Asymmetric transfer hydrogenation of aryl ketoesters with a chiral double-chain surfactant-type catalyst in water

Article abstract of DOI:10.1039/c7gc01639e

A chiral double-chain surfactant-type ligand was designed and synthesized. The rhodium catalyst formed from the ligand can self-assemble into chiral vesicular aggregates in water, which was applied to the ATH of a broad range of aromatic ketoesters in neat water and gave up to 99% yield and 99% ee. In addition, this double-chain surfactant-type catalyst could also be applied to the dynamic kinetic resolution (DKR) of bicyclic β-ketoesters in water.

Full text of DOI:10.1039/c7gc01639e

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Asymmetric transfer hydrogenation of aryl ketoesters with chiral
double-chain surfactant-type catalyst in water
Jiahong Li,a Zechao Lin, c Qingfei Huang, c Qiwei Wang, c Lei Tang, d Jin Zhu, c and Jingen Deng,* bc
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A chiral double-chain surfactant-type ligand was designed and reaction medium formed by associating surfactant-type
22-25
The hydrophobic and
synthesized. The rhodium catalyst formed from the ligand can catalysts themselves in water.10,
self-assemble into chiral vesicular aggregates in water, which was hydrophilic region of the surfactant-type catalyst play an
applied to ATH of broad range of aromatic ketoesters in neat significant role in the formation of aggregates, which can be
water and gave up to 99% yield and 99% ee. In addition, this easily changed by modifying the hydrophobic chain or the
double-chain surfactant-type catalyst could also be applied to the polar head according to specific substrates. Sometimes a little
dynamic kinetic resolution (DKR) of bicyclic
β
-ketoesters in water.
change of hydrophobic or hydrophilic region of amphiphilic
catalyst may make
a great difference of its catalytic
performance, with the catalytic site unchanged.8, 11
Amphiphilic surfactants, commonly used as additives or
catalysts, can form aggregates in water as nano- or micro-
reactors, such as micelle, vesicle. These microreactors can not
only increase the solubility of nonpolar substrates and
catalysts, but also accelerate the reaction rates and
stereoselectivities due to the effect of aggregate formation,
such as the hydrophobic interaction and the preorganizational
function of micelles.1-5 Thus, the use of micellar surfactants as
Our Previous Works:
OH
O
O
O
sigle-chain ligand
L1
∗
or
R
R
X
O
n
n = 0, 1, 2.
Aliphatic Ketone or ketoester
This Work:
OH
O
n
O
O
n
∗
additives or catalysts is attracting considerable attention in
3-4, 6-7
among which
R
double-chain ligand L4
R
O
aqueous reactions in recent years,1,
O
R
R
n = 0, 1, 2, 3
n = 0, 1, 2, 3
surfactant-type catalysts attract our interesting, especially.
However, most works in chiral surfactants-type catalyst
systems with high enantioselectivity used single chain
Aromatic Ketone
Figure 1
In our previous work,8, 11 we have developed one kind of
chiral single-chain surfactant-type catalyst L1-Rh, which may
self-assembles to micelles in water to form aqueous-micelles
as reaction media. And different kinds of aliphatic ketones and
ketoesters have been reduced with high yield and
enantioselectivities (Figure 1) using this kind of surfactant-type
catalyst. However, it takes a long time to complete those
reactions. Considering the activity of surfactant-type catalysts
strongly depend not only on the structure of the catalyst itself
but also on the structure and morphology of the
nanoaggregate, we designed and synthesized a novel double-
chain ligand L4 derived from Noyori’s diamine ligand TsDPEN
and hoped to develop a catalytic system with a different state
of aggregation from our previous micelle system. Herein, we
report the results of a study of accelerating reaction and
increasing stereoselectivities with surfactant-type catalysts
bearing two polar hydrophilic heads and two chains as
hydrophobic tail in asymmetric transfer hydrogenation (ATH)
of aromatic ketones.
surfactant-type catalyst8-15 and there have been
a few
successful reports on using double-chain amphiphilic
surfactant as asymmetric catalyst16-17. Moreover, compared to
the single-chain catalyst, the double-chain catalyst geneally
aggregates to vesicles rather than micelles in water.18-21 The
excellent catalytic performance of surfactant-type catalysts for
specific reaction lies not only in their catalytic functional group,
but also in the structure and morphology of their aggregates as
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O
O
aggregates formed from the two surfactant-type catalysts,
respectively, are given. Actually, the one chain catalyst L1-Rh-H
can self-assemble into spherical nanoscale micelle particles, in
which favours the hydrophobic interaction between the chain
of catalyst and the alkyl chain of organic substrate (Figure 3, c
and d).11 However, the nano aggregate of double-chain
catalyst L4-Rh-H has the structure of branch chain or bilayer
and a hollow core (Figure 3, a and b), which is nearly ten times
of the diameter compared to that of the micelle of L1-Rh-H.
The bilayer formed from vesicular L4-Rh-H, may significantly
enlarge the interfacial area of catalyst layer. For the ATH in
aqueous medium, the hydrogen source HCOONa may solubilize
in the internal and external aqueous phases of the bilayer in
vesicular catalyst (Figure 2, b), and the substrates may be
incorporated into the hydrophobic interior of the bilayer (SI,
Figure S2). Moreover, the presence of lipophilic double chains
N
N
2I
N
N
2I
N
H
N
H
O
S
O
S
H2
N
HN
H2
N
HN
O
O
L2
L1
O
O
N
N
N
N
2I
2I
N
H
N
H
O
S
O
S
H2
N
HN
H2N
HN
O
O
L3
L4
Figure 2. Chiral Surfactant-type Diamine Ligands.
Firstly, we examined the catalytic activity of four kinds of
surfactant-type ligands L1 L2 L3 L4 (Figure 2) at the same
,
,
,
conditions (Table 1). We were impressed by the fact that the
ATH of acetophenone (acp) with double-chain ligand L4
afforded a conversion of 94% in 10 min at 40oC, which was
faster than single-chain ligands L1 and L2 (entry 4 vs 1-2). while
the ATH of aliphatic ketone 2, single-chain ligand L1 showed
the best results among the four ligands (entry 5 vs 6-8), with
80% conversion and 88% ee in 120 min.11 Ligand L2 and L3
only offered moderate results in the ATH of both aromatic
in L4-R4-H leads to formation of
a more hydrophobic
microenvironment in the vesicular bilayers, which can enhance
the solubility of organic substrates in water. Thus, it is not only
increase the concentration of substrates but also the rate of
reactions in the vesicle.
ketone
5) compared with L4 and L1. It's worth mentioning that the
conversion in the ATH of with L4 is 94% in 3h, when the S/C
ratio is as high as 1000 (entry ), which is one of the fastest
1 (entries 2, 3 vs 4) and aliphatic ketone 2 (entry 6, 7 vs
1
9
ligands for the ATH of acp in the Noyori’s diamine ATH
system.26-27
Table 1. The activities of different ligands in the ATH of
aromatic ketone and aliphatic ketones.
OH
∗
O
1 R1 = C6H5
[RhCl2Cp*]2 , Ligand
HCOONa, H2O, 20°C
2 R2 = (CH2)5CH3
R
R
T
( min)
T
conv.
b(%)
Ee
c(%)
entry ligand ketone
(oC )
40
40
40
40
20
20
20
20
40
1
2
L1
L2
L3
L4
L1
L2
L3
L4
L4
1
1
1
1
2
2
2
2
1
10
43
61
41
94
80
71
61
63
94
96
97
95
97
88
80
82
79
97
10
3
10
4
10
5
120
120
120
120
180
Figure 3. TEM images of nano-aggregates of catalyst in water.
6
7
After investigating the ATH of acp, we studied the effect of
hollow nano-vesicle on the ATH of aromatic ketoesters,
compared to what have been test with the micellar ligand L1 in
8
9d
a reaction conditions: 0.004 mmol of ligand, 0.002 mmol of
metal precursor, 5.0 mL of H2O, 2 mmol of HCOONa, 0.4
mmol of ketones, S/C = 100. b Conversion was determined
by GC analysis using decane as internal standard.
Enantiomeric excess was determined by GC analysis.d S/C =
1000.
our previous work. 8 We tested the aryl α, β, γ, δ ketoesters
, and , respectively. The ATH of these four substrates
3,
4,
5
6
(Figure 4) shows that the ligand L4 has the advantages of
higher reaction speed, higher yield than that of ligand L1, and
has comparative enantioselectivites with L1. Especially, ehyl β-
c
oxo-β-phenylpropanoate
4 was completely converted to the β-
hydroxy ester in 2 h with excellent ee value (97%).
We speculated that the noticeable difference of
experimental result between ligands L1 and L4 may own to the
fact that the difference of structure and morphology of
aggregates assembled from catalysts L1-Rh and L4-Rh in water,
respectively. Thus, we did TEM research of the aggregates of
Optical active β-hydroxyacids and their derivatives are
versatile chiral building blocks for many key structural
elements in pharmaceuticals and natural products.28-31 The
reaction conditions were optimized in detail for the ATH of
(SI, Table S1 S5). Three metal precursors, [RuCl2(p-cymene)]2,
interrelation between the structure and the morphology of [Cp*IrCl2]2 and [Cp*RhCl2]2, which are commonly used in the
4
-
L4-Rh-H in water. As shown in Figure 3,
a simplified
2 | J. Name., 2012, 00, 1-3
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Table 2. ATH of βꢀketoesters with double-chain surfactant-type catalyst.
ATH, were tested using L4 as a ligand and the Rh-complex
turned out to show the best reactivity and enantioselectivity,
o
with 84% yield and 96% ee (Table S4, entry 4) at 30 C. The
influence of temperature of the reaction was also investigated.
To our delight, the enantioselectivity of ATH of
4 exhibited
time yield.b
e.e.d
(%)
entry
1
substrate
d.r.c
-
only a slight decrease as the temperature increased (SI, Table
S5), 96% ee was obtained at 40 oC. It is noticeable that
unmodified Rh-TsDPEN catalyst only gave 80% ee at the same
conditions32 but similar results were obtained in emulsions33
and also in organic solvents34 with Ru-TsDPEN as a catalyst.
Furthermore, 94% ee was obtained even at 50oC, and the
reaction rate was significantly elevated to 99% conversion in
30min. For the micellar catalyst L1-Rh-H, at the temperature
above 40oC, the activity was sharply decreased and the
aggregates were disrupted.11 These results indicate that the
vesicular aggregates of the catalyst L4-Rh-H are remarkably
thermostable and can be used as a nanoreactor for the ATH, in
which there is a more hydrophobic microenvironment.
(h)
(%)
1.5
99
96
83
94
86
94
93
97
R
R
R
R
R
R
R
4
2
3
4
5
6
7
1.5
1.5
3
98
99
91
98
96
52
-
-
-
-
-
-
7b
7c
7d
1.5
3
7e
7f
O
O
O
O
O
O
3
4
(1) L4, 25 oC, 2 h, S/C =100 :
L4,
25 oC, 2h, S/C =100 :
99% yield, 97% ee;
6
(3)
7g
>99% yield, 73% ee;
8
8
(2) L1, 25 oC, 4 h, S/C =100 :
(4) L1, 25 oC, 2 h, S/C =100 :
>99% yield, 74% ee
80% yield, 96% ee
8
6
6
81
94
77
85
98
91
-
-
-
-
-
-
74
96
95
93
98
97
R
R
R
R
R
R
7h
7i
7j
O
O
O
O
9
O
O
6
5
10
11
12
13
6
(7) L4, 40 oC, 48 h, S/C =100 :
(5) L4, 25 oC, 48 h, S/C =50 :
10% yield, 92% ee;
(8) L1, 40 oC, 48 h, S/C =100 :
83% yield, 89% ee;
(6) L1, 25 oC, 48 h, S/C =50 :
8
8
6
trace, N.D.
50% yield, 89% ee
7k
7l
Figure 4. The ATH of aromatic ketoesters with ligand L4
and L1
1.5
3
.
With the optimal catalyst and conditions in hand, the β-aryl
β-ketoester substrate scope was investigated at 40oC. As
shown in Table 2, for most of the ketoester substrates,
regardless of bearing electron-donating or -withdrawing
substituents at the para- or meta-position, high yields and
good to excellent enantioselectivities were obtained (entries 1-
9). Though, the enantioselectivities of other 2-substituted
ketoesters 7d and 7h showed the lowest ee values 86% and 74%
respectively (entries 4 and 8). However, in Carreira’s catalyst
system,35 comparable enantioselectivities were observed in
longer reaction time with higher catalyst load (S/C = 50) at low
7m
98
14
24
30
95
51
99:1
97:3
1R,2R
7n
99
15
7o
1R,2R
a
reaction conditions: 0.004 mmol of ligand L4, 0.002
mmol of [Cp*RhCl2]2 , 5 mL of H2O, HCOONa (2 mmol), 0.4
mmol of ketoesters 40 oC, S/C=100. isolated yield.
b
c
Diastereomeric ratio was determined by 1HNMR analysis. d
Enantiomeric excess was determined by HPLC analysis.
o
temperature (4 C). To our delight, the ATH of heteroaromatic
ketoesters 7l and 7m shows the best result, with 98% and 97%
ee respectively (entries 12 and 13). Noticeably, the product of
7m has also been used in the enantioselective synthesis of
Moreover, the catalyst Rh-L4-H has also realized the
dynamic kinetic resolution (DKR) of bicyclic β-ketoesters 7n
and 7o in water.36-37 The ATH of bicyclic β-ketoester 7n
afforded the product 9n with 95% yield, 98% ee and 99:1 of dr
(Table 2, entry 14), and the product 9o provided a result with
99% ee and 97:3 of dr (entry 15). The absolute configurations
of the products 9n and 9o were (1R, 2R) speculated by the 2D-
nuclear overhauser effect spectroscopy (NOESY).
duloxetine which is
a
dual inhibitor of serotonin,
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thermostability and more hydrophobic environment, which
may contribute to its higher activity. In addition, the Rh-L4
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Asymmetric transfer hydrogenation of aryl ketoesters with chiral double-chain surfactant-type catalyst
in water
1