Synthesis of N-phenyl β-amino acids via iridium-catalyzed asymmetric hydrogenation using mixed monodentate ligands

Article abstract of DOI:10.1016/j.tetasy.2010.11.026

The iridium-catalyzed asymmetric hydrogenation of N-phenyl-β- dehydroamino acid derivatives was examined using monodentate phosphoramidite ligands. The highest yields and enantioselectivities were obtained using a mixed ligand approach with PipPhos L1 and

Full text of DOI:10.1016/j.tetasy.2010.11.026

Tetrahedron: Asymmetry 22 (2011) 36–39
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Synthesis of N-phenyl b-amino acids via iridium-catalyzed asymmetric
hydrogenation using mixed monodentate ligands
Nataša Mrši c´ , Lavinia Panella , Adriaan J. Minnaard , Ben L. Feringa a, , Johannes G. de Vries a,b,
a
b
a,
⇑
⇑
⇑
a
University of Groningen, Stratingh Institute for Chemistry, Nijenborgh 4, 9747 AG Groningen, The Netherlands
DSM Innovative Synthesis BV, A Unit of DSM Pharma Chemicals, PO Box 18, 6160 MD Geleen, The Netherlands
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The iridium-catalyzed asymmetric hydrogenation of N-phenyl-b-dehydroamino acid derivatives was
examined using monodentate phosphoramidite ligands. The highest yields and enantioselectivities were
obtained using a mixed ligand approach with PipPhos L1 and achiral triphenylphosphine (full conversion,
Received 2 November 2010
Accepted 22 November 2010
Available online 25 January 2011
70% ee).
Ó 2010 Elsevier Ltd. All rights reserved.
1
. Introduction
Enantiomerically pure b-amino acids and their derivatives not
nation of acylated
compounds (N-aryl imines, quinolines, and quinoxalines).
N-Aryl b-amino acid derivatives are key structural elements of
many natural products and drug intermediates. One method to
prepare such compounds is to perform the asymmetric hydrogena-
tion of N-aryl b-enamino esters. Only a few examples of this reac-
a
-dehydroamino acid derivatives12 and of C@N
13
1
4
only exhibit broad biological activity but are also building blocks
for the synthesis of b-peptides, b-lactam antibiotics, and other chi-
ral pharmaceuticals. Peptides containing b-amino acids show high
stability toward enzymatic hydrolysis and are considered valuable
as promising pharmaceutical products. In addition, b-peptides
show interesting three-dimensional structures,2 and have played
an important role in advancing the understanding of enzyme
mechanisms, protein conformations, and properties related to
molecular recognition. As a result, the asymmetric synthesis of
b-amino acids has attracted significant attention.3
1
7
,15
tion have been reported in the literature.
Therefore, we decided
to examine the possibility of using Ir/phosphoramidite catalysts for
the enantioselective hydrogenation of N-arylated b-enamino
esters.
2
. Results and discussion
One of the most promising methodologies, also regarding an
industrial application, is the asymmetric hydrogenation of the
appropriate b-dehydroamino acid precursors catalyzed by homo-
Initial hydrogenation experiments were performed using
mol % of iridium precursor and 10 mol % of (S)-PipPhos L1 ligand,
5
at 5 bar of hydrogen pressure and room temperature, in dichloro-
methane (Table 1, entries 1, 2, and 8). In general rather low enanti-
oselectivities and conversions were obtained initially. The highest
ee was obtained in the hydrogenation of b-methyl N-phenyl ena-
mino ester 2, however, with low conversion (13% conversion,
4
geneous Rh or Ru complexes containing chiral phosphine ligands.
Whereas the asymmetric hydrogenation of acylated
a- and b-
dehydroamino acids is a standard method with many industrial
5
applications, the hydrogenation of unprotected b-dehydroamino
acids was developed much later. In recent years, several successful
metal catalysts for the highly enantioselective asymmetric hydro-
3
6% ee, entry 2). Only 8% ee was obtained in the hydrogenation
of b-phenyl substituted enamino ester 3 (46% conversion, entry 8).
Since reactions with Ir/PipPhos L1 at 5 bar of hydrogen pressure
gave modest conversions and ee’s, the following experiments were
performed at 25 bar of pressure. Various solvents as well as addi-
tives were screened in the asymmetric hydrogenation of b-methyl
N-phenyl enamino ester 2 and b-phenyl N-phenyl enamino ester 3.
Although in solvents, such as toluene, iso-propanol and THF, the
yield was often higher, the enantioselectivity was generally lower.
6
–9
genation of b-enamino acid derivatives have been reported.
A
study from the Merck/Solvias groups,8 using deuterium labeling
showed that the hydrogenation of unprotected b-dehydroamino
acid derivatives proceeds through the imine tautomer.
We have developed the use of monodentate phosphoramidite
ligands for the asymmetric hydrogenation of acylated
a- and b-
dehydroamino acids with excellent enantioselectivities.1
0,11
More
recently, we have reported that use of iridium complexes with
phosphoramidite ligands leads to excellent results in the hydroge-
The addition of I
2
led to a reduction in the enantioselectivity. The
reaction in DCM led to the highest enantioselectivity.
9
,16
17,18
Over the last decade, both Reetz et al.
and ourselves
have
⇑
Corresponding authors.
E-mail address: hans-jg.vries-de@dsm.com (Johannes G. de Vries).
shown that the use of mixtures of chiral monodentate ligands can
improve both the enantioselectivity and the reactivity in asymmetric
0
957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.11.026

N. Mrši ´c et al. / Tetrahedron: Asymmetry 22 (2011) 36–39
37
Table 1
Asymmetric hydrogenation of N-phenyl b-enamino esters using [Ir(COD)
]BArF/(S)-PipPhos L1a
2
5
1
mol% [Ir(COD)2]BArF
0 mol% (S)-PipPhos
NH
O
NH
*
O
R2
R2
R1
O
H2, rt, CH2Cl2, 16h
R1
O
1a-3a
1
-3
1
2
1
a, R = Me, R = Me
O
1
2
P
N
2a, R = Me, R = Et
O
3
a, R = Ph, R =Et
1
2
(
S)-PipPhos L1
Entry
1
Prod.
1a
Solvent
DCM
P (bar)
5
Conv.b (%)
21
eec (%)
20
2
3
4
5
6
2a
DCM
DCM
Toluene
IPA
THF
THF
5
25
25
25
13
45
73
98
99
36
25
16
5
2
3
d
7
25
100
8
9
1
1
1
1
3a
DCM
DCM
Toluene
IPA
THF
THF
5
25
25
25
25
25
46
34
47
48
73
43
8
20
17
9
20
0
0
1
2
3
d
a
b
c
Reaction conditions: 100
Conversion was determined by GC.
Enantiomeric excess was determined by HPLC.
l
mol enamino ester, 5
lmol [Ir(COD)
2
]BArF, 10
2 2 2
lmol (S)-PipPhos, 2.55 mL of solvent, CH Cl , 5 or 25 bar H , 16 h.
d
1
2
0 mol % I .
Table 2
Asymmetric hydrogenation of enamines using Ir catalysts with mixed ligandsa
hydrogenation reactions. It is also possible to use mixed complexes
based on a monodentate chiral ligand and a non-chiral phosphorus
ligand. Therefore, we decided to examine the possibility of using a
mixture of phosphoramidites with achiral P-ligands or amines.
5
mol% [Ir(COD)2]BArF
S)-PipPhos
Reactions were performed using 5 mol % of [Ir(COD)
2
]BArF and
NH
O
NH
*
O
(
R2
(S)-PipPhos L1 as a ligand, at 25 bar of hydrogen pressure and room
R2
R1
O
R1
achiral ligand
5 bar H2, rt, CH2Cl2, 16h
O
temperature. When an achiral phosphine was used as a second
ligand, the ratio between PipPhos L1 and achiral ligand was 2/1.
2
2, 3
2a, 3a
3 2
This was to prevent the formation of [Rh(PPh ) (COD)] as this com-
2
3
a, R1 = Me, R = Et
2
plex is capable of very fast hydrogenation and leads to a racemic
product. In the reactions where an amine was used as the second
ligand, the ratio of ligands was PipPhos L1/amine = 1:1, as in the
case of Crabtree’s catalyst.19 The results of the hydrogenation of
substrates 2 and 3 are presented in Table 2.
1
2
a, R = Ph, R = Et
Conv.b (%)
eec (%)
Entry
Prod.
Achiral ligand
—
Ir/LÃ/L
1/2/0
1/1/1
1
2
2a
45
100
25
0
Et
3
N
The addition of amine ligands, (S,S)-2-phenyl-1-(1-phenyl-
ethyl)-propylamine or triethylamine (entries 2, 3, 7, and 8)
resulted in excellent conversions, but no enantioselectivity was
observed with either substrate; When achiral phosphines were
added in combination with (chiral) PipPhos, the highest ee was
achieved using triphenylphosphine in the hydrogenation of both
3
1/1/1
94
0
Ph
PPh
N
H
Ph
4
5
6
7
3
1/2/1
1/2/1
1/2/0
1/1/1
100
48
34
65
16
20
2
(2-MeC
—
6
H
4
)
3
P
3a
Et
3
N
100
2
and 3 (up to 65% ee, entries 4 and 9). In the case of substrate 3,
the conversion was somewhat lower (54%, entry 9). Since the Pip-
Phos/PPh mixture induced the highest enantioselectivity in the
8
9
1/1/1
100
0
Ph
PPh
N
H
Ph
3
hydrogenation of N-phenyl b-enamino esters, we decided to per-
form this reaction at a higher concentration (1 mmol scale, 4 mL
of solvent) and lower catalyst loading, on substrate 1. Various achi-
ral P-ligands were tested in combination with PipPhos L1. The re-
sults are shown in Table 3.
3
1/2/1
1/2/1
54
15
45
5
10
(2-MeC
6
H
)
4 3
P
a
Reaction conditions: 100
PipPhos L1, 2.55 mL of CH Cl
Conversion was determined by GC.
l
2
mol substrate, 5
2
lmol [Ir(COD) ]BArF, 10 lmol (S)-
2
rt, 25 bar H , 16 h.
2
b
c
Enantiomeric excess was determined by HPLC.
Reactions were performed at 25 bar of hydrogen pressure and
2
room temperature, using 1 mol % of [Ir(COD) ]BArF, 2 mol % of Pip-
Phos L1 and 1 mol % of achiral ligand, in dichloromethane. The best
result was again obtained using triphenylphosphine in combina-
tion with PipPhos L1, providing full conversion and 70% ee (entry
1). With the use of tri-o-tolylphosphine L3 no conversion was

3
8
N. Mrši ´c et al. / Tetrahedron: Asymmetry 22 (2011) 36–39
Table 3
Achiral ligands screened in the asymmetric hydrogenation of 1
Table 4
a
Screening of phosphoramidite ligands in the asymmetric hydrogenation of 1a
1
2
1
mol% [Ir(COD)2]BArF
mol% PipPhos
1 mol% [Ir(COD)2]BArF
2 mol% L*
mol% Non-chiral ligand
1 mol% PPh3
NH
O
NH
*
O
NH
O
NH
*
O
2
5 bar H2, 16h
2
5 bar H2
O
O
O
O
CH2Cl2, rt,
CH2Cl2, rt, 16h
1
1a
1
1a
Entry
Achiral ligand
PPh
Conv.b (%)
eec (%)
1
2
3
4
5
6
7
8
9
3
L2
L3
L4
L4
L5
L6
L7
L8
L9
L10
100
0
100
5
100
29
6
100
42
26
70
—
52
—
63
42
nd
47
40
38
O
O
P
N
O
(2-MeC
(3-MeC
6
H
H
4
)
)
3
P
P
P
N
6
4
3
O
(2,4,6-MeC
(4-MeOC
(Naphth-1-yl)
(t-Bu)
(MeO)
Ph PO
(Me N)
6 2 3
H ) P
6
H )
4 3
P
P
3
3
P
L12
L13
3
P
3
1
0
2
3
PO
Ph
N
a
Ph
N
Reaction conditions: 1 mmol 1, 0.01 mmol [Ir(COD)
2
]BArF, 0.02 mmol (S)-Pip-
, 20 h.
O
Phos L1, 0.01 mmol achiral ligand, 4 mL of DCM, rt, 25 bar H
2
O
b
c
1
P
P
Conversion was determined by H NMR.
O
O
Enantiomeric excess was determined by HPLC.
Ph
Ph
obtained, while the use of other phosphines with a substituent at
the ortho-position also led to low conversions (L4 and L6, entries
(
S,R,R)-L14
(S,S,S)-L15
Conv.b (%)
4
and 6). Use of the bulky phosphine L7 led to low conversion
6%, entry 7). Full conversions and ee’s up to 63% were achieved
using phosphines with substituents at the meta- or para-positions
entries 3 and 5). These results suggest that o-substituted achiral
eec (%)
Entry
Ligand
PPh
3
(
1
2
3
4
5
6
7
8
L12
L12
L13
L13
L14
L14
L15
L15
À
+
19
7
4
6
nd
nd
46
10
0
(
À
phosphines as well as the bulky phosphine L7 are perhaps too
sterically demanding for coordination to the iridium together with
the PipPhos L1 ligand. When trimethylphosphite L8 was used with
PipPhos L1, full conversion was accomplished (47% ee), whereas
with addition of triphenylphosphine oxide L9 only up to 42%
conversion and 40% ee was achieved; the addition of HMPA only
led to a slight improvement (entries 8–10).
In addition to the achiral ligand screening, we examined the use
of four different phosphoramidite ligands in combination with tri-
phenylphosphine L2 in the hydrogenation of 1. The reactions were
performed using 1 mol % of iridium precursor, 2 mol % of phospho-
ramidite ligand, and 1 mol % of triphenylphosphine, at 25 bar of
hydrogen pressure and room temperature, in dichloromethane.
The results are presented in Table 4. Unfortunately, all the ligands
employed induced low conversions. The highest enantioselectivity
accompanied by very low conversion was obtained using phosphor-
amidite L13 in combination with triphenylphosphine (entry 4, 8%
conversion, 46% ee).
+
8
À
+
11
11
20
20
À
3
6
+
a
2
Reaction conditions: 1 mmol 1, 0.01 mmol [Ir(COD) ]BArF, 0.02 mmol L*,
0
.01 mmol PPh
3
, 4 mL of DCM, rt, 25 bar H
2
, 16 h.
b
c
Conversion was determined by 1H NMR.
Enantiomeric excess was determined by HPLC.
[
2
Ir(COD) ]BArF was obtained from Umicore and used as such.
NMR spectra were obtained on Varian Gemini-200 and Varian
AMX400 spectrometers. GC analysis was carried out on HP6890
using a flame ionization detector, while HPLC analysis was per-
formed on Shimadzu LC-10ADVP HPLC equipped with a Shimadzu
SPD-M10AVP diode array detector. The enantiomeric excess was
determined by HPLC with chiral columns (Chiralcel OD and AS-
H), in comparison with racemic products. Racemic products were
prepared by the Pd/C catalyzed hydrogenation of enamino-esters.
High resolution mass spectra were recorded on an AEI-MS-902
mass spectrometer.
3
. Conclusion
In conclusion, we have examined various in situ prepared irid-
ium catalysts in the hydrogenation of b-dehydroamino acid deriv-
atives. The highest enantioselectivity was obtained in a mixed
ligand approach using a mixture of the chiral phosphoramidite li-
gand PipPhos L1 and achiral triphenylphosphine L2 (full conver-
sion, 70% ee).
Reactions were performed in a stainless steal autoclave contain-
ing seven glass vessels (8 mL volume). These vessels were closed
with septum caps. Magnetic stir bars were placed inside of each
vessel and the needles were placed through the septa in order to
enable entrance of hydrogen. Vessels were filled under air and then
flushed with nitrogen before hydrogen pressure was applied.
2
0
10c
21
20
20
4
4
. Experimental
Ligands L1, L12,
L13, L14, and L15 were prepared
according to literature procedures.
.1. General remarks
Acknowledgment
All solvents were reagent grade and were dried and distilled, if
necessary, following standard procedures. Reagents were pur-
chased from Aldrich, Acros, Merck, or Fluka and used as received.
We would like to thank UMICORE for a generous gift of
[Ir(COD) ]BArF.
2

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2
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