O. Roy et al. / Journal of Fluorine Chemistry 167 (2014) 74–78
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Scheme 1. Fluorination of undec-1-ene by UF6.
Scheme 2. Cation-mediated fluorination of undec-1-ene.
the substrate [13] whereas, at reflux of ClF2CCFCl2, 1-adamantol
delivers a mixture of 1-fluoroadamantane (55%) and difluoroada-
mantanes (45%) [14]. Under the same conditions, cyclohexene
epoxide gives 2-fluorocyclohexanol in a poor yield (20%), along
with 25 undetermined products, whereas chloro- or bromo-
cyclohexane deliver a mixture of more than 40 non-fluorinated
products [14]. Fluorination becomes more efficient, though not
totally selective, at higher temperatures: for example, above 150 8C
and sometimes up to 600 8C, tetrachloromethane was transformed
into perchlorofluoromethanes, formal addition of F2 (eventually
followed by chlorine/fluorine exchange) occurs from various
chlorinated ethylenes [15–17] and, between 350 and 450 8C,
tetrafluoroethanes lead to pentafluoroethane [18]. Also, in contrast
to results reported by Olah et al. at 0 8C [10], acetic and
trifluoroacetic acids partially yield acyl fluorides at 40–60 8C
[19,20]. A more selective fluorination can be obtained when pellets
of NaF, KF or CaF2 are added to the system. For example, between
100 8C and 400 8C in the presence of such fluorides, simple alkanes
are highly fluorinated by UF6 in excess [21]. In the same way, when
opposed to UF6/CaF2 at 100 8C, alcohols, diols, epoxides and
cycloalkenes lead to polyfluorinated alkanes whereas acid, ester
and aldehyde moieties are transformed into trifluoromethyl
groups [22]. Such an enhancement of reactivity could be due to
the fact that, because of itÀs strong Lewis acidity, UF6 can add
As the reaction was carried out under anhydrous conditions
and the results were the same whatever the sample of UF6, a
simple hydrofluorination by HF (arising from partial hydrolysis
of UF6) can be ruled out. Nevertheless, the migration of fluorine
along the carbon chain, as well as the predominance of internal
fluorides, constitutes a strong argument for the occurrence of a
cation or a radical-cation. However, as no intermediate could be
quenched by alkylsilanes or phenylselenenyl chloride, the
neutralization of the cations by FÀ could be a quasi-concerted
process. Finally, as the results were similar with 1 or 0.5
equivalent of UF6, intermediate R-UF5 is suspected to react with
olefins as UF6 does. These observations lead us to the mechanism
proposed in Scheme 2.
Nevertheless, as UF6 is a strong oxidizer, the formation of a
radical-cation cannot be ruled out. Such a mechanism could
explain that several olefins (and eventually dienes) were detected
by gas chromatography and that the yield of fluoralkanes never
exceeded 66% (Scheme 3).
1,1-Disubstituted olefins, such as 2-methyl-undec-1-ene, or
internal olefins (7-tetradecene, (+)-p-menth-1-ene) were also
hydrofluorinated at room temperature but slower, so that 2
equivalents of UF6 were needed to complete the reaction within
16–19 h (respective yields: 35%, 51%, 23%). In this case, no
migration of fluoride was observed, that is consistent with the
higher stability of secondary and tertiary cations. Nevertheless, no
diastereomeric excess was obtained, that is also consistent with
the occurrence of a cation.
Then, reaction of UF6 with alcohols was examined at room
temperature. From the literature, no fluorination should be
expected since the only positive results were reported under
heating, eventually in the gas phase [13,14,22]. Indeed, in our
hands, aliphatic primary alcohols remained unaffected by UF6,
except 4-(biphenyl)methanol and 2-(phenyl)ethanol, which were
significantly converted but delivered a complex mixture among
which non-fluorinated products were largely predominant. In
these latter cases, oxidation probably matched extensively
fluorination, as reported by Olah et al. [7–11]. However, with a
secondary alcohol such as decan-2-ol, fluorination was predomi-
nant (46%) over some oxidation (10%) and dehydration (6%)
(Scheme 4).
À
fluoride to give UF7 or UF8 anions [23,24].
In order to rationalize these elements and to solve contra-
dictions between several reports, we reexamined the behavior of
depleted UF6 toward different functions, most often at room
temperature. As a large panel of functions was studied, the results
reported below were not fully optimized.
2. Results and discussion
As far as olefins are concerned, only the reaction of gaseous UF6
with polychlorinated olefins has been reported in the literature.
Thus, we studied the action of this reagent toward undec-1-ene in
CFCl2CF2Cl(F113)atroomtemperature. Itappearedthatamixtureof
1-, 2-, 3-, 4- and 5-fluoro-undecanes was obtained, among which 3-
fluoro and 4-fluoroundecanes were predominant (Scheme 1) [25].
Scheme 3. Radical-cation mediated fluorination of undec-1-ene.