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Direct conversion of perfluoroalkanes and perfluoroarenes to perfluoro
Grignard reagents
Christopher M. Beck, You-Jung Park and Robert H. Crabtree*†
Yale University, Department of Chemistry, 225 Prospect Street, New Haven, CT 06511, USA
Magnesium anthracene selectively reduces C
6
F
3
12 or C
6
F
6
The reaction of perfluorotoluene 2b with 1 equiv. of 1 at
241 °C produced perfluorotoluic acid 5b‡ exclusively as the
para isomer but in low yield (8.5%). The yields did not change
significantly between 278 and 0 °C or in the presence of an
excess of 1. Perfluoronaphthalene gave no identifiable organic
products in the presence of 1 at 278 °C. For comparison, Mg
powder (Aldrich Co., 50 mesh) was exposed to 2a or 2b under
analogous conditions. In this case, no reaction was observed
between the perfluoroarene and magnesium, further illustrating
the necessity for using 1 in Grignard synthesis.
and CF
3
C
6
F
11 or CF
3
C
6
F
5
to C
6
F
5
MgF and CF
C
6
F
4
MgF,
respectively
1
Magnesium anthracene MgC14
H
10 1, has found a wealth of
2
applications in organic chemistry. This compound is believed
to act as a soluble, activated form of magnesium metal,2 and
is often the compound of choice for the preparation of Grignard
reagents that are difficult or impossible to obtain directly from
a,b
2
magnesium metal. Here, we describe the use of 1 in new
synthetic routes to perfluoroaromatic Grignard reagents from
both perfluoroaromatic compounds as well as from the less
costly analogous saturated fluorocarbons.
Our success with these perfluoroarenes led us to look at the
corresponding perfluoroalkanes. When a THF solution of
4 equiv. of 1 at 0 °C was treated with perfluorocyclohexane 3a,
the initial orange solution rapidly turned dark brown. Sub-
Rare cases are known in which perfluoro Grignard reagents
have been synthesized from perfluoroaromatic compounds (but
not from perfluoroalkanes) and magnesium by entrainment with
2
sequent work-up with CO , then acid, followed by extraction
with dilute base afforded dark brown solids after removal of the
solvent. Direct sublimation of these solids at 115 °C in vacuo
3
4
a more reactive organic bromide, with or without transition
metal catalysis. Important work by Richmond and Pez and their
afforded white crystals which proved to be an inseparable
5
19
coworkers has shown that transition metal compounds and
6 5 2 6 4 2
mixture of C F CO H and 2,3,5,6-C HF CO H§ by F NMR
6
arene anions can reduce saturated perfluorocarbons in solution
spectroscopy in 14 and 4% overall chemical yield based on
fluorocarbon. This result is consistent with initial reduction of
perfluorocyclohexane to hexafluorobenzene which would then
be expected to react with an additional equivalent of 1 to yield
the corresponding Grignard reagent. Similarly, perfluoro-
methylcyclohexane 3b upon treatment with 4 equiv. of 1
yielded 5b in 5.7% yield, exclusively as the para isomer. As in
the case of the perfluoroarenes, no products were observed
when commercial Mg powder was treated with 3a or 3b in an
analogous manner. These transformations constitute, to our
knowledge, the only known conversions of perfluoroalkanes
directly to Grignard reagents. CAUTION: Fluorinated
but never previously to give such useful species as Grignard
reagents.
We now find that hexafluorobenzene 2a reacts cleanly with
1
equiv. of the orange species 1 in THF solution at 241 °C to
afford a solution, the blue color of which is believed to be
associated with the anthracene monoanion. Treatment of this
2
solution with CO followed by acid work-up, yielded penta-
fluorobenzoic acid 5a in moderate yield (34%), along with
unreacted 2a. The reaction appears to proceed via net insertion
of a Mg atom into the aromatic C–F bond, forming a Grignard
reagent which subsequently reacts with CO to form the
2
carboxylic acid (Scheme 1). Since 1 exists in a temperature
8
Grignards can spontaneously explode; appropriate care must
dependent equilibrium with anthracene and magnesium met-
therefore be exercised.
1b
al, it is unclear if the formation of the Grignard reagent occurs
directly or by prior production of a highly active form of
magnesium metal.
The authors are grateful to the Department of Energy and the
3M corporation for financial support.
R
F
R
R
F
Mg(C14H10)
THF, –41°C
4 equiv. Mg(C14H10
)
Table 1 Yields of 5 and reaction temperature for the reactions of 2 and 3
F
with 1a
THF, 0°C
Fluorocarbon
T/°C
Yield (%)
MgF
3a R = F
2a R = F
2b R = CF3
4a,b
3b R = CF3
2
2
3a
a
b
241
241
0
34.3
8.5
14.0b
5.7
1, CO2
, H+
2
3
b
0
R
F
a
Fluorocarbon was added to a pre-cooled THF solution of 1 and allowed to
stir for 15–40 min. Dry CO was introduced for ca. 40 min at the reaction
temperature and allowed to warm to room temperature under a CO flow.
The mixture was then quenched with dilute H SO and the THF removed in
vacuo. The product was extracted with Et O, and then extracted by shaking
with 1 m (aq) KOH. The basic extracts were acidified with dilute H SO and
extracted with Et O to yield the product. Yields reported here are for the
doubly sublimed, analytically pure solids. b Isolated as a mixture which also
2
2
2
4
2
CO2H
2
4
2
5
a,b
Scheme 1
contained 2,3,5,6-C
6
HF CO
4 2
H in 4% yield based on fluorocarbon.
Chem. Commun., 1998
693