Nickel-catalyzed monofluoromethylation of (hetero)aryl bromides via reductive cross-coupling

Article abstract of DOI:10.1039/c9cc03737c

A mild and efficient nickel-catalyzed direct monofluoromethylation of (hetero)aryl bromides by reductive cross-coupling has been developed. This method exhibits good efficiency, wide functional-group compatibility, and suitability for aryl and heteroaryl bromides with abundant industrial raw material BrCH₂F. This strategy provides an efficient way to synthesize monofluoromethylated molecules for drug discovery.

Full text of DOI:10.1039/c9cc03737c

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DOI: 10.1039/C9CC03737C
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Nickel-Catalyzed Monofluoromethylation of (Hetero)aryl
Bromides via Reductive Cross-coupling
Han Yin,‡a Jie Sheng,‡a,b Kai-Fan Zhang,a Zi-Qi Zhang,a Kang-Jie Biana and Xi-Sheng Wang*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A mild and efficient Nickel-catalyzed direct monofluoromethylation carboxylic ester6d-f and phenylsulfonyl,6g-j have been developed
of (hetero)aryl bromides by reductive cross-coupling has been rapidly.
developed. This method exhibits good efficiency, wide functional-
group compatibility, and suitable for aryl and heteroaryl bromides
with abundant industrial raw material BrCH2F. This strategy
Previous studies:
B
CH2F
cat. Pd or Ni
+
XCH2F
provides an efficient way to synthesize monofluoromethylated
molecules for drug discovery.
R
R
R
X
= halides
B
= B(OH)2, Bpin
X
Fluorinated organic compounds have been widely used in
pharmaceuticals and agrochemicals because fluorine or
fluorinated moieties could remarkably enhance the
lipophilicity, metabolic stability and bioavailability of the parent
molecules.1 Accordingly, the selective incorporation of fluorine
atom or fluorinated moieties into organic molecules has
attracted extensive interest of synthetic chemists.2 Among all
fluorinated functional groups, monofluoromethylene (CH2F)
has emerged as an important motif due to its widespread
existence in many biologically active molecules, such as
Afloqualone, Loflupane and Florfenicol. Actually, the traditional
methods for the synthesis of monofluoromethyl arenes (ArCH2F)
were nucleophilic monofluorination from benzylic alcohols3 or
halides.4 However, the limited availability of the corresponding
benzylic precursors restricted the application of such functional
group conversion processes in organic synthesis.5 As an
alternative, the monofluoromethyl functional group, known as
a large class of fluorine-containing building block, could be
introduced into arenes as a whole.2e-f During the past decades,
the indirect monofluoromethylation methods6 which usually
use monofluoromethylating reagents installed with easily
removed protective groups, including phosphoryl ester,6b-c
CHFR
cat. Ni
+
XCHFR
R
Reductive
cross-coupling
R
= H, alkyl
This work:
Br
CH2F
cat. Ni
+
BrCH2F
R
Het
R
Het
Reductive
cross-coupling
Scheme 1 Direct Monofluoromethylation by Cross-Coupling
As results for the development of direct and regiospecific
monofluoromethylation methods, transition-metal mediated or
catalyzed incorporation of monofluoromethyl group (CH2F) into
aryl substrates have been well established in the past decade.
Starting from the first example of stoichiometric amount of
palladium-mediated cross coupling of aryl boronates with
monofluoromethyl halides in 2009 by the Suzuki group,7 the
direct monofluoromethylations of aryl boron compounds with
fluoromethyl halides have been successfully developed via
palladium and nickle-catalysis by Hu8 and Zhang.9 Due to its high
atom- and step-economy and straightforward handling by
avoiding the use of organometallic reagents, nickel-catalyzed
reductive crossing-coupling has been used for fluoroalkylation
of aryl halides by Zhang10 and our group11a recently. However,
none of these methods could be compatible with heteroarenes,
which definitely hampered their application on drug design and
development. As one part of our continuous efforts11a-b to
develop high efficient monofluoroalkylation, herein, we
describe a nickel-catalyzed reductive cross-coupling between
heteroaryl halides and abundant industrial raw material
bromofluoromethane. This method has demonstrated high
efficiency, mild conditions, and good functional-group
a. Hefei National Laboratory for Physical Sciences at the Microscale and Department
of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026,
P. R. China. E-mail: xswang77@ustc.edu.cn
b. School of Environment and Civil Engineering, Dongguan University of Technology,
Dongguan, Guangdong 523808, P. R. China.
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
‡ These authors contributed equally to this work.
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compatibility for both aryl and heteroaryl bromides. The key to the absence of nickel catalyst or ligands when conducting the
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success was the combination of readily available bidentate and corresponding control experiments (entrDieOsI:2120-.12033)9./C9CC03737C
monodentate pyridine-type nitrogen ligands with nickel9,11a,12
Table 2 Substrate scope a
to in situ generate efficient catalysts which promote the
NiI2 (10 mol%)
dtbpy (12 mol%)
DMAP (24 mol%)
monofluoromethylation for broad scope of (hetero)aryl
bromides.13
Br
CH2F
+
BrCH2F
R
Het
Het
R
Mn (3 equiv)
N2, DMAc, 40 oC, 24 h
1
2
3
Table 1 Optimization of the reaction conditions a
CH2F
Ph
CH2F
CH2F
[Ni] (10 mol%)
CH2F
Br
CH2F
ligands (x mol%)
+
BrCH2F
Ph
Mn (3 equiv)
N2, DMAc, 40 oC, 24 h
Ph
Ph
b
b
3a
, 86%
3b
, 89%
3c
MeO
3d
, 74%
, 76%
1a
2
3a
MeO
CH2F
CH2F
Entry
[Ni]
Ligand(mol%)
Yield(%)b
CH2F
CH2F
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20d
21e
22
23
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiI2
NiCl2
Ni(OAc)2
4-CN-Py(24)
DMAP(24)
2,6-lutidine(24)
PPh3(24)
dmbpy(12)
dtbpy(12)
dombpy(12)
phen(12)
0
15
0
OMe
OMe
b
b
b
3h
, 84%
3e
, 83%
3f
3g
, 77%
, 82%
CH2F
O
CH2F
BnO
CH2F
Bz
CH2F
0
O
Bz
64
71
79
52
12
76
85 (86)c
72
69
78
74
50
39
19
68
41
76
0
3i
, 94%
3j
3k
3l
, 83%
, 77%
, 95%
CH2F
CH2F
O
Cl
CH2F
CH2F
N
dppe(12)
S
dombpy(12)/DMAP(24)
dtbpy(12)/DMAP(24)
dmbpy(12)/DMAP(24)
phen(12)/DMAP(24)
dtbpy(12)/2,6-lutidine(24)
dtbpy(12)/4-CN-Py(24)
dtbpy(12)/PPh3(24)
dtbpy(12)/DMAP(24)
dtbpy(12)/DMAP(24)
dtbpy(12)/DMAP(24)
dtbpy(12)/DMAP(24)
dtbpy(12)/DMAP(24)
none
b
3m
3o
3p
, 77%
, 68%
, 95%
3n
, 78%
CH2F
CH2F
O
N
CH2F
N
CH2F
N
S
3q
3r
3s
3t
, 93%
, 86%
, 80%
, 75%
CH2F
CH2F
O
CH2F
N
N
CH2F
, 79%
N
Ts
Ni(acac)2
NiI2
NiI2
NiI2
none
3u
3v
3w
3x
, 72%
, 87%
CH2F
, 93%
CH2F
CH2F
S
CH2F
N
N
S
N
O
Ts
, 65%
dtbpy(12)/DMAP(24)
0
3y
, 77%
3z
3aa
3ab
, 89%
CH2F
, 81%
a Standard reaction conditions: 1a (0.2 mmol, 1.0 equiv), 2 (2.5
equiv), [Ni] (10 mol%), Mn (3.0 equiv), DMAc (1 mL), 40 ℃, under
N2 atmosphere, 24 h. b Yields determined by 19F NMR using PhCF3
as an internal standard. c Isolated yield. d MeCN. e 60 ℃.
O
CH2F
O
O
O
N
Ph
N
CH2F
3ac
, 81%
3ad
, 77%
3ae
, 87%
a Standard reaction conditions: 1 (0.2 mmol, 1.0 equiv), 2 (2.5 equiv),
Our study commenced with 4-bromobiphenyl 1a as the pilot
substrate, and bromofluoromethane 2 as the coupling partner
in the presence of a catalytic amount of NiI2 (10 mol%) and Mn
(3 equiv) in DMAc (1 mL) at 40 ℃ for 24 hours. Different kinds
of nitrogen and phosphine ligands were first examined (entries
1-9; see the SI for details) and three kinds of bidentate pyridine-
type nitrogen ligands gave the desired monofluoromethylated
product 3a in approximate yields (64%-79%, entries 5-7).
Inspired by our previous results on combinatorial nickel
catalysis11, we then investigated various combinations of N/P
and N/N ligands. To our delight, a mixture of DMAP and dtbpy
(N/N) as a simple ligand combination further improved the yield
to 85% (entry 11), whereas other kinds of ligand combinations
were ineffective (entries 10-16; see the SI for details).
Additionally, careful screening of solvents, nickel sources and
temperature indicated DMAc, NiI2 and 40 ℃ were the best
choices (entries 17–21; see the SI for details). Lastly, it was
confirmed that none of the desired product 3a was detected in
NiI2 (10 mol%), dtbpy (12 mol%), DMAP (24 mol%), Mn (3.0 equiv),
b
DMAc (1 mL), 40 ℃, under N2 atmosphere, 24 h. Isolated yields.
Yield determined by 19F NMR using PhCF3 as an internal standard.
With the optimal conditions in hand, we then probed the
scope of aryl and heteroaryl bromides 1 with substrate 2. As
shown in table 2, the substituent groups on the phenyl ring,
including electron-donating groups such as Ph (3a, 3b) Me (3c,
3d), OMe (3f-h), OBn (3i) and OCOR (3j), and electron-
withdrawing groups such as ketones (3k-m) at different
positions on the phenyl ring of aryl bromides, were
monofluoromethylated smoothly in good to excellent yields
(68–94%). Meanwhile, aryl bromides installed with Cl (3n) or
polycyclic aryl group (3o) were also suitable for this
transformation. After the successful monofluoromethylation of
aryl bromides, our next concern was the direct
monofluoromethylation of heteroaryl compounds by reductive
cross-coupling that has never been achieved before.10-11,15 To
our delight, several heterocyclic substituents on the aryl
2 | J. Name., 2012, 00, 1-3
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bromides, such as carbazole (3p-q), morphine (3r), pyridyl (3s) power in DMAc, nor the subjection of 2 to standaVridewcAornticdleitOionlnines
and thiophene (3t), were also well tolerated in this catalytic in the absence of aryl bromide. Not surprisingly,
DOI: 10.1039/C9CC0373n7Co
reaction, providing the desired products in satisfactory yields hydrogenated product was detected in either case.
(75%-93%). Various kinds of heteroaryl bromides, such as On the basis of the results of the above experiments and
quinolyl (3u-v), indolyl (3w), dibenzofuryl (3x), dibenzothienyl previous reports9b-c, 11a, two reaction mechanisms, radical cage
(3y), benzoxazolyl (3z) and benzothiazolyl (3aa) were also rebound process (Path a) and radical chain mechanism (Path b),
compatible with this reaction, which thus paved the way for were proposed for this transformation. For Path a (Scheme 3a),
incorporation of monofluoromethyl group into such heteroaryl NiI2 was reduced to Ni0 (A) firstly, and an oxidative addition with
rings. It was worth noting that even cross-coupling with (hetero)aryl bromides 1 occurred to form Ni(II) complex
bromides on heteroaryl-rings, such as indolyl (3ab), quinolyl [(Ar)NiII(Ln)Br] (B) next. Then Ni(II) complex B was reduced to
(3ac) and pyridyl (3ad), which to the best of our knowledge had produce nickel complex [(Ar)NiI(Ln)] (C), which underwent the
never been achieved before due to the miscellaneous second oxidative addition with BrCH2F to produce
byproducts it might generate, afforded the expected products [(Ar)NiIII(Ln)CH2FBr] (E) through a radical cage rebound process,
in good to excellent yields (65%-81%). Indeed, we believe this in which the monofluoromethyl radical was generated via a
protocol might offer a solution for direct introduction of single electron transfer pathway. Finally, reductive elimination
fluorine-containing building blocks into heteroaryl rings. To of E provided the desired product and released the [BrNiI(Ln)]
demonstrate the synthetic potential and the functional group (F), which was further reduced by Mn to regenerate complex A.
tolerance of this transformation, this catalytic system has been Alternatively, a radical chain mechanism (Path b) was also
applied to the late-stage monofluoromethylation of possible in this transformation. The intermediate B could be
fenofibrate-derived aryl bromide (3ae), which afforded the oxidated by monofluoromethyl radical, which was generated by
corresponding monofluoromethylated product in excellent reaction of [BrNiI(Ln)] with BrCH2F diffused to the solution, to
yield (87%).
produce [(Ar)NiIII(Ln)CH2FBr] (E). Ni(III) complex E underwent
reductive elimination to provide the monofluoromethylated arene
and released [NiII(Ln)Br2], which could be reduced by Mn to
regenerated complex A.
Br
Br
CH2F
standard conditions
+
+
BrCH2F
(1)
(2)
TEMPO (1 equiv)
Ph
Ph
NiI2, Ligand
NiI2, Ligand
a
b
1a
2
3a
, 0%
Mn
Mn
standard conditions
CH2F
MnI2
0.5 MnBr2
MnI2
Ar Br
Ar Br
BrCH2F
MnBr2
LnNi0
1
LnNi0
(1S)-(1)-beta-Pinene
1
Ph
0.5 Mn
(2 equiv)
Mn
A
A
1a
2
4
, 3%
Ar
LnNiI Br
LnNiII Br
Ar
3a
+
, 85%
Br
F
LnNiII
B
Ar CH2F
LnNiII Br
1) Mn (3 equiv), DMAc,
N2, 40 oC, 24 h
B
0.5 Mn
0.5 MnBr2
CH2F
3
B
G
(3)
(4)
Br
BrCH2F
CH3F
2)2 M HCl, rt, 0.5 h
LnNiIII Ar
LnNiI Ar
C
2
2'
, N.D.
Br
CH2F
BrCH2F
LnNiIII Ar
LnNiI Br
E
Br
2
LnNiII Ar
+
CH2F
1) standard conditions
2) 2 M HCl, rt, 0.5 h
CH2F
BrCH2F
F
BrCH2F
CH3F
E
2
Ar CH2F
2'
, N.D.
2
D
3
Scheme 2 Mechanism studies
Scheme 3 Proposed reaction mechanism. a radical cage rebound
process. b radical chain process
To gain some insight into the mechanism of this
transformation, we next carried out a series of control
experiments. First, the reaction was completely quenched
when 1.0 equivalent of 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO) was added into the reaction, indicating that this
reaction might process via a radical path.14 To further confirm
this speculation, radical scavenger (1S)-(1)-beta-Pinene (2
equiv) was subjected into the standard conditions, which give
the cycle-opening product 4 smoothly, albeit with a pretty low
yield. Both findings demonstrate that a free monofluoromethyl
radical is generated in the catalytic cycle.6g, 15 Finally, several
verification experiments were performed to rule out the
possibility that this nickel-catalyzed transformation proceeded
through cross coupling of aryl bromides with monofluoroalkyl
manganese species, which was generated in situ between
BrCH2F (2) and Mn. Neither monofluoromethyl manganese
species were detected upon the treatment of 2 with manganese
In conclusion, we have developed
monofluoromethylation of aryl and heteroaryl bromides by
a nickel-catalyzed
reductive cross-coupling. Compared with the known direct
11a
monofluoromethylation methods,7-9,
our designed system
stands out with its facile operation, economic choice of readily
available substrates, and excellent functional groups tolerance,
especially for a variety of heteroaromatics and pharmaceutical.
Further exploration of the mechanistic details and the
application of this method to the modification of complex
biologically active molecules with fluorine groups are still
ongoing in our laboratory.
We gratefully acknowledge the National Basic Research
Program of China (973 Program 2015CB856600), the National
Science Foundation of China (21772187, 21522208) for financial
support.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Conflicts of interest
There are no conflicts to declare.
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