Synthesis and Reactivity of Fluoroalkyl Copper Complexes by the Oxycupration of Tetrafluoroethylene

Article abstract of DOI:10.1002/anie.201703923

The copper(I)-mediated generation of -OCF₂CF₂- moieties by the oxycupration of tetrafluoroethylene (TFE) using either copper aryloxides or alkoxides is disclosed. The key intermediates, 2-aryloxy-1,1,2,2-tetrafluoroethyl and 2-alkoxy

Full text of DOI:10.1002/anie.201703923

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International Edition: DOI: 10.1002/anie.201703923
German Edition: DOI: 10.1002/ange.201703923
Synthetic Methods
Hot Paper
Synthesis and Reactivity of Fluoroalkyl Copper Complexes by the
Oxycupration of Tetrafluoroethylene
Masato Ohashi, Takuya Adachi, Naoyoshi Ishida, Kotaro Kikushima, and Sensuke Ogoshi*
Abstract: The copper(I)-mediated generation of -OCF2CF2-
moieties by the oxycupration of tetrafluoroethylene (TFE)
using either copper aryloxides or alkoxides is disclosed. The
key intermediates, 2-aryloxy-1,1,2,2-tetrafluoroethyl and 2-
alkoxy-1,1,2,2-tetrafluoroethyl copper complexes, were
obtained from the reaction of the corresponding aryloxy and
alkoxy copper complexes with TFE, and their structures in
solution and in the solid state were unambiguously determined
by multinuclear NMR spectroscopy and X-ray diffraction
analysis. These copper complexes subsequently reacted with
aryl iodides (ArI) to afford ROCF2CF2Ar (R = aryl or alkyl) in
high yields.
O
rganofluorine compounds have attracted much attention,
mostly on account of their applications in a variety of research
areas, including pharmaceutical, agrochemical, and materials
science,[1] and consequently substantial efforts have been
devoted to the development of novel strategies for the
construction of fluorinated organic compounds. Among these,
fluoroalkyl ethers such as Ar-OCF2CF2-Ar’ (Ar/Ar’ = aryl)
have garnered special attention as they represent key
structures in insecticides and lubricants (Scheme 1a).[2] In
addition, this structural motif is fascinating with respect to
perfluoroalkoxylation of aromatic compounds.[3] A practical
approach to the construction of Ar-OCF2CF2-Ar’ moieties
would be the successive substitution of an aryloxy and an aryl
group for two halogen atoms in 1,2-dihalotetrafluoroethanes,
such as 1,2-dibromotetrafluoroethane (Halon-2402) or 1,2-
Scheme 1. Preparation of tetrafluoroethylene-bridging organofluorine
compounds.
starting material for the preparation of fluorinated com-
pounds, considering that it is a cost-effective and environ-
mentally benign feedstock with negligible GWP100 and ODP
values.[8] In this context, our group has developed new
transformations of TFE to furnish a variety of valuable
organofluorine compounds.[9] For example, we devised
a copper-mediated transformation of TFE into 1,2-diaryl-
tetrafluoroethane derivatives, in which the carbocupration of
TFE is a key step (Scheme 1c).[10] Given that the number of
reports on intramolecular oxycupration reactions remains
limited,[11] we subsequently envisioned that a fluoroalkylcop-
per species, with a terminal aryloxy group (ArOCF2CF2-Cu),
should be generated from the oxycupration of TFE by an
aryloxycopper species (Scheme 1d). The resulting fluoroal-
kylcopper species should be an ideal precursor for the
versatile construction of Ar-OCF2CF2-Ar’ derivatives. Con-
sequently, we set out to investigate the reactivity of copper
phenoxides and alkoxides toward TFE.
The reaction of [(phen)CuOPh] (1a; phen = 1,10-phenan-
throline), which was generated in situ from the reaction of
[CuMes][12] and phenol (2a) in the presence of phen,[13] with
TFE underwent the desired oxycupration, thus yielding
[(phen)CuCF2CF2OPh] (3a) in 86% yield upon isolation
(Scheme 2).[14] It should be mentioned that employing phen as
an auxiliary ligand is crucial for the oxycupration of TFE
toward a Cu-OPh fragment, as is evident from the reaction of
a mixture of [CuMes] and 2a with TFE in the absence of
phen, which did not afford the corresponding fluoroalkyl
copper complex. The role of phen would be to disperse the
aggregated structure of (PhOCu)n,[13c–g] and/or to generate
diiodotetrafluoroethane
(Scheme 1b).[2b,c,4,5]
However,
Halon-2402 has been banned from the chemical industry
because of its detrimental impact on the global environment,
and is reflected in the global warming potential (GWP100
=
1640) and its ozone depletion potential (ODP = 6.0).[6] More-
over, harsh reaction conditions are required to prepare 1,2-
diiodotetrafluoroethane by the addition of iodine to tetra-
fluoroethylene (TFE), and the yields are typically low (ca.
20%).[7] The development of alternative strategies for the
preparation of the target fluoroalkyl ethers is therefore in
high demand. TFE, which is conventionally employed as
a monomer for the generation of tetrafluoroethylene-based
polymers and co-polymers, should in contrast be an ideal
[*] M. Ohashi, T. Adachi, N. Ishida, K. Kikushima, S. Ogoshi
Department of Applied Chemistry, Faculty of Engineering
Osaka University
Suita, Osaka 565-0871 (Japan)
E-mail: ogoshi@chem.eng.osaka-u.ac.jp
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201703923.
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Table 1: Derivatization of 3a.[a]
Scheme 2. Preparation of fluoroalkylcopper species by the oxycupration
of TFE. THF=tetrahydrofuran.
bis(phenoxy)cuprate, [(PhO)2Cu]À as an equilibrium mix-
ture.[13a] The molecular structure of 3a was unambiguously
determined by X-ray diffraction analysis, which revealed that
the termini of the tetrafluoroethylene chain are capped with
a phenoxy and a (phen)Cu fragment (Figure 1).[15] In the
Figure 1. ORTEP drawings of 3a with thermal ellipsoids at the 30%
probability level. H atoms are omitted for clarity.
crystal lattice of 3a, the copper center adopts a monomeric,
slightly distorted Y-shaped coordination geometry, with
a bond angle sum of 359.68 around the copper center. A
multinuclear NMR analysis of a THF solution of 3a revealed
that 3a exists in solution as an equilibrium mixture of
a neutral form, [(phen)CuCF2CF2OPh] (3aneutral), and an ionic
form, [(phen)2Cu][Cu(CF2CF2OPh)2] (3aionic).[16] Such
a dynamic behavior of 3a in solution is consistent with
previous studies on fluoroalkyl copper complexes.[10a,17]
When isolated 3a was treated with a variety of iodoarenes
in THF at 608C, the corresponding 1-aryl-2-phenoxy-1,1,2,2-
tetrafluoroethanes were obtained in good to excellent yields
(Table 1a).[18] The reaction of 3a with iodobenzene (4a)
afforded 2-phenoxy-1,1,2,2-tetrafluoroethylbenzene (5aa) in
88% yield. The use of electron-deficient iodoarenes, such as
4-iodobenzotrifluoride (4b), 4-iodobenzonitrile (4c), and 4-
bromo-2-chloro-1-iodobenzene (4d), in the reaction with 3a
furnished the corresponding products (5ab–ad) in excellent
yields. The bromine and chlorine substituents in 5ad could be
used for further functionalization reactions. In contrast, the
use of iodoarenes bearing an electron-donating group (4e–g)
required prolonged reaction times for high product yields
(> 85%). 2-Iodopyridine (4h) could also be used in this
reaction, thus resulting in the formation of the corresponding
phenyl fluoroalkyl ether with a pyridyl group (5ah) in 98%
yield.
[a] Yields are those of isolated products. [b] Yields are estimated from 19
NMR spectroscopy. NBS=N-bromosuccinimide.
F
preparation of 1-aryl-2-aryloxy-1,1,2,2-tetrafluoroethanes
without the isolation of the key fluoroalkylcopper intermedi-
ate (Table 1b). Thus, the aryloxycopper species was initially
generated in situ from the reaction of [CuMes], phen, and the
respective phenol derivative (2a–e), before it was exposed to
TFE at 408C to undergo the oxycupration reaction. After
removing any excess of TFE, the reaction mixture was heated
to 608C for 2 hours in the presence of 1.2 equivalents of 4b.
As a result, the corresponding aryl fluoroalkyl ethers (5ab–
eb) were obtained in good to excellent yields. a-Naphthol
(2 f) could also be used in this one-pot reaction to afford 5 fb,
albeit that an extension of the reaction time was required to
complete the oxycupration step. In addition, the phenol
derivative 5ag, which was prepared from 3a and 4g, was also
susceptible to the oxycupration of TFE, and led to the
construction of a structure with consecutive -ArOCF2CF2-
units (5gb).
The fluoroalkyl copper species 3a could also be derivat-
ized by upon treatment with various electrophiles (Table 1c).
The reaction with I2 and NBS in THF at room temperature
resulted in a halogenation to furnish 1-iodo- (6) and 1-bromo-
In addition to the aforementioned stepwise reaction using
isolated 3a, a one-pot protocol could be applied to the
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Table 2: Reaction of 3a with MgBr2.
1,1,2,2-tetrafluoro-2-phenoxyethane (7), respectively. Treat-
À
ment with benzyl chloroformate induced a C C bond
formation reaction which led to fluorinated ester 8 in 97%
yield.
The present system, which involves the oxycupration of
TFE as a key step, was then applied to the formal synthesis of
the insecticide A (Scheme 1a). The key synthetic precursor
(9) could be prepared in 90% overall yield by the reaction of
[CuMes], commercially available 4-nitroguaiacol, and TFE
(Scheme 3). This synthetic protocol differed from the pre-
vious report, which used Halon-2402 for the preparation of 9.
Entry
Solvent
T [8C]
t [h]
Yield [%][a]
10
11
1
2
3
THF/[D8]THF
DMF/C6D6
DMF/C6D6
RT
40
RT
0.5
60.0
0.5
37
75
0
23
0
0
[a] Yields are estimated from 19F NMR spectroscopy. DMF=N,N-
dimethylformamide.
elimination, thus generating a fluorocarbene species, which
could subsequently undergo dimerization followed by fluo-
rine migration to afford 11.[10b] The gradual but selective
transformation of 3a into 10 was accomplished by treatment
of the former with MgBr2 in DMF/C6D6 at 408C for 60 hours
to yield the latter in 75% yield (entry 2). A similar reaction
conducted at room temperature did not generate any 10
(entry 3).[20] At room temperature, the coordination of DMF
to MgBr2 might decrease its Lewis acidity, thus compromising
the interaction between magnesium and fluorine. This result
inspired us to employ DMF as a solvent, given that it should
enable the oxycupration of 1a generated from a mixture of
copper(I) halide, metal phenoxide, and phen. The resulting
metal halide, which was generated as a result of trans-
metallation between the copper(I) halide and metal phenox-
ide, should then be inert in DMF solution toward the
oxycupration product. Indeed, our previous attempts to
generate [(phen)CuCF2CF2Ph] from the reaction of
PhMgBr with CuI in the presence of phen in THF were
unsuccessful, since the inevitable formation of magnesium
halides promoted a decomposition of the desired carbocup-
ration product to produce trifluorostyrene.[10a]
Scheme 3. Preparation of the key synthetic precursor 9 for the syn-
thesis of insecticide A.
Subsequently, the oxycupration of TFE was successfully
expanded to copper alkoxides: the reaction of [(phen)-
CuOtBu] (1h), which was generated in situ from [CuOtBu],
an equimolar amount of phen, and TFE, underwent the
desired oxycupration to yield [(phen)CuCF2CF2OtBu] (3h) in
85% yield upon isolation (Scheme 4). Treatment of 3h with4-
Although the exposure of a DMF solution of an equi-
molar mixture of CuCl, NaOPh, and phen to TFE afforded 3a
in 60% yield, the use of 2 equivalents of NaOPh improved the
yield of 3a to 99% yield (Table 3, entries 1 and 2). In contrast,
employing CuI instead of CuCl decreased in the yield of 3a to
46% (entry 3), which is probably due to the formation of an
analogue of the cationic cluster [K8Cu6(OtBu)12(DMF)8(I)]-
[I],[21] whose rigid structure might hamper the reaction with
Scheme 4. Preparation of fluoroalkylcopper complexes from the oxy-
cupration of TFE.
iodobiphenyl (4i) at 608C for 24 hours afforded 2-tert-butoxy-
1,1,2,2-tetrafluoroethyl-4-phenylbenzene (5hi) in quantitative
yield. Furthermore, the corresponding fluoroalkyl copper
species with a terminal ethoxy group (3i) could be isolated in
87% yield by the oxycupration of TFE with [(phen)CuOEt]
(1i), which was prepared by the reaction of [CuMes], phen,
and ethanol. The reaction of 3i with 4i furnished the
corresponding coupling reaction product (5ii) in excellent
yield.
Table 3: Preparation of 3a from CuX and NaOPh in the presence of
phen.
In anticipation of the formation of phenyl trifluorovinyl
ether (10),[10a] the reaction of 3a with MgBr2 was carried out.
However, when the reaction was conducted in THF at room
temperature, the yield of 10 was only 37%, while the
unexpected fluorinated compound 11 was generated in 23%
yield (Table 2, entry 1). In this reaction, MgBr2 should
initially interact with 3a, before a b-fluorine elimination is
facilitated by a six-membered transition state,[19] which would
lead to the formation of 10. At the same time, such an
interaction should concomitantly facilitate an a-fluorine
Entry
CuX
NaOPh
(x equiv)
Solvent
Yield [%][a]
3a
10
1
2
3
4
CuCl
CuCl
CuI
1.0
2.0
2.0
2.0
DMF/C6D6
DMF/C6D6
DMF/C6D6
THF/[D8]THF
60
99
46
0
0
0
0
CuCl
16
[a] Yields are estimated from 19F NMR spectroscopy.
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TFE. Conversely, 3a was not generated in THF (entry 4),
albeit that the formation of 10 (16%) was detected.
JSPS. We acknowledge the help of Dr. Kyoko Inoue for
measuring 19F NMR inversion recovery spectra.
Based on the fact that the reaction of CuI with 2 equiv-
alents of NaOtBu in the presence of phen yielded [Cu-
(OtBu)2][(phen)Na],[22] the result of entry 2 (Table 3) indi-
cated that [Cu(OPh)2] should be sufficiently reactive to
facilitate the oxycupration to TFE without the need of phen
as an auxiliary ligand for the copper center. Indeed, when
a DMF solution of a 1:2 mixture of CuCl and NaOPh was
exposed to TFE at room temperature for 8 hours in the
absence of phen, the 19F analysis clearly demonstrated that
two types of fluoroalkylcopper species, that is,
Conflict of interest
The authors declare no conflict of interest.
Keywords: copper ·ethers ·reaction mechanisms ·
structure elucidation ·synthetic methods
How to cite: Angew. Chem. Int. Ed. 2017, 56, 11911–11915
Angew. Chem. 2017, 129, 12073–12077
[CuCF2CF2OPh]
(12neutral
)
and
[(DMF)2Na]-
[Cu(CF2CF2OPh)2] (12ionic) were generated in 82% and
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Scheme 5. Preparation of fluoroalkylcopper complexes in the absence
of phen.
copper(I) phenoxide did not show any reactivity toward the
oxycupration of TFE (Scheme 2) and that the treatment of
CuCl with 2 equivalents of NaOtBu in DMF afforded
[(DMF)2Na][Cu(OtBu)2],[17,24]
[(DMF)2Na][Cu(OPh)2]
should be the active species towards the oxycupration of
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12ionic with 4b at 1008C afforded 5ab in 92% yield upon
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and a stoichiometric amount of copper(I) salt is required for
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to the preparation of fluoroalkyl copper species from readily
available reagents.
In conclusion, we have demonstrated the synthesis,
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oethyl and 2-alkyloxy-1,1,2,2-tetrafluoroethyl copper com-
plexes, which are generated from the oxycupration of TFE. A
variety of 1-aryloxy- and 1-alkyloxy-2-aryl-1,1,2,2-tetrafluor-
oehtanes could thus be synthesized using these copper
complexes as a fluoroalkylation reagent, and the synthetic
utility was demonstrated by the formal synthesis of the
insecticide A. Moreover, the use of a bis(aryloxy)cuprate in
DMF enabled the direct preparation of such fluoroalkyl
copper reagents in the absence of the auxiliary phen ligand.
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Acknowledgments
This work was supported by Grant-in-Aid for Scientific
Research (A) (No. A16H02276) and (B) (No. 16KT0057) and
Grant-in-Aid for Young Scientists (A) (No. 25708018) from
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cross-coupling reaction with iodoarenes would be not 3aionic but
3aneutral. This conclusion is consistent with the previous reports by
Vicic. See also the Ref. [17b]. For details, see the Supporting
Information. It should be mentioned that our kinetics experi-
ments could not completely rule out the possibility that both
3aneutral and 3aionic would be active toward the coupling reaction.
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in DMF/C6D6, 10 was generated in 9% yield.
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[23] For the characterization of the fluorocopper complexes and the
attempts to expand this reaction to copper(I)-catalyzed reac-
tions, see the Supporting Information.
[24] A. Lishchynskyi, V. V. Grushin, J. Am. Chem. Soc. 2013, 135,
12584.
[15] CCDC 1544176 (3a) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre.
[16] The equilibrium constants in THF, K = [3aneutral]2/[3aionic]2, were
2.23 ” 10À3 (at 258C) and 2.48 ” 10À3 (at 608C), respectively. For
details, see the Supporting Information.
Manuscript received: April 16, 2017
Revised manuscript received: May 25, 2017
Accepted manuscript online: June 6, 2017
Version of record online: July 14, 2017
Angew. Chem. Int. Ed. 2017, 56, 11911 –11915
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 11915