J. Xu et al. / Tetrahedron 71 (2015) 1712e1717
1713
Table 1
in IL medium has emerged as a practical and greener strategy for
Allylearyl coupling of allylic carbonate 1a with phenylboronic acid (2a) in an ILa
the amelioration of traditional cross-coupling reaction.16,17
Based on our continuous effort to the development of practical
and greener allylearyl coupling reactions, we envisioned that the
allylearyl coupling reaction of allylic carbonates with arylboronic
acids could perform in
a biphasic system of 1-butyl-3-
methylimidazolium hexafluorophosphonate ([bmim]PF6) and wa-
ter. We reasonedthatPdNPs, whicharegeneratedin situfromhomo-
coupling of arylboronic acids with palladium acetate in the reaction
system, could be stabilized and immobilized in IL phase. After the
reaction complete, the coupling product could be extracted with
organic solvents such as ether and the water-soluble byproducts of
boronates could be separated in water phase, and then the catalyst
recycling would be feasible (Scheme 1). Herein we report PdNPs-
catalyzed allylearyl coupling reaction of allylic carbonates with
arylboronic acids in ILs. The PdNPs can be successfully immobilized
in the IL layer, and reveal high catalytic activity for the allylearyl
coupling reaction. In addition, the PdNPs can be reused three times
without any loss of catalytic activity and stereospecificity.
Entry
Solvent
Conversion (%)b
1
2
3
4
5
6
7
8
THF
1,4-Dioxane
Ether
Toluene
MeOH
16
12
31
22
50
47
>99
>99
iPrOH
H2O
[bmim]PF6
[bmim]SbF6
[bmim]NTf2
[bmim]OTf
[bmim]BF4
[bmim]PF6/H2O
[bmim]PF6/ether
[bmim]PF6/toluene
[bmim]PF6/iPrOH
[bmim]PF6/H2O
[bmim]PF6/H2O
[bmim]PF6/H2O
9
>99
>99
38
10
11
12
13
14
15
16
17c
18d
19e,f
Complex mixture
>99
>99
>99
>99
>99
95
>99
a
Reaction conditions: Pd(OAc)2 (1 mol %), 1a (0.4 mmol), 2a (0.6 mmol), solvent
(0.4 mL of IL or IL/other solvent¼0.4 mL/0.4 mL), 25 ꢀC, 4 h.
b
Determined by 1H NMR of the crude reaction mixture.
Catalyst recycling of entry 13.
Catalyst recycling of entry 17.
Catalyst recycling of entry 18.
c
d
e
f
The reaction carried out for 18 h.
catalytic activity was decreased, but the reaction completed when
the reaction time prolonged to 18 h (entry 19).
As our expected, we found that the reaction system turned black
after few minutes no matter pure ILs or biphasic system. And for the
reaction in [bmim]PF6/water, water phase become clear after stop
the stirring, which means the catalyst immobilized efficiently in the
IL phase. In order to recognize the real catalytic species, the
immobilized catalyst was analyzed by the transmission electron
microscopy (TEM). The first sample made from after first run of
coupling reaction in [bmim]PF6/water biphasic system (entry 13).
As illustrated in Fig. 1, the TME analysis indicated that PdNPs were
generated and dispersed well with an average particle sized of
3.2 nm (Fig. 1a). The PdNPs was also generated in situ in neat
[bmim]SbF6 with an average particle sized of 2.8 and slightly broad
size distribution (Fig. 1b). After fourth run of the catalyst recycling,
the PdNPs still dispersed well in the IL phase (Fig. 1c). This result
indicated that lowered catalytic activity of the fourth cycle was not
resulted from the catalyst aggregation. Next, the leaching amount
Scheme 1. Synthetic strategy of PdNPs-catalyzed allylearyl coupling reaction in ionic
liquids.
2. Results and discussion
Initial investigations focused on examining the allylearyl cou-
pling reaction of allylic carbonate 1a with phenylboronic acid (2a)
as standard reaction partners in different solvents (Table 1). For
comparison, we firstly conducted the reaction in different organic
solvents. As our previous report,13b the reaction was highly effective
in water to afford the coupling product in full conversion (entry 7).
However, the reaction was less effective in organic solvents (entries
1e6). With these results in hand, we next examined the coupling
reaction in pure 1-butyl-3-methylimidazolium (bmim) based ILs
with different counter anions. To our delight, the coupling reaction
of 1a with 2a in the presence of 1 mol % of Pd(OAc)2 in [bmim]PF6
proceeded smoothly to afford allylearyl coupling product 3a in full
of the PdNPs was determined by ICP-AES analysis. 45.2
18.3 g of Pd were leached out to the ether layer and water phase,
respectively, during the phase-separation of the first run. After
second cycle, 13.8 g (ether layer) and 4.7 g (water phase) of Pd
were leached out. And after third cycle, 9.3 g (ether layer) and
1.1 g (water phase) of Pd were leached out. During the product
extraction of the fourth cycle, 23.7 g (ether layer) and 5.7
mg and
m
conversion with complete regio- and E/Z-selectivities, and no b-H
elimination product was observed (entry 8). The reaction per-
formed well in other ILs, such as [bmim]SbF6 (entry 9) and [bmim]
NTf2 (entry 10). However, low conversion was observed when the
reaction carried out in [bmim]OTf (entry 11), and complex mixture
was obtained in [bmim]BF4 (entry 12). As our expected, The re-
actions were tolerated in the biphasic system of [bmim]PF6 and
water to furnish the coupling product 3a in full conversion with
excellent selectivities (entry 13).18 The reaction was also highly
effective in biphasic system of [bmim]PF6 and insoluble organic
solvents, such as ether, toluene and isopropanol (entries 14e16)
Next, we tried catalyst recycling in biphasic system of [bmim]PF6
and water. As expected, the palladium catalyst was efficiently
immobilized in the IL, which can be reused three times without any
loss of catalytic efficiency (entries 17 and 18). In the fourth run, the
m
m
m
m
m
mg
(water phase) of Pd were leached out. These results clearly implied
that catalyst leaching is a reason for the decreased catalytic activity
for the catalyst recycling.
With optimized reaction conditions in hand, the generality of the
allylearyl coupling reaction was evaluated with a variety of allylic
carbonates 1 and arylboronic acids 2 in [bmim]SbF6 and [bmim]PF6/
water biphasic system (Table 2). Firstly, we examined the coupling
reaction of 1a with arylboronic acids having different electronic and
steric properties. Thereactionperformed well witharylboronicacids
with electron-donating groups to afford corresponding coupling