Pyridinium ions, hydrolysis

Amination of AT-alkylpyridinium salts with amide ions, which in principle should be easier than the reaction with the parent pyridine, has been little studied. The main reason for this is that solvent selection is difficult. Metal amides are only soluble in liquid ammonia (with which pyridinium salts react easily, vide infra), and pyridinium salts are soluble in solvents that are not suitable for use with metal amides. The A/ -methylacridinium cation undergoes direct imination to give (153) in 35% yield by treatment with potassium amide and iron (III) nitrate in liquid ammonia. Two other products (154) and (155) are also formed, probably by hydrolysis and subsequent disproportionation (Scheme 90). One might question whether sodamide is necessary to the above transformation in light of the fact that quin-olinium, isoquinolinium and certain pyridinium ions give cr-complexes (156), (157) and (158) in liquid ammonia alone at 0 °C (73JOC1949). 

The presence of l-(4-pyridyl)pyridinium ion in the crude reaction mixtures was detected by the observation of this same set of spectra in solutions prepared by hydrolysis of the reaction mixtures in acid solution, followed by treatment with strong base and heating (Figure 2). 

Further evidence for the presence of l-(4-pyridyl) pyridinium ion in the reaction mixtures was provided by a separation technique based on the precipitation of the l-(4-pyridyl)pyridinium ion in the presence of pyridine with Ph4B in a solution with a pH of 8. This procedure was necessary, since the ultraviolet spectrum of l-(4-pyridyl)pyridinium ion in a solution prepared by acid hydrolysis of the crude reaction products was completely obscured by the intense spectrum of pyridinium ion, which also was formed by the hydrolysis of the reaction products. However, as shown in Figure 3, the l-(4-pyridyl) pyridinium ion was easily identified after its separation as the tetraphenylborate(III) salt... 

The l-alkyI-2-, 3- and 4-carbamidopyridinyl radicals may be generated in water from the corresponding pyridinium ions by pulse radiolysis or radiolysis. The rate constant for the disappearance of 3 are pH-independent and close to diffusion controlled (Eq. 27). The rate constants for 4 are pH-dependent. Completely protonat 4 reacts with itself at rates somewhat less than diffusion controlled the rate constants decrease linearly with increasing pH (slope ca. 1) (Eq. 28). Rates in the range pH 8-11 could be followed using a pyridinyl ester. A minimum rate was found near pH 9.2 at higher pH values, the hydrolysis of the pyridinyl ester to the carboxylate controlled the rate (Eq. 29), since the pyridinyl carboxylate would react with itself in a diffusion controlled process (Eq. 30). 

The large negative entropies of activation and the large solvent isotope effects are no doubt intimately related. It is quite conceivable that these effects arise from a general catalysis by water of the water reaction. General base catalysis is known to occur in the hydrolysis of acetic anhydride by acetate, acetylp3rridinium ion by acetate (Bunton et al., 1961), acetylimidazole by imidazole, N-methyl,N -acetylimidazolium ion by N-methylimidazole, l-(N,N-dimethylcarbamoyl)pyridinium ion by pyridine (Johnson and Rumon, 1965), and ethyl haloacetates by weak bases (Jencks and Carriuolo, 1961). It is most reasonable then that the water reaction be similarily a base-catalyzed process. The isotope effects... 

Use the values found in Table 18-6 and Appendix G to calculate hydrolysis constants for the following cations of weak bases (a) (CH3)2NH2+, dimethylammonium ion (b) C5H5NH+, pyridinium ion (c) (CH3)3NH+, tri-methylammonium ion. 

Figure 3.9 Relative rates of hydrolysis of glycosyl pyridinium ions.

Taking into account that formation of the propiophenone 2 is the most important side reaction, we were interested in determining the exact nature of the active sites responsible for the hydrolysis of the acetal moiety. Aimed at this purpose, a characterization of the Bronsted and Lewis acidity of the centers was accomplished for the HY, ZnHY and ZnNaY samples by means of the pyridine adsorption method. Pyridine when adsorbed on solid acids, shows in the IR spectra specific bands assignable to pyridinium Ion (1540 cm ) and Lewis adducts (1450 cm ), which intensities are directly related to the population of both types of centers... 

Pyrylium perchlorate 38 forms colourless crystals which are prone to hydrolysis and decompose explosively at 275°C. It can be obtained from A -acceptor substituted pyridinium ions (see p 272), e.g. by the SO3 complex 35, on treatment with NaOH followed by addition of HCIO4. Initially, the orange-coloured sodium salt of 5-hydroxypenta-2,4-dienal (glutaconic dialdehyde in the enol form) 36 is formed which subsequently cyclizes via the red protonated species 37 [7] ... 

These conditions were met using as a reaction probe of the inner aqueous compartment of (DODA)X vesicles the alkaline hydrolysis of N-methyl-4-cyanopyridinium ion (MCP). The alkaline hydrolysis of N-alkyl-4-cyano-pyridinium ions (RCP) produces two products N-alkyl-4-pyridone (P) and N-alkyl-4-carboxamido pyridinium ion (A) (Scheme 3) [43]. The limiting P/A ratio in water is... 

Recall from Section 20.2 that pyridoxal 5 -phosphate reacts with the a amino group of an a-amino acid to form an imine, or Schiff base. When L-dopa reacts with PLP, the resultant imine undergoes decarboxylation, with the pyridinium ion of PLP acting as the electron acceptor. Hydrolysis then gives dopamine and regenerated PLP. The mechanism is shown in Figure 25.7. 

Pyridine, C5H5N (pKb = 8.82), forms a salt, pyri-dinium chloride, as a result of a reaction with HCl. Write an ionic equation to represent the hydrolysis of the pyridinium ion, and calculate the pH of 0.0482 M C5H5NH Cr(aq). 

Kinetics. The reaction of N-dodecyl 3-carbamoyl pyridinium bromide (I) with cyanide ion in the microemulsions was observed by following the 340 nm absorption maximum of the 4-cyano adduct (II). See equation (1). Following the work of Bunton, Romsted and Thamavit in micelles ( ), a 5/1 mole ratio of KCN to NaOH was employed to prevent cyanide hydrolysis. The pH of each reaction mixture was measured on a Coleman 38A Extended Range pH meter to insure that the system was sufficiently basic to allow essentially complete ionization of the cyanide. The appropriate amounts of cyanide and hydroxide were added to the mlcroemulslon sample within 10 minutes of running a reaction. Cyanide concentration varied between 0.02 and 0.08 M with respect to the water content. Substrate was Injected via a Unimetrics model 1050 syringe directly into a known volume of the yE-nucleophlle mixture in a 1.0 cm UV quartz cell. Absorbance at 340 nm was followed as a function of time on a Perkln-Elmer model 320 spectrophotometer at 25.0 + 0.3 C. Since the Initial bulk concentration of substrate was 10 M, cvanide was always present in considerable excess. 

A hydrogen attached to a pyridine or pyridine 1-oxide nucleus cannot be replaced directly by cyanide however, addition of cyanide to various quaternary salts constitutes an important class of reactions of synthetic importance. Before surveying these reactions in detail, the four main classes are outlined. In 1905, Reissert reported the first example, the reaction of quinoline with benzoyl chloride in aqueous potassium cyanide (Scheme 111) (05CB1603). This yielded a crystalline product, C17H12N2O, a Reissert compound (176) which afforded benzaldehyde and quinaldinic acid on acid hydrolysis (Scheme 111). Kaufmann (09CB3776) treated a 1 -methylquinolinium salt with aqueous potassium cyanide and observed 1,4-rather than 1,2-addition (Scheme 112), the Reissert-Kaufmann reaction. Reissert compounds are well known in the quinoline and isoquinoline series, but only rarely have even small yields been found in the pyridine series. On the other hand, cyanide ions add 1,4 with ease to pyridinium salts that have an electron withdrawing substituent at C-3. 

MS delivers both information about the mass and the isotope pattern of a compound and can be used for the structural analysis upon performance of MS/MS experiments. Therefore, it is a valuable tool for the identification and characterization of an analyte as well as for the identification of impurities. Potential applications are the identification of IL in fhe quality control or in environmental studies. Unwanted by-products formed during the s)mthe-sis or by the hydrolysis of components of the ILs can be identified by this method. The analysis of fhe IL itself is also a prerequisite for the analysis of compounds dissolved in fhese media, as will be ouflined in the section 14.4. Beside the identification of fhe ILs, a characterization of different properties like water miscibility and the formation of ion clusfers, providing valuable information abouf fhe molecular structure of the IL, can be performed by means of MS techniques. The majority of studies reported up to now have dealt with ILs encompassing substituted imidazolium or pyridinium cations, therefore fhe following discussion concentrates on these compounds unless otherwise stated. 

Halogenation. 3-Chloroindole can be obtained by chlorination with either hypochlorite ion or with sulfuryl chloride. In the former case the reaction proceeds through a 1-chloroindole intermediate (13). 3-Chloroindole [16863-96-0] is quite unstable to acidic aqueous solution, in which it is hydrolyzed to oxindole. 3-Bromoindole [1484-27-1] has been obtained from indole using pyridinium tribromide as the source of electrophilic bromine. Indole reacts with iodine to give 3-iodoindole [26340-47-6]. Both the 3-bromo and 3-iodo compounds are susceptible to hydrolysis in acid but are relatively stable in base. 

Acid-catalysed hydrolysis is the most common method for deprotecting isopro-pylidene derivatives and the add strength and reaction time can vary widely. The mildest conditions involve gently heating the substrate with pyridinium p-toluenesulfonate in aqueous media or methanol [Scheme 3.2. 3 In most cases aqueous trifluoroacetic acid [Scheme 3.3]4 dilute HC1 in THF, or an ion exchange resin such as Dowex SOW [Scheme 3,4]5 will remove them rapidly. In the latter case, note that the less hindered dioxolane hydrolysed preferentially. 

Before all these acetal-based protecting groups were introduced, the tetrahydropyranyl (THP) ether had found extensive use in organic synthesis. It can easily be synthesized from a variety of hydroxy-containing compounds like carbohydrates, amino acids, steroids and nucleotides by the acid-catalyzed reaction with dihydropyran. It is stable to bases, but the protection is removed through acidic hydrolysis with hydrochloric acid, toluenesulfonic acid or acidic ion-exchange resin (Scheme 27). In the case of acid sensitive substrates, e.g. containing an epoxide or a further acetal, pyridinium p-toluenesulfonate should be applied for particularly mild deprotection conditions. ... 

Using pyridinium triflate, the deprotonated amide intermediate was estimated to have pKa = 4.20. This low value is consistent with Zn(II) ion stabilization of the deprotonated amide. The formation of this deprotonated amide intermediate occurs in wet DMSO with a microscopic rate constant that corresponds to the intramolecular hydrolysis of coordinated nitrocefin catalyzed by [Zn2L3( i-0H)(N03)2]. Activation parameters for this reaction (A= 77.6 + 8.0kJM-1 and AS = —62.7 + 20.0 JM"1 K-1) are within error of those found for the same reaction in acetone DMSO... 

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