172467-78-6Relevant academic research and scientific papers
Highly Selective Fluorinating Agents: A Counteranion-Bound N-Fluoropyridinium Salt System
Umemoto, Teruo,Tomizawa, Ginjiro
, p. 6563 - 6570 (1995)
A series of alkyl- or (trifluoromethyl)-substituted N-fluoropyridinium-2-sulfonates 2a-h, differing in fluotinating power, were synthesized, and assessment was made of the effectiveness of each selective fluorinating agent.N-Fluoropyridinium-3- and -4-sulfonates 3 and 4 were also synthesized.Power-variables 2a-h were found to be highly selective fluorinating agents for a wide range of nucleophilic substrates such as activated aromatics, enol trialkylsilyl and alkyl ethers, active methylene compounds, activated olefins, and sulfides.Thus, phenol, naphthol, phenylurethane, and the trimethylsilyl ether of phenol were exclusively or highly selectively fluorinated at the o-position with 2f-h.Conjugated enol trialkylsilyl ethers of a steroid were regioselectively fluorinated at the 6-position with moderately powerful 2b-e.This regioselectivity increased with the bulkiness of the silyl part, and with the most bulky triisopropylsilyl group exclusive 6-fluorination was achieved.Preferential β-stereoselective fluorination at the 6-position was observed.N-Fluoropyridinium-2-sulfonates were activated with an acid.This acid-catalyzed fluorination led to the preferential p-fluorination of anisole.The present results can be explained based on the capacity of the 2-sulfonate anion to interact with the hydroxy group of phenol or naphthol, NH group of phenylurethane, silicon atoms of silyl ethers, or protons of acids.
Synthesis, Properties, and Reactivity of N,N'-Difluorobipyridinium and Related Salts and Their Applications as Reactive and Easy-To-Handle Electrophilic Fluorinating Agents with High Effective Fluorine Content
Umemoto, Teruo,Nagayoshi, Masayuki,Adachi, Kenji,Tomizawa, Ginjiro
, p. 3379 - 3385 (2007/10/03)
N,N′-Difluoro-2,2′-, -2,4′-, -3,3′-, -4,4′-bipyridinium and substituted N,N′-difluoro-2,2′-bipyridinmm bis(triflates), bis(tetrafluoroborates), bis(hexafluorophosphates), and bis(hexafluoroantimonates) 1-9 were synthesized in high yields by the direct fluorination of a mixture of a bipyridyl and a Lewis acid, a Br?nsted acid, or the alkali metal salt of an acid. The higher homologues, trimer 10 and polymer 11, were also synthesized. Unsubstituted or electron-donating group-substituted N,N′- difluorobipyridinium salts are stable nonhygroscopic crystals, while the electron-withdrawing group- substituted N,N′-diflurobipyridinium salts 3, 5, and 6 are moisture-sensitive crystals. Hydrolysis of 1b in boiling water gave 3,3′-dihydroxy-2,2′-bipyridyl. The reactivity determination indicated that the fluorinating capability decreased in the order 2,2′- ? 2,4' > 3,3′- ≈ 4,4′-isomer ? N-fluoropyridinium salt and that the two N-F moieties in a molecule were effective for fluorination. This fluorination occurred in a step-by-step manner, and the reactivity difference between the first and second fluorinations was very small. N,N′-Difluoro-2,2′-bipyridinium bis(tetrafluoroborate) (1b) is thus shown to be a highly reactive and easy-to-handle electrophilic fluorinating agent with the high effective fluorine content (103.3 g/kg) for preparing many fluoro organic compounds.
Power and structure-variable fluorinating agents. The N-fluoropyridinium salt system
Umemoto, Teruo,Fukami, Shinji,Tomizawa, Ginjiro,Harasawa, Kikuko,Kawada, Kosuke,Tomita, Kyoichi
, p. 8563 - 8575 (2007/10/02)
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 pKa 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.
