Pyridine nucleophilic strength

Lil in boiling pyridine or other weak nucleophilic bases can cleave alkyl esters to alkyl iodides and lithium carboxylates (Scheme 28). The reaction is mainly used for mild, aprotic cleavage of esters to car-boxylates. The high degree of dissociation for Lil and the nucleophilic strength of the iodide ion explain the reaction with esters, which is not useful with the other halides. Trimethylchlorosilane and sodium iodide also give alkyl iodides from esters. ... 

Bifunctional catalysis in nucleophilic aromatic substitution was first observed by Bitter and Zollinger34, who studied the reaction of cyanuric chloride with aniline in benzene. This reaction was not accelerated by phenols or y-pyridone but was catalyzed by triethylamine and pyridine and by bifunctional catalysts such as a-pyridone and carboxylic acids. The carboxylic acids did not function as purely electrophilic reagents, since there was no relationship between catalytic efficiency and acid strength, acetic acid being more effective than chloracetic acid, which in turn was a more efficient catalyst than trichloroacetic acid. For catalysis by the carboxylic acids Bitter and Zollinger proposed the transition state depicted by H. 

Data for the reactions of several cyclic tertiary amines with phenyl, 4-nitro-phenyl and 2,4-dinitrophenyl acetates, at 25°C and ionic strength 1.0, appear in Table 40, and as a Bronsted plot in Fig. 20. The usual irregularities of such plots for nucleophilic attack are evident. Linear relationships between log k and pKa are generally found for groups of compounds of closely similar structure, as for the substituted pyridines in Fig. 20. The data for the two tricyclic amines fall on separate curves, and the points for imidazole clearly fall on neither of the first two sets of lines. The separate lines for the reactions of particular classes of nucleophile are approximately parallel, as is usually found. 

Controlled addition of a suitable proton donor or electrophile (reductions) or nucleophile (oxidations) is often useful in determining a reaction mechanism. The strength of a proton donor may vary from perchloric acid through acetic acid and a phenol to an alcohol C acids, such as malonic ester, or N acids, such as urea, may also be used. Used as bases may be pyridine, carboxylate ions, alkoxides, or salts of malonic ester. Sometimes it is of interest to determine whether it is the basic or the nucleophilic properties of the compound that are important. The use of two bases with approximately the same pK values but widely differing in nucleophilicity, such as pyridine and a 2,6-dialkylpyridine, might answer the question. 

During oxidation in a nonaqueous solvent, the removal of a proton or an attack of a nucleophile on the oxidized substrate may be a chemical step in an ECE reaction. To explore which reaction is operating one can, in parallel voltammetric experiments, use two compounds that are equal in base strength but differ widely in nucleophilicity such a pair is pyridine and 2,6-dimethylpyridine. 

Convincing evidence has been put forward that the reaction pathway in the oxidative addition of two molecules of a pyridine derivative to 9,10-DPA [Eq. (31)] [53-58] includes Eqs. (1), (2), (6), and (9). By studying the number of electrons transferred ( obs) the oxidation of 9,10-DPA in the presence of different nucleophiles at the rotating disk electrode as a function of the rotation rate a>, it has been possible to show [53] that the strength of the nucleophile strongly affects the dependence of Wobs 011 Approximate pseudo-first-... 

The dependence of the observed rate constant on the identity of the departing ligand tends to be less than the dependence on the property of the incoming nucleophile, but is large enough to be easily detected. This dependence correlates with the strength of the metal-ligand bond. For the reaction of Pt(dien)X with pyridine in Table 5.4, this dependence follows the trend X = NOj > H,0 > CP > Br > P > Nj > SCN > NO/ > CN. " Since these substitutions are associative, this trend reflects the extent to which the M-X... 

Similarly the formula of pyridine reveals that it is a weak base, with a pK about 5, and the basic strength of substituted pyridines can quickly be calculated by the procedure of Clark and Perrin (1964). Further, the formula shows that there is a large deficiency of electrons in the 2, 4, and 6-positions, which are therefore liable to nucleophilic attack only the nitrogen atom is open to addition reactions electrophilic reactions are... 

---