T. Baran et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 47–52
51
Table 2
Recycling of OCMCS-3aPd and OCMCS-4aPd.
Yield (%)
The number of cycle
OCMCS-3aPd
OCMCS-4aPd
1
2
3
4
5
6
7
8
9
1
st
97
98
96
94
92
91
86
82
79
73
98
98
95
94
91
90
89
87
80
75
nd
rd
th
th
th
th
th
th
0th
Reaction conditions: 1.12 mmol of 4-bromoanisole, 1.87 mmol of phenyl boronic
◦
acid, 3.75 mmol of K2CO3, 0.04 mol% catalyst, 6 mL of toluene, 100 C, 48 h.
higher yields for aryl bromide in the C–C coupling reactions of chi-
tosan Schiff bases supported palladium catalyst. On the other hand,
lower yields were recorded for the reactions with aryl chlorides
(entries 9 and 10). This can be attributed to the poor reactivity of
aryl chlorides [34]. Furthermore, the catalysts were more efficient
in the reactions of compounds with electron withdrawing groups
(
entries 6 and 7). Turnover numbers (TON) were calculated [35] for
1
all the products and listed in Table 1. H NMR and GC–MS spectra of
synthesis of the biaryl compounds are given in the Supplementary
section (Fig. S4-S17).
3.8. Reusability test of catalyst
The reusability of the catalysts was tested and the findings are
given in Table 2. Upon achieving C–C coupling reactions, the cat-
alysts were removed from the reaction media with filtration and
the filtrate was rinsed with water and hot methanol to ensure
the catalytic activity of the catalysts. The recycling reactions were
carried out under the optimum conditions; it was observed that
the catalyst retained their activity even after ten run. After 10th
run, we observed that yield decrease to 75% (OCMCS-4aPd) and
7
3% (OCMCS-3aPd). Also at the end of the 10th run, hot filtrate
was analyzed with ICP-OES and very low palladium leaching was
observed (<1%). The leaching test indicated that the catalysts have
heterogeneous nature.
3.9. Mercury poisoning tests
It is already known that if addition of mercury stops the catalytic
activity, then the reaction mechanism is said to be heterogeneous. If
not, it is concluded that the reaction mechanism has a homogenous
nature [36–38]. The mercury poisoning tests showed that the reac-
tion mechanism follows a heterogeneous pathway (Table 1, entry
Fig. 6. XRD patterns of OCMCS-3aPd (a), OCMCS-4aPd (b).
1
1).
conditions (see Fig. S1-S3 and Table S1, S2 in Supplementary data),
the reaction of 4-bromoanisole with phenyl boronic acid was pre-
ferred as a model reaction. The studies revealed the optimum
4. Conclusion
conditions in the presence of the catalysts; amount of catalyst:
◦
0
.04% mol, temperature: 100 C, base system: K CO and reaction
In this study, two new O-carboxymethyl chitosan Schiff base
supported heterogeneous Pd(II) catalysts were synthesized. The
mode of bonding (FTIR), thermal stability (TG/DTG), surface mor-
phology (SEM/EDAX), crystallinity index (XRD) and metal content
(ICP-OES) of the catalysts were investigated. Moreover, to define
the geometry of the catalysts, UV–vis, magnetic moment and molar
conductivity measurements were performed. The catalytic activity
was tested for the first time in the synthesis of biaryls and the prod-
2
3
time: 48 h (see Fig. S1-S3 and Table S1, S2 in Supplementary data).
Under the optimum conditions, the catalytic activity was tested
in Suzuki coupling reaction of aryl halides with phenyl boronic acid.
These findings are presented in Table 1. An evaluation of the Suzuki
coupling reactions demonstrated that the reactions with aryl bro-
mide (entries 1–7) had a high yield with the exception of the entry
3
. Also, when aryl iodides were compared with aryl chlorides, it was
1
observed that the aryl iodides had higher yields (entry 8). The dif-
ferentiation observed in the reaction yield can be accounted for by
the bond dissociation energies and the size of the halide atoms [14].
In an earlier study by Makhubela et al. (2011), the authors reported
ucts were characterized with GC–MS and H NMR. The selectivity of
the catalysts was high in Suzuki coupling reactions. There reusabil-
ity studies showed that the catalysts retained their activity after ten
run. Leaching studies observed that very low leaching of Pd (<%1)