WEPA: a bio-derived medium for added base, π-acid and ligand free Ullmann coupling of aryl halides using Pd(OAc)2

Article abstract of DOI:10.1039/c8cc06940a

A bio-derived sustainable medium based on water extract of pomegranate ash (WEPA) has, for the first time, been developed for the homocoupling of aryl halides under palladium-assistance. Avoiding the requirement of an external base, ligand and π-acid, the use of the proposed renewable medium offers remarkable attributes like wide substrate scope, good to nearly quantitative yields of biphenyls with exceptional chemoselectivity and scale up viability.

Full text of DOI:10.1039/c8cc06940a

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WEPA: a bio-derived medium for added base,
p-acid and ligand free Ullmann coupling of aryl
halides using Pd(OAc)2†
Cite this: Chem. Commun., 2018,
54, 12333
Received 26th August 2018,
Accepted 8th October 2018
Jangam Lakshmidevi,a Rama Moorthy Appa,a Bandameeda Ramesh Naidu,a
b
a
S. Siva Prasad,a Loka Subramanyam Sarma
and Katta Venkateswarlu
*
DOI: 10.1039/c8cc06940a
rsc.li/chemcomm
A bio-derived sustainable medium based on water extract of (rt – 120 1C) besides offering high reaction rates and huge
pomegranate ash (WEPA) has, for the first time, been developed for substrate scope.4,6–9 Most of the palladium based homocoupling
the homocoupling of aryl halides under palladium-assistance. Avoiding reactions of aryl halides require the use of organic solvents4b,6 and
the requirement of an external base, ligand and p-acid, the use of the external bases.4b,6c–k,7 In addition, phosphine based ligands,6a–e
proposed renewable medium offers remarkable attributes like wide reducing agents7a,b,8 and (or) p-acids6a,f,g,8c,9 are often required.
substrate scope, good to nearly quantitative yields of biphenyls with
exceptional chemoselectivity and scale up viability.
Despite the increasing number of reports on ligand and (or)
base-promoted Ullman couplings, a very small number of
methods have been reported by the application of pure water/
Biaryls constitute an important category of organic compounds aqueous solvents as reaction media.7a–f,8a–c,9a Nevertheless, these
and are emerging structural motifs of pharmaceuticals, ligands, reported protocols also suffer from certain drawbacks like the
energy based materials, agrochemicals, dyes and bio-active requirement of p-acids such as CO2,8b,9a paraformaldehyde8c
natural products.1 Homocoupling (Ullmann coupling) of aryl reducing agents like zinc,8a,b magnesium8c and ascorbic acid7a,b
halides is a simple and convenient technique to access biaryls, and an external base.7a–f
and aryl halides are the most elementary, stable and readily
Green chemistry concerns the sustainability of human life
available substrates.2 Ullmann coupling has long been known and the environment, and also redesigning the materials and
(about a century ago)3 for the synthesis of biaryls and was processes which are the basis of our life and economy, making
initially reported by the use of a stoichiometric amount of them as safe as possible.13 In this context, the use of non-toxic
copper catalyst at elevated temperatures (4200 1C). However, solvents and renewable feedstocks are two of the twelve princi-
the requirement of excess amounts of copper, high tempera- ples by which green chemistry was constituted. A considerable
ture, slow rate of reaction (due to the low solubility of copper number of reports have appeared in the literature using bio-based
in organic solvents), and low tolerance to functional groups solvents including ethyl acetate, glycerol, cyrene, g-valerolactone
restricted the widespread use of copper catalysis.4 Many dedicated and 2-methyltetrahydrofuran for organic synthesis.14 There are
efforts have been focused on finding alternative catalysts operable very few reports in the literature on the use of nature derived
under less-vigorous conditions. In connection with this, various aqueous bases for organic synthesis, which can address several
transition metals such as nickel,5 palladium,4b,6–9 zinc,10 iron11 principles of green chemistry, in particular, the use of safer
and cobalt12 based catalysts have been developed as an alternative solvents and renewable feedstocks.14b,15 Inspired by the advan-
to copper. In particular, palladium holds great prominence tages of the nature derived aqueous feedstocks (base), we report
because it enables coupling at low to moderate temperature here a convenient and sustainable method for the synthesis of
biaryls via the Ullmann coupling of aryl halides catalyzed by
Pd(OAc)2 in WEPA under aerobic conditions. To the best of our
knowledge, this is the first report to use WEPA as an effective
a Laboratory for Synthetic & Natural Products Chemistry, Department of Chemistry,
Yogi Vemana University, Kadapa 516005, India.
(basic) medium by the successful utilization of agro waste such
as pomegranate peel.
E-mail: kvenkat@yogivemanauniversity.ac.in
b Nanoelectrochemistry Laboratory, Department of Chemistry, Yogi Vemana University,
Kadapa 516005, India
Herein, the potentiality of WEPA as a renewable medium is
systematically investigated using 4-iodoanisole (1a) as a model
substrate. Initially, 1 mol% of Pd(OAc)2 in WEPA (0.5 mL) and 1a
(1 mmol) was heated at 80 1C (which was found to be effective
after screening at different temperatures) and led to the
† Electronic supplementary information (ESI) available: Comparative analysis of
the reported Pd catalyzed Ullmann couplings of aryl halides, procedure for the
preparation of WEPA, general experimental procedure, recyclability, compound
characterization data, copies of 1H & 13C NMR, XPS and EDS spectra, SEM image,
CV data and particle size analysis. See DOI: 10.1039/c8cc06940a
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Table 1 Optimization of the reaction conditionsa
of papaya bark ash (WEPBA)18 and water extract of lemon fruit
shell ash (WELFSA),19 and the formation of 2a was observed in
81%, 48%, 83%, and 56% yields, respectively in 5 h (Table 1,
entries 14–17). This observation supports our assumption that
the pomegranate peel may be an effective medium for Pd-
catalyzed Ullman coupling. We speculate that the basic nature
of WEPA might be due to the presence of significant amounts of
nutrients. It was reported in the literature that pomegranate
peel was approved as a nutritious feed to cattle.20 To assess the
presence of nutrients in WEPA, X-ray photoelectron spectro-
scopy (XPS) of ash and energy-dispersive X-ray spectroscopy
(EDS) analysis of the extract (a white solid formed after the
evaporation of water) were carried out and the corresponding
spectra are provided in the ESI.† As can be seen, the presence of
K, Mg, Ca, C, O and Cl is evident from these spectra. Further-
more, these spectra indicate the presence of a high amount
(51.54 wt%) of K which is higher than that of other extracts,
WEB,16 WERSA17 and WEPBA.18 The presence of oxygen and
chlorine suggests that K, Mg and Ca may exist in the form of the
corresponding carbonates, oxides, and chlorides and impart
basicity to the WEPA and play a key role in the formation of
products. In addition, the pH of WEPA was found to be between
11.7 and 12.1 upon several repetitions, further supporting its
basic nature.
Catalyst
S. no. (mol%)
Medium
(mL)
Co-solvent Time Yieldb
(mL)
(h)
(%)
1
2
3
4
5
6
7
8
Pd(OAc)2 (1)
WEPA (0.5)
WEPA (0.5)
WEPA (0.5)
WEPA (1)
WEPA (1)
WEPA (1)
—
—
12
42
73
81
88
99
99
—
—
—
93
91c
68
60
81
48
83
56
—
—
—
—
—
Pd(OAc)2 (1)
Pd(OAc)2 (2)
Pd(OAc)2 (2)
Pd(OAc)2 (3)
Pd(OAc)2 (4)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
—
EtOH (0.5) 10
EtOH (0.5)
EtOH (0.5)
EtOH (0.5)
EtOH (0.5)
EtOH (0.5) 12
EtOH (0.5) 12
EtOH (0.5) 12
EtOH (0.5)
EtOH (0.5)
EtOH (0.5)
EtOH (0.5)
EtOH (0.5)
EtOH (0.5)
EtOH (0.5)
5
5
4
4
Water
9
WEPA (1)
WEPA (1)
WEPA (1)
WEPA (1)
WEPA (1)
WEB (1)
WERSA (1)
WEPBA (1)
WELFSA (1) EtOH (0.5)
10
11
12
13
14
15
16
17
18
19
20
21
22
PdCl2 (3)
5
5
5
5
5
5
5
5
Pd(PPh3)Cl2 (3)
Pd(dba)3 (3)
Pd(dppf)Cl2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
Pd(OAc)2 (3)
CuCl (3)
WEPA (1)
WEPA (1)
WEPA (1)
EtOH (0.5) 12
EtOH (0.5) 12
EtOH (0.5) 12
EtOH (0.5) 12
EtOH (0.5) 12
Cu(OAc)2 (3)
CuI (3)
CuSO4Á5H2O (3) WEPA (1)
CoCl2 (3) WEPA (1)
Under the successful optimized conditions, we tested the
applicability of this method to various aryl halides including
aryl iodides, aryl bromides and aryl chlorides and the results are
listed in Table 2. Aryl iodides are found to be more reactive than
a
b
Reaction conditions: 1a (1 mmol) at 80 1C. Isolated yield of 2a.
c
Formation of phosphine oxide as a side product was also observed.
formation of biphenyl (2a) in 42% isolated yield in 12 h bromides, which are in turn more reactive than aryl chlorides.
(Table 1, entry 1). Upon addition of 0.5 mL of ethanol to the Aryl iodides with both electron releasing (ER) (methoxy and
reaction mixture, the yield of 2a was increased to 73% in 10 h methyl) and electron withdrawing (EW) (chloro, acetyl, formyl
(Table 1, entry 2) and an increase in catalyst loading to 2 mol% and nitro) groups are found to form Ullmann coupling products
enhanced the yield to 81% in 5 h (Table 1, entry 3). At this stage, (2) in excellent yields (85–95%) (Table 2, entries 1, 2 and 4–8).
the increase of the WEPA (basic medium) to 1 mL formed 2a in Unsubstituted aryl iodide, iodobenzene (1d), was also found as
88% yield in 5 h (Table 1, entry 4) and an enhancement of a good substrate for the present conversion which formed
Pd(OAc)2 to 3 mol% improved the yield of 2a to nearly quanti- nearly quantitative yield of 2d (Table 2, entry 3). Mixed aryl
tative in 4 h (Table 1, entry 5). Further increase of the catalyst halide, 4-chloro-1-iodobenzene (1f), formed 2f in 95% yield with
amount to 4 mol% does not show any significant effect on the excellent chemoselectivity (Table 2, entry 5), i.e., the priority for
reaction progress (Table 1, entry 6). The reaction was then iodine in the presence of chlorine. Formation of the coupling
studied independently, in the absence of WEPA and Pd(OAc)2 product related to chlorine is not at all observed.
and no reaction was found (Table 1, entries 7–9), indicating that
We then investigated the influence of this method on aryl/
the use of Pd(OAc)2 and WEPA (basic medium) is crucial for the heteroaryl bromides and found the formation of biaryls in good
present conversion. We then screened other Pd-based catalysts to high yields (73–93) (Table 2, entries 9–27). High conversion
such as PdCl2, Pd(PPh3)Cl2, Pd(dba)3 and Pd(dppf)Cl2, under similar of arene bromides into biaryls was observed with ER (methoxy,
reaction conditions and found the formation of 2a in 93%, 91%, methyl and tert-butyl) (73–91%) as well as EW (phenyl, chloro,
68%, and 60% yields, respectively, in 5 h (Table 1, entries 10–13), fluoro, acetyl and formyl) (80–87%) groups (Table 2, entries 9–22),
indicating Pd(OAc)2 as the best among the screened catalysts. however, a protodebrominated product was formed as a minor
Moreover, the formation of phosphine oxide as the byproduct was product with 2-bromoanisole (1l), a major product with
observed when Pd(PPh3)Cl2 was used (Table 1, entry 11). The scope 2-bromotoluene (1n) (o-cresol, 4 is the minor one) and exclusively
of the reaction was also studied further by using transition-metal in the case of 2-bromobiphenyl (1q) and 2-bromobenzaldehyde
based salts such as CuCl, Cu(OAc)2, CuI, CuSO4Á5H2O and CoCl2 (1v). Bromobenzene (1x) was found to be the best substrate
under the present experimental conditions and no progress was among the aryl bromides to give 2d in 93% yield in 3 h (Table 2,
found (Table 1, entries 18–22).
entry 23). 9-Bromophenanthrene (1y) provided very good
Furthermore, this method was investigated with other bio- yield of biaryl, 2n (86%) (Table 2, entry 24), while, 9-bromo-
derived aqueous extracts such as water extract of banana anthracene (1z) gave the protodebromination product, 7 (Table 2,
(WEB),16 water extract of rice straw ash (WERSA),17 water extract entry 25).
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Table 2 Substrate scopea
when 1-bromo-4-chlorobenzene (1r) and 1-bromo-4-fluoro-
benzene (1s) were used as substrates (Table 2, entries 17 & 18).
Furthermore, this method was extended to aryl chlorides
(1ac–1af) and it was found that the corresponding reaction
proceeds with a long reaction time (12 h) and produced low
yields (25–35%) of products 2a, 2d and 2h (Table 2, entries 28–30).
1-Chloro-2,4-dinitrobenzene (1af) formed only the proto-
dechlorination product, m-dinitrobenzene (8), in 89% yield in
4 h (Table 2, entry 31). In one instance, the controlled reductive
dechlorination of 1af was reported with H2-Pd(PPh3)4-DMF,21
and copper powder-benzoic acid at 4200 1C,22 but, this method
is rather simple, benign and promises higher yields.
The present method was also tested and its successful
applicability for the multi-gram (up to 3 g) scale synthesis of
biaryl 2a (find at ESI†) was found. Additionally, the recyclability
tests are discussed in the ESI.†
The plausible mechanism of the present conversion is
exemplified in the following scheme (Scheme 1). The mecha-
nism of the palladium-catalyzed Ullmann coupling in WEPA
(catalytic cycle 1) is believed to be similar to the mechanism
proposed by Nadri et al.6d Initially, the reduction (I) of Pd(OAc)2
in WEPA forms an active Pd0 species (A) (evidenced by EDS and
cyclic voltammetric analysis, ESI†) and it adds two aryl halides
in two consecutive oxidative additions (viz., II and III) to form a
PdIV species, C. The intermediate C participates in reductive
elimination (IV) to give products, 2 and PdIIX2 (D) and D may be
finally reduced (V) to active Pd0 species, A, in WEPA (base).
In another possibility (catalytic cycle 2) the inter exchange of
the Ar and X groups (VI) between B generates dicoordinated
palladium intermediates, E and D. Finally, E may undergo
reductive elimination (VII) to generate A.
In summary, we have developed an effective and benign
method for the synthesis of biaryls from abundant aryl halides
via the application of novel renewable media, WEPA, in the pre-
sence of Pd(OAc)2. The present protocol avoids the use of toxic
solvents, external bases, phosphine ligands, p-acids and harsh
reducing agents. To the best of our knowledge, the present
development is the first report highlighting the use of nature-
derived basic media for Ullmann coupling. Furthermore, formation
S. no. Aryl halide (1)
Time (h) Product (2) Yieldb (%)
1
2
3-Iodoanisole (1b)
2-Iodoanisole (1c)
4
5
2
3
4
4
4
4
6
6
6
6
3
2b
2c
2d
2e
2f
2g
2h
2i
2a
2b
2c + 3
2e
4 + 5
2j
93
89
99
94
95
87
89
85
88
82
73 & 11
91
35 & 61
84
3
4
Iodobenzene (1d)
4-Iodotoluene (1e)
5
6
7
8
4-Chloro-1-iodobenzene (1f)
4-Iodoacetophenone (1g)
4-Iodobenzaldehyde (1h)
4-Nitroiodobenzene (1i)
4-Bromoanisole (1j)
3-Bromoanisole (1k)
2-Bromoanisole (1l)
4-Bromotoluene (1m)
2-Bromotoluene (1n)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
4-Bromo-4-tert-butylbenzene (1o) 6
4-Bromobiphenyl (1p)
2-Bromobiphenyl (1q)
1-Bromo-4-chlorobenzene (1r)
1-Bromo-4-fluorobenzene (1s)
4-Bromoacetophenone (1t)
4-Bromobenzaldehyde (1u)
2-Bromobenzaldehyde (1v)
3-Bromoacetophenone (1w)
Bromobenzene (1x)
9-Bromophenanthrene (1y)
9-Bromoanthracene (1z)
3-Bromopyridine (1aa)
2-Bromopyridine (1ab)
4-Chloroanisole (1ac)
6
3
6
2k
2d
2f
83
95
83
6
6
6
3
2l
80
83
87
94
2g
2h
6
6
3
2m
2d
2n
7
2o
2p
2a
2d
2h
8
81
93
86
90
80
81
32
35
12
10
10
10
12
12
12
4
Chlorobenzene (1ad)
4-Chlorobenzaldehyde (1ae)
1-Chloro-2,4-dinitrobenzene
(1af)
25
89
a
Reaction conditions: 1 (1 mmol), WEPA (1 mL), EtOH (0.5 mL),
b
Pd(OAc)2 (3 mol%) at 80 1C. Isolated yield.
Moreover, the efficiency of the present method was also
tested with the heteroaryl bromides, 3-bromopyridine (1aa) and
2-bromopyridine (1ab) and good yields (80% and 81%) of the
coupled products were found. High chemoselectivity over the
halogens (i.e., bromine over chlorine and fluorine) was observed Scheme 1 Plausible mechanism of Ullmann coupling in WEPA.
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