S. Bhat K, et al.
Applied Catalysis A, General 596 (2020) 117516
toxic, highly abundant, and environmentally benign, we conducted a
reaction in DMF/H O in 3:1 ratio (2 mL) as a solvent system at 100 °C,
2
which furnished a slightly low yield of 3c (72 %, entry 4, Table 1),
whereas the reaction in the absence of base was unsuccessful (entry 5,
Table 1). Reaction using Cs
2
CO furnished 87 % of 3c (entry 6, Table 1).
3
Scheme 1. Suzuki-Miyaura-Coupling Reaction.
Increasing the catalyst loading from 10 wt% to 20 wt%, resulted in the
formation of 3c in 88 % yield (entry 7, Table 1), whereas reducing the
catalyst loading to 5% afforded the 85 % of 3c (entry 8, Table 1). De-
creasing the reaction time to 3 h (entry 9, Table 1) or employing the
phenylboronic acid in 1.5 equiv. (entry 10, Table 1) furnished the 3c in
7
4 and 81 % yields, respectively. The best result (entry 2, Table 1) was
obtained with the use of K
2
CO (2 equiv.) as a base, 10 wt percentage of
3
catalyst, and water (2 mL) as the solvent at 100 °C for 6 h (See the
Supporting Information for the optimization studies were also per-
formed using iodobenzene (1a) and phenyl boronic acid (2a)).
3
. Substrate scope using Ti0.97Pd0.03O1.97 powder catalyst
First, we explored the scope of the Suzuki reaction using
Ti0.97Pd0.03O1.97 powder catalyst (Table 2). As can be seen, the reaction
proceeded well with a variety of aryl halides and arylboronic acid de-
rivatives. The reaction of iodobenzene with phenylboronic acid and m-
tolylboronic acid was facile furnishing the corresponding biaryl deri-
vatives 3a and 3b in 90 and 96 % yields, respectively. Similarly, the
reaction of para-methoxyiodobenzene, para-methyliodobenzene, and
para-fluoroiodobenzene with phenylboronic acid afforded the corre-
sponding biaryl derivatives 3c, 3d, and 3e in 92, 83, and 96 % yields,
respectively. The reaction of para-methoxyiodobenzene with (4-acet-
ylphenyl)boronic acid was also facile, affording the product 3f in al-
most quantitative yield. The reaction of halo substituted arylboronic
acids such as (4-fluorophenyl)boronic acid, (3-chlorophenyl)boronic
acid, and (4-chlorophenyl)boronic acid proceeded smoothly furnishing
the products 3 g, 3 h, and 3i in 99, 96, and 98 % yields, respectively. (4-
Fig. 1. Rietveld refinement of the XRD profile of freshly prepared catalyst.
powder catalyst as well as the Pd-coated on cordierite. The advantages
of the new catalyst for the Suzuki-Miyaura coupling reaction are, (i) the
catalyst is less expensive, (ii) air and moisture stable, (iii) the catalyst is
coated on cordierite monolith, which is easy to handle and easy to re-
cycle. Besides, the reaction was performed in ligand-free conditions, the
reaction proceeds smoothly in an aqueous medium, and the reaction is
compatible with heterocyclic molecules.
2
. Results and discussion
(
Methylthio)phenyl)boronic acid and (4-(tert-butyl)phenyl)boronic acid
The catalyst, Ti0.97Pd0.03
O
1.97, which is Pd2 ion substituted TiO
+
2
,
underwent a smooth Suzuki-Miyaura coupling with para-methox-
yiodobenzene forming the coupled products 3 j and 3k in 76 and 98 %
yields, respectively. These experiments reveal that the reactions of the
substrates with the electron-withdrawing or electron-donating groups
lead to the corresponding coupled products in good to excellent yields.
The reaction of aryl bromides with arylboronic acids were also facile
furnishing the corresponding biaryl derivatives in good to excellent
yields. A few examples have been presented in Table 2. The reaction of
2-bromonaphthalene with phenylboronic acid and 1-bromonaphtha-
lene with p-methoxy phenylboronic acid proceeded smoothly, fur-
nishing the corresponding biaryl derivatives 3 l and 3 m in 97 and 99 %
yields, respectively. The reaction of phenylboronic acid with para-
bromoacetophenone and meta-bromoprophiophenone proceeded
smoothly, affording the coupled products 3n and 3o in 85 and 85 %
yields, respectively. Similarly, the reaction of meta-methoxy-
bromobenzene with phenylboronic acid afforded the product 3p in 84
% yield. The reaction of bromobenzene with 3-cyanophenylboronic
acid furnished coupled product 3q in 64 % yield.
was synthesized based on earlier reports and confirmed by XRD, XPS
and EXAFS study [38]. XRD pattern and Rietveld refinement of the XRD
profile of freshly prepared catalyst is shown in Fig. 1. As confirmed by
the X-ray pattern, the catalyst Ti0.97Pd0.03O1.97 is a single-phase catalyst
[
38]. From the X-ray pattern, the average crystallite size of the catalyst
was found to be 8 ± 2 nm, which was calculated using the Scherrer
formula (0.95 λ/β cosθ), wherein β is FWHM of diffraction lines of the
X-ray pattern. The color of the catalyst is brown.
The catalyst was coated on a cordierite monolith by solution com-
bustion method [43]. Cordierite monolith is a ceramic material with a
cylindrical structure made up of many parallel channels of square
shape. The composition of cordierite monolith is Mg
Al
2 4
5
Si O18. Wash
coating of cordierite was performed to obtain coated cordierite with
γ–Al [43]. This operation has increased the surface area of cor-
dierite as γ–Al is one of the oxides with a high surface area. Then the
catalyst, Ti1-xPd 2-x, was coated over γ–Al to obtain a “ready to use
catalyst” coated over a cordierite monolith.
O
2 3
2 3
O
x
O
O
2 3
Having a pure catalyst in hand, we focused our attention on the
applicability of the catalyst for the Suzuki-Miyaura coupling reaction.
First, the screening studies were started to find optimal reaction con-
ditions for the reaction using 4–iodoanisole (1c, 0.4 mmol, 1 equiv.)
and phenylboronic acid (2a, 0.8 mmol, 2 equiv) as model substrates
For broadening the scope of the reaction, the reactions were carried
out with heterocyclic aromatic halides and heterocyclic aromatic
boronic acids. As can be seen, the reaction of phenylboronic acid with
6-bromoindole and 5-bromoindole proceeded well and furnished the
corresponding Suzuki-Miyaura coupled products 6 t and 6 u in 99 and
90 % yields, respectively. The reaction of 6-bromoindole with 3,4-me-
thylenedioxyphenylboronic acid was also facile, providing the coupled
product 6v in 98 % yield. The reactions of heterocyclic bromides, such
as 3-bromopyridine with phenylboronic acid, also proceeded well fur-
nishing the corresponding coupled products 6 w in 89 % yields. Our
attempt of using alkyl halides and alkylboronic acids for the Suzuki-
Miyaura coupling reactions using developed conditions were not suc-
cessful.
(
Table 1). The optimization studies were carried out using the powder
catalyst Ti0.97Pd0.03
O
1.97. The reaction of 1c with 2a using K
2
CO (2
3
equiv.) as a base, water (2 mL) as a solvent, and a powder catalyst
Ti0.97Pd0.03O1.97 (10 % by weight) at 100 °C for 15 h furnished the
corresponding coupled product 3c in 92 % yield (entry 1, Table 1). A
similar reaction at 100 °C for 6 h afforded the product 3c in 93 % yield
(
(
entry 2, Table 1). However, the reaction at 70 °C furnished 3c in 68 %
entry 3, Table 1). Although water is a preferred solvent, as it is non-
2