Oxidative coupling of 2-naphthol to (R)/(S)-BINOL by MCM-41 supported Mn-chiral Schiff base complexes

Article abstract of DOI:10.1039/c5ra03492b

Three Mn(III)-chiral Schiff base complexes supported on MCM-41 are found to be effective reusable catalysts for enantioselective oxidation of 2-naphthol to (R)- and (S)-BINOL (1,1′ bi-2-naphthol) in the presence of oxygen. The supported Mn(III)-complexes are characterized by PXRD, FTIR, solid state-NMR, BET, and cyclic voltammetry study. The homo-coupling reaction with oxygen as the oxidant is promoted by 20 mg of Mn(III) Schiff base complexes to afford binaphthols in nearly quantitative yields with high enantioselectivity of up to 91% ee. The catalytic activities of the homogeneous and heterogeneous chiral catalyst are found to be almost similar. However, the heterogeneous counterparts are found to be advantageous in terms of recyclability and storability. Oxygen partial pressure, the nature of the solvent, temperature and the amount of catalyst affect the catalytic oxidation process. High temperature and highly polar solvent are found to have adverse effects on the catalytic oxidation process.

Full text of DOI:10.1039/c5ra03492b

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PAPER
Oxidative coupling of 2-naphthol to (R)/(S)-BINOL
by MCM-41 supported Mn-chiral Schiff base
complexes†
Cite this: RSC Adv., 2015, 5, 33185
a
b
c
Kusum K. Bania,* G. V. Karunakar and Lanka Satyanarayana
Three Mn(III)-chiral Schiff base complexes supported on MCM-41 are found to be effective reusable
0
catalysts for enantioselective oxidation of 2-naphthol to (R)- and (S)-BINOL (1,1 bi-2-naphthol) in the
presence of oxygen. The supported Mn(III)-complexes are characterized by PXRD, FTIR, solid state-NMR,
BET, and cyclic voltammetry study. The homo-coupling reaction with oxygen as the oxidant is promoted
by 20 mg of Mn(III) Schiff base complexes to afford binaphthols in nearly quantitative yields with high
enantioselectivity of up to 91% ee. The catalytic activities of the homogeneous and heterogeneous chiral
catalyst are found to be almost similar. However, the heterogeneous counterparts are found to be
advantageous in terms of recyclability and storability. Oxygen partial pressure, the nature of the solvent,
temperature and the amount of catalyst affect the catalytic oxidation process. High temperature and
highly polar solvent are found to have adverse effects on the catalytic oxidation process.
Received 26th February 2015
Accepted 31st March 2015
DOI: 10.1039/c5ra03492b
www.rsc.org/advances
19
reusability. Since chiral catalysts are costly so it is equally
important to restore the catalytic activity for longer period of time
Activation of the C–H bond via oxidative coupling reaction has and also enhanced their recyclability. In current development of
1
. Introduction
20
1
–3
been a long standing goal in the eld of chemical research.
heterogenisation of homogeneous catalyst inorganic materials
21–23
Out of the various oxidative coupling reactions, the oxidative like MCM-41, zeolites, LDHs, silica
etc. plays a pivotal role in
0
coupling of 2-naphthol to axially chiral C2-symmetric 1,1 bi-2- designing of chiral heterogeneous catalyst. However, a limited
naphthol (BINOL) is considered to be one of the most versa- number of heterogeneous chiral Schiff base complexes have been
4,5
tile organic transformations. Chiral BINOL and its derivatives developed for such asymmetric oxidative reaction. Very recently,
are found to act as chiral auxiliaries and ligands in asymmetric chiral self-dimerized vanadium complexes on SiO were devel-
transformations. They also serves as backbone for various oped for the oxidative coupling of 2-naphthols with a maximum
2
6–8
24,25
biologically active natural products such as bioxanthracene enantioselectivity of 90% ee.
From our group we report for
9
(
ꢀ)-ES-242-4, cercosporin and calphostins A–D. Because of enantioselective oxidation of 2-naphthol via cation–p interaction
these importance of such molecule has fuelled the researchers in presence of zeolite-Y supported Fe-Schiff base complexes and
23,26
to develop efficient methodology and chiral catalyst for asym- also by trinuclear Fe-oxo carboxylates.
metric coupling of 2-naphthol. Subsequently many homoge-
Out of the various inorganic mantles MCM-41 has as well-
10,11
12,13
neous
and heterogeneous chiral catalysts
have been ordered pore arrays, large surface area, uniform pore size distri-
27
reported to catalyze such reaction with moderate to high yield butions and tunable pore diameter (4–15 nm). Because of these
and enantioselectivity. Chiral Schiff base complexes of iron unique properties MCM-41 has been extensively used for immo-
1
4,15
16
28,29
(
Fe),
ruthenium (Ru) and vanadium(V) appears to be bilization of chiral metal catalyst.
So in order to develop effi-
17,18
promising catalyst.
cient and recyclable chiral catalyst in this report we examined the
Although the homogeneous catalyst are found to have better catalytic activity of three chiral Mn(III)-Schiff base complex sup-
catalytic activity but they fails in terms of recovery and ported on MCM-41. The heterogeneous chiral catalyst is found to
be efficient catalyst for oxidative coupling of 2-naphthol in terms
of yield and enantioselectivity. Prior to this, these catalysts are
Department of Chemical Sciences, Tezpur University, Napaam, Assam, India, 784028. recycled for three consecutive cycles without any deactivation.
a
E-mail: kusum@tezu.ernet.in
b
Division of Crop Protection Chemicals, Indian Institute of Chemical Technology,
2
. Experimental method
Uppal Road, Hyderabad, Andhra Pradesh, India, 500607
c
Center for NMR and Structural Chemistry, Indian Institute of Chemical Technology,
2.1. Materials and physical measurements
Uppal Road, Hyderabad, Andhra Pradesh, India, 500607
Hexagonal siliceous MCM-41 is purchased from Sigma Aldrich.
The chemicals used for the synthesis of Schiff-base ligands and
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra03492b
1
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its corresponding Mn-complexes are manganese acetate, Schiff base complex, the working electrode is prepared by
Mn(OAc) $4H O (Qualigens), salicylaldehyde and its derivatives taking a 1 : 1 weight ratio of metal complexes in 1 mL of DCM.
2
2
0
0
(
Alfa Aesar) and 1,1 -binaphthyl-2,2 -diamine (Sigma Aldrich). This suspension is ultrasonicated for 15 min. 10 mL of this
For the preparation of modied MCM-41 we used pyridine dispersion is coated on glassy carbon electrode and 5 mL of 5%
carboxaldehyde and 3-aminopropyl-triethoxysilane purchased styrene (as binder from Aldrich) is added on these coating and
from Sigma Aldrich. All the solvents are puried prior to use. dried. The glassy carbon electrode is used as the working elec-
Powder X-ray diffraction patterns of the samples are recorded trode and Ag/AgCl/KCl (saturated) is used as reference elec-
on a Philips X'pert MPD diffractometer with Cu-Ka (l ¼ trode. The cyclic voltammogram are recorded in water using
˚
ꢁ
1
5
.5405 A) radiation with a step size of 0.02 2q and a step time of TBAP as electrolyte. The cyclic voltammogram of neat complex
s and a curved Cu-Ka monochromator under identical is taken in solution mode, using 0.01 M of the metal complexes
conditions. The BET surface areas are determined with the use in a 0.1 M TBAP. MAS-NMR measurements are performed on
of N2 sorption data measured at 77 K with a volumetric Bruker ultrasheld 500 MHz WB NMR spectrometer at 11.75 T
1
3
29
adsorption setup (NOVA 1000e, Quantachrome Instruments). eld producing C and Si Larmor frequencies at 125.77 and
The pore diameters of the samples are determined from the 130.31 MHz respectively. The samples are analyzed by placing
desorption branch of the N adsorption isotherm with the in a 3.2 mm zirconia rotor by regulating the spinning speed at
2
Barret, Joyner, and Halenda (BJH) method. The electronic 10 kHz. The applied pulse width corresponding to an angle of
absorption spectra are recorded using a Hitachi U-3400 spec- p/2 for both the nuclei is 2.3 ms, with a recycle delay of 3 s. The
1
3
trophotometer with a diffuse reectance attachment equipped chemical shi reference for C spectrum is set at 77 MHz
2
9
with an integrating sphere of 60 mm inner diameter. Mono- corresponding to CDCl peak, and for Si the reference is set at
3
chromatic light is used in the whole spectral region in order 0 ppm corresponding to the TSP single peak. The enantiomeric
to minimize the effect of uorescence. For recording the spectra excess (ee) of the product was determined by HPLC (High
of the MCM-41 immobilized metal complexes, the powdered Performance Liquid Chromatography) analysis using chiral
samples are placed in a black absorbing hole (10 mm, in columns (4 4.6 mm ꢂ 250 mm, DAICEL CHIRALCEL OD-H or
diameter and 3 mm deep) of a sample holder, and the surface is CHIRALPAK AD-H). CHNS/O-analyser, Perkin Elmer is used for
smoothed. The layer can be regarded as innitely thick, as elemental detection. Inductively Coupled Plasma (ICP) spec-
required by the Kubelka–Munk theory. The optical spectrums trometer analysis (Perkin Elmer, Model no. ICP Optima
are then recorded in the reectance mode. A Kubelka–Munk 2100 DV) and Atomic Absorption Spectroscopy (AAS, ICE 3500
3
0
(
KM) analysis is performed on the reectance data. The KM Thermo Scientic) are used to detect %Mn in heterogeneous
2
factor, F(R), is given by F(R) ¼ (1 ꢀ R) /2R ¼ k/s where R is the Mn(III)-Schiff base complexes.
diffuse reectance of the sample as compared to BaSO , k is the
4
molar absorption coefficient, and s is the scattering coefficient 2.2. Preparation of Schiff base ligands
of the sample. The infrared spectra in the range of 450–4000
0
In a 5 mL round-bottomed two-neck ask compound (S)-1,1 -
ꢀ
1
cm
are recorded on a Perkin-Elmer Spectrum 2000 FTIR
0
binaphthyl-2,2 -diamine (200 mg, 0.704 mmol, 1 equiv.) dis-
spectrometer. The spectra of the neat Mn-Schiff base complexes
and the MCM-41 immobilized complexes are recorded as KBr
pellets by mixing the samples well with KBr. The SEM and
elemental chemical analyses are performed by using JEOL JSM-
solved in 4 mL of toluene and 2-hydroxybenzaldehyde was
added (171 mg, 1.4 mmol, 2 equiv.). To this reaction mixture
catalytic amount of p-toluenesulfonic acid (13 mg, 0.07 mmol,
0
5
.1 equiv.) was added and the reaction mixture was stirred at
6390 LV at an acceleration voltage of 5–10 kV. The samples are
ꢁ
0–60 C for 4 hours. The reaction mixture colour was changed
deposited on a brass holder and sputtered with platinum. The
cyclic voltammograms of neat and MCM-41 supported metal
complexes are recorded using CHI-600A meter from CH
Instruments, and 0.1 M TBAP (tetra-butyl ammonium phos-
phate) is used as the supporting electrolyte. For recording the
cyclic voltammogram of the MCM-41 supported chiral Mn-
to yellow; this reaction mixture was cooled to room tempera-
ture. Toluene was removed under reduced pressure. The crude
residue was puried through a silica gel column using hexane
and ethyl acetate as eluent (10/3) to give pure chiral (S) ligand-1,
Scheme 1. The similar procedure was followed for the synthesis
of ligands 2 and 3. The structures of the three ligands were
Scheme 1 Synthesis of Schiff base ligand.
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13
1
13
conrmed from H and C NMR analysis. H and C NMR 2 g (ꢃ33.3 mmol) of MCM-41 and 0.3 g (1.67 mmol) of 3-ami-
spectrums of the three ligands are provided in ESI.†
nopropyltrimethoxysilane and in 20 mL of toluene was reuxed
ꢁ
at 80 C with constant stirring for 24 h. Aer cooling to room
temperature, the resultant white solid was collected aer 1 h.
The powder sample was then ltered and washed with diethyl
ether and nally dried under vacuum for 12 h. To this amino
2
.3. Preparation of Mn(III) Schiff base complexes (Mn-L1,
Mn-L2 and Mn-L3)
3 2 2
To an ethanolic solution of (10 mL) Mn(CH COO) $4H O
functionalized MCM-41 (white solid product), 2 mmol of
(10 mmol), a solution of either ligand L1/L2/L3 (5 mmol) was
ꢁ
4
1
-pyridine carboxaldehyde was added and stirred at 75 C for
2
prepared in absolute ethanol (50 mL) under N atmosphere and
6 h to obtain pyridine-carboxaldehyde-modied MCM-41.
stirred under reuxed condition for 4–5 h. The resulting reac-
tion mixture was then cooled to room temperature and solid
LiCl (0.64 g, 15 mmol) was added to it and stirred for an addi-
tional 4 h while the reaction mixture was exposed to air and
Solution of Mn-L1 or Mn-L2/Mn-L3 (1 mmol) prepared sepa-
rately in ethanol (10 mL) was added to this-modied MCM-41
(1 g), and the resulting suspension was reuxed for 48 h.
Resulting immobilized catalysts were ltered, washed thor-
oughly with dry toluene, and extracted repeatedly with meth-
anol and dichloromethane on a Soxhlet extractor until the
extraction liquid became colourless.

ltered. Aer complete evaporation of the solvent from the

ltrate, the resulting residue was extracted with dichloro-
methane. The organic phase was washed with water and dried
over Na SO . The drying agent was removed by ltration and the
2
4
solution was concentrated to yield the desired complexes,
Scheme 2.
2
.5. General procedure for oxidation of 2-naphthol
Oxidation of 2-naphthol to BINOL was carried out following the
12
previous reported procedure. In a typical catalytic process,
mmol of 2-naphthol along with 10 mL of methanol, 20 mg of
2.4. Preparation of immobilized chiral metal complexes
5
The MCM-41 supported chiral manganese complexes were catalyst were added into a 100 mL two-necked ask equipped
synthesized following the same procedure as we employed in with a condenser and an air pump. The reaction was started by
31
ꢀ1
our previous paper shown in Scheme 3. In a typical procedure passing the dry air with a stable ow rate of 80 mL min
Scheme 2 Synthesis of Mn-Schiff base complexes.
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Scheme 3 Synthesis of MCM-41 supported Mn-Schiff base complexes.
controlled by a ow meter into the bottom of reactor at reaction 3.1. Powder X-ray diffraction study (PXRD)
temperature. The progress of the reaction was monitored by
X-ray diffraction pattern of neat MCM-41 and its modied
counterparts are shown in Fig. 1. PXRD pattern of MCM-41
shows characteristic intense peak due to reection at (100)
plane and two low intense peak due to reections at (110) and
thin layer chromatography and aer the completion of the
reaction; the catalyst was ltered, washed with petroleum ether
ꢁ
and dried at 100 C overnight.
32
(200) planes. Decrease in peak intensities and shiing in 2q
value is observed aer modication with the linker and on
immobilization of the metal complexes. The intensities of all
peaks are found to decreases and intensities of 110 and 200
reections decreases more on immobilization of metal complex
over MCM-41, with little shiing in the 2q value, Fig. 1c and d.
Decrease in peak intensity and shiing of 2q value towards
lower value conrms the successful attachment of the chiral
metal catalyst with modied MCM-41.
2
.6. Rate dependence on 2-naphthol concentration
Kinetic study was performed by preparing a certain concentra-
tion of 2-naphthol (0.1 mmol to 1 mmol) in 2 mL toluene and
warmed to 60 C. The solution of the catalyst Mn-L1 (5.0 mg) in
toluene (2 mL) and solid Mn-L1–MCM-41 (5.0 mg) catalyst was
added at 60 C in either case. The area ratios of 2-naphthol was
analysed 5 times par 1 hour by HPLC to determine the
consumption rate of 2-naphthol in the initial stage of the
reaction. Similar kinetic measurements were also performed
ꢁ
ꢁ
nd
rd
3.2. CHN analysis and surface area
aer 2 and 3 run of the Mn-L1–MCM-41 catalyst to under-
stand recycled ability of the catalyst.
Carbon (C), hydrogen (H) and nitrogen (N) contents in the three
ligands, homogeneous and the heterogeneous complexes are
determined by CHN analysis. Amount of metal content in
homogeneous chiral complexes are estimated from UV-vis study
while amounts of Mn-loading in the immobilized complexes are
3. Results and discussion
In order to characterize the synthesized materials we have used determined by inductively coupled plasma (ICP) spectrometer
various spectrochemical and physicochemical techniques like analysis. The amount of C, H and N along with the amount of
PXRD, FTIR, BET, UV-vis/DRS, cyclic voltammetry and SEM for Mn-content approximately matches with the expected formula
their characterization. The details of which are described below (% calculated) of the metal complexes either in homogeneous
separately. Aer conrming the formation of the desired or heterogeneous phase. The ratio of C/N is also found to match
materials we apply them as catalyst for catalytic oxidation of approximately with the expected ratio, Table 1. This further
2-naphthol to BINOL. The results of catalytic activity are pre- reveals that metal complexes are immobilized over the surface
sented aer conferring the material characterization. of MCM-41.
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Fig. 1 XRD pattern of (a) MCM-41 (b) ammine functionalized MCM-41
(
c) pyridine carboxaldehyde–MCM-41 (d) metal complexes immobi-
lized in MCM-41.
MCM-41 and pyridine carboxaldehyde–MCM-41 shows
typical type IV isotherm having three well-distinguished
33
regions mono and multilayer adsorption on the pore walls
region A), capillary condensation (region B), and multilayer
adsorption on the outer surface (region C), Fig. 2a and b,
(
Fig. 2
N
2
-adsorption isotherm of (I) (a) MCM-41 (b) pyridine carbox-
respectively. The MCM-41 immobilized catalyst shows type III aldehyde–MCM-41 (c) Mn-L1–MCM-41 (d) Mn-L2–MCM-41. (II) Pore
size distribution in (p) MCM-41 (q) pyridine carboxaldehyde–MCM-41
isotherm, indicating incorporation of metal complex into the
(r) Mn-L1–MCM-41 (s) Mn-L2–MCM-41.
pores of MCM-41 by a multi-step graing method, Fig. 2c and
34
d. This is in agreement with the decrease of the pore volume of
MCM-41 aer incorporation of chiral Mn(III) Schiff base
complexes. BET surface area, pore diameter, and pore volume
are presented in Table 1. It is clearly observed that upon func-
tionalization and immobilization BET surface area decreases
signicantly in comparison to MCM-41 precursor. The meso-
porous diameter as well as the pore volume is also found to be
less in case of the modied MCM-41. For example on immo-
bilization of Mn-L2, the BET surface area get reduced from 1015
29
13
3
.3. FTIR and solid state Si and C NMR analysis
The FTIR spectrum of calcined MCM-41 is shown in Fig. 3a. The
bands 1064 and 3464 cm are characteristic for Si–O–Si and Si–
OH bond, respectively. In the FTIR spectra of amine func-
tionalized MCM-41, four additional peaks are due to n(N–H
ꢀ1
32
stretching), n(C–H, stretching), n(N–H bending) and n(CH
2
bending), are observed at 3274, 2940, 1567 and 1492 respec-
tively, (Fig. 3b). The functionalization of MCM-41 with amine
via APTS is conrmed by the presence of N–H stretching and
2
ꢀ1
˚
to 612 m g , the pore size from 37 to 23 A and pore volume
3
ꢀ1
from 0.576 to 0.448 cm g
.
–
CH2 bending modes. Aer treating with pyridine
Table 1 BET and CHN analysis of MCM-41, PyC–MCM-41, Mn-L1–MCM-41, Mn-L2/Mn-L3–MCM-41. %Mn loading is calculated from ICP
analysis. %Mn loading obtained from AAS and % calculated CHN analysis are shown in parenthesis
Elemental analysis (%)
Mn-loading
(mg/100 mg)
BJH pore
diameter ( A˚ )
Total pore volume
BET surface
area (m g
3
ꢀ1
2
ꢀ1
Compound
(cm g
)
)
C
H
N
MCM-41
PyC–MCM-41
L1
L2
Mn-L1
Mn-L2/Mn-L3
Mn-L1–MCM-41
Mn-L2/Mn-L3–MCM-41
—
—
—
—
37
30
—
—
—
—
22
23
0.944
0.576
—
—
—
—
0.465
0.448
1015
745
—
—
—
—
618
612
—
—
—
10.62 (72.9)
82.91
62.79
67.54 (70.54)
68.07 (65.77)
71.46 (74.57)
61.34 (61.78)
2.13 (8.16)
4.91
3.41
4.23 (4.23)
3.36 (3.08)
5.27 (5.26)
4.54 (3.77)
2.32 (18.9)
5.69
4.31
4.70 (4.70)
3.98 (3.72)
6.46 (7.91)
5.55 (5.06)
9.22 (9.24)
8.81 (8.82)
7.75 (7.79)
6.35 (6.41)
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Fig. 5b. The lower wavelength bands at 208 and 235 nm are due
to p / p transition, bands at 285 and 346 nm results from
charge transfer transition and that at 448 nm is because of d–d
transition. Diffuse reectance spectrum of the MCM-41 immo-
bilized complexes, also shows bands approximately in the
similar region, Fig. 5c and Table 2. The small change in the
wavelengths can be attributed to the effect of the MCM-41
matrix. MCM-41 anchored Mn-L1 shows absorption bands
resulting from p / p*, dp / pp and d / d transition. The
most signicant changes observed in case of Mn-L1 and Mn-L2/
Mn-L3 when supported on MCM-41 is the appearance of band
above 600 nm. The band above 600 nm can be attributed to
transition from a dp orbital (HOMO) to a pp-orbital of ligand
(LUMO) mostly concentrated on pyridine linkage.
3.5. Cyclic voltammetry study
Fig. 3 FTIR spectrum of (a) MCM-41 (b) APTS–MCM-41 (c) pyridine
carboxaldehyde–MCM-41 (d) typical FTIR spectrum of MCM-41 The electrochemical behaviour of the ligands and the neat and
immobilized metal complexes.
the MCM-41 supported metal complexes are studied by means
cyclic voltammetry. The cyclic voltammogram of the two ligands
L1 and L2/L3 are shown in Fig. 6. Both the ligands L1 and L2/L3,
carboxaldehyde, FTIR spectra shows peaks at 1647, 1548 and
ꢀ1
respectively shows one irreversible reduction peak at 0.236 V
and 0.239 V versus Ag/AgCl. In addition to this Br-substituted
ligand L2/L3 shows an additional peak at ꢀ0.511 V due to
reduction of –Br. This behaviour was observed for a wide range
1393 cm corresponding to C]N, C]C (aromatic) and C–N
stretching vibration. The absence of N–H vibrational band and
presence of C]N bands further conrms the condensation of
–
2
NH group of siloxyl moiety with –CHO group of pyridine
ꢀ
1
of scan rates from 0.005 to 0.1 V s . Hence, such a reduction
process should correspond to a totally irreversible electron
transfer. For all Schiff bases under study, this irreversible
reduction peak would be ascribed to an intramolecular reduc-
carboxaldehyde and it do not possesses any peaks due to N–H
vibrations. The functionalization of MCM-41 and its modica-
2
9
tion with pyridine carboxaldehyde are also conrmed by Si
1
3
and C NMR analysis, details of which are already presented in
35
tive coupling of the two imine groups to yield a piperazine.
31
our recently published work.
Such a process would involve self-protonation reactions where
the phenolic hydroxyl groups act as proton donors. Epc becomes
less negative according to the sequence H < Br, i.e. in order of an
increase in both electron-withdrawing and p-acceptor qualities
of the substituents.
Aer the conrmation of the formation of linker group on
silica matrix, the chiral Mn-Schiff base complexes are anchored
over MCM-41 via the formation of Mn–N bond with the linker
group, Scheme 2. The FTIR spectrum of the MCM-41 immobi-
lized complexes shows almost the similar bands with those of
ꢀ1
All the three Mn-Schiff base complexes exhibits a redox
the neat complexes in the region of 1600–1200 cm . The
III
II
ꢀ
1
couple (Mn /Mn ) in the range of ꢀ0.4 to 0.4 V. Redox poten-
tials values for the corresponding complexes are given in a
Table 3 and a typical cyclic voltammogram of Mn-L1 is shown as
a representative case in Fig. 7a. The presence of quasi reversible
one electron oxidation–reduction wave gives an additional
proof that compounds are of Mn(III). The slight difference in the
redox potential values of Mn-L1 with those of Mn-L2 or Mn-L3 is
due to the substituent effect on the phenyl ring. Moreover, for
all complexes studied, the cathodic wave corresponding to the
irreversible reduction of the ligand moiety is no longer
observed. This indicates that due to lack of transferable
presence of most characteristics bands above 1600 cm due to
C]N conrms the immobilization of the metal complexes over
1
3
MCM-41, Fig. 3d. The solid state C NMR spectrum of the
complexes also shows the characteristics peak corresponding to
that of the co-ordinated ligand. The slight change in chemical
1
3
29
shi C NMR and Si value in Fig. 4 is expected due to the
change in the chemical environment upon immobilization over
MCM-41.
3.4. UV-vis/DRS spectra
Typical UV-vis spectrum of ligand (L1), the neat Mn(III) metal hydroxylic protons, the reduction potentials of the ligands get
complex, Mn-L1 and that of corresponding MCM-41 supported shied to lower limit in comparison to the potential interval
metal complex, Mn-L1–MCM-41 are shown in Fig. 5 as repre- considered in the experimental measurements. The cyclic vol-
sentative case. The nature of transition for particular wave- tammogram of the corresponding MCM-41 supported
lengths is depicted in Table 2. For instant ligand L1 gives complexes also shows a quasi reversible redox couple which is
absorption bands at 226, 275, 316 and 348 nm, Fig. 5a. Bands at differed from those of the neat complexes. A representative
226 and 275 nm are due to p / p* while those in the higher voltammogram of which is shown in gure, Fig. 7c. Potential
wavelengths corresponds to n / p transition. Mn-L1 complex values for the immobilized complexes were found to be differed
shows absorption band at 208, 235, 285, 346 and 448 nm, from the parent complexes due to different co-ordination
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13
29
Fig. 4 (a) and (b) are C and Si NMR spectra for Mn-L1–MCM-41 while (c) and (d) are those for Mn-L2–MCM-41.
environment as well as due to matrix effect or connement 3.6. SEM-analysis
20
effect.
Scanning electron microscopic images of MCM-41 and MCM-
Cyclic voltammograms of the metal complexes were also
recorded in different scan rate ranging from 0.005 V to 0.1 V,
Fig. 7b and d. Lowering of the scan results in decrease in peak
current and a linear plot of square root of scan rate vs. peak
41 immobilized Mn(III)-Schiff base complexes are taken in
different magnication (ꢂ3000 to ꢂ10 000) are shown in Fig. 9.
From the SEM images it is observed that immobilization of
metal complexes through the above mentioned processes has
not lead to any change in morphology of MCM-41. Presence of
agglomerate particle clearly observed from highly magnied
SEM images can be attributed to the formation of metal of
complexes on surface of MCM-41. Similar observations were
(Fig. 8a) indicates the electron transport occurs through a
normal diffusion controlled process in solution. Similar to the
neat complexes, in case of the MCM-41 supported complexes
peak current is found to nish on lowering the scan rate. Linear
plot of scan rate vs. peak current further signies for surface
controlled process, Fig. 8b.
31
also made in our previous work as well as in various other
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conversion of 2-naphthol to BINOL. Enhancement of air ow
ꢀ1
up to 80 mL min led to almost 62% conversion of 2-naphthol
with Mn-L1 complex. Conversely, the rate of air ow was also
found to affect the %yield and enantioselectivity (%ee) of the
product. For instant in presence of Mn-L1 catalyst (10 mg)
ꢀ
1
when the rate of air ow was 10 mL min the %yield and the
ee (S-BINOL) was found to be 3% and 9%, respectively
%
ꢀ
1
whereas with rate ow of 80 mL min 57% yield and 67% ee,
was obtained, Table 4. Further increase in air ow did not lead
to any increase in conversion rate. While performing the
reaction with MCM-41 in presence of air no conversion of
2
-naphthol was observed upto several days indicating that
metal source is essential for the catalytic conversion. MCM-41
supported metal complex Mn-L1–MCM-41 under identical
condition however; result in 65% conversion with 62% yield
and 69% ee. The slight higher catalytic activity observed in case
of the heterogeneous catalyst can be attributed to the differ-
ence in active site availability under homogeneous and
heterogeneous catalyst.
Fig. 5 (a) and (b) represents the typical UV-vis spectra of ligands and
Mn(III) complexes, respectively. (c) Corresponding UV-vis/DRS of the
MCM-41 supported metal complexes.
Temperature, nature of solvent and catalyst amount plays
Thus the presence of agglomerate particle conrms an important role in catalytic oxidation processes. Effect of
28,29
31
reports.
the successful immobilization of Mn-Schiff base complexes temperature, solvent polarity and amount of catalyst was
over MCM-41.
monitored by considering Mn-L1 and Mn-L1–MCM-41 as test
catalysts. To observe the effect of temperature, we considered
ꢁ
a wide range of temperature viz room temperature (rt, 32 C)
4
. Catalytic activity
to ꢀ30 ꢁC and performed the reaction using toluene as
solvent. Results obtained from such observation are depicted
The three catalysts were screened for the oxidation of 2-naph-
hol performed under aerobic conditions and at different
catalytic condition. In a preliminary experiment, the reaction
of 2-naphthol (5 mmol) with Mn-L1 (10 mg) catalyst was per-
formed in toluene at ambient temperature in absence of air.
Reaction did not proceed at all upto 4 days indicating that
catalyst alone can not oxidize 2-naphthol. Introduction of air
in Table 5. In presence of Mn-L1 complex maximum yield
ꢁ
(
90%) and ee (83%) was obtained at ꢀ10 C. However, further
decrease in temperature resulted in decrease of enantiose-
ꢁ
lectivity. At ꢀ30 C both the yield and ee was found to decrease
dramatically. On increasing the temperature from 32 to 100
ꢁ
C, no signicant enhancement in catalytic activity was
ꢀ
1
ꢁ
observed. So we considered, ꢀ10
C as the optimize
into the reaction mixture at 10 mL min , leads to slow
Table 2 Assignment of UV-vis/DRS transitions in synthesized complexes
Compound
Wavelength
Transition
Ligand 1, L1
226, 275
16, 348, 365
235, 278
12, 348, 372
208, 235
p / p*
n / p*
p / p*
n / p*
p / p*
dp / pp*
d / d
3
Ligand 2 (L2) or ligand 3 (L3)
Mn-L1
3
285, 346
48
4
Mn-L2 or Mn-L3
212, 236
p / p*
dp / pp*
d / d
265, 388
61
4
Mn-L1–MCM-41
237, 281
p / p*
dp / pp*
d / d
dp / pp*(pyridine
carboxaldehyde)
p / p*
395
510
627
Mn-L2/L3–MCM-41
239, 277
386
505
617
dp / pp*
d / d
dp / pp*(pyridine
carboxaldehyde)
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Fig. 6 Cyclic voltammogram of ligand 1 and ligand 2/ligand 3.
Table 3 Redox potential values of the ligand and metal complexes
temperature for the catalytic reaction. It is pertinent to
mention herein that effect of temperature do not readily
follows the Arrhenius law. The anomalous change in %ee with
respect to temperature can be explained by considering
following two eqn (1) and (2). As per the eqn (1), relative rate
constant (ln K) for the formation of (S)- and (R) BINOL in the
ox
red
Compound
E
E
Ligand 1, L1
Ligand 2 (L2) or ligand 3 (L3)
Mn-L1
Mn-L2 or Mn-L3
Mn-L1–MCM-41
Mn-L2/L3–MCM-41
—
—
ꢀ0.030
ꢀ0.034
ꢀ0.166
ꢀ0.169
0.236
0.239
ꢀ0.178 oxidation of 2-naphthol can be correlated with ee's of the
ꢀ0.184
ꢀ0.271
ꢀ0.281
reaction and can be written in the form of differential Eyring
3
6
equation as shown in eqn (2).
ꢀ
ꢁ
ꢀ
ð100 þ %eeÞꢁ
K
s
ln
¼ ln
(1)
K
R
ð100 ꢀ %eeÞ
Fig. 7 Cyclic voltammogram of (a) Mn-L1 (b) that of in different scan rate (0.010 V to 0.1 V) (c) CV of Mn-L1–MCM-41 and (d) that of in different
scan rate (0.005 V to 0.1 V).
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Fig. 8 (a) Plot of square root of scan rate vs. peak current (for Mn-L1 complex), (b) plot of scan rate vs. peak current (for Mn-L1–MCM-41).
‡
and DDS remains constant under that situation. On the other
hand, if the enantio determining step is changing at different
‡
‡
temperatures, a non-linear curve is expected as DDH and DDS
values will no longer remains constant. In the present case the
plot of ln(K /K ) vs. 1/T shown in Fig. 10 is found to be
s
R
nonlinear. Therefore it can be considered that the enantio
determining step is changing over the range of temperatures
examined in the current state of art. Thus, the observed reversal
in absolute induction could arise from
a temperature-
dependent interchange of the enantio determining mecha-
nisms. In case of the heterogeneous complex, Mn-L1–MCM-41,
we observed (Table 5) almost similar trend in catalytic activity
with respect to change in temperature i.e. the sense of induction
is reversed in accordance with Arrhenius law and also improved
Fig. 9 SEM image of (a) MCM-41 (b) Mn-L1–MCM-41 (c) Mn-L2–
MCM-41. (d)–(f) are SEM images of the same respectively in high
magnification.
ꢁ
with decreasing temperature from room temperature to ꢀ10 C.
ꢀ
ꢁ
‡
‡
In general Arrhenius law holds well only if the process follows
the same reaction sequence. Once the temperature is increased
the rate of the reaction will increase but at the same time supply
of molecules to the surface may become slow and in that case
Arrhenius plot will change slope. In the present case reverse
temperature effect is possible since adsorption and surface
concentration is a function of temperature especially when the
K
s
ꢀDDH
DDS
ln
¼
þ
(2)
K
R
RT
R
If the mechanism of enantioselection remain constant over a
wide range of temperature variation then according to eqn (2), a
‡
s R
linear plot (ln(K /K ) vs. 1/T) would be expected provided DDH
a
Table 4 Oxidation of 2-naphthol using Mn-L1 and Mn-L1–MCM-41 as catalyst in different rate of O
2
flow
b
c
d
Conversion (%)
Yield (%)
R : S (%)
%ee (S)
ꢀ1
Air ow mL min
Mn-L1
Mn-L1–MCM-41
Mn-L1
Mn-L1–MCM-41
Mn-L1
Mn-L1–MCM-41
Mn-L1
Mn-L1–MCM-41
1
2
3
4
5
6
7
8
0
0
0
0
0
0
0
0
5
12
23
32
38
47
57
62
7
15
27
38
41
52
59
65
3
9
5
12
21
34
37
47
51
62
35 : 65
31 : 69
33 : 67
34 : 66
32 : 68
27 : 73
25 : 75
22 : 78
32 : 68
31 : 69
34 : 66
29 : 71
26 : 74
27 : 73
26 : 74
20 : 80
9
12
16
21
44
57
62
67
11
12
18
23
49
59
63
69
18
27
30
38
48
57
a
ꢁ
b
Reaction conditions: catalyst 10 mg, 2-naphthol 5 mmol, solvent 10 mL at 32 C. Determined by GC, conversion (%) ¼ [moles of reactant
c
d
converted] ꢂ 100]/[moles of reactant in feed]. Isolated yield. Determined by chiral HPLC analysis.
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ꢀ1
a
Table 5 Oxidation of 2-naphthol using Mn-L1 and Mn-L1–MCM-41 as catalyst and O
2
as oxidant (80 mL min ) in different temperature
b
c
d
Conversion (%)
Yield (%)
R : S (%)
%ee (S)
r
ꢁ
Temp ( C)
Mn-L1
Mn-L1–MCM-41
Mn-L1
Mn-L1–MCM-41
Mn-L1
Mn-L1–MCM-41
Mn-L1
Mn-L1–MCM-41
3
1
1
5
0
2
5
0
62
64
65
65
70
70
90
82
74
70
65
65
67
66
66
72
72
94
86
82
75
68
57
59
60
60
64
64
78
72
68
63
54
62
64
64
65
67
67
86
81
76
69
61
22 : 78
35 : 65
32 : 68
27 : 73
23 : 77
26 : 74
63 : 37
34 : 66
20 : 80
27 : 73
28 : 72
20 : 80
33 : 67
28 : 72
25 : 75
24 : 76
24 : 76
30 : 70
26 : 74
18 : 82
22 : 78
24 : 76
67
68
73
74
78
78
83
80
74
71
65
69
72
76
76
81
82
85
84
78
73
69
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
5
10
15
20
25
30
a
b
Reaction conditions: catalyst 10 mg, 2-naphthol 5 mmol, solvent 10 mL. Determined by GC, conversion (%) ¼ [moles of reactant converted] ꢂ
c
d
1
00]/[moles of reactant in feed]. Isolated yield. Determined by chiral HPLC analysis.
20 mg of the catalyst, Table 7. No signicant enhancement in
the catalytic process was observed on further increment of the
catalytic amount. Thus we found the optimum condition for the
ꢁ
reaction as temperature, ꢀ10 C, solvent, toluene and catalytic
amount to be 20 mg.
The reaction is then performed using the other homoge-
neous and MCM-41 supported chiral Mn(III) complexes under
the above optimized condition. The % of conversion and
selectivity (R : S ratio) of the reactions are given in Table 7.
Comparison of the catalytic activity of the homogeneous and
heterogeneous systems indicates that chiral Mn-complexes
immobilized in MCM-41 gives slightly better yield and ee.
This enhancement in catalytic activity on immobilization into
MCM-41 can be attributed to the chiral enhancement due to the
connement effect associated with such macroporous systems.
It is also reported that by anchoring or immobilizing the cata-
lysts onto micro porous and mesoporous materials, there is a
signicant increase in ee as compared to its homogenous
s R
Fig. 10 Plot of ln(K /K ) vs. 1/T (a) for reaction catalyzed by Mn-L1
complex and (b) for reaction catalyzed by Mn-L1–MCM-41.
molecule has to adsorb in particular mode to give stereospeci-
29,32
analogue.

city or other preferred products. Further, lowering of the
ꢁ
ꢁ
To examine the role of the Schiff base in reaction, the reac-
tion is performed in presence of MCM-41 and the two Schiff
base ligands. We rst performed the reaction using of 20 mg of
temperature from 32 C to ꢀ10 C, the partition coefficient (K)
values were found to increase from 2.96 to 3.68. However,
further decrease in temperature decreases the partition coeffi-
ꢁ
ꢁ
MCM-41 and 5 mmol of Schiff base complex at ꢀ10 C in
cient value. Thus the reaction was performed at ꢀ10 C within a
toluene. MCM-41 in presence of chiral ligands does not lead to
any coupling product. This shows that the metal complex is
required to bring out the enantioselective coupling of
pH range 6.5 to 6.8.
To observe the effect of solvent, we choose both polar and
non-polar solvents of different dielectric constant as listed in
Table 6. In presence of both homogeneous and heterogeneous
chiral Mn-L1 catalyst, conversion of 2-naphthol to BINOL was
found to proceed well in non-polar solvents. Reaction does not
at all proceed in water as solvent and apart from toluene the
2-naphthol.
Several mechanisms viz. radical–radical and radical–anion
couplings, have been proposed for oxidative coupling of
-naphthol to BINOL. The radical–anion coupling is considered
to be the most plausible mechanism for such cross-coupling
2
reaction proceed well in CCl with maximum yield upto (80%)
4
37,38
reaction.
It has been reported that radical–anion coupling
and enantioselectivity (72%). Thus, considering toluene as
suitable solvent for the oxidation process, we performed all the
reaction in toluene using the other homogeneous and hetero-
geneous catalyst. Same reaction was also performed by varying
the amount of Mn-L1 and Mn-L1–MCM-41 catalyst from 5 mg to
can be either a rst or second-order dependence of the rate on
15
substrate concentration. In order to understand the proper
reaction conditions for the cross-coupling reaction, we study
the kinetics of this Mn-Schiff base complexes mediated aerobic
oxidative coupling using Mn-L1 and Mn-L1–MCM-41 as the
catalyst. A linear plot is obtained on plotting the amount of 2-
25 mg keeping the amount of the substrate constant. Maximum
yield above 88% and ee upto 91%, S-BINOL was obtained with
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a
d
Table 6 Oxidation of 2-naphthol using Mn-L1 and Mn-L1–MCM-41 as catalyst and O
2
as oxidant in different solvents
b
c
Conversion (%)
Yield (%)
R : S (%)
Mn-L1
%ee (S)
Solvent
Mn-L1
Mn-L1–MCM-41
Mn-L1
Mn-L1–MCM-41
Mn-L1–MCM-41
Mn-L1
Mn-L1–MCM-41
CH
3
OH
OH
16.5
13.2
Nil
16.2
21.5
30.2
44.3
56
17.2
15.4
—
18.4
23.2
30.8
47.6
66
9.2
8.9
—
10.6
16.8
24.2
26.5
30
10
12.3
—
15.2
19.5
26.2
30.5
60
38 : 62
39 : 61
—
33 : 67
36 : 64
63 : 37
34 : 66
17 : 83
13 : 87
31 : 69
28 : 72
—
24 : 66
44 : 56
30 : 70
26 : 74
12 : 88
11 : 89
15
18
—
25
22
27
39
64
72
15
21
—
29
21
30
44
62
67
C
2
H
5
H
2
O
CH
DMF
CH Cl
CHCl
Hexane
3
CN
2
2
3
CCl
4
87
88
75
80
a
ꢁ
b
Reaction conditions: catalyst 10 mg, 2-naphthol 5 mmol, solvent 10 mL, temperature ꢀ10 C. Determined by GC, conversion (%) ¼ [moles of
c
d
reactant converted] ꢂ 100]/[moles of reactant in feed]. Isolated yield. Determined by chiral HPLC analysis.
Table 7 Oxidation of 2-naphthol using 20 mg of different catalysts
and air as oxidant in toluene at ꢀ10 C
ꢁ
5. Recyclability test
a
b
c
The homogeneous chiral Mn(III) could not be retrieve whereas
we could successfully recover the MCM-41 supported chiral
catalyst. Recyclability test were performed under similar
conditions using the immobilized catalyst Mn-L1–MCM-41 and
Mn-L2–MCM-41. The catalysts were separated by centrifugation
aer rst run and fresh reactants were added to the supernatant
for the second cycle. Analysis of the reaction mixture shows no
Catalyst
Time (h) Conv. (%) Yield (%) R : S
%ee
Oxidant (air)
26
36
24
36
36
36
42
42
42
—
—
30
—
—
—
83
86
85
94
89
88
—
—
15
—
—
—
79
82
82
88
87
87
—
—
50 : 50
—
—
Mn(OAc)
Mn(OAc)
Mn-L1
Mn-L2
Mn-L3
Mn-L1 + air
Mn-L2 + air
Mn-L3 + air
Mn-L1–MCM-41 + air 27
Mn-L2–MCM-41 + air 34
Mn-L3–MCM-41 + air 34
2
$4H
+ air
2
O
—
—
—
—
—
2
—
16 : 84 82(S) further change in the conversion of 2-naphthol to BINOL. No
82 : 18 78(R)
7 : 93 83(S)
19 : 81 91(S)
95 : 5 87(R)
trace amount of metal complex was obtained in the UV-vis study
of the supernatant. Aer separation, catalyst were washed
thoroughly with dichloromethane (toluene), dried, and sub-
jected to another cycle with fresh reactants under similar
conditions. The conversion, %yield and %ee were found to be
consistent. The above procedure is repeated for three cycles,
and we do not observe any substantial loss in the catalytic
activity of the immobilized catalyst. This indicates that the
chiral Mn(III) salen complex is strongly bonded to the nitrogen
atom of pyridine carboxaldehyde modied MCM-41 through the
8 : 92 85(S)
a
Determined by GC, conversion (%) ¼ [moles of reactant converted] ꢂ
b
c
1
00]/[moles of reactant in feed]. Isolated yield. Determined by chiral
HPLC analysis.
nd
rd
axial coordination. Kinetic study aer 2 and 3 cycle also
suggest for rst order reaction and the amount of 2-naphthol
consumed in both the cycles was found to be almost consistent
with respect to the rst cycle. However, aer third cycle i.e. in
fourth and h cycle we observed substantial decrease in the
catalytic activity. Results of the %conversion, %yield and %ee
nd rd th
th
obtained aer 2 , 3 , 4 and 5 run are provided in ESI Table
S1.† In order to understand the reason behind the loss in
1
3
catalytic activity aer third cycle, we performed C NMR anal-
ysis and cyclic voltammetry study on the recovered catalyst
1
3
Mn-L1–MCM-41, Fig. 12. C NMR conrms that the complexes
get ruptured aer third cycle. Cyclic voltammetry patterns as
well as the redox potential values were also found to be
completely different from the original complex. Thus the
change in redox potential values as well structural modication
aer completion of third cycle clearly supports the loss of
catalytic activity. %Mn loading as obtained from ICP and AAS
analysis (Table S2†) was also found to be less aer 3 cycle in
comparison initial catalysts suggesting a substantial metal
leaching. Thus, because of the change in redox potential values,
Fig. 11 Dependence of the rate on substrate concentration for the
reaction catalysed by Mn-L1 and Mn-L1–MCM-41.
ꢀ
1
naphthol consumed (mmol h ) against the concentration of
the substrate, Fig. 11. This suggests that the reaction follows a
rd

rst order kinetics in presence of both the homogeneous and
heterogeneous chiral Mn-L1 catalyst.
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input in designing of newer heterogeneous chiral catalyst for
asymmetric oxidative coupling of 2-naphthol.
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ꢁ
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