Pd-catalyzed nucleophilic fluorination of aryl bromides

Article abstract of DOI:10.1021/ja5009739

On the basis of mechanism-driven reaction design, a Pd-catalyzed nucleophilic fluorination of aryl bromides and iodides has been developed. The method exhibits a broad substrate scope, especially with respect to nitrogen-containing heteroaryl bromides, and proceeds with minimal formation of the corresponding reduction products. A facilitated ligand modification process was shown to be critical to the success of the reaction.

Full text of DOI:10.1021/ja5009739

Communication
pubs.acs.org/JACS
Pd-Catalyzed Nucleophilic Fluorination of Aryl Bromides
Hong Geun Lee, Phillip J. Milner, and Stephen L. Buchwald*
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States
S
* Supporting Information
ranged from 0.10% to 1.6% with an average of 0.5%. In only one
case was the reduction product formed in greater than 1% yield
(see the Supporting Information).9
To expand our previously reported strategy (X = OTf, M = Cs;
Scheme 1) to the reaction of aryl bromides, we hypothesized that
ABSTRACT: On the basis of mechanism-driven reaction
design, a Pd-catalyzed nucleophilic fluorination of aryl
bromides and iodides has been developed. The method
exhibits a broad substrate scope, especially with respect to
nitrogen-containing heteroaryl bromides, and proceeds
with minimal formation of the corresponding reduction
products. A facilitated ligand modification process was
shown to be critical to the success of the reaction.
Scheme 1. Catalytic Cycle of Pd-Catalyzed Fluorination
n recent years, the synthesis of fluorinated arenes has attracted
I
much attention from the community of synthetic organic
chemists as a result of the importance of these compounds in
pharmaceutical1 and radiological2 applications. Among the
various strategies for their preparation,3 nucleophilic fluorination
of aryl (pseudo)halides with a metal fluoride salt (MF) is ideal
because of the wide availability of substrates and easy access to
the fluoride source.4 In 2009, we reported a method for the
nucleophilic fluorination of aryl triflates, and to a lesser extent
aryl bromides, using Pd catalysts based on bulky biarylphosphine
ligands.5 However, only highly activated (i.e., ortho-substituted
and electron-deficient) aryl bromides could be successfully
fluorinated. More recently, we described an improved system for
the fluorination of aryl triflates using AdBrettPhos (1)-based Pd
precatalyst 2 (Figure 1).6
two modifications would be required. First, a more reactive metal
fluoride might be necessary to facilitate transmetalation, since the
Pd(II) center of an aryl bromide oxidative addition complex is
less readily substituted than that of a cationic triflate complex. We
postulated that AgF, containing the bromophilic Ag(I) counter-
cation, might drive the transmetalation by irreversible formation
of AgBr. Second, a substoichiometric amount of base might be
required to facilitate ligand activation. In our recent report, we
showed that base induces an in situ ligand modification during
the catalytic fluorination of aryl triflates (Scheme 2, A to B to C, X
Scheme 2. Ligand Modification during the Catalytic
Fluorination
Figure 1. Structure of ligands 1 and 3 and precatalyst 2.
Since our initial report, however, only a single method that
converts unactivated aryl halides to the corresponding aryl
fluorides (a Cu-mediated fluorination of aryl iodides) has been
described.7 However, this reaction, like many other transition-
metal-mediated fluorination methods, is hampered by the
formation of reduction products that are usually difficult to
separate from the desired aryl fluoride. Herein we report a
practical method for the Pd-catalyzed nucleophilic fluorination of
(hetero)aryl bromides that are not activated toward direct
nucleophilic substitutions or transition-metal-mediated reac-
tions.3b Our system is applicable to the fluorination of widely
available aryl bromides,8 and the process does not result in
significant reduction of the aryl halide. Reduction products were
not observed for a majority of the examples. In those cases for
which reduction products were formed, the amount of Ar−H
= OTf) and that the catalyst is effective only when it is supported
by the modified ligand.10 In addition, this event was shown to be
more pronounced in the case of aryl halide oxidative addition
complexes (Scheme 2, A to B, X = Cl or Br),11 suggesting that the
process is important in the catalytic fluorination of aryl bromides.
We reasoned that AgF might not be basic enough to induce the
elimination of HX (Scheme 2, B to C), rendering the ligand
Received: January 28, 2014
Published: February 21, 2014
© 2014 American Chemical Society
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modification inefficient. Therefore, it was initially hypothesized
that the introduction of a substoichiometric amount of base
would promote the formation of C [3·Pd(0)].
Table 2. Pd-Catalyzed Fluorination of Aryl Halides Using
Precatalyst 2
a
To test this hypothesis, the fluorination of 3-bromo-N,N-
dimethylaniline was attempted using precatalyst 2 as the Pd
source (Table 1).12 As expected, a metal fluoride alone was not
Table 1. Effect of Base in the Pd-Catalyzed Fluorination of an
a
Aryl Bromide Using Precatalyst 2
b
entry
MF
base
yield (%)
1
2
3
4
5
6
7
8
9
KF
−
−
−
0
0
CsF
AgF
AgF
AgF
AgF
AgF
AgF
AgF
0
NaHCO3
K2CO3
K3PO4
NaF
0
11
21
0
KF
71
65
CsF
a
Reaction conditions: 3-bromo-N,N-dimethylaniline (0.10 mmol), MF
(0.20 mmol), base (0.050 mmol), 2 (0.0020 mmol), cyclohexane (1
b
mL), 130 °C, 14 h. Determined by 19F NMR analysis.
a
effective in promoting the transformation (entries 1−3).
However, when AgF was used in combination with 0.5 equiv13
of K2CO3 (entry 5) or K3PO4 (entry 6), the desired aryl fluoride
product was formed in modest yield, while the use of NaHCO3
was ineffective (entry 4). Ultimately, fluoride bases (except NaF)
were shown to be the most effective for promoting the reaction in
high yield (entries 7−9). Even though KF and CsF are equally
competent, inexpensive and less hygroscopic KF was chosen as
the base for reasons of cost and practicality.
Isolated yields (averages of two runs) are shown. Reaction
conditions: aryl halide (1.0 mmol), AgF (2.0 mmol), KF (0.50
b
mmol), 2 (0.010−0.020 mmol), cyclohexane (10 mL), 14 h. Toluene
c
was used as the solvent. 0.50 mmol scale; yield was determined by 19
NMR analysis.
F
Table 3. Effects of Precatalyst and KF in the Pd-Catalyzed
Fluorination of a Heterocyclic Aryl Bromide
a
The optimized reaction conditions were applied to the
fluorination of a variety of aryl bromides and iodides (Table
2).14 Substrates with ortho (4 and 5), meta (6−13), and para
(14−19) substituents were fluorinated with comparable
efficiencies regardless of the electronic nature of the substituents.
Importantly, base-sensitive functional groups such as methyl
sulfone (6), fluorene (10), and ketone (14 and 15) groups were
tolerated, as were nitrile (7), ester (8), and amide (16) groups,
which are common in pharmaceutical compounds. In addition,
substrates that are not amenable to electrophilic fluorination
becauses of potential oxidation, such as alkyl sulfides (9) and
electron-rich amines (12 and 13), were also fluorinated in good
yields.15 Unactivated aryl iodides were also viable substrates for
this reaction (10, 14, and 17). Analogous to our fluorination of
aryl triflates, 4-bromoanisole, which is electron-rich and lacks
ortho substituents, provided a mixture of regioisomers (19).5,6
We next tried to extend this methodology to the fluorination of
heteroaryl bromides using 3-bromo-5-cyanopyridine as a
representative substrate. Even though the addition of KF
significantly improved the reaction, the conditions failed to
provide the fluorinated heteroarene in a satisfactory yield (Table
3, entries 1 and 2). To remedy this situation, we decided to use a
precatalyst with a new ligand that had been modified ex situ. We
postulated that with heteroaryl substrates, either the ligand
modification process is not facile or the resulting modified ligand
(analogous to 3) is not effective for the fluorination reaction.
b
entry
precatalyst
base
yield (%)
1
2
3
4
2
2
−
KF
11
48
21
72
22
22
−
KF
a
Reaction conditions: 3-bromo-5-cyanopyridine (0.10 mmol), AgF
(0.20 mmol), KF (0.050 mmol), 2 or 22 (0.0020 mmol), 2-MeTHF (1
b
mL), 130 °C, 14 h. Determined by 19F NMR analysis.
Therefore, by using a premodified ligand such as 3, we could
circumvent the modification process altogether, allowing us to
take full advantage of the established system.
A synthetic route to ligand 3 is described in Scheme 3.16 The
known oxidative addition complex 2011 underwent the desired
rearrangement, elimination, and oxidative addition cascade in the
presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1-
bromo-4-n-butylbenzene.10 The new complex 21 with the
modified ligand was treated with excess propane-1,2-diamine
to provide free phosphine 3 in 49% overall yield from 20. Ligand
3 was successfully converted to the corresponding cyclo-
octadiene-ligated Pd(0) precatalyst 22 using our reported
conditions.6,17
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a
Scheme 3. First Synthesis of Ligand 3 and Precatalyst 22
Table 4. Pd-Catalyzed Fluorination of Heterocyclic Aryl
Bromides Using Precatalyst 22
a
a
Reagents and conditions: (a) DBU, 1-bromo-4-n-butylbenzene, THF,
23 °C, 12 h, 81%. (b) Propane-1,2-diamine, THF, 23 °C, 12 h, 61%.
b
(c) [(COD)Pd(CH2TMS)2], pentane, 23 °C, 48 h, 79%. Ellipsoids
are shown at 50% probability.
Alternatively, a more robust and scalable synthesis was also
developed for access to 3 on a gram scale (Scheme 4). The 4-n-
a
a
Scheme 4. Scalable Synthesis of Ligand 3
Isolated yields (averages of two runs) are shown. Reaction
conditions: aryl halide (1.0 mmol), AgF (2.0 mmol), KF (0.50
b
mmol), 22 (0.010−0.030 mmol), 2-MeTHF (10 mL), 14 h. 0.10
c
d
mmol scale. Determined by 19F NMR analysis. TBME was used as
e
f
the solvent. Cyclohexane was used as the solvent. 0.50 mmol scale.
were prepared from the corresponding bromides (27−29),
although 3-bromopyridine itself furnished a 1:1 para/meta
mixture of aryl fluoride regioisomers (not shown). In line with
our previous observations, forcing conditions were required to
synthesize electron-rich fluoropyridine 29.6 Indole (30), quino-
line (31−33), and isoquinoline (34 and 35) derivatives were also
prepared in high yields. Heteroaryl bromides with more than one
nitrogen atom, such as pyrimidine (36), indazole (37), and
quinoxaline (38), were also suitable substrates. Finally, the
vascular disorder drug Nicergoline (Sermion)22 was fluorinated
to provide analogue 39. This result suggests that our method-
ology should be viable for the late-stage fluorination of advanced
intermediates where pharmaceutically relevant functional groups
are present. Notably, five-membered heteroaryl halides remain
challenging substrates for this methodology.
In conclusion, we have developed a new Pd-catalyzed method
for converting unactivated (hetero)aryl bromides and iodides to
the corresponding fluorides. The reaction proceeds without
significant formation of reduction byproducts, which are difficult
to separate from the desired product. The success of the reaction
stems from the use of AgF with added KF to promote the
reaction and from control of the ligand modification process we
previously observed. Future efforts will be focused on identifying
the exact role of KF during the reaction and applying this
methodology to the synthesis of 18F-containing radiotracers.
a
Reagents and conditions: (a) 23 (1.0 mol %), K3PO4, THF/H2O, 80
°C, 12 h, 99%. (b) Br2, DMF/CH2Cl2, 0 °C, 30 min, 76%. (c) t-BuLi,
THF, −78 °C, 1 h; added to 2-fluoro-1,4-dimethoxybenzene and n-
BuLi in THF, −78 to 0 °C, 2 h; Br2, 0 °C, 30 min, 65%. (d) t-BuLi,
THF, −78 °C, 1 h; CuCl; Ad2PCl, toluene, 140 °C, 48 h, 54%.
butylphenyl moiety was installed by Suzuki−Miyaura cross-
coupling of commercially available 4-n-butylphenylboronic acid
in the presence of XPhos-based precatalyst 23.18 The resulting
biaryl 24 was monobrominated to furnish 25, which was
converted to the desired product via triaryl 26 using our
previously reported protocol.19
The new precatalyst 22 (Table 3, entry 3) showed improved
reactivity compared to 2 (entry 1). Unexpectedly, the use of 22
alone with AgF (entry 3) was not as effective as the combined use
of 2 with KF and AgF (entry 2). Only when a combination of KF
and AgF was used with 22 was the product obtained in high yield
(entry 4). This result suggests that KF plays a role beyond simply
assisting the modification of 1.20
Employing 22 enabled the successful preparation of
fluorinated heteroarenes that are not accessible through
traditional SNAr reactions (Table 4). Importantly, the use of
22 rather than 2 for the fluorination of heteroaryl bromides
provided superior results in almost all instances (the yields
obtained using 2 under identical conditions are shown in
parentheses for selected cases).21 In addition, reduction products
were not observed (by GC) in the purified samples in any case,
with the exception of 30, for which 0.10% Ar−H was observed. 3-
Fluoropyridine derivatives with varying electronic properties
ASSOCIATED CONTENT
* Supporting Information
Experimental details and compound characterization data. This
material is available free of charge via the Internet at http://pubs.
acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
sbuchwal@mit.edu
■
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transmetalation with AgF in these cases. Unprotected amino/hydroxyl
groups are also not tolerated.
(15) Tang, P.; Furuya, T.; Ritter, T. J. Am. Chem. Soc. 2010, 132, 12150.
(16) Preliminary unpublished results suggest that 3 containing 4-n-
butylphenyl gives the highest yield compared with ligands modified with
other aryl groups.
(17) Similar to 2, precatalyst 22 is insoluble in most organic solvents.
Therefore, spectroscopic characterization was not possible. However, it
readily forms the oxidative addition complex 21 and 1,5-COD upon
exposure to 1-bromo-4-n-butylbenzene. See the Supporting Information
for details.
(18) Bruno, N. C.; Tudge, M. T.; Buchwald, S. L. Chem. Sci. 2013, 4,
916.
(19) (a) Hoshiya, N.; Buchwald, S. L. Adv. Synth. Catal. 2012, 354,
2031. (b) Fors, B. P.; Dooleweerdt, K.; Zeng, Q.; Buchwald, S. L.
Tetrahedron 2009, 65, 6576.
(20) Related fluoride effects have been documented. See: (a) Su, W.;
Raders, S.; Verkade, J. G.; Liao, X.; Hartwig, J. F. Angew. Chem., Int. Ed.
2006, 45, 5852. (b) Pan, J.; Wang, X.; Zhang, Y.; Buchwald, S. L. Org.
Lett. 2011, 13, 4974.
(21) It was discovered that 22 is not as effective as 2 for certain non-
heterocyclic substrates. This result suggests that 4-n-butylphenyl may
not be the best modifying group for the fluorination of non-heterocyclic
substrates. The origin of this effect is under investigation in our lab.
However, ligand 3 was chosen for study because of its ease of synthesis
and generality.
Notes
The authors declare the following competing financial
interest(s): MIT has obtained or has filed patents on some of
the ligands/precatalysts that are described in the paper from
which S.L.B. and former/current coworkers receive royalty
payments.
ACKNOWLEDGMENTS
■
Research reported in this publication was supported by the
National Institutes of Health under Award GM46059. This
content is solely the responsibility of the authors and does not
necessarily reflect the views of the National Institutes of Health.
P.J.M. thanks the National Science Foundation for a predoctoral
fellowship (2010094243). We also thank Amgen for an
educational donation. The X-ray diffractometer was purchased
with the help of funding from the National Science Foundation
(CHE 0946721). Dr. Peter Muller (MIT) is acknowledged for
̈
solving the X-ray structure of 3. We also thank Dr. Aaron Sather
and Mr. Nicholas Bruno (MIT) for assistance with the
preparation of the manuscript and GC analysis, respectively.
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(8) Aryl iodides are also viable substrates for this reaction. However, we
focused on aryl bromides since they are more widely available.
(9) The starting materials corresponding to 4 and 5 were contaminated
with 0.85% and 0.3% Ar−H, respectively. See the Supporting
Information for details.
(10) Maimone, T. J.; Milner, P. J.; Kinzel, T.; Zhang, Y.; Takase, M. K.;
Buchwald, S. L. J. Am. Chem. Soc. 2011, 133, 18106.
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(12) Other sources of Pd were also evaluated; we found that precatalyst
2 is the most effective Pd source. See the Supporting Information for
details.
(13) The optimal loading of KF was found to be 0.5 equiv. See the
Supporting Information for details.
(14) To date, this reaction is not effective for unactivated aryl chlorides
and substrates with bulky ortho substituents, likely because of slower
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