Synthesis of rare earth catalysts and their applications for enantioselective synthesis of heterocyclic β-amino alcohols

Article abstract of DOI:10.1021/jo2019015

A new family of chiral lanthanide complexes derived from (R)-binaphthol has been synthesized by a one-pot procedure using only commercially available substrates. These complexes were evaluated for the aminolysis of meso-epoxides and proved to be efficient enantioselective catalysts. The samarium complex coordinated by two (R)-binaphthoxide ligands was the most enantioselective catalyst of this series. β-Amino alcohols including heterocycles have been isolated with enantiomeric excesses up to 84%.

Full text of DOI:10.1021/jo2019015

Article
pubs.acs.org/joc
Synthesis of Rare Earth Catalysts and Their Applications for
Enantioselective Synthesis of Heterocyclic β-Amino Alcohols
Myriam Martin,† Ahmad El Hellani,† Jing Yang,† Jacqueline Collin,*,†,‡ and Sophie Bezzenine-Lafollee*,†
́
†Equipe de Catalyse Molec
́ ̂ ́
ulaire, ICMMO, UMR 8182, Batiment 420, Universite Paris-Sud, 91405 Orsay, France
‡CNRS, F-91405 Orsay, France
S
* Supporting Information
ABSTRACT: A new family of chiral lanthanide complexes
derived from (R)-binaphthol has been synthesized by a one-
pot procedure using only commercially available substrates.
These complexes were evaluated for the aminolysis of meso-
epoxides and proved to be efficient enantioselective catalysts.
The samarium complex coordinated by two (R)-binaphthoxide
ligands was the most enantioselective catalyst of this series. β-
Amino alcohols including heterocycles have been isolated with
enantiomeric excesses up to 84%.
for enantioselective Strecker reaction although its structure has
not been determined.
INTRODUCTION
■
Asymmetric catalysis provides highly economic access to
optically active compounds. The development of efficient
methods to obtain new chiral catalysts for enantioselective
carbon−carbon and carbon−nitrogen bond formation is an
important goal nowadays. In the last decades, heterobimetallic
catalysts have received particular attention since they enable
highly efficient asymmetric reactions. Shibasaki et al. have
developed various bifunctional asymmetric catalysts based on
binaphtholate ligands and rare earths.1 They have extended this
concept to another metal center and reported a new chiral
catalyst 1 containing aluminum and lithium, by reacting LiAlH4
with 2 equiv of binaphthol (Figure 1, M = Al).2 This complex
In our laboratory, a family of ionic rare earth amides based
on (R)-1,1′-binaphthyl-2,2′-bis(alkylamine) ligands with various
metals and N-alkyl substituents has been synthesized and
characterized.5 These complexes contain a complex anion with
a discrete [Li(THF)4]+ as counterion. They exhibit high
activities and enantioselectivities for asymmetric intramolecular
hydroaminations. Moreover, these catalysts are easily and
rapidly prepared from commercially available sources. This led
us to envisage the preparation of similar rare earth ate
complexes based on binaphtholate ligands as readily available
catalysts that could be evaluated for enantioselective carbon−
nitrogen bond formation such as the aminolysis of meso-
epoxides.
Numerous synthetic routes have been reported for the
preparation of enantioenriched α,β-amino alcohols.6 The
asymmetric ring-opening of epoxides by various nucleophiles
to prepare 1,2-disubstituted derivatives including α,β-amino
alcohols was reviewed in 2006.7 New catalytic systems for the
enantioselective aminolyzes of epoxides based on binol,8
bipyridine,9 or salen10 ligands and different metal precursors
have been developed in recent years. High enantiomeric
excesses have been recorded by Schneider for the ring-opening
of nonfunctionnalized epoxides by aromatic amines using
In(OTf)3 coordinated by a chiral bipyridine ligand9a−c and by
Kobayashi with Nb(OMe)5 coordinated by a tetradentate binol
ligand8d,e (up to 98% and 96%, respectively), yet only a few
examples of enantioselective ring-opening of functionnalized
epoxides were described, and these involved aniline as the sole
Figure 1. Structure of complexes 1.
catalyzed Michael and tandem Michael−aldol reactions with
enantiomeric excesses up to 99% and 91%, respectively. The
Shibasaki team has also synthesized heterobimetallic complexes
using group 13 elements (B, Ga, In) (Figure 1).3 These
complexes catalyzed the asymmetric ring-opening of meso
epoxides by thiols to give β-hydroxy sulfides with enantiomeric
excesses up to 97% in the case of gallium catalyst. Vallee
́
et al.4
have reported the preparation of a similar chiral scandium
complex (Figure 1, M = Sc) which has been successfully used
Received: September 14, 2011
Published: October 22, 2011
© 2011 American Chemical Society
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amine. Both systems described above were efficient, with the
highest enantioselectivities being recorded in niobium catalyzed
reactions (up to 89% ee).9a−c,8d,e,11
We were delighted to find a total conversion in β-amino
alcohol 7aa with 70% enantiomeric excess, the same value as
using samarium iodobinaphtholate. In order to simplify the
preparation of the catalyst, we decided to prepare 4a directly
from yttrium chloride without isolating ate alkylyttrium
complex 3 (Scheme 3).
In our previous studies, we have reported the desymmetriza-
tion of cyclic meso-epoxides by aromatic amines catalyzed by
samarium iodo binaphtholate.12 These reactions allowed us to
prepare the corresponding β-amino alcohols with enantiomeric
excesses up to 93% as the highest values reported for these
transformations. The ring-opening by aromatic amines of cyclic
meso-epoxides containing O and N functionalities led to new
heterocyclic β-amino alcohols with up to 70% ee.13
Scheme 3. Synthesis of Complexes 4 by a One-Pot
Procedure
Herein we report a new route toward rare earth
heterobimetallic derivatives which catalyze the synthesis of
heterocyclic β-amino alcohols in high enantiomeric excesses.
RESULTS AND DISCUSSION
■
The synthesis of rare earth derivatives from amido or alkyl
precursors has attracted an increased interest since it avoids the
preparation of salts from ligands.14 Tetraalkyl ate complexes of
yttrium, lutetium, and scandium have been recently isolated and
used for the synthesis of carbene complexes.15 We thus
considered that these ate complexes could be simple starting
materials for preparing heterobimetallic rare earth complexes
coordinated by two binaphtholate ligands. Yttrium was selected
to investigate this route since heterobimetallic complexes YMB
have been the focus of numerous studies by several teams.16
The reaction of 2 equiv of binaphthol 2 with yttrium ate
complex 3 in tetrahydrofuran at room temperature led to the
formation of a new complex 4a obtained as a white powder
Addition of 4 equiv lithium trimethylsilyl methide on yttrium
chloride in THF was followed by the addition of 2 equiv of
binaphthol. Solvent was evaporated, the residue was dissolved
in toluene and centrifugated to eliminate lithium chloride, and
after evaporation of toluene the solid was washed with hexane.
The resulting product exhibited the same 1H NMR spectrum as
4a prepared from 3, and it catalyzed the aminolysis of 2,5-
dihydrofuran oxide by p-anisidine affording β-amino alcohol
7aa with the same enantiomeric excess as above. Thus, an
efficient catalyst can be prepared directly from commercially
available yttrium chloride and lithium trimethylsilylmethide
without isolating the alkyl ate lithium rare earth complex.
Following the same route, we next tried to prepare a complex of
a metal of higher ionic radius for which the ate complex similar
to 3 has not been isolated to the best of our knowledge.
Performing the reaction as described above with samarium
chloride led to 4b as a yellow powder. Compound 4b was
evaluated for the enantioselective ring-opening of epoxides
under the same conditions as samarium iodobinaphtholate
catalyzed reactions (Scheme 4). The results are indicated in
1
after evaporation of the solvent (Scheme 1). The H NMR
Scheme 1. Synthesis of 4a from Tetraalkyl Complex 3
spectrum of this product in C6D6 shows a large signal for
aromatic protons indicating the coordination of binaphthol to
yttrium and a binaphthol/THF ratio of 1/1. The coordination
of binaphthol was confirmed by the absence of OH bond in the
FT-IR spectrum. Elemental analysis correspond to a structure
including two binol units, one lithium atom, and two molecules
of THF per yttrium. The structure could be an ate complex
with dissociation of the lithium cation or a structure similar to
Scheme 4. Aminolysis of Epoxide 5 Catalyzed by 4
that proposed by Vallee
́
for a scandium complex.4,17
To evaluate the catalytic activity of complex 4a for the
enantioselective aminolysis of epoxides we examined first the
ring-opening of 2,5-dihydrofuran oxide 5a by p-anisidine. We
previously studied this reaction using samarium iodobinaph-
tholate as catalyst and obtained 70% ee as the highest
enantiomeric excess.13 The reaction was thus performed
under the same conditions (at 40 °C) using catalytic amounts
of 4a (Scheme 2).
Table 1. Yttrium- and samarium-based catalysts 4a and 4b
yielded similar results for the ring-opening of 2,5-dihydrofuran
oxide by p-anisidine (Table 1, entries 1 and 2). We next
examined the aminolysis of other epoxides 5b and 5c by o-
anisidine using samarium compound 4b as catalyst (entries 6
and 7). All of the above reactions afforded enantiomeric
excesses similar to those given by samarium iodobinaphtholate
under the same conditions (68% for 7aa, 58% for 7bb, 52% for
7cb). Temperature has been shown to have a dramatic
influence on the asymmetric induction for the rare earth-
catalyzed ring-opening of epoxides. Thus, the influence of
temperature on the reaction of N-heterocycle-containing
epoxide with o-anisidine catalyzed by 4b has been examined
Scheme 2. Aminolysis of Epoxide 5a Catalyzed by 4a
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Table 1. Enantioselective Ring-Opening of Meso Cyclic Epoxides Catalyzed by 4a and 4b
b
a
entry
cat.
substrates
product
T (°C)
time (h)
conv (%)
yield (%)
ee (%)
1
2
3
4
5
6
7
4a
4b
4b
4b
4b
4b
4b
5a, 6a
5a, 6a
5b, 6b
5b, 6b
5b, 6b
5b, 6b
5c, 6b
7aa
7aa
7bb
7bb
7bb
7bb
7cb
40
40
0
40
40
40
20
20
20
23
100
100
10
70
55
68
66
27
63
50
51
65
25
40
60
25
50
30
36
62
20
55
100
100
a
b
Yield, reaction performed with 7 mol % of catalyst 4 and a ratio 6/5: 1.2 in CH2Cl2 or C2H4Cl2 according to reaction temperature. Products
obtained with R catalyst have the configuration 1R, 2R . See ref 16 for determination of the configuration.
(entries 3−6). The maximum value of 63% for the
enantiomeric excess of 7bb was recorded at 25 °C.
Table 3. Influence of Temperature on Asymmetric Induction
for the Aminolysis of Epoxides Catalyzed by 4a or 4b in the
Presence of Lithium Chloride
To determine if lithium chloride could influence the catalytic
properties of compounds 4 we examined the aminolysis of
epoxides using as catalyst a mixture of 4 prepared as described
above (separated from LiCl) with 4 equiv of lithium or
potassium chloride. This study has been achieved on the ring-
opening of epoxide 5b by o-anisidine 6b since conversion is
easier to measure than for the ring-opening of epoxide 5a which
is lost during the treatment of reaction mixture. The results are
indicated in Table 2. Interestingly, a good enantiomeric excess
a
c
T
(°C)
time
(h)
conv
(%)
yield
ee
b
entry
cat.
product
(%)
(%)
d
1
2
4a
4b
4c
4a
4a
4a
7aa
7aa
7aa
7ba
7ba
7bb
7bb
7bb
7ba
7ba
7bb
7bb
7bb
7cb
7cb
7ca
7ca
7cb
7cb
7cb
7cb
7cb
7cb
40
40
40
92
90
48
50
70
27
50
17
50
24
27
60
40
60
40
43
43
23
16
23
48
100
100
95
61
55
81
84
50
50
55
76
79
64
44
60
80
62
60
28
36
33
29
57
67
65
60
48
56
d
40
3
40
25
60
25
25
60
25
60
25
40
60
0
4
86
86
34
57
93
5
100
95
6
e
7
4a
100
100
100
100
100
100
100
95
Table 2. Influence of the Presence of Salts on the Formation
of Amino Alcohol 7bb Catalyzed by 4a or 4b
8
4a
4b
4b
4b
4b
4b
4a
4a
4b
4b
4b
4b
4b
4b
4b
4c
9
c
10
11
12
13
14
15
16
17
18
19
20
21
22
23
89
51
55
entry
catalyst
4a
time (h)
conv (%)
39
yield (%)
ee (%)
1
2
3
4
5
6
7
20
20
50
20
20
20
50
35
36
57
30
40
44
51
80
78
76
63
74
64
80
a
4a + LiCl
4a + LiCl
4b
40
b
95
50
40
25
60
−40
0
100
100
100
100
100
100
100
100
90
a
4b + LiCl
4b + KCl
4b + LiCl
73
50
a
100
b
100
a
Conversion for reactions performed at room temperature using 7 mol
% of catalyst, 28 mol % of LiCl or KCl, epoxide 5b, and amine 6b with
25
40
60
60
27
24
b
a ratio of 6b/5b 1.2 in CH2Cl2. Reaction performed using the in situ
c
prepared catalyst without separation of lithium chloride. Unoptimized
yields.
a
Yield, reaction performed with 7 mol % of catalyst 4 and a ratio of 6/
5 1.2 in CH2Cl2 or C2H4Cl2 according to reaction temperature.
of 80% was obtained at room temperature with yttrium
complex 4a (entry 1). The presence of 4 equiv of lithium or
potassium chloride per samarium or yttrium led to either the
same enantiomeric excess (entries 1, 2 and 4, 6) or to an
increased value (entry 5 vs 4). In the presence of lithium
chloride or potassium chloride the rate of the samarium-
catalyzed reaction was increased (entries 4−6). These results
indicate that the separation of the salts is not required for using
catalysts. After successive additions of lithium trimethylsilyl
methide and binaphthol on rare earth chloride the solvent was
evaporated and the solid directly employed in a catalytic
reaction. Under these conditions, high enantiomeric excesses of
76% and 80% have been respectively obtained with yttrium and
samarium catalysts (entries 3 and 7). We have thus established
a very rapid preparation of the catalysts which can be
synthesized and used in a one-pot procedure. All the following
results have been obtained with in situ prepared catalysts.
The following step was to check if the optimized conditions
for reactions catalyzed by 4a or 4b without lithium chloride
were also the best ones for the reactions with catalysts prepared
by the new procedure (Table 3). For the aminolysis of 2,5-
b
c
Enantiomeric excess measured by HPLC. Products obtained with R
d
catalyst have the configuration (1R, 2R). Low conversions were
observed after 48 h at room temperature. Reaction performed with 2
mol % of catalyst 4a.
e
dihydrofuran oxide 5a by p-anisidine 6a, both yttrium and
samarium catalysts containing lithium chloride allowed us to
prepare the β-amino alcohol 7aa with total conversion in the
same conditions (40 °C, 40 h) as without LiCl. To our delight,
the asymmetric induction was significantly increased with ee
values over 80% with both catalysts (Table 3, entries 1 and 2).
For the aminolysis of epoxide 5b by o- and p-anisidine the
influence of temperature on asymmetric induction has been
examined using samarium and yttrium catalysts. Only small
variations of enantiomeric excesses have been observed from 25
to 60 °C for β-amino alcohols 7ba and 7bb in yttrium-catalyzed
reactions (entries 4−8). When a lower catalytic ratio of 4a (2
mol %) was used, β-amino alcohol 7bb could be isolated in
high yield (93%) and without decrease of enantiomeric excess
(79%) at room temperature but in an increased reaction time
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(entry 7). Interestingly, with samarium catalyst 4b total
conversions have been observed at room temperature for
7bb. The highest enantiomeric excess for 7ba was obtained at
60 °C (entry 10) and for 7bb at room temperature (entry 11).
o-Anisidine furnished higher enantiomeric excesses than p-
anisidine (compare entries 4 and 6, 10 and 11). Under the
optimized conditions, yttrium and samarium compounds 4a
and 4b led to products 7ba and 7bb with similar activities,
while samarium was slightly more enantioselective than yttrium
(compare entries 5 and 10, 6 and 11). Heterobimetallic
catalysts 4 afforded higher asymmetric inductions for the
aminolysis of heterocycles containing epoxides than samarium
iodobinaphtholate that we previously studied. The aminolysis
of cyclohexene oxide 5c by o- and p-anisidine catalyzed by 4a
and 4b at different temperatures led to total or nearly total
conversions even at low temperatures due to the absence of
supplementary heteroatom. With yttrium catalyst, low enantio-
meric excesses with marginal variation with temperature were
observed in reactions involving o-anisidine 6b (entries 14 and
15). For samarium-catalyzed reactions low values of enantio-
meric excesses were similarly obtained using p-anisidine 6a at
room temperature or 60 °C (entries 16 and 17). Yet o-anisidine
6b furnished higher values for the enantiomeric excess with a
non monotonous variation with temperature, the maximum
value 67% ee being recorded at 0 °C (entries 18−22).
Scheme 5. Synthesis of Complexes 8 and 9
Ligand 6,6′-bisbromobinaphthol was used for preparing yttrium
heterobimetallic complex 9, which catalyzed the formation of
amino alcohol 7ba with an enantiomeric excess slighly higher
than corresponding binaphthol complex 4a but with lower
conversion (entries 5 and 6). For the aminolysis of 5b by o-
anisidine a slight decrease of asymmetric induction was
recorded with 6,6′-bisbromobinaphthol (entries 1 and 8). The
presence of bromo substituents in 3,3′ or 6,6′ positions on
binaphthol did not lead to improved activity or enantiose-
lectivity of heterobimetallic yttrium or samarium catalysts for
the aminolysis of an N-heterocyclic epoxide. However, only
preliminary experiments have been achieved, and further
studies varying both the structure of the ligand and the size
of the metal are in progress.
Heterobimetallic lithium samarium or yttrium binaphtholate
complexes are efficient catalysts for the enantioselective
aminolysis of meso-epoxides including a heterocycle. Optimiz-
ing the preparation of catalysts and conditions for the catalytic
reactions has allowed us to obtain enantiomeric excesses up to
84%. For a further improvement of the experimental procedure,
catalytic reactions were performed outside the glovebox.
Catalysts 4a and 4b were weighted in air and reactions were
run with non degassed solvent and substrates at room
temperature. To our delight, yttrium and samarium catalysts
4a and 4b afforded β-amino alcohol 7bb in good yields and
with 74% and 79% ee, respectively. Yttrium complex 4a
prepared without separation from LiCl was stored under argon
outside the glovebox and tested as catalyst for the aminolysis
leading to 7bb. No decrease in the yield or the enantiomeric
excess of 7bb was observed after 4 weeks of storage, indicating
these new catalysts can be employed in very convenient
conditions. Gratifingly, preparation of catalysts and catalytic
reactions are both realized following straightforward proce-
dures.
In order to study the influence of the structure of the catalyst
and to try to improve the enantioselectivity of the aminolysis of
epoxides we prepared a variety of heterobimetallic compounds
by changing the rare earth metal or ligands. We first examined a
scandium catalyst obtained by a one-pot addition of a solution
of lithium trimethylsilylmethide in tetrahydrofuran to scandium
trichloride followed by the addition of binaphthol as described
1
above (Scheme 3). H NMR spectra of compound 4c showed
the absence of signals corresponding to lithium trimethylsi-
lylmethide or binaphthol and OH band was not observed in
FT-IR spectrum. The catalyst was used in situ without
separation of lithium chloride for aminolyzes of epoxides.
The different catalysts were first compared for the aminolysis of
2,5-dihydrofuran oxide at 40 °C by p-anisidine (Table 3, entries
1−3). Lower values of enantiomeric excesses have been found
for scandium than for yttrium and samarium catalysts. Only for
the reaction of cyclohexene oxide 5c with o-anisidine 6b at 60
°C was a higher value of the enantiomeric excess of 7cb
obtained with scandium catalyst 4c than with samarium 4b
(entries 22 and 23).
Since changing yttrium and samarium by a rare earth of
smaller ionic radius like scandium did not allow us to enhance
the asymmetric induction for the aminolysis of epoxides
containing other heteroatoms, we turned our attention to the
variation of the ligand. We selected 3,3′- or 6,6′-bromo-
substituted binaphthol ligands to study the influence of
electronic or steric effects on yttrium and samarium catalysts.
The same procedure as for binaphthol-based heterobimetallic
complexes was followed (Scheme 5). All complexes have been
tested without elimination of LiCl, and the different catalysts
have been used in situ for the desymmetrization of epoxide 5b
(see Table 4). Comparison of yttrium catalysts 4a and 8a
coordinated, respectively, by binaphthol and 3,3′-bisbromobi-
naphthol yielded a decrease in the rate of formation of amino
alcohol 7bb and also a lower value of the enantiomeric excess
with the substituted ligand (entries 1 and 2). A similar trend
was observed with samarium catalysts 4b and 8b coordinated
by binaphthol and 3,3′-bisbromobinaphthol (entries 3 and 4).
Since compounds 4 proved to be interesting catalysts we
turned our efforts toward deeper insight on their structure.
These heterobimetallic compounds could have an ate structure
with dissociation of the lithium cation or a neutral structure
with lithium coordinated to oxygen. The latter has been
́
proposed for the scandium complex 1 by Vallee but was not
supported by structural studies.4 Similar binaphthyl amido rare
earth complexes of ate structure have been characterized.5 For
an aluminum heterobimetallic binaphthoxide derivative,
Shibasaki proposed either a neutral complex or an equilibrium
in solution between the two structures.2 A linked-BINOL
ligand was employed by the same group for the preparation of
different rare earth catalysts and especially a stable La-linked
BINOL containing two binol units coordinated to lanthanum.
This complex was successfully employed for the catalysis of
asymmetric Michael reactions,18 yet a large variety of
heterobimetallic complexes REMB including three binaphth-
oxide ligands and three alkali metals per rare earth metal have
been isolated and used for the catalysis of a wide range of
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Table 4. Influence of the Ligand on the Aminolysis of Epoxide 5b
a
entry
ligand
BINOL
cat.
product
T (°C)
time (h)
conv (%)
yield (%)
ee (%)
1
2
3
4
5
6
7
8
4a
8a
4b
8b
4a
9
7bb
7bb
7bb
7bb
7ba
7ba
7ba
7bb
25
25
25
25
25
25
60
25
50
72
50
72
90
46
48
46
95
50
57
40
51
nd
30
27
31
38
76
24
80
66
50
56
46
64
3,3′-Br BINOL
BINOL
100
50
3,3′-Br BINOL
BINOL
86
6,6′-Br BINOL
6,6′-Br BINOL
6,6′-Br BINOL
33
9
100
90
9
a
Yield, reaction performed with 7 mol % of catalyst in the presence of lithium chloride and a ratio of 6/5 1.2 in CH2Cl2 or C2H4Cl2 according to
reaction temperature.
reactions.19 These very stable complexes can be obtained by
different routes, and aqua and anhydrous complexes showed
different structures and reactivity.20 Mechanistic studies have
been performed on aza-Michael reactions catalyzed by the
yttrium complex YLB to determine whether three or two
binaphthol units are involved in the active catalytic species.21
To address the question of the formation of complex YLB
during the preparation of complex 4 and of YLB acting as main
catalytic species we decided to prepare the YLB complex 10
(Figure 2) and to compare the NMR spectra and catalytic
properties of 4a and 10.
Different experimental procedures were followed for the
preparation of catalysts 4a and 4b without LiCl. Compound 4a
was obtained from isolated alkyl ate complex 3. However, 4b
was in situ prepared from SmCl3 and separated from LiCl after
the addition of binaphthol. The presence of LiCl in complex 4b
could be detected on the IR spectrum (large band at 300 cm−1
not observed for complex 4a). Elemental analyses for 4b are in
agreement with a structure in which two heterobimetallic
samarium and lithium units are coordinated to one LiCl
molecule such as [C40H24LiO4Sm(THF)2]2·LiCl. In spite of
numerous attempts, crystals of 4a or 4b suitable for X-ray
structure determinations could not be grown and the exact
structure of these complexes as well as that of the other
catalysts described could not be determined.24 Yet as suggested
for similar heterobimetallic complexes,23 an equilibrium
between several species depending on the solvent, the rare
earth, and the presence of LiCl is possibly occurring. The non-
monotonous variations of ee with temperature for 7bb (Table
1) and 7ca (Table 3) catalyzed by 4b support such an
equilibrium.
Only a few enantioselective catalysts have been reported for
the aminolysis of epoxides including O- or N-heterocycles.
These epoxides exhibit low reactivity toward ring-opening
reactions due to the complexation of the heteroatom to the
Lewis acid catalyst. Kobayashi has found enantiomeric excesses
over 80% in niobium-catalyzed reactions for both epoxides
studied in this work using aniline as the sole electrophile and a
substituted binaphthol ligand.8e Aminolysis of epoxides 5a and
5b by p- or o-anisidine which allows further deprotection of the
nitrogen has only been described previously in our samarium
iodobinaphtholate-catalyzed reactions.13 Heterobimetallic cata-
lysts 4a and 4b were revealed to be more active and
enantioselective than samarium iodobinaphtholate, especially
for the N-heterocycle-containing epoxide. To the best of our
knowledge, rare earth alkaline heterobimetallic complexes have
not been used for the catalysis of aminolysis of epoxides, but in
various asymmetric reactions with this family of catalysts it has
been shown that both metals are involved.16b,20 For aza-
Michael additions, in particular, a cooperative Lewis acid−
Lewis acid mechanism was suggested.19c Similarly for GaLB-
and Ti−Ga-salen-catalyzed desymmetrisation of meso epoxides
by thiols or selenols it has been proposed that metals act as two
different Lewis acids coordinating to the substrates.3,19b,25
Further studies are necessary to determine if both rare earth
and lithium are involved in the active species for the aminolysis
of epoxides catalyzed by complexes 4.
Figure 2. Structure of complex YLB 10.
YLB was obtained following the anhydrous route described
by Aspinall.16a The aminolysis of epoxide 5b by o-anisidine at
room temperature using 7 mol % complex 10 as catalyst yielded
β-amino alcohol 7bb with 43% yield and 18% ee. The
heterobimetallic complex YLB was far less enantioselective than
complex 4a prepared without or with LiCl (80% ee and 76% ee,
respectively), which indicated that it cannot be the major active
catalytic species in the reaction catalyzed by 4a. In addition,
conversely to 10 which is highly soluble in CD2Cl2 and C6D6,
1
complex 4a has a poor solubility in these solvents. The H and
13C NMR spectra of 4a in THF-d8 are well resolved and
different from the 1H and 13C NMR spectra of YLB 10 (see the
Supporting Informations). It thus appears that complex 10 is
not formed during the synthesis of complex 4a in THF.22
Shibasaki found that the addition of LiOTf led to a dramatic
increase of the reactivity and the enantioselectivity of aldol−
Tischenko reactions catalyzed by LLB. This was explained by a
structural change and formation of a binuclar complex
[La2Li4(binaphthoxide)5] complex which was characterized by
X-ray structure analysis.23 For the aminolysis catalyzed by
yttrium complex 4a the presence of LiCl had no effect on the
enantioselectivity of the reactions (Table 2, entries 1−3), while
with 4b increased enantiomeric excesses were observed with
catalysts containing LiCl (Table 2, entries 4, 5, and 7).
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Article
4a: C40H24LiO4Y·(THF)2; white powder; yield 74%; IR (CsI,
Nujol)/cm−1 ν 1615, 1590, 1500, 1423, 1337, 1270, 1245, 1142, 1071,
1018, 994, 958, 860, 813, 743, 667, 576, 486; H NMR (250 MHz,
C6D6) δ (ppm) 1.11 (s, 8 H, 4 CH2), 3.07 (s, 8 H, 4 CH2), 7.96−6.60
(m, 24 H, Ar). Anal. Calcd for C48H40LiO6Y (808.687): C, 71.29; H,
4.99. Found: C, 70.93; H, 5.38.
CONCLUSIONS
■
1
We have prepared a new family of complexes which are readily
obtained within short times from commercial reagents and
using tetrahydrofuran in small amount as the sole solvent. The
heterobimetallic complexes of lithium and yttrium or samarium
are stable for months if stored under argon. Both catalysts used
in situ are efficient for the formation of β-amino alcohols and
samarium reveals slightly more enantioselective. β-Amino
alcohols resulting from the ring-opening of 2,5-dihydrofuran
oxide and of N-tert-butyloxycarbonyl-3-pyrroline oxide respec-
tively with p- and o-anisidine have been isolated with
enantiomeric excesses over 80%. Catalytic reactions could be
realized under mild conditions using as low as 2 mol % catalyst.
The study of the catalytic activity of these new complexes for
various enantioselective desymmetrisation of epoxides as well as
for different reactions is currently in progress.
Preparation of 4b with LiCl Removal. In a glovebox, samarium
chloride (1 mmol, 256 mg) was suspended in a Schlenk tube in THF
(5 mL) and stirred at room temperature for 30 min. To this mixture
was added slowly a solution of LiCH2TMS (4 mmol, 376 mg) in THF
(2 mL). After 30 min of stirring, a solution of 1,1′-binaphthol (2 mmol,
572 mg) in THF (3 mL) was slowly added. After 30 min of stirring,
THF was evaporated and toluene (5 mL) added. The white precipitate
of lithium chloride was separated by centrifugation. Toluene was
evaporated under vacuum, and the catalyst was isolated as a powder
and stored in the glovebox.
4b: [C40H24LiO4Sm(THF)2]2·LiCl; yellow powder; yield 54%; IR
(CsI, Nujol)/cm−1 ν 1615, 1590, 1500, 1424, 1338, 1272, 1242, 1143,
1071, 1042, 990, 956, 859, 818, 743, 666, 632, 575, 523, 483, 389, 300;
1H NMR (250 MHz, C6D6) δ (ppm) 1.13 (s, 8 H, 4 CH2), 3.38 (s, 8
H, 4 CH2), 8.66−6.13 (m, 24 H, Ar). Anal. Calcd for
C96H80ClLi3O12Sm2(1782.677): C, 64.68; H, 4.52; Cl, 1.99; Li, 1.17.
Found: C, 62.55; H, 4.68; Cl, 1.92; Li, 1.21.
Preparation of Catalysts without LiCl Removal. In the glovebox,
rare earth (III) chloride (1 mmol) was suspended in a Schlenk tube in
THF (5 mL) and stirred at room temperature for 30 min before the
slow addition of a solution of LiCH2TMS (4 mmol, 376 mg) in THF
(2 mL). After 30 min of stirring, a solution of the ligand (2 mmol) in
THF (3 mL) was slowly added and the reaction mixture stirred for 30
min. THF was evaporated under vacuum, and the catalyst was isolated
as a powder and stored in the glovebox.
EXPERIMENTAL SECTION
■
All manipulations were carried out under argon atmosphere using
standard Schlenk or glovebox techniques. Tetrahydrofuran and C6D6
were distilled from benzophenone−sodium and degassed immediately
prior to use. Toluene, hexane, dichloromethane, and dichloroethane
were distilled from CaH2 and degassed immediately prior to use.
Catalysts 4a and 4b have been prepared from enantiopure (R)-1,1-
binaphthol. Ligands for catalysts 8a or 8b and 9 were prepared
according to the literature from enantiopure (R)-1,1-binaphthol: (R)-
3,3′-dibromo-1,1-binaphthol26 and (R)-6,6′-dibromo-1,1-binaphthol.27
Yttrium ate complex Y(CH2TMS)4Li(THF)4 was prepared as
described in literature.15 Yttrium heterobimetallic complex 10 was
prepared according to the Aspinall procedure.16a Epoxide 5a was
synthesized according to the literature procedure.28 The tert-butyl 6-
oxa-3-azabicyclo[3.1.0]hexane-3-carboxylate 5b was obtained by
reaction of commercially available 3-pyrroline with tert-butyl
dicarbonate followed by epoxidation.29 Epoxides and amines were
distilled and degassed or recrystallized prior to use.
[4a + 3LiCl]·(THF)6: white powder; yield 92%; 1H NMR (300
MHz, CDCl3) δ (ppm) 1.25 (s, 24 H, 12 CH2), 3.32 (s, 24 H, 12
CH2,), 5.78−7.75 (m, 24 H, Ar); 1H NMR (250 MHz, C6D6) δ (ppm)
1.32 (s, 24 H, 12 CH2), 3.43 (s, 24 H, 12 CH2), 6.37−8.57 (m, 24 H,
Ar); 1H NMR (300 MHz, THF-d8) δ (ppm) 6.87−7.04 (m, 12 H, Ar),
7.32−7.35 (m, 4 H, Ar), 7.66−7.73 (m, 8 H, Ar); 13C NMR (360
MHz, THF-d8) δ (ppm) 119.3, 119.9, 123.7, 125.8, 126.1, 126. 8,
127.3, 128.0, 135.2, 160.5.
1H and 13C NMR spectra were operating at 360, 300, and 250 MHz
1
for H and at 90.6, 75, and 62.5 MHz for 13C in CDCl3. Chemical
1
[4b + 3LiCl]·(THF)6: yellow powder; yield: 78%; H NMR (250
1
shifts for H and 13C spectra were referenced internally according to
MHz, CDCl3) δ (ppm) 1.26 (s, 24 H, 12 CH2), 3.41 (s, 24 H, 12
CH2), 6.72−7.74 (m, 24 H, Ar); 1H NMR (250 MHz, C6D6) δ (ppm)
1.35 (s, 24 H, 12 CH2), 3.53 (s, 24 H, 12 CH2), 5.83−8.91 (m, 24 H,
the residual solvent resonances and reported in ppm. Infrared spectra
were recorded as KBr disks or in Nujol using CsI cells and reported in
cm−1. Optical rotations were measured at room temperature in cell of
1 dm length at the sodium D line (λ = 589 nm) and were reported as
1
Ar); H NMR (300 MHz, THF-d8) δ (ppm) 6.85 (s br, 4 H, Ar),
7.16−7.36 (m, 12 H, Ar), 7.70 (d, 7.9 Hz, 4 H, Ar), 8.17 (d, 7.9 Hz, 4
H, Ar); 13C NMR (360 MHz, THF-d8) δ (ppm) 119.7, 124.2, 125.9,
126.8, 127.6, 128.1, 137.2, 169.3.
follows: [α]20 (c in g per 100 mL, CHCl3). HPLC analyses were
D
performed with an UV detector and chiral stationary-phase columns
(Chiralcel OD-H, Chiralpak IA). All of the crude products were
purified by preparative thin-layer chromatography on silica gel 60
PF254 (heptane/ethyl acetate 50/50 for 7aa, 7ba, 7bb and 90/10 for
7ca, 7cb). Enantiomeric excesses were determined as we previously
described.12,13
Preparation of Catalysts. Preparation of 4a from Y-
(CH2TMS)4Li(THF)4. In a glovebox, yttrium ate complex 3 (1 mmol,
660 mg) was dissolved in a Schlenk tube in THF (5 mL) and a
solution of 1,1′-binaphthol (2 mmol, 572 mg) in THF (1 mL) was
added slowly. After 30 min of stirring, THF was evaporated under
vacuum and the complex isolated as a powder and stored in the
glovebox.
Preparation of 4a with LiCl Removal. In a glovebox, yttrium
chloride (1 mmol, 195 mg) was suspended in a Schlenk tube in THF
(5 mL) and stirred at room temperature for 30 min. To this mixture
was added slowly 4 equiv of LiCH2TMS (4 mmol, 376 mg) in THF (2
mL). After 30 min of stirring, THF was evaporated and toluene (5
mL) added. The white precipitate of lithium chloride was separated by
centrifugation. A solution of 1,1′-binaphthol (2 mmol, 572 mg) in
THF (3 mL) was added to the supernatant, and the reaction mixture
was stirred at room temperature for 30 min. The solution was
evaporated under vacuum and the complex isolated as a powder and
stored in the glovebox.
[4c + 3LiCl]·(THF)4: white powder; yield 85%; 1H NMR (300 MHz,
CDCl3) δ (ppm) 1.56 (s,16 H, 8 CH2), 3.26 (s, 16 H, 8 CH2), 6.71−
1
7.93 (m, 24 H, Ar); H NMR (250 MHz, C6D6) δ (ppm) 1.19 (s, 16
H, 8 CH2), 3.19 (s, 16 H, 8 CH2,), 6.44−7.97 (m, 24 H, Ar); 1H NMR
(300 MHz, THF-d8) δ (ppm) 6.28−6.46 (m, 1 H, Ar), 6.77−7.27 (m,
15 H, Ar), 7.43−7.56 (m, 3H, Ar), 7.64−7.85 (m, 5 H, Ar); 13C NMR
(360 MHz, THF-d8) δ (ppm) 113.8, 118.5, 120.1, 122.4, 123.9, 124.2,
125.7, 125.8, 125.9, 127.6, 128.0, 129.4, 134.8, 153.8, 162.2.
Typical Procedure for the Aminolysis of Epoxides. In a
glovebox, catalyst 4 (0.035 mmol) and molecular sieves 4 Å (100 mg)
were weighed in a Schlenk tube and dichloromethane (5 mL) was
added. o-Anisidine 6b (74 mg, 0.6 mmol) was added to the solution,
which was stirred at room temperature for 15 min. Outside the
glovebox, the reaction was heated to the desired temperature, and a
solution of tert-butyl 6-oxa-3-azabicyclo[3.1.0]hexane-3-carboxylate 5b
(92.5 mg, 0.5 mmol) in dichloromethane (1 mL) was then added by
syringe. After being stirred for the specified time, the reaction mixture
was hydrolyzed with HCl 0.1 M, diluted with CH2Cl2, and neutralized
with NaOH 0.1 M. The aqueous layer was extracted with CH2Cl2.
After concentration, the product was purified by thin-layer
chromatography with hexane/ethyl acetate mixtures. The crude
product was purified by preparative thin-layer chromatography on
silica gel (heptane/EtOAc 50/50). The enantiomeric excesses of β-
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The Journal of Organic Chemistry
Article
amino alcohols were determined by HPLC analysis as described
below.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: (J.C.) jacqueline.collin@u-psud.fr; (S.B-L.) sophie.
bezzenine@u-psud.fr.
(1R,2R)-4-Oxa-2-(4-methoxyphenylamino)cyclopentanol 7aa:
mp 130−132 °C; [α]20 = +8.1 (c 1.0, CHCl3) for 66% ee; HRMS
D
(EI) calcd for C11H15O3N (M) 209.1046, found 209.1054; IR (CaF2,
CHCl3) (cm−1) ν 3684, 3622, 3021, 2977, 2930, 1364, 1216, 1046; 1H
NMR (360 MHz, CDCl3) δ 3.67 (1H, dd, J = 9.6 Hz, J = 2.5 Hz), 3.76
(3H, s), 3.73−3.79 (1H, m), 3.82−3.86 (1H, m), 4.04 (1H, dd, J =
11.3 Hz, J = 4.3 Hz), 4.24−4.29 (2H, m), 6.64 (2H, d, J = 9.0 Hz),
6.81 (2H, d, J = 8.8 Hz); 13C NMR (75 MHz, CDCl3) δ 56.0, 62.9,
72.8, 74.8, 76.3, 114.9, 115.3, 140.8, 152.9; HPLC (Chiralpak ADH
column, flow rate: 0.7 mL·min−1, hexane/ethanol 85/15, λ 254 nm,
tR(minor) = 26.3 min, tR(major) = 30.8 min).
ACKNOWLEDGMENTS
■
We thank CNRS for financial support and MENERS for Ph.D.
grants for M.M. and J.Y.
REFERENCES
■
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D
0.96, CHCl3) for 55% ee; IR (KBr) (cm−1) ν 3385, 3338, 2975, 2923,
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3.74−3.97 (2H, m), 4.24 (1H, bs), 6.61 (2H, d, J = 8.7 Hz), 6.79 (2H,
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(4) Chavarot, M.; Byrne, J. J.; Chavant, P. Y.; Vallee, Y. Tetrahedron:
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(3R,4R)-tert-Butyl 3-hydroxy-4-(2-methoxyphenylamino pyrroli-
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A. Eur. J. Inorg. Chem. 2007, 1159−1168. (c) Aillaud, I.; Collin, J.;
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dine-1-carboxylate 7bb: mp 123−126 °C; [α]20 = +22 (c 1.0,
D
CHCl3) for 76% ee; HRMS calcd for C16H24O4N2 (M) 308.1731,
found 308.1725; IR (KBr)/cm−1 ν 3478, 2945, 1682, 1601, 1513,
1
1413, 1233, 1161, 1120, 1024, 742; H NMR (300 MHz, CDCl3) δ
1.44 (9H, s), 3.26−3.41 (2H, m), 3.58−3.62 (1H, m), 3.73−3.88 (2H,
m), 3.79 (3H, s), 4.16−4.33 (1H, m), 6.63−6.84 (4H, m); 13C NMR
(62.5 MHz, CDCl3, rotamers) δ 28.6, 50.4, 50.8, 51.9, 52.2, 55.5, 58.9,
59.2, 73.2, 74.0, 80.0, 109.7, 110.4, 117.4, 121.4, 136.5, 146.9, 155.2.
HPLC (Chiralpak IA column, flow rate: 0.5 mL·min−1, hexane/2-
propanol: 85/15, λ 254 nm, tR(minor) = 12.2 min, tR(major) = 13.6
min).
(6) For various routes to amino alcohols, see: (a) O’Brien, P. Angew.
Chem. Ed. Engl. 1999, 38, 326−329. (b) Bergmeier, S. C. Tetrahedron
2000, 56, 2561−2576. (c) Ohkuma, T.; Ishii, D.; Takeno, H.; Noyori,
R. J. Am. Chem. Soc. 2000, 122, 6510−6511. (d) Adderley, N. J.;
Buchanan, D. J.; Dixon, D. J; Laine,
42, 4241−4244.
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Eur. J. Org. Chem. 2006, 4979−4988.
́
D. I. Angew Chem., Int. Ed. 2003,
(1R,2R)-2-(4-Methoxyphenylamino)cyclohexanol 7ca: oil;
HRMS calcd for C13H20NO2 (M + H+) 222.1489, found 222.1495;
IR (KBr) (cm−1) ν 3677, 3529, 3366, 3021, 3013, 2938, 2861, 2836,
1612, 1512, 1465, 1450, 1401, 1296, 1239, 1221, 1180, 1136, 1067,
(8) For binol/titanium catalysts, see: (a) Kureshy, R. I.; Singh, S.;
Khan, N. H.; Abdi, S. H. R.; Suresh, E.; Jasra, R. V. Eur. J. Chem 2006,
1303−1309. (b) Kureshy, R. I.; Singh, S.; Khan, N. H.; Abdi, S. H. R.;
Agrawal, S.; Jasra, R. V. Tetrahedron Lett. 2006, 47, 5277−5279.
(c) Hongly, B.; Zhou, J.; Zheng, W.; Yinlong, G.; Tianpa, Y.; Kuiing,
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catalysts, see: (d) Arai, K.; Salter, M. M.; Yamashita, Y.; Kobayashi, S.
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Salter, M. M.; Yamashita, Y.; Kobayashi, S. J. Am. Chem. Soc. 2007,
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1
1038; H NMR (250 MHz, CDCl3) δ (ppm) 0.85 −1.10 (m, 1H),
1.12−1.40 (m, 3H), 1.60−1.80 (m, 2H), 2.0−2.18 (m, 2H), 2.60 (bs,
1H), 2.92−3.04 (m, 1H), 3.24−3.55 (m, 1H), 3.73 (s, 3H), 6.66 (d,
2H, J = 8.8 Hz), 6.76 (d, 2H, J = 8.8 Hz); 13C NMR (62.5 MHz,
CDCl3) δ (ppm) 24.2, 25.0, 31.5, 33.0, 55.7, 61.6, 74.3, 114.8, 116.3,
141.5, 152.8; HPLC (Chiralcel OD-H column, flow rate: 1 mL·min,
hexane/2-propanol: 85/15, λ 254 nm, tR(minor) = 7.9 min, tR(major)
= 11.2 min.
(1R, 2R)-2-(2-Methoxyphenylamino)cyclohexanol 7cb: oil;
HRMS calcd for C13H20NO2 (M + H+) 222.1489, found 222.1496;
IR (KBr) (cm−1) ν 3616, 3429, 3067, 2964, 1602, 1511, 1456, 1430,
(9) For bipyridine/indium catalyst, see: (a) Mai, E.; Schneider, C.
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Org. Chem. 2009, 2863−2871.
1
1341, 1247, 1180, 1121, 1050, 1030, 977, 945; H NMR (250 MHz,
CDCl3) δ (ppm) 0.98−1.13 (m, 1H), 1.20−1.50 (m, 3H), 1.62−1.81
(m, 2H), 2.02−2.13 (m, 2H), 2.79 (bs, 1H), 3.08−3.18 (m, 1H),
3.35−3.45 (m, 1H), 3.83 (s, 3H), 6.64−6.82 (m, 4H); 13C NMR (62.5
MHz, CDCl3) δ (ppm) 24.3, 25.1, 31.6, 33.1, 55.4, 59.6, 74.6, 109.8,
111.4, 117.3, 121.3, 137.6, 147.5; HPLC (Chiralcel OD-H column,
flow rate: 1 mL·min, hexane/2-propanol: 85/15, λ 254 nm, tR(minor)
= 7.3 min, tR(major) = 16.8 min).
(11) For a titanium/binolate/water catalyst promoted the
enantioselective reaction of 4,4-dimethyl-3,5,8-trioxabicyclo[5.1.0]
octane with aliphatic amines, see ref 8c.
(12) Carree
(13) (a) Martin, M.; Bezzenine-Lafollee
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́
, F.; Gil, R.; Collin, J. Org. Lett. 2005, 7, 1023−1026.
ASSOCIATED CONTENT
■
́
, S.; Gil, R.; Collin, J.
S
* Supporting Information
́
NMR spectra of complexes 4a + LiCl (THF-d8), 4b + LiCl
(THF-d8), 4a + LiCl (THF-d8), 4b, and 4c and comparison
NMR between complex 10 and complex 4a + LiCl (THF-d8).
This material is available free of charge via the Internet at
http://pubs.acs.org.
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Article
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alternatively envisaged.
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characteristic of this lithium salt (137 ppm) was observed only when 3
equiv of bis-lithium binaphtholate was added. This supports the
structure proposed for 4a. Conversely, Shibasaki in a similar
experiment observed the signal characteristic of the lithium salt after
the addition of 1 equiv of bis-lithium binaphtholate to YLB.
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