A novel green protocol for ligand free Suzuki-Miyaura cross-coupling reactions in WEB at room temperature

Article abstract of DOI:10.1039/c4gc02522a

A highly efficient green protocol for palladium acetate-catalysed ligand-free Suzuki-Miyaura cross-coupling reactions in neat 'water extract of banana (WEB)' was developed. This method offers a mild, efficient and highly economical alternative to the exis

Full text of DOI:10.1039/c4gc02522a

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A novel green protocol for ligand free
Suzuki–Miyaura cross-coupling reactions in
WEB at room temperature†
Cite this: DOI: 10.1039/c4gc02522a
Received 24th December 2014,
Accepted 16th January 2015
Preeti Rekha Boruah, Abdul Aziz Ali, Bishwajit Saikia* and Diganta Sarma*
DOI: 10.1039/c4gc02522a
www.rsc.org/greenchem
A highly efficient green protocol for palladium acetate-catalysed ing these Pd-ligand complexes is quite complicated because
ligand-free Suzuki–Miyaura cross-coupling reactions in neat ‘water the synthesis of the ligands requires time consuming multi-
extract of banana (WEB)’ was developed. This method offers a step sequences and thus it is expensive.7 In addition, most of
mild, efficient and highly economical alternative to the existing the Pd–ligand complexes usually form under an inert atmos-
protocols since the reaction proceeds in WEB at room temperature phere or harsh conditions and also have a limited substrate
in air at very short reaction times (5–90 min) under ‘ligand/external scope. Keeping all these factors in mind researchers have
base/external promoter/organic medium’ free conditions.
modified these initially utilized palladium catalysts using
bulky or electron rich phosphine ligands,8 nitrogen containing
ligands,9 N-heterocyclic carbenes,10 oximes,11 and imines.12 In
general, these catalysts are air and moisture sensitive and are
often difficult to prepare. Meanwhile, recent studies demon-
strated that for the reduction of chemical waste and the
number of synthetic steps as well as safer and more efficient
reaction conditions – in short to fulfil green chemistry proto-
cols – one of the most important aspects is the application of
environmentally friendly solvents such as water, ionic-liquids
or supercritical carbon dioxide. Water, in contrast to common
organic reaction media, is particularly favourable as it is
cheap, non-flammable, nontoxic and readily available and has
significance in environmentally benign processes.13 However,
most efficient protocols for SM cross-coupling reactions in
aqueous medium suffer from high catalyst loadings,14 elevated
reaction temperatures,15 addition of promoters/additives,16
complex workup procedures or require the addition of hazar-
dous organic solvents. Consequently, there is a need to design
a very simple, highly economical, commercially feasible as well
as environmentally sustainable route for the synthesis of
biaryls in order to meet their growing demand in diverse
sectors.
Herein, we present a highly novel protocol with broad appli-
cability regarding the range of substrates and functional group
tolerance for SM cross-coupling reactions. In this communi-
cation, WEB was chosen as reaction media without using any
external promoters, external base, phosphine ligands and
organic co-solvents or biphasic media. To the best of our
knowledge, this is the first report of a base/ligand/organic
solvent free environmentally friendly protocol for Suzuki
cross-coupling reactions carried out in WEB at room tempera-
ture. Here, we have prepared an aqueous extract of banana
The biaryl moiety is a common structural framework found
mainly in pharmaceuticals, agrochemicals and biologically
active natural compounds.1 The Suzuki–Miyaura (SM) cross-
coupling reaction of aryl halides with arylboronic acids is one
of the most powerful carbon–carbon bond forming methodo-
logies for constructing biaryl compounds in organic chem-
istry.2 Among the cross-coupling processes, the SM reaction is
particularly widely used, due to attractive attributes such as
the commercial availability, the air and water stability, the
functional-group compatibility and the nontoxic nature of
boronic acids. Generally, the palladium catalyst is a unique
choice for the Suzuki–Miyaura reaction for the reason that it
shows excellent catalytic activity and stability. Over the past
few years, extensive efforts have been devoted toward the devel-
opment of new ligands with the ability to provide excellent
levels of catalytic generality and activity in palladium-catalyzed
cross-coupling processes.3 A survey of the recent chemical lit-
erature revealed that many examples of nickel4 and copper5
complexes as catalysts exist for these cross-coupling reactions,
but their inherent scopes and reaction efficiencies were found
to be limited. In this context, to get highly satisfactory results,
the properties of many catalysts had to be tuned using some
unique ligands, such as phosphine complexes. However, phos-
phine ligands are sensitive to air and moisture with conversion
to phosphine oxide.6 Again, the synthetic procedure for prepar-
Department of Chemistry, Dibrugarh University, Dibrugarh-786004, Assam, India.
E-mail: bishwajitsaikia@gmail.com, dsarma22@gmail.com;
Tel: +91 9954314676, +91 9854403297
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4gc02522a
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Pd(OAc)2 furnished the cross-coupled product in 99% yield
within a very short reaction time (Table 1, entry 2). Decreasing
the catalyst loading to 0.25 mol% gave a respectable 93% yield
of the product in 30 min (Table 1, entry 3). 0.1 mol% Pd(OAc)2
was found to be sufficient to give a moderate conversion with
72% isolated yield of the product after 30 min (Table 1, entry
4). A further decrease in catalyst loading (to 0.02 mol%)
resulted in poor conversion even after increasing the reaction
time up to 24 h (Table 1 entry 5). No further increase in yield
was observed on increasing the reaction time. In the absence
of palladium acetate, the reaction did not proceed at all in
WEB (Table 1, entry 6). Pd(OAc)2 in WEB catalyzed the reaction
very efficiently producing excellent yields of products at room
temperature in a very short reaction time. After exploring a
wide array of conditions, 0.5 mol% Pd(OAc)2 in WEB was
found to be optimal (Table 1, entry 2) for the SM reaction.
To explore the scope of this reaction, a series of aryl bro-
mides were allowed to react with arylboronic acids under iden-
tical conditions. Both the electron-rich and the electron-
deficient aryl bromides showed an excellent reactivity and
furnished the products in high yields in short reaction times
(Table 2, entries 1–7) at room temperature. It is worth noting
that the catalytic system was tolerant to a broad range of func-
tional groups, such as –OMe, –CH3, –NO2, –CN, –CHO and
–COMe. Interestingly, both the electron-rich and the electron-
deficient arylboronic acids delivered the products with high
yields at room temperature under aerobic conditions (Table 2,
entries 4, 5 & 8). We are quite satisfied to see that the for-
mation of undesired homo-coupling product was very negli-
gible under these reaction conditions. The ability to operate
for a range of substrates at low catalyst loading is of great
importance in metal-catalyzed processes. Thus, our present
result is quite significant as the desired biaryls could be
achieved at room temperature using a neat aqueous extract of
Fig. 1 Preparation of WEB.
(scientific name: Musa balbisiana Colla; family: Musaceae;
species: Musa balbisiana) by first drying the banana peels fol-
lowed by burning them to ash. Water was added to the ash,
mixed well and then filtered. The filtrate is termed as WEB
here (Fig. 1).17 The resulting system shows superior catalytic
activity to a variety of reported catalyst systems. There is no
doubt that WEB has very great potential in green chemical pro-
cesses from the economic and environmental points of view in
the near future. The main objectives of this protocol are to
develop an efficient synthetic process for the facile conversion
of the Suzuki–Miyaura reaction under an oxygen atmosphere
at room temperature. Furthermore, the present method develo-
ped for the Suzuki–Miyaura reaction offers many advantages
including high conversion, short duration, high economy, the
involvement of non-toxic green substrates, etc. The operational
simplicity of the process without the need for special handling
is also noteworthy. This new Suzuki variant can be accom-
plished with excellent yields for a broad range of arylhalides
and arylboronic acids.
The efficiency of the catalytic system was evaluated by the
coupling of 4-bromoanisole and phenylboronic acid in WEB in
the absence of ligands or additives (Table 1, entries 1–6).18 To
optimize the reaction conditions, the effect of the amount of
Pd(OAc)2 on the coupling reaction was studied. From the
results in Table 1, we can see that an excellent yield of the
desired product was obtained in the presence of 1 mol%
Pd(OAc)2 for 5 min (Table 1, entry 1). Even 0.5 mol% of
Table 2 Pd(OAc)2 and WEB catalysed Suzuki–Miyaura cross-coupling
reaction of aryl halides with aryl boronic acids at room temperaturea
Table 1 Effects of catalyst quantity in the Suzuki–Miyaura reaction of
4-bromoanisole with phenylboronic acid in water extract of banana
(WEB) at room temperaturea
Entry
R1
R2
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
4-OMe
3-OMe
4-Me
4-Me
4-Me
4-NO2
4-CN
H
4-CHO
3-CHO
2-CHO
4-COMe
H
H
H
4-OMe
4-CF3
H
H
4-OMe
H
5
10
10
10
15
10
10
10
15
15
20
15
99
98
99
99
97
99
98
99
96
95
92
97
Catalyst (mol%)
Pd(OAc)2
Entry
Time
Yieldb (%)
1
2
3
4
5
6
1
5 min
5 min
30 min
30 min
24 h
99
99
93
72
0.5
0.25
0.1
0.02
—
9
10
11
12
H
H
H
50
24 h
No reaction
a Reaction conditions: arylhalide (1 mmol), arylboronic acid
(1.2 mmol), Pd(OAc)2 (0.5 mol%), in WEB (3 mL) at room temperature.
b Isolated yields.
a Reaction conditions: arylhalide (1 mmol), arylboronic acid (1.2 mmol),
catalyst, in WEB (3 mL) at room temperature. b Isolated yields.
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nal bases, external promoters, ligands etc. The scope of these
reactions is broad with respect to both coupling partners, and
we expect the methodology presented will find great utility in
academic and industrial applications in the near future.
Table 3 Pd(OAc)2 and WEB catalysed Suzuki–Miyaura cross-coupling
reaction of (hetero)aryl halides with aryl boronic acids at room
temperaturea
Acknowledgements
Entry
1
Ar–Br
Ar–B(OH)2
Time (min)
60
Yieldb (%)
88
DS is thankful to UGC, New Delhi for a start-up grant
[no. F.20-3(2)/2012(BSR)]. The authors also acknowledge the
Department of Science and Technology for financial assistance
under DST-FIST programme and UGC, New Delhi for Special
Assistance Programme (UGC-SAP) to the Department of Chem-
istry, Dibrugarh University.
2
3
60
90
90
86
Notes and references
a Reaction conditions: (hetero)aryl halide (1 mmol), arylboronic acid
(1.2 mmol), Pd(OAc)2 (0.5 mol%), in WEB (3 mL) at room temperature.
b Isolated yields.
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banana as a reaction medium and with relatively low catalyst
loading (0.5 mol%) without using any additives, external base,
ligand and organic co-solvents.
To check the efficiency of our catalytic system, we next
employed some heterocyclic systems as Suzuki–Miyaura coup-
ling partners (Table 3, entries 1–3) and we found that hetero-
cyclic systems also worked efficiently under this catalytic
system.
In order to demonstrate the practical importance and wide
application of the catalytic system consisting Pd(OAc)2/WEB, it
is essential to show the efficient recycling of the catalyst
system. The results obtained in our experiment confirmed that
it was possible to recycle and reuse the catalytic medium after
extraction of products with diethyl ether but the yields
obtained in successive steps were low (60% yield in 1st recycle).
The lowering of yield is due to the solubility of Pd(II) or Pd(0)
species in diethyl ether which is evidenced by the colour
change of the ether fraction from colourless to slightly black.
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[banana peels, trunk and rhizome of 1. Dwarf Cavendish
(species: Musa acuminate) and 2. Chinia (Manohar); Musa
sepientum] and we observed identical reactivity towards
Suzuki–Miyaura cross coupling reactions.
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17 Method of preparation of WEB: 5 g of banana ash was sus-
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well for 10 min. The mixture was then filtered through sin-
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mixture of aryl halide (1 mmol), arylboronic acid
(1.2 mmol) and Pd(OAc)2 (0.5 mol%) in WEB (3 mL) was
stirred for the indicated time at room temperature. After-
ward, the reaction solution was extracted four times with
diethyl ether (4 × 10 mL). The products were purified by
column chromatography over silica gel using n-hexane–
ethyl acetate (9 : 1 v/v) to obtain the desired coupling pro-
1
ducts. The products were characterized using H-NMR and
GC-MS.
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