Asymmetric Henry reaction catalysed by L-proline derivatives in the presence of Cu(OAc)2: Isolation and characterization of an in situ formed Cu(II) complex

Article abstract of DOI:10.1002/aoc.3123

Synthesis of asymmetric β-nitroalcohols by the Henry reaction is one of the most exploited carbon-carbon bond-forming reactions owing to the versatility of both functional groups for synthetic manipulation by functional group interconversion. Here we repo

Full text of DOI:10.1002/aoc.3123

Full Paper
Received: 11 July 2013
Revised: 15 December 2013
Accepted: 24 December 2013
Published online in Wiley Online Library: 6 February 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3123
Asymmetric Henry reaction catalysed by L-proline
derivatives in the presence of Cu(OAc)2: isolation
and characterization of an in situ formed
Cu(II) complex
H. Atoholi Semaa, Ghanashyam Beza* and Sanjib Karmakarb†
Synthesis of asymmetric β-nitroalcohols by the Henry reaction is one of the most exploited carbon–carbon bond-forming
reactions owing to the versatility of both functional groups for synthetic manipulation by functional group interconversion.
Here we report synthesis of a series of proline-derived compounds to study their catalytic activities for asymmetric Henry
reaction in the presence of Cu(OAc)2.H2O. The proline derivative, 2-((E)-(((S)-1-benzylpyrrolidinyl)diphenylmethylimino)
methyl)phenol 1 showed optimum catalytic activity. The catalytic species Cu(II)–1 complex, formed in situ, was isolated
and characterized by various spectroscopic techniques and X-ray crystallography to show a cis-N2O2 coordination geometry.
Asymmetric β-nitroalcohols were achieved without the use of added base, unlike most of the reported protocols. Copyright
© 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: Henry reaction; β-nitroalcohol; asymmetric; L-proline; copper(II) acetate
Introduction
Results and Discussion
The Henry (nitroaldol) reaction is an important carbon–carbon
bond-forming reaction to achieve β-nitroalcohol from alde-
hyde and nitroalkane. β-Nitroalcohols act as synthetic precur-
sors to several important compounds.[1] In particular, the
versatile nitro group can be converted into several other func-
tionalities by reduction,[2] Nef reaction[3] and nucleophilic
displacement[4a] to generate α-hydroxy ketones, aldehydes,
carboxylic acids, azides, sulphides and many other important
bifunctional compounds. Significantly, amino alcohols ob-
tained by reduction of β-hydroxy nitroalkanes have found
widespread utility as chiral ligands in asymmetric catalysis,
and as an important building block of natural products as
well as pharmaceuticals.[4b–g]
Ever since the first example on the enantioselective Henry
reaction reported by Shibasaki et al.,[5] several chiral metal
complexes[6] and chiral organocatalysts[7,8] have found applica-
tion in the Henry reaction. In particular, Lewis acid-based
catalysts employing Cu(I)[9] and Cu(II)[6f,10-12] have gained signifi-
cance owing to their low toxicity, ease of handling and ready
availability. Interestingly, in spite of bearing nitrogen-containing
ligands, most of these methods employing nitrogen-based
chiral ligands require an additional amine source as a co-
catalyst. Here we report the synthesis of a series of L-proline
derivatives (1–6) and their catalytic activity in asymmetric synthe-
sis of β-nitroalcohol in the presence of copper(II) salt. Moreover,
the in situ formed catalytic species, i.e. Cu(II)–1 (Scheme 1)
was isolated, and characterized by various spectroscopic
techniques and single-crystal X-ray crystallography.
With the key discoveries of L-proline as a catalyst for the first
intramolecular asymmetric aldol reaction reported by Eder
et al.,[13] and its intermolecular version by List et al. in 2000,[14]
L-proline has emerged as an efficient catalyst in various asym-
metric reactions.[15] L-Proline derivatives complexed with
zinc[16a-c] or copper[16d–i] have been utilized for the catalytic
asymmetric nitroaldol reaction but the application of L-proline-
based Cu(II) complex[16d–i] has not been widely explored,
especially with a reaction site containing tertiary amine. As stated
earlier, most of the Cu(II)-based catalysts are bidentate in nature
and hence require an additional amine source. We hypothesized
that the introduction of an additional donor atom in the form of
nitrogen in the ligand may eliminate the need for an added base.
To test our hypothesis, we designed three different types of
amines with N,N,O–, N,N- and N,O– donor ligands (Fig. 1).
The syntheses of amines 3–5 starting from L-proline are reported
in the literature.[17–19] Catalyst 1 was synthesized in 95% yield from
(S)-1-benzylpyrrolidin-2-yl)diphenylmethanamine 3[19] by treatment
*
Correspondence to: G. Bez, Department of Chemistry, North Eastern Hill Univer-
sity, Shillong 793022, India. E-mail: ghanashyambez@yahoo.com
†
Sanjib Karmakar contributed to the crystallographic studies.
a Department of Chemistry, North Eastern Hill University, Shillong-793022, India
b Sophisticated Analytical Instrument Facility, Gauhati University, Guwahati-
781014, India
Appl. Organometal. Chem. 2014, 28, 290–297
Copyright © 2014 John Wiley & Sons, Ltd.

Asymmetric Henry reaction catalysed by L-proline derivative
Ph
O
H
O2N
to the phenolic ―OH, respectively. Additionally, in the 1H NMR
spectrum the singlet at δ 14.81 ppm along with the doublet at δ
7.04ppm suggested the presence of phenolic ―OH, and the phe-
nyl group with substituent having + M effect. A typical IR peak at
1619 cmꢀ1 is due to azomethine C=N stretching,[20] molecular ion
peaks in electrospray ionization (ESI)-MS (447.15 [M+ 1]+ and
469.20 [M+ Na]+), and the elemental analysis data corroborated
the findings of 1H and 13C NMR data. The triazolyl amine 2 was syn-
thesized in near quantitative yield by click reaction[21] of the (S)-2-
(azidodiphenylmethyl)-1-benzylpyrrolidine 4[19] with phenyl
acetylene in toluene and t-butanol mixture in the presence of
copper iodide and diisopropylethylamine (DIPEA) (Scheme 2).
Ph
Catalyst 1,
Cu(OAc)2.H2O
Nitromethane
N
HO
N
Ph
HO
R
R
1
Ph
Ph
N
N
Cu
Ph
O
AcO
Cu(II)-1
Scheme 1. Synthesis of β-nitroalcohol.
1
The IR, H NMR, 13C NMR and ESI mass spectra (see supporting
information) confirmed the structure 2. The amino alcohol 6[22]
was synthesized from (S)-methyl-1-benzylpyrrolidine-2-carboxylate
7[18] by treatment with a large excess of methylmagnesium iodide
(Scheme 2).
For all the optimization processes, the reaction of 4-chlor
obenzaldehyde with nitromethane was taken as the pilot reaction
for the asymmetric Henry reaction. In order to screen the proline
derivatives (1–6), the pilot reaction in EtOH–CH2Cl2 (2:1) medium
was carried out in the presence of an equimolar ratio of the proline
derivatives and Cu(OAc)2.H2O. The results are summarized in Table 1.
The reaction took a much longer time for completion with catalysts
2–5, while the enantiomeric ratio was very poor with all the catalysts
except 1 (er = 79:21). Therefore we chose the proline derivative 1 for
further optimization.
Having identified the best catalyst, we studied the effect of
catalyst loading, solvent, reaction temperature and base (see
the Tables S1–S4 in the supporting information) to find that the
pilot reaction worked at its optimum when the reaction was
carried out in EtOH–CH2Cl2 at room temperature in the presence
of 20 mol% 1 and Cu(OAc)2.H2O without added base. In order to
study the substrate scope, the optimized procedure was applied
to the reaction of nitromethane with a variety of aryl aldehydes
(Table 2). Very good to excellent yields (up to 92%) were achieved
Ph
Ph
Ph
N
Ph
Ph
N
N
Ph
N
N
N
Ph
NH2
N
HO
Ph
N
Ph
Ph
Ph
3
2
1
Ph
N3
Ph
Ph
N
Ph
N
OH
Ph
HO
Ph
Figure 1. L-Proline-based amines.
with 2-hydroxybenzaldehyde in methanol for 18 h at room tempera-
ture (Scheme 2). In the 1H NMR spectrum the occurrence of a new
singlet at δ 8.05 ppm is due to the ―CH=N― proton, which
confirmed the formation of the Schiff base 1. This finding was also
supported by 13C NMR signals at δ 164.44 ppm and δ 162.11 ppm,
indicating the presence of ―CH=N― and the ipso carbon attached
Table 1. Screening of catalyst for asymmetric Henry reactiona
O
H
NO2
HO
Catalyst,Cu(OAc)2.H2O
Nitromethane
Cl
Cl
CO2Me
N
Ph
Ph
Lit19
Lit18,19
N
N
Ph
Entryb
Catalyst
Time (h)
Yieldc (%)
erd (%)
N3 Ph
H N
Ph
2
Ph
Ph
7
3
4
1
2
3
4
5
6
1
2
3
4
5
6
36
72
48
72
48
24
90
70
90
69
81
83
71:29
51:49
52:48
49:51
55:45
52:48
a
c
b
Ph
N
Ph
N
Ph
N
Ph
Ph
N
N
OH
Ph
Ph
Ph
N
HO
ap-Chlorobenzaldehyde (0.5mmol): CH3NO2: catalyst: Cu(OAc)2.
N
6
H2O= 1: 10: 0.2: 0.2.
2
bReaction was carried out in 2:1 ratio of ethanol-dichloromethane
(1ml) mixture.
Scheme 2. Synthesis of the catalysts 1, 2 and 6. Reagents and condi-
tions: (a) 2-hydroxybenzaldehyde, MeOH, 18 h, room temperature (r.t.),
95%; (b) phenyl acetylene, CuI, DIPEA, toluene, t-BuOH, 12 h, r.t., 96%; (c)
MeMgI, Et2O, 0 °C–r.t., 91%.
cIsolated yield after column chromatography purification.
dDetermined by HPLC with Chiralcel OD-H column.
Appl. Organometal. Chem. 2014, 28, 290–297
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H. A. Sema et al.
Table 2. Enantioselective Henry reaction of aldehydes with nitromethanea
Entry
Aldehyde
Prod.
T (h)
Yieldb (%)
erc (%)
Config.d
1
2
4-Tolualdehyde
8a
8b
8c
8d
8e
8f
72
84
84
36
36
36
36
36
36
36
90
85
75
90
90
92
80
85
85
80
73:27
70:30
60:40
74:26
71:29
73:27
68:32
62:38
62:38
68:32
R
R
R
R
R
R
R
R
R
R
4-Anisaldehyde
3
3,4-Methylenedioxybenzaldehyde
4-Fluorobenzaldehyde
4-Chlorobenzaldehyde
4-Bromobenzaldehyde
3-Nitrobenzaldehyde
4-Nitrobenzaldehyde
2-Nitrobenzaldehyde
3-Bromobenzaldehyde
4
5
6
7
8 g
8 h
8i
8
9
10
8j
aAldehyde (0.5 mmol):CH3NO2:catalyst:Cu(OAc)2.H2O = 1:10:0.2:0.2.
bIsolated yield after column chromatography purification.
cDetermined by HPLC using Chiralcel OD-H column.
dThe absolute (R) configuration was determined by comparison with literature values.[10a,23–25]
in most cases. Barring those (Table 2, entries 1–3) with electron-
donating substituents on the phenyl ring, reaction times
were very good (36 h, entries 4–10) and required reduced
times as compared to some reported Henry reactions (Table S5
in supporting information).[23a,26] No particular trend of
enantioselectivity with the change of substituent on the phenyl ring
was observed. Enantiomeric ratio ranged from a maximum of 74:26
(Table 2, entry 4) to a minimum of 62:38 (Table 2, entry 8). The
absolute configuration of nitroalcohols was found to be R- in all
cases.
When the reaction of nitroethane with aryl aldehydes was
carried out in the presence of Schiff base, 1 and Cu(OAc)2.H2O
(20 mol%) at room temperature, moderate diastereoselectivity (up
to anti/syn = 71:29) and appreciable enantioselectivities (eranti up to
16:84). Results (Table 3) showed that the nitroaldol products were
column chromatography. The complex Cu(II)–1 was characterized by
IR, UV–visible, fluorescence and single-crystal X-ray crystallography.
The formation of Cu(II)–1 was confirmed by comparing the UV–
visi
ble spectra of amine 1 (Fig. 2) and Cu(II)–1 complex (Fig. 3),
where the spectral changes were observed upon deprotonation
and coordination of 1 in Cu(II)–1. The electronic spectrum of 1
showed intensity of the bands at 319 nm and 416 nm, whereas
Cu(II)–1 showed bathochromic shifts of the bands at 381 nm and
621 nm. In 1, the band at 319nm has been assigned to imine n →
π* transitions[29] and the band at 416 nm indicated the keto-amine
tautomeric form of the Schiff base.[30] The spectrum of Cu(II)–1
showed an intense band at 381 nm, which could be associated with
ligand-to-metal charge transfer transitions. The fact that the imine
n → π* transitions are shifted to longer wavelengths upon coordi-
nation to the metal confirmed the coordination of the imine nitro-
gen to the metal atom.[31] A less intense shoulder at 621 nm in Cu
predominantly anti-selective, with the exception of
1-(benzo
[d][1,3]dioxol-5-yl)-2-nitropropan-1-ol (Table 3, entry 2). Here too, the
electronic nature of the substituents on the phenyl ring had only a mi-
nor effect on the diastereoselectivity. Electron-withdrawing sub-
stituents on the phenyl ring exhibited higher yields (entry 4, up
to 90%) and diastereoselectivities (entries 1 and 4 with anti/syn = 70/
30 and 71/29 respectively) in comparison to the electron-donating sub-
stituents (entries 2 and 6, up to 78% yield with anti/syn = 36/64 and 55/
45 respectively). 4-Chlorobenzaldehyde gave higher anti isomer
with 16:84 er (entry 1), while 4-ntrobenzaldehyde showed lower
anti isomer with 60:40 er (entry 3), and benzo[d][1,3]dioxol-5-
carbaldehyde gave low syn isomer with 59:41 er (entry 2).
Having successfully demonstrated the substrate scope of our pro-
tocol for asymmetric synthesis of nitroalcohols, we were curious to
know the structure of the catalytic core of the Cu(II)–1 complex that
was responsible for its catalytic activity. Given the fact that the most
of the nitrogen-containing catalysts that give stereoselective
synthesis in the presence of metal salts are known to form metal com-
plexes, we isolated the in situ formed Cu(II) complex Cu(II)–1 by elut-
ing the dark-green polar spot with CH2Cl2–MeOH mixture using
(II)–1 could be assigned to d → d transitions.
The formation of Cu(II)–1 was further confirmed by comparing
the fluorescence excitation and emission behaviour with 1. The
fluorescence spectrum of the amine 1 (Fig. 4) in dichloromethane
at a concentration of 1 × 10ꢀ4 M and a temperature of 298 K
showed bands at 489 nm and 501 nm upon excitation of bands
at 319 nm and 416 nm, respectively. The fluorescence spectrum
of 1 × 10ꢀ4 M solution of Cu(II)–1 (Fig. 5) in dichloromethane at
298 K showed a band at 432 nm upon excitation of 381 nm. It is
interesting to note that 1 and Cu(II)–1 showed appreciable lumi-
nescent properties similar to some reported Schiff base com-
pounds[32] and copper(II) complexes containing Schiff bases.[33]
In the IR spectrum, Cu(II)–1 showed a slight shift in absorption
bands; azomethine C N stretching[28] was observed at 1610 cmꢀ1
as opposed to 1619 cmꢀ1 in 1. The IR bands at 1381 and
1208 cmꢀ1 due to the phenolic C―O stretching in 1 appeared
at 1334 and 1202 cmꢀ1 in Cu(II)–1. This could be explained by
the ionic nature of the phenolic O―H bond that makes the
C―O stretching vibration stronger than usual, while the force
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Appl. Organometal. Chem. 2014, 28, 290–297

Asymmetric Henry reaction catalysed by L-proline derivative
Table 3. Henry reaction with nitroethanea
Nitroethane
Catalyst 1, Cu(OAc)2.H2O
HO
O
H
NO2
EtOH-CH2Cl2 (1:1), RT
R
R
anti-
+
OH
R
NO2
syn-
Yieldb (%)
anti/sync
eranti
e
Entry
Aldehyde
Prod.
T (h)
1
2
3
4
5
6
7
4-Chlorobenzaldehyde
3,4-Methylenedioxybenzaldehyde
4-Nitrobenzaldehyde
2-Nitrobenzaldehyde
3-Bromobenzaldehyde
4-Anisaldehyde
9a
9b
9c
36
60
44
60
36
42
36
85
70
88
90
74
78
80
70/30
36/64
54/46
71/29
67/33
55/45
61/39
16:84
59:41f
60:40
35:65
31:69
36:64
28:72
9d
9e
9f
Benzaldehyde
9 g
aAldehyde (0.5 mmol), catalyst 1 (20 mol%), Cu(OAc)2.H2O (20 mol%) and nitroethane (5 mmol) in EtOH-DCM, stirred at room temperature.
bIsolated yield.
cDetermined by HPLC using Chiralcel OD-H and Chiralpak AD-H column.
dHPLC retention time for syn and anti was determined by comparing with literature data.[7,27,28]
eEnantiomeric ratio of the major diastereomer.
fEnantiomeric ratio of syn diastereomer.
Figure 2. UV–visible spectrum of 1 in 2 × 10ꢀ4 ꢀ1 cmꢀ1 CH2Cl2 solution
M
at 298 K.
Figure 3. UV–visible spectrum of Cu(II)–1 in 1 × 10ꢀ4 ꢀ1 cmꢀ1 CH2Cl2
M
solution at 298 K.
in the caption to Fig. 6, which shows the molecular structure of
the Cu(II)-1 complex. In the crystal structure of Cu(II)–1, CH2Cl2
is present as solvent of crystallization in a 1:1 molar ratio. The
constant of the C―O bond decreased upon complexation to give
IR bands at lower frequencies. Moreover, the involvement of phe-
nolic C―O in complexation was also reflected by the
disappearance of the O―H band around 3058 cmꢀ1
.
absolute configuration of the stereogenic C9 carbon was deter-
22
mined to be (S), as for 1, which has an optical rotation, ½αꢁD
=
The single-crystal X-ray crystal structure was determined in
order to establish the structure, configuration and coordination
behaviour of Cu(II)–1. The dark-green single crystals were
obtained by slow diffusion of hexane into its dichloromethane
solution; X-ray data collection and refinement details are summa-
rized in Table 4. Selected bond lengths and angles are presented
ꢀ78.6 (c 0.900, CHCl3)). The molecular structure featured a
four-coordinated copper centre with the NNO donor set of
the Schiff base (phenolic oxygen, the imine nitrogen and the
benzylpyrrolidine nitrogen) and one O atom from the ―COOCH3
group. The nitrogen atoms are neutral donors, while the phenolic
Appl. Organometal. Chem. 2014, 28, 290–297
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H. A. Sema et al.
Table 4. Crystallographic and structure refinement parameters for Cu(II)–1
300
250
200
150
100
50
800
600
400
200
0
Chemical formula
C33H32CuN2O3, CH2Cl2
A
B
Formula weight
653.08
Crystal system, space group
Monoclinic, P21
0.30 × 0.43 × 0.52
Dark-green plates
9.6926(10)
Crystal size (mm3)
Crystal colour and habit
a (Å)
b (Å)
16.4156(15)
10.7442(10)
116.174(5)
c (Å)
β (°)
Volume (Å3)/Z
1534.2(3)/2
1.414
0
Dx (g cmꢀ3
)
400
500
600 500 600 700 800
μ (mmꢀ1
F(000)
)
0.918
Wavelength / nm
678
θ range (°)
2.1–30.0°
Figure 4. Fluorescence emission spectra of the Cu(II)–1 complex. Excita-
tion wavelengths were 319 and 416 nm for layers A and B, respectively.
Reflections collected
17 066
Independent reflections
Data with I > 2σ(I)/restraints/parameters
Final R indices [I > 2σ(I)]
R indices (all data)
8995 [Rint = 0.028]
6689/1/380
R1 = 0.045, wR2 = 0.107
R1 = 0.065, wR2 = 0.116
0.82/ꢀ0.69
0.004(11)
2000
1500
1000
500
0
Max., min. Δρ (e Åꢀ3
)
Flack parameter
400
500
600
700
Wavelength (nm)
Figure 5. Fluorescence emission spectra of amine 1. Excitation wave-
lengths were fixed at 381 nm.
oxygen atom and one oxygen of the acetate ion are anionic do-
nors to keep the oxidation state of central copper at +2. The
square-planar cis-N2O2 coordination sphere is slightly distorted
since the bond angles deviated from their ideal values of 90/180°
(Fig. 6). Copper is more closely bonded to the phenolic oxygen
(1.890(2) Å) than that of the acetate oxygen (1.943(2) Å), in agree-
ment with related complexes.[24,34] The distance between Cu1
and carbonyl-O3 (2.788(3) Å) is too long to be considered a sig-
nificant interaction. The Cu―N distance to the benzy
lpyrrolidine-N atom, 2.028(2) Å, is longer than the
corresponding value involving the imino-N atom, 1.947(2) Å.
The Cu―N(benzylpyrrolidine) distance is at the longer end ob-
served for other reported Cu―N distances, range 1.935(2)–2.004
(6) Å, as observed, for example, in the tridentate Schiff base ligand
N-(1-acetyl-2-propylidene)(2-pyridylmethyl) amine[19] and 2,2′-bip
yridine of Cu(II)[35] complexes. This slight variation may occur due
to the coordination of NNO donor sites in this complex (S)-9 as
compared to the reported NNO and NN-bonded copper
complexes.[35,36] It may be noted that we found only one such
Schiff base complex, albeit achiral, with a similar coordination
geometry in the crystallographic literature.[37]
Figure 6. Molecular structure of Cu(II)ꢀ 1 complex. Anisotropic displace-
ment ellipsoids are shown at the 40% probability level, with hydrogen
atoms and the solvent molecule omitted for clarity. Selected bond angles
(°): O1―Cu1―O2, 91.03(10); O1―Cu1―N1, 93.30(9); O1―Cu1―N2,
167.38(10); O2―Cu1―N2, 92.37(10); O2―Cu1―N1, 174.83(10);
N1―Cu1―N2, 84.01(9). Selected bond lengths (Å): Cu1―O1, 1.890(2);
Cu1―O2, 1.943(2); Cu1―N1, 1.947(2); Cu1―N2, 2.028(2).
Conclusion
We should successfully synthesized proline-based ligands, which
can be used in the enantioselective Henry reaction in the
presence of Cu(OAc)2.H2O. The results suggested that enan-
tioselective nitroaldol reaction can be accomplished with high
yields with moderate enantioselectivities, without adding any
additional bases. The study revealed the in situ formation of the
chiral complex Cu(II)–1 that actually catalysed and controlled
the stereoselectivity of the reaction.
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Appl. Organometal. Chem. 2014, 28, 290–297

Asymmetric Henry reaction catalysed by L-proline derivative
pyrrolidine at C-e), 2.26–2.19 (m, 2H, CH of pyrrolidine at C-e and
C-c), 1.81–1.76 (m, 1H, CH of pyrrolidine at C-c), 1.52–1.45 (m, 1H,
CH of pyrrolidine at C-d), 1.18–1.12 (m, 1H, CH of pyrrolidine at C-
d). 13C NMR (100 MHz, CDCl3): δ 164.44 (―N CH―C6H4―OH),
162.11 (―C―OH of phenol), 144.57 (Cipso, ―NCH2C6H5), 142.81,
140.38 (Cipso, ―(N)C(C6H5)2), 132.61, 132.46 (―N CH―C6H4―OH
at C-w and C-y), 132.09 (―NCH2C6H5 at C-h and C-l), 130.20
(―NCH2C6H5 at C-i and C-k), 130.09 (―NCH2C6H5 at C-j), 129.07,
128.69 (― (N)C(C6H5)2 at C-o), 128.57, 128.46 (― (N)C(C6H5)2 at C-
q), 128.13, 127.98 (― (N)CC6H5, C-n), 127.83, 127.19 (― (N)CC6H5,
C-r), 126.85 (―(N)C(C6H5)2, C-p), 126.44 (―N CH―C6H4―OH, C-t),
118.20 (―N CH―C6H4―OH, C-x), 117.78 (―N CH―C6H4―OH, C-
v), 77.75 (C-b of pyrrolidine ring), 71.91(―C―N CH―C6H4―OH, C-
a), 62.03 (―NCH2C6H5, C-f), 55.10 (C-e of pyrrolidine ring), 30.73 (C-
c of pyrrolidine ring), 24.02 (C-d of pyrrolidine ring). MS (ESI): m/z
447.15 [M + 1]+, 469.20 [M+23]+. Elemental analysis for C31H30N2O:
calcd C 83.37, H 6.77, N 6.27; found C 83.39, H 6.79, N 6.28.
Experimental
Materials and Instruments
The commercially available chemicals and reagents were used
without further purification. The IR spectra were recorded on a
PerkinElmer 983 spectrophotometer. H NMR (400 MHz) and 13C
1
NMR (100 MHz) spectra were recorded on an FT NMR Bruker ad-
vance II 400 MHz spectrometer using CDCl3 as solvent and TMS as
internal standard, unless otherwise stated. Mass spectra were
obtained from a Waters ZQ 4000 mass spectrometer by the ESI
method, while elemental analyses of the complexes were
performed on a PerkinElmer-2400 CHN/S analyser. Absorption
spectra were obtained at room temperature using a PerkinElmer
Lambda 25 UV–visible spectrophotometer. The fluorescence mea-
surement was carried out using a Hitachi F4500 spectrophotome-
ter. Enantiomeric ratios were determined by chiral HPLC analysis
on Daicel Chiralcel OD-H and AD-H chiral columns. Optical rotation
was measured on a PerkinElmer 341 polarimeter. Single-crystal X-
ray diffraction measurement was carried out at room temperature
on a Bruker Smart Apex II CCD diffractometer using Mo-Kα
radiation (λ = 0.71073 Å). The crystals was mounted on a Stoe-
Image plate diffraction system equipped with three-circle
goniometer, using Mo-Kα graphite monochromated radiation
(λ = 0.71073Å) at 296(2). K. SMART[38] software was used for data
acquisition. Data integration and reduction were undertaken with
SAINT[38] software. The structure was solved by direct methods
using the program SHELXS–97.[39] Refinement and all further calcu-
lations were carried out using SHELXL–97.[39] SADABS[40] software
was used for absorption correction (multi-scan). Structural illustra-
tion was drawn with ORTEP-3 for Windows.[41]
Synthesis of (S)-1-((1-benzylpyrrolidin-2-yl)diphenylmethyl)-4-phenyL-1H-
1,2,3-triazole, 2
p
o
q
n
r
d
e
c
b
n
o
q
m
m
a
p
N
l
g
N
f
r
k
j
N
s
N
h
v
u
t
i
w
z
x
y
2
((S)-1-Benzylpyrrolidin-2-yl)diphenylazidomethane, (S)-4 (1.512 g,
4.11 mmol) was dissolved in a toluene (16 ml) and t-BuOH (4 ml)
solvent mixture. To the stirred solution was added phenylacetylene
(0.629 g, 6.17 mmol), CuI (55 mg, 0.288 mmol) and DIPEA (1.161 g,
9.0 mmol). The reaction mixture was stirred at room temperature
overnight and progress of the reaction was monitored by TLC. After
completion of the reaction, solvents were removed under reduced
pressure and the residue was purified by column chromatography
on silica gel (60–120 mesh) using an EtOAc–hexane mixture as eluent
to obtain the pure product 2 as a white solid (1.647 g, 3.95 mmol,
Synthesis of 2-((E)-(((S)-1-benzylpyrrolidinyl)diphenylmethylimino)methyl)
phenol, 1
p
q
n
d
o
q
m
a
e
p
m
N b
f
HO
l
N
g
r
s
k
j
t
y
x
u
v
h
i
24
96%); m.p. 68–70 °C. ½αꢁD = +7.1 (c 1, CHCl3). IR (KBr, cmꢀ1): 3436,
w
1
3250, 3058, 2965, 1606, 1440, 1122, 870, 758, 704, 638. H NMR
1
(400 MHz, CDCl3): δ 7.82 (s, 1H, C CH―N), 7.81–7.79 (m, 2H, ―C6H5
on triazole ring at C-v and C-z), 7.60–7.57 (m, 2H, ―C6H5 on triazole
ring at C-w and C-y), 7.42–7.17 (m, 14H, ―NCH2C6H5, ― (N)C(C6H5)2,
―C6H5 on triazole ring at C-x), 6.91 (dd, J= 8 Hz, 2 Hz, 2H, ― (N)C
(C6H5)2 at C-n), 4.99 (dd, J= 9.2 Hz, 3.2 Hz, 1H, CH of pyrrolidine at
C-b), 3.55 (d, J= 16.8 Hz, 1H, ―NCH2C6H5 at C-f), 3.40 (d, J=16.8Hz,
1H, ―NCH2C6H5, C-f), 2.55–2.30 (m, 3H, CH of pyrrolidine at C-e,
CH2 of pyrrolidine at C-d), 1.93–1.88 (m, 1H, CH of pyrrolidine at C-
e), 1.36–1.28 (m, 2H, CH2 of pyrrolidine at C-c). 13C NMR (100 MHz,
CDCl3): δ 146.34 (C CH of triazole ring), 140.49 (Cipso, ―NCH2C6H5),
140.05 (Cipso, ― (N)C(C6H5)2), 130.78 (C CH of triazole ring), 130.10
(Cipso, ―C6H5 on triazole ring), 128.78, 128.68 (― (N)C(C6H5)2 at C-o
and C-q), 128.31, 128.15 ―(N)C(C6H5)2 at C-n and C-r), 128.07,
128.01 (―(N)C(C6H5)2 at C-p), 127.86 (―C6H5 of triazole ring at
C-w and C-y), 127.82 (―NCH2C6H5 at C-h and C-l), 127.74
(―NCH2C6H5 at C-i and C-k), 126.56 (―NCH2C6H5 at C-j), 125.67
(―C6H5 of triazole ring at C-v and C-z), 122.27 (―C6H5 of triazole ring
at C-x), 77.88 (CH of pyrrolidine at C-b), 70.47 (― (N)C(C6H5)2 at C-a),
62.43 (―NCH2C6H5 at C-f), 55.24 (CH2 of pyrrolidine at C-e), 31.52
(CH2 of pyrrolidine at C-c), 23.66 (CH2 of pyrrolidine at C-d). MS
In a two-necked round bottom flask equipped with magnetic
stirrer were placed ((S)-1-benzylpyrrolidin-2-yl)diphenylmethanamine,
(S)-3 (4.104 g, 12 mmol) and MeOH (20 ml). To this stirred solution
anhydrous Na2SO4 (2.88 mmol) and 2-hydroxybenzaldehyde
(1.53 ml, 14.4 mmol) were added at room temperature. Progress of
the reaction was monitored by thin-layer chromatography. On
completion after 18 h, the solvent was evaporated and the residue
was purified by column chromatography (5% ethyl acetate
in hexane) to obtain the product 1 as yellow solid (5.090 g,
22
95% yield); m.p. 126–127 °C. ½αꢁD = ꢀ78.6 (c 0.9, CHCl3). IR
(KBr, cmꢀ1): 3462, 3058, 2819, 1958, 1619, 1493, 1447, 1381, 1281,
1
1208, 1116, 1003, 890, 751, 704, 651, 592, 446. H NMR (400MHz,
CDCl3): δ 14.84 (s, 1H, ―OH of phenol), 8.08 (s, 1H, ―CH N―),
7.54–7.52 (m, 1H, ―N CH―C6H4―OH at C-y), 7.41–7.13
(m, 16H, ―NCH2C6H5, ― (N)C(C6H5)2 and ―N CH―C6H4―OH at
C-w), 7.07–7.05 (m, 1H, ―N CH―C6H4―OH at C-v), 6.84
(m, 1H, ―N CH―C6H4―OH at C-x), 4.07 (dd, J = 9.2, 2.4Hz, 1H,
CH of pyrrolidine at C-b), 3.45 (d, J = 13.2 Hz, 1H, ―NCH2C6H5),
3.20 (d, J = 13.2 Hz, 1H, ―NCH2C6H5), 2.82–2.78 (m, 1H, CH of
Appl. Organometal. Chem. 2014, 28, 290–297
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

H. A. Sema et al.
(ESI): m/z 471.07 [M + 1]+. Elemental analysis for C32H30N4: calcd C
81.67, H 6.43, N 11.91; found C 81.71, H 6.47, N 11.93.
(R)-1-(3-Bromophenyl)-2-nitroethanol, 8j: HPLC analysis (Chiralcel
OD-H column, 84:16 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
254 nm); retention times: major enantiomer (R) tr = 11.61 min, minor
enantiomer (S) tr = 17.64 min; er 68:32.
1-(4-Chlorophenyl)-2-nitropropan-1-ol, 9a: HPLC analysis (Chiralpak
General Procedure for Asymmetric Henry Reaction
AD-H column, 96:4 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
A 10 ml round-bottom flask containing a stirring bar was charged
with EtOH:CH2Cl2 (2:1, 1.0 ml), catalyst 1 (0.075 mmol, 20 mol%)
and [Cu(OAc)2.H2O] (0.075 mmol, 20 mol%) and the mixture was
stirred at room temperature for 4 h. Then nitroalkane (nitrometh-
ane/nitroethane) (3 mmol) was added to the resulting green
solution, followed by addition of aldehyde (0.3 mmol). The
mixture was then stirred at room temperature for a specified
time. After completion of the reaction, the solvents were
removed under reduced pressure. Purification of the residue by
column chromatography on silica gel (60–120 mesh) using ethyl
acetate and hexane as eluent gave the desired β-nitroalcohols.
215 nm); syn/anti ratio 70:30; retention times: anti: tr =30.76min
(minor), tr = 33.28 min (major), tr = 45.55 min (major), tr = 50.17 min
(minor); eranti 16:84.
1-(Benzo[d][1,3]dioxoL-5-yl)-2-nitropropan-1-ol, 9b: HPLC analysis
(Chiralpak AD-H column, 94:6 n-hexane:2-propanol, flow rate: 0.5 ml
minꢀ1, 215 nm); syn/anti ratio 36:64; retention times: anti: tr = 54.68
min (major), tr = 58.96 (minor) min, syn: tr = 63.83 min (major), tr =
106.16 min (minor); ersyn 59:41.
2-Nitro-1-(4-nitrophenyl)propan-1-ol, 9c: HPLC analysis (Chiralpak
AD-H column, 89:11 n-hexane:2-propanol, flow rate: 0.3 ml minꢀ1
,
215 nm); syn/anti ratio 54:46; retention times: anti: tr = 56.48 min
(major), tr = 69.64 min (minor), syn: tr = 71.82 min (major), tr = 79.08
min (minor); eranti 60:40.
Spectral data of Cu–1 complex
2-Nitro-1-(2-nitrophenyl)propan-1-ol, 9d: HPLC analysis (Chiralpak
Cu(II)–1: dark-green solid; m.p. 114–115 °C. IR (KBr, cmꢀ1): 3443,
3211, 2932, 2853, 2362, 1619, 1533, 1450, 1394, 1122, 758, 704,
625. Elemental analysis for C33H33CuN2O5: calcd C 65.93, H 5.53,
N 4.66; found C 65.98, H 5.59, N 4.67.
AD-H column, 94:6 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
215 nm); syn/anti ratio 70:30; retention times: anti: tr = 42.70 min
(minor), tr = 46.12 min (major), syn: tr = 57.10 min (minor), tr = 77.97 min
(major); eranti 35:65.
HPLC data of compounds
1-(3-Bromophenyl)-2-nitropropan-1-ol, 9e: HPLC analysis (Chiralpak
AD-H column, 96:4 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
(R)-1-(4-Methylphenyl)-2-nitroethanol, 8a: HPLC analysis (Chiralcel
OD-H column, 90:10 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
215 nm); syn/anti ratio 67:33; retention times: anti: tr = 27.81 min
(minor), tr = 31.29 min (major), syn: tr = 39.60 min (minor), tr = 45.27 min
(major); eranti 31:69.
215 nm); retention times: major enantiomer (R) tr =23.98min, minor
enantiomer (S) tr = 30.34 min; er 73:27.
1-(4-Methoxyphenyl)-2-nitropropan-1-ol, 9f: HPLC analysis
(Chiralpak AD-H column, 96:4 n-hexane:2-propanol, flow rate: 0.5 ml
minꢀ1, 215nm); syn/anti ratio 55:45; retention times: anti: tr = 44.58
min (minor), tr = 50.56 min (major), syn: tr = 67.85 min (minor),
tr = 78.59 min (major); eranti 36:64.
(R)-1-(4-Methoxyphenyl)-2-nitroethanol, 8b: HPLC analysis (Chiralcel
OD-H column, 85:15 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
210 nm); retention times: major enantiomer (R) tr = 25.69 min, minor
enantiomer (S) tr = 32.49 min; er 69:31.
(R)-1-(3,4-Methylenedioxyphenyl)-2-nitroethanol, 8c: HPLC analysis
(Chiralcel OD-H column, 84:16 n-hexane:2-propanol, flow rate: 0.5 ml
minꢀ1, 210 nm); retention times: major enantiomer (R) tr = 28.91 min,
minor enantiomer (S) tr = 35.94 min; er 60:40.
2-Nitro-1-phenylpropan-1-ol, 9 g: HPLC analysis (Chiralpak AD-
H column, 96:4 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
215 nm); syn/anti ratio 61:39; retention times: anti: tr = 30.85 min
(minor), tr = 33.54 min (major), syn: tr = 46.00 min (major),
tr = 50.66 min (minor); eranti 28:72.
(R)-1-(4-Fluorophenyl)-2-nitroethanol, 8d: HPLC analysis (Chiralcel
OD-H column, 90:10 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
215 nm); retention times: major enantiomer (R) tr = min, minor
enantiomer (S) tr = min; er 74:26.
Acknowledgements
(R)-1-(4-Chlorophenyl)-2-nitroethanol, 8e: HPLC analysis (Chiralcel
OD-H column, 92:8 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
The authors are grateful to UGC-RGNF for financial support in the
form of SRF bearing fellowship allotment No. 18-16(7)/Acad/
2006-772, SAIF NEHU for NMR analysis and SAIF Guwahati for
XRD analysis.
214 nm); retention times: major enantiomer (R) tr =29.78min, minor
enantiomer (S) tr = 37.84 min; er 71:29.
(R)-1-(4-Bromophenyl)-2-nitroethanol, 8f: HPLC analysis (Chiralcel
OD-H column, 84:16 n-hexane:2-propanol, flow rate: 0.5 ml minꢀ1
,
254 nm); retention times: major enantiomer (R) tr =17.45min, minor
enantiomer (S) tr = 21.12 min; er 73:27.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 290–297

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Appl. Organometal. Chem. 2014, 28, 290–297
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