Perfluorozinc aromatics by direct insertion of zinc into C-F or C-CL bonds

Article abstract of DOI:10.1016/S0040-4039(00)00527-X

For the first time perfluoroaromatic organozinc compounds were obtained by direct action of zinc on C-F bonds in the presence of metal salts (SnCl₂, CuCl₂, ZnBr₂). The reactions are accelerated by ultrasound. The scope of

Full text of DOI:10.1016/S0040-4039(00)00527-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 3817–3819
Perfluorozinc aromatics by direct insertion of zinc into C–F or
C–Cl bonds
Alexey O. Miller,a,∗ Vyacheslav I. Krasnov,b Dietmar Peters,a Vyacheslav E. Platonov b and
Ralf Miethchen a
aUniversity of Rostock, Department of Chemistry, Buchbinderstr. 9, D-18051 Rostock, Germany
bN.N. Vorozhtsov Institute of Organic Chemistry Siberian Division of Russian Academy of Sciences, Lavrentiev Ave. 9,
630090 Novosibirsk, Russia
Received 2 March 2000; accepted 27 March 2000
Abstract
For the first time perfluoroaromatic organozinc compounds were obtained by direct action of zinc on C–F bonds
in the presence of metal salts (SnCl2, CuCl2, ZnBr2). The reactions are accelerated by ultrasound. The scope of
the method can be extended to polyfluoroaromatics which contain other halogen atoms (Cl) in the aromatic ring.
© 2000 Elsevier Science Ltd. All rights reserved.
Organozinc compounds are well known organometallics1 but their use in synthetic organic chemistry
except for the Reformatsky reaction is limited as they were replaced by the more active organolithium
and organomagnesium compounds. Recently, the relative stable organozinc compounds are used as
precursors in order to prepare more reactive organometallics by transmetalation procedures.2 Therefore,
simple methods for the synthesis of organozinc compounds from accessible precursors are desired.
Perfluorinated zinc aromatics are directly formed from iodo- and bromoarenes and zinc or dimethylzinc.3
Activated arenes with C–Cl bonds can also be used.4,5 In contrast, there are no reports in the literature
on the direct formation of a C–Zn bond from a C–F bond in a perfluoroarene. There are few examples
concerning the direct insertion of other metals into a Carom.–F bond but the yields are poor.6,7
We have found that aryl zinc compounds 2a–d are formed at room temperature from perfluoroarenes
1a–d by reaction with zinc in DMF in the presence of SnCl2 (Scheme 1).8 The corresponding organozinc
compounds were characterized by 19F NMR spectra.9 All signals can be assigned as signals from fluorine
located in the aromatic part of the molecule or from the corresponding CF3 groups. The signals of
the ortho-fluorine in perfluorozinc arenes are characteristically shifted compared to the perfluorinated
precursor as reported in the literature for other perfluorometallic aromatics.10 Fluoride signals of Zn–F
were not observed in our spectra. Thus, the insertion of zinc into the C–F bond is accompanied by an
F–Cl exchange resulting in the formation of ArZnCl species.
∗
Corresponding author.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00527-X
tetl 16806

3818
Scheme 1.
In analogy to reports in the literature11 we observe an equilibrium between Ar2Zn and Ar-Zn-Hal
species in solution, but the content of the Ar-Zn-Hal species directly depends on the amount of the
catalyst SnCl2. The addition of ZnBr2 to the reaction solution shifted the equilibrium nearly completely
to the Ar-Zn-Hal species whereas KF does not influence the products ratio at all. Other solvents and salts
can also be used for the reaction. However, the reaction rate decreased in THF or if ZnBr2 or CuCl2 was
used. Furthermore, perfluorodiaryls were especially found as by-products if the reaction was promoted
by CuCl2. The reaction failed without addition of salts. It is possible to use ultrasound or heating to
accelerate the formation of the organozinc compounds. As far as the perfluorinated arene precursor is
concerned, we found the following range of substrate reactivity (Scheme 2):
Scheme 2. Percentage conversion of perfluoroarenes with Zn/SnCl2 in DMF (sonication, 20°C, 10 h)
Scheme 2 represents the conversion of the starting compounds into perfluorozincarenes under similar
conditions.12 Without the application of ultrasound the conversion of octafluorotoluene is reduced from
25% to less than 10%. No reaction was found for hexafluorobenzene and perfluoro-p-xylene at room
temperature even after 24 h. Pentafluoropyridine always gives, irrespective of the salt used, an amount of
perfluorobipyridyl. If water is added to the reagents mixture, hydrodefluorination takes place.
To confirm the formation of organozinc compounds 2 the reaction mixture was treated with water,
bromine and copper (II) chloride, respectively. The reaction products 3–5 (Scheme 3) are obtained in
good yield.13 The reaction of 4-tetrafluoropyridylzinc 2c with copper(I) chloride gives the corresponding
copperarene.14
Scheme 3.
It was also interesting to evaluate the reaction of chloropolyfluoroarenes under the conditions applied
for the formation compounds 2a–d. Thus, chloropentafluorobenzene reacts with zinc in the presence of
SnCl2 at room temperature to give pentafluorophenyl zinc chloride after 40 h in 60% yield. After five
days starting material was not detectable by 19F NMR spectroscopy. The ratio of C6F5ZnCl:(C6F5)2Zn
(11.5:1) determined in DMF:CDCl3 (1:1) at room temperature was similar to literature data.11

3819
Acknowledgements
The authors thank the Deutsche Forschungsgemeinschaft for financing the stay of Dr. Krasnov at the
University of Rostock, Department of Organic Chemistry, and the Russian Foundation for Fundamental
Research (Project N98-03-32966a) for financial support.
References
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8. Formation of organozinc compounds. A mixture of the perfluoroarene (6 mmol), Zn (30 mmol), and SnCl2 (1 mmol) in
dry DMF (8 ml) was treated in a closed flask with ultrasound12 or stirring. After the necessary time (5–24 h) 150–200 mg
of the solution were diluted with CDCl3 (∼1:1) and 19F NMR spectra were recorded.
9. 19F NMR (C6F6, DMF–CDCl3): Compound 2a (Y=Cl): 47.1 (2-F, 6-F), 26.9–27.1 (3-F, 5-F); J2,3=30.0 Hz, J2,5=18.0 Hz,
J2,6 and J3,5=7.7 Hz and 3.3 Hz. Compound 2b (Y=Cl): 105.8 (CF3, J=21.6 Hz), 45.9 (2-F, 6-F), 19.4 (3-F, 5-F); J2,3=30.5
Hz, J2,5=17.8 Hz, J2,6 and J3,5=7.3 Hz and 3.8 Hz. Compound 2c (Y=Cl): 64.6 (2-F, 6-F), 40.0 (3-F, 5-F); J2,3=34.2 Hz,
J2,5=26.7 Hz, J2,6=11.7 Hz, J3,5≈0 Hz. Compound 2d (Y=Cl): 105.9 (dd, J3F=J5F=22.4 Hz, 4-CF3), 103.1 (dd, J3F=19.2
Hz, J5F=1.3 Hz, 2-CF3) 47.8 (6-F) 43.6 (3-F), 30.6 (5-F), J3,6=20.5 Hz, J5,6=30.9 Hz. 4-Tetrafluoropyridylcopper: 63.5 (m,
2-F, 6-F), 45.7 (m, 3-F, 5-F).14 C6F5ZnCl: 45.9 (2-F, 6-F); 4.0 (4-F), −0.3 (3-F, 5-F); J2,3=31.8 Hz, J2,4=0 Hz, J2,5=13.9 Hz,
J3,4=19.3 Hz, J2,6 and J3,5=7.7 Hz and 3.1 Hz.
10. Herberg, S.; Lang, H.; Naumann, D. J. Fluorine Chem. 1987, 35, 267–274.
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12. Ultrasonic bath Sonorex RK 102 H (Bandelin), 35 kHz, 2×120 Welec, power input: 0.1 Wcal/cm3.
13. Reactions of organozinc compounds. The solution of 2a–d8 was diluted with aq. HCl or separated from the excess of
zinc and then added dropwise under stirring into a solution of Br2 (1.5 equiv.) and CuCl2 (1.2 equiv.) in 8 and 15 ml
DMF, respectively. The mixture was stirred for 1 h and diluted with water. The products 4a–d and 5b–c, respectively, were
separated by steam distillation. Then, liquids were separated and solid products extracted with CH2Cl2. The work-up of
nitrile 5a was made by addition of a fourfold amount of 2% aq. HCl followed by extraction with equal amounts of diethyl
ether. After drying (Na2SO4), evaporating of the solvent and recrystallisation from heptane:CCl4 (1:1) pure 5a was isolated
15
(mp 131–133°C; Ref. 129.5–131°C). The excess of bromine in case of 4a–d was removed by treatment with aqueous
Na2SO3 solution. Compound 4a: mp 77–78°C (heptane); Ref. 15 76–76.5°C. Identification of the compounds was made by
19F NMR spectroscopy.
14. Nguen, B. V.; Burton, D. J. J. Fluorine Chem. 1994, 67, 205–206.
15. Birchall, J. M.; Haszeldine, R. N.; Jones, M. E. J. Chem. Soc. C. 1971, 7,1343–1348.