Pyridinium cation, formation

Methyl propiolate and pyridine give a rather unstable 2 1 molar adduct which is the 1,2-dihydropyridine (112). The reaction sequence proposed to account for its formation is identical in principle to a similar scheme proposed earlier in the acridine series (Section II,A,2) and is also supported by the observation that the 1-benzoyl-pyridinium cation with the phenylacetylide anion yields (113). ... 

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. 

The rates of H-D exchange at the a-positions for a series of A-substituted pyridinium cations and pyridine 1-oxide derivatives in D20 at 75°C (Scheme 36) (70JA7547) correlate well with the Taft inductive parameter a, (p, = 15). A positively charged nitrogen in a ring is estimated to activate the a-position toward deprotonation and ylide formation by a factor of 1015. 

Other reports include the formation of annulated pyridinium salts <02RJOC424, 02 /O375>, activation of carbocations by pyridinium salts <02TL6841>, and Stille reactions on pyridinium cations <02SL1904>. Finally two reports detail pyridine-based brominating reagents. Muathen... 

There is no spectral evidence for pseudobase formation by the N-methyl-pyridinium cation in even the most basic aqueous solutions that are attainable. An oil which separates from solutions of this cation in concentrated aqueous base has been identified by PMR and IR spectroscopies as predominantly ionic N-methylpyridinium hydroxide.70 The UV spectra of the IV-methylquinolinium and N-methylisoquinolinium cations are pH-independent below pH 14, but both these cations undergo irreversible reactions in more basic aqueous solutions (Section V,D) so that pK + values are not directly measurable. Based on substituent effects in more highly substituted quinolinium and isoquinolinium cations, pKR+ values of 16.5 and 15.3 have been estimated26 for the N-methylquinolinium and Af-methyl-isoquinolinium cations respectively. The estimate for the latter cation is based on somewhat limited data and should be compared with pKR+ = 16.29, which has been measured in aqueous dimethyl sulfoxide solutions.90,91... 

It is unfortunate that there has been so little work devoted to quantitative measurements of cation-pseudobase equilibria in methanol and ethanol since these media have several advantages over water for the determination of the relative susceptibilities of heterocyclic cations to pseudobase formation. The enhanced stability of the pseudobase relative to the cation in alcohols compared to water is discussed earlier this phenomenon will permit the quantitative measurement of pseudobase formation in methanol (and especially ethanol) for many heterocyclic cations for which the equilibrium lies too far in favor of the cation in aqueous solution to allow a direct measurement of the equilibrium constant. Furthermore, the deprotonation of hydroxide pseudobases (Section V,B) and the occurrence of subsequent irreversible reactions (Sections V,C and D), which complicate measurements for pKR+ > 14 in aqueous solutions, are not problems in alcohol solutions. Data are now available for the preparation of buffer solutions in methanol over a wide range of acidities.309-312 An appropriate basicity function scale will be required for more basic solutions. The series of -(substituted phenyl)pyridinium cations (163) studied by Kavalek et al.i2 should be suitable for use as indicators in at least some of the basic region. The Hm and Jm basicity functions313 should not be assumed90 to apply to methoxide ion addition to heterocyclic cations because of the differently charged species involved in the indicators used to construct these scales. 

Beranek et ai.314 report a 600-fold enhancement in equilibrium constant for methoxide addition to pyridinium cations in 1 1 dimethyl sulfoxide-methanol relative to methanol. These workers have also measured the rates of dissociation of the methoxide adducts of N-phenylpyridinium cations (163) in dimethyl sulfoxide-methanol mixtures and have used the observation that the relative rates of dissociation appear to be independent of dimethyl sulfoxide content of the solvent to extrapolate these data to pure methanol. The rates of dissociation for all substituents other than X = 3-N02 or 4-NOz are too rapid in 100% methanol to allow direct determination by stopped-flow spectrophotometry. The presence of dimethyl sulfoxide shifts the equilibrium toward the adduct both by enhancing the rate of methoxide addition and by decreasing the rate of adduct dissociation. These solvent effects on pseudobase formation are similar to those observed... 

Methylbenzoate was obtained from iodobenzene in moderate to good yield with polymer-stabilised palladium nanoparticles as the catalyst.1391 Of the ionic liquids screened in the reaction, those based on the pyridinium cation generally gave better results than imidazolium-type liquids. The latter probably deactivate the catalyst by formation of stable carbene complexes. 

In reactions that involve bond formation using the lone pair of electrons on the ring nitrogen, such as protonation and quatemisation, pyridines behave just like tertiary aliphatic or aromatic amines. When a pyridine reacts as a base or a nucleophile it forms a pyridinium , cation in which the aromatic sextet is retained and the nitrogen acquires a formal positive charge. 

When mixtures of Mn(N03)2 and H-ZSM-5 were thermally treated for 2 h at 870 K, XPS revealed a significant decrease in the surface concentration of manganese [35]. Further evidence for introduction of Mn into H-ZSM-5 was provided by the ESR spectra of MnO/H-ZSM-5 and MnCl2/H-ZSM-5 mixtures after heat-treatment at 870 K and 770 K followed by rehydration at ambient temperature [14,35]. The spectra showed a signal with six hyperfine lines typical of Mn + in cation sites with Oj, symmetry. Also, TPDA, TPE and IR (consumption of acidic OH groups, decreased pyridinium ion formation upon pyridine admission to the heat-treated mixture, increased density of... 

Ritschel Bruice TC and York JL, JACS, 83, 1382-1387 (1961) cited in Brooke D, Guttman DE, Complex formation influence on reaction rate. IV. Studies on die kinetic behaviour of 3-carbedioxy-l-pyridinium cation, /. Pharm. Sci, 57, 1677-1684 (1968). 

The role of the IL is crucial in both the PdNP formation and stereospecifity of C—C coupling that could not be obtained in previous studies of PdNP-catalyzed Heck reactions [33e-g]. Salts of N-butyronitrile pyridinium cation react with PdCl2 to give dinitrile complexes that turn black upon addition of phenyltributyktan-nane, and the PdNPs formed catalyze Stille and Suzuki C—C coupling reactions. 

Neutral N-H or N-R dihydropyridines are employed both by nature (the NADH and NADPH dinucleotide coenzymes) and by synthetic chemists as selective reducing agents in transformations driven in large part by the tendency of dihydropyridines to achieve rearomatization. In the field of transition metal organometallic chemistry, dihydropyridines have been either formed on the coordination sphere of a metal to which they are bound through the C=C double bonds, or used as substrates. Ishitani studied the formation of intermediate complexes containing 1,4-dihydropyridines r coordinated to ruthenium fragments in the reaction of pyridinium cations... 

---