Ligand- and Base-Free Access to Diverse Biaryls by the Reductive Coupling of Diaryliodonium Salts

Article abstract of DOI:10.1002/ejoc.201600242

A ligand- and base-free, Pd-catalyzed protocol to access a wide range of symmetrical and unsymmetrical biaryls from stable diaryliodonium salts was developed. The reaction involved the use of an effective and recyclable Pd/polyethylene glycol-400 catalyst system to harness the aryl moieties of two diaryliodonium salts; the ensuing biaryls may be utilized to synthesize an array of useful compounds, including 5-aryluracils, carbazoles, chromenones, fluorenones, phenathridines, and boscalid analogues.

Full text of DOI:10.1002/ejoc.201600242

DOI: 10.1002/ejoc.201600242
Communication
Biaryls
Ligand- and Base-Free Access to Diverse Biaryls by the
Reductive Coupling of Diaryliodonium Salts
V. Arun,[a] P. O. Venkataramana Reddy,[a] Meenakshi Pilania,[a] and Dalip Kumar*[a]
Abstract: A ligand- and base-free, Pd-catalyzed protocol to ac- diaryliodonium salts; the ensuing biaryls may be utilized to syn-
cess a wide range of symmetrical and unsymmetrical biaryls
from stable diaryliodonium salts was developed. The reaction
involved the use of an effective and recyclable Pd/polyethylene
glycol-400 catalyst system to harness the aryl moieties of two
thesize an array of useful compounds, including 5-aryluracils,
carbazoles, chromenones, fluorenones, phenathridines, and
boscalid analogues.
Introduction
The biaryl framework is an important scaffold that is present in
various natural products, valuable drug candidates, chiral li-
gands, agrochemical agents, and advanced materials (Fig-
ure 1).[1] The most common approaches to prepare biaryls in-
clude traditional cross-coupling reactions (Scheme 1) involving
preactivated precursors, namely, aryl pseudohalides and aryl
metal reagents (e.g., Stille, Kumada, Suzuki, and Negishi reac-
tions).[2] However, some of these classical couplings often re-
quire over-stoichiometric amounts of the precursors that need
to be prepared in additional synthetic steps, the use of rela-
tively unfavorable solvents, and generate homocoupled by-
products and waste salts. In the recent past, these methods
have been supplanted by emerging and attractive C–H func-
tionalization strategies, namely, directing group (DG)-assisted
arylation,[3] cross-dehydrogenative coupling,[4] and transition-
metal (TM)-free cross-coupling reactions (Scheme 1).[5] Cross-
dehydrogenative couplings require activated arenes, excess
amounts of the arenes (as solvent), metal catalysts, oxidants,
high temperatures (>100 °C), and prolonged reaction times (20–
48 h). Generally, poor regioselectivities of the C–H arylations
and the use of excess amounts of the arenes are the major
obstacles associated with dehydrogenative coupling reactions.
TM-free cross-couplings employ aryl halides (1.0 equiv.), excess
amounts of the arenes (10.0 equiv.), a radical initiator (2–
3 equiv.), elevated temperatures (>140 °C), and long reaction
times (24–72 h). However, simple and atom-economical proto-
cols to prepare valuable biaryls from easily available starting
materials are highly desirable.
Figure 1. Some important molecules containing the biaryl motif.
In recent years, tremendous efforts have been directed to-
wards the chemistry of diaryliodonium salts because they have
high intrinsic electrophilicity relative to the corresponding aryl
Scheme 1. Existing and present strategies to prepare biaryls.
halides.[6] They are stable solids, inert to air and moisture, and
are also mild and nontoxic arylating agents that are easily pre-
pared from commercially available starting materials.[7] Owing
to their increasing significance, diaryliodonium salts have been
extensively studied for various TM-catalyzed and metal-free ary-
[a] Department of Chemistry, Birla Institute of Technology and Science,
Pilani 333031, Rajasthan, India
E-mail: dalipk@pilani.bits-pilani.ac.in
http://universe.bits-pilani.ac.in/pilani/dalipk/profile
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600242.
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lations and also in the construction of heterocycles.[8] Research Results and Discussion
originating from the groups of Sanford, Macmilian, and Gaunt
demonstrates the synthetic utilities of diaryliodonium salts
through copper- and palladium-mediated carbon–carbon bond
formation.[9] To the best of our knowledge, there are a few re-
ports involving the reductive coupling of diaryliodonium salts
to access biaryls involving the use of stoichiometric amounts of
methylmagnesium iodide and anhydrous nickel chloride under
an inert atmosphere[10] and activated Zn in the presence of
At the outset of this study, we chose diphenyliodonium triflate
(1a) as a model substrate to identify the optimum reaction con-
ditions. Initially, the coupling reaction was performed in the
presence of CuI (0.5 equiv.) at 140 °C for 24 h, but desired bi-
phenyl (2a) was not obtained (Table 1, entry 1). Trials with vari-
ous copper catalysts including CuI, copper(II) trifluoromethane-
sulfonate [Cu(OTf)2], CuCl, CuBr, Cu(OAc)2, and CuBr2 in 1,2-di-
chloroethane (DCE) or dioxane at 80 °C for 12 h were also un-
successful (Table 1, entries 2–7). We further explored the reac-
tion of 1a in the presence of a Pd catalyst by using microwave
irradiation. Microwave irradiation is a greener method to acti-
vate reactions than conventional heating, because it drives the
chemical transformations very rapidly, and the desired products
are obtained in high yields.[13] Reaction of 1a in the presence
of Pd(OAc)2 (10 mol-%) in DCE and DMF under microwave irra-
diation at 50 °C for 30 min also failed to deliver 2a. Surprisingly,
a palladium catalyst[10b] in combination with diethylzinc and
[10c]
Pd(OAc)2
and indium with Pd(OAc)2 under a N2 environ-
ment.[10d] Besides the limited substrate scope and the forma-
tion of a mixture of biaryls in some cases, these reaction condi-
tions involve the use of stoichiometric amounts of ligands, met-
als, and sensitive reagents. Sodium tetraarylborates, organo-
boranes, organostannanes, and arenes have also been coupled
with diaryliodonium salts by using PdCl2, Pd(PPh3)4, CuI, and Pd
complexes to prepare diverse biaryls.[11]
As a result of our ongoing research directed towards the under similar reaction conditions, upon changing the solvent
synthetic utilities of hypervalent iodine reagents,[12] we envi- from DMF to dioxane, anticipated product 2a was obtained in
sioned an operationally simple Pd(OAc)2/polyethylene glycol- 60 % yield (Table 1, entry 11). Notably, if the reaction of 1a was
400 (PEG-400) catalysis system to prepare biaryls by harnessing conducted in PEG-400, 2a was formed in 90 % yield within
the aryl moieties of two easily accessible and stable diaryliodo- 10 min. PEG-400 as a reaction medium has received great atten-
nium salts. This protocol proceeds under neutral conditions tion owing to its appealing and ecofriendly features.[14] No
without the use of any activating metal reagent, ligand, or base change in the yield was observed upon reducing the loading
to generate an array of biaryls in good to excellent yields. De- of Pd(OAc)2 from 10 to 5 mol-%. Under conventional heating
spite an additional synthetic step required to obtain the diaryl- (50 °C) conditions, the Pd-catalyzed reductive coupling of 1a
iodonium salts, the present protocol provides a convenient failed to produce 2a (Table 1, entry 14); however, upon raising
route to prepare biaryls in view of the associated drawbacks of the reaction temperature to 100 °C, 2a was obtained in 75 %
the direct coupling of aryl halides and arenes in C–H functional- yield (Table 1, entry 15). During optimization of the reaction
ization strategies.[5]
conditions, we also screened diphenyliodonium salts bearing
Table 1. Optimization of the reaction conditions to prepare biphenyl (2a).
Entry
Catalyst
(mol-%)
X
Solvent
Microwave
irradiation [W]
T
[°C]
Time
[min]
Yield [%][a]
Yield [%][a]
2a
3a
1
2
3
4
5
6
7
8
CuI (50)
CuI (50)
Cu(OTf)2 (50)
CuCl (50)
CuBr(50)
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTs
Br
DMF
dioxane
DCE
dioxane
dioxane
dioxane
dioxane
DCE
–
–
–
–
–
140
80
80
80
80
80
80
50
50
50
50
50
50
50
100
50
50
50
1440
720
720
720
720
720
30
30
30
30
30
10
10
10
120
10
–
–
–
–
–
–
–
–
–
–
–
–
70
10
70
60
–
–
–
10
–
–
CuBr2 (50)
–
–
Cu(OAc)2 (50)
Pd(OAc)2(10)
Pd(OAc)2(10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (10)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2 (5)
Pd(OAc)2(5)
Pd(OAc)2 (5)
Pd(OAc)2(5)
50
50
50
50
50
50
–
9
DMF
water
10
11
12
13
14
15
16
17
18
–
dioxane
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
60
90
90
n.r.[b]
75
–
–
50
50
50
90
90
50
10
10
–
40
BF4
[a] Yield of isolated product. [b] n.r.: no reaction.
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–
–
–
different counterions (e.g., OTf, BF4, Br–, and OTs) and found ticipated 3-phenylindole (2p) in 70 % yield. The developed
that diphenyliodonium triflate (1a) was the most suitable salt strategy was successfully extended to prepare an arylated pyr-
to obtain biaryls 2 in high yields. Diphenyliodonium tetrafluoro- imidine base by using the 5-phenyluracil iodonium salt (1q),
borate was better than the salts bearing bromide and tosylate which afforded 5-phenyluracil (2q) in 50 % yield. Arylated ura-
(–OTs) counterions (Table 1, entries 16–18). The scope of the cils are well known for their interesting medicinal properties.[16]
protocol was investigated by treating various symmetrical and
unsymmetrical diaryliodonium salts under the catalytic system.
Diaryliodonium salts 1a–z were prepared by known protocols
involving easily available arenes and iodoarenes.[7a] Iodonium
salts containing electron-donating groups such as methyl (see
compound 1b) and tert-butyl (see compounds 1c, 1j, lk, and
1r) were successfully coupled to afford corresponding biaryls
2b, 2c, 2j, 2k, and 2r in good to excellent yields (85–90 %,
Table 2). Iodonium salts 1d–i possessing halogen atoms (i.e., F,
Cl, Br, and I) were well tolerated under the optimal reaction
conditions and delivered corresponding biaryls 2d–i in good
yields (55–90 %). Halogen-bearing biaryls 2d, 2g, and 2i can
serve as useful precursors for various cross-coupling reactions
to give useful molecules in addition to their interesting organic
luminogen properties.[15] Gratifyingly, fused-ring naphthyl de-
rivatives 2n and 2o were prepared in yields of 75 and 80 %,
respectively. Diaryliodonium salts possessing nitro (see com-
pound 1l), ester (see compound lm), and amide (see compound
1s) functional groups were successfully prepared and converted
into corresponding biaryls 2l, 2m, and 2s in good yields (60–
82 %). To our delight, indolyl(phenyl)iodonium salt 1p gave an-
Phenyl(mesityl)iodonium triflate with a bulky mesityl moiety
led to biphenyl 2a rather than expected phenyl–mesityl cou-
pled product. This interesting observation motivated us to pre-
pare various symmetrical biaryls 2t–z (Table 3). In these reac-
tions, iodomesitylene (3b) behaved as a leaving group and an-
other aryl partner underwent self-coupling to deliver symmetri-
cal biaryls 2t–z. Released 3b was easily recovered from the reac-
tion mixture by hexane wash, and resulting 3b was successfully
reused to prepare diaryliodonium salts 1t–z. Notably, various
aryl(mesityl) iodonium salts bearing methyl (see compound 1t),
methoxy (see compound 1u), halo (see compounds 1v–x), ester
(see compound 1y), and acetyl (see compound 1z) substituents
were well tolerated under the reaction conditions and pro-
duced symmetrical biaryls 2t–z in good to excellent yields (70–
82 %). Though the protocol is fairly general, it may not be suit-
able to prepare highly oxygenated or amino-substituted bi-
phenyls owing to inaccessibility of the corresponding diaryl-
iodonium salts.
Table 3. Substrate scope of aryl(mesityl)iodonium triflates.[a,b]
Table 2. Substrate scope of diaryliodonium salts 1a–s.[a,b]
[a] Reaction conditions: 1t–z (1 mmol), Pd(OAc)2 (5 mol-%), PEG-400 (2 mL),
microwave irradiation (50 W), 50 °C, 10 min. [b] Yields of isolated products
are shown.
The synthetic usefulness and prowess of the methodology
was clear by the successful preparation of o-functionalized bi-
aryls 2l–m from corresponding iodonium salts 1l–m within
10 min (Scheme 2). Biaryl 2m was utilized to obtain fluorenone
4m (75 %) and chromenone 5m (60 %). Similarly, a useful pre-
cursor, o-nitro biaryl 2l was smoothly converted into phenan-
thiridine 4l (75 %) and boscalid analogue 6l (90 %) in good to
excellent yields.[17] Interestingly, biaryl 2l also provided a simple
and efficient route to the carbazole alkaloid glycosinine
(Scheme 3).[18] To demonstrate scalability (1 gram) of the devel-
oped protocol, we prepared 2a in 88 % yield from 1a under the
optimized reaction conditions (Table 2).
The recyclability of the Pd/PEG catalyst system was demon-
strated by the reaction of 1a under the optimized conditions.
Gratifyingly, the Pd/PEG system was successfully recovered and
reused six times without any loss of catalytic activity (see the
Supporting Information).[19]
[a] Reaction conditions: 1a–s (1 mmol), Pd(OAc)2 (5 mol-%), PEG-400 (2 mL),
microwave irradiation (50 W), 50 °C, 10 min. [b] Yields of isolated products
are shown.
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Scheme 2. Synthesis of druglike molecules; MsOH = methanesulfonic acid, HFIP = hexafluoroisopropanol, TFA = trifluoroacetic acid.
Scheme 3. Rapid access to the carbazole alkaloid glycosinine.
Additional experiments such as TEM analysis of the reaction
mixture and analysis of PdII in the PEG-400 solution exposed to
microwave irradiation by 1H NMR spectroscopy indicated the
formation of in situ Pd nanoparticles (see the Supporting Infor-
mation).[19] On the basis of our results and literature reports,[19]
a plausible mechanism for the formation of biaryls is depicted
in Scheme 4. It is believed that initial microwave irradiation of
a solution of palladium acetate in PEG-400 (reducing species)
generates Pd0 species A, which upon oxidative addition to the
diaryliodonium salt is believed to give arylpalladium species
B.[8d] This species is likely to rearrange into PdII species C. Sub-
sequently, reductive elimination of C generates the biaryl com-
pound and Pd0.
Scheme 5. Reaction of 3a under optimized conditions.
Conclusions
In summary, we successfully developed a ligand- and base-free
Pd-catalyzed synthesis of useful biaryls from easily accessible
and stable diaryliodonium salts. The highlights of the present
protocol include operational simplicity, mild reaction condi-
tions, broad substrate scope for symmetrical and unsymmetrical
biaryls, scalability, and the use of a recyclable Pd catalyst. The
potential utility of the developed method was demonstrated
by preparing valuable heterocycles such as 5-aryluracils, carb-
azoles, chromenones, fluorenones, phenanthiridines, and
boscalid analogues. Hopefully, this methodology will find appli-
cation in diverse fields of synthetic organic chemistry. Further
mechanistic investigation and exploration of this transforma-
tion to other heterocyclic systems are currently underway in
our laboratory, and the results will be disclosed in due course.
Scheme 4. Plausible mechanism.
Experimental Section
Typical Procedure for the Synthesis of Biaryls: A 10 mL micro-
wave vial was charged with a magnetic stir bar, diphenyliodonium
triflate (1a; 0.1 g, 0.232 mmol), Pd(OAc)2 (2.6 mg, 5 mol-%), and
PEG-400 (2 mL). The vial was kept under microwave irradiation for
10 min at 50 °C (power: 50 W, pressure: 345 kPa). Completion of
the reaction was confirmed by TLC (hexane), and the resulting con-
tents were taken into water and extracted with ethyl acetate (3 ×
3 mL). The combined organic layer was dried with anhydrous so-
Under the optimized reaction conditions, iodobenzene (3a)
did not lead to biphenyl (Scheme 5); furthermore, biphenyl was
not produced upon reductive coupling of 3a (see the Support-
ing Information). These observations suggest that the aryl iod-
ide may not be involved in any stage in the reductive coupling
of the diaryliodonium salts to biaryls 2. However, formation of
symmetrical biaryls 2t–z by reductive coupling of aryl(mesityl)-
iodonium triflates 1t–z needs further mechanistic investigation. dium sulfate. After removal of the organic solvent, the residue was
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Acknowledgments
The authors gratefully acknowledge the Department of Biotech-
nology (DBT), New Delhi for financial support and the Central
NMR facility BITS Pilani for spectroscopic data. V. A. and M. P.
are thankful to the Council of Scientific and Industrial Research
(CSIR), New Delhi for SRF awards.
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Keywords: Synthetic methods · Microwave chemistry · C–C
coupling · Biaryls · Hypervalent compounds · Iodine ·
Palladium
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Received: March 2, 2016
Published Online: April 1, 2016
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