Chiral silver amide catalyst for the [3+2] cycloaddition of α-amino esters to olefins

Article abstract of DOI:10.1002/anie.201008272

Silver lining: Highly exo-selective asymmetric [3+2]cycloaddition of α-amino ester Schiff bases with activated olefins proceeds in the presence of AgHMDS/1. The α-amino ester Schiff bases including those derived from aliphatic imines successfully reacted

Full text of DOI:10.1002/anie.201008272

DOI: 10.1002/anie.201008272
Asymmetric Catalysis
Chiral Silver Amide Catalyst for the [3+2] Cycloaddition of a-Amino
Esters to Olefins**
Yasuhiro Yamashita, Takaki Imaizumi, and Shu¯ Kobayashi*
The synthesis of highly substituted pyrrolidine derivatives is
very important in bioorganic and medicinal chemistry,
because they are useful building blocks for biologically
active molecules, and they sometimes show interesting
biological activity.[1] The asymmetric [3+2] cycloaddition of
a-amino ester Schiff bases with substituted olefins is one of
the most efficient methods for pyrrolidine preparation in an
optically pure form,[2] thus making it possible to introduce
various substituents on the pyrrolidine skeleton stereoselec-
tively. From the pioneering work of Grigg et al.,[3] chiral
catalyst systems based on a combination of chiral metal Lewis
acids and tertiary amine bases have been reported, and high
diastereo- and enantioselectivities have been attained in some
cases.[4–7] However, there are some problems that have yet to
be solved for this reaction: 1) addition of external amines is
needed in most cases; 2) major products of the cycloaddition
are endo, and there are few examples which provide exo
adducts selectively; 3) applicable substrates for this method
are still limited. In particular, successful examples using Schiff
bases derived from aliphatic aldehydes are very rare (see
below).
was found to be a very efficient catalyst for enanitioselective
[3+2] cycloaddition of a-amino ester Schiff bases with sub-
stituted olefins.
First, we investigated a [3+2] cycloaddition reaction of the
benzaldehyde Schiff base of glycine methyl ester (1a) with
methyl acrylate (2a) in the presence of a chiral complex
(10 mol%) prepared from AgHMDS and (R)-binap (L1) in
toluene at 308C. The desired reaction proceeded in 82% yield
with moderate enantioselectivity (Table 1, entry 1). In this
catalyst system, the endo adduct was obtained as the major
diastereomer (endo/exo = 72:18). In contrast, we investigated
other ligand structures and found that the use of segphos-type
ligands afforded the exo adduct preferentially (entries 3 and
4); in particular (R)-DTBM-segphos (L4) showed high
exo selectivity (endo/exo = 4:96) and good enantioselectivity
(84% ee, entry 4). Since typical chiral silver catalysts showed
endo selectivity, it is remarkable that the exo product was
Table 1: Asymmetric [3+2] cycloaddition reaction of glycine Schiff base
1a with methyl acrylate (2a).[a]
In a previous paper, we described that silver bis(trimeth-
ylsilyl)amide (AgHMDS; HMDS = hexamethyldisilazide)
when combined with a chiral ligand works well in the
enanitioselective [3+2] cycloaddition of a-aminophospho-
nate Schiff bases, which have less-acidic hydrogen atoms at
the a position, with substituted olefins.[8] High yields and
diastereoselectivities were attained without addition of exter-
nal bases. We thought that the key was the use of a basic silver
reagent, AgHMDS, which was indeed employed for the first
time in organic synthesis. In contrast, for the [3+2] cyclo-
addition of a-amino ester Schiff bases, which have more-
acidic hydrogen atoms at the a position, we thought that
AgHMDS might be too basic and thereby lead to low yields
and selectivities because of some undesired side reactions.
However, unexpectedly, a AgHMDS/chiral ligand complex
Entry Ligand Solvent t [h] Yield [%] endo/exo
ee [%]
endo exo
1[b,c]
2[b,c]
3[b,c]
4[b,c]
5[c]
6
L1
L2
L3
L4
L4
L4
L4
L4
L4
toluene
toluene
toluene
toluene
toluene
2
2
2
2
2
82
90
91
90
87
37
40
71
91
76
6
72:18
40:60
18:82
4:96
9:91
45
17
–
–
–
–
–
–
–
52
30
77
84
79
94
98
97
98
97
–
toluene 24
1:99
7
Et2O
Et2O
Et2O
Et2O
Et2O
24
24
24
48
72
<1:>99
<1:>99
1:99
<1:>99
>99:<1
8[d]
9[e]
10[e,f] L4
–
–
11[e,g]
–
[a] Asymmetric [3+2] cycloaddition of 1a with 2a at 0.2m at 08C in the
presence of 5 mol% of the chiral AgHMDS catalyst prepared from AgOTf
and KHMDS and the chiral phosphine ligand in situ unless otherwise
noted. [b] 10 mol% of AgHMDS and the ligand were used. [c] 308C.
[d] 0.4m. [e] 0.6m. [f] 1 mol% of the catalyst was used. [g] Used isolated
AgHMDS.
[*] Dr. Y. Yamashita, T. Imaizumi, Prof. Dr. S. Kobayashi
Department of Chemistry, School of Science
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)
Fax: (+81)3-5684-0634
E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp
[**] This work was partially supported by a Grant-in-Aid for Science
Research from the Japan Society for the Promotion of Science
(JSPS), the Global COE Program (Chemistry Innovation through
Cooperation of Science and Engineering), the University of Tokyo,
and theMEXT (Japan). We thank Takasago Int. Co. for supply of (R)-
DTBM-segphos.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201008272.
Angew. Chem. Int. Ed. 2011, 50, 4893 –4896
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4893

Communications
obtained selectively. With these promising results in hand, we
yields with excellent exo selectivities and over 90% enantio-
meric excesses (entries 3–5). The Schiff bases containing
heteroatoms in their aromatic rings also worked well without
significant loss in selectivity (entries 6 and 7). Bulky aromat-
ics, such as 1-naphthyl, did not affect the asymmetric
environment, and good to high exo selectivity and high
enantiomeric excess were observed (entry 8). The Schiff
base having an alkenyl substituent also reacted in high yield
with high diastereo- and enantioselectivities (entry 9). Nota-
bly, the Schiff bases prepared from other amino acid esters
worked well and the cycloadducts with quaternary carbon
centers were obtained with high stereoselectivities
(entries 10–12). Next, we tried to expand our methodology
to reactions of Schiff bases prepared from aliphatic aldehydes.
Usually, [3+2] cycloaddition reactions using aliphatic alde-
hyde Schiff bases are recognized to be very difficult. Aliphatic
imines, especially primary alkyl imines, are easily converted
into enamines in the presence of bases, which lead to
undesired side reactions such as self-condensation. In fact,
successful examples of asymmetric [3+2] cycloadditions of
aliphatic aldehyde Schiff bases of glycine esters are quite
limited, and high enantioselectivities were obtained only in
the cases of secondary alkyl aldehyde Schiff bases.[9] We
tested the reaction of the cyclohexanecarboxyaldehyde Schiff
base 1n and were delighted to find that the reaction
proceeded well in the presence of the AgHMDS catalyst to
afford the desired cycloadduct in good yield with high
enantioselectivity under the same reaction conditions for
the Schiff base prepared from benzaldehyde (entry 13). We
then tried to use the most challenging primary alkyl aldehyde
Schiff bases (entries 14–16) and found that the desired
reactions also proceeded well to give the desired adducts in
good yields with high enantioselectivities. It is noted that, to
the best of our knowledge, this is the first successful example
of asymmetric [3+2] cycloaddition of primary alkyl aldehyde
Schiff bases (see also Table 4).
optimized the reaction conditions so as to improve the
selectivity and the efficiency of the reaction. Even when the
reaction was conducted in the presence of 5 mol% of the
silver catalyst, the reaction proceeded smoothly in good yield
with a slight loss in the selectivity (entry 5). When the reaction
was conducted at 08C for 24 hours using 5 mol% of the Ag
catalyst, the desired product was obtained in only 37% yield
but with excellent exo selectivity and very high enantioselec-
tivity (entry 6). Several solvents were then examined, and it
was found that use of diethyl ether (Et2O) was the best as
excellent diastereo- and enantioselectivities [endo/exo =
< 1: > 99, 98% ee (exo)] were obtained (entry 7). Finally, we
found that the reactions run at higher concentrations were
effective in improving the yield without significant loss in the
selectivities (entries 8 and 9). Both a high yield and stereo-
selectivity were attained in the reaction run at 0.6m (entry 9).
Under these reaction conditions, 1 mol% of the catalyst
worked well and afforded the desired product (entry 10).
Interestingly, AgHMDS itself showed very low catalytic
activity in the absence of the phosphine ligand (entry 11).
With the optimal reaction conditions in hand, we exam-
ined the scope of the substrates (Table 2). First, several Schiff
bases of the glycine ester were tested in the reaction of methyl
acrylate (2a) in the presence of the Ag complex. The [3+2]
reactions using the Schiff base with electron-donating groups
on the aromatic ring gave almost identical results to the Schiff
base derived from benzaldehyde, and the desired products
were obtained in high yields with high stereoselectivities
(entries 1 and 2). Not only the Schiff bases having electron-
donating groups but also those having electron-withdrawing
groups reacted with 2a to afford the desired products in good
Table 2: Asymmetric [3+2] cycloaddition of 1 with olefin 2a.[a]
The current catalyst system was also successfully applied
to asymmetric [3+2] cycloadditions with other olefins 2
(Table 3). Methyl acrylate (2a) as well as acryl amides (2b
and 2c) reacted with 1a in high yields with high diastereo- and
enantioselectivities (entries 1 and 2). The reaction of methyl
vinyl ketone (2d) also gave the desired product with high
selectivity (entry 3). These results indicated that the coordi-
nation ability of the carbonyl oxygen atom of the olefins did
not affect asymmetric environment in the transition state.
Other olefins having electron-withdrawing groups were also
examined, and the olefins bearing sulfonyl (2e), phosphoryl
(2 f), and cyano (2g) groups reacted with 1a to afford the
corresponding pyrrolidine derivatives in high yields with high
selectivities (entries 4–6). Whereas a cis-disubstitued olefin,
dimethyl maleate (2h), was found to work well without any
significant decrease of selectivity (entry 7), a trans-disubsti-
tuted olefin, dimethyl fumarate (2i), reacted to afford the
desired product as a 1:1 diastereomer mixture with high
enantioselectivity (entry 8). Thus, the wide scope of both the
substrates, Schiff bases and olefins, using this chiral silver
amide system is remarkable. In addition, it is noteworthy that
the reactions proceeded smoothly without addition of any
external bases.
Entry R1
R2
1
3
Yield [%] exo/endo ee [%][b]
1
4-MeC6H4
H
H
H
H
H
H
H
H
H
1b 3ba 90
1c 3ca 93
1d 3da 82
1e 3ea 88
1 f 3 fa 98
1g 3ga 92
1h 3ha 90
1i 3ia 97
1j 3ja 90
>99:<1 99
>99:<1 98
>99:<1 95
2
3
4-MeOC6H4
4-FC6H4
4
5
6
7
4-BrC6H4
4-NCC6H4
3-pyridyl
2-furyl
97:3
92
>99:<1 96
>99:<1 90
>99:<1 94
8
1-naphthyl
94:6
99
=
9
(E)-PhCH CH
>99:<1 98
>99:<1 97
10
11
12
13
14
15
16
Ph
Ph
Ph
Me[c] 1k 3ka 78
iBu[c] 1l 3la 87
Bn[c] 1m 3ma 81
H
H
H
H
97:3
94:6
94
90
c-C6H11
(CH3)2CHCH2
PhCH2CH2
CH3(CH2)5
1n 3na 71
1o 3oa 64
1p 3pa 70
1q 3qa 62
>99:<1 97
>99:<1 88
>99:<1 92
>99:<1 82
[a] The reaction of 1 with 2a was conducted at 08C for 24 h at 0.6m in the
presence of the Ag catalyst (5 mol%) prepared from AgOTf and KHMDS
and L4 in situ. [b] Values are for the exo product. [c] An l-amino acid ester
was used.
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Angew. Chem. Int. Ed. 2011, 50, 4893 –4896

Table 3: Asymmetric [3+2] cycloaddition of 1a with olefins 2.[a]
aliphatic
imines,
successfully
reacted with several olefins to
afford the corresponding pyrroli-
dine derivatives in high yields with
high exo- and enantioselectivities.
Additional investigations into the
substrate scope and mechanistic
aspects are in progress.
Entry
2
R3
R4
R5
3
Yield exo/endo ee
[%]
[%][b]
1
2
3
4
5
6
2b
2c
2d
2e
2 f
2g
CO(NCH2CH2OCH2CH2-)
CONMe2
COMe
SO2Ph
P(O)(OEt)2
CN
H
H
H
H
H
H
H
H
H
H
H
H
H
3ab 92
3ac 97
3ad 82
3ae 83
3af 80
3ag 96
3ah 90
>99:<1 96
>99:<1 95
>99:<1 97
>99:<1 97
>99:<1 98
>99:<1 99
>99:<1 96
Received: December 30, 2010
Keywords: asymmetric synthesis ·
.
cycloadditions · heterocycles ·
homogeneous catalysis · silver
7[c]
8
2h[d] COOMe
COOMe
H
2i[e]
COOMe
COOMe 3ai
98
50:50
93 (90)[f]
[a] The reaction of 1a with 2 was conducted in the presence of the Ag catalyst (5 mol%) prepared from
AgOTf and KHMDS and L4 in situ at 08C for 24 h in 0.6m. [b] Values are for the exo product. [c] The Ag
catalyst was used at a 10 mol% loading. [d] Dimethyl maleate. [e] Dimethyl fumarate. [f] The ee value of
the endo product.
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Finally, the present catalyst system was compared with
other previously reported efficient catalyst systems (Table 4).
While the [3+2] cycloaddition of 1q with 2a proceeded
smoothly in the presence of the present AgHMDS system to
afford the desired product 3qa in good yield with good
enantioselectivity, other Ag catalyst systems (AgOTf + Et3N
or DBU, AgOAc with (R)-DTBM-segphos) did not work at
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Table 4: Reactivity of the silver catalysts in the reaction using 1q.[a]
Entry
Ag catalyst
Yield [%]
exo/endo
ee [%][b]
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1
2
3
4
AgHMDS
62
>99:<1
82
–
–
AgOTf + Et3N
AgOTf + DBU
AgOAc
n.r.
n.r.
n.r.
–
–
–
–
[a] The reaction of 1q with 2a was conducted in the presence of the Ag
catalyst (5 mol%) at 08C for 24 h at 0.6m. [b] Values are for the
exo product. n.r.=no reaction. DBU=1,8-diazabicyclo[5.4.0]undec-7-
ene.
In summary, we have demonstrated the successful asym-
metric [3+2] cycloaddition of a-aminoester Schiff bases with
several activated olefins. A silver complex prepared from
AgHMDS and a chiral phosphine ligand worked as an
efficient base catalyst without an additional tertiary amine.
Examination of the ligand structure has revealed that a large
and bulky bidentate phosphine ligand, (R)-DTBM-segphos,
was also effective for providing an asymmetric environment,
and a-amino ester Schiff bases, including those derived from
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Communications
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