Pd-Catalyzed Aerobic Oxidative Coupling of Thiophenes: Synergistic Benefits of Phenanthroline Dione and a Cu Cocatalyst

Article abstract of DOI:10.1021/jacs.0c09962

Substituted bithiophenes are prominent fragments in functional organic materials, and they are ideally prepared via direct oxidative C-H/C-H coupling. Here, we report a novel PdII catalyst system, employing 1,10-phenanthroline-5,6-dione (phd) as the ancillary ligand, that enables aerobic oxidative homocoupling of 2-bromothiophenes and other related heterocycles. These observations represent the first use of phd to support Pd-catalyzed aerobic oxidation. The reaction also benefits from a Cu(OAc)2 cocatalyst, and mechanistic studies show that Cu promotes C-C coupling, implicating a role for CuII different from its conventional contribution to reoxidation of the Pd catalyst.

Full text of DOI:10.1021/jacs.0c09962

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Pd-Catalyzed Aerobic Oxidative Coupling of Thiophenes: Synergistic
Benefits of Phenanthroline Dione and a Cu Cocatalyst
Stephen J. Tereniak,† David L. Bruns,† and Shannon S. Stahl*
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ABSTRACT: Substituted bithiophenes are prominent fragments in functional organic materials, and they are ideally prepared via
direct oxidative C−H/C−H coupling. Here, we report a novel PdII catalyst system, employing 1,10-phenanthroline-5,6-dione (phd)
as the ancillary ligand, that enables aerobic oxidative homocoupling of 2-bromothiophenes and other related heterocycles. These
observations represent the first use of phd to support Pd-catalyzed aerobic oxidation. The reaction also benefits from a Cu(OAc)2
cocatalyst, and mechanistic studies show that Cu promotes C−C coupling, implicating a role for CuII different from its conventional
contribution to reoxidation of the Pd catalyst.
alladium-catalyzed oxidative C−H/C−H coupling of
P
(hetero)arenes1 provides efficient access to biaryl
monomers used in high-performance polymers. Ancillary
ligands often improve the Pd catalyst performance when O2
is used as the oxidant.2,3 An important commercial application
of this concept is featured in the homocoupling of dimethyl
phthalate, generating a biphenyl tetracarboxylate precursor to
the polyimide resin, Upilex.4,5 The patented catalyst system of
Ube Industries consists of Pd(OAc)2 and phenanthroline
(phen), and uses Cu(OAc)2 as a cocatalyst.6 This reaction has
attracted considerable recent attention, with efforts focused on
use of alternative arene feedstocks, such as o-xylene,
identification of new catalyst systems and ancillary ligands,
and mechanistic studies.7−14 2,2′-Bithiophenes represent
another important target for oxidative coupling reactions.
These motifs are prevalent in organic materials, with
applications ranging from organic solar cells15,16 to organic
light-emitting diodes17 and organic field effect transistors
(Figure 1A).18 Pd-catalyzed oxidative homocoupling of
thiophenes has precedent;19−26 however, the most effective
substrates, including those compatible with aerobic conditions,
consist of electron-rich derivatives that are less useful in
organic materials applications. Substrates that feature bromide
or other electron-withdrawing substituents are unreactive or
perform poorly under existing oxidative homocoupling
conditions. A rare exception, reported by Mori and co-workers,
enables oxidative homocoupling of 2-bromothiophenes and
other derivatives (Figure 1B);19,20 however, it requires AgI as
the stoichiometric oxidant. The present study was motivated
by the prospect of identifying a ligand-supported Pd catalyst
system that enables use of O2 as the oxidant, while retaining
good reactivity with 2-bromothiophenes and related electron-
deficient heterocycles (Figure 1C). The resulting catalyst
system showcases the utility of 1,10-phenanthroline-5,6-dione
(phd) as a ligand that has not been used previously in Pd-
catalyzed aerobic oxidations. The discovery of this catalyst
system, the substrate scope, and mechanistic insights are
reported herein.
Figure 1. (A) Commercial organic polymers for materials science
applications containing a 2,2′-bithiophene unit. (B) Precedent for Pd-
catalyzed oxidative homocoupling of 2-bromo-3-alkylthiophenes with
stoichiometric AgI oxidants and (C) aerobic oxidation method
targeted in the present study.
Our initial efforts focused on evaluating Pd catalyst
compositions and reaction conditions for oxidative homo-
coupling of 2-bromo-3-hexylthiophene 1a with O2 to afford
5,5′-dibromo-4,4′-dihexyl-2,2′-bithiophene 2a. Substrate 1a
was selected because alkyl groups are commonly featured as
solubilizing groups in thiophene-containing organic electronic
materials, and bromide substituents are often desired for
subsequent coupling steps to access oligomeric or polymeric
products.27 Several previously reported conditions for aerobic
Received: September 17, 2020
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C−H/C−H biaryl coupling were screened,7,22,28 but these
proved ineffective (see section III of the Supporting
Information for full screening data). For example, 2a was
obtained in only 4% yield when using the previously reported
Pd-catalyzed aerobic conditions effective for the homocoupling
of electron-rich thiophenes. On the other hand, promising
reactivity was observed by combining features of the
commercial oxidative homocoupling of dimethyl phthalate,29
including use of an ancillary nitrogen ligand and Cu(OAc)2
cocatalyst and the use of DMSO as the solvent (Chart 1). In a
Chart 1. Ligand Effects in Pd/Cu-Cocatalyzed Aerobic C−H
a
Homocoupling of 2-Bromo-3-hexylthiophene
Figure 2. Assessment of catalytic components on thiophene C−H
homocoupling. Conditions: 1a (1.1 mmol), catalyst (3 mol %), 1.0
mL in DMSO, 1.1 atm pO2, 120 °C, 16 h.
1c react smoothly to afford 2,2′-bithiophenes that are useful as
intermediates or precursors to intermediates in materials
synthesis.34,35 A terthiophene 3c was isolated as a byproduct
from the coupling reaction of 1c (see section XI in the
Supporting Information). A 74% yield of 2d was observed
when using cyclohexyl acetate as a cosolvent to enhance the
solubility of 1d. 2-Chloro- and 2-bromothiophenes 1e and 1f
benefited from a lowered reaction temperature (100 °C).
Dihalogenated substrates gave moderate-to-good yields (2g−
2i). The 3,3′-difluorobithiophene 2i is a noteworthy product
because fluorination in this position has been shown to
enhance solar cell power conversion efficiency.36,37 Benzo[b]-
thiophenes38−40 also underwent oxidative coupling to afford 2j
and 2k.
a
Conditions: 0.17 mmol 2-bromo-3-hexylthiophene, 5 mol % ligand
(for L5−L11 and phd) or 10 mol % ligand (L1−L4), 0.25 mL in
DMSO. Yields determined by HPLC.
survey of different ancillary ligands, electron-rich pyridine
derivatives led to higher yields of 2a over electron-deficient
pyridines (L3 vs L1, Chart 1). Bidentate nitrogen-donor
ligands, including 2,2′-bipyridine, 1,10-phenanthroline, and
4,5-diazafluoren-9-one (L8), afforded similar, moderate yields
(26−57%). In contrast, 1,10-phenanthroline-5,6-dione (phd)
led to a good yield of 2a (80%). Phd has been used previously
as a catalyst itself for the aerobic oxidation of amines,30−32 but
this is the first successful use of phd in Pd-catalyzed aerobic
oxidation reactions.33 Pd salts with anionic ligands less basic
than acetate (trifluoroacetate, nitrate) were less effective, while
those with similar or higher basicity (benzoate, propionate,
pivalate) performed very similarly to Pd(OAc)2.
When the loadings of Pd(OAc)2, phd, and Cu(OAc)2 were
lowered to 3 mol %, the yield of 2a could be improved to 85%
by including 3 mol % 1,4-benzoquinone (BQ). Systematic
assessment of the different catalyst components (Figure 2)
highlighted the importance of each of the catalyst components
to access 2a in good yield. High conversion of the thiophene
starting material was observed with Pd(OAc)2/Cu(OAc)2, but
little product was obtained, suggesting phd promotes selective
formation of product 2a. The ternary catalyst system,
composed of Pd(OAc)2/Cu(OAc)2/phd, led to a substantially
improved yield (64%). As the inclusion of 3 mol % BQ does
not affect the initial rate of the reaction (see Figure S2), the
enhanced yield of 2a is attributed to improved catalyst stability.
The optimized catalyst system, consisting of 3 mol %
Pd(OAc)2/phd, Cu(OAc)2, and BQ was then tested in the
oxidative homocoupling of a series of thiophenes and related
heterocycles (Chart 2). Several 2-halo-3-alkylthiophenes 1a−
The 3,3′-diester derivative 2l is a precursor for materials
used in organic solar cell applications.41 Products 2m−2p
show that the catalyst system affords moderate-to-good yields
with both electron-deficient and electron-rich substrates.
Product 2p is a precursor to 5,5′-bis(carbaldehyde)-2,2′-
bithiophene, a common intermediate in Knoevenagel con-
densations and Horner−Wadsworth−Emmons and Wittig
olefinations used to prepare conjugated polymers.42−45
A
2,2′-bithiophene substrate 1q was coupled in good yield at 100
°C to yield quaterthiophene derivative 2q, suggesting that the
catalyst has promise for the synthesis of oligothiophene
intermediates. Bithiazole 2r has been utilized as an
intermediate for the synthesis of organic field effect transistors,
although it was accessed in modest yield (29%). The phd/Pd
catalyst system represents an important advance over the
previous nonligated Pd(OAc)2 aerobic catalyst system.22 The
latter method was evaluated for a number of the important
substrates in Chart 2 (1a, 1b, 1l, 1p, and 1q). In each case, the
product yield was <10%.
A gram-scale oxidative coupling of 1a under the standard
conditions generated 2a in 90% isolated yield. Moreover, the
coupling of 1a was performed with Pd(OAc)2 and phd
loadings of 0.5 mol %, with Cu(OAc)2 and BQ at 13 and 3 mol
1
% loading, respectively. A 70% H NMR yield of 2a was
B
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h
Chart 2. Substrate Scope for Phd/Pd(OAc)2-Catalyzed Aerobic Oxidative Coupling of Thiophenes
a
b
c
d
e
f
Solvent was a 1.2:1 mixture DMSO:CyOAc. 100 °C. 5 mol % catalyst. Mn(OAc)2·4H2O used instead of Cu(OAc)2·H2O. 24 h. R = 2-
g
(butyl)octyl; obtained with ∼7% unidentified impurity (see section XI of the Supporting Information for details). Isolated in 9:1 ratio with
monodeprotected 3p (see Supporting Information section XI for details) Conditions: Substrate (1.1 mmol, 1.1 M), 3 mol % each Pd(OAc)2, phd,
Cu(OAc)2·H2O, BQ, 1 mL reaction volume in DMSO, 120 °C, 1.1 atm O2, 16 h. H NMR yields, with isolated yields in parentheses.
h
1
obtained under these conditions. Both of these results have
important implications for larger scale applications of this
chemistry.
Several observations provide insight into the role of phd as
an ancillary ligand. When [phd] is varied at a fixed loading of 3
mol % Pd(OAc)2 and Cu(OAc)2, the reaction rate increases
linearly with [phd] until it maximizes at a phd:Pd ratio of
approximately 1:1 (Figure 3A.i). The rate diminishes beyond
this phd loading, and catalysis is completely inhibited at a
phd:Pd ratio of 3:1. Complementary data were observed at a
higher Cu(OAc)2 loading (12 mol %) while maintaining 3 mol
% Pd(OAc)2. The rate again maximizes at a phd:Pd ratio of
approximately 1:1 and decreases beyond this ratio. These
results, together with NMR titration experiments showing that
phd binds tightly to Pd(OAc)2 in 1:1 stoichiometry (see
Figures S10 and S11), suggest that phd coordinates
preferentially to Pd rather than Cu in the catalytic reaction.46
A primary deuterium kinetic isotope effect (KIE) is observed
when the homocouplings of 1a and 1a-d1 (deuterium at the C5
position) are monitored under standard conditions [3 mol %
Pd(OAc)2 and Cu(OAc)2]: kH/kD = 2.9 0.1. Changing the
Cu(OAc)2 loading has little impact on the KIE: kH/kD = 2.4
0.1 and 2.6 0.2 at 0.75 mol % and 10 mol % Cu(OAc)2,
respectively. These KIE values, which resemble those observed
in other Pd-catalyzed C−H oxidation reactions,47−52 and a
first-order dependence of the catalytic rate on [1a] (cf. Figure
S4) indicate that C−H activation is the catalytic turnover-
limiting step.
Other experiments provide insights into the role of
Cu(OAc)2. CuII salts are commonly used as redox cocatalysts
to promote Pd catalyst reoxidation by O2; however, less
conventional roles in which CuII influences substrate oxidation
steps mediated by PdII have also been documented.53 The data
in Figure 3A.i show that the rate is faster at higher Cu(OAc)2
loading, and systematic variation of [Cu(OAc)2] reveals a
saturation dependence (Figure 3A.ii). Addition of 10 equiv of
DOAc inhibits biaryl coupling, but it leads to deuterium
incorporation into the C5 position of 1a, consistent with
reversible C−H activation of thiophene under these con-
ditions. Cu(OAc)2 has only moderate impact on the H/D
exchange, increasing deuterium incorporation by a factor of 1.5
after 30 min (28% vs 42%; Figure 3B, top). The H/D exchange
and KIE data indicate that Cu(OAc)2 has minimal influence on
C−H activation. On the other hand, Cu(OAc)2 has a major
impact on C−C coupling: the oxidative coupling rate of 1a
differs by a factor of 15 in the presence and absence of 3 mol %
Figure 3. Mechanistic experiments probing the effect of Cu(OAc)2
loading on the rate (A) and the role of Cu(OAc)2 on C−H activation
and C−C coupling (B).
C
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Cu(OAc)2 (25 × 10−6 vs 1.7 × 10−6 M/s) (Figure 3B,
bottom). H NMR spectroscopic data provide evidence for
AUTHOR INFORMATION
Corresponding Author
■
1
direct interaction between Cu(OAc)2 and (phd)Pd(OAc)2 in
solution, likely involving the dione fragment of the Pd-bound
phd ligand.54−60 This evidence was obtained by comparing the
effect of adding 1 equiv Cu(OAc)2 to solutions of (phen)Pd-
(OAc)2 and (phd)Pd(OAc)2 in DMSO-d6. Whereas Cu-
(OAc)2 has very little effect on the spectrum of (phen)Pd-
(OAc)2, it leads to the appearance of multiple new para-
magnetically shifted peaks (upfield and downfield) in the
spectrum of (phd)Pd(OAc)2 (see Figures S12 and S13).
Collectively, the kinetic and spectroscopic data suggest that
CuII influences C−C coupling in a non-redox role. Such roles
could include serving as a Lewis acid to promote activation of a
second thiophene in a sequential C−H/C−H coupling
pathway, promoting transmetalation of a thiophenyl group
between two (phd)PdII−ArHet intermediates, or promoting
reductive elimination from the (phd)Pd(ArHet)2 intermediate
present in either mechanism. Support for this non-redox/Lewis
acid role was obtained by replacing CuII with redox inactive
MgII and ZnII acetate salts. The MgII and ZnII salts are not as
effective as CuII, but the 35% and 30% yields of 2a,
respectively, are significantly better than the 5% yield of 2a
obtained in the absence of a metal cocatalyst (see Table S10).
The improved performance of CuII relative to ZnII and MgII is
consistent with its enhanced Lewis acidity (cf. Irving-Williams
series), which could support more favorable interaction with
the phd dione fragment. This secondary binding site in the phd
ligand provides a rationale for the superior performance of phd
relative to the other ligands in Chart 1, and we postulate that a
phd-bridged bimetallic species, (AcO)2Cu(phd)Pd(OAc)2,
exhibits kinetic advantages over more conventional bimetallic
interactions, such as those involving bridging acetate ligands.
Shannon S. Stahl − Department of Chemistry, University of
WisconsinMadison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0002-9000-7665; Email: stahl@
chem.wisc.edu
Authors
Stephen J. Tereniak − Department of Chemistry, University of
WisconsinMadison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0003-4978-0020
David L. Bruns − Department of Chemistry, University of
WisconsinMadison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0003-2455-502X
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c09962
Author Contributions
†S.J.T. and D.L.B. contributed equally.
Notes
The authors declare the following competing financial
interest(s): A patent application based on this chemistry has
been submitted.
ACKNOWLEDGMENTS
■
This work was funded by the NSF through the CCI Center for
Selective C−H Functionalization (CHE-1205646 and CHE-
1700982) and by the NIH (NRSA Fellowship Award F32
GM119214 to S.J.T.). We acknowledge the NSF (CHE-
9709065) and the Paul and Margaret Bender Fund for support
of the NMR facility. We acknowledge the NIH (1S10
OD020022-1) for mass spectrometry support.
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