Mont-K-10-supported Pd catalyst for Suzuki–Miyaura reaction
It is clear from Table 1 that carbonate bases, namely K2CO3,
Na2CO3 and Cs2CO3, give comparable yields (Table 1, entries
1–3), and K2CO3 is considered for further optimization of reac-
tion conditions. In contrast, the reaction does not go to comple-
tion with the relatively weak base NaHCO3 (Table 1, entry 4). We
also tested some hydroxide bases (NaOH and KOH), but the yield
is very poor (Table 1, entries 6 and 7). An organic base also does
not improve the yield to any great extent (Table 1, entry 8). Fur-
ther screening with different amounts of K2CO3 confirms that 3
equiv. is the optimum for efficient coupling (Table 1, entries 9
and 10). Based on these observations we consider K2CO3 for fur-
ther optimization.
The next goal was to find a suitable solvent in which the
reaction can proceed at a good rate. A wide range of organic
and aqueous solvents were investigated and the results ob-
tained are listed in Table 2. Alcoholic solvents MeOH, EtOH
and i-PrOH are found to be very effective for the catalytic sys-
tem under investigation (Table 2, entries 1–3). We also exam-
ined 50% aqueous composition of the alcoholic solvents and,
to our surprise, excellent yields of the isolated cross-coupling
product are observed in almost all cases. But considering the
time factor, we take 50% aqueous isopropanol to be superior
(Table 2, entries 4 and 5). Interestingly, when water is used
alone as solvent the reaction does not complete even after
24 h and only 75% of the coupled product can be isolated
(Table 2, entry 6). The use of other organic solvents (DMF,
CH3CN, THF and DCM) is found to be less effective in our pro-
tocol (Table 2, entries 7–10).
In order to optimize the amount of catalyst and substrate, a series
of reactions were performed with 4-bromoanisole and pheny-
lboronic acid. The observations are listed in Table 3. It is found that
10 mg of Pd–clay catalyst is enough for the complete conversion of
0.5mmol of 4-bromoanisole (Table 3, entries 1–3). Further increas-
ing the amount of catalyst has no effect on the reaction system in
terms of yield and time (Table 3, entry 3), while decreasing its
amount to 5 mg leads to lower yield of isolated product (Table 3,
entry 2). We also optimized the effective amount of phenylboronic
acid required for the complete conversion of 4-bromoanisole. Max-
imum yield is observed with 1.2 equiv. of phenylboronic acid
(Table 3, entries 4 and 5).
Reusability Test
From a green chemistry perspective, the reusability of catalysts
makes them more attractive. We also investigated the reuse of
our catalyst. The reuse test was performed with 4-bromoanisole
and phenylboronic acid as substrate and we find that the catalyst
is reusable up to three cycles without any significant loss of catalytic
activity (Table 5, entries 1–3). However, after the third cycle the yield
is decreased (Table 5, entry 4). This may be due to the deactivation
of the catalyst during the course of the reaction and recovery pro-
cess. Although in the literature few reports are available of similar
recycling activity, the requirement of high reaction temperature
and harmful organic solvents detract from their applicability.[48–50]
To check the heterogeneity of the catalyst, we performed a hot fil-
tration test with 4-bromoanisole (0.5 mmol) and phenylboronic acid
(0.6 mmol). After 3 h of reaction the solid catalyst was separated from
the reaction mixture (65% conversion determined using GC-MS). The
filtrate was further stirred for 6 h. As expected no further increase in
the yield was detected (indicated by GC-MS). Inductively coupled
plasma atomic emission spectroscopic analysis of the filtrate collected
by filtration confirmed that Pd was absent from the reaction mixture.
Conclusions
We have developed a mild and efficient protocol for the Suzuki–
Miyaura cross-coupling reaction at room temperature under aer-
obic conditions using a palladium triphenylphosphine–mont-K-
10 catalyst. The catalyst system was found to be effective for a
wide array of functionalized aryl bromides and arylboronic acids.
The application of aqueous solvent in our protocol and the reus-
ability of the catalyst are significant advantages compared to
other existing catalyst systems for the Suzuki reaction.
Acknowledgements
We wish to thank the Department of Science and Technology, New
Delhi, for financial support (no. SR/FT/CS-0098/2009) as well as for
an INSPIRE research fellowship to A.G.
References
With the optimized reaction conditions we next explore the scope
with various electronically diverse aryl bromide and arylboronic acids.
The results clearly indicate that aryl bromide and arylboronic acids
having both electron-withdrawing and electron-donating groups
(OCH3, NO2, CH3 and COCH3) couple successfully to give good to excel-
lent yield of isolated cross-coupling product (Table 4, entries 1–13). To
check the compatibility of our protocol for heterocyclic boronic
acids and heterocyclic bromides, 2-methoxypyridine-5-boronic
acid was stirred with 4-bromoanisole and to our delight isolated
cross-coupled product is obtained in 97% yield (Table 4, entry 14).
Similar results are also obtained with 5-bromopyrimidine and 3-
bromothiophene (Table 4, entries 15 and 16). Excellent yield of
the product is also obtained for the coupling of 3-bromothiophene
with phenylboronic acid (Table 4, entry 17).
It is worth noting that the system is effective for gram-scale reac-
tion of aryl bromide and arylboronic acids. When 1000 mg of 4-
bromoanisole (5.35 mmol), 783.24 mg of phenylboronic acid
(6.42 mmol), 2215 mg of K2CO3 (16.05mmol) and 100mg of
Pd–mont-K-10 in 30 ml of i-PrOH–H2O (1:1) are stirred using our op-
timized conditions, the reaction completes in 8 h with an isolated
yield of 96%. Further, the catalyst can be recovered and applied
successfully for another cycle.
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Appl. Organometal. Chem. (2014)
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