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M. Navidi et al. / Journal of Organometallic Chemistry 743 (2013) 63e69
functionalization [57e59]. Schiff bases, which are an important
class of ligands with extensive applications in different fields [60],
also showed excellent catalytic activity when grafted on CNTs
[61,62].
We have already reported the results obtained for the solvent-
free synthesis of ynones using the air/moisture stable and reus-
able multi-walled carbon nanotubes functionalized with a Pd(II)e
Schiff base complex (PdeSchiff base@MWCNTs, Fig.1) [63]. In order
to further establish other heterogeneous palladium-catalyzed CeC
cross-coupling reactions with our catalyst, we herein report the
application of this catalyst in SuzukieMiyaura and Sonogashirae
Hagihara coupling reactions under aerial condition.
60 ꢀC under air atmosphere. After 2e7 h, the reactions were
completed (TLC). The mixture was cooled to room temperature,
Et2O (10 mL) and H2O (5 mL) were added, and filtered. The organic
phase was separated and dried over MgSO4. The solvent was
evaporated and the residual was purified by preparative TLC on
silica gel plates eluting with n-hexane/EtOAc ¼ 9:1. All the products
were known compounds and were identified by comparison of
their physical and spectroscopic data with those of authentic
samples.
2.4. Recycling of the catalyst in SuzukieMiyaura reaction
Recycling of the catalyst was performed upon the reaction of 4-
bromoanisole with phenylboronic acid under the condition dis-
cussed in the preceding section. After completion of the reaction in
the first run, the catalyst was separated from the reaction mixture
by filtration, washed with H2O (5 mL) and Et2O (10 mL) and dried in
vacuum at 70 ꢀC for 24 h. The resulting solid mass was reused for
another batch of the similar reaction. This process was repeated for
four runs.
2. Experimental
2.1. General
NMR Spectra were recorded with either a Bruker DRX-400 or
AQS-300 spectrometer with nominal frequencies of 400 and
300 MHz for proton or 100 and 75 MHz for carbon, respectively in
CDCl3 solutions. IR spectra were obtained using an ABB FTLA 2000
instrument. All reagents and solvents were commercially available
and used without any further purification.
2.5. General procedure for the SonogashiraeHagihara reactions
A mixture of aryl iodide (1.0 mmol), terminal alkyne (1.5 mmol),
Et3N (2.0 mmol), H2O (2 mL), and the catalyst (0.012 mmol,1.2 mol%
Pd) was stirred at 90 ꢀC under aerial conditions. The progress of the
reaction was monitored by TLC. After completion, H2O was evap-
orated, and CHCl3 (10 mL) was added to the reaction mixture, and
the catalyst was recovered with centrifugation. The organic layer
was washed with H2O (2 ꢁ 5 mL), dried over anhydrous MgSO4,
filtered and concentrated in vacuum. The crude product was further
purified by preparative TLC (silica gel) using n-hexane as eluent to
afford the desired product. All the products were characterized by
IR, 1H, and 13C NMR spectroscopy.
2.2. Preparation of PdeSchiff base@MWCNTs
The synthesis of the PdeSchiff base@MWCNTs catalyst was
conducted according to the procedure previously reported [63]. The
purchased CO2H-MWCNTs (100 mg) were suspended in a solution
of thionyl chloride (25 mL) and DMF (1 mL). The suspension was
stirred at 65 ꢀC for 24 h. The solid was then separated by filtration
and washed with anhydrous THF (30 mL), and dried in vacuum to
obtain COCl-MWCNTs. COCl-MWCNTs (50 mg) were added to a
solution of the previously prepared Pd(II)-Schiff base (100 mg) in
degassed CHCl3 (8 mL), and the suspension was refluxed for 20 h
under N2 atmosphere. The solid was then filtered and washed with
THF (3 ꢁ 10 mL) and CH2Cl2 (2 ꢁ 10 mL) and dried in vacuum to
afford the desired catalyst.
The catalyst was also characterized by attenuated total reflec-
tion infrared spectroscopy (ATR), Raman spectroscopy, ICP, XRD,
XPS, TEM, and TG-DTA data. The ATR spectrum of the catalyst
showed a band at 1708 cmꢂ1 associated with C]O stretch of the
ester linkage between the carbon nanotube and the Schiff base
complex. Moreover, the metal content of the complex was found to
be 16.20 ppm using ICP, and the ratio of Pd/N in the catalyst was
obtained to be 1.82%.
2.6. Selected spectral data of the compounds
2.6.1. Compound 2b
1H NMR (CDCl3, 300 MHz):
d
(ppm): 7.28e7.48 (m, 9H), 2.32
(s, 3H); 13C NMR (CDCl3, 75 MHz):
(ppm): 141.99, 141.96, 135.4,
130.3, 129.8, 129.2, 128.1, 127.3, 126.8, 125.8, 20.5; 3b: 1H NMR
(CDCl3, 300 MHz):
(ppm): 7.54e7.59 (m, 4H), 7.44 (t, J ¼ 7.3 Hz,
2H), 7.32 (t, J ¼ 7.3 Hz, 1H), 7.00 (d, J ¼ 8.6 Hz, 2H), 3.87 (s, 3H);
13C NMR (75 MHz, CDCl3):
(ppm): 159.2, 140.8, 133.8, 128.7,
128.2, 126.8, 126.7, 114.2, 55.3; 7b: 1H NMR (CDCl3, 300 MHz):
d
d
d
d
(ppm): 10.07 (s, 1H), 7.96 (d, J ¼ 8.0 Hz, 2H), 7.76 (d, J ¼ 8.2 Hz,
2H), 7.65 (d, J ¼ 8.0 Hz, 2H), 7.41e7.52 (m, 3H); 13C NMR (CDCl3,
75 MHz):
d (ppm): 192.1, 147.2, 139.7, 135.2, 130.3, 129.1, 128.5,
127.7, 127.4; 15b: 1H NMR (CDCl3, 300 MHz):
d (ppm): 8.01 (d,
2.3. General procedure for the SuzukieMiyaura reactions
J ¼ 8.1 Hz, 2H), 7.64 (d, J ¼ 8.1 Hz, 2H), 7.58 (d, J ¼ 8.5 Hz, 2H),
7.00 (d, J ¼ 8.5 Hz, 2H), 3.86 (s, 3H), 2.63 (s, 3H); 13C NMR (CDCl3,
An RB flask was charged with aryl halide (1.0 mmol), arylboronic
acid (1.5 mmol), K2CO3 (2 mmol), DMF/H2O (v/v ¼ 1:1, 4 mL) and
catalyst (0.1 mol% Pd); the mixture was magnetically stirred at
75 MHz):
d (ppm): 197.7, 159.9, 145.3, 135.3, 132.2, 129.0, 128.4,
126.6, 114.4, 55.4, 26.6; 2c: 1H NMR (CDCl3, 300 MHz):
d (ppm):
7.57 (d, J ¼ 3.9 Hz, 1H), 7.55 (d, J ¼ 2.1 Hz, 1H), 7.36e7.39 (m, 3H),
7.31e7.32 (m, 2H), 7.04 (dd, J ¼ 5.0, 3.8 Hz, 1H); 13C NMR (CDCl3,
75 MHz):
d (ppm): 132.0, 131.5, 128.5, 128.4, 127.3, 127.2, 123.3,
O
O
123.0, 93.1, 82.7; 6c: 1H NMR (CDCl3, 300 MHz):
d (ppm): 7.53
(dd, J ¼ 7.4, 2.4 Hz, 2H), 7.49 (d, J ¼ 8.7 Hz, 2H), 7.30e7.38 (m,
3H), 6.89 (d, J ¼ 8.7 Hz, 2H), 3.84 (s, 3H); 13C NMR (CDCl3,
75 MHz):
d (ppm): 159.6, 133.1, 131.5, 128.3, 128.0, 123.6, 115.4,
N
N
114.0, 89.4, 88.1, 55.3; 8c: 1H NMR (CDCl3, 300 MHz):
d (ppm):
7.94 (d, J ¼ 8.3 Hz, 2H), 7.61 (d, J ¼ 8.3 Hz, 2H), 7.55e7.58 (m, 2H),
Pd
7.36e7.39 (m, 3H),2.61 (s, 3H); 13C NMR (CDCl3, 75 MHz):
O
O
d
(ppm): 197.3, 136.2, 131.8, 131.7, 128.8, 128.5, 128.3, 128.2, 122.7,
Fig. 1. Heterogeneous PdeSchiff base@MWCNTs catalyst.
92.7, 88.6, 26.6.