Pyridinium polyhydrogen fluoride

We also developed a number of other useful new fluorinating reagents. They ineluded a convenient in situ form of sulfur tetrafluoride in pyridinium polyhydrogen fluoride, selenium tetrafluoride, and ey-anurie fluoride. We introdueed uranium hexafluoride (UFg), depleted from the U-235 isotope, which is an abundant by-product of enrichment plants, as an effective fluorinating agent. 

The preparation of e/n-difluoro compounds by the oxidative fluorodesul-furization ot 1,3-dithiolanes readily proceeds by treatment with a pyridinium polyhydrogen fluoride-Af-halo compound reagent the latter serves as a bromonium ion source [2], l,3-Dibromo-5,5-dimethylhydantoin is the most effective of several At-halo oxidants. It is believed that /V-halo compounds combine with hydrogen fluoride to generate in situ halogen fluorides, the oxidants. Formation of gem-difluorides from dithiolanes derived from ketones is efficient and rapid, even at -78 °C, whereas the reaction of dithiolanes derived from aldehydes requires higher temperature (0 °C) (equation 4). 

Aromatic thio orthoesters are successfully converted into trifluoromethyl arenes by treatment with a pyridinium polyhydrogen fluoride-A -halo imide reagent. The reactions are conducted at -30 to -20 °C, and the nature of A-halo imide is critical both 1,3-dibromo-5,5-dimethylhydantoin and A-bromosuccin-imide give similar yields of trifluoromethyl compounds [5] (equation 7)... 

Dithiocarbamates are almost quantitatively converted into trifluorometh-ylamines by reacting at ambient temperature with either tetrabutylammonium dihydrogen trifluoride, triethylamine trihydrogen trifluoridc, or pyridinium polyhydrogen fluoride and an 7V-halo imide reagent [75] (equation 19)... 

Substituted aryl hydrazones can be converted to geminal difluondes in satis factory yields by molecular fluorine [H4, iodine fluoride [ 5], and N bromo-suc-cinimide-pyridinium polyhydrogen fluoride or A -bromosuccinimide-polyvi-nylpyridinium polyhvdrogen fluoride [5(5] (equation 21) (Table 6)... 

PPHF = pyridinium polyhydrogen fluoride York, C. Prakash, G.K.S. Wang, Q. Olah. G.A. Svnlett. 1994, 425... 

FLUORINATIONS WITH PYRIDINIUM POLYHYDROGEN FLUORIDE REAGENT 1-FLUOROADAMANTANE... 

Pyridinium polyhydrogen fluoride and related polyhydrogen fluorides were developed as a modified liquid complex of anhydrous hydrogen fluoride for fluor-ination reactions.39 These complexes are ionic liquids. It was subsequently found40 that such ionic liquids with higher complexed HF content are advantageous reactions media for alkylation and other reactions. 

With water, the reaction was carried out in aqueous acetonitrile at 0°C, but the products (p-quinols) were formed in moderate yield better results were obtained with silylated phenols, especially with their tripropylsilyl ethers [13]. For fluorina-tion, pyridinium polyhydrogen fluoride was the source of fluoride this system was effective with a variety of substrates, including a phenolic steroid [15]. As with DIB, intramolecular cyclizations were also performed on several occasions. 

Hydroxylation of arenes. This reaction can be carried out conveniently with 30% H2O2 in pyridinium polyhydrogen fluoride at 0° (20-60% yield). In the case of alkylbenzenes predominate ortho-para substitution obtains. 1,2-Methyl shifts are sometimes observed. Loss of an alkyl group can occur to a slight extent by ipso hydroxyl attack. ... 

Treatment with hydrazine to give the hydrazone, and then CuBr2/t-BuOLi, generated the gem-dibromide." Oximes gives gem-dichlorides upon treatment with chlorine and BF3 OEt2, and then HCl." Some dithianes can be converted to gem-difluorides with a mixture of fluorine and iodine in acetonitrile." " Oximes give gem-difluorides with NO BF and pyridinium polyhydrogen fluoride." ... 

The oxidative fluorination of 4-alkylphenols to give the 4-fluoro-4-alkylcyclohexa-2,5-dien-l-ones can be achieved by using hypervalent iodine reagents, such as phenyliodo bis(trifluoroacetate) or phenyliodine diacetate in the presence of pyridinium polyhydrogen fluoride (equation 81) vide infra). ... 

Oxidative fluorination of the p-substituted phenol 371 was effected with PhI(OCOCF3)2 and pyridinium polyhydrogen fluoride (PPHF) to afford directly the hydroindolenone (372) (35%). In the case of 373, the corresponding 4-fluorinated cyclohexa-2,5-dienone (374) was produced in 43% yield. Cyclization of 374 to 375 was readily effected with Na2C03, as shown in Scheme 75. When MeOH was used instead of PPHF the corresponding methoxy compounds were obtained. The yields are higher than that of the fluoro compounds. 

Olah GA, Welch J (1975) Onium ions. XII. Heterolytic dediazoniation of benzenediazonium ions in halide ions in pyridinium polyhydrogen fluoride solution giving isomeric haloben-zenes reflecting ambident reactivity of benzenediazonium ions and intermediate phenyl cation as well as subsequent aryne formation. J Am Chem Soc 97 208—210... 

Hydrogen fluoride-Pyridine (Pyridinium polyhydrogen fluoride) 285... 

Aliphatic Nitrations. Nitronium salt nitrations were also extended to nitration of olefinic systems. Nitronium salts readily add to olefins, giving the corresponding nitrofluoroalkanes. This addition is particularly effective when carried out in solvent systems such as pyridinium polyhydrogen fluoride (lU). 

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