Pentakis -1 -azaprismane

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. 

Pentakis(pentafluoroethyl)-l-azaprismane 52 can be isolated in 91% yield by irradiation of the corresponding pyridine. The photolytic isomerization of alkylpyridines (e.g., 2-picoline to 3- and 4-picolines) is also believed to involve azaprismane intermediates. 

Upon irradiation with UV light (A > 200 nm) the two carbon-carbon double bonds of pentakis(pentafluoroethyl)-l-azabicyclo[2.2.0]hexa-2,5-diene (199 Section III,D,2) entered into an intramolecular (2 + 2)-cycloaddition reaction to give the corresponding 1-azaprismane (200).226 This highly strained molecule was thermally stable up to 160°, at which temperature it decomposed into a mixture of 199 and pentakis-(pentafluoroethyl)pyridine. In terms of conservation of orbital symmetry this high stability can be well understood. Just as in prismane... 

Photolysis of highly substituted pyridines gives rise to isolable and stable valence isomers. For instance, pentakis(pentafluoroethyl)pyridine 101 is converted almost quantitatively into the symmetrical 1-aza-DEWAR pyridine 102. Further irradiation brings about its rearrangement to the corresponding azaprismane derivative 104 [59] ... 

Full details of the preparation of pentakis(pentafluoroethyl)-l-azaprismane (30) in 91 % yield by u.v. irradiation of pentakis(pentafluoroethyl)pyridine (see p. 93) are now available so are those of the isolation (2% yield) of A difluoromethyl-... 

Polyperfluoroalkyl substitution greatly stabilizes both Dewar-pyridine and azaprismane valence isomers. Irradiation of pentakis(pentafluoroethyl)pyridme 134 in perfluoro-n-pentane with Hght of A, >... 

---