Palladium-catalyzed base-free Suzuki-Miyaura coupling reactions of fluorinated alkenes and arenes via a palladium fluoride key intermediate

Article abstract of DOI:10.1002/ejoc.201201405

A new strategy for C-C bond formation with organoboronates through C-F activation of fluorinated alkenes and arenes was developed. In this Pd-catalyzed Suzuki-Miyaura-type cross-coupling reaction, neither a base for enhancing the reactivity of the organoboron reagents nor a Lewis acid for promoting C-F bond activation was required. A fluoropalladium intermediate played an essential role in this reaction. In addition, a Ni(NHC) catalyst was efficient for C-C coupling through C-F bond activation of fluoroarenes. Pd⁰/PR ₃ complexes promote C-C bond formation with organoboronates through C-F bond activation of fluorinated alkenes. Mechanistic studies show that a Pd^II fluoride intermediate plays an essential role in this base-free cross-coupling reaction. Moreover, a Ni(NHC) catalyst is efficient for C-C coupling through C-F bond cleavage of fluoroarenes. Copyright

Full text of DOI:10.1002/ejoc.201201405

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201405
Palladium-Catalyzed Base-Free Suzuki–Miyaura Coupling Reactions of
Fluorinated Alkenes and Arenes via a Palladium Fluoride Key Intermediate
Masato Ohashi,[a] Hiroki Saijo,[a] Mitsutoshi Shibata,[a] and Sensuke Ogoshi*[a]
Keywords: C–F bond activation / Palladium / Cross-coupling / Boron / Fluorine
A new strategy for C–C bond formation with organoboron-
ates through C–F activation of fluorinated alkenes and ar-
enes was developed. In this Pd-catalyzed Suzuki–Miyaura-
type cross-coupling reaction, neither a base for enhancing
the reactivity of the organoboron reagents nor a Lewis acid
for promoting C–F bond activation was required. A fluoropal-
ladium intermediate played an essential role in this reaction.
In addition, a Ni(NHC) catalyst was efficient for C–C cou-
pling through C–F bond activation of fluoroarenes.
compounds (Scheme 1b). Herein, we report the first base-
free C–C bond-forming reactions of perfluoroalkenes with
arylboronates in the presence of a Pd0 catalyst. This reac-
tion was successfully expanded to other fluorinated alkenes
as well as fluorinated arenes.
Introduction
Over the past few decades, the transition-metal-mediated
activation of carbon–fluorine bonds has attracted much at-
tention because this bond is thermally and chemically ro-
bust and because this bond can undergo a number of
unique functionalization reactions for the preparation of in-
tricate compounds that are otherwise difficult to prepare.[1]
We have recently demonstrated the palladium-catalyzed
coupling reaction of tetrafluoroethylene (TFE) with aryl-
zinc compounds to yield (α,β,β-trifluoro)styrene deriva-
tives.[2] Our next concern is to apply the C(sp2)–F bond acti-
vation methodology to Suzuki–Miyaura-type C–C bond-
forming reactions, which generally tolerate a broad range
of functional groups.[3] So far, most of the reported Suzuki–
Miyaura-type reactions that occur through C–F bond acti-
vation, which are performed with either highly electron-de- Scheme 1. Possible path for the Suzuki–Miyaura coupling reaction
based on C–F bond activation reactions: (a) The previously re-
ficient organofluorine compounds or compounds bearing a
ported reactions conducted in the presence of a base, (b) the key
directing group, have been conducted in the presence of a
role of metal fluoride in novel base-free Suzuki–Miyaura-type reac-
tions, (c) C–F bond cleavage assisted by Lewis acids following C–
base (Scheme 1a),[4] whereas the fluoride anion itself is re-
garded as a good activator for neutral organoboron com-
pounds.[5,6] In fact, Widdowson noted the possibility that
the use of an extraneous base should be, in principle, cata-
lytic;[4a] however, such a reaction has not been developed.
We speculated that if the transition-metal fluorides (R-TM-
F) generated by direct oxidative addition of a C–F bond
would have reactivity that was high enough to act as a
fluoride donor, the development of base-free C–C couplings
with organoboron reagents would be a significant develop-
ment to Suzuki–Miyaura couplings with organofluorine
C bond formation in the presence of base.
Results and Discussion
We first began by seeking an active species that could
cleave the C–F bond of TFE without additives, because our
original protocol involving the use of Lewis acid additives
(MX) inevitably lost the chance to generate the transition-
metal fluoride intermediate in return for efficient C–F bond
cleavage, and the extraneous addition of base was thereby
required (Scheme 1c).[2,7,8] As a result, the thermolysis of
[(η2-CF2=CF2)Pd(PCy3)2] (1a)[9,10] in THF at 100 °C under
a N2 atmosphere underwent C–F bond activation of TFE
to yield expected trifluorovinylpalladium(II) fluoride 2 in
45% yield (Scheme 2). Analysis by NMR spectroscopy re-
vealed the concomitant formation of palladium 2-per-
[a] Department of Applied Chemistry, Faculty of Engineering
Osaka University, Suita, Osaka 565-0871, Japan
Fax: +81-6-6879-7394
E-mail: ogoshi@chem.eng.osaka-u.ac.jp
Homepage: http://www.chem.eng.osaka-u.ac.jp/~ogoshi-lab/
english/indexEN.html
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201405.
Eur. J. Org. Chem. 2013, 443–447
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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M. Ohashi, H. Saijo, M. Shibata, S. Ogoshi
SHORT COMMUNICATION
fluorobutenyl species 3.[11] The recovery of [Pd(PCy3)2]
(26%) indicated the existence of a coordination–dissoci-
ation equilibrium of TFE to palladium under the reaction
conditions. Thus, this reaction was carried out under a TFE
atmosphere (1 atm), and this resulted in an improved yield
of 2. By contrast, C–F bond activation did not proceed at
all by heating the PPh3 analogue, [(η2-CF2=CF2)Pd-
(PPh3)2] (1b); instead, the decomposition of 1b along with
the liberation of a molecule of TFE and the precipitation
of Pd black was observed.[2] In addition, the nickel ana-
logue, [(η2-CF2=CF2)Ni(PCy3)2] (4), was found to cleave
the C–F bond of TFE at 100 °C under a N2 atmosphere to
give corresponding trifluorovinylnickel fluoride 5 in 70%
yield (Scheme 2).[9] However, conducting this reaction un-
der a TFE atmosphere (1.0 atm) resulted in the formation
of difluorophosphorane, F2PCy3, and desired complex 5
was not obtained at all. Characteristic upfield-shifted reso-
nance attributable to a fluorine adjacent to a group 10
metal appeared at δ = –317.9 and –385.7 ppm in the 19F
NMR spectrum of 2 and 5, respectively. The structures of
palladium and nickel fluorides 2 (Figure 1) and 5, respec-
tively, were unambiguously determined by X-ray diffraction
analysis.[12,13] To the best of our knowledge, examples of
fluoropalladium complexes generated by oxidative addition
of a C–F bond are very rare.[4i,14] In addition, complexes
2 and 5 are the first examples of structurally well-defined
oxidative addition products of TFE on transition metals
without the use of Lewis acid additives.
Figure 1. ORTEP drawing of 2 with thermal ellipsoids at the 30%
probability level. H atoms are omitted for clarity.
unique to metal fluorides 2 and 5 among all the other corre-
sponding metal halides.[4e] In fact, the Pd0-catalyzed cou-
pling reaction of chlorotrifluoroethylene with 6a in the ab-
sence of base did not give any coupling product, probably
as a result of the generation of an unreactive trifluorovinyl-
palladium chloride intermediate.[15]
Scheme 2. Generation of trifluorovinylpalladium(II) and trifluoro-
vinylnickel(II) fluorides by direct oxidative addition of TFE.
We next examined the reaction of 2 with a stoichiometric
amount of 5,5-dimethyl-2-phenyl-1,3,2-dioxaborinane (6a)
to evaluate the degree of reactivity of 2 towards organobor-
ane reagents. Treatment of 2 with 6a (4 equiv.) in THF at
100 °C for 2 h gave desired coupling product (α,β,β-tri-
fluoro)styrene (7a) in 75% yield (Scheme 3). In contrast,
no C–C bond formation occurred, even after a prolonged
Scheme 3. Reactivity of group 10 trifluorovinyl halides towards 6a.
It seemed logical to use this reaction scheme to conduct
reaction time when 6a was treated with corresponding pal- a palladium- or nickel-catalyzed cross-coupling reaction of
ladium iodide 8a, which reacts with ZnPh2.[9] In addition, TFE with 6a. In the presence of Pd(dba)2 (10 mol-%) and
neither palladium bromide 8b nor palladium chloride 8c PCy3 (20 mol-%), the coupling reaction of TFE with 6a
underwent the coupling reaction with 6a.[9] Furthermore, took place at 100 °C in the absence of any additive to give
trifluorovinylnickel fluoride 5 also reacted with 6a to afford desired product 7a in 66% yield.[16] This result pointed out
7a in 61% yield, whereas corresponding nickel iodide 9a that the reaction proceeds even in the absence of a base,
did not give 7a at all. These observations clearly show that whereas the use of a base is generally indispensable for the
C–C bond formation with organoboron compounds is Suzuki–Miyaura coupling reaction to enhance the reactivity
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Suzuki–Miyaura Coupling of Fluorinated Alkenes and Arenes
of the organoboron reagents. The addition of cesium car- Table 1. Substrate scope.[a]
bonate did not affect the yield of 7a. In contrast, in the
absence of any additive, the use of [Ni(cod)2/PCy3] (10/
20 mol-%) gave only a small amount of 7a (5%). This is
probably rationalized by the fact that potential η2-TFE
nickel precursor 4 gradually decomposes under the TFE at-
mosphere (Scheme 2). Further optimization of the cross-
coupling reaction of TFE with arylboronates was con-
ducted by using a Pd0 species.[16] As a result, the reaction of
TFE with 6a at 100 °C in the presence of [Pd(dba)2/PiPr3] in
THF led to the formation of desired product 7a in 87%
yield.[17]
With the optimized reaction conditions, the scope of the
cross-coupling reaction was investigated with respect to the
arylboronates (Table 1). Reactions with naphthylboronates
(6b, Ar = 1-naphthyl; 6c, Ar = 2-naphthyl) gave 7b and
7c in 88 and 73% yield, respectively, and furthermore, the
reaction with 1-pyrenylboronate (6d) gave 7d in moderate
yield. In addition, the reactions with 4-trifluoromethyl-
phenyl-, 4-anisyl-, and 4-vinylphenylboronates (i.e., 6e–g)
also afforded the corresponding trifluorostyrene derivatives
(i.e., 7e–g) in good to moderate yields. Of the 4-halogen-
ophenylboronates employed, 4-fluorophenyl- and 4-chloro-
phenylboronates gave p-fluoro- and p-chloro-substituted
(α,β,β-trifluoro)styrenes 7h and 7i in 74 and 76% yield,
respectively, whereas no coupling product was obtained
with 4-bromophenylboronate, probably because of the oc-
currence of undesired oxidative addition of the C–Br bond.
The reaction with 2-benzofulylboronate (6j) yielded corre-
sponding product 7j in moderate yield. In some cases, rela-
tively lower isolated yields were obtained relative to the
yields calculated by NMR spectroscopy because of the high
volatility of the compounds at room temperature.
Although this catalytic reaction leaves much to be de-
sired with regard to the catalyst loading and the product
yield, it is of great significance in preparing trifluorostyrene
derivatives substituted with nitro, aldehyde, ester, and cyano
groups (i.e., 7k–n). These functional groups can easily react
with Grignard reagents that are required for the in situ
[a] General conditions: 6 (1.00 mmol), solvent (10.0 mL), TFE
preparation of organozinc reagents, and therefore, products
7k–n are difficult to synthesize from a coupling reaction
with organozinc reagents. In addition, bis(boronate) rea-
gents such as 4,4Ј-biphenyldiboronate (6o) can be used to
prepare monotrifluorovinyl compounds, and the unreacted
boronate moiety can be used in further cross-coupling reac-
tions to synthesize highly functionalized derivatives.[18]
This base-free coupling reaction with arylboronates can
be expanded to other organofluorine molecules. The reac-
tion of vinylidene fluoride with 6b proceeded in the pres-
ence of Pd0/PiPr3 to give 1-(1-fluorovinyl)naphthalene (11)
in 86% yield (Scheme 4a). In addition, the corresponding
reaction with hexafluoropropylene gave a mixture of re-
(3.5 atm). Yields were determined by analysis of the crude product
by 19F NMR spectroscopy with α,α,α-trifluorotoluene as an in-
ternal standard. The values in square brackets are the isolated
yields. [b] Using PnBu3 instead of PiPr3. [c] Reaction conditions: 6o
(0.30 mmol), solvent (1.5 mL), TFE (30 mg, 0.30 mmol). Analysis
by NMR spectroscopy revealed that 29% of 6o remained and that
the bistrifluorovinyl compound was generated in 7% yield. [d] Re-
action conditions: 6o (0.95 mmol), solvent (10.0 mL), TFE
(100 mg, 1.00 mmol). After the isolation procedure, 130 mg (36%)
of 6o was recovered and the bistrifluorovinyl compound was iso-
lated in 10% yield.
gioisomers 12 (Scheme 4b).[19] Although the Pd0 catalyst 13 in excellent yield. The use of hexafluorobenzene allowed
was ineffective,[20] [{Ni(iPr2Im)2}2(cod)] {iPr2Im = 1,3-bis- the formation of corresponding biphenyl 14. Moreover, this
(isopropyl)imidazolin-2-ylidene}[21] showed catalytic ac- base-free coupling protocol can be applied to fluorobenz-
tivity towards the base-free coupling reaction of fluoro- ene, which needed to be used as a solvent, to give 4-meth-
arenes (Scheme 4c). That is, the reaction of octafluorotol- oxybiphenyl (15). Unlike the catalytic reaction with TFE,
uene with 6f in THF at 100 °C for 10 h gave desired product the usefulness of the nickel species in the base-free coupling
Eur. J. Org. Chem. 2013, 443–447
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445

M. Ohashi, H. Saijo, M. Shibata, S. Ogoshi
SHORT COMMUNICATION
reaction with fluoroarenes might be rationalized by its high Pd0 species and the boron fluorides.[24] It should be em-
activity towards aromatic C–F bond cleavage, and more- phasized that no extraneous base was required at all in this
over, there are no pathways to its degradation.[22]
reaction, although extraneous base is generally a requisite
for the Suzuki–Miyaura cross-coupling reaction to promote
the transmetalation step with the organoboron reagent.
Conclusions
In summary, we demonstrated the Pd0/PR3-catalyzed
cross-coupling reaction of fluoroalkenes with arylboron-
ates, in which neither an extraneous base to enhance the
reactivity of the organoboronates nor a Lewis acid additive
to promote the oxidative addition of the C–F bond was
required. The key palladium(II) fluoride intermediate that
showed unique reactivity towards the organoboron com-
pounds was isolated. We also successfully expanded this
novel coupling reaction with arylboronates to fluoroarenes
by using a Ni0/NHC catalyst. These results may open new
avenues for the development of a base-free Suzuki–Miyaura
coupling reaction, including the in situ generation of a
metal fluoride intermediate through C–F bond activation.
Scheme 4. Group 10 metal catalyzed base-free cross-coupling reac-
tion of fluorinated compounds with arylboronate 6. Yields were
determined by analysis of the crude product by 19F NMR spec-
troscopy with α,α,α-trifluorotoluene as an internal standard. Iso-
lated yields are given in square brackets. Note that in the prepara-
tion of 15, C6H5F was used as the reaction solvent (1 mL).
Experimental Section
General Procedure for the Pd0-Catalyzed Coupling Reaction of TFE
with 5,5-Dimethyl-2-aryl-1,3,2-dioxaborinane (6): All catalytic reac-
tions were conducted in a pressure-tight NMR tube (Wilmad-
LabGlass, 524-PV-7) and monitored by 11B NMR and 19F NMR
spectroscopy. A mixture of 6 (0.10 mmol), Pd(dba)2 (5.6 mg,
0.01 mmol), PiPr3 (3.2 mg, 0.02 mmol), and α,α,α-trifluorotoluene
(12.0 μL, 0.097 mmol; as an internal standard for 19F NMR spec-
troscopy) was dissolved in THF/[D8]THF (4:1 v/v, 0.5 mL). The
resulting solution was transferred into the NMR tube, which was
then charged with TFE (3.5 atm, Ͼ0.30 mmol). The reaction mix-
ture was thermostatted at 100 °C until the reaction was terminated.
Monitoring the reaction and determination of the yield of 7 were
performed by 19F NMR spectroscopy.
On the basis of the results described above, the additive-
free, Pd-catalyzed monoaryl substitution of TFE might pro-
ceed as follows (Scheme 5). Coordination of a molecule of
TFE to Pd0 takes place to generate η2-TFE species A.
Then, the combination of Pd0 and the trialkylphosphane,
which is a strong σ-donor, enables the oxidative addition of
a C–F bond to Pd0 with no additives, and this generates
trifluorovinylpalladium(II) fluoride intermediate B. Trans-
metalation of B with the arylboronate[23] gives C. Reductive
elimination then affords 7 along with regeneration of the
Isolation of 7: A mixture of 6 (1.00 mmol), Pd(dba)2 (56.5 mg,
0.10 mmol), and PiPr3 (32.0 mg, 0.20 mmol) was dissolved in THF
(10.0 mL). The resulting solution was transferred into an autoclave
reactor, and then TFE (3.5 atm) was charged into the reactor. The
reaction mixture was thermostatted at 105 °C (in most cases, the
reaction bath temperature was set higher than that of the tube ex-
periment in view of the thermal gradient in the autoclave reactor)
for the given reaction time. After the unreacted TFE was purged
from the reactor (Caution! The reaction mixture must be handled
in a well-ventilated fume hood), the reaction mixture was concen-
trated in vacuo. Pentane was added to the residue, and the resulting
suspension was filtered through a short silica column. The filtrate
was concentrated in vacuo and purified by HPLC (CHCl3). The
desired compounds were isolated as a CHCl3 solution because of
their high vapor pressure and/or the occurrence of cyclodimeriza-
tion at higher concentrations. Yields were determined by 19F NMR
spectroscopy with α,α,α-trifluorotoluene as an internal standard.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures and analytical and spectro-
scopic data for all new compounds.
Scheme 5. A plausible reaction mechanism.
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Suzuki–Miyaura Coupling of Fluorinated Alkenes and Arenes
[8] M. Tobisu, T. Xu, T. Shimasaki, N. Chatani, J. Am. Chem. Soc.
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Acknowledgments
[9] Complexes 1a and
4 were prepared by exposing either
We thank Prof. Udo Radius and David Schmidt (Julius-Maximil-
ians-Universitat Wurzburg) for the gift of [{Ni(iPr2Im)2}2(cod)]
and fruitful discussions. This work was supported by the Ministry
of Education, Culture, Sports, Science, and Technology (MEXT)
of Japan through a Grant-in-Aid for Scientific Research (No.
21245028) and a Grant-in-Aid for Scientific Research on Innovative
Areas “Molecular Activation Directed toward Straightforward Syn-
thesis” (No. 23105546).
Pd(PCy3)2 or a mixture of Ni(cod)2 and PCy3 to a TFE atmo-
sphere (1 atm). Detailed synthetic procedures for 1a and 4 and
those for 8a–8c can be found in the Supporting Information.
[10] TFE is suspected to be carcinogenic. The reaction mixture
must be handled in a well-ventilated fume hood.
[11] Complex 3 was identified on the basis of the similarity of the 19
F
NMR patterns observed in the perfluoro2-butenylzinc species
CF3(ZnX)C=CFCF3. D. J. Burton, J. Fluorine Chem. 1994, 66,
81–85. Detailed spectroscopic data of 3 as well as the isolation
procedure of 2 can be found in the Supporting Information.
[12] The ORTEP drawings of 1a, 4a, and 5 can be found in the Sup-
porting Information.
[13] CCDC-899786 (for 1a), -899787 (for 2), -899788 (for 4), and
-899789 (for 5) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
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[16] See the Supporting Information for details of the optimization
of the catalytic reaction conditions (Table S1).
[17] GC–MS analysis of the crude product revealed that this cata-
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[18] Although not isolated, this protocol can be applied to the cou-
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boronates. See the Supporting Information for details of these
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[19] Dmowski reported the reaction of CF3CF=CF2 with PhMgBr
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Chem. Int. Ed. 2007, 46, 3741–3744.
[22] We also observed that NHC ligands can react with TFE. See
the Supporting Information (Table S1).
[23] C. Amatore, A. Jutand, G. Le Duc, Angew. Chem. 2012, 124,
1408; Angew. Chem. Int. Ed. 2012, 51, 1379–1382.
[24] Another possible mechanism involving concerted bimolecular
elimination via a five-membered transient intermediate may af-
ford 7, see: N. P. Mankad, F. D. Toste, J. Am. Chem. Soc. 2010,
132, 12859–12861.
Received: October 22, 2012
Published Online: November 29, 2012
Eur. J. Org. Chem. 2013, 443–447
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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