The Pentafluoroethyl Group

Substitution on a benzene or pyridine ring also does not have much effect upon these chemical shifts. 

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Ullman type coupling occurs between aryl halides and trifluoromethyl copper species generated by the action of copper iodide on sodium tnfluoroacetate [168, 169] or on methyl fluorosulfonyldifluoroacetate [170] (equation 145) Similarly the pentafluoroethyl group can be introduced from potassium pentafluoropropion-ate [171] (equation 146)... 

By applying bifunctional diols such as hydroquinone (52) doubly charged diphosphate anion (53) in which fluorine atoms are in meridional configuration with regard to the pentafluoroethyl groups was prepared (Scheme 20). 

Rapid collapse of an ion pair of carbonium ion (28) and fluoride should also result in cu-addition of HF across the allene 1,2-double bond such addition could give rise either to c(r-2/f-nonafluoropent-2-ene (25) or to its trawr-isomer (26), depending on the direction of approach of the reagent to the reaction site. Preferential formation of the c/s-product is ascribed to partial shielding of one side of the 1,2-bond by the pentafluoroethyl group (a Courtauld molecular model reveals the distinct possibility of such hindrance to attack). 

A rapid, nonphotochemical, transfer of the alkyl group has been observed at 44° C between [MeCo(BAE)H20)] and [Co"(tfen)] in both the forward and reverse directions, and from the ethyl, but not the pentafluoroethyl, analog and it was suggested that the reaction involved electron transfer accompanied by the transfer of the alkyl group (174a). Whatever the best description of the mechanism, it presumably involves direct transfer... 

It has proved possible actually to isolate the valence bond isomers from photolysis of some perfluoropyridines. Irradiation of (289) gives pentakis(pentafluoroethyl)-l-azabicyclo[2.2.0]hexa-2,5-diene (290), a colourless liquid (b.p. 176 °C), almost quantitatively (equation 186). Further irradiation gives pentakis(pentafluoroethyl)-l-azaprismane (291) which is also a stable colourless liquid (73JCS(Pl)l542). The remarkable stability of these products has been attributed in part to relief of steric strain when the planar adjacent bulky pentafluoroethyl groups take up a nonplanar position in the valence isomers. 

Fluoroalkylamines react with nitrous acid to produce the corresponding unstable fluoroahphatic diazomum ions Placement of the tnfluoromethyl group at a carbon position a, (i, or y to a diazomum ion was used to probe the inductive effect on the chemistry of the transient carbocation resulting from dediazomation [7] If the fluoroalkyl group is bound to the same carbon as the amino group, conversion to the more stable diazo compound occurs For example, 4-diazo-l,l,l,2,2-pentafluoro-3-pentafluoroethyl-3-tnfluoromethylbutane is obtained from the reaction of the poly-fluoroalkylamine salt with sodium nitrite [8, 9] (equation 8)... 

Elimination of the fluoride ion from the CF3 group of carbanion B leads to the formation of olefin 54 with a terminal multiple bond. Intramolecular cyclization of olefin 54 gives a minor amount (5%) of seven-membered heterocycles 5-pentafluoroethyl-6-trifluoromethyl-5,7-difluoro-l,4-dioxacy-cloheptene-6 52 and 5-pentafhioroethyl-6-trifluoromethyl-7,7-difluoro-l,4-dioxacycloheptene-5 53. 

Monoethanolamine reacts with perfluoro-2-methylpent-2-ene in the presence of triethylamine, forming 7-fluoro-5-pentafluoroethyl-6-trifluoro-methyl-2,3-dihydro[l,4]oxazepin 135 (OlJFC(llO)l 1). If the compound has bulky substituents in the -position relative to the amino group, attack at the internal double bond is sterically hindered, and the internal olefin isomerizes in the presence of the base to the terminal olefin. The attack of the N-nucleophilic center of monoethanolamine occurs at the carbon atom of the terminal double bond, giving 5-fhioro-3,3-dimethyl-7-pentafluoroethyl-6-trifhioromethyl-2,3-dihydro[l,4]-oxazepin 136 or 137 as the major product. 

Similarly, a 1,4-bonded Dewar pyridine was obtained in the photolysis of 2,4,6-trimethyl-3,5-bis(trifluoromethyl)pyridine. This Dewar pyridine is much less stable than the pentakis(pentafluoroethyl) counterpart. Thus, while it could be kept in a refrigerator for a few years, it isomerized to the aromatic counterpart on thermolysis. Treatment with sodium methoxide in deuteriomethanol resulted in the deuterium exchange in the a-methyl but not in the 7-methyl group, suggesting that the stability of this Dewar pyridine is partly due to the pull push interaction between the trifluoromethyl and the methyl groups on the double bond. Isomerization to the pyridine was catalyzed by acids and transition metals such as Fe or Rh. Complexes with Pd(II) or Pt(II) were isolated and determined to be square planar with a trans-N-u bond as indicated by X-ray analysis. The Dewar pyridine when coordinated as ligands on the metal isomerized to the aromatic form much faster than when noncoordinated (Scheme 36).89,90 Isomerization was... 

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