Rh-catalyzed highly enantioselective hydrogenation of nitroalkenes under basic conditions

Article abstract of DOI:10.1002/chem.201301049

Go catalytic! A highly enantioselective hydrogenation of β,β-disubstituted nitroalkenes and isomeric mixtures of nitroalkenes by using a Rh/DuanPhos catalytic system under basic conditions has furnished a convenient approach to β-chiral nitroalkanes, whic

Full text of DOI:10.1002/chem.201301049

DOI: 10.1002/chem.201301049
Rh-Catalyzed Highly Enantioselective Hydrogenation of Nitroalkenes under
Basic Conditions
Shengkun Li,[a, b] Kexuan Huang,[b] Jiwen Zhang,[a] Wenjun Wu,*[a] and Xumu Zhang*[b]
Since the first report by Knowles, Horner et al. in 1968,
catalytic asymmetric hydrogenation of alkenes has been one
of the most powerful approaches to chiral compounds and
great progress has been made.[1] Among the different al-
kenes, the hydrogenation of b,b-disubstituted nitroalkenes is
a challenging problem. The resulting b-chiral nitroalkane
from this “synthetic chameleon”[2] is a valuable synthetic
scaffold. The nitro group serves as a masked functionality
and can be easily transformed
like to report a novel rhodium catalytic system under basic
conditions for highly enantioselective hydrogenation of b,b-
disubstituted nitroalkenes. We believe that a monohydride
RhIH is responsible for the high enantioselectivity under
basic conditions and this new protocol is notable with sever-
al attractive features: 1) addition of base is necessary for
achieving both high enantioselectivity and high activity, and
up to>99% ee can be obtained (Scheme 1); 2) a number of
to amine, aldehyde, carboxylic
acid, nitrile oxide, and denitrat-
ed compound.[3] A variety of
asymmetric syntheses of the b-
chiral nitroalkanes by starting
from nitroalkenes have, there-
fore, been developed, such as
biocatalytic reduction,[4] metal-
or organocatalyzed transfer hy-
drogenations,[5] and enantiose-
lective conjugate additions.[6]
We envision that asymmetric
hydrogenation will furnish an
Scheme 1. Hydrogenation of b,b-disubstituted nitroalkenes and nitroalkene mixtures.
efficient and environmental
benign approach to prepare
these important chiral compounds. Recently, our group re-
ported the first enantioselective hydrogenation of b,b-disub-
stituted nitroalkenes with a Rh/Josiphos catalytic system.[7]
A number of chiral ligands were scanned and only a few
promising results were achieved, and the enantioselectivity
for most substrates was lower than 95% ee (ee=enantiomer-
ic excess) It is highly desirable to develop a more conven-
ient and highly enantioselective protocol. Herein, we would
chiral ligands (DuanPhos, TangPhos, Et-DuPhos, and Bina-
pine) can be used in highly enantioselective hydrogenation,
and this method is potentially practical for preparing chiral
nitroalkanes; and 3) mixtures of nitroalkenes can also be
used without erosion of either conversion or enantioselectiv-
ity.
Based on our previous work on the enantioselective hy-
drogenation of nitroalkenes,[7] the crucial role of additives[8]
in the asymmetric hydrogenation of certain alkenes was ex-
amined. Our investigation into the asymmetric hydrogena-
tion of nitoalkene 1a demonstrated that the addition of base
is necessary for improving the reactivity and enantioselectiv-
ity.
[a] S. Li, J. Zhang, Prof. Dr. W. Wu
College of Science
Northwest A&F University
Yangling, Shaanxi 712100 (P. R. China)
E-mail: wuwenjun@nwsuaf.edu.cn
As shown in Table 1, the [RhACTHNUTRGENNUG(nbd)2]SbF6/ACHTUNGTRNE(NUGN Sc, Rp)-Duan-
phos catalyst (nbd=norbornadiene) barely can promote the
hydrogenation in the absence of base, giving only 16% con-
version (Table 1, entry 1). Addition of 50 mol% of NEt3
brought a remarkable improvement of the enantioselectivity
and catalytic activity to 95% ee and 100% conversion, re-
spectively (entry 2). Fine-tuning of the amount of NEt3 dem-
onstrated that the conversion decreased sharply when the
amount of NEt3 is lower than 10 mol%, while no obvious
[b] S. Li, K. Huang, Prof. Dr. X. Zhang
Department of Chemistry and Chemical Biology and
Department of Pharmaceutical Chemistry
Rutgers, the State University of New Jersey
Piscataway, New Jersey 08854 (USA)
Fax : (+1)732-445-6312
E-mail: xumu@rci.rutgers.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301049.
10840
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Chem. Eur. J. 2013, 19, 10840 – 10844

COMMUNICATION
Table 1. Rh-catalyzed asymmetric hydrogenation of 1a with different
Table 2. Rh/Duanphos-catalyzed asymmetric hydrogenation of b,b-disub-
bases.[a]
stituted nitroalkenes.[a]
Entry Base [mol%]
H2 [atm] T [8C] Conv. [%][b] ee [%][c]
Entry Substrate
Product Conv. [%][b] ee [%][c]
1
2
3
4
5
6
7
8
–
50
50
50
50
50
50
50
50
50
50
50
20
50
50
50
50
50
50
50
50
50
50
50
40
16
100
100
95
42
40
40
44
38
63
80
95
94
94
96
96
96
96
97
92
94
98
1[d]
2
3
4
5
6
7
8
R=H (1a)
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
>99
>99
>95
>99
>99
>99
>99
>99
65
>99
>99
>99
>99
>99
92
97 (S) (À)
98 (À)
NEt3 (50)
NEt3 (20)
NEt3 (10)
NEt3 (5)
TMEDA (5)
urotropine (5)
NMP (5)
NMM (5)
piperidine (5)
diethylamine (5)
NEt3 (50)
R=3,5-(CF3)2 (1b)
R=3-CF3 (1c)
R=3-F (1d)
R=3-Cl (1e)
R=3-Br (1 f)
R=3-OMe (1g)
R=4-CF3 (1h)
R=4-F (1i)
R=4-Cl (1j)
R=4-Br (1k)
R=4-Me (1l)
R=4-Et (1m)
96 (À)
>99 (À)
97 (À)
98 (À)
96 (S) (À)
99 (À)
9
9[e]
10[f]
11
12
13
14
15
16[g]
17[g]
18[h]
95 (S) (À)
96 (S)(À)
>99 (S) (À)
95 (À)
10
11
12[d]
54
100
98(À)
[a] Unless otherwise mentioned, all reactions were carried out with a
R=4-tBu
N
97 (À)
[RhACHTUNGTRENNUNG(nbd)2]SbF6/ligand/substrate ratio of 2:2:100, in MeOH, for 18 h.
[b] The conversion of 1a was determined by 1H NMR spectroscopy and
calculated with the formula, 3a/(1a+2a+3a)ꢂ100%. [c] Enantiomeric
excess was determined by HPLC analysis on a chiral phase. [d] [Rh-
R-Ph= 2-Naphthyl (1o) 3o
R=2-F (1p)
R=2-Cl (1q)
R=2-OMe (1r)
98 (À)
3p
3q
3r
>99
88
68
96 (À)
91 (À)
96 (À)
ACHTUNGTRENNUNG(nbd)2]SbF6/ligand/substrate ratio of 1:1.1:100.
[a] Unless otherwise mentioned, all reactions were carried out with
0.1 mmols of substrate, with a [Rh(nbd)2]SbF6/ligand/substrate/NEt3 ratio
AHCTUNGTRENNUNG
of 1:1.1:100:50, in MeOH, at 408C, under 20 atm. of hydrogen for 18 h.
[b] The conversion higher than 99% was determined by HPLC analysis,
the others were detected based on 1H NMR spectroscopy. [c] Enantio-
meric excess was determined by HPLC analysis on a chiral phase. The
absolute configuration was assigned by comparison of the observed opti-
cal rotation with reported data. [d] 1.63 g of 1a was consumed. [e] 3%
Rh, 400% NEt3, 508C, 50 atm. H2. [f] 2% Rh, 200% NEt3, 508C, 20 atm.
H2. [g] 1.5% Rh. [h] 1.5% Rh, 508C, 40 atm. H2.
variation on enantioselectivity was detected (entries 3–5).
To determine the effect of different bases on the catalytic
activity and enantioselectivity, a number of organic base ad-
ditives (5 mol%), including N,N,Nꢁ,Nꢁ-tetramethyl-1,2-
ethane (TMEDA), hexamethylenetetramine (urotropine),
N-methylpyrrolidone (NMP), N-methylmorpholine (NMM),
piperidine, and diethylamine, were tested under similar con-
ditions, and comparable results have been achieved to that
of NEt3 (entries 6–11). The effect of inorganic base (K2CO3,
Cs2CO3, K3PO4, see the Supporting Information) on this re-
action was also investigated, and showed similar enhanced
activity and enantioselectivity. It is remarkable that in the
presence of 50 mol% of NEt3, full conversion and up to
98% ee can be achieved when the original experimental pa-
rameters including catalyst loading, hydrogen pressure, and
temperature were reduced to 1%, 20 atm., and 408C, re-
spectively (entry 12).
A variety of b-alkyl-b-aryl-nitroalkenes were hydrogenat-
ed to demonstrate the synthetic utilities of this methodology.
As shown in Table 2, the meta-substituents at the aromatic
ring of the substrates had a negligible effect on the activity,
no matter if they were electron-donating or -withdrawing,
and all the tested substrates were hydrogenated smoothly
with full conversion (Table 2, entries 2–7). A slight enhance-
ment of the ennatioselectivity was detected when electron-
withdrawing substitutents were introduced to the meta-posi-
tion, meta-fluoro-substituted substrate afforded the corre-
sponding chiral nitroalkane with up to >99% ee (entry 4).
The catalytic activity appears to be sensitive to the pattern
and electronic properties when the substituents were switch-
ed to the para-position. Outstanding selectivities of
ꢀ99% ee and full conversions were achieved in the hydro-
genation of para-trifluoromethyl and bromo-substituted ni-
troalkenes (entries 8 and 11). While higher loading of cata-
lyst or additive is necessary for fluoro- and chloro-substitut-
ed nitroalkenes to achieve comparable conversions (en-
tries 9 and 10). Substrates with electron-donating substitutes
at the para-position proceeded well under the standard con-
ditions, and a small improvement of enantioselectivity with
the increasing steric hindrance was detected (entries 12 vs.
13, 12 vs. 14). 2-Naphthyl nitroalkene showed similar reac-
tivity, affording the corresponding b-chiral nitroalkane with
up to 98% ee (entry 15). We were pleased to find that the
some ortho-substituted nitroalkenes, such as fluoro, chloro,
and methoxyl-substituted substrates, can also be hydrogenat-
ed in moderate to good conversion without a significant de-
crease in enantioselectivities (entries 16–18).
To explore the potential application of the [Rh-
AHCTUNTGREGN(NNU nbd)2]SbF6/DuanPhos/NEt3 catalytic system in the practical
asymmetric hydrogenation of nitroalkenes, further studies
were carried out and some impressive results were obtained
as follows: 1) This protocol can be easily implemented on a
gram scale under the standard hydrogenation conditions.
1.63 g of 1a were hydrogenated to the desired nitroalkane
without obvious erosion of either conversion or enantiose-
lectivity (Table 2, entry 1). 2) Study on the turnover number
(TON) of the hydrogenation of 1a showed that the transfor-
mation was complete with 1% catalyst at room temperature
with 99% ee, and with 0.25% catalyst at 508C (TON=400),
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W. Wu, X. Zhang et al.
Table 3. Rh-catalyzed asymmetric hydrogenation of nitroalkenes in pres-
ence of NEt3.[a]
only a slight decrease in ee (96% ee) was detected. When
the substrate to catalyst ratio was increased to 800, the
enantioselectivity still remained unchanged with 72% con-
version. This is the highest TON in either hydrogenation or
conjugate reduction of b,b-disubstituted nitroalkenes. 3) A
mixture of nitroalkenes are hydrogenated enantioselectively
to the corresponding nitroalkanes with 94–99% ee (Table 3,
entries 1–5). Compared with the pioneering work by Car-
reira[5c] on enantioselective conjugate reduction of isomeric
mixtures of nitroalkenes, the positive effect of NEt3 was de-
tected in our novel catalytic system, and the highly enantio-
selective hydrogenation can be carried out in one pot
through in situ equilibration. This will allow for a conven-
ient application in a large-scale synthesis of b-chiral nitroal-
kanes. 4) A number of commercially available chiral biden-
tate phosphorus ligands, such as DuanPhos, TangPhos, Et-
DuPhos, f-ketalPhos, and Binapine, can be used successfully
in the current protocol, affording the b-chiral nitroalkanes
with full conversion and good to excellent enantioselectivi-
ties (entries 6–8). Those features demonstrate that the cur-
rent protocol holds a great potential for convenient synthe-
sis of enantiopure nitroalkanes.
Entry
Ligand
Substrate
Product
ee
[%][a]
1[b]
2[b]
3[b]
4[b]
Duanphos
Duanphos
Duanphos
Duanphos
R=H(1a/2a=3.4:1)
R=3-Br (1 f/2 f=1.9:1)
R=3-OMe (1g/2g=2.3:1)
R=4-Br
3a
3 f
3g
3k
98
96
95
99
(1k/2k=2:1)
R=4-Me (1l/2l=3.5:1)
R=H (1a)
R=H (1a)
R=H (1a)
R=H (1a)
R=H (1a)
R=H (1a)
R=H (1a)
5[b]
6
7
8
9
Duanphos
Tangphos
Et-Duphos
f-Ketalphos
Binapine
Duanphos
Duanphos
Duanphos
Duanphos
3l
3a
3a
3a
3a
3a
3a
3a
3a
94
95
96
93
96
97
98
94
88
10[c]
11[d]
12[e]
13[f]
R=H (1a)
[a] See Table 2. [b] The ratio of isomers was determined by 1H NMR
spectroscopy. [c] In toluene. [d] In dichloromethane. [e] In ethyl actate.
[f] In THF.
Consistent with earlier reports,[5c] our tests also showed
immediate isomerization of b,b-disubstituted nitroalkenes in
the presence of NEt3. The rapid equilibrium was detected
for the asymmetric hydrogenation of the nitroalkenes was
shown in Scheme 2, in which the base additive possibly con-
verts a cationic RhIIIH2 species to a more active neutral
RhIH complex and thus promotes the hydrogenation of ni-
troalkenes through a monohydride pathway.
1
through H NMR spectroscopy (see the Supporting Informa-
tion). In our procedure, nitroalkenes were stirred with NEt3
for 5–10 min before the catalyst was added and we envi-
sioned that the substrate under-
went isomerization before hy-
drogenation. The success in
enantioselective hydrogenation
of nitroalkenes mixtures dem-
onstrated that tedious separa-
tion from isomers of nitroal-
kenes is unnecessary, making it
a practical synthetic procedure.
Variation of the amount of
NEt3 showed negligible effect
on the equilibrium of the iso-
merization of nitroalkene 1a,
whereas significant influence
was observed on the conversion
(Table 1, entries 1–5). We de-
duced that the NEt3 can result
in not only isomerization of ni-
troalkenes but also alternation
of the catalyst to a more active
species. A reasonable explana-
tion in the earlier report was
that heterolytic activation of
hydrogen was induced when the
cationic RhI was used in con-
junction with basic additives.[9]
We assume that a monohydride
RhIH complex is an active spe-
cies and a proposed mechanism Scheme 2. Proposed pathway for highly enantioselective hydrogenation of nitroalkenes.
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Rh-Catalyzed Highly Enantioselective Hydrogenation of Nitroalkenes
COMMUNICATION
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In conclusion, we have developed a highly enantioselec-
tive hydrogenation of b,b-disubstituted nitroalkenes and the
isomeric mixtures of nitroalkenes by in situ equilibration.
This novel Rh/DuanPhos catalytic system under basic condi-
tions will furnish a convenient approach to b-chiral nitroal-
kanes, which are otherwise not so easy to make. This new
method shows a great potential for a large-scale synthesis of
enantiopure nitroalkanes. A further study is underway with
the aim of exploring the mechanism of this novel catalytic
system and expanding substrate scope.
Experimental Section
General: A stock solution was made by mixing [RhACHTUNRGTNEUNG(nbd)2]SbF6 with (Sc,
Rp)-Duanphos in a 1:1.1 molar ratio in anhydrous methanol at room
temperature for 30 min in a nitrogen-filled glovebox. NEt3 solution in
methanol (0.1 mL, 0.05 mmol) was added to a vial containing the sub-
strate (0.1 mmol) and a stir bar in anhydrous methanol (1 mL). The re-
sulting yellow solution was stirred for 5 min at room temperature, and
then an aliquot of the catalyst solution (0.2 mL, 0.001 mmol) was trans-
ferred by syringe into the vial. The resulting vials were transferred to an
autoclave, which then underwent three cycles of flushing with hydrogen
gas by pressurizing to 20 atm. and then depressurizing. The reaction was
then stirred under H2 (20 atm.) at 408C for 18 h. The autoclave was
cooled to room temperature and the hydrogen gas was released slowly
and carefully. The solution was then concentrated and neutralized by ad-
dition of HCl aqueous solution (5 mL, 0.05 mmol). The aqueous layer
was extracted by CH2Cl2 (2 mLꢂ3), The combined organic extracts were
washed with a saturated solution of NaCl, dried over Na2SO4, and then
concentrated and passed through a short column of silica gel (eluant:
CH2Cl2). The ee values of all compounds were determined by HPLC
analysis on a chiral stationary phase.
Acknowledgements
This research was supported by the National Institute of Health
(GM58832), National Key S&T Research Foundation of China
(2010CB126105), China Scholarship Council, and scholarship award for
an excellent doctoral student granted by the ministry of education
(China) for S. Li.
Keywords: catalysis
· hydrogenation · monohydride ·
nitroalkenes · rhodium
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Chem. Eur. J. 2013, 19, 10840 – 10844