Pyridines acidity function

With the oxides which are nitrated as the cations the difficulties are much less serious for the use of an acidity function is not involved. Comparison of 2,6-dimethoxy- and 3,5-dimethoxy-pyridine i-oxide with wt-dimethoxybenzene (which is nitrated at the encounter rate)... 

Sulfonic esters are most frequently prepared by treatment of the corresponding halides with alcohols in the presence of a base. The method is much used for the conversion of alcohols to tosylates, brosylates, and similar sulfonic esters. Both R and R may be alkyl or aryl. The base is often pyridine, which functions as a nucleophilic catalyst, as in the similar alcoholysis of carboxylic acyl halides (10-21). Primary alcohols react the most rapidly, and it is often possible to sulfonate selectively a primary OH group in a molecule that also contains secondary or tertiary OH groups. The reaction with sulfonamides has been much less frequently used and is limited to N,N-disubstituted sulfonamides that is, R" may not be hydrogen. However, within these limits it is a useful reaction. The nucleophile in this case is actually R 0 . However, R" may be hydrogen (as well as alkyl) if the nucleophile is a phenol, so that the product is RS020Ar. Acidic catalysts are used in this case. Sulfonic acids have been converted directly to sulfonates by treatment with triethyl or trimethyl orthoformate HC(OR)3, without catalyst or solvent and with a trialkyl phosphite P(OR)3. ... 

The high selectivity that the system shows to pyrazine 20 compared to the stronger base pyridine, indicates that the diamine is chelated between the carboxylic acid functions as in 21. Spectroscopic evidence in the form of upfield shifts in the NMR spectra of the complexes supports such structures. Not only aromatic diamines are accommodated but also aliphatics such as l,4-diazabicyclo[2.2.2]octane (DABCO) in complex 22. Typically, exchange rates into and out of these complexes are such that they appear fast on the NMR time scale at ambient temperature, but exchange can be frozen out at low temperatures20. For DABCO, an activation barrier of 10.5 kcal M 1 was observed at Tc = 208 °K. 

A good correlation has been found between pKa values for a series of pyridine 1-oxides and the corresponding pyridines. Weakly basic pyridine 1-oxides that protonate in the H0 region have been shown to follow the HA acidity function. Some of these have been used to extend the HA scale and to determine its variation with temperature. 

Several derivatives have been prepared by acid-catalyzed cyclization of 4-hydrazinopyridines containing a carboxylic acid function. For example, cyclization in dilute acids of 130125 or 131a (R = H, Ar)19,126-128 gave the 3-hydroxypyrazolo[4,3-c]pyridines 132 (R1 = Ar, R2 = H) or N-oxides 133... 

In some processes, there is a need to immobilize dyes, either temporarily as in the case of filter and antihalation devices or permanently as for the images in colour image transfer processes. Because the dyes normally used in these applications contain acid functional groups, the corresponding mordants are basic in nature and are usually polymeric. Polyvinyl-pyridines have utility as mordants. Other useful compounds are illustrated by the pyridinium salt (95) (49USP2484430), the morpholinium salt (96) (72BRP1245952) and the imidazole (97) (72GEP2150136). 

Section V contains specific examples of procedures for Inflate synthesis some general comment about these procedures is worthwhile. Triflic anhydride is the reagent of choice for preparing triflates derived from sugars, whereas triflyl chloride is used more often when uucleosides are involved. Triflate formation with triflic anhydride requires addition of a base (usually pyridine) to the reaction mixture to neutralize the triflic acid produced [Eq. (4) 7]. Some triflates are reactive enough that pyridine can function as a nucleophile in the substitution process. In these instances, replacement of pyridine with a non-nucleophilic base, such as 2,6-di-r-butyl-4-methylpyridine, avoids this undesired reaction (Scheme 1) [7]. 

The variations of acidic properties in the surface layers and in the bulk solid catalysts after calcination, reduction, or coking were examined by pyridine Nls XPS [4,7] and by the pyridine infrared adsorption techniques, respectively. This provides a means to compare the changes in the characteristic BrBnsted and Lewis acidity functions after those treatment conditions. First of all, TPD of ammonia revealed that both coked and regenerated samples exhibited much decreased acidity as compared with either calcined or reduced samples before the reaction of n-heptane conversion in either N2 or H2 stream [7]. The adsorption of pyridine may cause further perturbation to the Pt4+ or Pt 2+ species in the zeolite as indicated by the increase in binding energies of Pt3d5/2 electrons, as shown in Table 3 and Figure 4,... 

It was found by Nis XPS studies of pyridine-adsorbed samples that after deactivation the surface acidic function changes in a different manner with the bulk acidity measured by infrared characteristic absorption bands of pyridine adsorbed samples [7], which would suggest different distributions of the acidic properties in the sample catalysts. The effects of additive elements on the overall acidic features of modified zeolite catalysts are dependent on sample pretreatment and/or reaction condition, which will contribute differently to the induced acidity on the surface and in bulk bifunctional properties, as examined by the reaction of n-heptane shown in Figure 1. 

Pyridine could function as a nucleophile in ozonization. It is soluble in hydrocarbons and is rather stable towards ozone (8, 11). Pyridine does have a dramatic effect on the course of ozonization. Slomp and Johnson (13) in their work on the ozonolysis of 4,22-stigmastadien-3-one propose that two moles of aldehyde are formed for each mole of double bond oxidized and that pyridine is oxidized to pyridine oxide. They also propose that pyridine oxide oxidizes aldehyde to acid with regeneration of pyridine. 

As it is generally the case with bifunctional catalysis processes, the balance between hydrogenating and acid functions determines for a large part the catalyst activity. This was quantitatively shown for series of bifunctional catalysts constituted by mechanical mixtures of a well dispersed Pt/Alumina catalyst and of mordenite samples differing by their acidity and their porosity (25). The balance between hydrogenating and acid functions was taken as nPt/nH+ the ratio between the number of accessible platinum atoms and the number of protonic sites determined by pyridine adsorption. 

The synthesis of the oxazole compound 45 starts with the coupling of the N-protected (/ )-methylcysteine compound 18 with threonine terf-butyl ester using bis(2-oxo-3-oxazolidi-nyl)phosphinyl chloride (BOP-Cl) [15] as a coupling reagent. Jones oxidation of the threonine hydroxy group leads to the ketoamide 44. The desired oxazole ring is closed by treatment with thionylchloride/pyridine. After deprotection, the oxazole, compound 45 is obtained. In the next step the oxazole compound 45 is coupled with the tris(thiazoline) compound 43 to yield the thioester 46. Now Fukuyama closes the fourth and last thiazoline ring (46 47). After conversion of the carboxylic acid function into a methyl-... 

More hydrophobic templates, such as 2,4-dichlorophenoxy acetic acid (Fig. 5.19), are best imprinted in combination with vinyl pyridine as functional monomer in... 

A conceptual alternative way to the activation of the carboxylic acid function is the reaction of carboxylic acids with amino groups activated as isocyanates - and isothiocyanates (equation 16). Preparation of these derivatives is racemization free. The reaction proceeds via mixed acid anhydrides in aromatic hydrocarbon solvents at elevated temperatures, and decarboxylation leads to the V-substituted amide. Pyridine as solvent enhances the conversion rate but increases also the amount of the urea side product via disproportionation. Application to peptide chemistry is limited, because peptide ester fragments tend to form hydantoins. ... 

The mycotoxin patulin was synthesized via the oxidation of a disubstituted furan in the laboratory of M. Tada. The required 2,3-disubstituted furan was conveniently prepared via the Feist-Benary reaction of acetonedicarboxylic acid dimethyl ester and chloroacetaldehyde in the presence of pyridine. Subsequent functional group modification and oxidation of this furan finally gave the natural product. 

The first examples of supercharged nucleotide analogues (85-87) have been described, in which methylenebisphosphonic acid containing an additional ioni-sable acidic function has been incorporated into p,y-bridged derivatives of adenosine triphosphate. The compounds and their protected precursors were obtained following acid-catalysed reaction of the respective precursors (88-90) with adenosine 5 -phosphoromorpholidate in pyridine in yields of 80,75 and 25%. 

Nonaqueous titrations have been used to estimate total acidity (e.g., Wright and Schnitzer, 1959). This approach utilizes an aprotic solvent such as pyridine or dimethylformamide, in which a very strong base such as ethoxide ion can be used to react with the acidic functional groups of the humic substance sample. The comparison of nonaqueous total acidity and aqueous total acidity should be tempered with caution and any efforts to subdivide nonaqueous total acidity into functional group classes by analogy with pKa values of acidic groups in aqueous solution are theoretically unfounded. Some reasons for concern are presented in the next paragraph. 

Several problems are inherent in this method, so the results should be considered as quite operational (Dubach et al., 1964 van Dijk, 1966 Stevenson and Goh, 1972 Holtzclaw and Sposito, 1979 Perdue, 1979 Perdue et al., 1980). First, unlike the barium hydroxide reagent used for total acidity, 0. IM calcium acetate is poorly buffered. The equilibrium pH, which determines the extent to which the acidic functional groups of humic substances will react, is dependent on the amount of humic substances added to the 50 mL of calcium acetate. Perdue et al. (1980) demonstrate that the binding of Ca to the humic substance sample displaces additional protons that do not react if sodium acetate or pyridine is used as the exchange base. It cannot be assumed that those excess protons are derived exclusively from carbbXyr groups. 

Acid-base properties of oxide surfaces are employed in many fields and their relationship with PZC has been often invoked. Adsorption and displacement of different organic molecules from gas phase was proposed as a tool to characterize acid-base properties of dry ZnO and MgO [341]. Hammet acidity functions were used as a measure of acid-base strength of oxides and some salts [342]. Acidity and basicity were determined by titration with 1-butylamine and trichloroacetic acid in benzene using indicators of different pAg. There is no simple correlation between these results and the PZC. Acid-base properties of surfaces have been derived from IR spectra of vapors of probe acids or bases, e.g. pyridine [343] adsorbed on these surfaces. The correlation between Gibbs energy of adsorption of organic solvents on oxides calculated from results obtained by means of inverse gas chromatography and the acceptor and donor ability of these solvents was too poor to use this method to characterize the donor-acceptor properties of the solids [344],... 

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