N-Fluoropyridinium

A novel route to 2-fluoropyridines involved the base-induced decomposition of substituted N-fluoropyridinium salts. Abstraction of the 2-H produces a singlet carbene (11) that removes F from a counterion. This is in contrast to the reaction with C nucleophiles, which are fluorinated, and is attributed to the high stability of C—F compared to O—F and N—F (89JOC1726). 

A Useful Fluorinating Agent, N-Fluoropyridinium Triflate, and o-Fluorination of a Ketone... 

The difficulty of perfluorinating pyrrole is enhanced in the porphyrins, which have reduced 7r-density on the a-carbons. Although direct fluorination appears promising, it is not yet fully developed [88JCS(P1)1735 89CL263 90TL6847]. The use of N-fluoropyridinium triflate resulted only in meso-fluorination (92TL1069). 

Meinert demonstrated that fluorination of pyridine at low temperatures gives the ionic salt N-fluoropyridinium fluoride, a compound that was reported to be explosive at 0°C (Fig. 59) [154]. However, direct fluorination of variously substituted pyridines is possible and good yields of the corresponding 2-fluoro-pyridines (Figs. 60 and 61) [155] are obtained, offering an attractive alternative to the usual halogen-exchange and Balz-Schiemann routes to these products. These reactions probably proceed via N-fluoropyridinium salts (Fig. 62) which are activated towards nucleophilic attack. 

N-Fluoropyridinium triflate is stable and can be stored indefinitely under a dry atmosphere. It slowly decomposes in water. The submitters report that it has a half-life of 13 days in D2O at room temperature. 

N-Fluoropyridinium triflate Pyridinium, 1-fluora-, salt with trifluoromethanesulfbnio acid (1 1) (12) (107263-95-6)... 

Although a number of electrophilic fluorinating reagents are known, most are toxic, unstable, and difficult to handle. These new, stable N-fluoropyridinium salts are readily obtained in 60-80% yield by reaction of a pyridine with F2/N2(l/9) in CH3CN in the presence of sodium triflate. 

Oxidation of pyridinecarboxylic acids.1 Reaction of a pyridinecarboxylic acid or the ester with 10% F2 in N2 results in oxidation to the corresponding 2-pyridone in 50-75% yield. An N-fluoropyridinium salt is probably an intermediate. 

With the increasing importance of organofluorine compounds in the development of new materials, pharmaceuticals, and agricultural chemicals, N-fluoropyridinium salts should find extensive use as mild and selective fluorinating agents. 

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