Bhadury et al.
1187
cations and has been employed as the starting monomer for
the manufacture of numerous classes of commercially im-
portant organic fluoropolymers, including Teflon, Neoflon,
and Viton (13). HFC-227ca can be employed as a potential
CFC replacement under the Montreal Protocol (14). We
have studied reactions for the preparation of the above-
mentioned organofluorine compounds, for which simple and
practical synthetic procedures were not readily available.
Highly volatile fluorinated olefins such as PFIB, TFE, and
perfluoro-2-butene (E and Z) are, in general, known to be pro-
duced by high-temperature disproportionation of perfluorin-
ated hydrocarbons and polymers (12, 15). While pyrolysis of
fluorocarbons usually leads to complex mixtures, other meth-
ods reported require high temperature and use of corrosive
materials (13). Therefore, alternate synthetic procedures to
Table 1. Ylide-based synthesis of organofluorine compounds.
Reagent system Substrate Product
Ph P/CF Br /Zn CF COCF @1.5H O (CF ) C=CF
2
3
2
2
3
3
2
3 2
Ph P/CF Br /Zn CF BrCF I
CF =CF
3
2
2
2
2
2
2
2
Ph P/Zn
CF BrCF I
CF =CF
3
2
2
2
Ph P/CF Br /Zn CF CF CF CF I
3
2
2
3
2
2
2
Ph P/CF Br /Zn CF CF CFICF
CF CF=CFCF
3
2
2
3
2
3
3
3
Ph P/CF Br /Zn CF CF CF I
CF CF CF H
3
2
2
3
2
2
3
2
2
Note: GC yields are 84–96% based on starting compound; isolated
yields are given in Experimental section.
synthesize these fluoroolefins would be highly useful.
In
hexafluoroacetone gas did not produce any significant
amount of PFIB. This may presumably be due to the fact
that anhydrous hexafluoroacetone (bp –26 °C), a low-boiling
gaseous substance at ambient temperature, would have very
low contact time with the reagents and may have escaped
from the reaction mixture. A reported procedure in which
hexafluoroacetone (sesquihydrate) was successfully used in
place of anhydrous hexafluoroacetone in catalytic epoxida-
tion of alkenes (17) led us to employ the sesquihydrate in-
stead of hexafluoroacetone gas for generation of PFIB. This
also meets the requirement that substrate be present before
the addition of the zinc metal to the phosphonium salt. Al-
though the sesquihydrate predominantly exists as a diol, pre-
sumably the equilibrium between thediol and the ketonic
form is shifted towards the right in the aprotic solvent DMF.
This enables the ylide to consume the ketone to yield PFIB
at a rapid rate (Scheme 1). The synthetic route provides a
convenient and safe method for the generation and isolation
of the highly toxic compound PFIB.
this context, the reactions of fluorinated phosphonium ylides
generated in situ from quaternary salts in DMF with
appropriate substrates have been investigated. Thus, difluoro-
methylene triphenylphosphonium ylide reacts with hexa-
fluoroacetone (sesquihydrate) in a Wittig reaction to generate
PFIB. Other compounds produced are tetrafluoroethylene
from 1-bromo-2-iodotetrafluoroethane, tetrafluoroethylene
and perfluorocyclobutane from 1-iodononafluorobutane,
perfluoro-2-butene from 2-iodononafluorobutane, and HFC-
2
27ca from 1-iodoperfluoropropane. We have also studied the
formation of TFE from the quaternary phosphonium salt
generated by the reaction of triphenylphosphine and 1-bromo-
2
-iodotetrafluoroethane. The syntheses of these highly volatile
organofluorine compounds employing different reagent sys-
tems are summarized in Table 1. The compounds have been
characterized by H NMR, F NMR, and mass spectral data,
shown in Table 2, which were consistent with their structure
and reported literature values (10, 13, 16).
1
19
The in situ ylide formation reaction in DMF has been car-
ried out for the conversion of carbonyl compounds into the
corresponding terminal 1,1-difluoroolefins. We have em-
ployed this reaction to convert several activated aromatic ke-
tones and aldehydes into olefins (3). The Wittig reaction has
now been successfully extended to yield yet another highly
volatile aliphatic difluoroolefin, PFIB, a highly toxic Sched-
ule 2 chemical.
Facile conversion of hexafluoroacetone (sesquihydrate) into
PFIB has been achieved when the ketone is subjected to a
Wittig reaction with difluoromethylene triphenylphosphonium
ylide. The reaction is based on zinc metal dehalogenation of
in situ generated (bromodifluoromethyl)triphenylphosphon-
ium bromide salt (I, Scheme 1) in DMF (3). The salt was
formed when an equimolar amount of dibromodifluor-
omethane was added all at once to a well-stirred solution of
triphenylphosphine at room temperature in dry DMF. This
was followed by the addition of 0.5 mol of hexafluoro-
acetone (sesquihydrate) to the salt solution at room tempera-
ture. Then 1 mol of zinc powder was added slowly into the
reaction mixture. Within 1 to 2 min an exothermic reaction
ensued due to the formation of organozinc bromide (II) in
equilibrium with difluoromethylene triphenylphosphonium
ylide (III). The PFIB generated was collected in a trap
cooled by a liquid-nitrogen/methanol mixture. It should be
noted that an initially attempted synthesis of PFIB via a
Wittig reaction of the in situ generated ylide with
For the synthesis of TFE, the substrate 1-bromo-2-
iodotetrafluoroethane was added to a room temperature solu-
+
–
tion of the salt [Ph P –CF Br]Br , which had been generated
3
2
in situ by the reaction of Ph P and CF Br in DMF. This was
3
2
2
followed by the addition of zinc metal, resulting in a highly
exothermic reaction due to the formation of an organozinc
halide/fluoromethylene ylide system. The nucleophilic ylide
under the reaction conditions presumably abstracts the more
labile positive iodine from the substrate to give a carbanion
(IV, Scheme 2). Subsequent elimination of bromide ion from
the adjacent carbon atom forms TFE. Alternatively, TFE
could also be generated from the quaternary phosphonium
+
–
salt [Ph P –CF –CF Br]I (V, Scheme 2) obtained in situ by
3
2
2
the reaction of Ph P and CF BrCF I. The organozinc halide
3
2
2
VI, obtained after the addition of zinc metal, collapses spon-
taneously to yield TFE by the elimination of a molecule of
Ph P from the carbon atom adjacent to the carbanion center
3
(Scheme 2).
Similarly, when 1-iodononafluorobutane was added into a
+
–
solution of the salt [Ph P –CF Br]Br followed by zinc
3
2
metal, a mixture of volatile products consisting of TFE and
perfluorocyclobutane was generated. As the result of the
nucleophilic attack of the ylide on the positive iodine of the
substrate, the carbanion (VII, Scheme 3) is first generated.
Fragmentation and fluoride ion elimination from the termi-
nal carbon atom of the conjugated system lead to the forma-
tion of TFE. The nucleophilic attack by this carbanion on
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2004 NRC Canada