Copper-mediated aerobic fluoroalkylation of arylboronic acids with fluoroalkyl iodides at room temperature

Article abstract of DOI:10.1021/ja301705z

A Cu-mediated ligandless aerobic fluoroalkylation of arylboronic acids under mild conditions is described for the first time. The reaction tolerates a wide range of functional groups, allowing for further transformation. Mechanistic studies suggest that [R_fCu] is the active Cu species that forms the desired perfluoroalkylarenes and that [R_fCu] is generated from [PhCu] by either an oxidative addition/reductive elimination mechanism or nucleophilic substitution via a halogen "ate" intermediate.

Full text of DOI:10.1021/ja301705z

Communication
pubs.acs.org/JACS
Copper-Mediated Aerobic Fluoroalkylation of Arylboronic Acids with
Fluoroalkyl Iodides at Room Temperature
Qingqing Qi, Qilong Shen,* and Long Lu*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling
Road, Shanghai 200032, PRC
S
* Supporting Information
Herein, we report a ligandless Cu-mediated fluoroalkylation of
ABSTRACT: A Cu-mediated ligandless aerobic fluoro-
alkylation of arylboronic acids under mild conditions is
described for the first time. The reaction tolerates a wide
range of functional groups, allowing for further trans-
formation. Mechanistic studies suggest that [RfCu] is the
active Cu species that forms the desired perfluoroalkylar-
enes and that [RfCu] is generated from [PhCu] by either
an oxidative addition/reductive elimination mechanism or
nucleophilic substitution via a halogen “ate” intermediate.
arylboronic acids with fluoroalkyl iodides under aerobic
conditions (Scheme 1). Preliminary mechanistic studies
revealed that the reaction proceeds via a [RfCu] species
generated by reaction of [ArCu] with RfI.
Scheme 1. Preparation of Perfluoroalkyl Arenes
n the past two decades, we have witnessed the widespread
Iuse of fluoroalkylated arenes in materials science, argochem-
istry, and the pharmaceutical industry.1 The “magic effect” of
fluorine on the chemical, physical, and biological properties of
organic compounds has made the incorporation of fluorine a
routine strategy. Thu, the development of new methodologies
for preparing fluoroalkylated arenes has become a subject of
great interest.2,3 Recently, significant advances in trifluorome-
thylation of arenes have been achieved through a radical
pathway or transition-metal-catalyzed coupling.4−8 For exam-
ple, radical trifluoromethylation of simple arenes and hetero-
cycles was performed using visible light or BuOOH as the
radical initiator, albeit with poor regioselectivity. More precise
installation of the trifluoromethyl group was accomplished by
metal-mediated coupling. A halide or boron on an aromatic ring
can be replaced with a trifluoromethyl group under mild
reaction conditions.4−7 On the other hand, few general
methods for perfluoroalkylation or difluoromethylation of
arenes have been reported. Preparation of perfluoroalkylated
or difluoromethylated arenes typically relies on Cu-mediated
coupling of RfI or R′CF2I with ArI, discovered by McLoughlin
and Thrower in the mid-1960s.3,9,10 Only recently, Daugulis
reported a Cu-catalyzed reaction of aryl iodide and 1H-
perfluoroalkane reagents in the presence of zinc bis-2,2,6,6-
tetramethylpiperidide,11 and Hartwig reported a 1,10-phenan-
throline (Phen)-ligated perfluoroalkylcopper reagent that reacts
with aryl iodides or arylboronic acids to give perfluoroalkyl
arenes in good yields.6 Shreeve12 and Qing13 independently
reported Pd-catalyzed α-arylation of difluoromethyl ester
enolates. Amii reported a Cu-catalyzed coupling of aryl iodides
with α-silyldifluoroacetate to give aryldifluoroacetates in
moderate to good yields.14 Baran et al. developed a new
difluoromethyl reagent for radical difluoromethylation of arenes
and heteroarylenes.15 However, direct coupling of arylboronic
acids with perfluoroalkyl iodides has not been reported to date.
Perfluoroalkyl iodides are known to undergo oxidative
addition to Pd(0),16 and reductive eliminations from the Pd
complexes [(Xantphos)Pd(Ar)(CF3)] and [(BrettPhos)Pd-
(Ar)(CF3)] to give ArCF3 in high yields were reported by
Grushin17 and Buchwald,5 respectively. In addition, pioneering
work by Fu et al. demonstrated that alkyl iodides can be
coupled with arylboronic acids in the presence of Pd or Ni
catalysts.18 In light of these advances, we envisioned that if RfI
could be directly coupled with arylboronic acids in the presence
of a Pd or Ni catalyst with a suitable ligand, it might offer a new
and efficient entry to perfluoroalkyl or difluoromethyl arenes.
We initially focused on the Pd- or Ni-catalyzed coupling
reaction of phenylboronic acid with n-C4F9I in the presence of a
variety of phosphine and nitrogen ligands such as Xantphos,
Brettphos, Ruphos, 2,2′-bipyridine (bipy), and PyBox. How-
ever, no trace of the desired product was observed even at 90
°C in several solvents such as diglyme, DMF, or dioxane.
Surprisingly, when 0.5 equiv of CuI was used in DMF, the
desired product was detected in 7% yield by 19F NMR
spectroscopy and GC/MS (Table 1, entry 1). With careful
control of the conditions, it was discovered that the presence of
air is very important for the reaction, as no desired product was
observed when the reaction was conducted in an argon
atmosphere. Interestingly, when the reaction was conducted in
an oxygen atmosphere, only the homocoupling side product
1,1′-biphenyl, was observed. The yield of C4F9Ph increased to
65% when DMSO was used as the solvent (entry 2). Further
screening of different Cu sources disclosed that Cu powder was
the most efficient catalyst, producing ArRf in 76% yield when
8
7
t
Received: February 21, 2012
Published: March 29, 2012
© 2012 American Chemical Society
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with RfI to give the corresponding perfluoroalkyl arenes in
moderate to good yields. The reaction can be scaled up easily.
For example, the reactions of phenylboronic acid with C4F9I
(82 mmol) and ClC4F8I (30 mmol) afforded yields of 58 and
60%, respectively. The difluoromethyl group has been utilized
as a thiol or hydroxyl isostere in drug design and has great
potential in medicinal chemistry. It was found that the current
methodology can be easily extended to the coupling of
ICF2CO2Et and ICF2CONEt2 with arylboronic acids to give
the difluoromethylated compounds in good yields. Interest-
ingly, CuI could also mediate the same reactions but generally
gave lower yields. In addition, the Cu-mediated coupling of
arylboronic acids with RfI is tolerant of a variety of functional
groups attached to the arylboronic acid, such as ester,
enolizable ketone, aldehyde, nitro, and halides, including
chloride, bromide, and iodide. It is worth noting that bromide
and iodide were not compatible with McLoughlin and
Thrower’s conditions.9 In addition, this method takes place
on unprotected systems in air at room temperature (rt) without
any need for solvent purification; thus, it has great potential
applications for medicinal chemists in parallel syntheses of
fluorinated bioactive compounds.
Table 1. Optimization of Conditions for Cu-Mediated
Coupling of Arylboronic Acids with Perfluoroalkyl Iodides
a
To gain some insight about the mechanism and to identify
the Cu species that mediates the formation of perfluoroalky-
lated arenes, we monitored by 19F NMR spectroscopy the
reaction of phenylboronic acid with C4F9I under the standard
conditions. [C4F9Cu] was clearly observed in 10−20% yield in
the reaction solution (Figure 1 in the Supporting Information).
The stoichiometric reaction of independently prepared
[C4F9Cu] with 3.0 equiv of phenylboronic acid in DMSO in
air at rt formed the perfluoroalkylated benzene in 90% yield as
determined by 19F NMR spectroscopy (eq 1).19 These
experiments suggest that [C4F9Cu] is the active Cu species
formed in the catalytic reaction.
a
Reaction conditions for eor entries 1−8: phenylboronic acid (1.6
mmol), C4F9I (0.82 mmol), Cu catalyst (0.5 equiv; for entry 8, 1 equiv
was used) in solvent (3.0 mL) at rt for 12 h. For entries 9−31:
phenylboronic acid (0.82 mmol), C4F9I (0.41 mmol), Cu catalyst (1.5
equiv), additive (1.0 equiv) in DMSO/DMF (3.0/0.6 mL) at rt for 12
b
h. Yields determined by 19F NMR analysis of the crude reaction
mixture with an internal standard. 1.2 mol of phenylboronic acid was
used. Unpurified mixed solvents. 40 mol %.
c
d
e
We speculated that the active Cu species [C4F9Cu] could be
generated via two different pathways (Scheme 2). In pathway
A, C4F9I could oxidatively add to [Cu0] through a well-known
radical mechanism to form [C4F9Cu] and CuI.20 Alternatively,
in pathway B, [ArCu], formed by transmetalation of ArB(OH)2
to CuI or reaction of ArB(OH)2 with Cu in the presence of air,
could react with RfI by either an oxidative addition/reductive
elimination mechanism or a nucleophilic substitution via a
halogen “ate” intermediate to form [C4F9Cu] and CuI.
5:1 DMSO/DMF was used as the solvent (entries 5−8).
Interestingly, Cu foil was equally effective, giving the desired
product in 59% yield, while Cu wire gave a low yield of 27%.
The use of other metals (e.g., Zn, Fe, Cd, Al) resulted in no
detectable formation of the desired product. The choice of
solvent was very important for the reaction. No trace of
product was observed when the reaction was conducted in
MeOH, dimethoxyethane, 1,2-dichloroethane, dioxane, or
acetonitrile (entries 9−13). Addition of phosphine ligands
such as PPh3, DPPF, DPPE, or DPPB completely shut down
the reaction, while addition of BINAP gave the coupled product
in a slightly lower yield (54%; entries 14−18). Notably, the
yield of the reaction decreased significantly when Phen, bipy, or
2,6-lutidine was added (entries 19−21). In contrast to most
Suzuki−Miyaura cross-coupling reactions, where bases such as
K3PO4 or K2CO3 are indispensable, reactions in the presence of
bases such as Li2CO3, K3PO4, KOAc, and NaHCO3 gave lower
yields (entries 22−25), while other bases such as K2CO3, KF,
Cs2CO3, NaOtBu, LiOH, or DBU proved to be completely
ineffective, giving essentially no desired product (entries 26−
31).
To probe whether [RfCu] is formed via a free Rf• radical, we
conducted two different sets of experiments. In the first set,
freshly prepared Cu powder from either reduction of CuSO4
with Zn or an acid activation process was reacted with C4F9I in
DMF or DMSO under argon or air at rt. No formation of
[C4F9Cu] was observed by 19F NMR spectroscopy under these
conditions (eq 2), in agreement with previous reports that
reactions of Cu with perfluoroalkyl iodides typically require
temperatures higher than 60 °C.2e In the second set, three
separate reactions were conducted by addition of a radical
inhibitor (hydroquinone), an electron-transfer scavenger (1,4-
With the optimized conditions, we next examined the
substrate scope of the cross-coupling reactions between RfI and
arylboronic acids (Table 2). Various boronic acids were treated
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Table 2. Scope of Cu-Mediated Coupling of Arylboronic
Acids with Perfluoroalkyl Iodides
Scheme 2. Possible Pathways for the Formation of RfCu
a
Addition of hydroquinone or 1,4-dinitrobenzene had negligible
impact on the yield of this reaction. However, even with 1.0
equiv of the additive, the reaction still proceeded smoothly after
14 h to afford the desired product in 27 or 30% yield,
respectively. In addition, when 1.0 equiv of diallyl ether was
added to the reaction of phenylboronic acid with C4F9I under
the standard conditions, no radical-initiated cyclization product
was observed. These experiments clearly rule out the formation
of [C4F9Cu] via a radical pathway.
To probe whether [ArCu] is involved in the formation of
[RfCu], we studied the stoichiometric reaction of [ArCu] with
C4F9I at rt (eq 4). [PhCu] was prepared as an ether solution at
−10 to 0 °C according to a procedure reported by Costa.21 We
noticed that [PhCu] is not so stable at rt in ether solution but
relatively much more stable in DMSO. The reaction of [PhCu]
with C4F9I was conducted by first evaporating the diethyl ether
at −5 °C, adding C4F9I in DMSO at −10 °C, and finally
warming to rt under argon for 4 h or air for 1 h. The reaction of
[PhCu] afforded [C4F9Cu] in yields of 33% under argon and
14% under air, as determined by 19F NMR spectroscopy.22 The
formation of PhI was also confirmed by GC−MS. These results
provide evidence that [RfCu] is generated from [PhCu],
although at this stage it is unclear whether it is formed through
an oxidative addition/reductive elimination mechanism23 or a
nucleophilic substitution via a halogen “ate” intermediate.
To figure out whether [ArCu] is formed under the catalytic
reaction conditions, we studied the stoichiometric reaction of
PhB(OH)2 with Cu in the presence or absence of air. Reaction
of PhB(OH)2 with Cu powder formed biphenyl in 63% yield in
the presence of air.24 However, only an 8% yield of biphenyl
was isolated under an argon atmosphere. These experiments
suggest that the formation of [PhCu] is likely under the current
reaction conditions.
On the basis of these preliminary results, we propose the
reaction mechanism outlined in Scheme 3. Arylboronic acid is
first converted to [ArCu] in the presence of Cu powder and air.
This cuprate then reacts with RfI by either an oxidative
addition/reductive elimination mechanism or nucleophilic
substitution via a halogen “ate” intermediate to form
[C4F9Cu], which undergoes oxidative arylation with arylbor-
onic acid in the presence of air to form the perfluoroalkylated
arene product.
a
Reaction conditions: arylboronic acid (1.2 mmol), C4F9I (0.82
mmol), Cu powder (0.82 mmol) in 5:1 DMSO/DMF (3.6 mL) at rt
for 12 h. Isolated yields are shown. Yields in parentheses were
determined by 19F NMR analysis for reactions that used CuI (0.5
equiv) as catalyst in DMSO (3.0 mL).
dinitrobenzene), or a radical clock (diallyl ether) to the Cu-
mediated coupling of phenylboronic acid with C4F9I (eq 3).
In summary, we have developed the first Cu-mediated
ligandless aerobic fluoroalkylation of arylboronic acids under
mild conditions. No bases are required, and the reaction can be
easily scaled up. In addition, the reaction tolerates a wide range
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Scheme 3. Proposed Mechanism
of functional groups, allowing for further transformation.
Mechanistic studies suggest that [RfCu] is the active Cu
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
shenql@mail.sioc.ac.cn; lulong@mail.sioc.ac.cn
■
(11) Popov, I.; Lindeman, S.; Daugulis, O. J. Am. Chem. Soc. 2011,
133, 9286.
Notes
The authors declare no competing financial interest.
(12) Guo, Y.; Shreeve, J. M. Chem. Commun. 2007, 3583.
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2012, 134, 1494. (b) After our submission, Fier and Hartwig reported
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ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
National Basic Research Program of China (2012CB821600,
2010CB126103), the Special Fund for Agro-scientific Research
in the Public Interest (201103007), the National Key
Technologies R&D Program (2011BAE06B05), the Shanghai
Scientific Research Program (10XD1405200), the Key Program
of the National Natural Science Foundation of China
(21032006), the Shanghai Pujiang Program (11PJ1412200),
and the SIOC Startup Fund.
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