Valence-bond isomer chemistry. Part IV. The valence-bond isomers of pentakis(pentafluoroethyl)pyridine

Abstract

Reaction of pentafluoropyridine with tetrafluoroethylene and caesium fluoride in dimethylformamide produces, inter alia, perfluoro-4-ethyl-, -2,4-diethyl-, -2,4,5-triethyl-, -2.3.4.6-tetraethyl-, and -pentaethyl-pyridines. In perfluoro-n-pentane solution, the last compound is isomerized by U.V. light to pentakis(pentafluoroethy1)-1 Lazabi-cycl0[2,2,0] hexa-2.5-diene. which in turn forms the corresponding 1 -azatetracyclo[2,2,0,02~s,0S~] hexane. These valence-bond isomers show substantial thermal resistance to rearrangement ; in hexafluorobenzene solution, the half-life at 170°C of the hexadiene is 104 h and that of the hexane is 1.1 h.PHOTOCHEMICAL isomerization of benzene derivatives provides a convenient route to the non-planar valencebond isomers , para-bonded benzenes (bicyclo[2,2 ,O] hexa-2,5-dienes) , benzvalenes (tricyclo[3 , 1 ,O ,02,6] hex-3-enes) , and prismanes (tetracyclo[2,2,0,02~6,03~5]hexanes). Perfluoroalkylation leads to increased thermal stability, and the hexakis-trifluoromethyl and -pentafluoroethyl derivatives are the most stable examples known.2Little is known concerning the valence-bond isomers of six-membered nitrogen heterocycles. N-Methyl-2pyridone forms a $am-bonded isomer upon U.V. irradiation in solution,3 the isomeric picolines and lutidines are photochemically in t erconver tible in the gas phase , possible via prismane intermediates: and it has recently been reported that pyridine is isomerized to an unstable $am-bonded isomer (Scheme 1) which decays back to SCHEME I pyridine with a half-life of 2.5 min at 25 oC.6Nucleophilic displacement of fluoride in peduoroaromatic compounds by perfiuorocarbanions (from fluoro-olefins and casium fluoride in aprotic solvent) provides a convenient route to perfluoroalkyl derivatives.6 Pentafluoropyridine has been reported to react with tetrafluoroethylene to give low yields of pentakis-(pent aff uoroet hyl)pyridine,6* together with perfluoroethylpyridines, C,F,(C,F,),,N(n = 1 4 ) .7 We find 1 (a) Part 111, M.

Footsteps of a fluorine chemist

“…The group of Haszeldine and co-workers [23] had shown that azaprismanes may be obtained from perfluoroalkylpyridine derivatives and we set out to detect and identify rearrangements corresponding to those observed for the pyridazine system. We constructed a system with extensive substituent labeling by starting with perfluoro-3,5-bistrifluormethylpyridine then, by employing the Negative Friedel-Crafts chemistry that was mentioned above, we introduced a pentafluoroethyl group at the 4-position and then perfluoroisopropyl at the sterically less demanding 2,6-positions (Fig.…”

Robert Neville Haszeldine. 3 May 1925—13 October 2016

“…phosphites and arsines to give dimeric carbonyls, which proved to be among the most selective and active hydroformylation catalysts known (50).…”

Low temperature IR spectroscopy and photochemistry of matrix-isolated α-pyridil

“…In turn, efficient isomerization of Kekulé ring structures to their Dewar analogues have been found to take place in matrices for related compounds, like ␣-pyrones and ␣-thiopyranones [50][51][52][53], while some Dewar pyridines, including the unsubstituted one, could also be observed experimentally as result of photochemical transformation of their most stable Kekulé isomers [54][55][56][57][58][59][60][61][62]. On the other hand, to the best of our knowledge, experimental observation of Hückel pyridine isomers was only reported once, in a study where the ultrafast dynamics of isomerization of pyridine (and a few other analogue compounds) was investigated by femtosecond-resolved mass spectrometry complemented by DFT/ab initio calculations [62].…”