Fluorinated Vinylsilanes from the Copper-Catalyzed Defluorosilylation of Fluoroalkene Feedstocks

Article abstract of DOI:10.1002/anie.201710866

Herein, a copper-catalyzed C?F bond defluorosilylation reaction of tetrafluoroethylene and other polyfluoroalkenes is described. Mechanistic studies, based on a series of stoichiometric reactions with copper complexes, revealed that the key steps of this defluorosilylation reaction are 1) the 1,2-addition of a silylcopper intermediate to the polyfluoroalkene and 2) a subsequent selective β-fluorine elimination, which generates a Cu?F species. The β-fluorine elimination is facilitated by Lewis acidic F?Bpin, which is generated in situ during the defluorosilylation.

Full text of DOI:10.1002/anie.201710866

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Accepted Article
Title: Fluorinated Vinylsilanes from the Copper-Catalyzed
Defluorosilylation of Fluoroalkene Feedstocks
Authors: Hironobu Sakaguchi, Masato Ohashi, and Sensuke Ogoshi
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Fluorinated Vinylsilanes from the Copper-Catalyzed
Defluorosilylation of Fluoroalkene Feedstocks
Hironobu Sakaguchi, Masato Ohashi, and Sensuke Ogoshi*
Abstract: Herein, a copper-catalyzed C–F bond defluorosilylation
reaction of tetrafluoroethylene and other polyfluoroalkenes is
described. Mechanistic studies, based on a series of stoichiometric
reactions with copper complexes, revealed that the key steps of this
defluorosilylation reaction are: i) the 1,2-addition of a silylcopper
intermediate to the polyfluoroalkene, and ii) a subsequent selective -
fluorine elimination, which generates a Cu–F species. The -fluorine
elimination is facilitated by the Lewis acidic F–Bpin, which is
generated in situ during the defluorosilylation.
than the corresponding vinylboranes. Although silylcuprations
across C–C multiple bonds are usually reliable and powerful,[9]
silylcuprations of fluorinated alkenes remain unexplored thus far.
Herein, we describe a novel copper-catalyzed transformation of
polyfluoroalkenes into vinylsilane derivatives via cleavage of the
C–F bond.
Organofluorine compounds have attracted much attention on
account of their remarkable applications in pharmaceutical and
materials sciences.[1] So far, most research has been aimed at the
development of methods for the selective introduction of either
fluorine atoms or fluorinated building blocks in organic molecules.
Practical approaches usually include fluorinated organosilicon
reagents
such
as
the
Ruppert–Prakash
reagent
(trifluoromethyltrimethylsilane), which are easy to handle and
store, and exhibit high stability and low toxicity.[2] Fluorinated
vinylsilanes have shown great promise as a powerful tool for the
introduction of fluorinated vinyl moieties in organic molecules,
given that vinylsilanes represent versatile building blocks in
Scheme 1. Synthesis of fluorinated vinylsilanes.
organic synthesis.[3] However,
a straightforward synthetic
approach to fluorinated vinylsilanes remains elusive because
almost all relevant starting materials are either expensive or not
easily available (Scheme 1).[4]
Polyfluoroalkenes such as tetrafluoroethylene (TFE) and its
analogues (1a–1f) are ideal starting materials as they represent
economical organofluorine feedstocks.[1f] However, their use is
mostly limited to the production of polymers or refrigerants for air
conditioning. During our studies on the development of novel uses
for TFE as an ideal fluorinated C2 building block,[5] we
In the presence of 10 mol% CuOtBu and 10 mol% 4,5-bis(di-
phenylphosphino)-9,9-dimethylxanthene (Xantphos), the reaction
of Me2PhSi–Bpin (2a) with TFE (1a) at 100 °C afforded
trifluorovinylphenyldimetylsilane (3a) in 58% yield under
concomitant formation of the undesired 2-trifluorovinyl-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (4a) in 30% yield (Table 1, entry
1).[10] After screening some potential ligands, we discovered that
this catalytic system exhibited the best performance when 1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) was used as a
ligand (entries 2–4, Table S1). Reduction of the catalyst loading
to 5 mol% (IPr)CuOtBu decreased the rate of the reaction (entry
5). The progress of the catalytic reaction was monitored by 19F
NMR spectroscopy, which revealed that 3a was not formed during
the early stages of the reaction (Figure 1, A). During this period,
only 2-silyl-1,1,2,2-tetrafluoroalkylcopper(I) (5; vide infra) was
generated in situ as a possible intermediate. Subsequently, 5
could be generated by 1,2-addition of a silylcopper intermediate
to TFE.[11] The formation of 3a was observed after 7200 s and,
ultimately, 3a was obtained in 60% yield after 19200 s. We believe
that this conversion increase should be induced by F–Bpin
generated in situ during the defluorosilylation. Upon adding 10
mol% of F–Bpin generated from H–Bpin and NEt3∙3HF[12] as an
additive, the reaction proceeded immediately (Figure 1, B), and
the yield of 3a increased to 82% (Table 1, entry 6). This result
suggests that F–Bpin plays a key role in facilitating the -fluorine
elimination that leads to (IPr)CuF. The use of (IPr)CuF instead of
(IPr)CuOtBu resulted in a similar yield of 3a, whereby the addition
of F–Bpin is not required. This may be rationalized by the in-situ
accomplished
the
synthesis
of
2-aryl-1,1,2,2-
tetrafuoroethylcopper species via the carbocupration of TFE.[6a]
We also disclosed a Lewis acid-promoted -fluorine elimination of
such fluoroalkylcopper(I) species to yield trifluorostyrene
derivatives.[6b]
Furthermore,
the
copper-catalyzed
defluoroborylation of polyfluoroalkenes via borylcupration has
very recently been reported.[6c,7,8] However, when using TFE and
its analogues as substrates, some of the defluoroborylated
products could not be isolated due to their high volatility and
sensitivity toward water. Against this background, we envisioned
that the silylcupration of TFE, followed by -fluorine elimination
leading to Cu–F species, should enable a catalytic transformation
into fluorinated vinylsilanes, which are expected to be more stable
[a]
Mr. H. Sakaguchi, Prof. Dr. M. Ohashi, Prof. Dr. 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 for this article is given via a link at the end of
the document.
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generation of F–Bpin from the reaction of (IPr)CuF with 2a as the
On the other hand, (3,3,3-trifluoro-1-propen-2-yl)benzene (1g)
underwent an SN2’-type allylic rearrangement to yield a 3,3-
difluoro-2-phenylallylsilane derivative (3g) in excellent yield. In
contrast, using -trifluorostyrene, chlorotrifluoroethylene,
perfluoropropoxyethylene, or octafluorocyclopentene did not
result in any reaction or side reaction (Scheme S2–S4).
first step of the catalytic reaction.[13]
Table 1. Cu(I)-catalyzed defluorosilylation of TFE (1a) with PhMe2Si–Bpin (2a)a
Table 2. Substrate Scopea
aReactions were conducted in a pressure-tight NMR tube and monitored by 19
F
NMR spectroscopy. bdppf: 1,1’-bis(diphenylphosphino)ferrocene.
Figure 1. Reaction rates in the absence (A: ■; Table 1, entry 5) and presence
(B: ○; Table 1, entry 6) of 10 mol% F–Bpin (generated in situ from H–Bpin and
NEt3∙3HF); reactions were monitored by 19F NMR spectroscopy; conditions:
Me2PhSi-Bpin (0.10 mmol), TFE (3.5 atm, > 3.0 eq), (IPr)CuOtBu (0.01 mmol),
THF-d8 (0.5 mL), 100 °C.
With the optimal reaction conditions in hand, the scope and
limitations of this copper-catalyzed defluorosilylation reaction
were examined for a variety of fluoroalkenes (Table 2). We found
that not only 1a but a variety of other fluoroalkenes are
susceptible to this reaction. When hexafluoropropylene (1b) was
used, the reaction proceeded efficiently and yielded the
corresponding mono-defluorosilylated product (3b) as a mixture
of E/Z regioisomers (Table 2, entry 2). Furthermore, with
perfluorobutadiene (1c), the corresponding fluorovinylsilane (3c)
was obtained in moderate yield (entry 3). It is worth noting that
polydefluorosilylated products were not generated under excess
amount of these polyfluoroalkenes. In addition to these
perfluoroalkenes, the use of vinylidene fluoride (1d) afforded the
respective silylated product (3d) in good yield, albeit (IPr)CuOtBu
had to be employed instead of (IPr)CuF (entry 4). Furthermore,
trifluoromethylated monofluoroalkenes 1e and 1f with fluorine
atoms at the vinyl and allyl positions were defluorosilylated
selectively at the C(sp2)–F bond to afford silylated products 3e
and 3f respectively, in moderate yield (entries 5 and 6). It should
be mentioned that vinyl C–H silylation products were not observed.
aYields determined by 19F NMR spectroscopy; isolated yields given in
parentheses; E/Z ratio given in square brackets. b5 mol% (IPr)CuOtBu was used
instead of 5 mol% (IPr)CuF. c1 eq of 1g and 1.5 eq of 2a were used.
Subsequently, we carried out a series of stoichiometric
reactions to gain deeper insight into the reaction mechanism.[14]
The
structurally
well-defined
silylcopper(I)
complex
(IPr)CuSiMe2Ph,[15] prepared in situ from the reaction of
(IPr)CuOtBu with 2a, reacted smoothly with TFE to generate 2-
silyl-1,1,2,2-tetrafluoroalkylcopper(I) complex 5 in 98% yield
(Scheme 2). The molecular structure of 5 was unambiguously
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determined by single-crystal X-ray diffraction analysis (Figure
2).[16]
Scheme 2. Synthesis of 2-silyl-1,1,2,2-tetrafluoroalkylcopper(I) complex 5.
Scheme 3. -fluorine elimination reactions with 5 (n.d.: not detected).
Furthermore, we examined the reactivity of Cu(I) complexes
5 and 6 toward 2a. In the presence of 1e as a silylcopper
scavenger, the reaction of 5 with 2a occurred at 100 °C to furnish
defluorosilylated 3a in 48% yield (Scheme 4). This result clearly
rules out another possible route to 3a during the catalytic reaction
via a transmetallation between 6 and 2a, i.e., 6 reacts with 2a to
selectively afford the defluoroborylated product 4a in 47% yield
under the same reaction conditions. It is worth noting that only the
corresponding defluorosilylated product (3e) was observed in
both reactions. These results clearly indicate that the silylcopper
species is regenerated from the reaction of the Cu complex with
2a.
Figure 2. Molecular structure of 5 with thermal ellipsoids set at 30% probability;
hydrogen atoms omitted for clarity, and only selected atoms labelled. Selected
bond lengths (Å) and angle (deg): C1–C2 1.522(3), Cu–C2 1.931(2), Cu–C3
1.897(2), C2–Cu–C3 175.16(9).
To clarify the reaction pathway leading to 3a, as well as the
key role of F–Bpin in the catalytic reaction, the reactivity of 5 was
investigated further. The thermolysis of 5 in THF at 100 °C
afforded trifluorovinylcopper(I) complex 6 in 48% yield instead of
the expected catalytic reaction product 3a (Scheme 3, A). The
formation of 6 was attributed to a -fluorine elimination leading to
F–SiMe2Ph.[17] This behavior of silylcupration product 5 stands in
stark contrast to that of the transient borylcupration analogue
pinBCF2CF2Cu(IPr) (7). Treatment of (IPr)CuBpin with TFE in
THF at room temperature led to the quantitative formation of 6
within a few minutes, and intermediates (including 7) were not
detected during the reaction.[6c] The differences in reactivity
between 5 and 7 toward the -fluorine elimination leading to 6
may reflect differences in the fluorophilicity of the SiMe2Ph and
Bpin moieties. On the other hand, 5 underwent a different type of
-fluorine elimination in the presence of F–Bpin at 40 °C, which
afforded 3a in 85% yield.[18] It is worth noting that 5 remained
intact at the same temperature in the absence of F–Bpin (Scheme
3, B). This result strongly suggests that the facile formation of 3a
during the catalytic reaction in the presence of F–Bpin is the result
of a Lewis-acid-promoted bimolecular -fluorine elimination.[6b,19]
Scheme 4. Reactivity of Cu(I) complexes toward 2a.
On the basis of the results of the aforementioned
stoichiometric experiments, we would like to propose a feasible
reaction mechanism (Scheme 5).[20] The transmetallation of
fluorocopper A with 2a should afford silylcopper B and F–Bpin.
Subsequently, the silylcupration of TFE should afford
fluoroalkylcopper C, under subsequent generation of
a
deflurosilylated product and regeneration of the fluorocopper
species upon -fluorine elimination promoted by F–Bpin.
However, in the case where -fluorine elimination affords a
fluorosilane, fluorovinylcoppper D should be generated, which
would react with 2a to afford a silylcopper and a defluoroborylated
product.
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[1]
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The thus obtained silylated products may also serve as useful
synthetic units for the introduction of fluoroalkene moieties
(Scheme 6).[21] For example, the copper-mediated cross-coupling
reaction
of
trifluorovinylphenyldimethylsilane
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Scheme 6. Cross-coupling reaction between 3a and iodobenzene.
In summary, we have developed a novel copper-catalyzed
defluorosilylation reaction of fluoroalkenes with silylborane. Our
mechanistic studies indicate that the key steps of this
defluorosilylation reaction are the 1,2-addition of a silylcopper
intermediate to the polyfluoroalkene, and a subsequent -fluorine
elimination leading to the corresponding vinylsilane and the
regenerated fluorocopper. We also discovered the role of F–Bpin,
which is generated in situ during the defluorosilylation, and
facilitates the -fluorine elimination. Further studies, including
detailed mechanistic investigations and further synthetic
applications, are currently underway in our laboratory.
Acknowledgements
[5]
This research was supported by JSPS KAKENHI Grant Number
16H02276 (S.O.); JSPS KAKENHI Grant Number 16KT0057 and
17H03057 (M.O.); JSPS KAKENHI Grant Number 16J00573
(H.S.); H.S. is grateful for support as a JSPS Research Fellow.
Keywords: silylcupration · organofluorine compounds·
organosilicon compounds · copper complex · -fluorine
elimination
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10.1002/anie.201710866
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The generation of 5 was confirmed by 19F NMR, see Figure S1.
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Scheme S14). See the Supporting Information for details.
[14] Stoichiometric reaction using HFO-1234yf (1e) as a substrate was also
conducted. Me2PhSiCH2CF(CF3)Cu(IPr) (8) and CH2=C(CF3)Cu(IPr) (9)
were isolated. See the Supporting Information for details.
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[9]
[16] CCDC
1564010 (5), 1578017 (8) and 1578089 (9) contain the
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obtained free of charge from The Cambridge Crystallographic Data
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[20] A possible reaction mechanism for the defluorosilylation of 1,1-substitute
fluoroalkenes (1d and 1e), based on several stoichiometric reactions,
was depicted on Scheme S8. See the Supporting Information for details.
[21]
3a is applicable to not only cross-coupling reaction but also other
transformation reactions. For 1,2-addition to aldehyde, see a) M. Fujita,
M. Obayashi, T. Hiyama, Tetrahedron 1988, 44, 4135; b) M. Fujita, T.
Hiyama, J. Am. Chem. Soc. 1985, 107, 4085. For metalative C(sp2)–F
bond functionalization, see c) H.-J. Tsai, C.-W. Hsieh, S.-Y. Wu,
Phosphorus, Sulfur, and Silicon and the Related Elements 2007, 182,
491; d) J. Kvíčala, R. Hrabal, J. Czernek, I. Bartošovꢀ, O. Paleta, A.
Pelter, J. Fluorine Chem. 2002, 113, 211; e) P. Martinet, R. Sauvetre, J.-
F. Normant, J. Organomet. Chem. 1989, 367, 1.
[10]
The reaction mixture must be handled in a well-ventilated fume hood.
See the Supporting Information for details.
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10.1002/anie.201710866
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Hironobu Sakaguchi, Masato Ohashi,
and Sensuke Ogoshi*
Page No. – Page No.
Fluorinated Vinylsilanes from the
Copper-Catalyzed Defluorosilylation
of Fluoroalkene Feedstocks
Copper-catalyzed defluorosilylation reaction of fluoroalkene feedstocks is disclosed.
Mechanistic studies revealed that the key steps of this defluorosilylation reaction
are: i) the 1,2-addition of a silylcopper intermediate to the polyfluoroalkene, and ii) a
subsequent selective -fluorine elimination, which generates a Cu–F species. The
-fluorine elimination is facilitated by the Lewis acid F–Bpin, which is generated in
situ during the defluorosilylation.
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