Cobalt-Catalyzed C-F Bond Borylation of Aryl Fluorides

Article abstract of DOI:10.1021/acs.orglett.8b03167

A mild and practical cobalt-catalyzed defluoroborylation of fluoroarenes is presented for the first time. The method permits straightforward functionalization of fluoroarenes, with high selectivity for borylation of C-F over C-H bonds, and a tolerance for aerobic conditions. Furthermore, two-step ¹⁸F-fluorination was achieved for expanding the scope of ¹⁸F-positron emission tomography probes.

Full text of DOI:10.1021/acs.orglett.8b03167

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Cobalt-Catalyzed C−F Bond Borylation of Aryl Fluorides
Soobin Lim,†,‡,⊗ Dalnim Song,†,‡,⊗ Seungwon Jeon,†,‡,⊗ Youngsuk Kim,†,‡ Hyunseok Kim,‡
Sanghee Lee,∥,# Hyungdo Cho,‡ Byung Chul Lee,*,∥,⊥ Sang Eun Kim,∥,⊥,# Kimoon Kim,†,‡,§
and Eunsung Lee*,†,‡,§
†Center for Self-assembly and Complexity, Institute for Basic Science (IBS), Pohang 37673, Republic of Korea
§
‡Department of Chemistry and Division of Advanced Materials Science, Pohang University of Science and Technology, Pohang
37673, Republic of Korea
∥Department of Nuclear Medicine, Seoul National University Bundang Hospital, Seoul National University College of Medicine,
Seongnam 13620, Republic of Korea
⊥Center for Nanomolecular Imaging and Innovative Drug Development, Advanced Institutes of Convergence Technology, Suwon
16229, Republic of Korea
#Department of Transdisciplinary Studies, Graduate School of Convergence Science and Technology, Seoul National University,
Suwon 16229, Republic of Korea
S
* Supporting Information
ABSTRACT: A mild and practical cobalt-catalyzed defluorobor-
ylation of fluoroarenes is presented for the first time. The method
permits straightforward functionalization of fluoroarenes, with high
selectivity for borylation of C−F over C−H bonds, and a tolerance
for aerobic conditions. Furthermore, two-step 18F-fluorination was
achieved for expanding the scope of 18F-positron emission
tomography probes.
luorine atoms engender a range of unique physical and
chemical properties in such widely used fluorinated
otherwise requires a multistep synthesis to the targeted
pharmaceuticals with a boronic ester group (Scheme 1a).3
In 2015, the Martin group reported defluoroborylation of
fluoroarenes using B2neop2 (bis(neopentylglycolato)diboron),
catalytic [Ni(COD)2] (COD = 1,5-cyclooctadiene), and
tricyclohexylphosphine (PCy3) under high temperature (110
°C), while in the same year, the Niwa and Hosoya group also
reported the defluoroborylation using B2pin2 (bis(pinacolato)-
diboron) with a similar catalytic system.8a,b Subsequently, Niwa
and Hosoya developed a related method for defluoroborylation
of fluoroarenes, catalyzed by air-stable copper salts.9 Although
these methods offer great opportunities for a new organic
transformation and easy access to radiochemical tracers, the
operation under inert conditions and the lack of functional
tolerance toward alcohol and amines may slow down a practical
utility. Therefore, along with these developments, we have been
trying to find another method for defluoroborylation of
fluoroarenes, aiming for milder conditions and better tolerance
for functional groups.
F
substances as pharmaceuticals, agrochemicals, and materials.1
Fueling these applications have been significant synthetic
advances made over the past decades to enable manipulation
of C−F bonds. In particular, a variety of aromatic C−F bond-
forming methods now offer rapid access to complex fluorinated
molecules,2 and late-stage 18F-fluorination reactions are being
developed to expand the scope of positron emission tomography
(PET) tracers.3
Lagging behind the development of C−F bond-forming
reactions, cleavage and functionalization of C−F bonds had
traditionally been a challenging goal,4 as C−F bonds are one of
the strongest bonds in organic molecules.5 Therefore,
functionalization of aromatic C−F bonds had traditionally
relied on highly activated, electron-poor substrates6 and/or
strongly reducing conditions.7 In this regard, development of a
mild and selective C−F bond functionalization method would
enable straightforward diversification of various fluorine-
containing molecules. In particular, enabling late-stage C−F
functionalization would provide an alternative synthetic strategy
toward complex molecules as C−F bonds are resistant to most
chemical transformations.2
C−F bond borylation is our particular interest as arylboronic
esters are a versatile functional group that enables further
transformation. In particular, combined with the late-stage 18F-
fluorination reaction of arylboronic esters, C−F bond borylation
would enable the straightforward conversion of readily available
fluorinated pharmaceuticals into 18F-labeled PET tracers, which
In 2011, the Holland group demonstrated the chemoselective
activation of the C−F bond in fluorobenzene using a low-valent
cobalt complex under mild conditions (Scheme 1b).10
Encouraged by this result, we set out to develop defluorobor-
ylation of fluoroarenes using Co catalysis. However, most of our
attempts to catalyze borylation of aryl fluorides using the original
Received: October 4, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03167
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. C−F Bond Activation and Borylation
Table 1. Borylation of 4-Fluorobiphenyl (1a) with HBpin:
Catalyst Screening and Condition Optimization
a
b
entry
deviation from above
yield (%)
1
2
3
4
5
6
7
8
none
12 h
99
89
99
1
38
15
86
58
10
61
0
c
c
12 h with stock solution of CoCl2 and L3
L1 instead of L3
L2 instead of L3
B2Pin2 instead of HBPin
1.5 equiv HBPin
Na instead of Mg
KC8 instead of Mg
1 equiv Mg
DMF instead of THF
toluene instead of THF
no CoCl2
no ligand
no Mg
9
10
11
12
13
14
15
16
0
0
1
0
9,10-dihydroanthracene (1.2 equiv) as an additive
99
Holland system10 with various borylation reagents and
reductants failed, although a stoichiometric reaction of an
arylcobalt(II) complex with boron sources afforded the
borylated product in low yield (see the Supporting Information
(SI)). Nevertheless, we found a combination of a simpler β-
diketiminate ligand and CoCl2 to achieve the first cobalt-
catalyzed defluoroborylation of aryl fluorides. The new cobalt-
based system operates under mild condition and shows high
selectivity against C−H bond borylation (Scheme 1c).11
We began our research by screening the reactivity of cobalt
complexes (L1Co(II)X, X = biphenyl or chloride), supported by
Holland’s ligand L1 (Scheme 1a), with various reducing agents
and pinacolborane (HBpin) or B2pin2 as borylating agents
(Tables S1 and S2). We found that the cobalt aryl complex could
produce the corresponding borylated aryl product with HBpin.
Inspired by the initial results, we found that a simple
combination of CoCl2 and a β-diketiminate ligand (L3)
produced the desired defluoroborylation product 2a in a
catalytic manner and 99% yield under optimized conditions:
0.1 equiv of CoCl2, 0.15 equiv of L3, 1.5 equiv of Mg, and 2.0
equiv of HBpin (Table 1, entry 1). The reduced reaction time
resulted in lower yield (Table 1, entry 2), but the use of CoCl2
and L3 stock solution could shorten the reaction time (Table 1,
entry 3). Decreased amounts of HBpin or Mg resulted in lower
yields (Table 1, entries 7 and 10). The use of other β-
diketiminate ligands, B2pin2 as a borylating agent, Na or KC8 as
reductants, and DMF or toluene as solvents, gave considerably
lower yields of 2a (Table 1, entries 4−6, 8, 9, 11, and 12). In the
absence of CoCl2, ligand or magnesium, no or trace amount of
product was observed (Table 1, entries 13−15).
a
b
0.174 mmol of 1a and 3.0 mL of solvent were used. 1H NMR yield
c
using 1,3,5-trimethoxybenzene as an internal standard. CoCl2 and L3
were dissolved in THF after heating at 60 °C for complexation, and
the solution was used. See the SI for detailed reaction conditions.
2j, 2k and 2p) in addition to the fluoroarene substrates with
methoxy and tertiary amine functions (2l, 2m, 2n, 2p, and 2r).
Using B2Pin2 as a borylating agent instead of HBpin left the
olefin of 4-fluoro-α-methylstyrene untouched and gave 2i in
74% yield. The method also catalyzed defluoroborylation of
fluorinated heterocycles such as indole, pyrazole, quinoline, and
pyridine without any protection (2s, 2t, 2u, and 2v), although
the reactions required 20 mol % of CoCl2. In the case of 2-(4-
fluorophenyl)indole (1q), defluoroborylation provided bory-
lated product 2q using 10 mol % of CoCl2. Fluoroestrol
derivative (1w) was borylated in 57% yield to afford 2w as an
example of a complex secondary aliphatic alcohol. Interestingly,
while previously reported transition-metal-catalyzed defluor-
oborylations required proper protecting groups,8a,b unprotected
free alcohols and amines were tolerated in this system (2e, 2j, 2k,
and 2o). Furthermore, it can uniquely take place in substrates
containing styrene, free indole, pyridine, and aliphatic alcohol
(2i, 2s, 2v, and 2w), unlike previously reported defluorobor-
ylation.8 However, fluoroarenes with electron-withdrawing
groups, such as ketone, nitro, nitrile, and ester (Figure S2),
were not borylated under the optimized conditions with the
exception of the trifluoromethyl group. Trifluoromethyl-
substituted fluoroarenes could be borylated by this method,
but the yields were very low (less than 16%, see the SI).
Defluoroborylation of 1-(4-fluorophenyl)pyrrolidin-2-one re-
sulted in its deoxygenation, speaking for considerably reducing
nature of the reaction conditions (Figure S3).
To assess the scope of the cobalt catalysis, various aryl
fluorides were subjected to defluoroborylation under the
conditions optimized for 2a (Scheme 2). Simple aryl fluorides,
such as 4-fluorobiphenyl (1a), 2-fluorobiphenyl (1b), and 1-
fluoronaphthalene (1c), converted in 71−94% isolated yields to
the corresponding arylboronic esters. Defluoroborylation of
4,4′-difluoro-1,1′-biphenyl (1d) efficiently provided diborylated
product 2d. Fluoroarenes with unprotected alcohol and amino
groups were successfully borylated with this cobalt system (2e,
Importantly, the reaction operates well under aerobic
conditions as demonstrated by the reactions of 4-fluorobiphenyl
(1a), 4-fluoro-4′-hydroxybiphenyl (1e), and 3-(4-fluorophen-
yl)-1H-pyrazole (1t), which successfully producing the
corresponding boronic esters in good 1H NMR yields (Scheme
2; see the SI for detailed reaction conditions). Thus, this method
B
DOI: 10.1021/acs.orglett.8b03167
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
data indicate that magnesium could not only reduce a cobalt
complex for activating C−F bond of aryl fluoride but somehow
form a Grignard reagent with an aryl group. In addition, we
observed the borylated product (5%) after addition of HBpin to
the reaction mixture (Figure S9). Since the yields of both
hydrodefluorinated and borylated products were not significant,
detailed mechanistic study on the catalysis is currently under
investigation.
Scheme 2. Cobalt-Catalyzed Borylation of Aryl Fluorides
The preparation of 18F-labeled aryl fluorides from the
corresponding aryl boronates, which were not labeled
previously, using the modified copper-mediated 18F-fluorination
methods have been developed by the Gouverneur and Sanford
groups (Scheme 3).3h−j,12 In addition to the previous nickel- or
a
Scheme 3. 18F-Fluorination of Aryl Boronic Esters
a
Reaction conditions: Substrates (10 μmol), Cu(OTf)2 (3.3 μmol),
pyridine (0.1 mmol), 18-crown-6 (0.8 μmol), CsHCO3 (0.5 μmol),
DMF (0.3 mL). [18F]fluoride (8.0−8.5 MBq).
copper-catalyzed defluoroborylation methods,8b,9,13 the present
defluoroborylation approach can also expand the scope of useful
18F-labeled PET probes by two-step facile preparation method
(defluoroborylation and 18F-fluorination).
In summary, a mild and practical cobalt-catalyzed defluor-
oborylation of fluoroarenes is reported for the first time. The
method enables straightforward functionalization of a diverse set
of aromatic fluorides with high selectivity for borylation of
aromatic C−F over C−H bonds. Most importantly, this method
tolerates unprotected functional groups such as alcohols and
amines and operates well under aerobic condition without a
significant loss in yield. In addition, this method was successfully
applied for 18F-fluorination to expand the scope of useful 18F-
PET probes.
a
0.174 mmol of substrates was used. See the SI for detailed reaction
b
conditions. 1H NMR yield using dibromomethane or 1,3,5-
trimethoxybenzene as an internal standard. Under aerobic con-
ditions. Under aerobic conditions using commercial GR grade THF
(H2O content <0.05%). Using 4,4′-difluoro-1,1′-biphenyl (1d) as the
substrate. B2pin2 (1 equiv), NaF (1 equiv), and L2 (0.3 equiv) were
c
d
e
f
used instead of HBpin and L3. The reaction temperature was 80 °C.
g
0.043 mmol of substrates and 0.75 mL of solvent were used.
is indeed beneficial from easy operation without a significant loss
in yield, while previous transition-metal-catalyzed defluorobor-
ylation systems require inert conditions.8a,b This methodology
was successfully utilized for a larger scale synthesis (2a).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b03167.
■
S
Since the borylation yield was not affected when 9,10-
dihydroanthracene was added to the reaction, the radical does
not seem to be involved in the reaction (Table 1, entry 16). To
figure out the role of magnesium for the borylation reaction,
L1Co(II)biph was borylated with HBpin in the presence of
magnesium. However, magnesium did not affect the reaction
yield (Figure S7), which indicates that the borylation process
may not be associated with further reduction of L1Co(II)biph.
We also confirmed that 1a was completely consumed even
without pinacolborane (Figure S8). When the reaction mixture
without HBpin was quenched with water, only 20% of
hydrodefluorinated product was observed (Figure S8). The
Detailed experimental procedures, spectroscopic data for
all new compounds, and X-ray data for 2t (PDF)
Accession Codes
CCDC 1836643 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
C
DOI: 10.1021/acs.orglett.8b03167
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Authors
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*E-mail: leebc@snu.ac.kr.
*E-mail: eslee@postech.ac.kr.
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ORCID
Youngsuk Kim: 0000-0002-7378-2301
Kimoon Kim: 0000-0001-9418-3909
Eunsung Lee: 0000-0002-1507-098X
Author Contributions
⊗S.L., D.S., and S.J. contributed equally to this work.
Notes
̈
Int. Ed. 2015, 54, 9075. (h) Kallane, S. I.; Teltewskoi, M.; Braun, T.;
Braun, B. Organometallics 2015, 34, 1156. (i) Bakewell, C.; White, A. J.;
Crimmin, M. R. J. Am. Chem. Soc. 2016, 138, 12763. (j) Kim, Y.; Lee, E.
Chem. Commun. 2016, 52, 10922. (k) Zhou, J.; Kuntze-Fechner, M. W.;
Bertermann, R.; Paul, U. S.; Berthel, J. H.; Friedrich, A.; Du, Z.; Marder,
T. B.; Radius, U. J. Am. Chem. Soc. 2016, 138, 5250. (l) Chen, W.;
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Institute for Basic Science (IBS)
[IBS-R007-D1], the Korean Health Technology R&D Project
through the Korean Health Industry Development Instituted
(KHIDI), funded by the Ministry of Health & Welfare, Republic
Korea (No. HI14C-1072-010014), and the National Research
Foundation of Korea (NRF) grant funded by the Korea
government (2018R1A4A1024713).
̈
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