Combinatorial Nickel-Catalyzed Monofluoroalkylation of Aryl Boronic Acids with Unactivated Fluoroalkyl Iodides

Article abstract of DOI:10.1021/acs.orglett.7b02012

A combinatorial nickel-catalyzed cross-coupling between arylboronic acids and unactived 1-fluoro-1-iodoalkanes has been developed, which demonstrated high efficiency, mild conditions, and excellent functional-group compatibility. Readily available nitrogen and phosphine ligands were combined with a nitrogen source, which in situ generated a variety of easily tunable catalysts to promote the fluoroalkylation for broad scopes of both coupling partners. This new strategy on combinatorial catalysis offers new solutions for nickel-catalyzed cross-coupling reactions.

Full text of DOI:10.1021/acs.orglett.7b02012

Letter
pubs.acs.org/OrgLett
Combinatorial Nickel-Catalyzed Monofluoroalkylation of Aryl
Boronic Acids with Unactivated Fluoroalkyl Iodides
Jie Sheng, Hui-Qi Ni, Ge Liu, Yan Li, and Xi-Sheng Wang*
Hefei National Laboratory for Physical Sciences at the Microscale and Department of Chemistry, University of Science and
Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, China
*
S Supporting Information
ABSTRACT: A combinatorial nickel-catalyzed cross-coupling between
arylboronic acids and unactived 1-fluoro-1-iodoalkanes has been
developed, which demonstrated high efficiency, mild conditions, and
excellent functional-group compatibility. Readily available nitrogen and
phosphine ligands were combined with a nitrogen source, which in situ
generated a variety of easily tunable catalysts to promote the fluoroalkylation for broad scopes of both coupling partners. This
new strategy on combinatorial catalysis offers new solutions for nickel-catalyzed cross-coupling reactions.
ransition-metal-catalyzed cross-coupling reactions have
alkyl) have emerged as interesting moieties, as the incorporation
T
emerged as a powerful strategy for selective construction
of a fluorine atom at the benzylic position could dramatically
1
7
of carbon−carbon bonds in recent decades. Because of their key
roles in the catalytic cycle to enhance both efficiency and
selectivity of such reactions, the discovery of efficient ligands has
modify the biological activity of nonfluorinated compounds. For
example, application of this strategy afforded monofluorinated
mitochondrial complex 1 (Figure 1), an inhibitor for coronary
2
come into major focus in transition-metal catalysis. While there
is no universal ligand to accommodate all substrates and
reactions, the design and synthesis of new skeletons or selective
modification of the specific positions of known skeletons has
been used as the most common tactics to develop novel ligands.
However, these methods for tuning the steric and electronic
effects of catalysts normally require multistep synthesis, which
thus limits their utility in organic synthesis. Meanwhile, the self-
assembly of a central metal with two different ligands would
3
create various easily tunable catalysts, which offers an alternative
Figure 1. Representative bioactive molecules containing monlfluor-
oalkylated arenes.
solution to promote the target reaction via structural
modification of catalysts. Such heteroleptic catalysts with two
different ligands may be more active and selective than the
4
homocombinational ones. In addition, this approach could
7a
artery disease with significantly improved IC50 bioactivity;
a
easily generate greater catalyst diversity via a simple change of the
combination of readily available ligands, which will make the fine-
tuning of steric and electronic effects of catalysts more
convenient. While combinatorial transition metal catalysis was
normally considered only as a valuable path to discover optimal
remarkable improvement in bioactivity was also detected when
CH at the benzylic position was replaced with CHF in
2
compound B, which dramatically changed the IC of the
50
7b
enzyme combination agent MMP-13. Despite its importance,
the synthetic utility of ArCHF-alkyl is still very limited. Recently,
a direct C−H fluorination at the benzylic position has been
developed by the Groves, Lectka, Chen, and Tang groups, but
still suffered from relatively poor functional-group compati-
3b,c,5
catalysts via high-throughput screening,
such easily tunable
catalysts also demonstrate greater utility due to their in situ
generation for a broader scope of substrates. Accordingly, the
development of highly efficient catalysts via molecular
recognition and self-assembly of different readily available
ligands with metals offers an efficient solution to unresolved
problems on transition-metal catalysis, especially for catalytic
activity and reaction scope.
8
bility. Meanwhile, the Gandelman group reported the only
example of nickel-catalyzed cross-coupling of triphenylborane
with 1-halo-1-fluoroalkanes to make monofluoroalkylated
9
arene. However, the requirement of normally expensive and
not readily available triarylboranes as the coupling partner will
thus hamper its utility in organic synthesis. And most
importantly, neither method is suitable for late-stage fluoroalky-
The selective introduction of fluorine or fluorinated moieties
into organic molecules has long been realized as a powerful
strategy in drug design and development, as the fluorinated
compounds can drastically enhance the metabolic stability,
6
lipophilicity, and bioavailability of parent compounds. Among
Received: July 2, 2017
all fluorinated groups, monofluoroalkylated arenes (ArCHF-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02012
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
lation of complex molecules because of the limited availability of
the corresponding transformable precursors.
(entry 7). To improve the yield further, other nickel sources were
next examined under this combinational P/N ligand catalytic
Herein, we report the first example of combinatorial P/N
ligands nickel-catalyzed monofluoroalkylation of arylboronic
acids with unactivated fluoroalkyl iodides, in which high catalytic
reactivity, mild conditions, and broad scope have been
system, and Ni(OTf) was demonstrated as the optimal choice
2
with a 91% yield (entry 14). Meanwhile, the amount of ligand
was carefully studied, but both increasing and decreasing ligand
loading only produced lower yields (entries 15−16; for details,
see the Supporting Information (SI)). Lastly, control experi-
ments confirmed that none of the desired products were detected
in the absence of a nickel catalyst or ligands (entries 17−18).
As nitrogen and phosphine ligand combinations demonstrated
their decisive role in this catalytic monofluoroalkylation, a
number of frequently used, commercially available ligands,
including 12 nitrogen ligands (N1−N12) and 15 phosphine
ligands (P1−P15), were chosen for combinatorial screening to
discover the optimal catalysts. As shown in Figure 2, among all
1
0
demonstrated. The key to success is the combination of
different kinds of readily available nitrogen and phosphorus
11
ligands with a nickel source, which in situ generated a variety of
easily tunable catalysts to promote the fluoroalkylation between a
broad scope of both coupling partners. This new strategy on
combinatorial catalysis will thus offer new solutions for nickel-
catalyzed cross-coupling reactions.
We commenced our initial investigation with phenylboronic
acid (1a) as the pilot substrate and 1-fluoro-1-iodo ethylbenzene
(2a) as the coupling partner in the presence of a catalytic amount
of Ni(NO ) ·6H O (5 mol %) and K CO in 1,4-dioxane at 80
3
2
2
2
3
°
C. A careful investigation of ligands, including different kinds of
nitrogen and phosphine ligands, has been carried out first. While
phosphine ligands exhibited almost no catalytic reactivity,
nitrogen ligands afforded the desired fluoroalkylated product
3
(
%
a, but with low yields (Table 1, entries 1−6). For example, PPh3
10 mol %) gave none of fluoroalkylated arene 3a and bpy (5 mol
) afforded 3a in only 16% yield. The mixture of PPh and bpy as
3
a simple ligand combination greatly improved the yield to 57%,
which clearly indicated that combinational P/N ligand catalysis
could be the best choice to promote this fluoroalkylation reaction
a
Table 1. Ni-Catalyzed Monofluoroalkylation: Optimization of
Conditions
Figure 2. Combinational screening for discovery of efficient catalysts.
a
a
Reaction conditions: 1a (2.0 equiv), 2a (0.2 mmol, 1.0 equiv),
Ni(OTf) (5 mol %), ligand (5−10 mol %), K CO (3.0 equiv), 1,4-
2
2
3
1
9
dioxane, 80 °C, 24 h. Yield was determined by F NMR spectroscopy
using PhOCF as an internal standard. P1−P15: PPh , PCy , P(4-
3
3
3
OMeC H ) , P(4-Me C H ) , P(NEt ) , PAd Bn, PAd (n-Bu), BINAP,
6
4
3
6
4
3
2
3
2
2
b
dppe, dppp, dpph, dppf, Xantphos, X-phos, Dave-phos; N1−N12: bpy,
dtbpy, dmbpy, dombpy, phen, Neocuproine, BPhen, DMEDA,
TMEDA, L1, terpy, DMAP.
entry
Ni source
ligand (mol %)
bpy (5)
yield (%)
1
2
3
4
5
6
7
8
9
Ni(NO ) ·6H O
16
13
8
3
2
2
Ni(NO ) ·6H O
dtbpy (5)
phen (5)
L1 (5)
3
2
2
Ni(NO ) ·6H O
3
2
2
these choices, bpy/PPh , bpy/dppe, bpy/Xantphos, bpy/X-
Ni(NO ) ·6H O
4
3
3
2
2
phos, phen/PCy , dtbpy/dppe, and Bphen/dppe could furnish
Ni(NO ) ·6H O
DMEDA (5)
0
3
3
2
2
the desired product 3a with excellent yields (>95%), and
approximately 10 more ligand combinations could afford good
yields (about 85%). The diversity of ligand combinations
provided extensive approaches to tune the properties of the
nickel complex from both electronic and steric effects, which thus
offered a novel solution to solve the problems of catalytic activity
and substrate scope.
Ni(NO ) ·6H O
PPh (10)
0
3
2
2
3
Ni(NO ) ·6H O
bpy (5)/PPh (10)
57
trace
80
77
trace
53
88
94(91)
89
86
0
3
2
2
3
NiCl2
NiBr2
NiI2
bpy (5)/PPh (10)
3
bpy (5)/PPh (10)
3
10
11
12
13
14
15
16
17
18
bpy (5)/PPh (10)
3
Ni(acac)2
Ni(OAc)2
bpy (5)/PPh (10)
3
bpy (5)/PPh (10)
3
With these optimal catalysts in hand, we next examined the
scope by verifying the functional groups installed on the
NiCl ·DME
bpy (5)/PPh (10)
2
3
Ni(OTf)2
Ni(OTf)2
Ni(OTf)2
bpy (5)/PPh (10)
3
arylboronic acids. As shown in Figure 3, when bpy/PPh was
bpy (5)/PPh (15)
3
3
selected as the ligand of choice, various electron-donating
bpy (10)/PPh (10)
3
substituents, including Me (3b, 3i), n-C H (3c), t-Bu (3d), Ph
bpy (5)/PPh (10)
5
11
3
(
3e, 3j), PhO (3f), MeS (3g), and TMS (3h) at the 4- or 3-
Ni(OTf)2
0
position on the phenyl ring of arylboronic acids were
monofluoroalkylated smoothly in good to excellent yields
(81−97%). However, electron-withdrawing groups were not
be well tolerated under such conditions, giving only low to
a
Unless otherwise noted, the reaction conditions were as follows: 1a
0.4 mmol), 2a (0.2 mmol, 1.0 equiv), [Ni] (5 mol %), ligand (5−10
(
b
mol %), K CO (0.6 mmol), 1,4-dioxane (2.0 mL), 80 °C, 24 h. Yield
determined by F NMR spectroscopy using PhOCF as an internal
standard; numbers in parentheses were yields of isolated products.
2
3
19
3
moderate yields using this bpy/Ni/PPh catalyst. Given that the
3
alteration of ligand combinations definitely changed the steric
and electronic effects of the catalyst system, other ligand
combinations were screened to solve this problem (for details,
see the SI). As expected, F (3l, 3q), Cl (3m, 3r, 3s), CF (3n),
3
ester (3o), and ketone (3p) were completely compatible with
B
DOI: 10.1021/acs.orglett.7b02012
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
employed accordingly (3ab, 3ac). Meanwhile, other polycyclic
arenes were well tolerated in this reaction with different
arylboronic acids (3ad). Additionally, the electronic properties
of aromatic rings (3ae, 3af) in monofluoroalkyl iodides had no
decisive influence on the reaction efficiency, and all these
fluoroalkylating reagents could be coupled to arylboronic acids
successfully with different ligand combinations changed in
response. The 1-fluoro-1-iodoalkanes with the addition of
other functional groups, such as ethers (3ag) and long-chain
alkanes (3ah), were also well tolerated in this combinatorial
catalytic system. Notably, this combinatorial nickel catalyst
system showed high reactivity with ester- and sulfone-activated
10h
monofluoromethyl halides.
Both BrCFHCO Et (2i) and
2
ICFHSO Ph (2j) were smoothly coupled with arylboronic acids
2
in excellent yields (3ai, 3aj), and electron-withdrawing
substituents, which required a higher catalyst loading and ligand
10h
batch charging in our previous report, were well tolerated in
this catalytic system with excellent yields (3yi, 3zj).
To gain some insight into the mechanism of this trans-
formation, a series of control experiments were next carried out.
First, the reaction was completely quenched when 1.0 equiv of
2
,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) was added into
the reaction system (eq 1, Figure S1, see the SI), which showed
that this reaction may proceed via a radical path. To confirm this
speculation, radical clock 4 was subjected to the standard
conditions, giving the cycle-opening product 5 in 38% yield (eq
2
, Figure S1, see the SI). This result demonstrated that a
monofluoroalkyl radical did exist in the catalytic cycle. Next, the
replacement of the Ni(II) species with Ni(0) afforded 3a in
almost the same yield (90%, eq 3, Figure S1, see the SI), which
12
indicated a possible Ni(0)/Ni(II) catalytic cycle. To determine
3
1
how the Ni(0) species was generated in the catalytic system, P
NMR was used to monitor the reaction. A 31P signal found at
2
2.7 ppm, the same as the signal detected in the bpy/Ni(COD)2/
PPh system, appeared in this bpy/Ni(OTf) /PPh catalyst
3
2
3
system in the presence of PhB(OH) and K CO . Taking also
2
2
3
into account that little biphenyl could be detected in the reaction
system by TLC and GC-MS, both results suggested that the
catalytically active Ni(0) species was in situ generated via the
Ni(II)-catalyzed homocoupling of arylboronic acid in the
13
presence of base (for details, see the SI). However, at this
stage, a Ni(I)/Ni(III) catalytic cycle could not be excluded.
To demonstrate the synthetic potential and the functional
group tolerance of this transformation, this catalytic system has
been applied to the late-stage monofluoroalkylation of complex
bioactive molecules. As shown in Figure 4, the ezetimibe-derived
Figure 3. Ni-catalyzed monofluoroalkylation: substrate scopea a Reac-
tion conditions: 1 (2.0 equiv), 2 (0.2 mmol, 1.0 equiv), Ni(OTf) (5 mol
2
%
), bpy (5 mol %)/PPh (10 mol %), K CO (3.0 equiv), 1,4-dioxane
3 2 3
b
c
(
2.0 mL), N , 80 °C, 24 h. Phen (5 mol %)/PCy (10 mol %). bpy (5
2 3
d
mol %)/P(4-MeO C H ) (10 mol %). bpy (5 mol %), dppe (5 mol
6
4 3
e
f
%
). bpy (5 mol %), X-phos (10 mol %). dtbpy (5 mol %)/PPh (10
3
g
19
mol %). Yield was determined by F NMR spectroscopy using
PhOCF as an internal standard. The compound is unstable upon
3
h
purification with silica gel chromatography. 48 h.
this nickel-catalytic system when phen/PCy was selected as the
3
ligand combination. This idea has also been used to realize the
monofluoroalkylation of ortho-substituted arylboronic acids,
which showed Me (3v), F (3w), and Cl (3x) on the 2-position of
the phenyl ring afforded excellent yields with bpy/Ni(OTf)2/
Figure 4. Fluoroalkylation of ezetimibe derivative 6.
boronic acid 6, made from ezetimibe in four steps, was
fluoroalkylated successfully to 7 in excellent yield (89%). This
late-stage modification of complex molecules clearly indicated
the great potential of this method in drug discovery and
development as an efficient and expedient tactic to make
fluorine-containing analogues.
P(4-OMePh) used as the catalyst.
3
To test whether this method could be extended to different
kinds of monofluoroalkylating reagents other than 2a, a great
number of monofluoroalkyl iodides were next demonstrated in
our reaction system. The length of the alkyl chain had no obvious
influence on the efficiency of the reaction, while the
combinatorial catalysts with different ligand combinations were
In summary, we have developed a novel method for
combinatorial nickel-catalyzed monofluoroalkylation of arylbor-
C
DOI: 10.1021/acs.orglett.7b02012
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
onic acids, in which the combination of nickel with phosphine
and nitrogen ligands was crucial to increase the catalytic activity
for monofluoromethylation. Both coupling partners were well
tolerated in this catalytic system by tuning the electronic and
steric properties of catalysts via combination of readily available
ligands. Mechanistic investigations indicated a Ni /Ni catalytic
cycle involving a monofluoroalkyl radical were generated in situ.
Further exploration of the mechanistic details of this catalytic
cycle and application of this method to fluorine-containing
modification of complex biologically active molecules are
ongoing in our laboratory.
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ASSOCIATED CONTENT
Supporting Information
■
(
(
b) Xia, J.-B.; Zhu, C.; Chen, C. J. Am. Chem. Soc. 2013, 135, 17494.
c) Bloom, S.; Pitts, C. R.; Woltornist, R.; Griswold, A.; Holl, M. G.;
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02012.
Lectka, T. Org. Lett. 2013, 15, 1722. (d) Bloom, S.; McCann, M.; Lectka,
T. Org. Lett. 2014, 16, 6338. (e) Xu, P.; Guo, S.; Wang, L.; Tang, P.
Angew. Chem., Int. Ed. 2014, 53, 5955.
Experimental procedure and characterization of all new
compounds (PDF)
(9) (a) Jiang, X.; Sakthivel, S.; Kulbitski, K.; Nisnevich, G.; Gandelman,
M. J. Am. Chem. Soc. 2014, 136, 9548. (b) Jiang, X.; Gandelman, M. J.
Am. Chem. Soc. 2015, 137, 2542.
AUTHOR INFORMATION
Corresponding Author
(10) (a) Doi, H.; Ban, I.; Nonoyama, A.; Sumi, K.; Kuang, C.; Hosoya,
T.; Tsukada, H.; Suzuki, M. Chem. - Eur. J. 2009, 15, 4165. (b) Guo, C.;
Yue, X.; Qing, F.-L. Synthesis 2010, 2010, 1837. (c) Zhao, Y.; Gao, B.; Ni,
C.; Hu, J. Org. Lett. 2012, 14, 6080. (d) Fujiwara, Y.; Dixon, J. A.;
O’Hara, F.; Funder, E. D.; Dixon, D. D.; Rodriguez, R. A.; Baxter, R. D.;
Herle, B.; Sach, N.; Collins, M. R.; Ishihara, Y.; Baran, P. S. Nature 2012,
■
*
E-mail: xswang77@ustc.edu.cn.
ORCID
Xi-Sheng Wang: 0000-0003-2398-5669
Notes
4
92, 95. (e) Zhao, Y.; Ni, C.; Jiang, F.; Gao, B.; Shen, X.; Hu, J. ACS
Catal. 2013, 3, 631. (f) Liang, Y.; Fu, G. C. J. Am. Chem. Soc. 2014, 136,
520. (g) Hu, J.; Gao, B.; Li, L.; Ni, C.; Hu, J. Org. Lett. 2015, 17, 3086.
h) Su, Y.-M.; Feng, G.-S.; Wang, Z.-Y.; Lan, Q.; Wang, X.-S. Angew.
Chem., Int. Ed. 2015, 54, 6003.
11) For examples of phosphines used as additives to enhance the
5
(
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(
We gratefully acknowledge the Strategic Priority Research
Program of the Chinese Academy of Sciences (Grant No.
XDB20000000), the National Basic Research Program of China
973 Program 2015CB856600), the National Science Founda-
tion of China (21602213, 21522208, 21372209), and the
Fundamental Research Funds for the Central Universities
catalytic reactivity, see: (a) Everson, D. A.; Shrestha, R.; Weix, D. J. J. Am.
Chem. Soc. 2010, 132, 920. (b) Xiao, Y.-L.; Guo, W.-H.; He, G.-Z.; Pan,
Q.; Zhang, X. Angew. Chem., Int. Ed. 2014, 53, 9909. For examples of
nitriles used as additives to enhance the catalytic reactivity, see: (c) Ge,
S.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 16330. (d) Ge, S.; Green,
R. A.; Hartwig, J. F. J. Am. Chem. Soc. 2014, 136, 1617. (e) Yin, G.;
Kalvet, I.; Englert, U.; Schoenebeck, F. J. Am. Chem. Soc. 2015, 137,
(
(
WK2060190046) for financial support.
4
164. For examples of pyridine or pyridine derivatives used as additives
to enhance the catalytic reactivity, see: (f) Biswas, S.; Weix, D. J. J. Am.
Chem. Soc. 2013, 135, 16192. (g) An, L.; Xiao, Y.-L.; Min, Q.-Q.; Zhang,
X. Angew. Chem., Int. Ed. 2015, 54, 9079.
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