Iridium-Catalyzed Enantioselective Hydrogenation of β,β-Disubstituted Nitroalkenes

Article abstract of DOI:10.1002/adsc.201500723

A highly efficient, iridium-catalyzed, enantioselective hydrogenation of β,β-disubstituted nitroalkenes has been developed. Using a complex consisting of iridium and (S,S)-f-spiroPhos as the catalyst, a variety of β,β-disubstituted nitroalkenes were successfully hydrogenated to the corresponding chiral nitroalkanes with excellent enantioselectivities (up to 98% ee) and high turnover numbers (TON=1000).

Full text of DOI:10.1002/adsc.201500723

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DOI: 10.1002/adsc.201500723
Very Important Publication
Iridium-Catalyzed Enantioselective Hydrogenation of
b,b-Disubstituted Nitroalkenes
Man Liu,+a Duanyang Kong,+a Meina Li,a Guofu Zi,a and Guohua Houa,*
a
Key Laboratory of Radiopharmaceuticals, College of Chemistry, Beijing Normal University, Beijing 100875, Peopleꢀs
Republic of China
Fax: (+86)-10-5880-2075; e-mail: ghhou@bnu.edu.cn
+
These authors contributed equally to this work.
Received: July 31, 2015; Revised: September 25, 2015; Published online: December 9, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500723.
Abstract:
A
highly efficient, iridium-catalyzed,
tion,[7] transfer hydrogenation,[8] biocatalysis.[9] How-
ever, the direct asymmetric hydrogenation of b,b-dis-
ubstituted nitroalkenes has been relatively less ex-
plored. One recent example of a rhodium-catalyzed
asymmetric hydrogenation of b,b-disubstituted nitro-
alkenes has been reported by Zhang et al, and excel-
lent enantioselectivities were achieved. However,
beside the high catalyst loading (e.g., 3 mol% cata-
lyst), it was shown that the catalytic system was sensi-
tive to steric hindrance, and thus for substrates having
a substituent in the ortho-position of the phenyl ring
it resulted in poor to moderate conversion or enantio-
selectivity.[3] It has been shown that iridium-based cat-
alysts are more active to the asymmetric hydrogena-
tion of certain classes of compounds including unfunc-
tionalized olefins,[2b,f] ketones,[10] unsaturated carboxyl-
enantioselective hydrogenation of b,b-disubstituted
nitroalkenes has been developed. Using a complex
consisting of iridium and (S,S)-f-spiroPhos as the
catalyst, a variety of b,b-disubstituted nitroalkenes
were successfully hydrogenated to the correspond-
ing chiral nitroalkanes with excellent enantioselec-
tivities (up to 98% ee) and high turnover numbers
(TON=1000).
Keywords: asymmetric catalysis; enantioselectivity;
hydrogenation; iridium; nitroalkenes
Asymmetric hydrogenation represents one of the ic acids[11] and imines.[12] We have also previously
most economical and industrially revalvent synthetic shown that the asymmetric hydrogenation of b-acyl-
methods for the production of enantiomerically pure amino nitroolefins and a,b-unsaturated nitriles cata-
chiral compounds.[1,2] Chiral functional groups can be lyzed by the Ir- or Rh-(S,S)-f-spiroPhos complex re-
transformed into various important moieties present sulted in enantioselectivities of up to 98% ee and
in pharmaceuticals and biologically active mole- 99.9% ee, respectively.[13] However, to the best of our
cules.[1] The asymmetric hydogenation of b,b-disubsti- knowledge, an efficient iridium catalyst for the asym-
tuted nitroalkenes results in chiral nitroalkanes, metric hydrogenation of nitroalkenes without any ad-
a class of important intermediates in the synthesis of ditional chelating groups has not yet been reported.
pharmacueticals.[3] It is well known that nitro groups When we evaluated the hydrogenation of b,b-disubsti-
can be easily transformed into a variety of functional tuted nitroalkenes with the Ir-(S,S)-f-spiroPhos cata-
groups including amines, aldehydes, carboxylic acids, lyst, excellent enantioselectivities (up to 98% ee) and
nitrile oxides, and denitrated compounds.[4] These high turnover numbers (TON=1000) were achieved,
building blocks, such as chiral carboxylic acids and for the sterically hindered substrates as well
amines, are present in many different commercial (Scheme 1).
pharmaceuticals including the well-known non-steroi-
dal anti-inflammatory drug (NSAID), (+)-ibupro- hydrogenation of nitroalkenes, (E)-(1-nitroprop-1-en-
fen,[5] and the muscle relaxant, baclofen.[6]
2-yl)benzene 1a was chosen as a model substrate and
To explore the possibility of the direct asymmetric
Due to the utility of chiral nitroalkanes in chemical was initially hydrogenated under 20 atm of hydrogen
synthesis, a variety of synthetic methods to produce pressure in CH2Cl2 at 508C using the complex consist-
chiral nitroalkanes starting from nitroalkenes has ing of (S,S)-f-spiroPhos and [Ir(COD)Cl]2. Full con-
been developed, such as asymmetric conjugate addi- version with excellent enantioselectivity was achieved
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Scheme 1. Ir-catalyzed asymmetric hydrogenation of b,b-di-
substituted nitroalkenes 1.
in 12 h (Table 1, entry 1). For comparison, bidentate
diphosphine ligands such as Binap, DuanPhos, Seg-
Phos, JosiPhos and f-Binaphane (Figure 1) were also
screened, but low conversions and poor enantioselec-
tivities were obtained (entries 2–7). The remarkable
Figure 1. Structures of the phosphine ligands for hydrogena-
activity and enantioselectivity of the Ir-(S,S)-f-spiro- tion of (E)-(1-nitroprop-1-en-2-yl)benzene 1a.
Phos complex can possibly be attributed to the elec-
tron-rich property and the rigidity of the ligand with
the privileged spirobiindane skeleton, which was de- non-polar solvents including tetrahydrofuran (THF),
veloped by Zhou and co-workers.[14] After screening 1,2-dichloroethane (DCE), toluene and dichlorome-
of the appropriate phosphine ligands, the effect of sol- thane (CH2Cl2) were shown to give similar enantiose-
vent was explored. In fact, reactions conducted in lectivities, ranging from 90% to 95% ee (entries 8–
Table 1. Ir-catalyzed asymmetric hydrogenation of (E)-(1-nitroprop-1-en-2-yl)benzene 1a, optimization of the reaction condi-
tions.[a]
Entry
Ligand
Solvent
Temperature [oC]
P(H2) [atm]
Conversion [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
(S,S)-f-spiroPhos
(S)-MonoPhos
(S)-f-Binaphane
(S)-Binap
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
50
50
50
50
50
50
50
50
50
50
50
50
30
50
50
20
20
20
20
20
20
20
20
20
20
20
20
20
10
20
>99
4
29
6
20
21
37
>99
>99
>99
98
96
16
61
35
20
1
14
95
94
95
93
34
95
96
96
(R)-DM-SegPhos
(S,R)-DuanPhos
(R)-JosiPhos-1
(S,S)-f-spiroPhos
(S,S)-f-spiroPhos
(S,S)-f-spiroPhos
(S,S)-f-spiroPhos
(S,S)-f-spiroPhos
(S,S)-f-spiroPhos
(S,S)-f-spiroPhos
(S,S)-f-spiroPhos
9
toluene
DCE
10
11
12
13
14
15[d]
dioxane
MeOH
CH2Cl2
CH2Cl2
CH2Cl2
10
50
95
>99
[a]
Reaction conditions: [Ir(COD)Cl]2/phosphine/substrate ratio=0.5:1:100, 20 atm H2, 12 h.
Conversion determined by GC analysis.
Determined by chiral GC using a Supelco Alpha-Dex 225 column (30 m”0.25 mm”0.25 mm).
6 h.
[b]
[c]
[d]
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Iridium-Catalyzed Enantioselective Hydrogenation
Table 2. Ir-catalyzed asymmetric hydrogenation of b,b-disub-
stituted nitroalkenes 1.[a]
groups and electron-withdrawing substituents, includ-
ing halides (i.e., F, Cl and Br) all showed high enan-
tioselectivities ranging from 91–98% ee. In addition,
aromatic ring effects showed little influence, both
meta- and para-substituted phenyls could be hydro-
genated with excellent enantioselectivities (entries 2–
9). Importantly, ortho-substituted substrates could
also be hydrogenated to the products with compara-
ble or higher enantioselectivities. The substrate 1j
with an ortho-F group in the phenyl ring afforded 2j
with as high as 95% ee. The substrate 1k bearing
a bulky ortho-methyl group was also hydrogenated to
produce the corresponding product 2k with the excel-
lent enantioselectivity (97% ee), but a longer reaction
time was required for complete conversion (entry 11).
Intriguingly, the substrate 1o prepared from 1-tetra-
lone provided the chiral nitroalkane 2o with full con-
version and the highest enantioselectivity, 98% ee
(entry 15). However, when both R1 and R2 were ali-
phatic groups, a significant erosion of the ee value
was observed albeit with full conversion (entry 16).
This catalyst could also be applied to the asymmetric
hydrogenation of the heterocyclic substrate 1q afford-
ing the product with a slight lower ee value (entry 17).
Moreover, we were pleased to find that when the cat-
alyst loading was lowered to 0.1 mol% (TON=1000),
a similar enantioselectivity was also achieved with full
conversion (entry 18).
To evaluate the potential practical application of
this method, the asymmetric hydrogenation of b,b-di-
substituted nitroalkenes was also carried out on
a gram scale. Under the optimized reaction condi-
tions, substrate 1b (1.0 g) was smoothly hydrogenated
providing the conresponding product in excellent
yield (98% yield) with maintained enantioselectivity,
95% ee.
Furthermore, this method was successfully applied
to the synthesis of chiral amines and ibuprofen. The
hydrogenation products could be readily converted to
chiral amines by simple reduction of the nitro group
Entry R1
R2 Product Conversion ee
[%][b]
[%][c]
1
2
3
4
5
6
7
8
C6H5 (1a)
Me 2a
Me 2b
Me 2c
Me 2d
>99 (98)
>99 (99)
>99 (96)
>99 (98)
>99 (97)
>99 (99)
>99 (98)
>99 (97)
>99 (99)
>99 (98)
>99 (96)
>99 (99)
>99 (99)
>99 (98)
96 (S)
95 (S)
96 (S)
93 (S)
97 (S)
91 (S)
98 (S)
95 (S)
96 (S)
94 (À)
97 (S)
95 (À)
95 (S)
94 (S)
4-FC6H4 (1b)
4-ClC6H4 (1c)
4-BrC6H4 (1d)
4-MeOC6H4 (1e) Me 2e
4-MeC6H4 (1f) Me 2f
4-i-BuC6H4 (1g) Me 2g
3-ClC6H4 (1h) Me 2h
3-MeOC6H4 (1i) Me 2i
2-FC6H4 (1j)
2-MeC6H4 (1k)
1-naphthyl (1l)
2-naphthyl (1m) Me 2m
C6H5 (1n)
9
10
11[d]
12
13
14
Me 2j
Me 2k
Me 2l
Et 2n
15
2o
>99 (99)
98 (À)
16
cyclohexyl (1p) Me 2p
>99 (96)
>99 (97)
>99 (96)
69 (R)
90 (R)
95 (S)
17
2-thienyl (1q)
4-ClC6H4 (1c)
Me 2q
Me 2c
18[e]
[a]
Unless otherwise mentioned, all reactions were carried
out at a [Ir(COD)Cl]2/(S,S)-f-spiroPhos/substrate ratio of
0.5:1:100 in CH2Cl2 at 20 atm H2 and 508C for 6 h.
[b]
1
Conversion determined by H NMR spectroscopy or GC
analysis; data in parentheses are isolated yields.
Determined by chiral GC analysis.
Reaction for 17 h.
[c]
[d]
[e]
S/C=1000, 808C, 100 atm.
11). However, use of polar solvents resulted in de- in high yield and with maintained enantioselectivity.
creased conversion and enantioselectivities (entry 12). For instance, the hydrogenation product 2a was re-
Lower temperature and lower hydrogen pressure re- duced to the corresponding chiral amine with almost
sulted in incomplete conversion, albeit with almost no the same ee value. In addition, the chiral (+)-ibupro-
loss of enantioselectivity (entries 13 and 14). In addi- fen could also be prepared by this method from sub-
tion, complete conversion can be achieved in only 6 h strate 1g in high yield and excellent enantioselectivity,
(entry 15).
96% ee (Scheme 2).[15]
Encouraged by the promising results obtained in
In conclusion, a highly efficient and enantioselec-
the hydrogenation of the substrate 1a, a variety of tive catalyzed hydrogenation of b,b-disubstituted ni-
b,b-disubstituted nitroalkenes 1b–1q was then exam- troalkenes by an Ir-(S,S)-f-spiroPhos complex has
ined for the hydrogenation under the optimized reac- been developed. This new method provides a straight-
tion conditions and results are shown in Table 2. Nei- forward method for the synthesis of chiral nitroal-
ther electronic effect nor position of the substituent kanes with excellent enantioselectivities (up to 98%
on the aromatic ring (i.e., para, meta or ortho-) had ee) and high turnover numbers (TON=1000). Further
an obvious influence on the enantioselectivity. For ex- studies of the extension of this catalyst system to
ample, substrates bearing electron-donating substitu- other types of substrates are underway and will be re-
ents including methyl or methoxy (entries 6 and 7) ported in due course.
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carefully. The solution was concentrated and passed through
a short column of silica gel to remove the metal complex.
The ee values of all compounds 2 were determined by GC
or HPLC analysis on a chiral stationary phase.
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (grant nos. 21272026,
21172022 and 21472013), the Fundamental Research Funds
for the Central Universities, Specialized Research Fund for
the Doctoral Program of Higher Education of China, Pro-
gram for Changjiang Scholars and Innovative Research
Team in University, and Beijing Municipal Commission of
Education.
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Experimental Section
General Information
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1
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