Iridium-catalysed asymmetric hydrogenation of enamides in the presence of 3,3-substituted h8-phosphoramidites

Article abstract of DOI:10.1002/adsc.200800799

In situ generated iridium complexes containing bulky 3,3′-substituted H8-phosphoramidites as ligands catalyse the asymmetric hydrogenation of non-functionalised enamides in high yield and good to excellent selectivity.

Full text of DOI:10.1002/adsc.200800799

UPDATES
DOI: 10.1002/adsc.200800799
Iridium-Catalysed Asymmetric Hydrogenation of Enamides in the
Presence of 3,3’-Substituted H8-Phosphoramidites
a
b
c
c
Giulia Erre, Stephan Enthaler, Kathrin Junge, Daniele Addis,
c,
and Matthias Beller *
CAT, Catalytic Center ITMC, RWTH-Aachen, Worringerweg 1, 52074 Aachen, Germany
UniCat, Technische Universitꢀt Berlin, Department of Chemistry, Straße des 17. Juni 135, 10623 Berlin, Germany
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢀt Rostock, Albert-Einstein-Str. 29a, 18059 Rostock, Germany
Fax : (+49)-(0)381-128-151113; phone: (+49)-(0)381-128-1113; e-mail: matthias.beller@catalysis.de
a
b
c
Received: December 21, 2008; Revised: April 6, 2009; Published online: May 28, 2009
[
5]
Abstract: In situ generated iridium complexes con-
taining bulky 3,3’-substituted H8-phosphoramidites
as ligands catalyse the asymmetric hydrogenation of
non-functionalised enamides in high yield and good
to excellent selectivity.
experience in the hydrogenation of aryl-enamides
and their attractive potential to form chiral amines,
they were selected as subject of our further investiga-
[
6]
tions. Since the original work of Kagan, a number of
studies on the Rh- or Ru-catalysed asymmetric hydro-
genation of arylenamides has been published. While
in the beginning research mainly has been focused on
the design of chelating phosphorus ligands like
Keywords: asymmetric catalysis; enamides; hydro-
genation; iridium; phosphoramidite ligands
[
6,7]
Me-DuPhos, Me-BPE, DIOP, BINAP, and others
an increasing number of monodentate P ligands was
[
5,8]
successfully applied in this area.
In this work, we report on the successful use of ster-
ically demanding phosphoramidite ligands 1 in the iri-
dium-mediated hydrogenation of several aryl- and
Introduction
Asymmetric hydrogenation constitutes an important alkyl-enamides. A thorough study on the influence of
method for the economic and environmentally benign additives to the catalytic system has been conducted
formation of chiral centres. In the last decades numer- with remarkable consequences on the activity and
ous catalytic systems based on precious metal com- enantioselectivity of the system.
plexes have been developed for the effective and
highly enantioselective hydrogenation of various
olefin classes. Typically, Rh and Ru complexes are Results and Discussion
used, which require coordinating groups, such as car-
bonyl, adjacent to the double bonds to direct their ac- So far only two studies on the hydrogenation of
[9]
tivity and to achieve high enantioselectivity. In con- simple enamides by iridium catalysts are known. At
trast to that, mainly Pfaltz and co-workers have best moderate enantioselectivities of up to 60% ee
shown in their elegant work that Ir catalysts can be have been reported for an iridium complex containing
[
10]
used successfully in the hydrogenation of non-func- a mixed phosphine olefin ligand.
[
1]
tionalised olefins. More recently, also examples of
selective and active catalysts based on iridium have nylvinyl)acetamide 2a in the presence of phosphor-
been used in the hydrogenation of acetamido esters. amidites 1 was chosen (Scheme 1). The catalytic ex-
As model reaction, the hydrogenation of N-(1-phe-
AHCTUNGTRENNUNG
[
2]
In detail, de Vries et al. described the use of iridium
complexes with bulky phosphoramidites to hydroge-
ACHTUNGTRENNUNGn ate acetamidocinnamate with up to 98% ee. Soon
after, our group used novel 3,3’-disubstituted H8-bi-
naphthophosphoramidites which achieved excellent
results in the hydrogenation of a-and b-amino acid
[3,4]
precursors.
of the new catalytic system, further investigations on
In order to demonstrate the relevance
a more broad scope of functionalised alkenes, such as Scheme 1. Monodentate 3,3’-substituted phosphoramidites 1
b-amino acid precursors, were performed. Given our based on H8-binaphthol.
Adv. Synth. Catal. 2009, 351, 1437 – 1441
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1437

Giulia Erre et al.
UPDATES
[a]
[a]
Table 1. Hydrogenation of N-(1-phenylvinyl)acetamide 2a.
Table 2. Effect of the addition of anionic salts (NaX).
Entry
Additive
Conv. [%]
ee [%] (S)
1
2
3
4
5
6
7
8
NaCOOH
Na SO
SDS
>99
>99
>99
90
>99
>99
>99
65
90
88
87
2
91
93
92
68
2
4
I2
Entry
Ligand
Conv. [%]
ee [%]
NaBF4
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
1g
14
27
>99
85
>99
60
>99
–
NaClO
NaPF
NaBAr
4
18 (S)
84 (S)
56 (S)
78 (S)
44 (S)
82 (S)
6
F
[a]
Reactions were carried out at 108C under 10 bar pressure
of hydrogen for 16 h with 2.5 mmol [Ir(cod)Cl] , 5 mmol
ligand 1c, 5 mmol NaX or I and 0.25 mmol substrate in
.0 mL toluene. NaBAr =sodium tetrakis AHCTUNGTERNNUNG[ 3,5-bis(tri-
fluoromethyl)phenyl]borate, SDS=sodium dodecyl sul-
AHCTUNGTRENNUNG
2
2
2
F
[
a]
All reactions were carried out at 108C under 10 bar pres-
sure of hydrogen for 16 h with 2.5 mmol [Ir(cod)Cl] , 5
mmol ligand and 0.25 mmol substrate in 2.0 mL toluene.
A
H
N
T
E
N
N
fate.
2
often advantageous. Several anionic salts were tested
periments were performed in an 8-fold parallel reac- such as NaPF , NaClO , NaBF , NaBAr , etc. We
6
4
4
F
[
11]
tor array with a reactor volume of 3.0 mL.
Initial were pleased to find that many of these salts induce
investigation on the effect of the variation of the an improvement of the enantioselectivity (Table 2, en-
ligand substituents immediately gave a remarkable tries 5–7). Especially, the addition of NaClO induced
4
outcome (Table 1). In fact, under typical conditions a remarkable increase of enantioselectivity of up to
(
2 mol% of [Ir
ACHTUNGTRENNUNG( cod)Cl] as catalyst precursor and 93% ee. Worthy of note is the scarce activity of the
2
4
mol% ligand in toluene at 108C, 10 bar of hydrogen BAr -modified system, which has been active in other
F
[
15]
for 16 h) ligands 1a–g showed significantly different hydrogenations reported by Pfaltz.
behaviours. Ligand 1a lacking substitution in 3,3’-posi- Next, a more detailed study on the kinetics of the
tions does not form an active catalytic species reaction was conducted at normal pressure and 258C
(
Table 1, entry 1).
(Scheme 2). Here, we monitored closely the activity
[16]
On the other hand substitution with the p-methoxy- of the reduction with different catalyst loadings.
phenyl, naphthyl or p-fluorophenyl group induced full Among the highly enantioselective ligands, the
conversion within 16 h and good enantioselectivity of pMeO-phenyl-substituted phosphoramidite 1c pre-
up to 84%. Peculiarly, the more basic mesityl-substi- sented the highest TOF. Notably, the addition of salts
tuted ligand 1f, which has been used with best success like NaClO or NaBF improves the catalytic activity
4
4
[
3]
in the reduction of acetocinnamates, gave here poor (average TOF estimated from the consumption curve:
performance concerning both activity and enantiose- 22 h ). An observable improvement in activity is re-
À1
lectivity of the catalyst. Bromide ligand 1b is not
active as previously reported, while the simple
phenyl-substituted 1d is moderately active but far less
enantioselective than its parent aryl-substituted li-
gands 1c, 1e and 1g. Notably, a beneficial effect of the
3
,3’-disubstitution on enantioselectivity has also been
reported with BINAP ligands in the asymmetric hy-
drogenation of N-(1-phenylvinyl)acetamide.
[
12]
From the results shown, it was not clear whether
the active system was still a monodentate iridium-
phosphoramidite complex. However, reactions with
two equivalents of ligand did not show any increase
in enantioselectivity. Nevertheless, a thorough investi-
gation on the effect of additives was conducted. A
representative list of sodium salts added to
[Ir ACHTUNGTRENNUNG( cod)Cl] /1c is reported in Table 2. It is known
2
[13] [14]
from the works of Crabtree and Pfalz that iridi-
um complexes can form active dimeric complexes.
Thus, the addition of a non-coordinating anion, which Scheme 2. Hydrogen consumption versus time at different
might stabilise the more selective monomeric form, is reaction conditions.
1438
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ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2009, 351, 1437 – 1441

Iridium-Catalysed Asymmetric Hydrogenation of Enamides
UPDATES
Scheme 3. Selection of synthesised aryl- and alkyl-enamides 2b–k.
[
a]
corded also after reduction of the catalyst loading to Table 3. Hydrogenation of aryl- and alkyl-enamides.
À1
À1
1
mol% (average TOF: 21 h vs. 11 h ).
Finally, the catalyst system modified with NaClO4
was tested on a broader selection of enamides. The
different a-aryl-enamides 2i, 2j and 2k were synthes-
ised according to literature procedures by reduction
of the corresponding oximes with iron powder in the
[
17]
presence of acetic anhydride.
Enamide substrates
Entry
Substrate
Conv. [%]
ee [%]
2
a–2f were prepared using a modified synthetic proto-
[
5a,18]
1
2
2b
2c
2c
2c
2d
2e
2f
2f
2g
2h
2h
2i
>99
78
35
>99
21
99
52
44
>99
99
90
17
18
>99
58
96
88 (S)
81 (S)
81 (S)
82 (S)
64 (S)
70 (S)
49 (À)
64 (À)
15 (À)
17 (+)
12 (+)
60 (À)
51 (À)
7 (À)
col starting from the corresponding nitrile.
The
enamides 2g and 2h were synthesised according to the
described procedure (vide infra) by acylation of, re-
spectively, 2-phenylethylamine or tryptamine with
neat acetic anhydride, subsequent ring formation via
Bischler–Napieralski-reaction in the presence of
P O , and finally double bond shift to the exocyclic
position by acetylation of the imine with acetic anhy-
[
[
b]
c]
3
4
5
6
7
8
9
[19]
[b]
2
5
[5b,20,21]
[c]
dride.
The different aryl- and alkyl-enamides
Scheme 3) were subjected to hydrogenation using
mol% of [Ir(cod)Cl] /1c as catalyst adding 2 mol%
10
11
2
12
13
14
15
16
17
18
(
2
[
[
b]
b]
2i
ACHTUNGTRENNUNG
2
[
d]
2j (6/5)
2j (9/1)
2j (9/1)
2k
NaClO (Table 3). Among the enamides 2b–d bearing
4
[d]
[d]
29 (À)
14 (À)
70 (+)
73 (+)
a methoxy group on different positions of the phenyl
ring, the highest enantioselectivity is detected for
para-substitution (Table 3, entries 1, 2 and 5). Further
attempts to improve the catalyst activity by changing
the additive (Table 3, entries 3, 8, 13 and 16) or reduc-
ing the catalyst loading (Table 3, entries 4 and 10) did
not improve the outcome of the reaction. Interesting-
ly, even the alkyl-substituted enamide 2k gave a com-
paratively high enantioselectivity of up to 73% ee
>99
>99
[b]
2k
[
a]
Reactions were carried out at 108C under 10 bar pres-
sure of hydrogen for 16 h with 2.5 mmol [Ir(cod)Cl] ,
mmol ligand 1c, 5 mmol NaClO4 and 0.25 mmol sub-
strate in 2.0 mL toluene.
AHCTUNGTRENNUNG
2
5
[b]
[c]
[d]
+
5 mmol NaBF4.
Iridium:enamide=1:100.
Enamide 2j was synthesised and used in different E/Z
(
Table 3, entry 18).
ratios as given in parentheses.
Conclusions
sterically hindered alkyl-enamides are selectively re-
In situ generated iridium complexes of bulky H8-bi- duced.
naphthophosphoramidites 1 are active and highly
enantioselective catalysts for the hydrogenation of
non-functionalised enamides. The catalyst system is
Experimental Section
sensitive to the addition of non-coordinating salts
NaClO , NaBF , etc.). Under optimised conditions All manipulations were performed under an argon atmos-
(
4
4
a number of aryl-substituted enamides as well as phere using standard Schlenk techniques. Toluene was dis-
Adv. Synth. Catal. 2009, 351, 1437 – 1441
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1439

Giulia Erre et al.
UPDATES
tilled from sodium benzophenone ketyl under argon. [Ir- References
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(cod)Cl] (purchased by STREM) was stored under argon.
2
Sodium salts were all commercially available and used with-
out further manipulations. Enamides 2a–k were synthesised
following the literature procedures mentioned above.
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19, 1519–1522; Angew. Chem. Int. Ed. 2007, 46, 1497–
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1
500.
[
[
[
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A. Spannenberg, M. Beller, Chem. Asian J. 2008, 3,
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8
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2
a: ees were determined by HPLC (ChiralPak AD-H):
(
9
R)-3a 6.6 min and (S)-3a 8.8 min (eluent: n-heptane/ethanol
5:5; flow: 1.0 mLmin ).
À1
2
b: ees were determined by HPLC (ChiralPak AD-H):
(
R)-3b 20.3 min and (S)-3b 29.5 min (eluent: n-heptane/eth-
anol 98:2; flow: 1.5 mLmin ).
À1
[
2
c: ees were determined by HPLC (ChiralPak AD-H):
(
R)-3c 30.6 min and (S)-3c 33.6 min, (eluent: n-heptane/eth-
anol 95:5; flow: 1.0 mLmin ).
À1
2
d: ees were determined by HPLC (ChiralPak AD-H):
(
R)-3d 11.4 min and (S)-3d 17.3 min (eluent: n-heptane/eth-
anol 95:5; flow: 1.0 mLmin ).
À1
2
e: ees were determined by HPLC (ChiralPak AD-H):
(
S)-3e 12.7 min and (R)-3e 16.9 min (eluent: n-heptane/etha-
nol 95:5; flow: 0.8 mLmin ).
À1
2
f: ees were determined by HPLC (Chiralcel OD-H):
(
À)-3f 27.0 min and (+)-3f 32.4 min (eluent: n-heptane/eth-
À1
anol 98:2; flow: 1.0 mLmin ).
2
g: ees were determined by HPLC (ChiralPak-AD-H):
(
À)-3g 11.0 min and (+)-3g 13.7 min (eluent: n-heptane/eth-
5
142–5143; j) F.-Y. Zhang, C.-C. Pai, A. S. C. Chan, J.
À1
anol 95:5; flow: 1.0 mLmin ).
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2
h: ees were determined by HPLC (Chiracel OJ-H): (+)-
3
9
h 37.5 min and (À)-3h 46.9 min (eluent: n-hexane/ethanol
À1
5:5; flow: 1.0 mLmin ).
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i: ees were determined by HPLC (Chiralpak AD-H):
(
9
À)-3i 8.2 min and (+)-3i 9.2 min (eluent: n-heptane/ethanol
À1
5:5, flow: 1 mLmin ).
2
j: ees were determined by HPLC (Chiralcel OJ-H): (+)-
3
9
j 11.9 min and (À)-3j 13.6 min (eluent: heptan/ethanol
À1
5:5, flow: 1 mLmin ).
2
k: ees of 3k were determined by GC (Lipodex E 25 m):
(
+)-3k 49.0 min, (À)-3k 50.4 min.
2
003, 5, 4137–4139; g) P. Hannen, H.-C. Militzer, E. M.
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Adv. Synth. Catal. 2009, 351, 1437 – 1441
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1441