Enantioselective ring opening of meso-epoxides with aromatic amines catalyzed by dinuclear magnesium complexes

Article abstract of DOI:10.1002/cjoc.201201008

The dinuclear magnesium complexes generated in situ from the reaction of chiral multidentate semi-azacrown ether ligands with n-Bu₂Mg were found to be efficient catalysts for enantioselective ring-opening of meso-epoxides with aniline derivativ

Full text of DOI:10.1002/cjoc.201201008

FULL PAPER
DOI: 10.1002/cjoc.201201008
Enantioselective Ring Opening of meso-Epoxides with Aro-
matic Amines Catalyzed by Dinuclear Magnesium Complexes†
Hongli Bao,a,b Zheng Wang, a Tianpa You,b and Kuiling Ding*,a
a State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Road, Shanghai 200032, China
b Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China
The dinuclear magnesium complexes generated in situ from the reaction of chiral multidentate semi-azacrown
ether ligands with n-Bu2Mg were found to be efficient catalysts for enantioselective ring-opening of meso-epox-
ides with aniline derivatives, affording the corresponding chiral β-amino alcohols in good yields with up to 90%
ee.
Keywords meso-epoxides, ring-opening, asymmetric catalysis, dinuclear complex, magnesium
Introduction
epoxides, using chiral binuclear magnesium complexes
as the precatalysts. Such a complex was generated in
situ from the reaction of Trost-type chiral multidentate
semi-azacrown ether ligand[7] with n-Bu2Mg, and had
been demonstrated previously by our group to be a
highly efficient catalyst for the copolymerization of
cyclohexene oxide with CO2 [Figure 1, (a)].[8] We envi-
sioned that such a binuclear complex may act as a
Brønsted base-Lewis acid bifunctional catalyst for dual
activation[9] of both the amine and the epoxide, so that
an orientated intramolecular delivery of the nucleophilic
amine (or amido ligand) can occur with good stereo-
chemical control at the two proximal metal centers
within the chiral cavity of the complex [Figure 1, (b)].
Notably, previous mechanistic studies on metal cata-
lyzed asymmetric ring-opening of epoxides often re-
vealed the involvement of some bimetallic species in the
transition states, and hence several types of homo- or
hetero-binuclear complexes or dimeric precatalysts have
been successfully developed for the cooperative cataly-
sis in asymmetric ring opening of epoxides with hy-
droxyarenes,[10] silyl azides,[11] water and alcohols,[12] or
aryl selenols and thiols[13] as the nucleophiles, respec-
tively.
Catalytic asymmetric epoxide ring-opening reactions
are powerful methods in chemistry for the syntheses of
various optically enriched 1,2-difunctionalized com-
pounds with two adjacent stereocenters, which can be
extensively used as chiral building blocks in the syn-
theses of natural products and pharmaceutical drugs.[1]
Consequently, a broad range of nucleophiles have been
investigated in the ring opening reactions of meso or
racemic epoxides, and many asymmetric catalytic pro-
tocols have been developed to date.[2] The asymmetric
ring opening reaction of epoxides with amine nucleo-
philes constitutes a simple route to optically enriched
β-amino alcohols, which are of great synthetic utility for
the synthesis of biologically active compounds or as
chiral ligands in asymmetric synthesis.[3] Although a
number of chiral catalysts (mostly including chiral
Lewis acid complexes) have been developed for this
reaction over the years,[4] in most cases the reactions are
still plagued by problems such as high catalyst loading,
narrow substrate scope, and/or unsatisfactory enanti-
oselectivity. In this regard, we have recently reported a
mechanistic study on the Ti(IV)/BINOLate complex
catalyzed enantioselective aminolytic ring opening of
3,5,8-trioxabicyclo[5.1.0]octane,[5] and have developed
a Mg(II)/BINOLate complex that is efficient for enanti-
oselective catalysis of ring opening aminolysis of meso-
epoxides with both aromatic and aliphatic amines.[6] As
a continuation of our studies along this line, in the pre-
sent work we investigated the asymmetric catalytic
aminolytic ring-opening desymmetrization of meso-
Results and Discussion
Initial studies of the aminolytic ring-opening reac-
tions were performed using cyclohexene oxide (1a) as
the model substrate and aniline (2a) as the amine nu-
cleophile. The reactions were carried out at r.t. for 24
*
E-mail: kding@mail.sioc.ac.cn
Received October 16, 2012; accepted November 16, 2012; published online December 27, 2012.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201201008 or from the author.
Dedicated to the Memory of Professor Weishan Zhou.
†
Chin. J. Chem. 2013, 31, 67—71
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
67

Bao et al.
FULL PAPER
R'
O
23). Finally, the system with the Mg/L2 ratio of 1.2∶1
was found to be the optimal in terms of the overall
catalytic performance, giving the β-amino alcohol 3a in
80% yield with 88% ee (Table 1, Entry 19).
R
Ar
Ar
Ar
O
O
N
Ar
Ar
Ar
O
N
O
N
Mg
O
Ar
Mg
Mg
Ar
Mg
R'OH
-RH
N
O
Chart 1 Multidentate ligands in this study
for epoxide/CO2 copolymerization
L1: R = CH3, Ar = Ph
L2: R = CH3, Ar = 4-CF3-C6H4
L3: R = NO2, Ar = Ph
L4: R = NO2, Ar = 4-CF3-C6H4
L5: R = CH3O, Ar = 4-CF3-C6H4
L6: R = CH3, Ar = 3,5-(CF3)2-C6H3
L7: R = CH3, Ar = 4-Ph-C6H4
Ar
Ar
Ar
Ar
previous results
(a)
OH
HO
N
N
OH
R
"R
H
epoxide
amine
Ph
O
"R
Ar
Ar
O
HN
L8: R = H,
Ar = Ph
H
Ar
Ar
-RH
O
CF3
Mg
O
Mg
Ph
Ph
N
N
Ph
Ph
OH
HO
N
proposed dual activation
in this work
(b)
HO
CF3
N
O
OH
N
Figure 1 Proposed dual activation mode for asymmetric ami-
nolysis of epoxides catalyzed by the dinuclear Mg complexes.
L9
L10
h in the specified solvent using the Mg(II) complex gener-
ated in situ from the combination of ligand L1－L10
(Chart 1) and n-Bu2Mg as the metal precursor, and the
results are summarized in Table 1. For the reactions
conducted in toluene, a great variation in catalytic per-
formance was observed among the ligand series L1－
L10 (Table 1, Entries 1－10). While the reactions in-
volving bis(α,α-diarylprolinol) ligands L1, L2, and L5
－L8 afforded the product 3a with moderate to good ee
values in the yields of 22%－50% (Table 1, Entries 1, 2,
5－8), the catalyst containing ligand L3 or L4 with a
nitro group on the phenolic backbone, the methyl-
capped ligand (S,S)-L9, or (S)-L10 that bears only one
diarylprolinol moiety on the phenolic backbone, gave no
desired product under the otherwise identical conditions
(Table 1, Entries 3, 4, 9, and 10). Ligand L2 with CF3
groups on the diarylprolinol motifs turned out to be op-
timal, which was thus used for subsequent reaction
studies. Gratifyingly, further modification of reaction
conditions by changing the solvent (Table 1, Entries
12－16) led to a substantial enhancement of the cata-
lytic activity. Among the solvents tested, n-pentane was
found to be the best one for L2/Mg catalyzed aminoly-
sis of 1a by aniline, affording the product 3a in 73%
yield with 84% ee (Table 1, Entry 13). Interestingly, the
reaction did not proceed in THF (Table 1, Entry 16),
probably as a result of catalyst deactivation caused by
the occupation of the Mg site by the strongly coordinat-
ing THF molecules. Since the relative amount of the
ligand and the metal can exert a profound influence on a
catalytic transformation, we have also further examined
the effect of n-Bu2Mg/L2 molar ratios on this reaction
(Table 1, Entries 17－23). Intriguingly, even though the
reaction with a Mg/L2 ratio of 2∶1 afforded the high-
est yield of 3a (87%), the corresponding enantioselec-
tivity (72% ee) was still less satisfactory (Table 1, Entry
Table 1 Optimization of the asymmetric ring opening reaction
of epoxide 1a and aniline catalyzed by the complexes of phenol-
bridged bis(α,α-diarylprolinol) ligands with n-Bu2Mga
OH
O
Ligand (5.0 mol%)
Bu2Mg (X mol%)
NHPh
+
PhNH2
toluene, r.t., 24 h
2a
1a
3a
Entry Ligand
Solvent
X
Yieldb/%
eec/%
1
2
L1
L2
L3
L4
L5
L6
L7
L8
L9
L10
L2
L2
L2
L2
L2
L2
L2
L2
L2
Toluene 7.5
Toluene 7.5
Toluene 7.5
Toluene 7.5
Toluene 7.5
Toluene 7.5
Toluene 7.5
Toluene 7.5
Toluene 7.5
Toluene 7.5
36
76
86
50
3
Trace
Trace
46
ND
ND
87
4
5
6
37
40
7
27
53
8
22
64
9
Trace
Trace
73
ND
ND
81
10
12
13
14
15
16
17
18
19
20
Hexane
7.5
Pentane 7.5
Heptane 7.5
73
84
75
75
Decane
THF
7.5
7.5
82
72
Trace
73
ND
88
Pentane 5.0
Pentane 5.5
Pentane 6.0
Pentane 7.0
72
88
80
88
72
84
68
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© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2013, 31, 67—71

Enantioselective Ring Opening of meso-Epoxides with Aromatic Amines
Continued
Continued
Entry Ligand
Solvent
X
Yieldb/%
eec/%
Entry Epoxide
R2 (2)
Yieldb/% (3) eec/%
21
22
23
L2
L2
L2
Pentane
Pentane
8.0
9.0
79
84
87
78
73
72
9
1a
1a
1a
1b
1c
1d
m-IC6H4 (2i)
Bn (2j)
81 (3i)
88
－
－
65
88
63
e
10
－
e
Pentane 10.0
11
i-Pr (2k)
Ph (2a)
－
a
All reactions were performed at r.t. for 24 h in 1 mL of the
specified solvent, with 0.5 mmol of PhNH2 and 0.75 mmol of 1a
(1.5 equiv) in the presence of 5.0 mol% of the ligand and X mol%
of n-Bu2Mg (both relative to that of aniline). Yield of the iso-
lated product. c Determined by chiral HPLC.
12
24 (3j)
83 (3k)
67 (3l)
13d
14d
Ph (2a)
Ph (2a)
b
a All reactions were performed in pentane at r.t. for 24 h, with 0.5
mmol of the amine and 0.75 mmol of the epoxide (1.5 equiv.) in
the presence of 5.0 mol% of L2 and 6.0 mol% of n-Bu2Mg (both
Under the optimized reaction conditions, the
(S,S)-L2/n-Bu2Mg catalyzed enantioselective ring
opening aminolysis was extended to various epoxide
and amine combinations (Table 2). The catalyst was
found quite efficient for the reactions of 1a and a num-
ber of aniline derivatives, affording the corresponding
β-amino alcohols in high yields with good ee values (up
to 90% ee, Table 2, Entries 1－9). Unfortunately, no
reactivity was observed in the cases of aliphatic amines
(Table 2, Entries 10 and 11), which are a notoriously
challenging class of nucleophiles for such reactions
since their strong Lewis base character can often result
in catalyst deactivation. Further examination of the ep-
oxide substrates adaptability was performed using ani-
line as the amine partner (Table 2, Entries 12－14).
While cis-stilbene oxide 1c was transformed into its
corresponding ring-opened product 3k with high yield
and good ee value (Table 2, Entry 13), the reactions
involving epoxides 1b and 1d were somewhat less sat-
isfactory, giving their corresponding products either in
poor yield (Table 2, Entry 12) or in moderate ee value
(Table 2, Entry 14).
b
relative to that of the amine). Yields of the isolated products.
c Determined by chiral HPLC. d Catalyst loading: 10 mol% of L2
and 12.0 mol% of n-Bu2Mg. e No product was isolated.
Based on these results, we postulate a plausible
catalytic cycle for asymmetric ring-opening of the
meso-epoxide 1a by aniline (Figure 2). Treatment of the
ligand with n-Bu2Mg gives rise to the dinuclear Mg
complex a,[7q,8] which on exposure to the aniline leads to
the formation of the amido Mg species b which initiates
the catalytic cycle. Coordinating activation of the epox-
ide on the second Mg center, followed by an intra-
molecular nucleophilic attack of the adjacent amido
ligand in c, affords the amino alkoxide complex d. De-
protonation of aniline releases the product 3a and re-
generates the dinuclear species b, which initiates further
catalytic cycles.
Conclusions
We have developed a catalytic protocol for the
asymmetric ring-opening of meso-epoxides with aniline
derivatives as nucleophiles, in the presence of dinuclear
Mg complexes generated in situ from the combination
of phenol-bridged bis(α,α-diarylprolinol) ligands and
n-Bu2Mg. Under the optimized conditions, the corre-
sponding chiral β-amino alcohols were obtained in good
yields with up to 90% ee.
Table 2 Enantioselective aminolysis of the epoxides catalyzed
by the (S,S)-L2/n-Bu2Mg complexa
OH
n-Bu2Mg (6.0 mol%)
(S,S)-L2 (5 mol%)
O
NHR2
R2NH2
R1
O
+
R1
R1
pentane, r.t., 24 h
R1
3a－3l
1a－1e
2a－2k
Experimental
O
O
General
O
Ph
Ph
All reactions and manipulations were performed us-
ing standard Schlenk techniques or in a glovebox. All
solvents and the reactants were purified and carefully
dried according to standard methods prior to use. NMR
spectra were recorded in deuteriochloroform with a
Bruker AM-300 (1H NMR 300 MHz; 13C NMR 75 MHz)
spectrometer. Chemical shifts were reported in ppm
relative to the internal standard tetramethylsilane (δ＝0)
1a
1b
1c
1d
Entry Epoxide
R2 (2)
Yieldb/% (3) eec/%
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
Ph (2a)
80 (3a)
69 (3b)
80 (3c)
81 (3d)
68 (3e)
61 (3f)
90 (3g)
74 (3h)
88
76
90
79
82
78
85
81
m-MeOC6H4 (2b)
o-MeOC6H4 (2c)
p-MeOC6H4 (2d)
m-MeC6H4 (2e)
o-MeC6H4 (2f)
1
for H NMR and deuteriochloroform (δ＝77.0) for 13C
NMR spectroscopy. ESI mass spectra were obtained by
using a Shimadzu LCMS-2010EV spectrometer. HRMS
spectra were recorded on a Waters MicroMass GCT
Mass Spectrometer or an IonSpec 4.7 Tesla FTMS in-
strument. Optical rotations were measured with a
p-EtC6H4 (2g)
3,5-Me2C6H4 (2h)
Chin. J. Chem. 2013, 31, 67—71
© 2013 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
69

Bao et al.
FULL PAPER
OH
Ar
Ar
Ar
NHPh
Ph
NH
OH
Ar
HO
N
NHPh
Ar
Ar
Ar
O
O
N
N
OH
R
Mg
Ar
Mg
O
Ar
Ar
Ar
O
N
O
N
PhNH2
N
O
Mg
Ar
Mg
O
-3 equiv. C4H10
PhNH2
2 equiv. Bu2Mg
R
b
Bu
BuH
d
Ar
Ar
Ar
O
O
Mg
R
Mg
Ar
O
H
Ph
N
N
O
HN
Ar
O
Ar
O
O
H
Mg
Ar
Mg
Ar
N
N
O
R
a
R
c
Figure 2 Proposed catalytic cycle for the ring opening of cyclohexene oxide with aniline catalyzed by the dinuclear Mg complexes.
Perkin-Elmer 341 automatic polarimeter. Infrared spec-
tra were obtained with a BIO-RAD FTS-185 Fourier
transform spectrometer in KBr pellets. The ee values
were determined by HPLC analyses on chiral columns.
HPLC analyses were carried out on a JASCO 1580 liq-
uid chromatograph with a JASCO CD-1595 detector
and an AS-1555 autosampler. Silica gel (300－400
mesh) was used for column chromatography. Melting
points were measured on a RY-I apparatus and uncor-
rected.
matography on silica gel using petroleum ether/ethyl
acetate (V/V＝8∶1 to 4∶1, containing ca. 5% triethyl-
amine) as the eluent to afford the pure amino alcohol
product.
Acknowledgement
We thank NSFC (Nos. 21172237, 21032007,
21121062) and the Major Basic Research Development
Program of China (No. 2010CB833300) for support of
this work.
Synthesis of the chiral ligands L1－L10
The phenol-bridged bis(α,α-diarylprolinol) ligands
(S,S)-L1 － L8 (Trost-type ligands) and the methyl-
capped ligand (S,S)-L9 were synthesized by following a
reported procedure,[14] and ligand (S)-L10 was prepared
according to a similar procedure (for details, see Sup-
porting Information).
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(Pan, B.; Qin, X.)
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