Highly Diastereo- and Enantioselective Michael Addition of Nitroalkanes to 2-Enoyl-Pyridine N-Oxides Catalyzed by Scandium(III)/Copper(II) Complexes

Article abstract of DOI:10.1002/chem.201502129

A C₂-symmetric Schiff-base ligand, derived from tridentate-Schiff-base, was developed and successfully applied to the asymmetric Michael addition of nitroalkanes to 2-enoyl-pyridine N-oxides. With this newly catalytic system, an unprecedented diastereoselectivity was obtained in the asymmetric Michael addition of nitroalkanes to 2-enoyl-pyridine N-oxides. In addition, a switch in enantioselectivity was achieved by using this newly catalytic system and our previous catalyst. After a facile reduction, the optically active adduct was converted to a biologically active dihydro-2H-pyrrol 4a. Furthermore, a connection of two tridentate-Schiff-base subunits proved to be an effective strategy in ligand design. Complex chemistry! A C₂-symmetric Schiff-base ligand, derived from tridentate-Schiff-base, was developed and successfully applied to the asymmetric Michael addition of nitroalkanes to 2-enoyl-pyridine N-oxides with unprecedented diastereoselectivity (see scheme).

Full text of DOI:10.1002/chem.201502129

DOI: 10.1002/chem.201502129
Communication
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Organic Synthesis |Hot Paper|
Highly Diastereo- and Enantioselective Michael Addition of
Nitroalkanes to 2-Enoyl-Pyridine N-Oxides Catalyzed by
Scandium(III)/Copper(II) Complexes
Lijun Li, Sheng Zhang, Yanbin Hu, Yanan Li, Chong Li, Zhenggen Zha, and Zhiyong Wang*[a]
enantioselective version of this reaction (Scheme 1).[2g–n] Mean-
Abstract: A C2-symmetric Schiff-base ligand, derived from
tridentate-Schiff-base, was developed and successfully ap-
plied to the asymmetric Michael addition of nitroalkanes
to 2-enoyl-pyridine N-oxides. With this newly catalytic
system, an unprecedented diastereoselectivity was ob-
tained in the asymmetric Michael addition of nitroalkanes
to 2-enoyl-pyridine N-oxides. In addition, a switch in enan-
tioselectivity was achieved by using this newly catalytic
system and our previous catalyst. After a facile reduction,
the optically active adduct was converted to a biologically
active dihydro-2H-pyrrol 4a. Furthermore, a connection of
two tridentate-Schiff-base subunits proved to be an effec-
tive strategy in ligand design.
while, 2-enoyl-pyridine N-oxides have been extensively ex-
plored in asymmetric transformations as a popular Michael ac-
ceptor since the pioneering work of Jørgensen, Pedro, and
Singh.[3] Very recently, the enantioselective addition of nitrome-
thane to 2-acylpyridine N-oxides has been reported by the
Pedro group.[4] However, the 1,4-addition of nitroalkanes to 2-
enoyl-pyridine N-oxides has not been explored yet (Scheme 1).
In this context, conjugate addition of nitroalkanes to 2-enoyl-
pyridine N-oxides was chosen as a model reaction to address
the issue of diastereoselectivity.
To solve the diastereoselectivity issue of this reaction, a C2-
symmetric Schiff-base ligand was designed, which was based
on our previously developed chiral tridentate-Schiff-base L1
(Scheme 2). In our previous work, this efficient Schiff-base
system showed high performance in various asymmetric trans-
The Michael addition of stabi-
lized carbanions to electron-defi-
cient alkenes catalyzed by transi-
tion metals has emerged as
a powerful tool in organic syn-
thesis for constructing carbon–
carbon bonds.[1] Although im-
pressive progress in the transi-
tion-metal-catalyzed asymmetric
Michael additions has been
made in the past decades, there
remains a great challenge in the
diastereoselectivity issue of Mi-
chael additions.[2] It is notewor-
thy that a satisfactory diastereo-
selectivity in the Michael addi-
tion of nitroalkanes to a,b-unsa-
Scheme 1. Enantioselective Michael addition of nitroalkanes to 2-enoyl-pyridine N-oxides.
turated ketones still was a chal-
lenging problem despite the
immense developments in the
formations including Henry and Fridel–Crafts reactions.[5] How-
ever, the limitation of tridentate-Schiff-base L1 as a ligand has
also been recognized gradually. We found that chiral ligand L1
was generally used with post-transition metals, such as copper
and zinc. Due to the difference of ionic radius between post-
transition metals and early-transition metals (especially for rare
earth metals), we envisaged that chiral ligand L2, which was
readily obtained by a connection of two tridentate-Schiff-base
subunits, may complex with early transition metals and be ap-
[a] L. Li, S. Zhang, Y. Hu, Y. Li, C. Li, Prof. Z. Zha, Prof. Dr. Z. Wang
Hefei National Laboratory for Physical Sciences at Microscale
CAS Key Laboratory of Soft Matter Chemistry & Collaborative Innovation
Center of Suzhou Nano Science and Technology
Department of Chemistry, University of Science and Technology of China
Hefei, Anhui 230026 (China)
E-mail: zwang3@ustc.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502129.
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enantioselectivity, but when ligand L1 was employed only
a trace amount of product 3aa was observed. To further opti-
mize the reaction conditions, L2b was chosen as an optimal
ligand and the effect of solvent on the asymmetric induction
was also explored (entries 3, 5–8). THF was found to be the
most effective solvent in the Michael addition in terms of the
yields and levels of asymmetric induction. Interestingly, when
we replaced the ligand L2b with twice the amount of L1, race-
mic product was obtained with low yield (entry 9). This result
illustrated that the 1,3-diaminopropane linked Schiff-base
ligand L2b played a different role in this reaction compared to
two equivalents of L1. Other early transition metals were also
tested, but no better result was observed (entries 10–11). After
screening different reaction conditions, both of enantiomers of
3aa can be obtained with 92 and 95% ee, respectively, when
chiral copper(II) and scandium(III) catalysts were employed (en-
tries 4–5).
Scheme 2. Our strategy to design chiral ligand L2.
plied in various asymmetric reactions (Scheme 2).[6] In this com-
munication, we found that the chiral ligand L2 indeed com-
plexed with early transition-metal scandium(III). With this
newly designed catalyst system, asymmetric Michael addition
of nitroalkanes to 2-enoyl-pyridine N-oxides was first devel-
oped with high diastereo- and enantioselectivity.
With these results in hand, we decided to probe the scope
of the asymmetric Michael addition of nitromethane 2a to 2-
enoyl-pyridine N-oxides with chiral scandium and copper cata-
lysts. The scandium-catalyzed Michael addition proceeded with
good to excellent enantioselectivities to give the chiral pyri-
dine N-oxides products 3aa–3la in moderate to good yield.
Remarkably, the 4-CF3-substituted substrate 1h, which had not
previously been explored in asymmetric transformation, was
also reacted efficiently to afford 3ha with excellent enantiose-
lectivity (92% ee). In addition, the opposite configuration of
the products could also be obtained with high control of enan-
tioselectivity (90–93% ee) (Table 2, entries 1, 6, 8, 10) when we
applied the chiral copper complex to this reaction.
In a first series of experiments we tested our previous li-
gands and newly designed ligands (Table 1, entries 1–4) in the
Michael addition of nitromethane to 2-enoyl-pyridine N-oxide
1a in the solvent of toluene at room temperature. We found
that both the newly developed ligands L2a/L2b and our previ-
ous ligand L3 were able to catalyze this reaction with opposite
Table 1. Optimization of the enantioselective Michael addition of nitrome-
thane to 2-enoyl-pyridine N-oxides.[a]
Table 2. Enantioselective Michael addition of nitromethane to 2-enoyl-pyri-
dine N-oxides.[a]
Entry
R
3
Yield [%][b]
ee [%][c]
1[d]
2
3
4
Ph
3aa
3ba
3ca
3da
3ea
3 fa
3ga
3ha
3ia
89 (82)
87
83
61
81
72 (63)
68
74 (85)
71
69 (74)
77
65
95 (À92)
Entry
Ligand
Metal salt
Solvent
Yield [%][b]
ee [%][c]
3-MeC6H4
3-OMeC6H4
3-NO2C6H4
3-FC6H4
3-ClC6H4
3-BrC6H4
4-CF3C6H4
4-ClC6H4
4-BrC6H4
2-ClC6H4
2-naphthyl
90
1
2
3
4[d]
5
6
7
8
9[e]
10
11
L1
Cu(OTf)2
Sc(OTf)3
Sc(OTf)3
Cu(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Yb(OTf)3
Y(OTf)3
Tol.
Tol.
Tol.
Tol.
THF
MTBE
EtOAc
CHCl3
THF
THF
THF
trace
31
58
82
89
76
73
61
43
78
46
n.d.
57
75
À92
95
87
93
92
À3
47
25
92
90
93
L2a
L2b
L3
L2b
L2b
L2b
L2b
L1
5
6[d]
7
96 (À90)
94
8[d]
9
92 (À93)
95
10[d]
11
12
3ja
3ka
3la
94 (À91)
89
L2b
L2b
88
[a] Reactions were conducted using 0.3 mmol of 1 and 0.3 mL of 2a with
10 mol% of Sc(OTf)3, 10 mol% of Cs2CO3 and 10 mol% of ligand in THF
(1.2 mL) at RT. [b] The product was isolated by flash chromatography.
[c] The ee value of the product was determined by HPLC analysis on a chiral
stationary phase. [d] The data in the parentheses was observed when the
reaction was performed with chiral copper catalyst (see details in the Sup-
porting Information (p. S6–7)).
[a] Reactions were conducted using 0.1 mmol of 1a and 0.1 mL of 2a with
10 mol% of metal salts, 10 mol% Cs2CO3 and 10 mol% of ligand in solvent
(0.4 mL) at RT for 48 h. [b] The product was isolated by flash chromatogra-
phy. [c] The ee value of the product was determined by HPLC on a chiral
stationary phase. [d] The reaction was performed for 5 days. [e] 20 mol%
ligand was used. MTBE=methyl tert-butyl ether.
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(entry 17). In addition, nitropropane also proved to be a good
substrate in this reaction in terms of diastereo- and enantiose-
lectivity (entry 18).
Table 3. Diastereo- and enantioselective Michael addition of nitroalkanes
to 2-enoyl-pyridine N-oxides.[a]
To showcase the synthetic utility of this process, we applied
this Michael addition to the synthesis of dihydro-2H-pyrrol 4a,
which was considered as an analogue of nicotine (Scheme 3).[7]
The adduct 3aa underwent a facile reduction procedure,
which involved the reduction of the nitro group and in situ
cyclization, affording biologically active dihydro-2H-pyrrol 4a
with a slight erosion of enantiomeric excess.
Entry
R
3
Yield [%][b] d.r. [anti/syn][c] ee [%][d]
1[e]
2
Ph
4-FC6H4
3ab 87 (94)
3mb 86
11:1 (1:1.6)
9:1
93 (À80:À82)
93
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
4-ClC6H4
4-BrC6H4
4-MeC6H4
4-OMeC6H4
3-MeC6H4
3-OMeC6H4
3-NO2C6H4
3-FC6H4
3-ClC6H4
2-FC6H4
2,4-OMeC6H3 3qb 89
2-naphthyl
2-thienyl
2-furyl
n-hexyl
Ph
3ib
3jb
76
72
10:1
12:1
10:1
9:1
6:1
8:1
4:1
5:1
4:1
9:1
11:1
8:1
3:1
3:1
25:1
9:1
92
95
92
91
91:76
95
91:89
91:88
98:n.d.
93
92
95
93:67
92:37
95
To get insight into the reaction mechanism and the coordi-
nation environment around the scandium center, the relation-
ship between the ee values of chiral ligand and the corre-
sponding ee values of the product 3aa was studied.[8] Interest-
ingly, a clear negative nonlinear effect, which was quite
different from previously reported work, was observed in the
process catalyzed by scandium.[9] This nonlinear effect demon-
strated that the chiral scandium catalyst formed a dimeric
structure in the solvent. In addition, the homochiral dimer was
more easily formed and stabilized in the system, thus leading
to the homochiral dimer showing low activity for the reaction.
On the other hand, 1H NMR spectroscopic experiments were
also performed to further understand the structure of chiral
scandium(III) complex.[10] Based on these results, and the abso-
lute configuration of the product 3ca,[11] a possible transition-
state model was proposed as shown in Figure 1.
3nb 84
3ob 89
3bb 86
3cb 86
3 db 64
3eb 81
3 fb
3pb 79
73
3lb
3rb
3sb 76
3tb
3ac
84
75
69
72
93
[a] Reactions were conducted using 0.3 mmol of 1 and 0.3 mL of 2b or 2c
with 15 mol% of Sc(OTf)3, 15 mol% of Cs2CO3 and 15 mol% of ligand in
THF (1.2 mL) at RT. [b] The product was isolated by flash chromatography.
[c] The d.r. (d.r.=anti/syn) value was determined by 1H NMR spectroscopy.
[d] The ee value of the product was determined by HPLC analysis on
a chiral stationary phase. [e] The data in the parentheses was observed
when the reaction was performed with chiral copper catalyst (see details
in the Supporting Information (p. S7)).
In conclusion, a highly diastereo- and enantioselective Mi-
chael addition of 2-enoyl-pyridine N-oxides with nitroalkanes
was first developed. To the best of our knowledge, this level of
diastereoselectivity is unprecedented in the asymmetric 1,4-ad-
Having surveyed the scope of the Michael addition
of nitromethane to 2-enoyl-pyridine N-oxides, we
were poised to investigate other more challenging
nitroalkanes, such as nitroethane and nitropropane
(Table 3). To temper the lower reactivity of these ni-
troalkanes, we improved the catalyst loading to
15 mol%. Under these conditions, a wide range of
electron-donating (Table 3, entries 5–6) and -with-
drawing substituted (entries 2–4) aromatic substrates
were well tolerated with enantiomeric excesses up to
Scheme 3. The synthetic utility of the Michael addition of nitroalkanes to 2-enoyl-pyri-
dine N-oxides.
95%. To our delight, unprecedented diastereoselectivities (9:1–
12:1 d.r.) were observed in these examples. Furthermore, the
variation of the substituent position on aromatic rings of sub-
strates 1 was compatible in this efficient process, thereby
giving access to the products with excellent enantioselectivity
(91–98% ee) and moderate to good diastereoselectivity (d.r.
4:1–11:1) (entries 7–13). We were pleased to find that the sub-
strate bearing 2,4-OMe groups was also tolerated in this pro-
cess with 92% ee and 11:1 d.r.. To further evaluate the scope of
substrates, we replaced the phenyl with the 2-naphthyl and
heterocyclic group in substrate and the corresponding prod-
ucts 3lb, 3rb–sb were furnished with a decrease of d.r. value.
Notably, an exceptionally high diastereoselectivity (25:1) and
95% ee were furnished in the case of aliphatic aldehyde-de-
rived substrate 1t, albeit with a slight decrease of yield
Figure 1. The relationship between the ee values of chiral ligand and the
corresponding ee values of the product 3aa and the proposed transition-
&
state model. : chiral scandium catalyst.
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ficient method, biologically active dihydro-2H-pyrrol 4a was
readily synthesized. More importantly, we developed a novel
C2-symmetric Schiff-base ligand L2b, which showed excellent
catalytic activity and high stereoselectivity in this reaction
when used with scandium(III) triflate. Moreover, a relatively
rare negative nonlinear effect was observed in this process,
which could be helpful for further investigation of the reaction
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Experimental Section
General procedure
A mixture of Sc(OTf)3 (14.8 mg, 0.03 mmol) and the ligand (L2b,
21.4 mg, 0.03 mmol) in THF (1.2 mL) with Cs2CO3 (9.8 mg,
0.03 mmol) was stirred at room temperature for 2 h under nitro-
gen. Compound 1a (67.6 mg, 0.3 mmol) and 2a (0.3 mL) were
then added and the resulting mixture was stirred at room tempera-
ture. After the reaction was complete (monitored by TLC analysis),
the solvent was removed under vacuum. The residue was purified
by silica gel column chromatography with petroleum ether/ethyl
acetate (1:3) as the eluent affording the product 3aa as a yellow
oil (76.4 mg, 89% yield, 95% ee). The enantiomeric excess of the
product was determined by chiral HPLC analysis (Chiralpak AD-H
column).
[4] M. Holmquist, G. Blay, M. C. MuÇoz, J. R. Pedro, Org. Lett. 2014, 16,
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[6] For the design of chiral ligands used with post-transition metals, see:
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Acknowledgements
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The authors are grateful to the National Nature Science Foun-
dation of China (2127222, 91213303, 21432009, 21472177).
[8] The nonlinear effect of copper catalyst was also studied. See Sec-
tion 1.7.1 (p. S9–10) of the Supporting Information.
Keywords: catalysis · diastereoselectivity · Michael addition ·
nitroalkanes · scandium
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Information Section 1.7.2 (page S10–11).
[11] The absolute configuration of the products was proven by X-ray crystal
structure analysis of product 3ca. See Section 1.5 (p. S8) of the Support-
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tallographic 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.
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Received: May 31, 2015
Published online on July 21, 2015
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