Protonated pyridine

UV spectrum changes dramatically. The salt causes maxima at 251 (4.16), 286 (4.16), 387 (3.56), and 473 (3.37) nm. NMR data for a sample in CF3COOD were presented. It is interesting to note that the nonmethylated derivatives of 130 and 131 are present in solution above pH 6.5 as betaines, forming a protonated pyridine ring and an olate group (91MI3). 

Kishimoto et al. (1974, 1981) found a general acid catalysis by protonated pyridines in coupling reactions of the 1-naphthoxide ion if weakly electrophilic diazonium ions were used. In this case it is likely that the general acid protonates the carbonyl oxygen of the o-complex, with a concerted or stepwise deprotonation at the 4-position (transition stage 12.150). 

Several metallophthalocyanines have been reported to be active toward the electroreduction of C02 in aqueous electrolyte especially when immobilized on an electrode surface.125-127 CoPc and, to a lesser extent, NiPc appear to be the most active phthalocyanine complexes in this respect. Several techniques have been used for their immobilization.128,129 In a typical experiment, controlled potential electrolysis conducted with such modified electrodes at —1.0 vs. SCE (pH 5) leads to CO as the major reduction product (rj = 60%) besides H2, although another study indicates that HCOO is mainly obtained.129 It has been more recently shown that the reduction selectivity is improved when the CoPc is incorporated in a polyvinyl pyridine membrane (ratio of CO to H2 around 6 at pH 5). This was ascribed to the nature of the membrane which is coordinative and weakly basic. The microenvironment around CoPc provided by partially protonated pyridine species was suggested to be important.130,131 The mechanism of C02 reduction on CoPc is thought to involve the initial formation of a hydride derivative followed by its reduction associated with the insertion of C02.128... 

Isonicotinamide, 5. This compound was sufficiently soluble to allow SERS spectra to be obtained at the 50 mM level in 0.10 M KC1 and 0.10 M KC1 + 0.10 M HC1 at -0.20 V. The spectra resembled those seen with other pyridines. In particular, an intense band at 1600 cm-1 was seen with the neutral electrolyte and it was replaced by a band at 1640 cm-1 in the acidic electrolyte. Of the two basic sites, only the ring nitrogen will be protonated in 0.10 M HC1 (22) so, with this compound also, the 1640 cm-1 band appears to be due to the protonated pyridine. No carbonyl band was seen in either spectrum. 

Protic acids, in cationic polymerization of cyclic siloxanes, 22 560 PROTO (-)-Protoemetine, 2 84, 85 Protonated ozone, 7 7 774-775 Protonated pyridines, 27 100-101 Protonation, 75 653-654... 

Figure 4.26 IR spectra showing absorbance bands due to pyridine adsorption on a H-FAU sample. PyrH is protonated pyridine on Bronsted sites, Pyr L is pyridine coordinated to Lewis acid sites, Pyr phys is physisorbed pyridine. All spectra recorded at 25 °C.

Electrochemical reduction of 1-alkylpyridinium salts 1 leads to Are addition of one electron with the formation of a ii-delocalised radical-zwitterion. This is a formally neutral species. Botlr this species and the N-protonated pyridine radical-anion are essentially n-delocalised radicals. The radical-zwitterion from 1-methylpyridinium shows a long wavelength absorption band in water with 7, ax 750 nm [19]. The nitrogen ring radical-zwitterions take up further electrons at more... 

The band centered at 3615 cm disappears by adsorption of pyridine and further outgassing at 150°C, thus confirming its assignment to stretching vibration of acidic OH groups. For samples with a different Si/Al ratio, a direct correlation is observed between the intensity of this band and that of protonated pyridine at 1540 cm (Table I). 

Table 28 Free Radical Acylation of Protonated Pyridines ...

Protonated pyridines and derivatives readily undergo acylation at C-2 or C-4 (Table 28) (76MI20503). Acyl radicals are usually generated either by hydrogen abstraction from aldehydes (Scheme 210), or by oxidative decarboxylation of a-keto acids (Scheme 211). In the former case (Scheme 210) with acridine as the substrate, reduction can take place to give a dihydroacridine. 

Fig. 28. 36-MHz l5N CP/MAS spectra of pyridine-15N on zeolite HY. The experimental conditions were all the same for (a) and (b), except that sample b was extensively dealuminated by increasing the activation temperature to SSITC (400°C for sample a). Both spectra were acquired at 77 K to prevent chemical exchanges on the NMR time scale, (a) The single resonance at —176 ppm as well as its associated sidebands indicates protonation of pyridine by the Brpnsted sites, (b) In addition to the protonated pyridine, four additional resonances at -68, -88, —116, and -140 ppm are also seen, indicating complexation of pyridine with different extraframework Lewis sites.

Pyridine is half protonated in aqueous phosphate buffer at pH 5.2. If you mix 45 mL of phosphate buffer with 55 mL of methanol, the buffer must have a pH of 3.2 to half protonate pyridine. Suggest a reason why. 

In nitrogen heteroaromatics, upfield protonation shifts are found for carbons a to nitrogen, while those in / and y positions are deshielded on protonation [94, 99,100]. This is shown in Fig. 3.5 for quinoline [94]. The protonation shifts for C-/1 and C-y can be rationalized in terms of the cannonical formulae of protonated pyridine [73 d], while the upfield shifts for C-a are probably due to the lower n character of the N — C-a bond. The curves in Fig. 3.5 representing the pH dependence of 13C shifts resemble titration curves. pK values and, in the case of amino acids, the isoelectric points pi can be obtained from the point of inflection of the (5 versus pH plot for each individual carbon [84, 94, 98]. 

No protonated pyridine (which resonates at 174 ppm) was found on initial measurement. The author attributes the subsequent appearance of this signal (on exposure to air) to the formation of carbonate or bicarbonate species which serve as counterions to protonated pyridine. 

The structure and nature of acidity of sulphated zirconia has been recently investigated (ref. 8-10). Untreated zirconium hydroxide which is amorphous, crystallises 1n the monoclinic form at 350°C. On the contrary, the sulphate treated zirconium hydroxide crystallises at higher temperature (500ooC) into tetragonal form, which has significantly greater surface area as compared to the monoclinic zirconia. The infra red spectra of absorbed pyridine indicate presence of only coordinatively bonded pyridine and not the protonated pyridine implying that the zirconia is predominantly of Lewis... 

X = H, I, Co, Hg, C02H R = alkyl, hydroxyalkyl, aminoalkyl, acyl Scheme 82 Additions to protonated pyridines... 

Scheme 6 Decarboxylation of MTh (1) to HBnTh (2), showing protonated pyridine capturing the carbanion, preventing reversion. Reprinted with permission from Reference 3. Copyright 2009 American Chemical Society.

The synthesis of 2- and 4-functionalized pyridines, the most useful outcome of Gif chemistry, is based on selective addition of nucleophilic carbon-centered radicals at positions 2and 4of protonated pyridines. 2/4-Pyridyl moieties are regarded as high-value intermediates in the synthesis of compounds of industrial and phar-... 

IV sites. A protonated pyridine has never been observed, and the Lewis acid sites on titanium oxides cannot be converted into Br0nsted sites by water vapor adsorption (217). Although Jones and Hockey (216) suggest that the chemistry of surface hydroxyl on rutile corresponds more closely to that of the OH" ion rather than that of the hydroxyl group, no surface reactions similar to that observed with alumina [Eq. (14)] have since been reported. 

---