Highly enantioselective construction of the α-chiral center of amides via iridium-catalyzed hydrogenation of α,β-unsaturated amides

Article abstract of DOI:10.1002/adsc.200900080

The chiral center at the a-position of amides is installed in excellent enantioselectivity via the iridium-catalyzed asymmetric hydrogenation of αβ-unsaturated amides under mild conditions. Even aliphatic amides are suitable substrates. The presence of a

Full text of DOI:10.1002/adsc.200900080

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DOI: 10.1002/adsc.200900080
Highly Enantioselective Construction of the a-Chiral Center of
Amides via Iridium-Catalyzed Hydrogenation of a,b-Unsaturated
Amides
Wei-Jing Lua and Xue-Long Houa,*
a
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 354 Fenglin Road, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-5492-5100; e-mail: xlhou@mail.sioc.ac.cn
Received: February 4, 2009; Published online: April 16, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900080.
Abstract: The chiral center at the a-position of
amides is installed in excellent enantioselectivity via
the iridium-catalyzed asymmetric hydrogenation of
a,b-unsaturated amides under mild conditions.
Even aliphatic amides are suitable substrates. The
presence of a hydrogen atom on the nitrogen of the
amide is important for the enantioselectivity of the
reaction.
hydrogenation reaction, in particular, the a-chiral
center of carbonyl compounds has been successfully
established via Ir-catalyzed hydrogenation of a,b-un-
saturated ketones as well as carboxylic acids.[8,9] How-
ever, only lower enantioselectivities were obtained
when a,b-unsaturated amides were used.[10] Recently,
Bolm[8b] and we[9] realized the Ir-catalyzed hydrogena-
tion of a,b-unsaturated ketones in high enantioselec-
tivity. Based upon the results, we investigated the hy-
drogenation of a,b-unsaturated amides. In this com-
munication, we would like to report our preliminary
results on the asymmetric hydrogenation of a,b-unsa-
turated amides with aromatic and/or aliphatic sub-
stituents at the a- and b-positions using chiral Ir com-
plexes; a wide range of amides with an a-carbon
Keywords: amides; enantioselectivity; hydrogena-
tion; iridium; P,N ligands
Amides with an a-chiral center are an important class chiral center in high enantioselectivity were obtained.
of compounds because not only are they widely pres- The importance of the presence of hydrogen on the
ent as important subunit in natural products[1] but nitrogen of the amide group is also demonstrated.
also they can be transformed into many other useful
Initially, the asymmetric hydrogenation of a,b-unsa-
compounds such as ketones, amines, etc.[2] Many pro- turated amide 1a was carried out by using 2 mol% of
cedures have been developed to install a chiral center 3a as catalyst in dichloromethane (DCM) under
at the a-position of amides via alkylation, however, 50 bar of H2 pressure at room temperature [Eq. (1)].
chiral auxiliaries have been used in most cases.[2,3]
A
Chiral amide 2a was obtained with completed conver-
catalytic version of the enantioselective alkylation of sion but the ee value was only 21%.
amides with an a-chiral center has only been realized
To improve the enantioselectivity, amides 1 with
for a few examples,[4] one of the reasons is the racemi- different substituents on nitrogen were tested
zation of the chiral carbon via enolization in strong (Table 1). The results showed that all reactions pro-
basic conditions. A great challenge remains regarding
the effective synthesis of chiral amides enantioselec-
tively. One approach to address this challenge is the
reaction under neutral conditions. In this regard, hy-
drogenation seems to be a good choice. The transition
metal-catalyzed asymmetric hydrogenation as a prac-
tical protocol has attracted much attention in recent
years.[5] Among them, chiral iridium complexes have
shown their unique properties both in asymmetric in-
duction and catalytic activity.[6–9] A variety of com-
pounds have been used as substrates in Ir-catalyzed
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Highly Enantioselective Construction of the a-Chiral Center of Amides
COMMUNICATIONS
Table 1. The asymmetric hydrogenation of amides 1 with dif-
ferent substituents on N catalyzed by Ir complex 3a.[a]
Table 2. The influence of ligands with different structure in
the asymmetric hydrogenation of unsaturated amide 1e.[a]
Entry
1, R1, R2
Conv. [%][b]
ee [%][c]
Entry
Catalyst
Conv. [%][b]
ee [%][c]
1
2
3
4
5
6
7
a, -(CH2CH2)2O
b, Et, Et
c, Ph, H
d, n-Bu, H
e, i-Bu, H
f, Bn, H
g, CH2CH2OMe, H
100
100
100
100
100
100
100
21
30
54
42
55
49
45
1
2
3
4
3a
3b
4a
4b
5a
5b
5b
5c
5d
5e
5e
5f
5f
100
100
100
100
100
100
100
100
100
100
94
55
70
81
79
79
93
90
62
5
6
7[d]
8
[a]
Reaction conditions: 1 (0.1 mmol), catalyst (2 mol%), H2
(50 bar), DCM 2 mL. The reaction mixture was stirred at
room temperature for 24 h.
9
62
10[d]
11
12[d]
13
93
86[e]
91
[b]
[c]
1
Determined by H NMR.
100
41
Determined by chiral HPLC.
Nd.[e,f]
[a]
Reaction conditions: 1e (0.1 mmol), catalyst (2 mol%),
H2 (50 bar) DCM 2 mL. The reaction mixture was stirred
at room temperature for 24 h.
ceeded to complete conversion. Better enantioselec-
tivities were realized when one substituent on nitro-
gen was H in spite of the other substituent being aro-
matic (entry 3) and aliphatic groups having different
steric hindrance (entries 4–7) while amides 1a and 1b
with two substituents on the nitrogen gave the prod-
ucts in lower ee (entries 1 and 2).
[b]
[c]
[d]
[e]
[f]
1
Determined by H NMR.
Determined by chiral HPLC.
0.6 mol% of catalyst was used.
0.1 mol% of catalyst was used.
Not determined.
Based upon these results, the influence of different
type of ligands on the enantioselectivity were investi-
gated using amide 1e as substrate (Figure 1, Table 2). furnished product 2e in 79% ee (entry 5), while 5c
The catalysts 4 derived from PHOX ligands[7b,11] pro- and 5d with Ph and Bn as substituents on the oxazo-
vided product in better ee than the catalysts 3 derived line ring provided chiral amide 2e in 62% ee (entries 8
from benzylic-substituted ligands developed by our and 9). The electronic property of the substituent on
group[12] (entries 3, 4 vs. entries 1, 2). Interestingly, the the phenyl ring on the P atom has only a slight effect
catalysts 5 derived from Fc-PHOX[13] gave even on the enantioselectivity. Catalysts 5e and 5f with
better results, among which 5b and 5e with a t-Bu MeO and CF3 at the 4-position of phenyl ring, respec-
group on the oxazoline ring and phenyl and 4-me- tively, provided products with close ees (entry 10 vs.
thoxyphenyl substitutents on P the atom, respectively, entry 12). The studies on the effect of common sol-
provided the product in 93% ee, the highest value vents (toluene, CHCl3, THF, AcOEt, CH2Cl2) using
among the catalysts screened (entries 6 and 10). Cata- 1e as substrate and 5b as catalyst under the conditions
lyst 5a with an i-Pr substituent on the oxazoline ring of Eq. (1) revealed that toluene (93% ee), AcOEt
(92% ee) and CH2Cl2 (93% ee) are suitable solvents
(not showed in the Table 2).
Under the above reaction conditions, different sub-
strates were subjected to the hydrogenation reaction
using 5b as catalyst [Eq. (2), Table 3]. All amides 1c–
p with different aromatic substituents at the b-posi-
Figure 1. The structures of the catalysts.
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Wei-Jing Lu and Xue-Long Hou
COMMUNICATIONS
Table 3. Asymmetric hydrogenation of a,b-unsaturated
amide 1 catalyzed by 5b.[a]
gen provided products with the same configuration at
the a-position of the amide [Eq. (3)].
Entry
Substrate
[Ir] (mol%)
Conv. [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
1s
1t
2
2
2
2
2
2
2
2
2
3
2
2
2
2
2
2
2
2
2
2
2
3
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
99
96
84
93
95
90
96
97
87
98
96
97
96
95
95
93
84
9
10
11
12
13
14
15
16
17
18
19
20
21
22
The utilities of amides with an a-chiral center ob-
tained from hydrogenation are demonstrated by the
transformation of 2c and 2u into chiral amine 7a and
chiral ketone 7b in high yields and with no decrease
of the ee values [Eq. (4)].
87
95
75
1u
1c
1t
97[d]
Nd.[d,e]
98[d]
85
100
1t
[a]
Reaction conditions: 1 (0.1 mmol), catalyst (2 mol%), H2
(50 bar), solvent 2 mL at room temperature. The reaction
mixture was stirred for 24 h to assure the complete con-
version of 1.
[b]
[c]
[d]
[e]
1
Determined by H NMR.
Determined by chiral HPLC.
The hydrogenation pressure is 1 atm.
Not determined.
tion and with different aryl and alkyl or even hetero-
The absolute configuration of a-carbon of amides
ACHTUNGTRENNUNG
atom substituents at the a-position provided the cor- 2t,[14a] 2v and 2w[14b] was determined as (S) by compar-
responding amides with an a-chiral center in com- ison of their analytical data with those of authentic
plete conversion and in high enantioselectivities (en- samples reported in the literature.[14]
tries 1–14). It should be noted that amides with alkyl
In summary, the asymmetric hydrogenation of a,b-
substituents at both the a- and b-positions are also unsaturated amides has been realized by using a
suitable substrates to afford chiral aliphatic amides in chiral Ir catalyst, providing amides with an a-chiral
complete conversion and in high ee (entries 15–19) center in high enantioselectivity. Even aliphatic
while worse results were obtained by using the corre- amides are suitable substrates. The presence of hydro-
sponding aliphatic a,b-unsaturated ketones as substra- gen on the nitrogen of the amide is important for the
tes.[8b,9]
enantioselectivity of the reaction. The usefulness of
The hydrogenation of amides 1c and 1t also pro- these chiral amides with an a-chiral center has also
ceeded smoothly under ambient H2 pressure, provid- been demonstrated. Investigation on the details of the
ing the products in slightly increased ee (entries 20 reaction and the role of the H atom on the nitrogen
and 22 vs. entries 1 and 18).
atom of the amide are in progress.
The hydrogenation can also be carried out on a
gram-scale. When 1.314 g of 1t (7.5 mmol) were hy-
drogenated with 0.2 mol% 5b as catalyst under 50 bar
hydrogen pressure at room temperature for 24 h,
1.276 g (96% yield) of 2t were produced in 94% ee.
The stereochemistry of the substituent on nitrogen
has little effect on the selectivity of the reaction. Hy-
drogenation of the amides 1v and 1w having chiral
Experimental Section
General Procedure for the Asymmetric
Hydrogenation
A
mixture of catalyst 5b (0.002 mmol), substrate 1
substituents with different configurations on the nitro- (0.1 mmol), and DCM (2 mL) in a tube containing a mag-
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Highly Enantioselective Construction of the a-Chiral Center of Amides
COMMUNICATIONS
netic stir bar was placed in an autoclave under air and the
autoclave was sealed. The autoclave was pressurized to
50 bar with H2 and the solution was stirred at room temper-
ature for 24 h. The pressure was then carefully released and
the reaction mixture was passed through a short column of
silica gel with ethyl acetate/petroleum ether. The solution
was then concentrated and the resulting oil or solid was ana-
lyzed.
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Acknowledgements
This work was financially supported by the Major Basic Re-
search Development Program (2006CB806106), National
Natural Science Foundation of China (20532050, 20672130,
20821002), Chinese Academy of Sciences, Croucher Founda-
tion of Hong Kong, and Science and Technology Commis-
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