Pyridinium Compounds, Ylides, Pyridine N-Oxides

Pyridinium cations, role in mechanisms of nucleophilic aliphatic substitution 90CSR83. 

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One Nitrogen Atom (it is self-subdivided into Pyridines, Pyridinium Compounds, Ylides, Pyridine N-Oxides, Applications of Pyridines, Bipyridines and Related Systems, Hydropyridines, Biologically Active Pyridines and Hydropyridines, Pyridines Annulated with Carbocycles, Pyridines Annulated with Heterocycles). 

One of the oldest and most popular methods for oxidizing activated halides is the three-step Krbhnke oxidation. The individual steps are (i) quatemization of the halide with pyridine (ii) deprotonation in base and reaction of the resulting pyridinium ylide with N.N-dimethyl-4-nitrosoaniline and (iii) acid hydrolysis of the nitrone to the carbonyl compound (Scheme 5). 

The oxidation of alkyl halides to carbonyl compounds with pyridine or 2-picoline N-oxide is a popular and general method, applicable even to unactivated substrates. The reaction may be performed in two ways. In the first, the halide is heated with the N-oxide in the presence of a base such as sodium hydrogen carbonate. In the second, the intermediate N-alkoxypyridinium salt is isolated before base treatment. The reaction has been shown by labeling to proceed via the pyridinium ylide, or, in the case of picoline N-oxide, via the anhydrobase (Scheme 8). Some typicd examples are shown in equations (23)-(25). ... 

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