A new copper (I)-tetrahydrosalen-catab zed asymmetric henry reaction and its extension to the synthesis of (S)-norphenylephrine

Article abstract of DOI:10.1002/chem.200601262

A new chiral hydrogenated salen catalyst has been developed for the asymmetric Henry reaction which produces the expected products in moderate to high yields (up to 98%) with excellent enantioselectivities (up to 96% ee). A variety of aromatic, heteroarom

Full text of DOI:10.1002/chem.200601262

FULL PAPER
DOI: 10.1002/chem.200601262
A New Copper(I)–Tetrahydrosalen-Catalyzed Asymmetric Henry Reaction
and Its Extension to the Synthesis of (S)-Norphenylephrine
[
a]
[a]
[a]
[a]
[a, b]
YanXio ng ,
Fei Wang, Xiao Huang, Yuehong Wen, and Xiaoming Feng*
Abstract: A new chiral hydrogenated
salen catalyst has been developed for
the asymmetric Henry reaction which
produces the expected products in
moderate to high yields (up to 98%)
with excellent enantioselectivities (up
to 96% ee). A variety of aromatic, het-
eroaromatic, enal, and aliphatic alde-
hydes were found to be suitable sub-
strates in the presence of hydrogenated
salen 1 f (10 mol%), (CuOTf) ·C H
8
tended to the synthesis of (S)-norphe-
nylephrine in 67% overall yield, start-
ing from commercially available m-hy-
droxybenzaldehyde. Based on experi-
mental investigations and MM+
calculations, a possible catalytic cycle
including a transition state (8 or A) has
been proposed to explain the origin of
reactivity and asymmetric inductivity.
2
7
(5 mol%), and 4 Š molecular sieves.
This process is air-tolerant and easily
manipulated with readily available re-
agents, and has been successfully ex-
Keywords: asymmetric catalysis
copper Henry reaction
norphenylephrine · salen
·
·
·
Introduction
tively catalyze the Henry reaction by concurrent activation,
realized by the combined use of discrete Lewis acid and
base as structurally independent entities.
The nitroaldol (Henry) reaction is one of the most powerful
[1]
reactions in constructing CÀC bonds in organic chemistry,
Meanwhile, of recent interest are the hydrogenated deriv-
atives of the Schiff base-type ligands and their complexes
providing efficient access to valuable functionalized structur-
[11]
al motifs, such as 1,2-amino alcohols and a-hydroxy carbox-
with transition-metal ions.
Comparing the properties of
[2,3]
ylic acids.
In the last few years, great efforts have been
salen and tetrahydrosalen ([H ]salen), the latter has in-
4
devoted towards the implementation of asymmetric versions
of the Henry reaction and significant progress has been ach-
creased N basicity and framework flexibility to more easily
[12]
in a folded fashion, which might produce strong asymmet-
ric inductivity in some reactions. However, they have been
much less developed to date. In our previous study, hydro-
genated Schiff base ligand has been successfully employed
[4–10]
ieved.
tion, Shibasaki reported the first efficient method by making
Along a similar principle of cooperative activa-
[7]
use of the two-center catalysis, and Trost revealed a novel
[8]
[11c]
family of dinuclear zinc complexes, catalyzing the reaction
between nitromethane and aldehydes. Subsequently, Evansꢀs
in the asymmetric cyanosilylation of aldehydes.
I
Herein, we wish to report a new Cu –[H ]salen-catalyzed
4
[9]
copper acetate–bis(oxazoline) catalyst and Palomoꢀs zinc
asymmetric Henry reaction, which delivers excellent enan-
tioselectivities for aromatic, heteroaromatic, enal, and ali-
phatic aldehydes. In addition, this method provides several
[10]
triflate–amino alcohol complex were both found to effec-
practical advantages: 1) chiral [H ]salen ligand 1 f is readily
4
modifiable and can be easily synthesized in two simple steps
and in 94% overall yield, 2) in contrast to the instability of
[
a] Dr. Y. Xiong, F. Wang, Dr. X. Huang, Dr. Y. Wen, Prof. Dr. X. Feng
Key Laboratory of Green Chemistry & Technology
(
Sichuan University), Ministry of Education, College of Chemistry
the salen ligand in acid medium, chiral [H ]salen ligand can
4
Sichuan University, Chengdu 610064 (China)
Fax : (+86)28-8541-8249
E-mail: xmfeng@scu.edu.cn
be recovered in near quantitative yield after the reaction by
simple aqueous acid/base workup and thus can be reused
(
4
see Experimental Section), and 3) m-hydroxybenzaldehyde
d was directly employed as a starting material for the syn-
[
b] Prof. Dr. X. Feng
State Key Laboratory of Biotherapy, West China Hospital
Sichuan University, Chengdu 610041 (China)
thesis of (S)-norphenylephrine which has therapeutic appli-
cations in a variety of diseases, for example, in the treatment
of hypertension, pain, and depression.
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
[13]
Chem. Eur. J. 2007, 13, 829 – 833
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
829

ture, which led us to determine
that the combined use of 4 Š
molecular sieves and a temper-
ature of 458C were both crucial
to reactivity and enantioselec-
tivity (Table 1, entry 9 versus 8
and entry 10 versus 9). Thus, we
found that treatment of 4-nitro-
benzaldehyde with nitrome-
thane in the presence of
[
H ]salen 1 f (10 mol%),
4
(
CuOTf) ·C H (5 mol%), and
2 7 8
4
Š molecular sieves enabled
the excellent and efficient for-
mation of product in 95% yield
with 91% ee (Table 1, entry 10).
Results and Discussion
Initially, building on the [H ]salen–Cu
A
H
R
U
G
Under the optimal reaction conditions, a variety of aro-
matic, heteroaromatic, aliphatic, and enal aldehydes were
found to be suitable substrates, with the Henry reaction
giving the corresponding nitroaldol adducts in moderate to
good yields with excellent enantioselectivities (Table 2). It is
4
2
radentate R,R ligands (1a–f) were examined by using the
Henry reaction of 4-nitrobenzaldehyde with nitromethane
as the model reaction (Table 1). Moderate bulky tBu groups
Table 1. Asymmetric Henry reaction of 4-nitrobenzaldehyde with nitro-
methane under indicated conditions.
Table 2. Asymmetric Henry reaction of aldehydes with nitro compound
under optimum conditions.
[
a]
Entry
Ligand Catalyst Metal
[mol%]
T
t
Yield ee
[a]
[b]
[c]
Entry
1
Aldehyde
t [h] Yield [%]
ee [%]
91 (S)
90 (S)
91
93 (S)
90
94
94 (S)
94
96 (S)
92
92 (S)
93 (S)
90
91 (S)
91
[b]
[c]
A
C
H
T
R
E
U
N
G
[8C] [h] [%]
[%]
4-nitrobenzaldehyde (4a)
30
95
1
2
3
4
5
6
7
8
9
1
1
1
1a
1b
1c
1d
1e
1 f
1 f
1 f
1 f
1 f
2a
3a
20
20
20
20
20
20
20
10
10
5
Cu
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OAc)
Cu(OTf)
(CuOTf)
(CuOTf)
(CuOTf)
(CuOTf)
(CuOTf)
A
H
R
U
G
2
2
2
2
2
2
13
13
13
13
13
13
13
13
13
45
45
45
9
9
9
9
9
9
9
9
9
37
43
28
41
34
40
41
46
52
62
52
67
[d]
9
3
2
3
4
3-nitrobenzaldehyde (4b)
2-nitrobenzaldehyde (4c)
3-hydroxybenzaldehyde (4d)
3-phenoxybenzaldehyde (4e)
4-chlorobenzaldehyde (4 f)
3-chlorobenzaldehyde (4g)
2-chlorobenzaldehyde (4h)
2,4-dichlorobenzaldehyde (4i)
benzaldehyde (4j)
1-naphthaldehyde (4k)
2-naphthaldehyde (4l)
4-phenylbenzaldehyde (4m)
furan-2-carbaldehyde (4n)
thiophene-2-carbaldehyde (4o)
cinnamaldehyde (4p)
24
30
30
60
19
48
30
13
60
60
72
56
60
60
56
72
60
20
93
95
75
61
77
73
92
96
64
66
84
63
86
38
44
74
57
98
[e]
5
6
7
8
9
2
trace n.d.
A
T
E
G
2
2
2
2
2
·C
·C
·C
·C
·C
7
7
7
7
7
H
H
H
H
H
8
8
8
8
8
21
33
83
93
91
71
2
[
d]
[
[
[
d]
0
1
2
30 95
30 88
30 20
10
11
12
d]
d]
5
5
[f]
[
a] All reactions were run on a 0.2 mmol scale of 4-nitrobenzaldehyde in
the mixture of 0.8 mL of methanol and 0.6 mL of nitromethane. [b] Iso-
lated yield. [c] Determined by HPLC analysis. [d] In the presence of
13
14
15
16
[
e]
91
92
90
88 (S)
91
1
0 mg of 4 Š MS.
[
[
[
g]
e]
h]
17
18
19
hexanal (4q)
isobutyraldehyde (4r)
4-nitrobenzaldehyde (4a)
at the 3’,5’-position of the phenolic ring of ligand 1 f were
beneficial to the enantioselectivity (Table 1, entry 6
versus 1–5). To find an effective metal complex for the cata-
[
a] Reactions were carried out on a 0.2 mmol scale of aldehydes in the
mixture of 0.8 mL of methanol and 0.6 mL of nitromethane in the pres-
ence of 10 mg of 4 Š MS. [b] Isolated yield. [c] Determined by HPLC
analysis. The absolute configurations were established as S by comparison
of the sign of the optical rotation values with that in the literature (see
reference [9]). [d] In the presence of 50 mL of water. For the detailed ex-
perimental procedure see the Supporting Information. [e] In the mixture
of 0.4 mL of tBuOH and 0.3 mL of nitromethane. [f] tBuOH as the sol-
vent. [g] In the mixture of 0.2 mL of methanol and 0.15 mL of nitrome-
thane. [h] Reactions were carried out on a 0.2 mmol scale of 4-nitroben-
zaldehyde in the mixture of 0.8 mL of methanol and 0.6 mL of nitro-
ethane in the presence of 10 mg of 4 Š MS; syn/anti 15:85 was deter-
[14]
lytic addition, we conducted systematic screening studies
and established that chiral ligand 1 f and (CuOTf) ·C H
8
2
7
could effectively promote the enantioselective addition
Table 1, entry 8). Complexed with (CuOTf) ·C H ,
(
2
7
8
[
H ]salen ligand 2a, based on the cyclohexane-1,2-diamine
4
moiety and salen ligand 3a, gave poor enantioselectivities
Table 1, entries 11 and 12). These studies also included ex-
aminations of various additives and the reaction tempera-
(
1
mined by HPLC and H NMR spectroscopy.
830
www.chemeurj.org
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 829 – 833

Synthesis of (S)-Norphenylephrine
FULL PAPER
noteworthy that not only the steric hindrance of the substi-
tution at the aromatic ring, but also its electronic nature has
no obvious effect on enantioselectivity (Table 2, entries 1–3,
6–9 and 11–12). 4-Nitrobenzaldehyde could also react
smoothly with nitroethane with high diastereoselectivity
(
(
syn/anti 15:85) and excellent enantioselectivity (91% ee)
Table 2, entry 19). Interestingly, even in the presence of ad-
ditional water (50 mL), the chemical efficiency and enantio-
selectivity of the model reaction were maintained (Table 2,
entry 1). When using the enantiomer of 1 f as a ligand, the
model reaction afforded the R product 5a in 95% yield with
90% ee.
As an interesting synthetic utility of this method, m-hy-
droxybenzaldehyde 4d was directly used as the starting ma-
terial for the synthesis of (S)-norphenylephrine with the
Henry reaction giving the nitroaldol adduct (S)-5d in 75%
yield with 90% ee (Table 2, entry 4). By catalytic hydrogena-
tion, nitro compound 5d was readily converted into (S)-nor-
2
0
phenylephrine in 90% yield (Scheme 1, [a] =À1.75 (c=0.4
D
Scheme 1. Synthesis of (S)-norphenylephrine.
[
15]
20
D
in CH OH); lit. [a] =À1.7 (c=5.8 in CH OH, 98% ee)).
3
3
When 3-phenoxybenzaldehyde 4e was employed as a sub-
strate, the reaction also provided the corresponding product
5
e in moderate yield with excellent enantioselectivity
(
Table 2, entry 5).
In this paper, the Henry reaction was simple in operation
Figure 1. MM+ optimized geometries for [H
salen 3a-CuOTf (II) complexes.
4
]salen 1 f-CuOTf (I) and
with readily available reagents and no special precautions
were taken to exclude water or air from the reaction vessel.
By using the HyperChem program package, the geometries
of [H ]salen 1 f-CuOTf (I) and salen 3a-CuOTf (II) com-
plexes were optimized at the MM+ level (Figure 1). The
bonyl of 4-nitrobenzaldehyde is much more accessible to a
nucleophilic group than the Si face as the interaction of the
Si face and the attacking group will strongly increase repul-
sion between phenyl subunits as in transition state B.
4
CuO N plane in the former shows a heavier tetrahedral dis-
2
2
tortion and the two phenolic rings are bent upwards and
downwards, respectively.
Based on our preliminary experimental investigations,
modeled complex I and the previously reported steric and
electronic considerations, the explanation of the asymmet-
ric induction mechanism can be correctly predicted
Conclusion
[9]
I
We have developed a new chiral Cu –[H ]salen catalyst for
4
(
Figure 2). In the catalytic cycle, due to the strong coordina-
the asymmetric Henry reaction with good substrate generali-
ty. The products from this reaction are produced in moder-
ate to high yields with excellent enantioselectivities. This
pathway is air-tolerant and easily manipulated with readily
available reagents. Furthermore, this method has been suc-
cessfully applied to the synthesis of (S)-norphenylephrine,
starting from commercially available m-hydroxybenzalde-
hyde. Further efforts will be devoted to improve the reactiv-
ity and enantioselectivity as well as to synthesize norepi-
nephrine and epinephrine analogues.
tion ability of the nitro group to soft metal, nitromethane is
activated and deprotonated to generate the active nucleo-
phile through a possible complex 7. Once the nitroaldol re-
action proceeds from the transition-state model 8 to 6, the
intermediate 6 again works as a chiral Brçnsted base cata-
lyst for the second cycle. On the basis of the observed abso-
lute configuration of 2-nitro-1-(4-nitrophenyl)ethanol
(
Table 2, entry 1), a possible transition state (8 or A) has
been proposed. In transition state A, the Re face of the car-
Chem. Eur. J. 2007, 13, 829 – 833
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
831

X. Feng et al.
Acknowledgements
We thank the National Natural Science Foundation of China (Nos.
0225206, 20390055 and 20372050) and the Ministry of Education, P R
China (No. 104209 and others) for financial support.
2
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4
Experimental Section
Typical experimental procedure: Ligand 1 f (13.0 mg, 0.02 mmol), 4 Š
molecular sieves (10 mg), and (CuOTf) ·C H (5.2 mg, 0.01 mmol) were
2 7 8
added to methanol (0.8 mL) at ambient temperature. Stirring continued
for 10 min and then 4-nitrobenzaldehyde (30.2 mg, 0.2 mmol) and nitro-
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[
3
concentrated in vacuo to give a glutinous phase. CH
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peared. The mixture was then extracted with CH Cl (3”10 mL) and the
organic extracts were combined, washed with brine, dried over anhydrous
Na SO , and evaporated in vacuo. The residue was purified by flash chro-
matography by using EtOAc/PE (1:4; PE=petroleum ether b.p. 60–
2 2
Cl (2 mL) and HCl
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5
A
C
H
T
R
E
U
N
G
R
D
(minor)=19.70 min, S: t (major)=24.23 min; [a] =+31.68 (c=0.14 in
A
H
R
N
[
10b]
25
CH
recover the [H
NaHCO
was added dropwise to the aqueous phase until pH ꢀ10. The
mixture was extracted with CH Cl (3”10 mL), and the organic layer was
dried over Na SO and filtered. The solvent was evaporated to afford
2
Cl
2
, 91% ee) (lit.
[a]D =À38.48 (c=1.3 in CH
2 2
Cl , 84% ee)). To
3
4
]salen ligand 1 f from the aqueous phase, saturated
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2
2
4
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www.chemeurj.org
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 829 – 833

Synthesis of (S)-Norphenylephrine
FULL PAPER
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A
H
R
U
G
2
A
H
R
U
G
2
, and Zn(OAc) were also examined, with either inferior re-
sults or complete failure of the reaction observed.
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Received: September 2, 2006
Published online: October 27, 2006
Chem. Eur. J. 2007, 13, 829 – 833
ꢁ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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