Power and structure-variable fluorinating agents. The N-fluoropyridinium salt system

Article abstract of DOI:10.1021/ja00179a047

The usefulness of the N-fluoropyridinium salt system as a source of fluorinating agents was examined by using substituted or unsubstituted N-fluoropyridinium triflates 1-11, N-fluoropyridinium salts possessing other counteranions 1a-d and 3a, and the counteranion-bound salts, N-fluoropyridinium-2-sulfonates 12 and 13. Electrophilic fluorinating power was found to vary remarkably according to the electronic nature of the ring substituents. This power increased as the electron density of positive nitrogen sites decreased, and this was correlated to the pK_a values of the corresponding pyridines. By virtue of this variation, it was possible to fluorinate a wide range of nucleophilic substrates differing in reactivity. It is thus possible to fluorinate aromatics, carbanions, active methylene compounds, enol alkyl or silyl ethers, vinyl acetates, ketene silyl acetals, and olefins through the proper use of salts pentachloro 6 through 2,4,6-trimethyl 2, their power decreasing in this order. All the reactions could be explained on the basis of a one-electron-transfer mechanism. N-Fluoropyridinium salts showed high chemoselectivity in fluorination, the extent depending on the reactive moiety. In consideration of these Findings, selective 9α-fluorination of steroids was carried out by reacting 1 with tris(trimethylsilyl ether) 73 of a triketo steroid. Regio- or stereoselectivity in fluorination was determined by a N-fluoropyridinium salt structure. Steric bulkiness of the N-F surroundings hindered the ortho fluorination of phenols and aniline derivatives, while the capacity for hydrogen bonding on the part of the counteranions prompted this process, and the counteranion-bound salts 12 and 13 underwent this fluorination exclusively or almost so. Both bulky N-fluoropyridinium triflates 2 and 7 preferentially attacked the 6-position of the conjugated vinyl ester of a steroid from the unhindered β-direction to give a thermally unstable 6β-fluoro isomer. On the basis of these results, N-fluoropyridinium salts may be concluded to constitute a system that can serve as a source of the most ideal fluorinating agents for conducting desired selective fluorination through fluorinating capacity or structural alteration.