The solid complex Zn(CF3)Br·2DMF as an alternative reagent for the preparation of both, trifluoromethyl and pentafluoroethyl copper, CuCF3 and CuC2F5

Article abstract of DOI:10.1016/j.jfluchem.2009.10.011

Trifluoromethylcopper and pentafluoroethylcopper are prepared conveniently via the reaction of the solid complex Zn(CF₃)Br·2DMF with copper(I) bromide in N,N-dimethylformamide. The reactions of both copper species with 2,4-dinitrochlorobenzene,

Full text of DOI:10.1016/j.jfluchem.2009.10.011

Journal of Fluorine Chemistry 131 (2010) 212–216
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Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
The solid complex Zn(CF3)Brꢀ2DMF as an alternative reagent for the preparation of
both, trifluoromethyl and pentafluoroethyl copper, CuCF3 and CuC2F5
a
b
a
Mikhail M. Kremlev a, , Wieland Tyrra
, Aleksej I. Mushta , Dieter Naumann , Yurii L. Yagupolskii
*
b,**
a Institute of Organic Chemistry, National Academy of Sciences of the Ukraine, Murmanskaya St. 5, UA-02094 Kiev, Ukraine
b Institut fu¨r Anorganische Chemie, Universita¨t zu Ko¨ln, Greinstr. 6, D-50939 Ko¨ln, Germany
A R T I C L E I N F O
A B S T R A C T
Article history:
Trifluoromethylcopper and pentafluoroethylcopper are prepared conveniently via the reaction of the
solid complex Zn(CF3)Brꢀ2DMF with copper(I) bromide in N,N-dimethylformamide. The reactions of
both copper species with 2,4-dinitrochlorobenzene, 4-iodonitrobenzene, 4-nitrobenzyl iodide, 4-
bromobenzoic acid ethyl ester, 4-iodobenzoic acid ethyl ester, 2-iodopyridine and 2-iodopyrimidine
have been studied. The structure of 2,4-dinitrotrifluoromethylbenzene has been elucidated.
ß 2009 Elsevier B.V. All rights reserved.
Received 2 July 2009
Received in revised form 15 October 2009
Accepted 23 October 2009
Available online 4 November 2009
Keywords:
Solid trifluoromethylzinc bromide
Trifluoromethylcopper
Pentafluoroethyl copper
Halide substitution
C–C coupling
1. Introduction
stabilize this trifluoromethylcopper reagent; warming to room
temperature shows no change with only a single 19F NMR signal at
The study and development of methods for the introduction of
fluorinated moieties into organic molecules to change their
properties by design is one of the challenges in synthetic
chemistry. Fluorine-containing physiologically active substances
and the methodologies making them easily accessible are of great
interest with special respect to the introduction of perfluoroalkyl
groups [1].
From the view point of trifluoromethylation of organic halides,
it is a real task to invent how to generate efficiently and
conveniently trifluoromethylcopper species. For example, Kobaya-
shi et al. directly prepared it by the reaction of trifluoroiodo-
methane with activated copper in DMF [2] or HMPA [3].
Kondratenko et al. [4] generated trifluoromethylcopper using
Hg(CF3)2 as a trifluoromethyl source by transmetallation to copper.
Trifluoromethylcopper has also been prepared via in situ metath-
esis of trifluoromethylcadmium or -zinc reagents with copper(I)
salts [5]. Trifluoromethylcadmium or -zinc species were synthe-
sized by the reactions of difluorodihalomethanes with activated
cadmium or zinc powder [6]. These authors noted that addition of
HMPA (hexamethyl phosphoric acid triamide) is necessary to
ꢁ28 ppm. Subsequently, signals for CuC2F5 (ꢁ84.7 (CF3);
ꢁ113.7 ppm (CF2)) became visible upon prolonged stirring at
ambient temperature. The amount of CuC2F5 formed was
quantitatively based on the assumption that 2 equiv. of CuCF3
produce 1 equiv. of CuC2F5 [5].
Some publications [7–11] show that the formation of CuCF3 can
be achieved using the commercially available compound Me3SiCF3
either starting from copper alkoxides [8,9] or the system of heavier
copper halides and an alkali fluoride [7,10,11].
Most methods employed use expensive or toxic reagents [CF3I,
Me3SiCF3, Hg(CF3)2] or required autoclaves.
Earlier the method of preparation of the complex Zn(CF3)Brꢀ2L
(L = DMF or CH3CN) from bromotrifluoromethane and zinc dust in
DMF or acetonitrile was developed [12]. In solution, this complex
undergoes Schlenk-type equilibria between Zn(CF3)Br, Zn(CF3)2
and ZnBr2; the ratio of Zn(CF3)Br and Zn(CF3)2 is strongly solvent-
dependent (19F NMR). It was mentioned in one Ph.D. Thesis [13]
that treatment of the complex Zn(CF3)Brꢀ2DMF with copper(I)
iodide led to the formation of CuCF3 but no reactions of CuCF3
prepared in such a manner were reported.
The main advantage of the complex Zn(CF3)Brꢀ2L compared
with that prepared in situ is that it can be stored in an inert
atmosphere for a long period (about 6–12 months) in an ordinary
refrigerator without noteworthy decomposition. Therefore, it is
not necessary to prepare this compound freshly every time
when it is needed. In contrast to the acetonitrile adduct [14],
* Corresponding author.
** Corresponding author. Tel.: +49 221 4703276; fax: +49 221 4703276.
E-mail addresses: kremlyov1@yandex.ru (M.M. Kremlev), tyrra@uni-koeln.de
(W. Tyrra).
0022-1139/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2009.10.011

M.M. Kremlev et al. / Journal of Fluorine Chemistry 131 (2010) 212–216
213
Zn(CF3)Brꢀ2DMF exhibits significantly lower reactivity and only
the reactions with iodine chloride [12] and some inorganic
compounds [14] are documented.
stirring for 4 h at +50 8C (alternatively 12 h at ambient
temperature), CuC2F5 was the only species detected in the 19F
NMR spectrum.
ꢂC
2ZnðCF3Þ ꢀ 2DMF þ 4CuBr4 hꢁ;5!0 CuC2F5 þ 2ZnBr2 þ 4DMF
2. Results and discussion
DMF
2.1. Formation of CuCF3 and CuC2F5
þ CuF2ð?Þ þ . . .
The addition of copper(I) bromide to
a
solution of
Zn(CF3)Brꢀ2DMF in molar ratio of 1:1 in DMF led to the formation
of exclusively one trifluoromethylcopper species within 3 h at
ambient temperature.
2.2. Reactions of CuCF3 and CuC2F5 with aryl halides
Trifluoromethylcopper reacts with activated aryl chlorides and
iodides to give the corresponding trifluoromethylated compounds
1–4 in good yields.
ZnðCF3ÞBr ꢀ 2DMF þ CuBr3ꢁh!; rtCuCF3 þ ZnBr2 þ 2DMF
DMF
The signal of the CF3-group occurs in the 19F NMR spectrum at
ꢁ32 ppm. Within the next 5 h, this ‘‘CuCF3’’ derivative is
completely converted into Cu[CuIII(CF3)4] [15].
The reaction of trifluoromethylcopper with inactivated aryl
halides is accompanied by the competitive conversion of
trifluoromethylcopper into pentafluoroethylcopper. As the result
a mixture of trifluoromethyl- and pentafluoroethyl compounds is
obtained. For example, the reaction of trifluoromethylcopper
prepared by this method with 2-iodothiophene led to a mixture of
60% 2-trifluoromethylthiophene and 40% 2-pentafluoroethylthio-
phene. Treatment of trifluoromethylcopper with 2-iodopyridine at
80 8C for 6 h gave a mixture of 2-trifluoromethylpyridine and 2-
pentafluoroethylpyridine in a 93% and 7% ratio. The maintenance
of trifluoromethyl- and pentafluoroethyl derivatives were deter-
mined by 19F NMR spectroscopy in both the mixtures.
Addition of copper(I) bromide to
a
solution of
Zn(CF3)Brꢀ2DMF in a molar ratio of 2:1 in DMF at ambient
temperature after 20 min allowed the detection of both
CuCF3 (ꢁ32.0 ppm) and Cu[Cu(CF3)2] (ꢁ28.4 ppm) together
with a small quantity of Cu[CuIII(CF3)4] (ꢁ35.1 ppm) in the 19F
NMR spectrum. In accordance with the observations described
[5], signals for CuC2F5 at ꢁ85.0 ppm (CF3) and ꢁ113.9 ppm
(CF2) were detected which intensified upon prolonging the
reaction time and rising the temperature to +50 8C. After

214
M.M. Kremlev et al. / Journal of Fluorine Chemistry 131 (2010) 212–216
If a 1:1 molar ratio of Zn(CF3)Brꢀ2DMF and CuBr is chosen, the
The reaction of 4-iodonitrobenzene, 4-bromobenzoic acid
ethyl ester, 2-iodopyridine and 2-iodopyrimidine with penta-
fluoroethylcopper gave the corresponding pentafluoroethyl
derivatives 5–8. It has to be mentioned, that CuCF3 did not
give a considerable amount of CF3 and C2F5 derivatives (about 2–
reactions with 2-iodothiophene or 2-iodopyridine at 60 8C for 5 h
gave mixtures of the trifluoromethyl and pentafluoroethyl deriva-
tives in 80:20 ratios in both cases determined by 19F NMR
spectroscopy.
Again, a selective conversion of trifluoromethylcopper into
pentafluoroethylcopper is achieved by stirring of a 1:2 (CF3ZnBrꢀ
2DMF/CuBr)reactionmixtureeitheratambienttemperaturefor12 h
orat50 8Cfor4 h.Thereagentpreparedinsuchamannermaybeused
for the selective transfer of pentafluoroethyl groups to aryl halides.
4%, 19F NMR reaction control) in reactions with aromatics
containing electron releasing groups such as methyl or methoxy.
The question why no significant amounts of CuC3F7 are formed
will be subject of future investigations.
The perfluoroalkyl group transfer might be formulated as an
addition–elimination mechanism involving copper(III) species as
intermediates taking into account that CuRf [15,16] as well as CuR
[17] derivatives are easily oxidized even at low temperature.
However, alternative routes are also conceivable.
Alkaline hydrolysis of the esters 4, 6 followed by acidification
allowed the synthesis of the corresponding benzoic acids 9, 10.
2.3. Solid-state structure of 2,4-dinitrotrifluoromethylbenzene (1)
˚
Fig. 1. Molecular structure of 1. Selected bond lengths [A] and angles [8] (with
estimated standard deviations in parentheses): C1-C11 1.513(6); C11-F111
1.320(5); C11-F112 1.32(1); C11-F113 1.336(5); C4-N41 1.490(6); N41-O411
1.217(9); N41-O412 1.22(1); C6-N61 1.47(1); N61-O611 1.223(5); N61-O612
1.215(4); O411-N41-O412 125.4(3); O611-N61-O612 125.3(2); C5-C6-N61-O612
45.0(3); C1-C6-N61-O611 46.7(3).
Compound 1 crystallizes in the monoclinic space group P21/c
(no. 14) with 4 molecules per unit cell. Its bond length and angles
do not exhibit any peculiarities in comparison with the related
structure of 2,4-dinitrochlorobenzene [18]. The molecule is quasi-

M.M. Kremlev et al. / Journal of Fluorine Chemistry 131 (2010) 212–216
215
planar with one NO2 group (4-position) lying in the plane, while
the other is twisted by approximately 458 out of the plane (Fig. 1).
Selected bond lengths and angles are listed in the caption of Fig. 1.
MgSO4 and the solvent was evaporated on a water bath. The
remainder was distilled under reduced pressure or crystallized.
3.2.1. 2,4-Dinitrotrifluoromethylbenzene (1)
Yellow crystals. Yield 1.20 g (51%), m.p. 47–48 8C (pentane), lit.
3. Experimental
m.p. 48 8C [4]. 19F NMR:
d
ꢁ60.7 (s, CF3); 1H NMR:
d 7.80 (m, 1H,
Schlenk techniques were used throughout all manipulations.
Purifications were carried out in ambient atmosphere. NMR
spectra of compounds isolated were recorded on a Bruker Avance
II 300 (1H, 300.1 MHz; 19F, 282.4 MHz; 13C, 75.4 MHz) spectro-
meter in CDCl3 solutions unless quoted. External standards were
phenyl), 8.40 (m, 1H, phenyl), 8.70 (m, 1H, phenyl) [25].
X-ray crystal structure analysis of 1: C7H3F3N2O4
˚
(FW = 236.11), monoclinic, P21/c (no. 14), a = 7.327(1) A,
3
˚
˚
˚
˚
b = 10.060(2) A, c = 13.799(2) A,
Z = 4, Dcalc. = 1.732 g cmꢁ3, T = 293(2) K. The final R factor was
0.0348 (Rw = 0.0925 for all data) for 2013 reflections with I > 2 (I).
b = 117.12(1) A, V = 905.3(3) A ,
used in all cases (1H, 13C: Me4Si; 19F: CCl3F). Chemical shifts (
d
) are
s
given in ppm; couplings (J) in Hz. Assignment of NMR signals was
carried out using 1D (1H, 19F, 13C{1H}, 13C{19F})and 2D (COSY,
HSQC, HMBC) NMR techniques. Visible melting points were
determined using a Stuart melting point apparatus SMP10. The
solid complex Zn(CF3)Brꢀ2DMF was prepared from bromotrifluor-
omethane and zinc powder in DMF [12]. Copper(I) bromide was
freshly prepared from copper sulphate, potassium bromide and
sulphur dioxide or Na2SO3 in distilled water prior to use following
Ref. [19].
Crystallographic data for the structure have been deposited with
the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-736284. Copies of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK (fax: +44 1223 336033 or e-mail: deposit@ccdc.ca-
m.ac.uk).
3.2.2. 1-Nitro-4-trifluoromethylbenzene (2)
Colourless crystals. Yield 1.32 g (69%), m.p. 39–40 8C (pentane),
lit. m.p. 40–41 8C [4]. 19F NMR:
(‘‘d’’, 2H, H-3,5, JHH = 8.4 Hz), 8.35 (‘‘d’’, 2H, H-2,6, JHH = 8.4 Hz)
d
ꢁ63.6 (s, CF3); 1H NMR:
d 7.83
3.1.1. Preparation of copper(I) bromide
[26].
A mixture of 101.2 g (0.45 mol) of CuBr2 and 500 ml of water
was stirred for 20 min at ambient temperature; then 28.9 g
(0.45 mol) of copper powder was added. The reaction mixture was
stirred for 1 h at ambient temperature, afterwards for 2 h at 50–
60 8C before a solution of 1 g of Na2SO3 in 15 ml of water was
added. The reaction mixture was stirred at 50–60 8C for another
hour. The reaction mixture was cooled to ambient temperature;
the precipitate was filtered off, washed with water, methanol,
diethyl ether and dried under reduced pressure at 60 8C for 2 h. The
yield was 121.0 g (93%) of colourless to very pale blue CuBr.
All compounds are characterized by melting or boiling points,
elemental analyses and NMR spectroscopic data. For known
compounds, only melting or boiling points as well as 1H and 19F
NMR data are listed, while values of elemental analyses are
omitted.
3.2.3. 1-Nitro-4-(2,2,2-trifluoroethyl)benzene (3)
Colourless crystals. Yield 1.40 g (68%), m.p. 63–65 8C (pentane),
lit. m.p. 65 8C [4]. 19F NMR:
d
ꢁ65.3 (t, CF3, JFH = 10.7 Hz); 1H NMR:
d
3.50 (q, 2H, CH2, JHF = 10.7 Hz), 7.50 (‘‘d’’, 2H, H-3,5, JHH = 8.7 Hz),
8.20 (‘‘d’’, 2H, H-2,6, JHH = 8.7 Hz) [27].
3.2.4. 4-Trifluoromethylbenzoic acid ethyl ester (4)
Colourless liquid. Yield 1.46 g (67%), b.p. 90–92 8C (10 mm Hg).
lit. b.p. 78–80 8C (5.0 Torr) [28]. 19F NMR:
d
ꢁ63.62 (s, CF3); 1H
NMR:
d 1.40 (t, 3H, CH3, JHH = 7.2 Hz), 4.38 (q, 2H, CH2,
JHH = 7.2 Hz), 7.58 (‘‘d’’, 2H, H-3,5, JHH = 9.0 Hz), 7.90 (‘‘d’’, 2H, H-
2,6, JHH = 9.0 Hz) [26].
3.3. Preparation of pentafluoroethylcopper and its reaction with
organic substrates
3.1.2. Crystal structure determination
To a well stirred mixture of Zn(CF3)Brꢀ2DMF (3.60 g, 10 mmol)
in 20 ml DMF freshly prepared copper(I) bromide (2.88 g,
20 mmol) was added at room temperature. After stirring for
30 min, the formation of CuCF3 was complete and the reaction
mixture was heated under stirring at 50 8C for 4 h, until the
formation of CuC2F5 was complete. The corresponding halogen
containing derivative (5 mmol) was added and the reaction
mixture was stirred for 8 h at 85–90 8C. The reaction was
terminated after signals of CuC2F5 were no longer detected in
the 19F NMR spectra. The mixture was poured into 15% aqueous
ammonia (100 ml) and the organic layer was extracted with
diethyl ether. The ether extract was washed with water, dried over
MgSO4 and the solvent was evaporated. The remainder was
distilled under reduced pressure or crystallized.
Compound 1 forms single crystals which were sealed in glass
capillaries and the suitability was checked with the help of an IP-
diffractometer (STOE IPDS I). The same device was used to collect
the reflection data of the respective best specimen using graphite-
monochromated Mo-K radiation (71.073 pm). The data were
a
corrected for Lorentz and polarization effects. The programs used
in this work are Stoe’s X-Area [20], including X-RED [21] and X-
Shape [22] for data reduction and numerical absorption correction,
and SIR-92 [23] and SHELXL-97 [24] for structure solution and
refinement. All hydrogen atoms were placed in idealized positions
and constrained to ride on their parent atom.
3.2. Reactions of trifluoromethylcopper with organic substrates
To a well stirred mixture of Zn(CF3)Brꢀ2DMF (3.6 g, 10 mmol) in
20 ml DMF freshly prepared copper(I) bromide (2.88 g, 20 mmol)
was added at room temperature. After stirring for 3 h, the
formation of CuCF3 was complete. The corresponding halogen
containing derivative (10 mmol) was added and the reaction
mixture was stirred for 8 h at 85–90 8C. The reaction was
terminated after signals of CuCF3 were no longer detected in the
19F NMR spectra. The mixture was poured into 15% aqueous
ammonia (100 ml) and the organic layer was extracted with
diethyl ether. The ether extract was washed with water, dried over
3.3.1. 1-Nitro-4-pentafluoroethylbenzene (5)
Yellow liquid. Yield 0.63 g (53%), b.p. 43 8C (9.4 ꢃ 10ꢁ2 mbar),
lit. b.p. 50 8C (0.4 mm Hg) [29]. 19F NMR:
d
ꢁ85.0 (s, 3F, CF3),
ꢁ115.9 (s, 2F, CF2); 1H NMR:
d 7.83 (‘‘d’’, 2H, H-3,5, JHH = 9.0 Hz),
8.39 (‘‘d’’, 2H, H-2,6, JHH = 9.0 Hz) [30].
3.3.2. 4-Pentafluoroethylbenzoic acid ethyl ester (6)
Colourless liquid. Yield 0.94 g (70%), b.p. 105–107 8C
(10 mm Hg) [7]. 19F NMR:
d
ꢁ85.2 (s, 3F, CF3), ꢁ115.9 (s, 2F,
CF2); 1H NMR:
d 1.40 (t, 3H, CH3, JHH = 7.2 Hz), 4.38 (q, 2H, CH2,

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M.M. Kremlev et al. / Journal of Fluorine Chemistry 131 (2010) 212–216
JHH = 7.2 Hz), 7.55 (‘‘d’’, 2H, H-3,5, JHH = 9.0 Hz), 7.88 (‘‘d’’, 2H, H-
2,6, JHH = 9.0 Hz).
authors are indebted to Dr. Hendrik T.M. Fischer for solving the
structure of 1.
3.3.3. 2-Pentafluoroethylpyridine (7)
References
Colourless liquid. Yield 1.14 g (58%), b.p. 149–151 8C, lit. b.p.
1
2
152 8C [31]. 19F NMR:
d
ꢁ83.4 (t, CF3, JF,C = 286 Hz, JF,C = 38 Hz,
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d
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(‘‘d’’, 1H, H-6). 13C NMR:
d 119.0 (CF3); 111.2 (CF2); 147.8 (C-2,
2JF,C = 25 Hz); 121.9 (C-3, JF,C = 4 Hz); 137.2 (C-4); 126.3 (C-5);
3
149.9 (C-6).
3.3.4. 2-Pentafluoroethylpyrimidine(8)
Colourless liquid. Yield 1.20 g (69%), b.p. 44–46 8C (9.6 ꢃ
1
2
10ꢁ2 mbar). 19F NMR:
d
ꢁ82.6 (t, CF3, JF,C = 287 Hz, JF,C = 37 Hz,
3JF,F = 1.8 Hz); ꢁ117.7 (q, CF2, 1JF,C = 256 Hz, 2JF,C = 35 Hz,
3JF,F = 1.8 Hz); Lit. 19F NMR:
d
ꢁ80.4 (CF3), ꢁ115.0 (CF2) [25]; 1H
3
NMR,
NMR:
d
d
(ppm): 8.92 (d, 2H, H-4,6); 7.53(t,1H, H-5, JH,H 5 Hz). 13C
118.8 (CF3); 109.5 (CF2); 157.0 (C-2); 123.1 (C-5); 157.9 (C-
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and 45% aqueous KOH (3 ml) was heated at 115 8C for 1 h. The
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with hydrochloric acid to pH 4–5. The precipitate was filtered,
washed with water, dried and crystallized.
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3.4.1. 4-Trifluoromethylbenzoic acid (9)
Colourless crystals. Yield 0.17 g (90%), m.p. 219–221 8C
(benzene), lit. m.p. 220–221 8C [32]. 19F NMR:
d
ꢁ61.3 (s, CF3);
1H NMR:
d 7.70 (d, 2H, H-3,5, JHH = 8.4 Hz), 7.88 (d, 2H, H-2,6,
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Colourless crystals. Yield 0.22 g (92%), m.p. 154–156 8C
(benzene), lit. m.p. 155–156.5 8C [34]. 19F NMR (DMSO-d6):
d
2
ꢁ84.0 (s, 3F, CF3, 1J F,C = 287 Hz, JF,C = 39 Hz), ꢁ114.1 (s, 2F, CF2,
1JF,C = 254 Hz, 2JF,C = 37 Hz); 1H NMR (DMSO-d6):
d 7.84 (‘‘d’’, 2H, H-
2,6, JHH = 8.4 Hz), 8.16 (‘‘d’’, 2H, H-3,5, JHH = 8.4 Hz); 13C NMR
(DMSO-d6):
119.0 (q, CF3, 1JF,C = 287, 2JF,C = 39 Hz), 113.0 (q, CF2,
1JF,C = 254, 2JF,C = 37 Hz), 135.3 (s, C-1), 130.5 (s, C-2,6), 127.2 (s,C-
d
N.O. Brace, Chem. Abstr. 66 (1967) 2388y.
3
2
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Acknowledgements
Financial support of this work by the Deutsche Forschungsge-
meinschaft (grant 436 UKR 113) is gratefully acknowledged. The