Direct preparation of oximes and Schiff bases by oxidation of primary benzylic or allylic alcohols in the presence of primary amines using Mn(III) complex of polysalicylaldehyde as an efficient and selective heterogeneous catalyst by molecular oxygen

Article abstract of DOI:10.1007/s13738-017-1131-z

The present work introduced the new strategy for direct preparation of Schiff base as well as oxime compounds through oxidation of primary benzylic or allylic alcohols in the presence of amines by complexation of Mn(III) to a polymeric Schiff base ligand based on polysalicylaldehyde (PSA-Schiff base-Mn(III) complex). As a new environmentally benign protocol, manganese heterogeneous polymeric catalytic system demonstrated promising oxidation of alcohols in ethanol using molecular oxygen. PSA was synthesized through polycondensation reaction of 2-hydroxy-5-chloromethyl-benzaldehyde and then treated with 2-aminophenol to form polymeric ligand. Average molecular weight of PSA was studied by an analytical method as well as GPC analysis. Formation of the catalyst was characterized step by step by FTIR, UV–Vis, ¹H NMR, TGA, CHN and EDX analyses. Loading amounts of metal ions as well as leaching amount of the catalysis were studied by ICP-OES instrument. The catalyst shows up to high yields for oxidation of primary and secondary primary benzylic or allylic alcohols to carbonyl compounds, especially direct imine formation in a mild, inexpensive and efficient method which can be successfully recovered from the reaction mixture and reused for several times without any remarkable reactivity loss. Effect of solvent, temperature, catalyst amount and oxygen donors along with some blank experiments to elucidation of catalyst activity was evaluated in this work. Also chemoselectivity behavior of the catalyst was investigated with some combinations.

Full text of DOI:10.1007/s13738-017-1131-z

J IRAN CHEM SOC
DOI 10.1007/s13738-017-1131-z
ORIGINAL PAPER
Direct preparation of oximes and Schiff bases by oxidation
of primary benzylic or allylic alcohols in the presence of primary
amines using Mn(III) complex of polysalicylaldehyde as an
efꢀcient and selective heterogeneous catalyst by molecular oxygen
1
2
1
Milad Kazemnejadi · Alireza Shakeri · Mohammad Mohammadi ·
Marzieh Tabefam
3
Received: 16 November 2016 / Accepted: 2 May 2017
©
Iranian Chemical Society 2017
Abstract The present work introduced the new strategy
for direct preparation of Schiff base as well as oxime com-
pounds through oxidation of primary benzylic or allylic
alcohols in the presence of amines by complexation of
Mn(III) to a polymeric Schiff base ligand based on poly-
salicylaldehyde (PSA-Schiff base-Mn(III) complex). As a
new environmentally benign protocol, manganese heteroge-
neous polymeric catalytic system demonstrated promising
oxidation of alcohols in ethanol using molecular oxygen.
PSA was synthesized through polycondensation reaction of
metal ions as well as leaching amount of the catalysis were
studied by ICP-OES instrument. The catalyst shows up to
high yields for oxidation of primary and secondary primary
benzylic or allylic alcohols to carbonyl compounds, espe-
cially direct imine formation in a mild, inexpensive and
efꢀcient method which can be successfully recovered from
the reaction mixture and reused for several times without
any remarkable reactivity loss. Effect of solvent, tempera-
ture, catalyst amount and oxygen donors along with some
blank experiments to elucidation of catalyst activity was
evaluated in this work. Also chemoselectivity behavior of
the catalyst was investigated with some combinations.
2
-hydroxy-5-chloromethyl-benzaldehyde and then treated
with 2-aminophenol to form polymeric ligand. Average
molecular weight of PSA was studied by an analytical
method as well as GPC analysis. Formation of the cata-
Keywords Alcohol oxidation · Polysalicylaldehyde ·
Schiff base · Oxime · Mn polymeric complex
1
lyst was characterized step by step by FTIR, UV–Vis, H
NMR, TGA, CHN and EDX analyses. Loading amounts of
Introduction
Electronic supplementary material The online version of this
article (doi:10.1007/s13738-017-1131-z) contains supplementary
material, which is available to authorized users.
Schiff base (C=N, also known as azomethine or imine)
and oxime (=N–OH) compounds are one of the most
applicable compounds in chemistry and medicine and
widely used in organic transformation and pharmaceu-
tical synthesis [1]. In coordination chemistry, they are
known as powerful ligands for various transition metals
and due to the versatility preparation, their roles as efꢀ-
cient catalysis in wide variety of ꢀelds are inevitable [2].
Schiff base compounds and their complexes have note-
worthy properties in medicine which among them can be
point to: antitumor [2], antifungal [3], antibacterial [4–6],
antimicrobial [7], antiviral [8], anticonvulsant [9], anti-
cancer [10, 11], neoplasm inhibitors [12, 13], and antitu-
bercular [14] properties. Moreover, they showed appli-
cations in electronic devices [15, 16], catalysis [17, 18],
solvatochromism in solution [19]. Schiff base complexes
*
Marzieh Tabefam
M_Tabefam@sbu.ac.ir
Milad Kazemnejadi
miladkazemnejad@yahoo.com
Alireza Shakeri
Alireza.shakeri@khayam.ut.ac.ir
Mohammad Mohammadi
mmohammadinurabad@yahoo.com
1
2
3
Department of Chemistry, College of Sciences, Golestan
University, Gorgan, Iran
Faculty of Chemistry, University College of Science,
University of Tehran, Tehran, Iran
Department of Phytochemistry, Medicinal Plants and Drugs
Research Institute, Shahid Beheshti University, Tehran, Iran
1
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J IRAN CHEM SOC
can be polymerized [20] or linked to various nanoparti-
cles [21–23] to nailing desire purposes such as catalysis
only conquer the above-mentioned drawbacks, but also can
be recovered from the reaction mixture.
[
24, 25] and coating [26].
Traditionally, imine compounds were prepared from
Salicylaldehyde is one of the safe, readily available
organic compounds in health and reactivity [54] point of
view and can be extracted from natural sources such as
buckwheat [55]. Also, salicylaldehyde is known as one of
the constituents of beaver castoreum [56]. Previously, oxi-
dative polycondensation reaction has been used for prepa-
ration of polysalicylaldehyde [57]. In the present work, we
synthesized polysalicylaldehyde by polycondensation reac-
tion of 2-hydroxy-5-chloromethyl-benzaldehyde and then
designed a polymer-Mn complex through complexation of
a PSA-Schiff base ligand to Mn(III) metals as a new het-
erogeneous, low-cost polymeric catalyst for oxidation of
different alcohols as well as direct preparation of various
Schiff base (such as salen-type ligands) and oxime com-
pounds in the presence of molecular oxygen and ethanol as
green oxidant and green solvent, respectively.
condensation of aldehyde and amines; but in last dec-
ades, aerobic oxidation of alcohols and amines (known as
cross-coupling of alcohols with amines) was another new
approach in order to form imines [27]. However, traditional
method has some limitations including using unstable
aldehyde as well as using Lewis acid as catalyst in some
cases and removing water formation during the reaction
[27–33]. Oxidation of alcohols that lead to the preparation
of carbonyl compounds are an important class in organic
synthesis due to their applications in production of vari-
ous pharmaceutical and chemical materials [34, 35]. In the
other hand, Schiff base compounds which formed through
condensation of amines and carbonyl compounds were
under-effect to oxidation of alcohols; because in this point
of view, alcohols are more commercially available, cheaper
and stable than aldehydes [36]. So, the attentions have been
attracted to oxidation of alcohols in the presence of amines
to formation of Schiff base compounds as an alternative
method which covers the above-mentioned drawbacks of
traditional method [36]. Various protocols and attempts
to oxidation of alcohols (primary and secondary) in order
to form carbonyl compounds show the importance of this
basic reaction in organic synthesis [37].
Experimental
Materials and instrumentation
All materials were purchased from Sigma-Aldrich and
Merck corporations and used as received without any puri-
ꢀcation. FTIR spectra were obtained using a BRUKER
EQUINOX 55 FTIR spectrophotometer using KBr pellet.
Analytical thin layer chromatography (TLC) was carried
Due to lower toxicity as well as low cost of Mn com-
plexes than other transition metal complex systems, they
attracted great deal of attentions to themselves in order
to oxidation and epoxidation protocols [38, 39]. Various
methods have been reported for Mn(III) and Mn(II) salts
1
out on glass plates covered with silica gel. The H-NMR
spectra were recorded in CDCl3 and DMSO-d6 using
300 MHz instrument. All chemical shifts are reported in δ
units downꢀeld from TMS. Elemental analyses (C, H, N)
were performed on Perkin Elmer-2004 instrument. UV–
Vis analyses performed on UV Spectrolab BEL photon-
ics. Thermal studies (TGA-DTG) have been performed
on a NETZSCH STA 409 PC/PG in nitrogen atmosphere
with a heating rate of 20 °C/min in the temperature ranges
of 25–750 °C. ICP analysis was performed by VARIAN
VISTA-PRO CCD simultaneous ICP-OES instrument. Gas
chromatography (GC) analyses were performed on a Shi-
madzu-14B gas chromatography equipped with HP-1 capil-
[40–43] as well as their complexes for oxidation of alco-
hols [44]. Mn-salen-type complexes have been known as
efꢀcient reagents for catalytic oxidation reactions such as
epoxidation of oleꢀns, oxidation of alcohols and aldehydes
[45–47]. A few Mn catalysts have been reported for cross-
coupling of alcohols with amines some of which are as fol-
lows: (1) manganese octahedral molecular sieves (OMS-2)
[48], (2) MnOx/HAP [49] and (3) meso-Cs/MnOx [50].
Previously, Nasseri et al. [51] used polymer-bounded
Mn(III) complex base on cellulose for selective oxidation
of benzyl alcohols. Recently, Räisänen et al. [52] studied
catalytic homogeneous oxidation of alcohol by Mn(OAc)2.
Chen et al. [53] have reported various methods for imine
formation through oxidation of alcohols.
Although various methods have been reported for oxi-
dation of alcohols, most of them involve some drawbacks
such as lack of selectivity, high consumption of energy,
including various by-product, toxic, expensive and unsta-
ble materials, corrosive and dangerous procedure and low
efꢀciency. So, ꢀnding a method to be felt for oxidation of
alcohols to corresponding aldehydes or ketones that not
lary column (30 m, 0.25 mm, 0.25 μm) and N as carrier
2
gas. Gel permeation chromatography (GPC) was acquired
by Knauer advanced scientiꢀc instrument, Germany, with
RI detector (Smartline 2300) PL gel 10 µm, 10E3 A° col-
umn. Monodispersed poly(methyl methacrylate) (PMMA)
standards were used for calibration. Injected volume was
20 µL. An Oxford INCA 350 energy-dispersive X-ray
microanalysis system connected with the Hitachi S-4800
ꢀeld emission scanning electron microscope was used for
the energy-dispersive X-ray spectrometry (EDX) measure-
ment. Melting points were measured using Electrothermal
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J IRAN CHEM SOC
IA 9000 melting point apparatus. The Schiff base and
oxime products were characterized by their melting points,
elemental analysis, H NMR and FTIR spectra and com-
with distilled water (3 × 10 mL) in order to complete
elimination of acid and inorganic salts impurities (such
as KOH and KCl that can be formed in polycondensa-
tion reaction and release of HCl molecules) and put in
the oven (50 °C) overnight. Final mass for 2 was 0.83 g
1
pared with literature values.
Preparation of PSA and PSA‑Schiff base ligand
(m.p. > 400 °C). PSA (1 g, M ~ 2278) along with 2-ami-
w
nophenol (6 mmol) was added to 10 mL of ethanol. The
reaction was stirred at room temperature for 12 h until
pale orange product was obtained which represented the
formation of the intended Schiff base ligand. The product
was ꢀltered, washed with methanol (2 × 5 mL) and iso-
lated (1.6 g) after drying in the oven (50 °C).
The average molecular weight of the PSA was meas-
ured through acylation of hydroxyl groups (end group
analysis), an analytical method that determined hydroxyl
number (%OH) by Eq. (1): [59, 60]
2
-Hydroxy-5-chloromethyl-benzaldehyde was synthe-
sized and puriꢀed according to the procedure described
in [58]; in a 100-mL round-bottom ꢁask, salicylaldehyde
(
10 mmol), paraformaldehyde (0.49 g, 16.4 mmol) and
HCl 37% (80 mmol) along with several drop of concen-
trated H SO as a catalyst at 70°C were mixed together
and stirred for 20 h (Scheme 1). The reaction mixture
was cooled to room temperature, then water (20 mL) was
added to the mixture, and the product was extracted into
CH Cl (20 mL). Some anhydrous Na SO was used for
2
4
2
2
2
4
(
V − V ) × f × 0.0085 × 100
1 2
drying the organic phase. Remaining CH Cl removed
%OH =
2
2
(1)
mp
with rotary evaporator. Then, a pale purple product 1
as powder was isolated (9.5 mmol, 95% yield). Polym-
erization of 2-hydroxy 4-chloromethyl-benzaldehyde 1
where V (mL) and V (mL) are the consumed volumes of
1
2
potassium hydroxide required for titration of blank and
polymer sample, respectively, f is the coefꢀcient of KOH
solution 0.5 N that is equal to 0.732 [61], and ꢀnally mp
(g) is mass of polymer sample weighted for titration.
The molecular weight obtained by this method will be
(
Scheme 1) (6 mmol) was done in the presence of con-
centrated KOH (50% (v/v), 20 mmol) at 80°C from which
after stirring for 16 h a dark yellow solid (PSA, 2) was
obtained. The mentioned solid was ꢀltered and washed
Scheme 1 General Scheme for preparation of PSA-Schiff base-Mn(III) complex 4 (proposed structure)
1
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J IRAN CHEM SOC
numerical molecular weight (M ) that was measured by
insertion of %OH into Eq. (2) [59, 60]:
for PSA clearly conꢀrms the formation of PSA through
polycondensation reaction.
n
5
6.11 × 1000
M =
n
(2)
Complexation of Mn to PSA‑Schiff base ligand
%
OH
To measure the hydroxyl number of PSA, 0.017 g of
PSA was weighted and dissolved in pyridine/acetic anhy-
dride mixture (88:12) along with stirring at 50 °C for 8 h.
One or two droplets of phenolphthalein indicator were
added and the reaction mixture then titrated with KOH
PSA-Schiff base-Mn(III) complex was prepared by the addi-
tion of ethanol (15 mL) to PSA-Schiff base ligand (0.40 g)
at 70 °C (Underwent reꢁux). The reaction was stirred for
20 min, and then Mn(OAc) (2 mmol) was added wisely dur-
2
ing 10 min. Stirring lasted for another 4 h. The brown sedi-
ments were ꢀltered and washed with methanol (2 × 5 mL).
The resultant product was put on oven overnight and then
isolated as brown powder (Isolated weight: 0.69 g).
(
(
0.5 N). Hydroxyl number of PSA was measured 25.62
V − V = 0.7 mL, Eq. 1). So the M was calculated by
2
1
n
Eq. 2, equal to 2190 for PSA.
Molecular weight of the PSA also was studied with
gel permeation chromatography (GPC). The results
obtained from GPC are tabulated in Table 1. The numeri-
General procedure for oxidation of alcohols
cal molecular weight (M ) by GPC was equal to 2226.
In a 10-mL round-bottom ꢁask equipped with O -ꢀlled
n
2
Small differences (Just 36 unit) between analytical and
instrumentational measuring (GPC) of number average
balloon (~1 atm.), a mixture of alcohol (1 mmol), absolute
ethanol (3 mL) and 10 mg of catalyst 4 (3.2 mol%) was
stirred at room temperature. After the completion of the
reactions that monitored by thin layer chromatography
(TLC), the catalyst was ꢀltered off and the reaction mix-
ture was extracted to CH Cl and were puriꢀed with silica
gel plate chromatography (Scheme 2). The oxidation of
primary alcohols proceeds to aldehyde stage and was not
observed any carboxylic acid product identiꢀed by TLC.
The reactions also were performed in the absence of cata-
lyst or in the absence of oxidant in the same conditions.
molecular weight (M ) of PSA exhibited the accuracy
n
of the methods as well as obtained amount for M . PSA
n
has well polydispersity index (PSI = 1.43). An expla-
nation for this low PSI was step-growth polymerization
of 2-Hydroxy-5-chloromethyl-benzaldehyde in alkali
medium. The alkali medium neutralized HCl molecules
that formed during the condensation reaction. This act as
a driving force for forward of the polymerization reaction
and provided such low PSI. Obtained molecular weight
2
2
Table 1 Data obtained from GPC analysis of PSA
Types of molecular weight
Weight average molecular weight
2278
Number average molecular weight 2226
Z average molecular weight
Z + 1 average molecular weight
Polydispersity index
2328
2377
1.430
2244
1.028
1.056
Peak molecular weight
Z average/weight average
Z + 1 average/weight average
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J IRAN CHEM SOC
O / 1 atm
2
O / 1 atm
2
OH
R1
R2
+
R -NH
3
2
OH
R2
R1
Plate
silica gel
Chromatography
O
R1
R2
Organic phase
MeOH
4
Catalyst
Ext. DCM
Aldehyde
or keton
R3
N
R
1
Schiff baseR
2
Scheme 2 An experimental view for oxidation of alcohols as well as direct imine formation
General procedure for direct preparation of Schiff
base and oxime compounds catalyzed by PSA‑Schiff
base‑Mn(III) complex
Conversion and selectivity of each product were calcu-
lated by the following equations:
(
initial mol% ) − (final mol% )
conversion (mol% ) =
× 100
4)
initial mol%
In an oven-dried 10-mL round-bottom ꢁask, 10 mg catalyst
(
4
(3.2 mol%) along with 1 mmol amine or hydroxyl amine
was added to 7 mL absolute ethanol at ambient temperature.
Then, 1 mmol of alcohol was added to the reaction mixture
GC peak area of desired carbonyl
GC peak area of all products
Aldehyde selectivity =
× 100
and stirred under O atmosphere (~1 atm.). After 14–60 min,
2
(5)
yellow to red sediments were appeared which show successful
oxidation of alcohols followed by imine formation. The prod-
uct was ꢀltered and added to 20 mL of hot methanol which
just dissolved the imine compound. The ꢀltration of the reac-
tion mixture left a solid on the ꢀlter that was the catalyst 4. The
remained solution set aside overnight for crystallization, and
the recovered catalyst was reused for the next run (Scheme 2).
Activity of the catalyst was expressed as turn-over fre-
quency (TOF) and calculated from the moles of benzyl
alcohol converted per mole of Mn contained in the catalyst
using following equation:
Results and discussion
Characterization of PSA
FTIR spectra of the samples well explained their structures
and are shown in Fig. 1a–d. Advent of a peak at 1481 cm
in the spectrum of 1 was more related to the bending
vibration of methylene groups (–CH –) in m-substitution
−
1
2
than that of aldehyde group (Fig. 1a). Another character-
−
1
istic peak for 1 was 725 cm for vibration of C–Cl which
demonstrated successfully the formation of chloromethyl-
ene substitution on salicylaldehyde. The presence of C–Cl
moles (Converted benzyl alcohol)
TOF =
(3)
moles of Mn (Active site) × reaction time (h)
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J IRAN CHEM SOC
Fig. 1 FTIR spectrum of
-hydroxy-5-chloromethyl-
2
benzaldehyde 1 (a), PSA 2 (b),
PSA-Schiff base ligand 3 (c)
and PSA-Schiff base-Mn(III)
complex 4 (d)
vibration in the all spectra at the region of ~725–763 cm−1
demonstrated chloromethylene groups in polymer termi-
Chelation of Mn(III) to N and O atoms in PSA causes to
shifting of C=N stretching vibrations to lower wavenum-
−
1
−1
nal chains (Scheme 2). Also, two peaks at 800–900 cm
were assigned to 1,2,4-trisubstituted pattern on the ben-
bers at 1589 cm (Fig. 1d) [62]. Furthermore, consider-
able reduction of hydroxyl stretching (great elimination
of H-bonding) at catalyst 4 illustrates that the chelation of
Mn metals also happens through oxygen atoms. Vibrations
correspond to Mn–O and Mn–N were found at 547 and
−
1
zene ring. A series vibration around 2750 and 2850 cm
represented the C–H of aldehyde stretching (Fig. 1a, b).
Also, the spectra showed C–H bending vibrations between
−
1
−1
1
400 and 1480 cm . Vibration of C–H for benzene rings
586 cm , respectively [63].
−
1
was found at 3000–3040 cm for the samples. Due to the
polymerization of salicylaldehyde, stretching vibration of
Physical properties of the samples including elemental
analysis and color are given in Table 1.
−
1
hydroxyl groups at 3200 cm have been reduced (Fig. 1b,
dotted circle) which exhibited the formation of ether
bonds. After the formation of Schiff base moiety on PSA,
a broad peak related to O–H phenolic with H-bonding
was appeared in the spectrum of PSA-Schiff base ligand
Elemental analysis also proved the formation of 1
(Table 2). Also, it showed the presence of nitrogen atom in
amount of 11.84% for PSA-Schiff base ligand which exhib-
ited the presence of N result in imine formation. Also after
coordination of Mn to PSA-Schiff base ligand, the nitrogen
percentage reduced to 9.03% which shows the presence of
Mn in the complex.
−
1
(
Fig. 1c). Vibration of imine bond for c at 1612 cm
also was another conꢀrmation for imine bond formation.
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J IRAN CHEM SOC
Table 2 Elemental analysis
results and colors of the
compounds
Compounds
Color
Elemental analysis
%C
%H
%N
2
-Hydroxy-5-chloromethyl-benzaldehyde
Purple
Yellow
Orange
Brown
56.82 (56.33)a
70.11
4.25 (4.14)
4.35
–
PSA
–
PSA-Schiff base ligand
PSA-Schiff base-Mn(III) complex
70.80
5.27
11.84
9.03
65.14
3.90
a
Theoretically calculated
Fig. 2 1H NMR (300 MHz,
DMSO-d ) (a) 2-hydroxy-
6
5
-chloromethyl-benzaldehyde 1
and PSA 2 (b)
1
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J IRAN CHEM SOC
Fig. 3 a UV–Vis spectra of Mn(OAc) , PSA, PSA-Schiff base ligand and PSA-Schiff base-Mn complex and b TGA analysis of PSA, PSA-
2
Schiff base ligand and PSA-Schiff base-Mn complex
2-Hydroxy-5-chloromethyl-benzaldehyde 1 has been
previously synthesized, and due to the presence of two
directing groups on the phenyl, the chloromethylene group
1
is substituted at 5-position as shown in scheme 1 [20]. H
NMR of 2-hydroxy-5-chloromethyl-benzaldehyde 1 and
PSA 2 is shown in Fig. 2. Resonance for methylene pro-
tons was found at 4.58 ppm. Methylene protons for PSA
2
4
7
were represented with three peaks at 4.09, 4.74 and
.98 ppm. Aromatic protons were set in the region of 6.97–
.57 ppm for 1 and 7.11–8.3 ppm for PSA. Chemical shifts
for aldehyde and hydroxyl protons were found at 9.09 and
1
1
1.06 ppm for 1. H NMR spectrum of PSA shows two
peaks for aldehyde at 10.22 and 1.40 ppm due to different
chemical environments. Hydroxyl groups at terminal PSA
chains exhibit a single peak at 11.33 ppm (Fig. 2).
Fig. 4 EDX spectra of PSA-Schiff base-Mn complex
The electronic spectra for the samples were scrolled at
region of 200–800 nm. Figure 3a shows UV–Vis spectra
for Mn(OAc) , PSA, PSA-Schiff base ligand and PSA-
Mn(II) complex. PSA shows a single peak at ~336 nm
The data obtained from electronic studies along with brown
color reported for Mn(III) complexes [67] conꢀrm the Mn
metal was +3 in valence.
2
*
with a shoulder at 296 nm which can be assigned to π→π
*
and n→π related to phenyl and carbonyl groups, respec-
Thermal studies of the PSA, PSA-Schiff base ligand
and PSA-Schiff base-Mn(III) complex are shown in
Fig. 3b. The spectra show good thermal stability for all
three samples; remaining weight for the samples was
50–55% until 750 °C. Thermal decomposition of PSA
(Fig. 3b) occurs only in one step in the temperature span
of 200–525 °C corresponding to 47% weight loss. After
that, the weight loss continues to 750 °C with a gentle
slope which shows carbonization of the polymer. Advent
of an additional step in the TGA curve of PSA-Schiff
base ligand was attributed to decomposition of the Schiff
base moiety from PSA chain and conꢀrmed the Schiff
base formation on the PSA framework. The mentioned
decomposition occurs in temperature span of 99–360 °C
(8% weigh loss). Second stage which starts at 420 °C
tively. PSA-Schiff base ligand exhibits three adsorption
bands at 298, 356 and 444 nm. These adsorptions were due
*
*
to n→π and π→π , resulting in the formation of imine
*
bond and also π→π transitions for benzene rings [64].
Mn(OAc) shows a single peak at 296 nm which was attrib-
2
uted to the Mn(II) adsorption. Elimination of this peak in
PSA-Schiff base-Mn(III) complex and advent of a broad
peak with center of 428 nm show the changing valence of
Mn(II) to (III) [65, 66] as well as metal to ligand charge
*
transfer (MLCT). Furthermore, shift of π→π adsorption
for imine bond in catalyst 4 to longer wavelengths was as a
result of coordination of Mn(III) to PSA-Schiff base ligand
[67]. Adsorptions of π electrons related to benzene rings
appeared as shoulder at 374 nm for the catalyst 4 (Fig. 3a).
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J IRAN CHEM SOC
shows decomposition of PSA. Insertion of Mn on PSA-
Schiff base ligand leads to about 25% (230–510 °C)
weight loss in the thermal spectrum of PSA-Schiff base-
Mn(III) complex (Fig. 3b) in ꢀrst stage. According to
thermal degradation of Schiff base moiety in PSA-Schiff
base ligand, this amount was consistence with loading
amount of Mn obtained from ICP analysis. Second step
also was belonging to PSA decomposition.
Loading amount of Mn on the PSA-Schiff base ligand
was measured by inductive coupled plasma (ICP) ana-
lyzer. The experiments indicate that 17.56 mg of Mn was
loaded on 100 mg of PSA-Schiff base ligand framework.
The elements of the catalyst 4 were detected by
energy-dispersive spectroscopy (EDX) analysis (Fig. 4).
Related information including intensity % and weigh %
of the elements is shown at inset table in Fig. 4. EDX
spectrum clearly conꢀrmed the Mn complex formation on
PSA framework. Also, the spectrum showing the attend-
ance of Cl atoms set at the end chains of PSA framework
required for oxidation reactions catalyzed by PSA-Schiff
base-Mn(III) complex 4, we investigated these effective
parameters for oxidation of alcohols. For this purpose, the
oxidation of benzyl alcohol was choosing as model reac-
tion, and so temperature, catalyst amount, solvent and type
of oxygen donors were toward our optimization steps and
the corresponding results are given in Table 3. Temperature
had the least inꢁuence for the oxidation of benzyl alcohol
which raising the temperature from room temperature to
60 °C or 60–78 °C (Reꢁux) in ethanol as solvent did not
show higher efꢀciency (Table 3, entries 3, 5 and 6). The
effect of temperature for the other solvent was same as eth-
anol. On the other hand, solvent plays a vital role in the
oxidation reactions where the oxidation did not take place
at solvent-free conditions (Table 3, entry 15). Ethanol illus-
trated the best performance at room temperature among the
used solvents (Table 3, entry 5). The reaction did not show
any progress in water as solvent (Table 3, entry 1). Besides
ethanol, other solvents such as DCE, CH CN, CH Cl and
3
2
2
(
Fig. 4).
n-hexane showed high progresses for oxidation of benzyl
alcohol. Benzaldehyde conversion was low in DMF and
toluene (Table 3, entries 8, 9). 3.2 mol% of the catalyst 4
was sufꢀcient in order to nailing to the most possible efꢀ-
ciency (Table 3, entry 5, 96% yield). Amounts more or less
than 3.2 mol%, yields lower efꢀciencies (Table 3, entries 2
and 4) for oxidation reaction at room temperature in etha-
nol. Progress of the reactions was screened by TLC which
Optimization of effective parameters for oxidation
reactions
Catalytic oxidation of alcohols is strongly affected by
parameters such as oxygen donors, temperature, amount of
the catalyst and solvent [67]. To ꢀnd optimum conditions
Table 3 Effect of solvent for
oxidation of benzyl alcohol
% Yieldb
Entry
Solvent
Temperature (°C)
Catalyst amount (mol%)
Time (min)
a
1
2
3
4
5
6
7
8
9
H O
80
3.2
1.6
3.2
6.4
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
3.2
60
30
30
30
30
30
60
60
60
30
60
60
60
90
90
0
2
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
DMF
Toluene
THF
R.T.
88
R.T.
96
R.T.
93
60
96
Reꢁux
Reꢁux
80
95
90 (89)c
20
60
44
10
11
12
13
14
15
Reꢁux
Reꢁux
Reꢁux
Reꢁux
Reꢁux
R.T.
60 (57)c
88 (86)c
85 (84)c
86 (85)c
80 (83)c
0
DCE
CH CN
3
CH Cl
2
2
n-Hexane
Solvent-free
a
Reaction conditions: benzyl alcohol (1 mmol), catalyst 4, time, temperature, solvent (3 mL) (except
entry 15)
b
Isolated yield
c
Yield at room temperature
1
3

J IRAN CHEM SOC
Fig. 5 Experiments in order
to show a effect of oxidizing
agent and b catalyst activity;
−
1
c TOF (h ) calculated for
various oxidizing agents; d TOF
−
1
(
h ) calculated for different
*
catalysts. Reaction condi-
tions: benzyl alcohol (1 mmol),
catalyst 4 (3.2 mol %), room
temperature, ethanol (3 mL),
*
*
oxidant (2 mmol), 30 min.
KHSO5. * Reaction condi-
tions: benzyl alcohol (1 mmol),
catalyst (Except none), room
temperature, ethanol (3 mL), O2
balloon, 30 min
**
complex was shown by oxone and O as oxygen donors
2
−1
in terms of TOF (TOF = 60 h ). So, the optimum condi-
tions obtained from the optimization experiments for oxi-
dation of alcohols as well as direct preparation of Schiff
base/oxime compounds come as follows: ethanol (Solvent),
room temperature, 3.2 mol% of catalyst 4, time and O bal-
2
Scheme 3 Preparation of N-salicylidene-o-aminophenol (saphH2)
loon as oxidizing agent.
For the elucidation of activity of the catalyst 4, the
monomeric complex 5, N-salicylidene-o-aminophenol
resulting in the reaction of 2-aminophenol with salicylal-
dehyde was prepared with previously described procedure
[68, 69] and complexed to Mn and used as homogeneous
catalyst for oxidation of benzyl alcohol at optimum condi-
tions (Scheme 3). As shown in Fig. 5b, monomeric com-
plex 5 did not show good selectivity and yield for oxidation
of benzaldehyde with regard to its homogeneous perfor-
mance. Conversion to benzaldehyde was 68%, and also
benzoic acid was obtained. Furthermore, recovering of the
catalyst 5 was a serious problem and suffered from tedious
work-up of the carbonyl compounds. Oxidation of manga-
nese may be responsible for low selectivity and activity in
5 [70–72] which using solid support for Mn complex not
only reduced the formation of oxomanganese species but
also improved the recycling ability of the catalyst with low
activity loss [72–75].
in optimum conditions the oxidation of benzyl alcohol was
done for 30 min (Table 3, entry 5).
Effect of various oxidant and also catalyst activity on
oxidation of benzyl alcohol is given in Fig. 5a, b, respec-
−
1
tively, as donut charts. Also, TOF amounts (h ) for every
oxidant as well as used catalysts were calculated and are
shown in Fig. 5c, d, respectively. Effect of oxidant was
studied by some oxidizing agents: air, tert-BuOOH,
NaOCl, oxone, H O and KIO (Fig. 5a). In air atmosphere,
2
2
4
without O balloon equipment, yield of oxidation reduced
2
to 73%. Selectivity also lost in air atmosphere. Moreover,
in order to clarify the role of O molecules we checked the
2
reaction at argon atmosphere and the conversion was fur-
ther reduced to 12% (not shown in Fig. 5a). Oxone showed
the well performance equal to efꢀciency with O as oxidiz-
2
ing agent (Fig. 5a). But due to green and readily accessi-
ble O than oxone, we choice the ꢀrst one for the oxidation
reactions. The most activity of the PSA-Schiff base-Mn(III)
On the basis of reported structure for 5 [70] along with
achieved results from FTIR, UV–Vis, TGA and EDX
2
1
3

J IRAN CHEM SOC
Table 4 Oxidation of alcohols to carbonyl compoundsa
b
−1
)
Entry
Alcohols
Product
Time (min)
Yield (%)
B.P./M.P. (°C)
Found
TOF (h
Lit. [Refs.]
178 [67]
OH
O
1
2
30
22
96
94
177
60.00
79.40
OH
O
47–48
48 [67]
58 [67]
Cl
Cl
3
4
OH
O
40
25
90
98
57
41.98
72.92
NO2
NO
2
OH
NH2
245
248 [67]
MeO
N
O
5
6
7
20
45
42
97
88
93
194
196 [76]
91.86
98.21
52.84
OH
OH
OH
OH
OH
O
124–126
129–131
125–128 [77]
128–130 [76]
O N
O
2
N
OH
2
O
OH
O N
O N
2
2
8
9
OH
OH
O
30
19
95
98
6–8
4 [78]
59.37
95.70
MeO
MeO
OH
OH
OH
O
41–43
41–42 [79]
OH
MeO
MeO
O
1
1
0
1
OH
O
O
31
43
90
87
164
244
162 [80]
248 [67]
54.08
37.76
O
OH
1
1
2
3
OH
O
36
52
95
75
114
114–116 [81]
202 [80]
49.47
26.94
HO
O
OH
O
200–201
1
4
OH
O
60
70
154
155 [67]
21.88
1
3

J IRAN CHEM SOC
Table 4 continued
Entry Alcohols
b
−1
Product
Time (min)
45
Yield (%)
B.P./M.P. (°C)
Found
TOF (h
)
Lit. [Refs.]
6–8 [67]
1
5
OH
O
77
5–8
32.08
1
6
7
O
–
–
–
–
–
–
–
–
–
–
–
–
O
1
MeO
a
Reaction conditions: benzyl alcohol (1 mmol), catalyst 4 (0.032 mmol Mn, 3.2 mol %), room temperature, ethanol (3 mL), O2 balloon
(~1 atm.), time
b
Isolated yield
analysis, the most probably structure for catalyst 4 is shown
at scheme 1.
Furthermore, we checked the blank experiment, in the
absence of the catalyst 4 which didn’t observe any noticea-
achieved. Also, we can point to direct preparation of salen
derivatives, the most known strong and applicable ligand
so far, with high yields in suitable time (Table 5, entries
2-4) by the present method. Moreover, catalytic oxida-
tion of 1,4-benzenedimethanol yields terephthaladehyde
in amount of 95% (Table 5, entry 8) which in the presence
of 2-aminophenol the bis-imino compound was prepared.
However, due to steric hindrance of ketones, they provided
lower efꢀciency than aldehydes for preparation of ketoxi-
mes (Table 5, entries 9, 10) and need to heat in some cases.
The oxidation was inert to amine and aldehyde sub-
strates. This property provides the advantageous of the
oxidation conditions for selective preparation of aldehydes
as well as imine compounds. Moreover, chemoselectivity
behavior of the catalyst was studied by oxidation of ben-
zylic alcohol, n-butanol and ally alcohol through 4 combi-
nations as shown in Table 6.
The results demonstrated high selectivity for oxidation
of benzyl alcohol in the presence of aliphatic alcohol or
mixture of aliphatic/allyl alcohols (Table 6). Benzaldehyde
was the major product in the reaction mixtures of benzyl
alcohol with ally alcohol (Table 6, entry 1, > 99%), ben-
zyl alcohol with n-butanol (Table 6, entry 2, 94%) and
with mixture of n-butanol/allyl alcohol (Table 6, entry 4,
97%). Also the catalyst exhibited selectivity for oxidation
of n-butanol than ally alcohol (Table 6, entry 3, 67%).
Retrieval and recoverability of a catalyst is one of the
signiꢀcant matters for a heterogeneous catalysis which
manifest the stability, and metal leaching of the catalyst
and was related to some issues such as cost-effective and
clean in a reaction [22]. Stability and recoverability of
the catalyst was examined on the model reaction (Oxida-
tion of benzyl alcohol) in obtained premium conditions.
The catalyst was successfully recovered after completion
of the reaction, rinsed with water and reused for eight runs
ble product (Fig. 5b). Conversion with Mn(OAc) and PSA
2
also was 36 and 20%, respectively, for benzaldehyde for-
mation. Activity of the PSA-Schiff base-Mn complex as an
efꢀcient catalyst can be displayed in terms of TOF which
the catalyst has higher TOF amounts than Mn(OAc) and
2
Cat. 5. These experiments well addressed the activity of the
PSA-Schiff base-Mn(III) complex as an efꢀcient and prom-
ising catalyst for oxidation of alcohols.
The oxidation of a variety of primary and secondary
alcohols to corresponding carbonyl compounds carried out
in obtained optimum conditions and the data is shown in
Table 4. The best result was belonging to 4-methoxy benzyl
alcohol with 98% yield for 25 min (Table 4, entry 4). Oxi-
dation reactions showed dependency on electron donating/
withdrawing substituent on benzene ring in which electron-
rich substrates provide higher oxidation yields (Table 4
entries 4, 8, 9). Instead, electron-deꢀcient substrates have
lower efꢀciency (Table 4, entries 3, 6, 7). It believes donat-
ing groups stabilized the intermediates produced along the
reaction pathway for the incorporation of oxygen into the
molecule [51]. Aldehydes did not bear the oxidation which
obviously demonstrated selectivity of the method (Table 4,
entries 16 and 17). Secondary alcohols represented lower
efꢀciency than primary alcohols (Table 4, entries 13-15).
TOF amounts were calculated for every reaction, and the
related data are given in Table 4 which represent good
activity for the catalyst 4.
Direct synthesis of imines and oximes were taken with
various primary and secondary alcohols in the presence of
amines and PSA-Schiff base-Mn(III) complex as a cata-
lyst. Up to high yields whether for oximes or imines were
1
3

J IRAN CHEM SOC
Table 5 Direct preparation of Schiff base compounds catalyzed by PSA-Schiff base-Mn(III) complexa
b
−1
)
Entry
1
Alcohols/ketone
Amines
Schiff base/oxime product
Time (min) Yield (%) m.p. (°C) [Refs.] TOF (h
OH
NH2
NH2
N
144
88
92
95
92
[81]
11.98
19.79
20.83
2c
OH
OH
126–129 [30]
154–156 [82]
N
N
H N
2
OH HO
3c, d
OH
OH
90
H N
NH2
N
N
2
OH HO
4c, d
OH
OH
83
45
88
95
51–53 [83]
19.92
39.58
N
N
NH2 NH2
OH HO
5
OH
OH
NH2
185–186 [70]
N
OH
OH
HO
6
7
NH2
OH
44
95
98
95
~400 [84]
41.38
18.79
OH
HO
MeO
HO
N
MeO
HO
OH
NH2
N
162–165 [85]
8e
NH2
OH
110
90
83
71
213–215 [86]
58–62 [87]
14.17
14.80
OH
N
N
HO
HO
OH
OH
9d
NH OH
N
2
O
H
1
1
1
0d
OH
NH OH
HO
95
30
38
62
96
92
85–88 [87]
60–62 [88]
72–75 [89]
12.26
60.00
44.92
2
N
1
OH
OH
NH OH
OH
2
N
OH
2
OH
Cl
NH OH
OH
N
2
Cl
1
3

J IRAN CHEM SOC
Table 5 continued
Entry Alcohols/ketone
OH
b
−1
Amines
Schiff base/oxime product
Time (min) Yield (%) m.p. (°C) [Refs.] TOF (h
120 65 60–62 [90] 10.16
)
1
3
NH OH
HO
N
2
a
Reaction conditions: alcohol (1 mmol), amine (1 mmol), catalyst 4 (0.032 mmol Mn, 3.2 mol%), room temperature, ethanol (3 mL), O bal-
2
loon (~1 atm.), time
b
Isolated yield
c
2
mmol aldehyde was used
The reaction performed at reꢁux conditions
mmol amine was used
d
e
2
Table 6 Chemoselectivity behavior of the catalyst 4 in oxidation of various types of alcoholsa
Entry
Sample
Selectivity (%)b
Benzaldehyde
Conversion (%)c
Propanol
n-Butanol
1
Benzyl alcohol + allyl alcohol
Benzyl alcohol + n-butanol
>99
94
–
Trace
–
93
96
48
95
2
–
6
3
4d
Allyl alcohol + n-butanol
33
3
67
0
Benzyl alcohol + n-butanol + allyl alcohol
97
a
Reaction conditions: 0.5 mmol from each alcohol, catalyst 4 (10 mg, 3.2 mol%), room temperature, ethanol (3 mL), O balloon (~1 atm.), 1 h
2
b
c
d
GC analysis
For major product
Catalyst: 15 mg
The recovered catalyst after eighth run was charac-
terized with FTIR and UV–Vis techniques (Fig. 7). As
shown at Fig. 7, UV-Vis spectrum of the recovered cat-
alyst hasn’t changed than the fresh catalyst. Adsorption
related to Mn(III) was found at 428 nm for the recovered
catalyst as same as the fresh catalyst. With compari-
son between FTIR spectra of primary catalyst (Orange
curve) and used catalyst (Green curve), it could be con-
cluded that the structure of the catalyst remains intact
during the reactions and any change was not observed in
the main peaks for recovered than primary catalyst; for
−
1
−1
example, 1612 cm related to imine bond and 547 cm
−
1
and 586 cm related to vibrations of Mn–O and Mn-N,
respectively. No change in spectra of the recycled cata-
lyst as well as negligible leaching metal and loss in activ-
ity conꢀrm the stability of the catalyst 4.
Fig. 6 Yield and leaching amounts of the catalyst for reaction cycles
Also, in order to measure the exact species responsible
for the observed reactions and to measure the extent of Mn
leaching after the reactions, we have used the hot ꢀltration
test [91–93]. For this goal, the model reaction (oxidation
of benzyl alcohol) was carried out in the presence of PSA-
Schiff base-Mn(III) complex under optimized conditions.
The catalyst was removed in situ from the reaction mixture
without noticeable reactivity or weight loss. Also amounts
of metal leaching were measured by ICP analysis after
each run. Metal leaching form the catalyst 4 reached to
3
.8% after 8 runs which shows 0.5% in average per cycle
(Fig. 6).
1
3

J IRAN CHEM SOC
Fig. 7 UV–Vis and FTIR spectra of PSA-Schiff base complex and recovered PSA-Schiff base complex after eighth run
Table 7 Comparison among efꢀciency of PSA-Schiff base-Mn(III) complex with the reported catalysts in the literature for preparation of
N-benzylidenebenzylamine
Entry
Catalyst
Solvent/T (°C)
Time (h)
Yield (%)a
[Refs.]
1
2
3
4
5
6
7
Cu(ClO ) .H O (5 mol%)
Toluene/70
-b/R.T.
19
72
56
24
12
24
2.4
90
99
87
23
>99
97
92
[34]
4
2
2
Pd(OAc) (1 mol%)
[35]
2
PNP-type Ru pincer complex (0.2 mol%)
Toluene/Ref.
MeOH/R. T.
[94]
Au/TiO (0.02 mol%)
[95]
2
CuI (1 mol%)
CH CN/R. T.
[36]
3
Fe(NO ) /TEMPO
Toluene/80
[96]
3
3
PSA-Schiff base-Mn(III) complex (3.2 mol%)
Ethanol/R. T.
Present work
a
GC yield
b
Neat
after 44% conversion (GC), and the reactants were allowed
to undergo further reaction in the solution. Continuation of
the reaction (Without catalyst 4) under the same conditions
showed 48% conversion after 1 h. These results indicate
that after the removal of heterogeneous catalyst, its leach-
ing amount into the reaction mixture should be low and the
catalytic reactions in absence of the catalyst were low, con-
Conclusion
PSA was synthesized by self-polycondensation reac-
tion of 2-hydroxy-5-chloromethyl-benzaldehyde in the
presence of concentrated KOH. PSA-Schiff base ligand
was prepared through reaction of PSA with 2-aminophe-
nol. Then, PSA-Schiff base-Mn(III) complex as an efꢀ-
cient catalyst for oxidation of alcohols was prepared by
complexation of PSA-Schiff base ligand with a Mn salt.
ꢀrming that the catalyst acts heterogeneously in the reac-
tion and only slight leaching take place during the reaction.
To elucidation of advantage, novelty and uniqueness of
PSA-Schiff base-Mn(III) complex, we comprised our cata-
lyst with the reported catalysts in the literature for direct
preparation of N-benzylidenebenzylamine. So, the reaction
of benzyl alcohol with benzyl amine was selected as the
model reaction. The results exhibited in Table 7 appeared
superiority of PSA-Schiff base-Mn(III) complex over the
other systems in environmental and economic aspects.
N-benzylidenebenzylamine was formed by Schiff base-
Mn(III) complex at short reaction time in ethanol at room
temperature.
1
The catalyst was characterized with FTIR, UV–Vis, H
NMR, CHN, ICP-OES, TGA and EDX analyses. Average
molecular weight and PDI of PSA were found 2278 and
1.4, respectively. The catalyst can be prepared through
simple procedure, cheap and commercially accessible
organic materials which can be selectively catalyze oxi-
dation of primary and secondary alcohols to carbonyl
compounds as well as direct formation of Schiff base and
oxime compounds specially salen-type ligands through
an efꢀcient, mild, heterogeneous, recoverable, cost-effec-
tive and novel method by molecular oxygen in ethanol
1
3

J IRAN CHEM SOC
as a green oxidant and solvent, respectively. The catalyst
demonstrated well performance in the presence of oxone.
Moreover, homogenous monomeric type of the catalyst
exhibited lower efꢀciency than PSA-Schiff base-Mn(III)
complex. Chemoselectivity studies on the catalyst toward
oxidation of various types of alcohols revealed very high
selectivity for oxidation of benzyl alcohol than n-butanol
and allyl alcohol. The catalyst can be simply recovered
by simple ꢀltration from the reaction mixture and reused
for eight runs without any notable reactivity loss. Low
leaching, short reaction times, stability, up to high yields
and selectivity were some properties of the catalyst dis-
cussed in this work.
22. M. Salavati-Niasari, E. Esmaeili, H. Seyghalkar, M. Bazargan-
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