Y. Masuyama et al. / Journal of Molecular Catalysis A: Chemical 352 (2012) 81–85
85
Table 2
Reusability of PdHAP observed in 10 repetitive uses.
Repetitive use
1st
2nd
3rd
4th
5th
6th
7th
8th
9th
10th
3a, yield (%)a
3b, yield (%)a
83
79
82
77
80
76
77
73
74
75
77
64
79
65
79
61
76
53
74
52
a
Isolated yields after allowing to react at 50 ◦C for 24 h.
The reusability of PdHAP for the Suzuki–Miyaura-type cross-
coupling reaction of either 1a or 4-bromoanisole (1b) with 2 was
investigated under the same conditions as those of entry 4 in Table 1
(Table 2). The catalytic activity of PdHAP for the reaction of 1a was
maintained in ten repetitive uses, while that for the reaction of 1b
declined gradually.
catalyst. Palladium(II) introduced by the ion-exchange of cal-
cium(II) into an insoluble inorganic matrix HAP functioned as
a catalyst for the Suzuki–Miyaura-type cross-coupling reaction
of aryl bromides with potassium aryltrifluoroborates at 50 ◦C in
methanol under air. The heterogeneous palladium catalyst (PdHAP)
was used ten times repeatedly, though its catalytic activity grad-
ually declined. Water can also be used as a solvent, though the
reaction rate in water is slower than that in methanol. XRD and XRF
analyses and TEM images revealed that most of palladium species
would still be supported as isolated ions on the repeatedly used
PdHAP, in spite of the changes of the structure and composition
of PdHAP surface at the late stages in the repetitive uses and the
partial leaching of Pd species at the initial stages in the repetitive
uses.
The participation of Pd species leaching from PdHAP was inves-
tigated during the repetitive uses as described below. To the
suspension of fresh PdHAP or used PdHAP (after being used once or
three times) in MeOH (1 mL) was added PPh3 (0.01 mmol). After the
suspension was shaken at 50 ◦C for 2 h under nitrogen atmosphere,
the methanol solution was separated from the PdHAPs by filtration.
The cross-coupling reaction of 1a (1.3 mmol) and 2 (1.0 mmol) in
each of the methanol filtrates including another PPh3 (0.04 mmol)
was carried out with K2CO3 (3.0 mmol) at 50 ◦C for 24 h to pro-
duce 3a in a 20% yield (fresh PdHAP), a 5% yield (PdHAP after being
used once), or a trace (PdHAP after being used either three times or
five times). The results suggested that palladium species would be
leached from the PdHAPs at the initial stages in the repetitive uses,
whereas little leaching would occur at the late stages in the repeti-
tive uses. Thus, apart from the reaction with fresh PdHAP, palladium
species on the HAP matrix must catalyze the Suzuki–Miyaura-type
cross-coupling reaction. This PdHAP-catalyzed Suzuki–Miyaura-
type cross-coupling reaction would proceed similarly to usual
Suzuki–Miyaura cross-coupling reactions, after the reduction of
Pd(II) species on the HAP matrix to Pd(0) species with PPh3 or 2 [1].
The variation of palladium in repeatedly used PdHAP was ana-
lyzed by X-ray fluorescence (XRF) spectrometer. The P/Ca molar
ratio of repeatedly used PdHAP (after being used three times or five
times) was 0.19 or 0.14 respectively, and was quite different from
ratios, a Pd/Ca molar ratio (0.0048 after being used three times or
0.0043 after being used five times) of repeatedly used PdHAPs did
not differ much from that of fresh PdHAP (0.0049). The powder
X-ray diffraction (XRD) spectra of calcined HAP, fresh PdHAP and
used PdHAPs are shown in Fig. 1. The XRD peaks for fresh PdHAP
were the same as those of calcined HAP. After the material had been
repeatedly used, new peaks of 2ꢀ = 28◦, 47◦, 56◦, 69◦, 77◦ and 88◦
appeared along with the loss of the structural properties of HAP sur-
faces. Since the new peaks were the same as those of CaF2, most of
the surface of PdHAP at the late stages in the repetitive uses would
be covered with CaF2. Thus, the repeatedly used PdHAPs were found
to keep most of the palladium species on HAP matrices, though the
structure and composition of HAP moiety in the original PdHAP had
changed with repeated uses.
Acknowledgements
This study was supported by the Cooperative Research Program
of Catalysis Research Center, Hokkaido University (Grant # 2010-
B-24).
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We developed a C–C bond-forming reaction via the formation
of organometallic intermediates with a reusable heterogeneous