Aromatic C-H coupling with hindered arylboronic acids by Pd/Fe dual catalysts

Article abstract of DOI:10.1039/c3sc51206a

An aerobic oxidative coupling of arenes/alkenes with arylboronic acids (C-H/C-B coupling) using catalytic Pd(ii)-sulfoxide-oxazoline (sox) ligand and iron-phthalocyanine (FePc) has been developed. This dual catalyst system enables the synthesis of sterica

Full text of DOI:10.1039/c3sc51206a

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Aromatic C–H coupling with hindered arylboronic acids
by Pd/Fe dual catalysts†
Cite this: DOI: 10.1039/c3sc51206a
a
a
a
ab
*
*
Kazuya Yamaguchi, Hiroki Kondo, Junichiro Yamaguchi and Kenichiro Itami
An aerobic oxidative coupling of arenes/alkenes with arylboronic acids (C–H/C–B coupling) using catalytic
Pd(II)–sulfoxide–oxazoline (sox) ligand and iron–phthalocyanine (FePc) has been developed. This dual
catalyst system enables the synthesis of sterically hindered heterobiaryls and styrene derivatives under
air without stoichiometric co-oxidants. Additionally, this chemistry demonstrated an advance toward an
enantioselective biaryl coupling through C–H functionalization.
Received 2nd May 2013
Accepted 12th June 2013
DOI: 10.1039/c3sc51206a
www.rsc.org/chemicalscience
Scheme 1). This is achieved by using a Pd(II)–sulfoxide–oxa-
zoline/iron–phthalocyanine (FePc) dual catalyst. In this paper,
we rst describe the experimental background that led us to
an aerobic catalytic system, then a discussion of substrate
scope, and nally, an approach toward an enantioselective
biaryl coupling through C–H functionalization.
Introduction
The concept of C–H arylation has received much attention from
the organic chemistry community because it is a green and ideal
way to introduce aryl groups to organic molecules without pre-
functionalization.1 Many protocols have already been estab-
lished thus far for various aryl nucleophiles and electrophiles.
One representative C–H arylation reaction involves the Pd-
catalyzed C–H/C–B coupling between unfunctionalized arenes
and arylboronic acids.2–4 Although oxygen (air) is the ideal
terminal oxidant to allow for a greener process, this C–H/C–B
coupling reaction generally requires the use of stoichiometric
co-oxidants such as Cu(II) halides, Ag(I) salts (Ag2O, Ag2CO3) and
1,4-benzoquinone (BQ), 2,2,6,6-tetramethylpiperidine 1-oxyl
(TEMPO) and o-chloranil.5
Results and discussion
We began our study by searching for a non-TEMPO co-oxidant
for the aerobic C–H/C–B coupling of heteroarenes and
hindered arylboronic acids. Gratifyingly, we initially found
that BQ could function as co-oxidant when the reactions are
conducted in DMSO as a solvent. For example, C–H/C–B
To exacerbate this problem, these catalysts display low
reactivity, and occasionally reactions do not proceed when
using sterically hindered substrates in either the (hetero)arene
or the arylboronic acid coupling partner. In order to address
this problem, we have developed a ligand-enabled C–H coupling
for the synthesis of hindered heterobiaryls using a Pd–bisox-
azoline–TEMPO catalytic system.6 However, the catalytic system
used therein is not completely catalytic since it requires TEMPO
as a stoichiometric co-oxidant.
We now report the discovery of a catalytic system without
any stoichiometric co-oxidant for the coupling of hetero-
arenes with hindered arylboronic acids (C–H/C–B coupling,
aDepartment of Chemistry, Graduate School of Science, Nagoya University, Chikusa,
Nagoya 464-8602, Japan; Web: http://synth.chem.nagoya-u.ac.jp/. E-mail: itami.
kenichiro@a.mbox.nagoya-u.ac.jp; junichiro@chem.nagoya-u.ac.jp; Fax: +81-52-
788-6098; Tel: +81-52-788-6098
bInstitute of Transformative Bio-Molecules (WPI-ITbM), Nagoya University, Chikusa,
Nagoya 464-8602, Japan
† Electronic supplementary information (ESI) available: Detailed experimental
procedures, and spectral data for all compounds, including scanned images of
1H and 13C NMR spectra. CCDC 937410. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3sc51206a
Scheme 1 Pd-catalyzed (hetero)biaryl formation by C–H/C–B coupling.
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coupling of 2,3-dimethylthiophene (1A: 1.0 equiv.) and (2,4,6-
trimethylphenyl)boronic acid (2a: 4.0 equiv.) took place in the
presence of Pd(OAc)2 (10 mol%) and BQ (1.0 equiv.) in DMSO
at 80 ꢀC for 12 h under air to furnish the corresponding
coupling product 3Aa in 83% yield (Table 1, entry 1). In order
to render the coupling reaction catalytic in BQ, we then
¨
applied Backvall's “triple” catalytic system using metal–mac-
rocycles, such as FePc and Co(salophen), as a co-catalyst to re-
oxidize hydroquinone to BQ applying air (oxygen) as the
terminal oxidant.7 Indeed, when the reaction was conducted Fig. 1 Effect of DMSO.
in the presence of a catalytic amount of FePc (5 mol%), effi-
cient coupling occurred even with a catalytic amount of BQ
(entry 2). The yield decreased to 46% when the loading
amount of FePc was lowered to 1 mol% (entry 3) and the
reaction did not work at all without it (entry 4), demonstrating
its indispensability. Curiously however, the loading amount of
BQ did not inuence the yield of 3Aa (entry 5) and the yield
was maintained without any BQ at all (entry 6). This reveals
that the FePc catalyst does not function by oxidizing hydro-
quinone. Obviously, the reaction did not proceed without
Pd(OAc)2 (entry 7) and was very sluggish under a nitrogen
atmosphere (entry 8).
We studied the effect of the ligand for the reaction between
dimethylthiophene (1A) and arylboronic acids (2a and 2b) in
the presence of 10 mol% Pd(OAc)2, 10–20 mol% ligand (L1–
10), and 5 mol% FePc in DMF at 80 ꢀC under air for 12 h.
When DMSO was used as the ligand, the yield of 3Aa was
83% whereas the yield of 3Ab was 24% (L1). Tetramethylene
sulfoxide (TMSO) and White's ligand,9 which are effective
for palladium-catalyzed C–H oxidation reactions, were inef-
fective for this reaction (L2 and L3). Other bis-sulfoxide and
Upon investigation of solvent effects, an intriguing
phenomenon was observed (Fig. 1). When the solvent was
changed from DMSO to DMF, the yield of product 3Aa
decreased to 32% (entry 2), and other solvents were not
effective either.8 However, when only 100 mol% DMSO was
added (i.e., no longer as solvent), the yield was maintained,
and the same effect was observed for 20 mol% DMSO. It was
assumed that the DMSO functions as a ligand, and therefore
we examined the effect of the ligand for the C–H/C–B coupling
reaction (Scheme 2).
Table 1 Discovery of Pd/Fe dual catalysis for the C–H arylation of thiophenes
with arylboronic acidsa
Pd(OAc)2
Entry
(mol%)
BQ (mol%)
FePc (mol%)
Yield (%)
1
2
3
4
5
6
7
8b
10
10
10
10
10
10
—
10
100
10
10
10
5
—
—
—
—
5
1
—
5
5
5
5
83
81
46
Trace
84
83
ND
10
a
Conditions: 1A (0.25 mmol), 2a (1.0 mmol), Pd(OAc)2 (0.025 mmol), BQ
(0.025 mmol), FePc (0.0125 mmol), DMSO (0.2 mL), 80 ꢀC 12 h, under
air. The C4/C5 regioselectivity was always found to be greater than
b
95% selective at C4. The reaction was conducted under a nitrogen
atmosphere.
Scheme 2 Discovery of a suitable “hybrid” ligand for oxidative C–H coupling.
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mono-sulfoxide ligands (L4 and L5)10 were only effective
With optimal conditions in hand, we explored the substrate
when 2a was used. Bis-oxazoline ligands L6 and L7, which had scope of the C–H/C–B coupling of various heteroaromatics and
accelerated a hindered C–H/C–B coupling reaction in our arylboronic acids for the synthesis of hindered heterobiaryls
previous report,6 slightly increased the yields of 3Aa and 3Ab. (Scheme 3).
Other bidentate ligands such as L8 and L9 were not effective at
(2,6-Dimethylphenyl)boronic acid (2c) and sterically more
all. Aer screening further ligands, we noticed that sulfoxide hindered (2-isopropylnaphthalen-1-yl)boronic acid (2d) gave
and oxazoline ligands are effective, and we therefore hypoth- the corresponding coupling products 3Ac and 3Ad in good
esized that a hybrid ligand should be much more effective for yields. Thiophene derivatives 1B–E afforded the coupling
this reaction. To this end, sox ligand (L10)11 was applied to the products in moderate to good yields (3Ba, 3Cb, 3Db, 3Ea,
reaction, which proceeded smoothly with both arylboronic and 3Eb). The coupling of indole derivatives 1F–H with
acid substrates to give 3Aa and 3Ab in 86% and 92% yields, hindered arylboronic acids gave heterobiaryls 3Fa, 3Ga, 3Ha,
respectively.
and 3Hb in slightly lower yields compared with the thio-
phene series, but was regioselective, favoring the C3 position
of indole (87 to >95% C3-selectivity). Pyrroles 1I and 1J as
well as a furan derivative 1K can be used as heteroarenes to
afford the corresponding products in good yield with virtu-
ally complete C2-selectivity at the 5-membered heteroarene
(3Ia, 3Ib, 3Jb, 3Jd, and 3Ka).12 At present, the origin of the
regioselectivity (C2 or C3) for the reaction site of hetero-
arenes is unclear.
When alkenes were used instead of heteroarenes, the reac-
tion also proceeded smoothly to give the corresponding
coupling products (5La, 5Ma, and 5Na) in good to excellent
yields (Scheme 4).13 However, the oxidative coupling did not
proceed with 1,2-disubstituted alkenes.
Finally, this catalytic C–H coupling reaction was applied to
an enantioselective C–H coupling reaction (Fig. 2).14,15 The
coupling reaction of 2,3-dimethylthiophene (1A) and (2-iso-
propylnaphthalen-1-yl)boronic acid (2d) was conducted in the
presence of 10 mol% Pd–sox catalyst and 5 mol% FePc in
ꢀ
dimethylacetamide (DMAc) at 70 C under air. As a result, the
enantiomeric excess of the corresponding coupling product
3Ad was 61% using our optimal conditions.8 Under these
conditions (70 ꢀC, 24 h), racemization of 3Ad does not take
place.6 Although the enantioselectivity was not increased
compared to our previous report6 the reaction yield was
greater.
Scheme 3 Scope of heteroaromatics and arylboronic acid coupling partnersa.
a
Scheme 4 Oxidative coupling of alkenes with hindered arylboronic acidsa.
Conditions:
1
(0.25 mmol),
2
(1.0 mmol), Pd(OAc)2 (0.025 mmol), L10
b
(0.025 mmol), FePc (0.0125 mmol), DMF (0.2 mL), 80 ꢀC, 12 h, under air. The
a
Conditions: 1 (0.25 mmol), 2 (1.0 mmol), Pd(OAc)2 (0.025 mmol), L10 (0.025
regioselectivity of C3/C2 ¼ 88 : 12. c The regioselectivity of C3/C2 ¼ 87 : 13.
1
d
mmol), FePc (0.0125 mmol), DMF (0.2 mL), 80 ꢀC, 12 h, under air. b 1 (0.25 mmol),
2 (1.0 mmol), Pd(OAc)2 (0.025 mmol), FePc (0.0125 mmol), DMSO (0.2 mL), 80 ꢀC,
12 h, under air.
(0.25 mmol), 2 (1.0 mmol), Pd(OAc)2 (0.025 mmol), FePc (0.0125 mmol), DMSO
(0.2 mL), 80 ꢀC, 12 h, under air.
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Synthesis of hindered heterobiaryls, leading to moderate
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Conclusions
In summary, an efficient catalytic system (Pd–sox/FePc) that
operates with air as a terminal oxidant for the coupling of are-
nes/alkenes with arylboronic acids (C–H/C–B coupling) has
been discovered. This catalytic system enabled the synthesis of
hindered heterobiaryls in good yields, and provided an
approach toward an enantioselective C–H coupling reaction.
Elucidation of the mechanism of Pd/Fe catalyzed C–H arylation
and the application of enantioselective C–H biaryl coupling in
synthesis are currently ongoing in our laboratory.
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Acknowledgements
This work was supported by the Funding Program for Next
Generation World-Leading Researchers from JSPS (220GR049 to
K.I.) and Grants-in-Aid for Scientic Research on Innovative
Areas “Molecular Activation Directed toward Straightforward
Synthesis” (25105720 to J.Y.) from MEXT. We thank Dr Yasu-
tomo Segawa for assistance in the X-ray crystal structure anal-
ysis. Prof. Makoto Fujita and Dr Yasuhide Inokuma (The
University of Tokyo) are acknowledged for the structure deter-
mination. We thank Dr Yoshihiro Ishihara for helpful
discussions.
Notes and references
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