic salts turned deep blue-purple (Kuzuya et al. 1993). Single-line ESR spectra were recorded in the resnlting powder. No ESR spectra were observed in any of the dipyridinium salts when mechanical vibration was conducted with a Teflon-made ball in a Teflon-made blender nnder otherwise identical conditions. When observed, the ESR signals were quickly quenched on exposnre to air and the starting dicationic salts were recovered. Each of the resulting powders was dissolved in air-free acetonitrile, and the ESR spectra of the solution were recorded after the material had been milled nnder anaerobic conditions. Analysis of the signal hyperflne structure confirmed the formation of the corresponding cation-radicals, which are depicted in Scheme 5.5. 

Tickle, 1899). Which of the two oxygens was the site of proton addition became clear only when Hantzsch (1919) demonstrated the close optical analogy between the salts and methiodides of dimethyl-pyrone and pyridinium salts and argued that a benzene-like ring arises in the pyroxonium salts [191]. The discussion of the structure... 

Generally, the structures of the 1,2,3,5-thiatriazolines (2) and (3) are inferred from the mode of synthesis. The structure of l,2,3,5-thiatriazolo[5,4-fl]pyridine 3-oxide (10) has been studied in detail and the structural data strongly suggests that (10) exists as a pyridinium salt <63CB2519, 70CB1918>. 

In the case of the salts of organometallic cations, the 2a anion adopts only a trans configuration in the crystal structures. However, in all of the pyridinium salts, there is clear evidence for variable amounts of cis-trans disorder, which is somewhat unusual for such thiophenedithiolene complexes. The trans configuration is still predominate for structures of the [BzPy] and [BrBzPy] salts, while the structure of the [FBzPy] salt exhibits 77% cis conformation. In addition to the cis-trans disorder, the 2a anion in the [BrBzPy] salt also exhibits 20% orientation disorder. ... 

The indolizines constitute the core structure of many naturally occurring alkaloids, such as (-)-slaframine, (-)- dendroprimine, indalozin 167B and coniceine. There are a number of different routes to the synthesis of indolizines and they are most commonly synthesised by sequential N-quaternisation, intramolecular cyclocondensation reactions or the cycloaddition reaction of /V-acyl/alkyl pyridinium salts. 

It has been claimed (335) that preparation of an acid form catalyst by the thermal decomposition of pyridinium salts results in a cubic crystal structure and increases the surface area and pore volume. For example, the surface area of H4PM011VO40 increases from 1.0 to 5.3 m2 g 1 by the creation of macropores having radii of 103— 104 A. As a result of macropore formation, higher yields are obtained (Fig. 62). The formation of acetic acid, CO and C02 at high conversion is suppressed by treatment of the catalyst with pyridine. The application of this method to acidic Cs salts further improves the activity and selectivity. 

As might be anticipated, the majority of solvent effects of the structures of pyridines relate to conformational changes and, where appropriate, this has been mentioned above. It may be further exemplified by the conformational changes observed on addition of alcohols and fluorinated alcohols to solutions of the pyridinium salts such as 51 in water <2000JA738>. Helical coils are favored by the fluorinated alcohols as the co-solvent destabilizes the exposed hydrophobic side chains in other conformations along with favoring the helical conformation entropically. The former effect is less marked in nonfluorinated alcohols. 

Generally, the structures of the 1,2,3,5-thiatriazoles (114) and (113 X = S) are inferred from the mode of synthesis. The structure of l,2,3,5-thiatriazoIo[5,4-a]pyridine 3-oxide (117) has been discussed in more detail. 2-Hydrazinopyridine is formed upon hydrolysis and the IR spectrum exhibits an NH stretching vibration at 3280 cm-1. This suggests the interesting possibility of tautomeric and zwitterionic structures (Scheme 8). Methylation with diazomethane gives the 1-methyl derivative which apparently is best formulated as the zwitterionic A4-thiatriazoline (118). UV spectroscopic properties of (117) (Amax 235, 290 and 342) and (118) (Amax 235, 290 and 345 nm) reveal a close structural similarity (63CB2519). A comparative NMR spectroscopic study of (117) and the A3-l,2,3,5-thiatriazo-line (119 R1 = H, R2 = Me) supports the description of (117) as a pyridinium salt (Scheme 9) (70CB1918). 

The general features for conformational structure and steric effects on the barriers are maintained. Fairly high barriers to rotation were determined in isomeric pyridinium salts 74a and 74b, in which X and Y are hydrogens... 

The molecular (space-filling) models in Fig. 1 illustrate the location of the anionic donors I- and Co(CO)4- relative to the cobalticenium acceptor for optimal orbital overlap with the LUMO in the equatorial plane (34). For the pyridinium salts of Co(CO)4", the analogous charge-transfer interaction of the tetracarbonylcobaltate donor places it above the aromatic acceptor planes for optimal orbital overlap with the ti-LUMOs of Q+ and NCP+. Such X-ray crystallographic structures indicate that these charge-transfer salts consist of contact ion pairs that are directionally constrained for optimum CT interaction in the crystal lattice. 

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