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N. Lu et al. / Tetrahedron Letters 49 (2008) 371–375
fluorinated solvents can thus be avoided, the homoge-
neous catalysis achieved, and the Pd-catalysts recovered
and reused.
Pd complex 2a was not used for the reactions under
the thermomorphoric mode because it is still slightly
soluble at room temperature. The Pd complexes 2b–c
were then selected as good candidates for the subsequent
catalytic experiments to examine the temperature
dependency of the reactions.
As shown in Scheme 1, the reaction of [PdCl2(CH3CN)2]
with three highly fluorinated bipyridine derivatives,
4,40-bis(RfCH2OCH2)-2,20-bpy 1a–c, resulted in the Pd
complexes,
[PdCl2(4,40-bis-(RfCH2OCH2)-2,20-bpy)]
As shown in Scheme 2, the Pd-catalyzed Heck arylation
of methyl acrylate/acrylic acid with aryl iodides was
selected to demonstrate the feasibility of recycling usage
with 2b–c as the catalyst using DMF as the solvent
under thermomorphic mode, at ca 140 °C for 3–7 h
varying in each run. At the end of each cycle, the prod-
uct mixtures were cooled to 25 °C and were centrifuged;
then catalysts were recovered by decantation. The recov-
ered 2b–c was added with DMF, substrates, and base to
proceed to the next cycle. The products were quantified
with GC analysis by comparison to internal standard
(NMP). The results of Heck reaction exhibited a 100%
selectivity favoring the trans product. As one example
shown in Table 1, the Heck reaction of C6H5I (3a) with
methyl acrylate (4a) could be catalyzed by 2b with good
recycling results for a total of eight times. To our
knowledge, this is the first example of Pd-catalyzed
Heck reaction under the thermomorphic mode.
2a–c, as pale yellow solids, where Rf = n-C9F19 (a),
n-C10F21 (b), n-C11F23 (c), respectively. The solubility
of Pd complexes 2a–c in DMF as a function of temper-
ature is shown in Figure 1. The solubility was measured
by the variable temperature NMR spectrometer, where
the temperatures were varied from ꢀ30 to 90 °C (also
see Supplementary data). We recorded the NMR
integration of the peak of the C5(50)-protons in
2,20-bpy and compared its value to that of the peak
integration of x-proton (from terminal HCF2 group)
of the standard, HCF2(CF2)3CH2OH. It was shown that
compound 2a was slightly soluble in DMF at 25 °C, but
2b and 2c were not soluble. The solubility of 2a in DMF
began from virtually zero at or below ꢀ10 °C, increased
slowly with the temperature from ꢀ10 to 60 °C and
increased remarkably at >60 °C. The solubility curves
of 2b and 2c, however, began from virtually zero below
40 °C and increased only at higher temperature, that is,
90 °C, as limited by the NMR spectrometer employed in
the variable temperature NMR experiments. Upon
changing from 25 °C to 90 °C there was ca sevenfold
increase of solubility for 2a and several orders of
magnitude increase of solubility for 2b–c. Therefore,
With electron releasing OMe and Me substitutents on
iodobenzene, excellent yields, and recyclabilities could
also be achieved for the Heck arylation on 4a as cata-
lyzed by 2b under the thermomorphic mode. The results
with 2c as the catalyst were very similar to those with 2b
as the catalyst, although the temperature of reactions
was ca 10 °C higher in the case of 2c as the catalyst.
2.5
Pd complex 2a
Pd complex 2b
Pd complex 2c
2.0
Table 1. Recycling results of 2b-catalyzed Heck reaction of 3a and 4a
1.5
1.0
0.5
0.0
-0.5
under thermomorphic mode
Cycle no.
Time
(h)
Temperature
(°C)
Yield
(%)
TON
TOF
1
2
3
4
5
6
7
8
3
6
6
6
6
7
7
7
140
140
140
140
140
140
140
140
100
100
100
100
100
100
100
100
1000
1000
1000
1000
1000
1000
1000
1000
333
167
167
167
167
143
143
143
-20
0
20
40
60
80
100
Temp (°C)
Reaction conditions: DMF (3 mL); 3a (2.5 mmol, 510 mg), 4a (5.0
mmol, 355 mg), NEt3 (5.0 mmol, 510 mg), 0.1 mol % 2b (3.64 mg).
Figure 1. Solubility of Pd complexes 2a–c as
temperature.
a function of
Thermomorphic mode
Catalyst 2b/2c
130 -160 oC
X
OY
O
X
I
HNEt3I
NEt3
OY
O
X
3a
3b
3c
Y
H
OMe
Me
4a Me
4b H
Scheme 2. The Pd-catalyzed Heck reaction under thermomorphic mode.