92
X. Liu et al. / Catalysis Communications 67 (2015) 90–94
Fig. 2. Time profile of the reduction of acetophenone with different amounts of azeotrope.
Fig. 1. UV–vis spectra of a) the [PEG-4000-C8MIM][TsDPENDS] in water; b) the
{(p-cymene)RuCl2}2 in water; and c) the [PEG-4000-C8MIM][Ru-TsDPENDS] in water.
color in ethyl acetate (Fig. 3b), and catalyst can catalyze smoothly and
efficiently the reduction of acetophenone to the chiral 1-phenylethanol.
As the reaction completed and temperature decreased, the catalyst was
precipitated completely from the reaction system as solid and the ethyl
acetate phase reverts to colorless (Fig. 3c). Therefore, this catalysis system
showed the properties of thermoregulated phase-separation, which can
offer an effective and convenient route to isolate the product and improve
the recyclability of the catalyst.
Sequentially, the effects of the addition of HCOOH-NEt3 azeotropic
mixture on the reaction activity and phase separation behavior were
examined over [PEG-4000-C8MIM] [Ru-TsDPENDS] catalyst (Fig. 2).
When the volume of azeotrope increased from 0.1 mL to 0.2 mL, the
final conversion increased from 73% to 92%. However, when azeotrope
increased to 0.3 mL, the final conversion reduced to 86%. By continuing
the increase of the amount of azeotrope, the final conversion declined to
66%. The results indicated that more addition of the azeotrope was not
favorable for the AHT reaction. With excessive formic acid, the reaction
proceeded very slowly, probably because the protonation of the amino-
group of the diamine ligand followed by its decoordination from the Ru
atom [22,23]. Acid–base titration analysis indicated that a small amount
of formic acid still remained after the reaction when the addition of the
azeotrope was more than 0.2 mL. Besides, adding too much azeotrope
(more than 0.2 mL in this case) led to the poor thermoregulated separa-
tion due to the good miscibility of [PEG-4000-C8MIM][Ru-TsDPENDS]
with the residual formic acid.
The thermoregulated property of different catalysts [PEG-m-
CnMIM][Ru-TsDPENDS] in ethyl acetate was first screened, the transfer
hydrogenation of acetophenone was chosen as the model reaction
(Table 1). At first, three ionic liquid-regulating ruthenium complexes
([PEG-2000-CnMIM][Ru-TsDPENDS], n = 1, 4 and 8, respectively)
have been prepared. Unfortunately, these catalysts were all partially sol-
uble in ethyl acetate at room temperature and cannot be precipitated
from ethyl acetate even at 0 °C (Table 1, entries 1–3). However, when
the chain length of PEG-2000 was replaced by that of PEG-4000, it
was found that three ruthenium complexes ([PEG-4000-CnMIM][Ru-
TsDPENDS], n = 1, 4 and 8, respectively) exhibited excellent
temperature-dependent behavior in ethyl acetate (Table 1, entries 4–
6). Furthermore, the ionic liquid [PEG-4000-C8MIM][Ru-TsDPENDS]
with long alkyl chain (n = 8) proved to offer a better solubility in
ethyl acetate during the reaction conditions and exhibited a higher con-
version, in comparison with [PEG-4000-C1MIM][Ru-TsDPENDS]
(Table 1, entry 4 vs 6). An explanation is that the stronger hydrophobic
interaction of the ionic liquid and substrate results in more accessibility
of the substrate to catalytically active anion center. However, if the ionic
liquid with longer alkyl chain (n = 12) was used, it resulted in a poor
thermoregulated separation behavior. Especially, the absence of the
ionic liquid cation could not promote the hydrogenation reaction due
to the very poor solubility of sulfonated diamine ligand in ethyl acetate
(Table 1, entry 7).
Next, the thermoregulated behavior of [PEG-4000-C8MIM][Ru-
TsDPENDS] was examined in the ATH of acetophenone. Before the reac-
tion, the catalyst sank to the bottom of the flask (Fig. 3a). After adding
azeotrope, acetophenone and increasing temperature to 40 °C, the
resulting mixture became a homogeneous phase with a reddish brown
3.3. Substrate generality and reusability of the catalyst
Under the optimal conditions, we subsequently examined the
substrate scope with a wide range of ketones. A variety of aromatic
ketones were smoothly hydrogenated with high reactivities and
enantioselectivities using a thermoregulated catalyst [PEG-4000-
C8MIM][Ru-TsDPENDS]. The reaction kinetic curves of the reactive sub-
strates like acetophenone and p-bromoacetophenone, and less reactive
substrate p-methyl-acetophenone were shown in Fig. 16S, indicating
that the aromatic ketones with electron withdrawing group gave higher
reactivity (Table 2, entries 1–4), while the aromatic ketones with elec-
tron donating group lowered reactivity (Table 2, entries 8, 9), which
was in agreement with the previous report [24]. It was worth noting
that the substituted groups had an obvious influence on the reaction
rate, but not on the enantioselectivity (Table 2, entries 2–4, 8, 9).
In the hydrogenation of the heteroatom aromatic ketones (Table 2,
entries 5 and 6), chiral alcohol was obtained in high conversion and
enantioselectivity. Good conversion and enantioselectivity were also
observed in the reduction of 2-acetonaphthone (Table 2, entries 7).
Subsequently, the recyclability of the catalyst was examined by
choosing the acetophenone as a model substrate in ethyl acetate. The
Table 1
The catalytic activity, enantioselectivity and thermoregulated performance of different
catalysts in ATH of acetophenone.
Entries Catalysts
Con.%a Ee.%b Thermoregulated
phase separation
1
2
3
4
5
6
7
[PEG-2000-C1MIM][Ru-TsDPENDS] 67.8
95
96
96
94
96
95
51
No
No
No
Yes
Yes
Yes
No
[PEG-2000-C4MIM][Ru-TsDPENDS] 77.6
[PEG-2000-C8MIM][Ru-TsDPENDS] 99.5
[PEG-4000-C1MIM][Ru-TsDPENDS] 64.5
[PEG-4000-C4MIM][Ru-TsDPENDS] 94.8
[PEG-4000-C8MIM][Ru-TsDPENDS] 97.5
[Ru-TsDPENDS]
25.9
a
Reactions were carried out at 40 °C using 0.5 mmol of acetophenone and 0.005 mmol
Ru(II) complex in 1 mL ethyl acetate and 0.2 mL HCOOH-NEt3 azeotropic mixture for 10 h,
4 equivalents of HCOOH as hydrogen donor.
b
GC analysis was performed with a β-DEX™120 capillary column (30 m × 0.25 mm,
0.25 μm film) and dodecane as internal standard.