Cationic aromatics

The pyrazole ring is particularly difficult to cleave and, amongst the azoles, pyrazoles together with the 1,2,4-triazoles are the most stable and easiest to work with. This qualitative description of pyrazole stability covers the neutral, anionic and cationic aromatic species. On the other hand, the saturated or partially saturated derivatives can be considered as hydrazine derivatives their ring opening reactions usually involve cleavage of the N—C bond and seldom cleavage of the N—N bond. It should be noted, however, that upon irradiation or electron impact the N—N bond of pyrazoles can be broken. 

MO calculations, 7, 364 Oxetene, 2-isopropylidene-polysubstituted rearrangement, 7, 377 Oxetene, tetramethyl-structure, 7, 366 Oxetenes, 7, 363-402 decomposition, 7, 375 metabolism, 1, 243 molecular dimensions, 7, 366 thermal stability, 7, 370 Oxetenyl anions aromaticity, 7, 371 Oxetenyl cations aromaticity, 7, 371 Oxichlororaphine occurrence, 3, 196 Oxichromic developers in colour photography, 1, 378-379 Oxidation... 

Laser Raman (3) and UV diffuse reflectance (4) measurements have shown that the cation/aromatic ring interaction becomes stronger as the size of the cation increases. The Laser Raman results were interpreted as being due to electrostatic fields within the... 

Attention should be paid to possible competitive protonation of the nitrogen atom of the ionophore, which suppresses electron transfer (even more efficiently than cations) and proton may thus have a larger effect on quantum yield than other cations. Aromatic nitrogen, as in 2 and 6, have lower pKa than aliphatic amines and thus can be used in neutral media. 

In conclusion, structures containing polyiodide anions, with cationic aromatic ligands as counter parts of formulae [(L)(HL+)] (I ) are known to be synthesized by the treatment of the appropriate amide with HI [26-28], In contrast, the complexes with PYOH, in the present case, were formed by the direct reaction of 2-hydroxypyridine with di-iodine in a molar ratio of 2 1 and 1 2. This is a redox reaction, where 2-hydroxy-pyridine firstly is oxidized to pyridinone-2 radical cation. In the case of 2-hydroxy-pyridine however, peroxide structures are not formed like disulphides in the case of PYSH. Polyiodide anions are simultaneously produced in this case This should be a consequence of redox differences between -SH and OH groups and may be proven a useful pathway for the synthesis of polyiodide materials. 

II. Polar Cycloadditions in Which Cationic Aromatic Systems Act as Electrophiles. ... 

The quinolizinium ion, the parent compound of the aromatic quinolizines, is a cationic aromatic system like the pyrylium or thia-pyrylium cation. It is isoelectronic with naphthalene. The parent... 

The parfait-distillation method uses a sequential series of adsorbents to remove contaminants from water and vacuum distillation to recover unadsorbed materials. This method recovers a wide range of neutral, cationic, anionic, and hydrophobic contaminants. The first adsorbent, porous polytetrafluoroethylene (PTFE), removed humic acid and a broad range of hydrophobic compounds. PTFE was followed by Dowex MSC-1 and then Duolite A-162 ion-exchange resins. A synthetic hard water spiked parts-per-billion concentrations with 20 model compounds was used to evaluate the method. Poorly volatile, neutral, water-soluble species (glucose) cationic aromatics and most hydro-phobic compounds were recovered quantitatively. Model ampho-terics were removed from the influent but were not recovered from the adsorption beds. The recovery of model acids and bases ranged from 22% to 70% of the amount applied. 

Chromosorb T shows the direct correlation of capacity with the oc-tanol-water partition coefficient expected of hydrophobic adsorbents (6). Its overall affinity for hydrophobic water contaminants is similar to that of Amberlite XAD-8 (Rohm and Haas), but it has additional affinity for humic acid and for water-soluble, cationic, aromatic dyes. Chromosorb T is more easily cleaned than the XAD resins, and it is more inert, contributing essentially no contaminants to eluates. Table IV illustrates the adsorption and recovery of a series of hydrophobic test solutes at 50 ppb in 8 L of synthetic hard water on a 50-mL bed of Chromosorb T. 

Related symmetrical systems, involving a-cyclodextrin threaded onto alkyl chains that are terminated by cationic aromatic (heterocyclic) end groups, are also known. Formation of the above species involves threading of the 4,4 -bipyri-dinium groups through the a-cyclodextrin such that the alkyl chain lies in the cavity, with the cationic bipyridinium end groups extending on either side. 

Crown compounds having cyclophane-skeletons and called crownophanes , are reviewed. They form characteristic host-guest complexes due to their crown-cyclophane hybrid structures. Significant stabilization of complexes is often reported through the ion-dipole, cation-aromatic n-electron and n-n stacking interactions, size-and-shape complementarity, and hydrophobic interaction. 

In conclusion, structures containing polyiodide anions, with cationic aromatic ligands as counter parts of formulae [(L)(HL+)] (1 ) are known to be synthesized by the treatment of the appropriate amide with HI [26-28], In contrast, the complexes with PYOH, in the present case, were formed by the direct reaction of... 

A Benzamidine Receptor. Cationic aromatic guests may bind to synthetic receptors by a combination of hydrogen-bonding and jc-stacking interactions. Figure 11 displays the complex of benzamidinium chloride with a hexagonal lattice cleft that is bridged by an aromatic spacer unit (27). Results of titration studies in 95 5... 

Cations M3 (M = Ii...Cs) and Hj are the simplest o-aromatic (n = 0 for the 4w-l-2 rule) systems. Aromaticity in the Li3 was initially assessed by Alexandrova and Boldyrev [100]. It has only one completely delocalized a-molecular bonding orbital, which makes this cation aromatic (Figure 14.4). 

---