C2 symmetric copper (II) complexes of L-valine and L-phenyl alanine based chiral diimines for catalytic asymmetric Henry reaction

Article abstract of DOI:10.1016/j.tetlet.2021.153090

New C2 symmetric chiral diimines were synthesized from the amino acids L-valine and L-phenyl alanine. In situ copper (II) complexes of the chiral diimine ligands were found to catalyze asymmetric Henry reaction. The chiral nitro aldols were formed in excellent yield (99%) and ee (99%). The synthetic utility of the chiral catalysts were screened with various substituted prochiral aromatic and heteroaromatic aldehydes. Possible catalytic cycle for the chiral diimine copper complex catalyzed asymmetric Henry reaction has been proposed. The stereoselectivity of the asymmetric Henry reaction was discussed based on the transition state in the catalytic cycle.

Full text of DOI:10.1016/j.tetlet.2021.153090

Tetrahedron Letters 72 (2021) 153090
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
C2 symmetric copper (II) complexes of L-valine and L-phenyl alanine
based chiral diimines for catalytic asymmetric Henry reaction
K. Vinoth Kumar a, Peter Daniel Nixon b, S. Jesu Johnson a, Nallamuthu Ananthi a,
⇑
a Department of Applied Chemistry, Karunya Institute of Technology and Sciences, Coimbatore 641114, Tamil Nadu, India
b Center for Nano Sciences and Genomics, Karunya Institute of Technology and Sciences, Coimbatore 641114, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 January 2021
Revised 8 April 2021
Accepted 13 April 2021
Available online 16 April 2021
New C2 symmetric chiral diimines were synthesized from the amino acids L-valine and L-phenyl alanine.
In situ copper (II) complexes of the chiral diimine ligands were found to catalyze asymmetric Henry reac-
tion. The chiral nitro aldols were formed in excellent yield (99%) and ee (99%). The synthetic utility of the
chiral catalysts were screened with various substituted prochiral aromatic and heteroaromatic aldehy-
des. Possible catalytic cycle for the chiral diimine copper complex catalyzed asymmetric Henry reaction
has been proposed. The stereoselectivity of the asymmetric Henry reaction was discussed based on the
transition state in the catalytic cycle.
Keywords:
Asymmetric Henry reaction
Chiral diimines
Ó 2021 Elsevier Ltd. All rights reserved.
Chiral copper complex
L
-Valine
L
-Phenyl alanine
Introduction
Role of symmetry in chiral catalysts is one of the imperative fac-
tors in the enantioselectivity of catalytic asymmetric transforma-
Chiral nitroaldols are synthetically important compounds for
deriving various useful organic intermediates, potential drugs
and biologically important compounds. Chiral nitro aldols can be
easily converted into other useful chiral compounds such as chiral
amino alcohols, chiral amino sugars, chiral nitro compounds, chiral
substituted carboxylic acids etc [1,2]. Catalytic asymmetric Henry
reaction can be utilized to prepare interesting chiral nitroaldols.
Transition metal chiral catalysts possess advantages such as utility
in both homogeneous and heterogeneous conditions due to their
multiple oxidation state adaptability, their highly selective nature
and ambient reaction conditions [3].
Among the transition metal complexes utilized as catalysts for
catalytic asymmetric Henry reaction, copper complexes have been
found to possess excellent catalytic activity. Various chiral ligands
have been complexed with copper and employed as catalysts in
catalytic asymmetric Henry reaction which include chiral diamine
ligands [4,5], chiral tridendate ligands [6,7], chiral oxazoline based
ligands [8–10], chiral Schiff-base ligands [6,11,12], chiral salen
ligands [13–16], chiral amino alcohol based ligands [7,17–19], chi-
ral N,N’-dioxide based ligand [20], sparteine based ligand [21],
bisisoquinoline ligands [22,23], and cinchona alkaloid based
ligands [24,25].
tions. Symmetry in chiral catalysts reduces the undesirable side
reactions and promotes the expected transformation to occur
[26–28].
Among the symmetric chiral catalysts, C2 symmetric chiral cat-
alysts have been identified as potential catalysts for asymmetric
catalysis [26]. C2 symmetric chiral catalysts which have been pro-
ven to be efficient asymmetric catalysts are the well known Jacob-
sen catalyst [29], bisoxazoline [8], phospholane [30], salen [31],
BINOL [32], BINAP [33], diamines [34] type chiral catalysts.
Amino acids have many advantages to be used as chiral source
for the preparation of chiral catalysts for asymmetric catalysis
which include easy availability, low cost, simple functionalization,
availability of both enantiomers, ambient reaction conditions,
environmentally benign nature and recyclability [35]. These
advantages are very well needed for the design of potential chiral
catalysts. Amino acid based chiral catalysts are good at complexing
with transition metals and hence they are capable of promoting
several types of transition metal catalyzed asymmetric reactions.
Amino acids have been derived into various kinds of chiral scaf-
folds such as C2-symmetric N,N’-dioxides, guanidine amides, bis-
pidine-based diamines and organic salts. These compounds have
been employed as catalysts in various asymmetric transformations
[36,37]. L-Valine based chiral catalysts have been used for a range
of asymmetric reactions to obtain optically active chiral intermedi-
⇑
Corresponding author.
E-mail address: ananthi@karunya.edu (N. Ananthi).
ates. In the year 2010, we reported L-diphenylvalinol based salen
https://doi.org/10.1016/j.tetlet.2021.153090
0040-4039/Ó 2021 Elsevier Ltd. All rights reserved.

K. Vinoth Kumar, Peter Daniel Nixon, S. Jesu Johnson et al.
Tetrahedron Letters 72 (2021) 153090
chiral copper complex catalyzed asymmetric Henry reaction [38].
Air and water stable
lized as catalyst in the asymmetric nitroaldol reaction [39].
L
-Phenyl alanine derived Schiff base was uti-
-Phe-
L
nyl alanine derived thiazole type chiral ligands possessing
tetrahydroisoquinoline were utilized as catalysts in asymmetric
Henry reaction [40].
Aromatic amino acid
are few. In this work, two C2-symmetric chiral diimines were syn-
thesized from the amino acids -Valine and -Phenyl alanine by
reacting them with isophthalaldehyde. The -Valine derived ligand
L-Phenyl alanine derived chiral catalysts
Fig. 1. Structure of chiral diimine ligands 1b and 2b.
L
L
L
was already utilized in the asymmetric synthesis of unsaturated
lactams by ring closing metathesis [41]. The insitu copper (II) com-
plexes of these chiral diimines were found to catalyze asymmetric
Henry reaction. The resulting chiral nitroaldols were formed in
excellent yield and ee.
Results and discussion
Scheme 2. Catalytic asymmetric Henry reaction catalyzed by 1b (or) 2b-Cu
(OAc)2ꢀH2O.
L-Valine (1) and L-Phenyl alanine (2) based chiral diimines (1b
and 2b) have been synthesized. The carboxylic acid groups of the
amino acids were protected by converting them to ester function-
ality (1a and 2a). The amino groups in the ester protected amino
acids were functionalized by reacting with isophthalaldehyde in
2:1 ratio respectively under mild reaction conditions (Scheme 1).
The structures of the resulting chiral diimines (1b and 2b) are
shown in figure 1.
The synthesized chiral diimine ligands (1b and 2b) were
screened for the catalytic activity in asymmetric Henry reaction.
Initially benzaldehyde and nitromethane were made to react with
5 mol% of chiral diimine 1b with Cu(OAc)2ꢀH2O in the presence of
triethyl amine in ethanolic medium at ꢁ77 °C. (Scheme 2).
Low temperature reaction yielded only the racemic nitro aldol
in 50% yield (Table 1, Entry 1). Then the reaction was carried out
at 0 °C with same amount of catalytic system (5 mol%) with same
solvent. Reaction at 0 °C yielded the chiral nitroaldol in 60% yield
with 10% ee (Table 1, Entry 2). The temperature of the reaction
was further increased to room temperature which resulted in the
formation of chiral nitro aldol in 75% yield with 60% ee (Table 1,
Entry 3). The reaction temperature was further increased to
78 °C. At 78 °C, the chiral nitro aldol was formed in 75% yield with
40% ee (Table 1, entry 4). From the results of temperature optimiza-
tion of the reaction, it was found that room temperature gave the
chiral nitroaldols in good yield and ee (Table 1, Entry 3).
(Table 1, entries 5, 7 & 8). The yield and ee of the chiral nitro aldols
formed using nitroethane is comparable with that of nitromethane
(Table 1, entries 13 & 14).
Nitromethane was also tried as solvent, but there was no con-
version (Table 1, Entry 6). After finding the optimized reaction con-
ditions which resulted in the chiral nitro aldol in better yield and ee
for the chiral diimine 1b, with the same reaction conditions chiral
diimine 2b was also explored for the catalytic activity in the same
reaction.
In order to study the effect of base on the asymmetric Henry
reaction between nitromethane and benzaldehyde, the reaction
was carried out with various amounts of triethylamine as well as
with few organic and inorganic bases like Hunigs base, NMM,
NaOH and K2CO3. Hunig’s base resulted in the chiral nitro aldol
in comparable yield and ee as that of triethylamine (Table 2, entry
2). This may be due to the negligible difference between the pKb
values of Hunig’s base and triethylamine. NMM was found to be
less effective compared to triethylamine and Hunig’s base (Table 2,
entry 3). And also it was found that 1 mol% of triethylamine was
enough to achieve good yield and ee. Increasing the amount of tri-
ethylamine above 1 mol% also resulted in the same highest yield
and ee that resulted for 1 mol% of triethylamine. NaOH showed
very less activity than the other bases tried (Table 2, entry 4). With
K2CO3 the reaction was not successful (Table 2, entry 5).
Chiral diimine 2b-Cu(OAc)2ꢀH2O catalyzed the asymmetric
Henry reaction and gave the chiral nitro aldol in excellent yield
(99%) and ee (99%) with 10 mol%, 15 mol% and with stoichiometric
amount of the catalytic system (Table 1, Entries 10,11,12). The sub-
strate scope of the chiral diimine 2b-Cu(OAc)2ꢀH2O was further
The influence of the amount of catalyst was studied by increas-
ing the amount of the catalyst from 5 mol% to 10 mol%. 10 mol% of
the catalytic system gave the chiral nitro aldol in 95% yield with
90% ee (Table 1, Entry 5). Further increase in the amount of the cat-
alyst (15 mol%) and the stoichiometric amount of the catalyst
resulted in the formation of the chiral nitro aldol in same yield
and ee as that with 10 mol% of the catalytic system in ethanol
Scheme 1. Synthesis of chiral diimine ligands 1b and 2b.
2

K. Vinoth Kumar, Peter Daniel Nixon, S. Jesu Johnson et al.
Tetrahedron Letters 72 (2021) 153090
Table 1
Optimization of reaction conditions for the asymmetric Henry reaction catalyzed by 1b (or) 2b with Cu(OAc)2ꢀH2O.a
S. No
Ligand
Amount of catalyst
Temp (°C)
Yield (%)b
ee (%)c
Abs. config.d
1
2
3
4
1b
1b
1b
1b
1b
1b
1b
1b
2b
2b
2b
2b
2b
2b
5 mol%
5 mol%
5 mol%
5 mol%
ꢁ77
0
50
60
75
75
95
5
95
95
80
99
99
99
90
90
0
–
S
S
S
S
–
S
S
S
S
S
S
S
S
10
60
40
90
0
90
90
70
99
99
99
90
90
25
78
25
25
25
25
25
25
25
25
25
25
5
10 mol%
10 mol%
15 mol%
100 mol%
5 mol%
10 mol%
15 mol%
100 mol%
10 mol%
100 mol%
6e
7
8
9
10
11
12
13f
14g
a
b
The ratio of ligand 1b/2b-Cu(OAc)2ꢀH2O: nitromethane: triethylamine: benzaldehyde was 0.1 mmol: 1 mmol: 0.01 mmol: 1 mmol.
Isolated yield after column chromatography.
The enantiomeric excess was determined by HPLC analysis using chiralcel OD-H column.
The absolute configuration was assigned to the major enantiomer by comparison with the sign of the specific rotation given in the literature data [42].
Reaction was carried out with nitromethane as solvent.
c
d
e
f,g
Reactions were carried out using nitroethane.
efficiently and gave the corresponding chiral nitro aldols in excel-
lent yield and ee (Table 3, entries 1–9). The possible catalytic cycle
for the asymmetric Henry reaction catalyzed by the chiral diimines
1b and 2b with Cu(OAc)2ꢀH2O is given in Scheme 3.
Table 2
Effect of bases on the asymmetric Henry reaction.a
S. No
Base
Yield (%)b
ee (%)c
1
2
3
4
5
Triethylamine
Hunig’s base (N, N’-Diisopropylethylamine)
NMM (N-Methyl Morpholine)
NaOH
K2CO3
99
99
50
10
0
99
90
45
10
0
Initially the coordination of nitronate anion with copper com-
plex of the chiral diimine takes place through the oxygen of the
nitronate anion (1c, 2c). Then oxygen atom of the carbonyl group
of benzaldehyde coordinates with copper forming a six membered
transition state (1d, 2d). In the transition state (Scheme 4), the
attack of the nitronate anion on the carbonyl group of benzalde-
hyde takes place (1e, 2e) leading to the formation of chiral 2-
nitro-1-phenylethanol. The transition state in the catalytic cycle
of the asymmetric Henry reaction has been taken into considera-
tion for studying the optimised face attack (which can lead to high
ee) of nitronate anion on the carbonyl group of benzaldehyde
which can eventually lead to the stereoselectivity of the asymmet-
ric Henry reaction (Scheme 4). The Re face attack of the nitronate
anion on the carbonyl group of benzaldehyde is responsible for
the preferential formation of (S) enantiomer ((S)-2-nitro-1-pheny-
lethanol) which may be favoured by the steric factor of phenyl
group (Scheme 4).
a
The ratio of ligand 2b-Cu(OAc)2ꢀH2O: nitromethane: base: benzaldehyde was
0.1 mmol: 1 mmol: 0.01 mmol: 1 mmol. bIsolated yield after column chromatog-
raphy. cThe enantiomeric excess was determined by HPLC analysis using chiralcel
OD-H column.
analyzed by carrying out the asymmetric Henry reaction on vari-
ous substituted prochiral aromatic and hetero aromatic aldehydes.
Hetero aromatic aldehydes also were converted to the corre-
sponding chiral nitro aldols in good yield and ee (Table 3, entries
10, 11 & 12). From the results of the asymmetric Henry reaction
with substituted aldehydes, it was found that the aldehydes
possessing electron withdrawing groups underwent the reaction
Table 3
Asymmetric Henry reaction between substituted aromatic, aliphatic and hetero aromatic aldehydes with nitromethane using 10 mol% 2b-Cu(OAc)2ꢀH2O as catalyst.a
Entry
Aldehyde
Yield(%)b
ee (%)c
Abs. configurationd
1
2
3
4
5
6
7
8
Benzaldehyde
99
99
99
99
99
99
99
99
80
80
80
80
82
84
99
99
95
90
88
90
90
68
70
60
30
34
45
50
S
S
o-Nitro benzaldehyde
m-Nitro benzaldehyde
p-Nitro benzaldehyde
p-Chloro benzaldehyde
p-Methoxy benzaldehyde
o-Chloro benzaldehyde
p-Methyl benzaldehyde
4-Hydroxy 3-methoxy benzaldehyde
Pyrrole-2-carboxaldehyde
Furfural
S [43]
S
S
S [44]
S
S [43]
S
9
10
11
12
13
14
S
S [43]
S [43]
S [42]
S [42]
Thiophene-2-carboxaldehyde
3-Methyl butyraldehyde
3-Methyl propionaldehyde
a
b
c
The ratio of ligand 2b-Cu(OAc)2ꢀH2O: Nitromethane: Triethylamine: Benzaldehyde was 0.1 mmol: 1 mmol: 0.01 mmol: 1 mmol.
Isolated yield after column chromatography.
The enantiomeric excess was determined by HPLC analysis using chiralcel OD-H column.
d
The absolute configuration was assigned to the major enantiomer by comparison with the sign of the specific rotation given in the literature data [42].
3

K. Vinoth Kumar, Peter Daniel Nixon, S. Jesu Johnson et al.
Tetrahedron Letters 72 (2021) 153090
solution of isophthalaldehyde was added drop wise in 2:1 ration
upon stirring. The reaction mixture was then refluxed with stirring
at 65 °C for 3 h. After the completion of the reaction monitored by
TLC, the reaction mixture was filtered and the solvent was
removed using rotary evaporator to yield the corresponding chiral
diimines (1b and 2b).
General procedure for the asymmetric Henry reaction catalyzed by 1b/
2b – Cu(OAc)2ꢀH2O
Chiral diimine 1b or 2b was dissolved in anhydrous ethanol on
stirring. To this Cu(OAc)2ꢀH2O was added in 1:1 ration and stirred
for one hour for the complex formation. After the color of the reac-
tion medium turned from yellow to green, nitro methane was
added and stirred. To this triethylamine was also added. Then ben-
zaldehyde was added drop wise and the reaction mixture was stir-
red at room temperature for 12 h. After the completion of the
reaction confirmed by TLC, 1 M HCl was added to the reaction mix-
ture to neutralize the medium. Then the solvent was removed
using rotary evaporator. The resulting aqueous solution was
extracted with ethylacetate. The organic layer was dried using
anhydrous sodium sulphate. The product chiral nitroaldol was
purified using column chromatography using silica gel as an adsor-
bent with 95:5 hexane:ethylacetate as an eluant.
Scheme 3. Possible catalytic cycle for the catalytic asymmetric Henry reaction
catalysed by 1b or 2b-Cu(OAc)2ꢀH2O.
Conclusion
New C2 symmetric chiral diimines were synthesised from
amino acids L-valine and L-phenyl alanine. The in situ copper (II)
complexes of the chiral diimines were found to catalyse asymmet-
ric Henry reaction of prochiral aromatic and hetero aromatic alde-
hydes. The chiral nitro aldols were formed in 99% yield and 99% ee.
The synthetic utility of the catalyst was screened by carrying out
the asymmetric Henry reaction on substituted prochiral aromatic
and hetero aromatic aldehydes. The corresponsding chiral nitro
aldols were formed in excellent yield and ee. Aldehydes possessing
electron withdrawing groups undergone the reaction efficiently
compared to aldehydes possessing electron donating group. The
preferential formation of (S)-2-nitro-1-phenylethanol was dis-
cussed based on the transition state involved in the catalytic cycle.
Re face attack of the nitronate anion on the carbonyl group of ben-
zaldehdye in the transition state favours the formation of S
enantiomer.
Scheme 4. Transition state in the catalytic asymmetric Henry reaction.
Declaration of Competing Interest
Experimental section:
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
General procedure for the preparation of aminoacid ester
hydrochlorides (1a & 2a)
Acknowledgements
To the methanolic solution of the amino acid (1 or 2), thionyl
chloride (SOCl2) was added drop wise under 0 °C with stirring.
Then the reaction mixture was stirred for 12 h at room tempera-
ture. After confirming the completion of the reaction by TLC, the
solvent and excess thionyl chloride were removed using rotary
evaporator. The corresponding aminoacid methyl ester hydrochlo-
ride 1a and 2a were obtained as colourless crystals in quantitative
yield.
The authors thank DST SERB, Government of India for the
research grant released for the project no. DST-SERB (SB/FT/CS-
074/2012).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tetlet.2021.153090.
General procedure for the preparation of chiral diimines (1b & 2b)
References
To the methanolic solution of the amino acid methyl esters (1a
and 2a separately) small amount of sodium hydrogen carbonate
was added and stirred for half an hour until the pH of the reaction
medium becomes neutral or slightly basic. To this, methanolic
[1] C. Matt, A. Wagner, C. Mioskowski, Novel transformation of primary nitro
alkanes and primary alkyl bromides to the corresponding carboxylic acids, J.
Org. Chem. 62 (1997) 234–235.
4

K. Vinoth Kumar, Peter Daniel Nixon, S. Jesu Johnson et al.
Tetrahedron Letters 72 (2021) 153090
[2] M.A. Poupart, G. Fazal, S. Goulet, L.T. Mar, Solid-phase synthesis of peptidyl
trifluoromethyl ketones, J. Org. Chem. 64 (1999) 1356–1361.
[23] Y.Q. Ji, G. Qi, Z.M.A. Judeh, Enantioselective nitroaldol reaction catalyzed by
chiral C1-tetrahydro-1,10-bisisoquinoline–copper(I) complexes, Tetrahedron:
Asymmetry 22 (2011) 929–935.
[24] M. Liu, S.J. Ma, Z. Tian, H. Wu, L. Wu, X. Xu, Y. Huang, Y. Wang, Modular
bipyridine ligands coupled with Cinchona alkaloids for Cu(II)-catalyzed
asymmetric Henry reactions, Tetrahedron: Asymmetry 24 (2013) 736–743.
[25] L. Zhang, H. Wu, Z.Y. Yang, X. Xu, H. Zhao, Y. Huang, Y. Wang, Synthesis and
computation of diastereomeric phenanthroline–quinine ligands and their
application in asymmetric Henry reaction, Tetrahedron 69 (2013) 10644–
10652.
[3] C. Palomo, M. Oiarbide, A. Laso, Recent advances in the catalytic asymmetric
nitroaldol (Henry) reaction, Eur. J. Org. Chem. 16 (2007) 2561–2574.
[4] R. Matsubara, S. Kobayashi, Enamides and enecarbamates as nucleophiles in
stereoselective C-C and C–N bond forming reactions, Acc. Chem. Res. 41 (2008)
292–301.
[5] B. Saito, G.C. Fu, Enantioselective alkylꢁalkyl Suzuki cross-couplings of
unactivated homobenzylic halides, J. Am. Chem. Soc. 130 (2008) 6694–6695.
[6] G.R. Qiang, T.H. Shen, X.C. Zhou, X.X. An, Q.B. Song, Synthesis of novel chiral
tridentate schiff-base ligands and their Applications in catalytic asymmetric
henry reaction, Chirality 26 (2014) 780–783.
[26] C. Moberg, The role of symmetry in asymmetric catalysis, Israel Journal of
Chemistry 52 (2012) 653–662.
[7] F. Xu, C. Lei, L. Yan, J. Tu, G. Li, Copper-chiral camphor b-amino alcohol complex
catalyzed asymmetric Henry reaction, Chirality. 27 (2015) 761–765.
[8] G. Desimoni, G. Faita, K.A. Jorgensen, C2-Symmetric chiral bis (oxazoline)
ligands in asymmetric catalysis, Chem. Rev. 106 (2006) 3561–3651.
[9] L.M. Stanley, M.P. Sibi, Chiral bisoxazoline ligands, in: Q.L. Zhou (Ed.),
Privileged Chiral Ligands and Catalysts, Wiley-VCH, Weinheim, 2011, pp.
171–219.
[27] J.K. Whitesell, C2 symmetry and asymmetric induction, Chem. Rev. 89 (1989)
1581–1590.
[28] S.E. Gibson, M.P. Castaldi, Applications of chiral C3 symmetric molecules,
Chem. Commun. 3045–3062 (2006).
[29] W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobsen, Enantioselective epoxidation
of unfunctionalized olefins catalyzed by salen manganese complexes, J. Am.
Chem. Soc. 112 (1990) 2801–2803.
[10] J.Y. Mao, X. Nie, M. Wang, Q. Wang, B. Zheng, B. Qinghua, Z. Jiangchun,
Catalytic asymmetric nitroaldol (Henry) reactions with copper
(II)/cyclopropane based bisoxazoline complexes, Tetrahedron: Asymmetry 23
(2012) 965–971.
[11] C. Li, F. Guo, K. Xu, S. Zhang, Y. Hu, Z. Zha, Z. Wang, Copper catalyzed
enantioselective Friedel Crafts alkylation of pyrrole with isatins, Org. Lett. 16
(2014) 3192–3195.
[30] T.P. Clark, C.R. Landis, Recent developments in chiral phospholane chemistry,
Tetrahedron: Asymmetry 15 (2004) 2123–2137.
[31] T.R.J. Achard, L.A. Clutterbuck, M. North, Asymmetric catalysis of carbon-
carbon bond forming reactions using metal (salen) complexes, Synlett 12
(2005) 1828–1847.
[32] M. Shibasaki, S. Matsunaga, BINOL, in: Qi-Lin Zhou (Ed.), Privileged chiral
ligands catalysts, Weinheim; Wiley-VCH, New York, 2011, pp. 295–332.
[33] R. Noyori, H. Takaya, BINAP: An efficient chiral element for asymmetric
catalysis, Acc. Chem. Res. 23 (1990) 345–350.
[34] S. Selvakumar, D. Sivasankaran, V.K. Singh, Enantioselective Henry reaction
catalyzed by C2-symmetric chiral diamine–copper(II) complex, Organic and
Biomolecular Chemistry 7 (2009) 3156–3162.
[12] M.L. Landry, D.X. Hu, G.M. McKenna, N.Z. Burns, Catalytic enantioselective
dihalogenation and the selective synthesis of (ꢁ)-deschloromytilipin A and
(ꢁ)-danicalipin A, J. Am. Chem. Soc. 138 (2016) 5150–5158.
[13] F. Li, Z.J. Zheng, J.Y. Shang, K.Z. Jiang, G.Q. Lai, J.X. Jiang, L.W. Xu, A Chiral Cu-
salan catalyst with
a rotatable aromatic p-Wall: Molecular recognition-
oriented asymmetric Henry transformation of aromatic aldehydes, Chem.
Asian J. 7 (2012) 2008–2013.
[35] M.Y. Liu, S.B. Hong, W. Zhang, W. Deng, Expedient copper catalyzed borylation
reactions using amino acids as ligands, Chinese Chemical Letters. 26 (2015)
373–376.
[14] J.D. White, S. Shaw,
A new catalyst for the asymmetric Henry reaction:
Synthesis of b-nitroethanols in high enantiomeric excess, Org. Lett. 14 (2012)
6270–6273.
[36] X. Liu, S. Dong, L. Lin, X. Feng, Chiral amino acids-derived catalysts and ligands,
Chin. J. Chem. 36 (2018) 791–797.
[15] M. Kannan, T. Punniyamurthy, Effect of ligand N, N-substituents on the
reactivity of chiral copper(II) salalen, salan, and salalan complexes toward
asymmetric nitroaldol reactions, Tetrahedron: Asymmetry 25 (2014) 1331–
1339.
[16] A. Dixit, P. Kumar, S. Singh, Synthesis of chiral salalen ligands and their insitu
generated Cu complexes for asymmetric Henry reaction, Chirality 12 (2018) 1–
12.
[17] D.D. Qin, W.H. Lai, D. Hu, Z. Chen, A.A. Wu, Y.P. Ruan, Z.H. Zhou, H.B. Chen,
Highly enantioselective Henry reactions of aromatic aldehydes catalyzed by an
amino alcohol–copper(II) complex, Chem. Eur. J. 18 (2012) 10515–10518.
[18] W. Chen, Z.H. Zhou, H.B. Chen, Efficient synthesis of chiral benzofuryl b-amino
alcohols via a catalytic asymmetric Henry reaction, Org. Biomol. Chem. 15
(2017) 1530–1536.
[37] K. Micskei, T. Patonay, L. Caglioti, G. Palyi, Amino acid ligand chirality for
enantioselective syntheses, Chem. Biodivers. 7 (2010) 1660–1669.
[38] N. Ananthi, U. Balakrishnan, S. Velmathi, Salicylaldimine based Copper (II)
complex potential catalyst for asymmetric Henry reaction, Arkivoc xi (2010)
370–379.
[39] C. Gan, G. Lai, Z. Zhang, Z. Wang, M.M. Zhou, Efficient and enantioselective
nitroaldol reaction catalyzed by copper Schiff-base complexes, Tetrahedron:
Asymmetry 17 (2006) 725–728.
[40] S.S. Pawar, S.K. Chakka, P.G. Andersson, H.G. Maguire Kruger GEM, T.
Govender, Comparison of tetrahydroisoquinoline (TIQ) thiazole and
oxazoline ligands for asymmetric Henry reactions, S. Afr. J. Chem. 65 (2012)
23–29.
[41] C. Fiorelli, D. Savoia, Asymmetric synthesis of d-substituted a, b-unsaturated d-
[19] B.V.S. Reddy, S.M. Reddy, S. Manisha, C. Madan, Asymmetric Henry reaction
catalyzed by a chiral Cu(II) complex: a facile enantioselective synthesis of (S)-
2-nitro-1-arylethanols, Tetrahedron: Asymmetry 22 (2011) 530–535.
[20] X.H. Liu, L.L. Lin, X.M. Feng, Chiral N, N0-dioxides: New ligands and
organocatalysts for catalytic asymmetric reactions, Acc. Chem. Res. 44
(2011) 574–587.
lactams by ring closing metathesis of enantiomerically pure N-acryloyl-
homoallylic amines, J. Org. Chem. 72 (2007) 6022–6028.
[42] D.A. Evans, D. Seidel, M. Rueping, H.W. Lam, J.T. Shaw, C.W. Downey, A new
copper acetate-bis(oxazoline)-catalyzed enantioselective Henry reaction, J.
Am. Chem. Soc. 125 (2003) 12692–12693.
[43] B. Qin, X. Xiao, X. Liu, J. Huang, Y. Wen, X. Feng, Highly enantioselective Henry
[21] H. Maheswaran, K.L. Prasanth, G.G. Krishna, R.K. Sridhar, B, Kantam ML,
Enantioselective nitroaldol (Henry) reaction using copper(II) complexes of (ꢁ)-
sparteine, Chem. Commun. 39 (2006) 4066–4068.
[22] Y.Q. Ji, G. Qi, Z.M.A. Judeh, Efficient asymmetric copper(I)-catalyzed Henry
reaction using chiral N-alkyl-C1-tetrahydro-1,10-bisisoquinolines, Eur. J. Org.
Chem. 25 (2011) 4892–4898.
(Nitroaldol) reaction of aldehydes and a-keto esters catalyzed by N, N’-
dioxide –copper (I) complexes, J. Org. Chem. 72 (2007) 9323–9328.
[44] H. Maheswaran, K. Leon Prasanth, G. Gopi Krishna, K. Ravikumar, B. Sridhar, M.
Lakshmi Kantam, Enantioselective nitroaldol (Henry) reaction using copper (II)
complexes of (-)- sparteine, Chem. Commun. 4066–4068 (2006).
5