Pyridine, perfluoroalkylated

Cobalt trifluoride fluorination corresponds to the electron-transfer mechanism via a radical cation. RF groups attached to the ring enhance the stability of intermediate dienes and monoenes. Perfluoroalkyl pyridines, pyrazines, and pyrimidines were successfully fluorinated but pyridazines eliminated nitrogen. The lack of certain dienes was attributed to the difference in stability of FC=C and RFC=C and steric effects [81JCS(P1)2059]. 

These types of compounds can also be formed via a Wittig reaction [19]. Triphenylphosphine can be quatemized with a suitable alkyl iodide, and the resultant perfluoroalkylated phosphonium salt will react with aldehydes to give fluorinated alkenes which are easily hydrogenated (Scheme 3.4). This methodology has recently been expanded to the formation of perfluoroalkylated pyridines [20],... 

The two examples just considered seem to suggest that the photoreaction of perfluoroalkylated pyridines would give the 1,4-bonded Dewar isomers. However, the photoreaction of 2,6-bis(heptafluoroisopropyl)-3,5-bis(penta-fluoroethyl)-4-trifluoromethylpyridine gave a product whose structure... 

In 1977, the 2,5-bonded Dewar isomers of perfluoroalkylated pyridine were isolated by Chamber s group (145) 156). The photolysis was carried out in the gas phase. Therefore, thse isomers must be the primary products. Their distributions seems to be determined by the electronic interaction between the substituents on the double bonds and steric effects. 

Due to the availability of perfluoroalkyl iodides, this method quickly became the method of choice for the regioselective synthesis of perfluoroalkylated heterocycles. Chen and Tamborski used this reaction for the preparation of variety perfluoroalkyl-pyridines," azines," and triazines." For example, pyridines 72-73 and pyrimidine 74 were synthesized by addition of perfluorohexyl iodide to the agitated mixture of the corresponding heteroaryl bromide, cooper bronze powder in DMSO solvent under nitrogen at elevated temperature (Fig. 7.26)." ... 

Keywords Fluoropyridmes Perfluoroalkyl pyridines Umemoto reaction Balts-Schiemann reaction Pentaflnoropyridine Substitntion reaction P -pyridines radiobiology Cyclization... 

The present review contains the literature published since 2009 for 2012. TUI 2009 four reviews on this field have been published, completely [1, 2] or in part [3] devoted to methods syntheses C-F pyridines and perfluoroalkyl pyridines [2,4]. To display full information about synthesis fluorinated pyridines in the present review earlier classical works also are included. 

Alkyl-5-arylisoxazoles (303) were prepared by the regiospecific reaction of appropriate 1,3-diketones (302) (R = alkyl or perfluoroalkyl) with hydroxylamine hydrochloride in pyridine (79MI41601). 

Direct perfluoroalkylation of heteroaromatics occurs with RFI when sodium hydroxymethane sulfinate (Rongalite) is present. 3-Perfluoroal-kylcoumarins can be obtained (90CC1781). The distribution of isomers from substituted pyridines is compatible with a radical reaction (90TL2711). 

Pyridines gave poor yields, whereas quinolines and pyrimidines were successfully perfluoroalkylated (91T6231). 

The exchange reaction of perfluoroalkyl iodides and iodopentafluorobenzene with dialkylzinc in the presence of Lewis base, such as diglyme or pyridine, afforded the corresponding bis(trifluoromethyl)zinc and bis(pentafluoro-phenyl)zinc in high yield [41] (Scheme 12). 

Alkyl and aryl thiols afforded with perfluoroalkyl iodonium salts in the presence of pyridine the corresponding sulphides in good to excellent yields [25], Several sulphides and sulphoxides upon reaction with trifluoroethyl phenyliodonium triflate were converted into 2,2,2-trifluoroethyl sulphonium salts [26]. 

Several amines such as anilines, pyridine, etc., were perfluoroalkylated affording the corresponding ammonium or pyridinium salts [26-28], Other substrates which underwent perfluoroalkylation included triphenylphosphine [26], sodium nitrite [29], potassium thiocyanate [29] and potassium selenocyanate [29]. 

The reaction of perfluoroalkylcopper compounds, prepared in situ from the perfluoroalkyl iodide and copper, with ip -hybridized organic halides [Eq. (73)] in refluxing pyridine utilizes the thermal stability of the intermediate copper compounds and their failure to react with sp -hybridized halides (29, 30). 

A Dewar isomer of unsubstituted pyridine was identified by Wilzbach, but it was very unstable.63 Other Dewar isomers of six-membered heterocycles, isolated and fairly stable, are substituted with perfluoroalkyl groups. However, 1,2-dihydroheterocyclic compounds such as a-pyrone or a-pyridone derivatives are easily photoisomerized to bicyclic Dewar isomers, which are useful for the synthesis of cyclobutadienes. The photochemistry of 1,2-dihydroheterocyclic compounds will be discussed, followed by those of fully aromatic compounds. 

These examples show that the 2,5-bonded Dewar pyridine can be formed even if it is substituted with perfluoroalkyl groups. In the second example in Scheme 38 another 2,5-bonded isomer was not formed. This fact shows that a steric effect is important. 

FITS reagents reacted very easily with thiols in the presence of pyridine to give the perfluoroalkyl sulfides in good to high yields. S-perfluoroalkylation was selective in the case of polyfiinctional substrates. In the presence of groups such as hydroxy, carboxy, alkoxycarbonyl or dialkylamino, the sulfur atom was always selectively substituted.216 (Table 5.10)... 

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