Asymmetric palladium-catalyzed directed intermolecular fluoroarylation of styrenes

Article abstract of DOI:10.1021/ja412881j

A mild catalytic asymmetric direct fluoro-arylation of styrenes has been developed. The palladium-catalyzed three-component coupling of Selectfluor, a styrene and a boronic acid, provides chiral monofluorinated compounds in good yield and in high enantiom

Full text of DOI:10.1021/ja412881j

Communication
pubs.acs.org/JACS
Asymmetric Palladium-Catalyzed Directed Intermolecular
Fluoroarylation of Styrenes
Eric P. A. Talbot,*,†,‡,§ Talita de A. Fernandes,‡ Jeffrey M. McKenna,*,† and F. Dean Toste*,‡
†Novartis Institutes for Biomedical Research, Horsham,West Sussex, RH12 5AB, United Kingdom
‡Department of Chemistry, University of California, Berkeley, California 94720, United States
S
* Supporting Information
We had previously shown that sp3 C−F occurred from gold(III)-
fluorides11 and Ritter and Sanford have elegantly demonstrated
ABSTRACT: A mild catalytic asymmetric direct fluoro-
arylation of styrenes has been developed. The palladium-
catalyzed three-component coupling of Selectfluor, a
that C−F bonds could be generated from the reductive
elimination of high-valent palladium.12 Therefore, we reasoned
styrene and a boronic acid, provides chiral monofluori-
nated compounds in good yield and in high enantiomeric
excess. A mechanism proceeding through a Pd(IV)-
fluoride intermediate is proposed for the transformation
and synthesis of an sp3 C−F bond.
that the directing group could control the regioselectivity and
stabilize the high-valent metal intermediate in conjunction with
the bipyridine ligand, diverting it from an oxidative Heck-type
coupling reaction toward C−F bond formation.13
On the basis of this hypothesis, and starting from commercially
available 2-vinylbenzoic acid, we sought an effective directing
group (DG) that could provide the desired control of
regiochemistry and product distribution. The PdCl2/BiPy-
catalyzed reaction of 1a with p-tolylboronic acid and
luorine substituents have become widespread in their use in
F
drugs,1 their presence in a plethora of agrochemicals2 and in
high-performance materials. The introduction of fluorine affects
a multitude of properties, and it is well-known that C−(F)x
substituents can induce conformational changes, alter pKa and
dipole moments, increase the lipophilicity, and beneficially alter
the oral bioavailability of a drug.2,3 Given this perspective, their
introduction has attracted considerable attention from the
synthetic community, and the safe and selective introduction
of these moieties into the desired scaffold is key to the further
expansion in these areas of research. The asymmetric synthesis of
sp3 C−F bonds4 is most commonly achieved through α-
fluorination of ketones and aldehyde derivatives with the use of
organocatalysts,5 phase transfer catalysis,6 or by way of ring-
opening of strained heterocycles.7 Few examples of enantiose-
lective transition-metal-catalyzed fluorination exist, and where
they do, the metal generally serves as a Lewis acid.8 Herein, we
report a palladium-catalyzed arylative fluorination and its
application toward the enantioselective construction of sp3 C−
F bonds.
15
Selectfluor,14 in wet CH2Cl2 containing tert-butylcatechol,16
afforded the benzylic fluoride 1b in 57% yield (Scheme 1).
a
Scheme 1. Screening of Suitable Directing Group
During the course of our studies on the Selectfluor promoted
gold-catalyzed intramolecular aminoarylation of styrene 1a with
p-tolylboronic acid,9 we noted that small amounts of benzylic
fluoride 1b was formed when bimetallic gold−palladium
complexes were employed as catalysts. Switching to palladium
chloride as a catalyst and employing bipyridine (BiPy) as a ligand
allowed for 1b to be formed as the major product. We envisioned
that this product arose from a pathway in which the
sulfonamide10 served as chelating group, rather than a
nucleophile, to a high-valent palladium intermediate (eq 1).
a
Yields after chromatographic purification.
Moreover, we were pleased to observe that under identical
conditions, the desired fluorinated product was formed when the
DG was 2,6-difluoroaniline, pentafluoroaniline,17 or 8-amino-
quinoline18 (2a−4a, Scheme 1). In contrast, no product was
detected with 5a, and only cyclized nonarylated isoxindole-type
products were observed with 6a and 7a. Finally, decomposition
and trace of product was observed with 8a, likely a result of
competing oxidation of the arylthioether.
Received: December 18, 2013
Published: March 11, 2014
© 2014 American Chemical Society
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Journal of the American Chemical Society
Communication
a
With these results in hand, we decided to focus our
investigation using 8-aminoquinoline (AQ) as the directing
group. This versatile directing group had proven easy to remove
and offered more flexibility to extend the chemistry.18 Screening
of several palladium sources revealed Pd(OAc)2 as the optimal
choice, with Pd(TFA)2, PdCl2, and PdBr2 all producing increased
amounts of the undesired competing oxidative Heck process (see
Supporting Information for product distribution) at the
expensive of the desired fluoride (Table 1, entries 1−4). N,N-
Scheme 2. Substrate Scope for AQ Directing Group
Table 1. Optimization of Fluoroarylation of Styrene 4a
c
d
a
entry
1
catalyst
PdCl2
ligand
L1
solvents
additive
none
yield 4b
CH2Cl2/H2O
(1/0.1)
54
2
3
4
5
6
7
Pd(OAc)2
PdBr2
L1
L1
L1
−
CH2Cl2/H2O
(1/0.1)
none
none
none
none
none
none
67
54
61
0
CH2Cl2/H2O
(1/0.1)
Pd(TFA)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
CH2Cl2/H2O
(1/0.1)
CH2Cl2/H2O
(1/0.1)
a
Yields after chromatographic purification.
L2
L3
CH2Cl2/H2O
(1/0.1)
56
76
CH2Cl2/H2O
(1/0.1)
such as methyl, methoxy, and amino afforded the corresponding
coupling products in good yields. Benzylic fluoride 13 was
generated in 45% yield after 24h with 2,6-dimethylphenylboronic
acid as the substrate, indicating that steric hindrance has an effect
on the reaction. The inductive electron-withdrawing/mesomeri-
cally donating chloro group was also well tolerated, and 10 was
formed in 81% yield after 13h. Electron-poor ester-substituted
boronic acids reacted slowly but still afforded the desired
fluorides in modest to good yields. It should also be noted that
reaction of p-tolylboronic acid and Selectfluor under the optimal
conditions with the (E)-propenyl amide (R2 = Me) yielded 20 as
a single diastereoisomer. On the other hand, the palladium-
catalyzed reaction of alkynyl, alkenyl, or alkyl boronic acids failed
to deliver the desired product.20
8
9
Pd(OAc)2
Pd(OAc)2
L3
L3
CH2Cl2
none
none
23
82
CH2Cl2/H2O
(1/0.2)
b
10
11
12
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
L3
L5
L3
CH2Cl2/H2O
(1/0.2)
P1:
30 mol %
91 (86)
82
CH2Cl2/H2O
(1/0.2)
P1:
50 mol %
CH2Cl2/H2O
(1/0.2)
P2:
30 mol %
74
a
Determined by correlation between HPLC-MS and crude 1H-NMR
b
c
analysis. Value in parentheses reflects isolated yield. L1: bipyridine;
L2: 4,4′-dimethoxy-2,2′-bipyridine, L3: 4,4′-di-tert-butyl-2,2′-bipyri-
dine. P1: bis(2-ethylhexyl) hydrogen phosphate; P2: dibenzyl
d
hydrogen phosphate.
In light of the above results, having defined conditions to
control the regiochemistry and product distribution, we
undertook the evaluation of a range of different N,P- and N,N-
chiral ligands. Unfortunately, using these ligands resulted in
erosion of the yield and slow conversion to the desired product;
attempting to heat the reaction failed to increase the conversion.
The most encouraging N,N-chiral ligand (pyridyl-oxazolidine
ligand L*) in combination with the quinolone-based directing,
afforded 4b in good enantiomeric excess (81%; er 90.5:9.5) but
at the expense of yield (15%) (Scheme 3). Given the issues that
were emerging using the quinolone-based directing group in
conjunction with the N,P- and N,N-chiral ligands, we
reinvestigated the requirements of our directing group. To that
end, we were encouraged to find that the palladium-catalyzed
enantioselective fluoroarylation of the 2,6-difluoroanilide and the
2,4,6-trimethylphenyl derivatives gave a much improved yield
without significant changes to perturbing the enantioselectivity.
The dramatic improvement was seen with the simplified anilides;
an aromatic group with no ortho-substituent or even a simple
methylamide now gave ee in excess of 90% and with the 4-
methoxyanilide directing group giving fluororide 27 in 74% yield
Bipyridine-type ligands were vital to achieving the fluorinative
reaction manifold, with 4′4-di-tert-butylpyridine proving optimal
(entry 7); absence of coordinating ligand produced solely the
conventional Heck product (entry 5).19 Solvent affects were
dramatic with methylene chloride being the solvent of choice; no
coupling products were formed in toluene, DMF, THF, CH3CN,
and EtOAc. Water was found to increase the reaction rate, and
therefore the ideal solvent consisted of a biphasic mixture of
1.0:0.2 CH2Cl2:water (entries 7−9). Addition of an organic
phosphate (30 mol %) as a phase transfer catalyst further
increased the rate of the fluoroarylation and diminished the
impurities seen in the reaction.18k Under the optimized reaction
conditions, the palladium-catalyzed oxidative arylation of styrene
4a furnished fluoride 4b in 86% isolated yield after 24 h at room
temperature (entry 10).
Having the optimized conditions in hand, a range of boronic
acids19 were subjected to the oxidative conditions using
Selectfluor in CH2Cl2/water (5/1:v/v) to evaluate the scope of
the reaction. As shown in Scheme 2, this protocol was efficient
with various arylboronic acids bearing electron-donating groups
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Journal of the American Chemical Society
Communication
Scheme 3. Substrate Scope for Enantioselective
Fluoroarylation
Scheme 4. Proposed Mechanism for the Directed
Fluoroarylation of Styrene
a
ate.21 Transmetalation with the boronic acid and thereafter
coordination and insertion yields a β-arylated Pd(II) species. The
regiocontrol of insertion is provided by the directing group
coordination and the electronics of the substrate. The
coordination of the directing group and the N,N-ligand may
also stabilize this intermediate and retard the competing β-
hydride elimination process thereby promoting oxidation.22
Oxidation to the high-valent Pd(IV) intermediate is achieved
using Selectfluor. Reductive elimination yields the desired
a
Yields after chromatographic purification and enantioselectivities
determined by chiral SFC or HPLC. Absolute Configuration assigned
by analogy to X-ray structure of 33.
monofluorinated product and the catalytic [N−N−Pd(II)
]
and 96% ee. Following these results, a range of boronic acids
were tested on the 4-methoxyaniline substrate, and pleasingly,
desired products (26−32) were obtained in good yield and high
ee with the exception of the bulky 2,6-dimethylboronic acid.
To demonstrate the potential of this fluorinated compound as
a useful precursor for synthesis, deprotection of the amino-
quinoline 4b was achieved using Boc anhydride with DMAP,
followed by mild deprotection with LiOH and hydrogen
peroxide in water/THF to give the desired acid 34 in 62%
yield over two steps (eq 2).18h Additionally, the 4-methox-
yanilide directing group employed in the enantioselective variant
was converted to amide 35 be reaction with ceric ammonium
nitrate (CAN) (eq 3).
complex.
In conclusion, using amide-based directing groups, we have
developed a palladium-catalyzed fluoroarylation of styrenes. The
reaction allows for the synthesis of a range of enantioenirched
benzylic fluorides by a three-component coupling of styrenes,
Selectfluor, and boronic acids.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and compound characterization data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
■
S
AUTHOR INFORMATION
■
Corresponding Authors
jeff.mckenna@novartis.com
eric.p.talbot@gsk.com
fdtoste@berkeley.edu
Present Address
§GlaxoSmithKline, Gunnels Wood Road Stevenage Herts SG1
2NY, U.K.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Novartis Education, Diversity and
Inclusion (ED&I) office for a presidential postdoctoral fellow-
ship (E.P.A.T.). F.D.T. thanks NIHGMS (RO1 GM073932) for
financial support, and T.A.D. is grateful or a postdoctoral
fellowship from CAPES. We are grateful to H. Douglas, G.
A proposed mechanism for the reaction is outlined in Scheme
4. As already noted, fluoroarylation was only achieved in the
presence of an N,N-ligand; thus, the catalytic cycle is initiated
through formation of an N,N-ligated palladium(II) intermedi-
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Journal of the American Chemical Society
Communication
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Browne and C. Page (Novartis) for analytical support. We
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ray crystallography and support from NIH Shared Instrument
Grant (S10-RR027172).
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