Enamine-metal lewis acid bifunctional catalysis: Application to direct asymmetrie aldol reaction of ketones

Article abstract of DOI:10.1002/ejoc.200900678

Unprecedented bifunctional enamine-metal Lewis acid, catalysts have been developed. In this bifunctional catalytic system, a tridentate ligand tethered with a chiral secondary amine was designed to solve the acid-base self-quenching problem leading to cat

Full text of DOI:10.1002/ejoc.200900678

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200900678
Enamine–Metal Lewis Acid Bifunctional Catalysis: Application to Direct
Asymmetric Aldol Reaction of Ketones
Zhenghu Xu,[a] Philias Daka,[a] Ilya Budik,[a] Hong Wang,*[a] Fu-Quan Bai,[b] and
Hong-Xing Zhang[b]
Keywords: Organocatalysis / Aldol reactions / Lewis acids / Lewis bases / Amino acids / Tridendate ligands
Unprecedented bifunctional enamine–metal Lewis acid cata- lyze aldol reactions of ketones efficiently in high yields and
lysts have been developed. In this bifunctional catalytic sys-
tem, a tridentate ligand tethered with a chiral secondary
amine was designed to solve the acid–base self-quenching
problem leading to catalyst inactivation. This new bifunc-
tional enamine–metal Lewis acid catalyst was found to cata-
good to excellent diastereoselectivities and enantioselectivi-
ties.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
have been well defined. The major challenge lies in the
acid–base self-quenching reaction leading to catalyst inacti-
vation. The general approach to solve this problem is to
finetune the reaction conditions and catalysts, such as using
a “hard” Lewis acid and a “soft” Lewis base. Several exam-
ples of this sort have been reported,[7] in which a proline/
pyrrolidine molecule was mixed with a transition metal to
establish the bi/multifunctional catalytic system. We are
using a different strategy to solve this problem. In our ap-
proach, the Lewis acid and Lewis base are incorporated
into one molecule. To achieve this, we use a tridentate li-
gand (Figure 1, 1, 2, and 3) tethered with a chiral secondary
amine, for example, -proline, in combination with transi-
tion metals. We envision that the tridentate ligand will
“trap” the incoming metal preventing the coordination of
the secondary amine to the metal, so that the bifunctional
system is built up within one molecule. The metal intro-
duced into the chiral tridentate scaffold will act as a Lewis
Introduction
Since the rebirth of organic catalysis in 2000,[1] asymmet-
ric organocatalysis has been growing at a breathtaking pace.
A large number of asymmetric carbon–carbon and carbon–
heteroatom bond-forming reactions, such as the aldol reac-
tion, Mannich reaction, Michael reaction, and even compli-
cated cascade reactions have been achieved with unprece-
dented ease.[2] Proline has been playing a major role in en-
amine-based catalysis.[1–4] Mechanistic studies suggest that
both the pyrrolidine ring and the carboxylic acid group are
essential to the reaction. In this view proline can be re-
garded as a Lewis base/Brønsted acid “bifunctional cata-
lyst”.[5] In the past several years, major efforts on modifying
proline catalysts have been focused on modification of the
carboxylic acid group by introducing electronic and/or ste-
ric factors to optimize the interaction between the Brønsted
acid and the aldol acceptor through hydrogen bonding. We
want to “replace” the Brønsted acid with a metal Lewis
acid and develop a novel class of metal Lewis acid–enamine
bifunctional catalysts with the intention to bridge more
traditional transition-metal catalysis with the newly estab-
lished prosperous area of organocatalysis.
Multifunctional catalytic systems for asymmetric organic
reactions have the potential to bring higher reactivity, selec-
tivity, and versatility.[6] Partly due to their inherent com-
plexity, few Lewis acid/Lewis base bifunctional systems
[a] Department of Chemistry and Biochemistry, Miami University,
Oxford, OH 45056, USA
Fax: +1-513-529-2824
E-mail: wangh3@muohio.edu
[b] State Key Laboratory of Theoretical & Computational Chemis-
try, Institute of Theoretical Chemistry, Jilin University,
Changchun 130023, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900678.
Figure 1. Tridentate ligands.
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Z. Xu, P. Daka, I. Budik, H. Wang, F.-Q. Bai, H.-X. Zhang
SHORT COMMUNICATION
acid, and it will also serve to assemble the molecule into a structure shows that coordination of the secondary amine
relatively rigid structure, a desired property for bifunctional on the pyrrolidine moiety of ligand 2 to the metal would be
catalysts. Furthermore, the Lewis acidity and the secondary very difficult due to its distance (Figure 1).
structure for substrate interaction can be easily tuned by
introducing different metals. Herein we wish to report the
preliminary results of the first example of enamine–metal
Lewis acid bifunctional catalysts for the direct asymmetric
aldol reactions of ketones.
Results and Discussion
Three tridentate ligands (1, 2 and 3) were prepared (Fig-
ure 1). These ligands can be obtained from pyridine-2,6-di-
amine (Scheme 1, see Supporting Information for the syn-
Scheme 1. Synthesis of ligand 2.
thesis of 1–3). Ligand 1 with C2 symmetry has two -proline
moieties incorporated, and ligand 2 and 3 have C1 sym-
metry and carry only one -proline moiety. To prove the
Table 1. Metal screening.[a]
principle, we decided to use the intermolecular aldol reac-
tion as the model reaction. Preliminary experiments focused
on benchmark screening of different metals in combination
with ligand 1 (Table 1). The reaction between acetone (ex-
cess) and 4-nitrobenzaldehye was carried out at room tem-
perature. The bifunctional catalysts were prepared by stir- Entry
Metal
Ligand
Time [h] Yield [%][c] ee [%][d]
1[8]
2
–
1
1
1
1
1
1
1
1
1
1
4
5
5
48
24
36
48
48
24
48
72
48
48
72
72
60
74
73
45
84
63
83
85
81
48
58
0
43
53
28
38
58
42
48
47
70
75
≈
ring ligand 1 with metal salts in a molar ratio of 1:1 in
corresponding solvent(s) for 2–4 h at room temperature. All
the bifunctional catalysts showed activities. Co(ClO4)2 and
Zn(OAc)2 displayed high activities in terms of yield and re-
action time. However, the enantioselectivities were modest
(42 and 53%ee; Table 1, Entries 2 & 6). Counteranions of
the metal salts also played a role in determining the yield
and enantioselectivity. Whereas Cu(SbF6)2 and Cu(NO3)2
gave reasonable ee values of 75 and 70% in modest yield,
Cu(OTf)2 and Cu(ClO4)2 resulted in enantioselectivities of
48 and 47%, respectively, albeit in high yields (Table 1, En-
tries 7–10). These results strongly suggest that metals par-
ticipate in the reactions. To further prove the bifunctional
nature of this catalytic system, we prepared N-Cbz-pro-
tected ligand 4 and ligand 5 bearing no pyrrolidine moiety
(Figure 1). The aldol reactions carried out with 4 and 5 in
Zn(OAc)2
Zn(ClO4)2
Ni(OAc)2
Ni(ClO4)2
Co(ClO4)2
Cu(OTf)2
Cu(ClO4)2
Cu(NO3)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
Cu(SbF6)2
3
4[b]
5[b]
6
7
8
9[b]
10
11
12
13[e]
0
20
≈
≈
[a] Reactions were carried out with acetone (2 mL) and aldehyde
(0.2 mmol). [b] DMSO (1 mL) was added. [c] Isolated yield. [d] De-
termined by chiral HPLC. [e] Pyrrolidine (20 mol-%) was added.
Having established the bifunctional nature of the cata-
the presence of Cu(SbF6)2 did not give any aldol products lytic system, we decided to optimize the reaction conditions
after 72 h, suggesting that the pyrrolidine ring is critical for of the aldol reaction catalyzed by ligand 1 with Cu(SbF6)2,
the reaction (Table 1, Entries 11 & 12). When 5, Cu- as it gives the highest enantioselectivity (75% ee; Table 1,
(SbF6)2, and pyrrolidine were mixed in a 1:1:1 molar ratio, Entry 10). Higher asymmetric induction was achieved when
the aldol reaction proceeded very slowly due to the coordi- THF was used as solvent (Table 2, Entries 1–4; see Support-
nation of the free pyrrolidine to the copper, and the aldol ing Information for a full list of reaction conditions). When
product was obtained in only 20% yield (Table 1, Entry 13), 2 mL of THF and 0.5 mL of acetone were used, a good ee
in sharp contrast to the reactions carried out with 1/metal value of 83% was obtained, albeit with a longer reaction
systems (Table 1, Entries 2–10) and free ligand 1[8] (Table 1, time (72 h) and low yield (31%). When 2 mL/1 mL of THF/
Entry 1), indicating that the tethered pyrrolidine moiety in acetone was used, the yield increased to 51% without sacri-
ligand 1 did not participate in the coordination to the metal ficing the ee value; addition of a small amount of water
and the acid–base self-quenching reaction did not happen helped increase the yield, but the ee value decreased slightly.
as expected. To gain more understanding of the catalyst Using a higher concentration largely increased the yield,
structure, the structure of the CuII complex was optimized but to our disappointment, the ee value was also lowered
with the DFT variant hybrid density functional theory slightly (Table 2, Entry 4). On the basis of these observa-
(B3LYP) in conjunction with the 6-31+G(d) basis set by tions, ligand 1 was modified by replacing one of the -pro-
using the Gaussian 03 program package. The optimized line moieties with a sterically more bulky -valine (2) and a
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Direct Asymmetric Aldol Reaction of Ketones
benzene carrying -phenylalanine (3) to optimize the cata- Table 3. Aldehyde scope of the direct aldol reaction of acetone.[a]
lytic performance. The asymmetric catalytic aldol reactions
carried out with both 2 and 3 resulted in much higher yields
and ee values in shorter reaction times, with 2 displaying
slightly superior enantioselectivity to 3 (Table 2, Entries 5–
Entry
R1
Product, Yield [%][c]
ee [%][d]
8). Even though it is not clear why C2 ligand 1 displayed
both poorer activity and enantioselectivity, we speculate
that it may arise from a competition between the two pro-
line moieties. When lower loading of 2 (10 mol-%) was
used, both activity and enantioselectivity decreased
(Table 2, Entry 9). Lowering the temperature (0 °C) helped
increase the ee value slightly (90% ee; Table 2, Entry 7).
1[b]
2[b]
3[b]
4
5
6
7
8
9
4-NO2C6H4
3-NO2C6H4
2-NO2C6H4
4-CNC6H4
4-COOMeC6H4
4-ClC6H4
2,6-Cl2C6H3
2-naphthyl
4-MeC6H4
7a, 75
7b, 75
7c, 71
7d, 95
7e, 92
7f, 60
7g, 73
7h, 73
7i, 48
90
89
93
87
87
85
91
74
58
Table 2. Optimization of the model reaction conditions.[a]
[a] The reactions were carried out with aldehyde (0.2 mmol) and
acetone (0.3 mL) in THF (0.6 mL) at r.t. for 24–72 h. [b] At 0 °C.
[c] Isolated yield. [d] Determined by chiral HPLC.
When cyclohexone was used as the aldol donor (Table 4),
the same trend was observed. In general, the reactions of
electron-deficient aldehydes proceeded in very high yields
(80–96%), good to excellent diastereoselectivity (anti/syn,
THF
[mL]
Acetone
[mL]
Entry Ligand
Time [h] Yield [%][e] ee [%][f]
1
1
1
1
1
2
2
2
3
2
2
2
2
0.6
2
0.6
0.6
0.6
0.6
0.5
1
1
0.3
1
0.3
0.3
0.3
0.3
72
60
60
48
48
24
48
48
72
31
51
61
84
93
93
75
92
95
83
82
79
75
88
87
90
85
83
2
3[b]
4
Table 4. Aldehyde scope of the direct aldol reaction of cyclic
ketones.[a]
5
6
7[c]
8
9[d]
[a] Reactions were performed with 0.2 mmol aldehyde. [b] H2O
(0.3 mL) was added. [c] At 0 °C. [d] 10 mol-% of catalyst. [e] Iso-
lated yield. [f] Determined by chiral HPLC.
Entry R1
x
anti/syn[b]
Product,
ee [%][d]
(anti)
Yield [%][c]
1
2
3
4
5
6
7
8
9
10
4-NO2C6H4
1
1
1
1
1
1
1
1
1
0
8:1
9:1
8:1
10:1
8:1
6:1
Ͼ30:1
6:1
5:1
8a, 96
8b, 96
8c, 96
8d, 91
8e, 82
8f, 80
8g, 95
8h, 71
8i, 70
8j, 96
91
81
83
84
80
82
86
76
71
92
Compound 2 was then selected in conjunction with
Cu(SbF6)2 as the bifunctional catalytic system for further
investigation, and the aldehyde scope was examined for the
asymmetric aldol reactions of acetone (Table 3). Under op-
timized conditions both electron-rich and electron-poor
aromatic aldehydes reacted smoothly with acetone to give
desired aldol products 7. The aldol products were obtained
in high yields (60–95%) and good enantioselectivity (85–
91%ee) for electron-deficient aldehydes (Table 3, Entries 1–
7); electron-rich aldehydes (Table 3, Entries 8 & 9), however,
gave decreased yields (48–73%) and enantioselectivity (58–
74%ee).
3-NO2C6H4
2-NO2C6H4
4-CNC6H4
4-COOMeC6H4
4-ClC6H4
2,6-Cl2C6H3
2-naphthyl
C6H5
4-NO2C6H4
4:1
[a] The reactions were carried out with aldehyde (0.2 mmol) and
ketone (0.3 mL) in THF (0.6 mL) at r.t. for 24–72 h. [b] Deter-
mined by 1H NMR spectroscopy. [c] Combined yield. [d] Deter-
mined by chiral HPLC.
Table 5. Examples of the direct aldol reaction of 2-butanone.[a]
Entry
R1
Product, Yield [%][c]
ee [%][d]
Product, Yield [%][c]
anti/syn[b]
ee [%][d] (anti)
1
2
3
4-NO2C6H4
3-NO2C6H4
2-NO2C6H4
9a, 36
9b, 34
9c, 20
88
85
84
10a, 59
10b, 54
10c, 40
Ͼ30:1
Ͼ30:1
25:1
Ͼ99
Ͼ99
99
[a] The reactions were carried out with aldehyde (0.2 mmol) and ketone (0.3 mL) in THF (0.6 mL) at r.t. for 24–48 h. [b] Determined by
1H NMR spectroscopy. [c] Isolated yield. [d] Determined by chiral HPLC.
Eur. J. Org. Chem. 2009, 4581–4585
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Z. Xu, P. Daka, I. Budik, H. Wang, F.-Q. Bai, H.-X. Zhang
tracted with EtOAc. After removal of the solvent, the residue (the
SHORT COMMUNICATION
6:1Ǟ30:1), and good enantioselectivity (80–91%ee;
Table 4, Entries 1–7). Cyclopentanone (Table 4, Entry 10)
was also examined with 4-nitrobenzaldehyde furnishing
96% yield and 92%ee, in lower diastereoselectivity
(anti/syn, 4:1).
1
mixture was analyzed by H NMR spectroscopy to determine dia-
stereoselectivity where applicable) was purified by column
chromatography on silica gel (hexane/ethyl acetate) to give the pure
products. All aldol products are known compounds, and their spec-
troscopic data are identical to those reported in the literature. The
ee values were determined by chiral HPLC analysis.
When 2-butanone served as the aldol donor, excellent
enantioselectivity
(99%ee)
and
diastereoselectivity
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures, characterization data for all
new compounds, and HPLC data.
(anti/syn, Ͼ30:1) were obtained with branched product 10
(Table 5). Linear product 9 was generated in good enantio-
selectivities (84–88%ee).
These reactions gave predominately (R)-aldol products.
The absolute configuration predicted from the proposed
transition state model (Figure 2) matches the experimental
results. Even though the detailed mechanism needs further
investigation, on the basis of the data obtained we speculate
that the metal (CuII) serves as a Lewis acid activating the
aldehyde, and the pyrrolidine ring serves as a Lewis base
and forms an enamine with the ketone. The enamine at-
tacks from the Re face of the aldehyde to give the (R)-aldol
product.
Acknowledgments
We thank Professor Mike Novak for useful discussions. Financial
support was provided by Miami University.
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Direct Asymmetric Aldol Reaction of Ketones
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Received: June 18, 2009
Published Online: July 20, 2009
Eur. J. Org. Chem. 2009, 4581–4585
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