N Fluoropyridinium fluoride

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. 

Several 2-substituted pyridines 73 can be prepared by reactions of N-fluoropyridinium triflate (72) with triethylamine in various solvents. A novel carbene has been proposed as the reactive species (87TL2705). Treatment of N-fluoropyridinium fluorides 74 with trinitromethane gives trinitromethyl-... 

Kiselyov AS, Strekowski L (1993) Synthesis of 2-substituted pyridines by the reaction of N-fluoropyridinium fluoride with trimethylsUyl derivatives. J Oig Chem 58(16) 4476-4478... 

Due to these safety problems, the search for new, easy-to-handle electrophilic fluorinating agents has become an important area of research. Some of these new reagents have been tested in the fluorination of enamines to give a-fluoro ketones. These include l-fluoropyridin-2(l//)-one (9),116 N-fluoroquinuclidinium fluoride (NFQNF, 10),117 N-fluoropyridinium salts 11,118 and N-fluorobis(trifluoromethanesulfonyl)aminc (Tf2NF. 12).42... 

Some nucleophiles add to Af-fluoropyridinium salts to give dihydropyridines in which elimination of fluoride occurs in situ to give the 2-substituted pyridine, thus avoiding the need for a dehydrogenation step. The main disadvantages of this method are that the preparation of the pyridinium salts require the use of elemental fluorine and that some carbanions give only modest yields due to competitive reactions such as C-fluorination. However, silyl enol ethers do react efficiently stabilised heteronucleophiles (phenolate, azide) can also be used. Addition to N-fluoropyridinium salts shows a strong preference for attack at an a-position. ... 

---