Pyridines proton exchange

The formation of an intramolecular H-bond is supported by the slower rate of amide proton exchange in pyridine/10% CD3OD. The influence of the stereochemistry on turn formation and turn geometry has been investigated. Seebach and coworkers have demonstrated that dipeptide sequences consisting... 

The equilibria governing the complex formation and oxygen and proton exchange in these systems are given by Scheme 1, where CN denotes the total free cyanide, i.e., HCN/CN-, and is used as such throughout this chapter. In the complex formation in Eq. (2) X represents different entering nucleophiles such as NCS , F , CN-, and pyridine (py). 

Fig. 6.4 Reversible interconversion of amino acid and keto acid. Conjugation of the imine bond in the aldimine with the electron sink of the pyridine ring plus protonation of the pyridine nitrogen as well as the metal ion - all this results in weakening of the C-H bond of the amino acid residue. Thus, also catalyzed is a-proton exchange, racemization of a chiral center at the a-carbon atom and decarboxylation of the appropriate amino acid. ...

The pyridine-catalyzed aromatic proton exchange with deuterium provides a simple indication of the ability of a phenol to participate in chromene formation. Only those phenols which undergo exchange react with the unsaturated carbonyl compound, the attack occurring at the positions of deuteration (64JA2084). 

Chemical shifts for aromatic azoles are recorded in Tables 17 and 18. Fast tautomerism renders two of the 13C chemical shifts equivalent for the NH derivatives (Table 17a), as in the proton spectra (Table 8a). However, data for the N-methyl derivatives (Table 17b) clearly indicate that the carbon adjacent to a pyridine-type nitrogen shows a chemical shift at lower field than that adjacent to a pyrrole-type TV-methyl group (in contrast to the H chemical shift behaviour). Solid-state studies on imidazole (and pyrazole) show there are three distinct signals for the annular carbon atoms (imidazole C(2), 136.3 C(4), 126.8 C(5), 115.3 ppm). Proton exchange does not occur in the solid, hence the spectra describe the structure in the crystal. Comparison with the corresponding chemical shifts for 1-methylimidazole (137.6, 129.3, 119.7 ppm) implies that tautomerism has been frozen in the solid state <1981CC1207>. Solid-state examination of 2,2/-bis-17/-imidazole also reveals frozen tautomerism. 

Pyridines in their transition metal complexes coordinate as )] (N), q (C,C), (N,C), or q ligands (Figured). By far the most common coordination mode is (N), in which the lone electron pair on the nitrogen donates to a Lewis acidic metal center (see Lewis Acids Bases). The M-N bond lies in the plane of the pyridine ring (Figure 6). Few reactivity studies of (N) pyridine ligands have been reported. One of potential interest for HDN is the base-catalyzed exchange of the pyridine protons for deuterium... 

Another example is the proton exchange between H2O and HDO at low water concentrations in organic solvents. t values were obtained by line shape analysis T was an approximately linear function of water concentration. At about 1.1 M water, t Vs was 6.7 (in nitromethane), 0.91 (acetonitrile), 1.0 (acetone), 1.6 (dioxane), 25 (pyridine), 8.3 (dimethylsulfoxide), > 100 (triethylamine). The proton exchange may be catalyzed by HsO" or OH", or by solvent lyonium or lyate species, but for the neutral solvents a water trimer (for which some evidence exists) may be involved as shown (S represents the cvganic solvent) ... 

Triethylamine easily deprotonates the ring. The deprotio metalla-cyclobutadienes are more easily isolated after addition of pyridine ligands. Deuterium exchange on the p carbon of W(C3-/-Bu2D)[OCH(Cp3)2]3 with HOCH(Cp3)2 was demonstrated. Proton exchange probably occurs via elimination of DOCH(Cp3)2 and readdition of HOCH(Cp3)2. Even the presence of dimethoxymethane causes the elimination of HOCH(Cp3)2 [Eq. (206)]. In this case an equilibrium between the metallacyclobutadiene and its deprotonated counterpart could be observed. Por the phenyl-sub-... 

Interesting synthetic approach to IH- and 5//-1,3-diazepines based on the photolysis of 2-azido trifluoromethyl pyridines 77 was reported recently.The photolysis of pyridines 77 (or tetrazolo[l,5-a]pyridines) results in the intermediate formation of l,3-diazacyclohepta-l,2,4,6-tetraenes 78 (Fig. 10.34). The reaction of 78 with alcohols affords 2-alkoxy-l//-l,3-diazepines 79, existing as mixture of NH tautomers, with free energies of activation for 1,3-proton exchange estimated to be 14—16 kcal/mol. " In the reaction of 77 with secondary amines, a thermodynamically stable 2-diaUcylamino-5//-l,3-diazepines 80 form in most cases, however, kinetic isomer 81 (R = z-Pr = 1 6-CF3 isomer) was isolated and fully characterized. The photolysis of 77 in the presence of water leads to the formation of the corresponding... 

Kaneko and coworkers reported that a BPG electrode modified with PVP membrane containing CoPc was selective to (and enhanced) CO2 reduction (with the formation of CO and H2) than if CoPc only was employed . The enhanced catalytic activity in the presence of PVP was discussed in terms of (i) electron donation of the pyridine group (of PVP) to the central Co metal in CoPc by axial coordination, (ii) proton exchange property of PVP, and (iii) the increased local concentration of CO2 in the PVP layer. ... 

Geormezi M, Chochos CL, Gourdoupi N, Neophytides SG, Kallitsis JK (2011) High performance polymer electrolytes based on main and side chain pyridine aromatic polyethers for high and medium temperature proton exchange membrane fuel cells. J Power Sources 196(22) 9382-9390... 

Yang JS, Xu YX, Zhou L et al (2013) Hydroxyl pyridine containing polybenzimidazole membranes for proton exchange membrane fuel cells. J Membr Sci 446 318-325... 

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