Asymmetric ring opening of meso-epoxides with aromatic amines catalyzed by a new proline-based N,N′-dioxide-indium tris(triflate) complex

Article abstract of DOI:10.1002/adsc.200700474

The catalytic asymmetric ring opening of meso-epoxides with aromatic amines was achieved using a new proline-based N,N′-dioxide-indium tris(triflate) complex in high yields (up to 99%) with excellent enantioselectivities (up to 99% ee) under mild conditio

Full text of DOI:10.1002/adsc.200700474

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DOI: 10.1002/adsc.200700474
Asymmetric Ring Opening of meso-Epoxides with Aromatic
Amines Catalyzed by a NewProline-Based N,N’-Dioxide-Indium
Tris(triflate) Complex
G
Bo Gao,a Yuehong Wen,a Zhigang Yang,a Xiao Huang,a Xiaohua Liu,a
and Xiaoming Fenga,b,*
a
Key Laboratory of Green Chemistry & Technology (Sichuan University), Ministry of Education, College of Chemistry,
Sichuan University, Chengdu 610064, Peopleꢀs Republic of China
State Key Laboratory of Biotherapy, Sichuan University, Chengdu 610041, Peopleꢀs Republic of China
b
Fax : (+86)-28-8541-8249; e-mail: xmfeng@scu.edu.cn
Received: September 26, 2007; Revised: December 25, 2007; Published online: February 5, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The catalytic asymmetric ring opening of
meso-epoxides with aromaticamines was ahcieved
using a new proline-based N,N’-dioxide-indium tris-
been obtained using this strategy by the promotion of
BINOL-based metal complexes[7a–g] or chiral bipyri-
dine-metal complexes.[7h–j]
A
Compared with other asymmetric reactions, the cat-
alysts developed for this reaction are still limited and
some effective ligands[7f–j] are usually prepared under
rigorous conditions. So, a new catalyst should be fur-
ther developed in terms of effectivity and practicabili-
ty. On the other hand, chiral N-oxide ligands are
excellent enantioselectivities (up to 99% ee) under
mild conditions. The coordination ability of N,N’-di-
oxide 1c was investigated by X-ray and NMR analy-
sis. A plausible seven-coordinate transition state
model was proposed. The chiral N,N’-dioxides sur-
veyed were synthesized from proline through only
three conventional steps. The procedure could be
run on a gram-scale without any loss of enantiose-
lectivity. This protocol provides a highly practical
and useful tool for the bulky preparation of optical-
ly pure b-amino alcohols.
emerging for their distinctive impact in the field of
[8]
asymmetriccatalysis
and have been well developed
in the reduction of ketones,[9] the allylation of alde-
hydes,[10] the epoxide opening,[11] and the aldol con-
densation.[12] Herein, we describe the first example of
proline-based N,N’-dioxide-InACHTRE(UNG OTf)3 complex-cata-
lyzed asymmetricring opening of meso-epoxides with
aromaticamines.
Keywords: amino alcohols; aminolysis; asymmetric
catalysis; N,N’-dioxide-indium tris
plex; meso-epoxides
A
As a part of our ongoing program, several proline-
based N,N’-dioxide catalysts have been successfully
applied to some nucleophilic additions, such as the cy-
anation of ketoimines[13a,b] and carbonyl com-
pounds,[13c–e] the allylation of ketones.[13f] In these cat-
alyses, all of the N,N’-dioxide ligands used were pre-
Chiral b-amino alcohols are useful building blocks for pared via coupling of two amino acids units at the N-
biologically active compounds,[1] as well as important termini.[14] Since an amino acid has two reactive func-
[2]
auxiliaries and ligands in asymmetricsynthesis. Sev- tional groups, it was logical to devise a new class of li-
eral methods for the direct asymmetric synthesis of gands (Figure 1) in which two prolines are connected
these units have been developed, such as functional at the C-termini by a diamine. Our efforts were en-
group manipulation of a chiral synthon,[3] aminohy- couraged by the fact that these N,N’-dioxides could
droxylation of olefins,[4] the addition of a-hydroxy ke- be synthesized from proline through three conven-
tones to imines,[5] aminolytickineticresolution of
tional steps, namely, reductive amination, isobutyl
[6a]
trans-aromaticepoxides
and racemic terminal epox- chlorocarbonate-assisted coupling and directed amine
ides.[6b,c] Among them, the catalytic asymmetric ring oxidation.
opening of epoxides with amines is one of the most
Then, the aminolysis of meso-stilbene oxide with
rational and straightforward strategies in combination aniline was carried out using N,N’-dioxides 1a–h as
with availability and atom economy. Indeed, some chiral ligands and In
simple and multifunctional b-amino alcohols have results are summarized in Table 1. Initial ligand
(OTf)3 as metal reagent.[15] The
ACHTREUNG
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385

Bo Gao et al.
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Table 1. Enantioselective ring opening of meso-stilbene
oxide with aniline.[a]
Entry Ligand Catalyst
Loading
Solvent
T
Yield ee
[oC] [%][c] [%][d]
[mol%][b]
1
2
3
4
5
6
7
8
1a
1b
1c
1d
1e
1c
1c
1c
1c
1c
1c
1c
1c
1f
1g
1h
5
5
5
5
5
3
1
3
3
3
3
3
3
3
3
3
THF
THF
THF
THF
THF
THF
THF
Et2O
PhOCH3
toluene
CH2Cl2
THF
THF
THF
THF
THF
0
0
0
0
0
0
0
0
0
0
0
99
72
99
99
75
99
48
7
59
26
91
51
77
91
91
24
30
16
90
86
91
52
40[e]
83[e]
9
6
3
98
10
11
12
13
14
15
16
30 99
À20 95
0
0
0
27
26
95
Figure 1. N,N’-dioxides 1a–h.
[a]
All reactions were performed with meso-stilbene oxide
(0.2 mmol), aniline (0.3 mmol) and 4 Š MS (20 mg) in
solvent (0.1 mL) for 69 h.
screening of 1a–d revealed that 1c was effective to
afford the product 3a in 99% yield with 91% ee.
When the substituent at N-termini was changed to 1-
ethylpropyl 1a, cyclopentyl 1b and cycloheptyl 1d, dis-
tinct enantioselectivity sacrifice (over 30% ee) was
observed (entries 1, 2 and 4 vs. entry 3). Meanwhile,
[b]
[c]
[d]
[e]
1.1:1 molar ratio of ligand to In
Isolated yield.
ACHTRE(UNG OTf)3.
Determined by HPLC on Chiralcel OD-H column.
The absolute configuration of major product was
(1R,2R).
1e also showed
a
decreased enantioselectivity
(entry 5). It suggested that there should be a strict
conformation match between the annulation frame-
work of the diamine and the cyclohexyl substituent.
mer of 1c) displayed a good enantioselectivity with
Using 1c-InACHTREUNG(OTf)3 complex as catalyst, the reaction excellent yield (entry 16). Notably, the absolute con-
conditions were further optimized. When 3 mol% cat- figuration of the major product was predominated by
alyst were used, the reactivity and enantioselectivity the proline fragment (entries 6 and 14 vs. entries 15
were maintained (entry 6). Further reducing the cata- and 16).
lyst loading to 1 mol% gave the same level of enan-
Encouraged by the above results, a variety of other
tioselectivity, but a decreased reactivity (entry 7). A substrates were then examined (Table 2). Substrates
solvent survey revealed that THF and CH2Cl2 (en- with both electron-donating and electron-withdrawing
tries 6 and 11) provided favorable results, while Et2O, groups at ortho or para positions of the anilines gave
PhOCH3 and toluene (entries 8–10) led to rather poor rise to excellent asymmetric induction (entries 2–8, 10
results. Excellent yields and enantiomeric excesses and 12). 3-Chloroaniline (entry 9) and 3-bromoaniline
could be achieved smoothly at 08C and À208C (en- (entry 11), however, exhibited somewhat lower enan-
tries 6 and 13), but a decreased enantiomeric excess tioselectivities (86% ee). Condensed ring amine also
was obtained at 308C (entry 12).
showed high reactivity and enantioselectivity
To determine whether cooperative effects of config- (entry 13). Aliphaticamines did not yield the desired
uration involving the cyclohexane-1,2-diamine and products, presumably because stronger nucleophilicity
proline can give rise to a more effective catalyst, 1f–h made them coordinate irreversibly to the indiumACHTREUNG(III)
were investigated under the conditions of entry 6. As complex. Other aromatic epoxides 2b and 2c reacted
shown in Table 1, enantiomers 1f and 1g gave disap- with aniline and 2-methoxyaniline in high yields and
pointing results with respect to reactivity and enantio- enantioselectivities as well (entries 14–17). Cyclohex-
selectivity (entries 14 and 15), while 1h (the enantio- ene oxide (2d) delivered the products 3q and 3r in
386
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AsymmetricRing Opening of meso-Epoxides with AromaticAmines
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(OTf)3 complex.[a]
Table 2. Enantioselective ring opening of meso-epoxides with amines promoted by 1c-InACHTREUNG
Entry
Epoxide
Amine
Product
Yield [%][d] ee [%][e]
1
2a
PhNH2
3a[b] R=H
99
91
2
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2-CH3O-C6H4NH2
2-CH3O-C6H4NH2
4-CH3O-C6H4NH2
2-CH3-C6H4NH2
4-CH3-C6H4NH2
4-i-Pr-C6H4NH2
4-CF3O-C6H4NH2
3-Cl-C6H4NH2
3b[b] R=2-CH3O 99
3b[b] R=2-CH3O 81
98
99
94
91
96
91
92
86
94
86
93
3[f]
4
3c[b] R=4-CH3O
3d R=2-CH3
3e[b] R=4-CH3
3f R=4-i-Pr
3g R=4-CF3O
3h R=3-Cl
99
90
99
90
99
95
99
94
96
5
6
7
8
9
10
11
12
4-Cl-C6H4NH2
3-Br-C6H4NH2
4-Br-C6H4NH2
3i[b] R=4-Cl
3j R=3-Br
3k[b] R=4-Br
13
2a
napthalene-1-amine
3l[b]
92
88
14
15
2b
2b
PhNH2
3m[b] R=H
82
90
89
97
2-CH3O-C6H4NH2
3n R=2-CH3O
16
2c
PhNH2
3o[b] R=H
86
93
17
18
2c
2-CH3O-C6H4NH2
PhNH2
3p R=2-CH3O
3q[c] R=H
89
93
98
22
2d
19
2d
2d
2d
2-CH3O-C6H4NH2
PhNH2
2-CH3O-C6H4NH2
3r[c] R=2-CH3O
3q[c] R=H
88
90
82
24
76
57
20[g]
21[g]
3r[c] R=2-CH3O
[a]
All reactions were performed under the conditions of entry 6 (Table 1), unless otherwise noted.
Absolute configuration of the major product was assigned as (1S,2S) by comparison of the rotation values in the litera-
[b]
ture.
[c]
Absolute configuration of the major product was assigned as (1R,2R) by comparison of the rotation values in the litera-
ture.
Isolated yield.
[d]
[e]
[f]
Determined by HPLC (see Supporting Information).
The reaction was performed with meso-stilbene oxide (1.177 g, 6 mmol) and 2-methoxylaniline (1.02 mL, 9 mmol) in THF
(3 mL) with 4 Š MS (600 mg) for 70 h at 08C using 1c (99.9 mg, 0.198 mmol) and In
Ligand 1e was used.
ACHTREU(NG OTf)3 (102.2 mg, 0.180 mmol).
[g]
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387

Bo Gao et al.
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high yields and good enantioselectivities using 1e (en- dried under vacuum at room temperature, the
tries 20 and 21) instead of 1c (entries 18 and 19) 1H NMR spectra showed an obvious up-field shift for
under identical reaction conditions. In addition, the the amide proton from 11.88 ppm to 8.34 ppm.[18] This
ring opening of meso-stilbene oxide with 2-methoxy- indicated that, in the N,N’-dioxide-In
ACHTRE(UNG III) complex,
ACHTREUNGaniline was tested on a gram scale and afforded the the intramolecular hydrogen bonds (Figure 2) should
product in 99% ee and good yield (entry 3), which be dissociated and the two N-oxides might be stabi-
could provide a highly practical and useful tool for lized via coordination with In3+. The 13C NMR spectra
the preparation of optically pure b-amino alcohols.
also showed an obvious up-field shift for the amide
To gain some information about the coordination carbon from 168.1 ppm to 166.4 ppm.[18] This strongly
abilities of N,N’-dioxide 1c, suitable crystals for X-ray suggested that there should be a coordination be-
analysis were grown from THF/hexane.[16] In the unit tween amide and In3+.
cell (C28H50N4O5), which involved only one diastereo-
Based on the above findings, a plausible seven-co-
isomer (Figure 2), each N-oxide is syn to the amide at ordinate transition state model[19] was proposed
(Figure 3), in which the N-oxides and amide oxygens
Figure 3. Proposed transition state model.
of 1c coordinated to In3+ in a tetradentate manner to
form two six-membered chelate rings, the triflyl
groups maintained on In3+ were blocked by two cyclo-
hexyl substituents, respectively, and the incoming
meso-stilbene oxide was attached to In3+ from the fa-
Figure 2. X-ray crystal structure of N,N’-dioxide 1c·H2O.
vorable backward side, meanwhile, the aniline was
fixed by the hydrogen bond with the amide proton
the same proline fragment and is respectively stabi- and preferred to attack the adjacent (R)-carbon of
À
lized by intramolecular hydrogen bond of H1 O2 the epoxide.
À
(1.876 Š) and H2 O4 (1.855 Š), and each cyclohex-
In summary, we have developed a novel proline-
(OTf)3 complex-catalyzed asym-
ane annulation adopts a chair conformation. The ob- based N,N’-dioxide-In
A
served differences of bond lengths and angles be- metricring opening of meso-epoxides with aromatic
tween the two proline fragments reveal a pseudo-C2- amines, in which chiral b-amino alcohols were ob-
symmetricgeometry of 1c. The unit cells are aggregat- tained in high yields (up to 99%) and excellent enan-
ed to a hexagonal system by two intermolecular hy- tioselectivities (up to 99% ee). This protocol provides
drogen bonds between one H2O and two N-oxides in a highly practical and useful tool for the bulk prepara-
adjacent cells. When the coordination abilities of 1c tion of optically pure b-amino alcohols. Further ef-
are considered, the X-ray structure suggests that the forts are being devoted to elucidating the detailed
amide nitrogens are unlikely to take part in coordina- mechanism and investigating the scope of the applied
tion to the hard In3+ because their lone pairs are delo- catalyst.
calized in amide carbonyl p-bonds. This is indicated
by the short bond length of N
G
ACHTRE(UGN amide) com-
À
pared to that of N
A
Experimental Section
À
À
1.276 Š vs. N1 C12 1.430 Š, and N2 C18 1.405 Š vs.
[17]
À
N2 C17 1.501 Š).
General Remarks
The coordiantion abilities of 1c were further deter-
mined by NMR analysis. When one equivalent of 1c
All manipulations were carried out under a nitrogen atmos-
and InACHTREUNG
(OTf)3 was treated with 4 Š MS in THF and phere using standard Schlenk or glove-box techniques. 1H
388
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AsymmetricRing Opening of meso-Epoxides with AromaticAmines
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and 13C NMR spectra were recorded at 400 MHz or
600 MHz and 100 MHz, respectively. The chemical shifts are
reported in ppm downfield to CDCl3 (d=7.27) for H NMR
was warmed to room temperature and purified by silica gel
chromatography (ethyl acetate:methanol=1:1) to give the
pure 1c as a white solid; yield: 1.01 g (95%); mp 154–1568C;
1
1
and relative to the central CDCl3 resonance (d=77.0) for
[a]2D5: À16.48 (c 0.390, CH2Cl2); H NMR (600 MHz, CDCl3):
1
13C NMR. Coupling constants in H NMR are in Hz. HR-
d=1.12–1.15 (m, 2H), 1.34–1.51 (m, 9H), 1.67–1.69 (m,
2H), 1.81–1.94 (m, 8H), 2.01–2.04 (m, 2H), 2.18 (m, 4H),
2.35–2.45 (m, 7H), 2.54–2.56 (m, 2H), 3.24–3.33 (m, 2H),
3.35–3.42 (m, 4H), 3.69 (m, 2H), 3.90 (m, 2H), 11.42 (br,
2H); 13C NMR (100 MHz, CDCl3): d=19.9, 22.4, 25.2
(25.19), 25.2 (25.25), 25.3, 27.8 (27.75), 27.8 (27.82), 28.4,
28.8, 30.9, 49.5, 64.5, 71.8, 74.8, 77.2, 167.6; ESI-HR-MS:
m/z=505.3754, calcd. for C28H48N4O4 +H+: 505.3754.
MS was recorded on Bruker-APEX-2 (SIMS). Optical rota-
tion data were obtained using a commerical polarimeter and
reported as follows: [a]TD (c g/100 mL, solvent). Melting
points were measured on an electrothermal digital melting
point apparatus and uncorrected. Enantiomeric excesses
(ee) were determined by HPLC using corresponding com-
mercial chiral columns as stated in the data of products with
UV detection at 254 nm and the retention times were com-
pared to those of the corresponding racemic samples.
Typical Aminolysis Procedure
To a stirred mixture of ligand 1c (0.0066 mmol), InACHTREUNG(OTf)3
(0.006 mmol) and 4 Š MS (20 mg) in THF (0.1 mL) were
added the epoxide (0.2 mmol) and amine (0.3 mmol) at 08C.
Then the reaction was performed at 08C for 69 h. After
completion, the reaction mixture was purified by silica gel
chromatography with mixtures of petroleum ether-ethyl ace-
tate as eluent to give the pure amino alcohol. All known
compounds were compared with data reported in the litera-
ture; all new compounds were fully characterized.
Representative Synthetic Procedure and Analytical
Data for N,N’-Dioxide 1c
Step 1:[10d] Palladium on carbon (10% by wt, dry, 0.12 g) was
suspended in methanol (10 mL) in a dry 50-mL flask and
flushed with N2. To this suspension was charged l-proline
Acknowledgements
We thankthe National Natural Science Foundation of China
(No. 20732003) and Sichuan University for financial support.
We also thankthe Sichuan University Analytical & Testing
Center for NMR and X-ray crystal analysis.
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Adv. Synth. Catal. 2008, 350, 385 – 390
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[14] One exception, see ref.[13a]
[15] Some typical metal reagents were also screened under
the conditions of entry 3 (Table 1): Sc
90% yield and 21% ee, InBr3 led to 12% yield and
55% ee, Zn(OTf)2 led to 10% yield and 50% ee,
ACHTRE(UNG OTf)3 led to
AHCTREUNG
(CuOTf)2·toluene led to 2% yield and 10% ee with a
configuration inversion.
[16] Further attempts to obtain X-ray grade crystal of the
1c-InACTHER(UNG OTf)3 complex were unsuccessful.
[17] CCDC 658823 contains the supplementary crystallo-
graphicdata for this paper. These data can be obtained
free of charge from the Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk./data_request/cif.
[18] NMR spectra, see Supporting Information.
[8] For reviews on chiral N-oxides in asymmetriccatalysis,
see: a) G. Chelucci, G. Murineddu, G. A. Pinna, Tetra-
hedron: Asymmetry 2004, 15, 1373; b) A. V. Malkov, P.
ˇ
´
Kocovsky, Eur. J. Org. Chem. 2007, 29.
[9] I. A. OꢀNeil, C. D. Turner, S. B. Kalindjian, Synlett
1997, 777.
[19] a) InACHTREU(NG III) can bind phosphines to form four-coordinate
species and nitrogen- or oxygen-derived ligands to pro-
duce five- and six-coordinate species, see: S. France,
M. H. Shah, A. Weatherwax, H. Wack, J. P. Roth, T.
Lectka, J. Am. Chem. Soc. 2005, 127, 1206, and referen-
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120, 6419; b) A. V. Malkov, M. Orsini, D. Pernazza,
ˇ
´
K. W. Muir, V. Langer, P. Meghani, P. Kocovsky, Org.
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ces cited therein; b) a pentagonal-bipyramidal InACHTREUNG(III)-
bipyridine complex has been identified by X-ray dif-
fraction, see: M. V. Nandakumar, A. Tschçp, H.
Krautscheid, C. Schneider, Chem. Commun. 2007,
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390
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Adv. Synth. Catal. 2008, 350, 385 – 390