Pyridinium zwitterion

The 3-hydroxy group is very acidic for a phenol but this is not the result of orientation - all isomeric hydroxypyridines show pX s of about 2 for ionisation of the hydroxy group. This effect is presumably due to formation of a zwitterionic structure rather than to any mesomerism. In pyridoxal the pX for OH is 4.2 and that for the pyridinium nitrogen 8.7. The pX of imines is normally 6-7 and although pyridoxal Schiff bases are tiibasic the important species near neutrality are the protonated pyridinium zwitterion (XXVI) or its tautomer (XXVII) and these same structures with the pyridinium nitrogen unprotonated. The pX for loss of a proton from the intramolecularly hydrogen-bonded system is ca. 9 so the anion (XXVIII) must also exist to a significant extent. 

Use of heterocycles, in particular tetrathiaflilvalenes and pyridinium zwitterions, in unimolecular electronic devices 99ACR950. 

Fig. 1. LB film spectra of the N-octyl-4-pyridinium zwitterion showing the progressive photochromic bleaching of the charge transfer band.

The spectra of the pyridinium and quinolinium analogues are superimposed in Figure 2 to show three photochromic bands with exploitable maxima at 495, 565 and 614 nm. The absorption maximum at 495 nm of the pyridinium zwitterion corresponds to minima in the spectra of the other materials whereas the maxima at 565 and 614 nm of the quinolinium zwitterions only partially overlap the adjacent charge transfer bands. The fact that there... 

The extrapolated viscosity follows the order D > E > B in both series of mesogens. Similar analysis of pyridinium zwitterions 71 and 72 in ClEster host showed that they both increase viscosity y, of the phase, and the increase is larger than for the sulfonium The effect is host dependent and in 6-CHBT the sulfonium zwitterions 5 and 73 have significantly lower impact on viscosity. 

Of derivatives of compound 3, the zwitterions 245 react with DMAD if magnesium bromide is added to give aminopyridine derivatives (00H(53)265). Compounds such as 102 are opened by acid to give quaternary pyridinium salts (92LA885). There are various interconversions of 2-substituted triazolopyridin-3-ones into 1-substituted zwitterions. An example is the reaction of 246 (83BCSJ2969) the reverse conversion is also reported. 

Zwitterionic 2-oxo-l,2,3,4-tetrahydropyrido[l,2-ft]pyridazine 78 was obtained from 6-(3-methoxycarbonylpropionyl)-l,l-dioxo-l,2-thiazine 76 with hydrazine hydrate via pyridinium betaine 77 (99JPR37). 

A theoretical study based on PM3 frontier molecular orbital (FMO) and potential energy surface (PES) analysis at the restricted Hartree-Fock (RHF)/6-31+G level was performed to examine the reaction of l-amino-2-ethoxycarbonyl-pyridinium mesitylenesulfonate and acrylonitrile in the presence of Hilnig s base leading to the formation of l,2-dihydropyrido[l,2-A]pyridazinium inner salt 17 <1999JOC9001>. The calculations indicated that both the [3+2] cycloaddition reaction and the ring expansion occurred in a concerted manner rather than through a stepwise mechanism via a zwitterionic intermediate 16 (Scheme 1). 

Most studies of micellar systems have been carried out on synthetic surfactants where the polar or ionic head group may be cationic, e.g. an ammonium or pyridinium ion, anionic, e.g. a carboxylate, sulfate or sulfonate ion, non-ionic, e.g. hydroxy-compound, or zwitterionic, e.g. an amine oxide or a carboxylate or sulfonate betaine. Surfactants are often given trivial or trade names, and abbreviations based on either trivial or systematic names are freely used (Fendler and Fendler, 1975). Many commercial surfactants are mixtures so that purity can be a major problem. In addition, some surfactants, e.g. monoalkyl sulfates, decompose slowly in aqueous solution. Some examples of surfactants are given in Table 1, together with values of the critical micelle concentration, cmc. This is the surfactant concentration at the onset of micellization (Mukerjee and Mysels, 1970) and can therefore be taken to be the maximum concentration of monomeric surfactant in a solution (Menger and Portnoy, 1967). Its value is related to the change of free energy on micellization (Fendler and Fendler, 1975 Lindman and Wennerstrom, 1980). 

Treatment of pyridinium salts 140 either with K2CO3 or with NaH afforded 4-oxidopyrido[l,2-h)pyridazinium zwitterions 141 [81JCS(P1) 1495],... 

The pyridinium salt NAD 19a and its reduced form NADH 20a are important co-factors for many enzymes, fhe reduced form is involved in enzyme mediated reductions where it is converted to NAD. In natural systems, NAD is converted back to NADH by another enzyme-controlled process. Attempts to effect the direct electrochemical conversion of NAD to NADH are not very successful. Reduction on a mercury cathode at -1.1 V see on the first one-electron reduction wave leads to the radical-zwitterion, which reacts further to give dimers. Three stereoisomers of the 4,4 -dimer account for 90 % of the mixture and three 4,6 -dimers form the remainder [78]. Reduction at -1.8 V on the second reduction wave produces only 50 % of enzymatically active 1,4-NADH. The NAD analogue 19b shows related behaviour and one-electron reduction affords two diastereoisomers... 

Whereas zwitterionic forms may be invoked to explain the properties of 2- and 4-thiopyrones, the 3-thiopyrone can only exist as the betaine (42) and is therefore to be compared with the pyridinium system for a bonding and structural likeness. A theoretical treatment (73JPR690) has been published using CNDO/2 methods, and the calculated parameters were found to agree well with experimentally determined spectral properties. 

More drastic conditions, i.e. fuming HN03 in (CF3C0)20 were required for the preparation of some zwitterionic pyridinium analogs of o-iodosobenzoic acid (Scheme 15) [53]. 

Whereas a,/ -unsaturated ketones afforded with DIB a-hydroxy-/ -methoxy dimethylacetal derivatives (Section 3.2.2), some steroidal ketones of this kind showed a deviation when treated with o-iodosylbenzoic acid for example, 4-androstene-3,17-dione gave a mixture of two methoxy derivatives and a diene [5]. Several sulphides were oxidized efficiently to sulphoxides by o-iodosylbenzoic acid in acetic acid-sulphuric acid, at room temperature [3]. o-Iodosylbenzoic acid is an excellent reagent for the rapid, catalytic cleavage of reactive esters, especially phosphates, some of which are in stock in big quantities for use as potential nerve gases. This kind of reactivity has drawn considerable attention, and several analogues of the parent acid showed better catalytic activity among them, a series of structurally interesting pyridinium 1,5-zwitterions should be mentioned [6] ... 

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). 

Base-catalyzed hydrogen exchange (e.g., in pyridinium cation, can occur in zwitterions or ylids). 

Formation of zwitterionic intermediate. 2-(-Hydroxyalkyl) groups on pyridinium ions 743 can be removed by nucleophiles via ylide, 744. 

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