Iridium-catalyzed highly enantioselective hydrogenation of exocyclic α,β-unsaturated carbonyl compounds

Article abstract of DOI:10.1002/adsc.201000185

By using the iridium complex of a phosphine-oxazoline ligand with an axis-unfixed biphenyl backbone, a highly enantioselective hydrogenation of the C=C bond of exocyclic α,β-unsaturated carbonyl compounds to afford α-chiral cyclic ketones, lactones and lactams was developed.

Full text of DOI:10.1002/adsc.201000185

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DOI: 10.1002/adsc.201000185
Iridium-Catalyzed Highly Enantioselective Hydrogenation of
Exocyclic a,b-Unsaturated Carbonyl Compounds
Fengtao Tian,a Dongmei Yao,a Yuanyuan Liu,a Fang Xie,a and Wanbin Zhanga,*
a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240,
Peopleꢀs Republic of China
Fax : (+86)-21-5474-3265; e-mail: wanbin@sjtu.edu.cn
Received: March 10, 2010; Revised: June 24, 2010; Published online: August 16, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000185.
Abstract: By using the iridium complex of a phos-
phine-oxazoline ligand with an axis-unfixed biphen-
yl backbone, a highly enantioselective hydrogena-
tion of the C=C bond of exocyclic a,b-unsaturated
carbonyl compounds to afford a-chiral cyclic ke-
tones, lactones and lactams was developed.
ed the enantioselective hydrogenation of a,b-unsatu-
rated amides by using an iridium complex derived
from ferrocenylphosphine-oxazoline (Fc-PHOX).[10]
However, there are few examples focused on the es-
tablishment of a-chiral carbon centers of cyclic car-
bonyl compounds, especially for lactones and lactams,
via asymmetric hydrogenation.[4,9] Even in these limit-
ed examples, the asymmetric hydrogenation is strong-
ly substrate dependent and there is no general solu-
tion for different kinds of cyclic substrates. Herein,
we disclose our preliminary results of the highly enan-
tioselective hydrogenation of exocyclic a,b-unsaturat-
ed carbonyl compounds by using an iridium complex
Keywords: asymmetric synthesis; cyclic carbonyl
compounds; hydrogenation; iridium; oxazoline
Cyclic carbonyl compounds with a-chiral carbon cen- with axis-unfixed biphenylphosphine-oxazoline li-
ters are an important group of compounds in organic gands 1 (Scheme 1).
synthesis and medicinal chemistry.[1] For the synthesis
of these optically active compounds, enzyme-mediat-
ed reactions and chemical syntheses from optically
active starting materials have usually been used.[2,3]
However, most of the reported methods are limited
in substrate scope, and high enantioselectivity has
seldom been obtained. The most straightforward and
simplest approach to these compounds would be
asymmetric hydrogenation of cyclic a,b-unsaturated
carbonyl compounds but much less efforts have been
focused on this option.[4,5] In most recent years, iridi-
um complexes with chiral phosphine-oxazoline li-
gands have attracted much attention because of their
easy availability and high reactivity and enantioselec-
tivity in the hydrogenation of unfunctionalized ole-
fins[6] or olefin substrates with less strongly coordinat-
ing groups[7–10]. In this context, Zhou found that the
C=C bond of an a,b-unsaturated carboxylic acid
could be hydrogenated with high enantioselectivity by
using an iridium spirophosphine-oxazoline (SIPHOX)
complex;[8] Bolm and Hou found that the C=C bond
of a,b-unsaturated ketones can be hydrogenated with
high enantioselectivity by using an iridium phosphine-
oxazoline (PHOX) complex;[9] Hou further investigat- Scheme 1. Complexation behavior of ligands 1a–f.
Adv. Synth. Catal. 2010, 352, 1841 – 1845
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Fengtao Tian et al.
Table 1. Optimization of the reaction conditions.[a]
COMMUNICATIONS
In the past decade, chiral phosphine-oxazoline li-
gands with an axis-fixed binaphthyl backbone were
developed and showed excellent chirality transfer
properties in asymmetric catalysis.[11] However, in
most cases, only one of the two diastereomers of the
ligands works effectively due to the configurationally
matching-mismatching effect. Bearing this in mind, a
novel class of phosphine-oxazoline ligands 1 with an
axis-unfixed biphenyl backbone was developed by us
in recent years, which exist as an equilibrium mixture
of diastereomers in solution as a result of rotation
around the internal bond of the biphenyl. When these
ligands coordinated to Pd(II), interestingly, only one
of the two possible diastereomeric complexes was
formed and showed excellent enantioselectivities in
Pd-catalyzed asymmetric allylic alkylation.[12] This
result indicated that all of the ligands could be fully
and effectively utilized in the asymmetric catalysis.
Based on the above results, the complexation be-
Entry Complex Solvent Pressure
[atm]
Conversion ee
[%][b]
[%][c]
1
2
3
4
5
6
2a
2a
2a
2a
2a
2a
2a
2a
2a
toluene 20
CH2Cl2 20
TBME[d] 20
MeOH 20
toluene 10
toluene 50
toluene 20
toluene 20
toluene 20
toluene 20
toluene 20
toluene 20
toluene 20
toluene 20
toluene 20
100
100
100
100
85
100
100
57
96
94
85
30
96
96
95
93
95
99
82
86
91
83
78
7[e]
8[f]
9[g]
10
9
10[h] 2a
11
12
13
14
15
2b
2c
2d
2e
2f
100
100
100
100
100
havior of our ligands 1 with [IrACTHUNRGTENUNG(COD)Cl]2 in CH2Cl2
at ambient temperature followed by counterion ex-
change with NaBARF was investigated (Scheme 1).
As expected, all of the ligands 1a–f afforded only one
of the two possible diastereomeric complexes 2a–f, re-
[a]
Reaction conditions: 3a (0.25 mmol), catalyst (1 mol%),
solvent (2.0 mL). All of the reactions were carried out
under hydrogen at room temperature for 24 h.
Determined by H NMR spectroscopy.
Determined by HPLC using a chiral Daicel Chiralcel
OJ-H column.
spectively, according to their H NMR and 31P NMR
1
spectra. The formed diastereomer 2a was easily crys-
tallized from chloroform/hexane and the X-ray dif-
fraction analysis showed that it has an S configuration
in its axial chirality (Figure 1).[13]
With these complexes in hand, the asymmetric hy-
drogenation of exocyclic a,b-unsaturated ketone 3a
was initially investigated by using 1 mol% of the Ir
complex 2a in toluene under 20 atm of H2 pressure at
room temperature. To our delight, it provided ketone
4a in full conversion and 96% ee after 24 h (Table 1,
[b]
[c]
1
[d]
[e]
[f]
TBME: tert-butyl methyl ether.
Reaction time was 1 h.
Catalyst loading was 0.2 mol%.
[g]
[h]
Catalyst counterion was ClÀ.
À
Catalyst counterion was PF6 .
entry 1), which indicated that the reaction preferen- while toluene showed somewhat higher enantioselec-
tially reduces the C=C bond under the aforemen- tivity (Table 1, entries 1–4). The hydrogen pressure
tioned conditions. Screening of the solvents revealed has some effect on the reactivity and little effect on
that toluene and dichloromethane were the more suit- the enantioselectivity (Table 1, entries 1, 5, 6). Short-
able solvents than TBME and MeOH in this reaction, ening the reaction time also afforded full conversion
and almost the same enantioselectivity (Table 1,
entry 7). However, a lower catalyst loading gave a
lower conversion and slightly decreased enantioselec-
tivity even after 24 h (Table 1, entry 8). Moreover, a
screening of catalyst counterions was carried out.
When the counterions of the complex 2a were
À
changed to ClÀ and PF6 , the conversions were de-
creased to 10% and 9%, although the enantioselectiv-
ities went up to 95% ee and 99% ee, respectively
(Table 1, entries 9 and 10). On the basis of these re-
sults, the reaction conditions described in entry 1
were chosen for screening of other Ir complexes with
different substituents either on the oxazoline ring or
on the phosphine phenyl ring. The results indicated
that the enantioselectivity in the asymmetric hydroge-
nation of 3a was greatly affected by the properties of
the substituents on the oxazoline ring and the P-
Figure 1. X-ray structure of complex (aS)-2a. The anion and
hydrogen atoms are omitted for clarity.
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Iridium-Catalyzed Highly Enantioselective Hydrogenation
Table 2. Hydrogenation of exocyclic a,b-unsaturated ketones 3a–i.[a]
[a]
Reaction conditions: substrate (0.25 mmol), catalyst (1 mol%), toluene (2.0 mL).
All of the reactions were carried out under hydrogen at room temperature for
24 h.
Determined by H NMR spectroscopy.
Determined by HPLC using Chiral Daicel Chiralcel OJ-H or OD-H column.
[b]
[c]
1
phenyl ring and ligand 1a is the best one for the hy- chiral carbon centers play a significant role in organic
drogenation of 3a (Table 1, entries 11–15). synthesis and medicinal chemistry, yet their prepara-
Then, several other exocyclic a,b-unsaturated ke- tion through enantioselective hydrogenation was
tones 3b–i were examined with complex 2a under the rarely reported,[4] especially for lactones, for which
above optimized conditions. All of the substrates af- only one report has appeared but which is limited to
forded the corresponding chiral cyclic ketones with two aliphatic substrates.[4a] Hydrogenation of lactones
full conversions (Table 2). The reduction of enones 5a–c gave 6a–c in full conversions and excellent enan-
3a–c with different ring sizes gave the corresponding tioselectivities of 95% ee, 94% ee and 94% ee, respec-
ketones in 96% ee, 75% ee and 70% ee, respectively tively (Table 3, entries 1–3). When the ring size was
(Table 2, entries 1–3), which revealed that ring size changed from 5- to 6-membered, the enantioselectivi-
has a significant effect on the enantioselectivity. Sub- ty decreased to 71% ee (Table 3, entry 4). The hydro-
strates 3d, e with different substituents on the aryl genation of lactam substrates was initially tested with
ring were hydrogenated in 96% ee and 95% ee, re- 5e under the above reaction conditions, and to our
spectively, which showed that the substituent pattern disappointment, it gave no conversion, probably be-
has little effect on the enantioselectivity (Table 2, en- cause of its poor solubility in toluene (Table 3,
tries 4 and 5). Substrates 3f with an indanone skeleton entry 5). Then, the amide nitrogen atom was protect-
and 3g with a tetralone skeleton gave 95% ee and ed with an Ac or Bn group to improve its solubility in
75% ee, respectively (Table 2, entries 6 and 7). Sub- toluene. The hydrogenation of the resulting substrates
strates with aliphatic substituents of the indanone 5f and 5g proceeded smoothly and excellent enantio-
skeleton gave 67% ee and 71% ee, respectively selectivities of 97% ee and 98% ee, respectively, were
(Table 2, entries 8 and 9).
obtained (Table 3, entries 6 and 7). Similar to the exo-
Encouraged by the above exciting results for exocy- cyclic a,b-unsaturated ketones and lactones, sub-
clic a,b-unsaturated ketones, we extended the sub- strates 5h and 5i with different substituent patterns
strate scope to exocyclic a,b-unsaturated lactones and also gave an excellent enantioselectivity of 98% ee
lactams. Enantiopure lactones and lactams with a- (Table 3, entries 8 and 9). However, the substrate with
Adv. Synth. Catal. 2010, 352, 1841 – 1845
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Fengtao Tian et al.
COMMUNICATIONS
Table 3. Hydrogenation of exocyclic a,b-unsaturated lac-
tones and lactams 5a–j.[a]
pressure needed), and finally pressurized to 20 atm. The re-
action mixture was stirred for 24 h, and then the hydrogen
gas was slowly released. The conversion of the substrate was
1
determined by H NMR spectroscopic analysis of the crude
reaction mixture, and the product was purified by chroma-
tography using a petroleum/ethyl acetate mixture (10:1) as
eluents. The enantiomeric excess was determined by HPLC
on a Chiral Daicel Chiralcel column. The HPLC conditions
and the spectral data of all compounds are provided in the
Supporting Information.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20772081), the Science and Technology
Commission of Shanghai Municipality (09431902500), and
Nippon Chemical Industrial Co., Ltd. We also would like to
thank Prof. T. Imamoto for helpful discussions.
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[13] Crystallographic data for (aS)-2a: C70H52BF24IrNOP,
Mr =1613.11,
T=293(2) K,
orthorhombic,
P2(1)2(1)2(1), a=12.7654(7) ꢉ, b=14.2837(8) ꢉ, c=
37.502(2) ꢉ, b=90.00(2)8, V=6838.0(7) ꢉ3, Z=4,
1calcd. =1.567 MgmÀ3, m=2.085 mmÀ1, 36314 reflections
collected, 9927 independent reflections (Rint =0.0955),
Final R indices [I>2s(I)]: R1 =0.0485, wR2 =0.0937.
CCDC 762042 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Adv. Synth. Catal. 2010, 352, 1841 – 1845
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1845