2.4.6- Trimethylpyridinium salt

Table 6 Chemical shifts for l-R-2,4,6-trimethylpyridinium salts in trifluoroacetic acid and the difference (D) between shifts for the a- and y-methyl groups [144, 145]...

The use of lower temperatures in the bromate-induced monobromopenta-hydroxylation of benzene by catalytic photoinduced charge transfer osmylation favours formation of the neo diastereoisomer of the deoxybromoinositol. Photolysis of N-(diphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate and N-[bis(4-methylphenyl)amino]-2,4,6-trimethylpyridinium salts induces nucleophilic addition of various 7i-nucleophiles such as electron rich alkenes to the o- and p-positions of one of the phenyl rings.These observations are thought to imply the presence of the diarylnitrenium ion (Ar2N ) as intermediate, but evidence is also presented for the involvement of radical species, as well as for the formation of indoles and indolinones. Some MO calculations are also reported. 

In recent years, several heterocyclic N-F compounds have been advertised as high-performance fluorination reagents. In particular, A -fluoro-2,4,6-trimethylpyridinium salts (103), iV-fluoroquinuclidine fluoride (104), and l-chloromethyl-4-fluoro-l,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoborate) ("Select-fluor", 105) have received wide attention (Scheme 1-72). Their drawback is the same as that of other dedicated reagents. Either adequate equipment to handle elemental fluorine is required to make them oneself or, if they are commercial, their price (in the 1000-5000 /mol range) is prohibitive for preparative scale applications. Moreover, the N-F reagent do not match the reactivity of elemental fluorine or perchloryl fluoride. 

A similar C-N interchange has also been reported with 3,5-dicyano-I,2,6-trimethylpyridinium salts (121), leading to the formation of 3-acetyl-2-(methylamino)pyridine derivative 122 (Scheme IV.48) (95T8599). 

Methyl-l,2,3-triazinium salts 8 reacted with 2 moles of 7V,./V-diethylprop-l-ynamine to give the 2,6-bis(diethylamino)-l,3,5-trimethylpyridinium salt 9.284... 

At rt l-fluoro-2,4,6-trimethylpyridinium triflate (Id 0.14g, 1 mmol) was added in several portions to a THF solution of the sodium salt of diethyl phenylmalonate [prepared in situ by treating diethyl phenyl-malonate (0.236 g, 1 mmol) with 60% NaH in oil (0.024 g, 1 mmol) in THF (2 mL) at 0°C]. After 30 min the reaction mixture was poured into dil HC1 and extracted with Et20. The extract was washed with aq NaHCO, and then with H20, dried (anhyd MgS04), filtered, and evaporated. The resulting residue was column chromatographed (silica gel, CH2Cl2/hexane 1 1) to give an oil yield 83%. 

Treatment of the sodium salts of 5-methyl(or-ethyl,-benzyl)-l,3-oxazine-4,6-dione 19 with l-fluoro-2,4,6-trimethylpyridinium triflate (Id) in tetrahydrofuran/hexamethylphosphoric triamide at — 78 °C gives the 5-fluoro derivatives 20 in high yield.58... 

Various steroid-derived enamines (e.g., 14) have been fluorinated with either (V-fluoropyridini-um trifluoromethanesulfonate (11a) or A -fluoro-2,4,6-trimethylpyridinium trifluoromethane-sulfonate (11b).118 A study of these fluorination reactions indicated that the yield of fluorinated compound is directly related to the electronic nature of the amine substructure. Thus, the yield is higher when less basic cnamino derivatives are used, according to the order morpholino > piperidino > pyrrolidin-1 -yl.118 Moreover, the milder V-fluoropyridinium salt lib gives better yields than salt 11a. 

Dicarbonyl compounds are selectively fluorinated a to the two carbonyl groups with 1-fluoro-4.6-bis(trifluoromethyl)pyridinium 2-sulfonate (2) at room temperature. In sharp contrast, the fluorination of carbonyl compounds with l-fluoro-2,4,6-trimethylpyridinium trifluoro-methanesulfonate (3) requires heating and Lewis acid catalysis (Table 6). The reactivity of the pyridinium N-F reagent is dependent on the electronic nature of the substituents on the ring.41 When two equivalents of. V-fluoropyridinium salts were used for the fluorination of 1,3-dicarbonyl compounds, the major product was the 2,2-difluoro derivate (Table 7). 

Another approach to the conversion of aryltrifluoroborate salts into aryl fluorides also used a common copper salt to promote the reactions however, this chemistry used an electrophilic source of fluorine (Scheme 7.58) [84]. The most effective source of F was found to be A-fluoro-2,4,6-trimethylpyridinium ttiflate. Similar to the authors previous work with arylstannanes, the key to the synthesis was prestirring the F source and the copper complex prior to the introduction of the aryltrifluoroborate salts. Using this system, a wide range of aryltrifluoroborate salts bearing ethers, halogens, aldehydes, ketones, amides, and esters were converted into aryl fluorides. 

Yu developed a similar method of Pd-mediated directed electrophilic fluorination of C-H bonds using iV-benzyl triflimide derivatives. Key to the success of Yu s chemistry is the use of 2,4,6-trimethylpyridinium fluoride as the triflate salt, which was identified through a screen of fluorinating agents. 

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