Synthesis and characterization of a novel oxo-bridged binuclear iron(iii) complex: Its catalytic application in the synthesis of benzoxazoles using benzyl alcohol in water

Article abstract of DOI:10.1039/c8nj00921j

In the present work, the synthesis and characterization of a novel binuclear oxo-bridged iron(iii) complex, (FeL^APIP)₂O, where L^APIP is the deprotonated form of a tetradentateo-aminophenol-iminopyridine ligand, is described. The iron complex was characterized using different techniques including X-ray crystallography, infrared spectroscopy, UV-Vis, magnetic susceptibility and cyclic voltammetry. The X-ray structure analysis revealed that in the structure of the (FeL^APIP)₂O complex, each iron(iii) is coordinated in a distorted square pyramidal arrangement by an oxo group, three amine nitrogens and one oxygen atom of the o-aminophenolate ligand. The variable-temperature magnetic measurement exhibits strong antiferromagnetic coupling between two iron(iii) centers. Cyclic voltammetry measurements showed two kinds of quasireversible events, suggesting the formation of a phenoxyl radical species in the region of the anodic peaks and a metal-centered redox (Fe^III/Fe^II) process at low potential. The catalytic activity of this Fe-complex was evaluated in coupling of 2-aminophenol and benzyl alcohols for the one-pot synthesis of benzoxazoles. The catalyst system showed high catalytic activity in this transformation and benzoxazole derivatives were obtained in good to excellent yields. tert-Butyl hydroperoxide was used as an oxidant and 1.2 mol% of catalyst was needed to accomplish the reaction in water as the green solvent.

Full text of DOI:10.1039/c8nj00921j

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Synthesis and characterization of a novel oxo-bridged binuclear
iron(III) complex: Its catalytic application in synthesis of
benzoxazoles using benzyl alcohol in water
Received 00th January 20xx,
Accepted 00th January 20xx
Elham Safaei,*,a Zahra Alaji,b Farhad Panahi,*,a,c Andrzej Wojtczakd and Janez Zvonko Jagličiće
DOI: 10.1039/x0xx00000x
In the present work, the synthesis and characterization of a novel binuclear oxo-bridged iron(III) complex, (FeLAPIP)2O,
where LAPIP is the deprotonated form of a tetradentateo-aminophenol-iminopyridine ligand, is described. The iron complex
was characterized using different techniques including X-ray crystallography, infrared spectroscopy, UV–Vis, magnetic
susceptibility and cyclic voltammetry. The X-ray structure analysis revealed that in the structure of (FeLAPIP)2O complex,
each iron(III) is coordinated in a distorted square pyramidal arrangement by an oxo group, three amine nitrogen's and one
oxygen atom of o-aminophenolate ligand. The variable-temperature magnetic measurement exhibits strong
antiferromagnetic coupling between two iron(III) centers. Cyclic voltammetry measurements showed two kinds of
quasireversible events, suggesting the formation of a phenoxyl radical species in the region of the anodic peaks and a
metal-centered redox (FeIII/FeII) process at low potential. The catalytic activity of this Fe-complex was evaluated in coupling
of 2-aminophenol and benzyl alcohols for the one-pot synthesis of benzoxazoles. The catalyst system showed high
catalytic activity in this transformation and benzoxazoles derivatives were obtained in good to excellent yields. tert-Butyl
hydroperoxide was used as an oxidant and 1.2 mol% of catalyst was needed to accomplish the reaction in water as the
green solvent.
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activation of dioxygen, while in ribonucleotide reductase the
reductive activation of dioxygen is concomitant with one
electron oxidation of a tyrosine residue to form uncoordinated
tyrosyl radical in close proximity to the iron(III) centre.8-11
Introduction
Tyrosyl radicals have been found in important proteins such as
ribonucleotide reductase (Fe),8 galactose oxidase (Cu),9 and in
photosystem II of photosynthesis (Mn)10 to play an important role
in catalysis in a diverse manner due to a redox non-innocent nature.
Phenol-containing ligands, which have the potential ability to form
one electron oxidized phenoxy radical ligands, afford 3d metal
complexes to promote a variety of unprecedented reactions.11-16
While there are many examples of structurally characterized
dinuclear iron complexes,1,17-20 the study on their catalytic
activities is limited. More recently, studies have shown that
well designed binuclear iron complexes provide an original
strategy in the search for new catalysts.21-24
Binuclear oxo-bridged non-haem iron complexes (containing
the Fe-O-Fe unit) have received considerable attention in
recent decade.1-3 They provide spectroscopic and structural
properties of the diferric sites in the proteins involved in the
storage (Haemerythrin)4 or the reductive activation of
dioxygen (methane monooxygenase,5 and ribonucleotide
reductase6,7).
Deoxyhemerythrin reversibly binds dioxygen within an active
site consisting of a (µ-hydroxo)bis(µ-carboxylato)diiron(II) core
and five terminal histidines. Dioxygen binds to the diferrous
center at the remaining vacant coordination site with
concomitant electron and proton transfer to form a (µ-oxo)
diferric center with a coordinated hydroperoxide moiety that is
hydrogen bonded to the oxo bridge.6,7 In methane
monooxygenase, the diiron centre is involved in the reductive
Benzoxazoles are important pharmacophores and there are
many applications of them in medicinal chemistry and modern
drug discovery. A large number of their derivatives were
synthesized.25-29 Benzoxazoles show a wide range of biological
applications
including
anti-fungal,
anti-inflammatory,
anthelminthic, analgesic, anti-bacterial, anti-viral, anti-
trypanosomatic, anticonvulsant, cytotoxic, anti-diabetics and
antiulcer activities.30-33 Also, there are benzoazole-based
compounds
which
show
interesting
photophysical
properties,34,35 and for this reason they are widely used in
organic electronic devices. In spite of the progress in
application of benzoxazoles, most of the reported methods for
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the synthesis of this class of compounds suffer from Synthesis of HLAP
disadvantages such as use of toxic and unrecyclable catalysts
To a solution of 3,5-DTBQ (3,5-di-tert-butylcyclohexa-3,5-
diene-1,2-dione) (0.22 g, 1.0 mmol) in acetonitrile (4.0 mL) was
added 2-aminobenzyl amine (0.122 g, 1.0 mmol), and the
reaction solution was stirred for 30 minutes at room
temperature in the presence of air. Afterward, it afforded a
yellow precipitate which is (HLAP).46
and solvents, prolonged reaction times, harsh reaction
conditions and difficult work-up procedures.36-43 Thus
development of new synthetic methodology and catalyst
systems for efficient synthesis of this class of compounds is
highly desired in organic synthesis.
In this work, we report the in situ synthesis of a binuclear
iron(III) complex (FeLAPIP)2O with an o-aminophenol-pyridine
based ligand. In order to evaluate the catalytic activity of
Synthesis of (FeLAPIP)2O complex
(FeLAPIP)2O in organic transformations, it was checked it in the The obtained HLAP ligand was used to synthesize (FeLAPIP)2O
reaction between benzyl alcohols and 2-aminophenol for one- complex by the following template reaction. First, to a stirred
pot synthesis of benzoxazoles.
suspension of HLAP (0.324 g, 1.0 mmol) in acetonitrile was
added pyridine-2-carboxaldehyde (0.096 mL, 1.0 mmol), then
FeCl2.nH2O (0.253 g, 1.0 mmol) was added to afford o-
aminophenol iron complex. After adding 3.0 equivalents of
NEt3, the color changed from brown to red. The mixture was
stirred for 2h and then the precipitate was filtrated. More
Experimental
General
All chemicals were purchased from Merck and Aldrich
chemical companies and used as received. Known benzoxazole
products were characterized by comparison of their spectral
and physical data with those reported in the literature. 3, 5-
DTBQ (3, 5-di-tert-butylcyclohexa-3, 5-diene-1, 2-dione) was
synthesized according to a modified literature procedure.20
Pure methanol and dichloromethane were used for synthesis
and crystallization. The FT-IR spectra were recorded in the
solid state on a FT-IR Bruker Vector 22 spectrophotometer in
the 400−4000 cm-1 range. Elemental analyses were performed
by the Research Institute of Petroleum Industry (RIPI). UV-Vis
absorbance digitized spectra were collected using a Pharmacia
Biotech spectrophotometer. The electronic spectra of the
mentioned complex were recorded in CH2Cl2. Magnetic
susceptibility was measured from powder samples of solid
material at the temperature range 2-300 K with a SQUID
susceptometer (Quantum Design MPMS-XL-5) in a magnetic
field of 1000 Oe. Cyclic Voltammetry (CV) measurements were
purification
in
methanol/dichloromethane
attained
microcrystal of (FeLAPIP)2O. The FT-IR of this complex has the
t
following key peaks (Figure S1): 2952 (C–H, Bu), 1594 (C=C),
1463 (C=N) and 748 (=C–H bending) cm-1. ESI-MS m/z
(Asymmetric part, [FeLAPIP]+): 499.3 (Figure S2). Elemental
Analysis, calculated: C, 66.28; H, 6.56; N, 8.28. Found: C, 66.88;
H, 6.86; N, 7.88.
General procedure for synthesis of benzoxazoles in the
presence of (FeLAPIP)2O complex as catalyst
To the suspension of benzyl alcohol (1.1 mmol) and 2-
aminophenol (1.0 mmol) in EtOH:H2O (1:1) (5.0 mL), TBHP in
water (0.4 mL) and (FeLAPIP)2O (0.012 g, 1.2 mol %) was added
and the resulting mixture heated to 80 °C for 12h. Then the
reaction was followed by TLC. After completion of the
reaction, the mixture was cooled down to room temperature.
The aqueous phase was extracted with diethyl ether (10 mL, 2
times) and the combined organic phases dried over Na2SO4.
The benzoxazole product was purified by column
chromatography (hexane/ethyl acetate) to obtain the desired
performed with
a
computer controlled Auto Lab
electrochemical system (ECO Chemie, Ultrecht, Netherlands)
equipped with a PGSTA 30 model and driven by GPES (ECO
Chemie). A glassy carbon electrode with a surface area of
0.035 cm2 was used as a working electrode and a platinum
electrode served as the counter electrode. The reference
electrode was an Ag wire. Ferrocene was added as an internal
standard after completion of a set of experiments, and
purity. The resulting products were identified by H NMR, 13C
1
NMR spectroscopy.
Results and discussion
potentials are referenced vs. the ferrocenium/ferrocene Synthesis and Characterization of the iron complex (FeLAPIP)2O
couple (Fc+/Fc). Brown crystals of (FeLAPIP)2Ocomplex suitable
The ligand HLAPIP was synthesized via the sequential
for the X-ray diffraction experiment were obtained from the
condensation of 2-aminobenzylamine with 3,5-DTBQ and then
CH2Cl2/CH3OH solution. The X-ray data were collected with an
pyridine-2-carboxaldehyde. Next, it was reacted immediately
with FeCl2.H2O to afford the Fe complex (FeLAPIP)2O (Scheme 1).
Oxford Sapphire CCD diffractometer using MoKα radiation λ
=0.71073 Å, at 293(2) K, by ω-2θ method. Structure was solved
During the formation of complex, addition of methanol to the
by direct methods and refined with the full-matrix least-
in situ generated imine bond resulted in the connection of
squares method on F2 with the use of SHELX201644 program
methoxy group to the C14 of ligand.
package. The analytical absorption correction was applied
(CrysAlis Pro 171.38.43 program package45 Rigaku OD, 2015)
with maximum and minimum transmission 0.895 and 0.791.
Hydrogen atoms were located from the electron density maps
and their positions were constrained during the refinement.
2 | J. Name., 2012, 00, 1-3
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O
NH2
OH
O
NH2
CH3CN
30 min
N
+
NH2
HLAP
H3CO
O
N
O
N
N
O
Fe
N
O
1)
H
N
Fe
HLAP
N
N
FeCl2.H2O , Et3N
2 h in CH3CN
2)
OCH3
Figure 1. Molecular structure of (FeLAP)2O. H atoms have been omitted
for clarity. Thermal ellipsoids are set at 50 % probability.
3) Crystals grown from
CH OH
/CH2Cl2
(FeLAPIP)2O
3
Scheme 1. The synthetic pathway toward(FeLAPIP)2O complex
The shortest bond to Fe is formed by the oxo O2, with the Fe1-
O2 distance of 1.7769(7) Å. This value is in agreement of the
reported bond length for Fe-O oxo bridge species.49,50 The Fe1-
O2-Fe1 [-x,y,-z+1/2] angle on the oxo bridge is 145.53(13)°. In
the pyramid base, the Fe1-O1 bond of 1.9369(15) Å involving
the phenolate oxygen is significantly shorter than the Fe-N
bonds (Table 2). This value (1.93 Å) is longer than those values
in Iron–phenolate complexes ( 1.86 Å and 1.87 Å).51 Among the
Fe-N bonds, the shortest is Fe1-N2, 2.0105(18) Å due to the
negative charge introduced by amidophenolate (N2, is three
coordinate, sp2 hybridization and not protonated). Significant
deformation of the N3O base of the coordination pyramid is
found with the displacement of the Fe ion towards the oxo
bridge by 0.6498(10) Å.
The electronic absorption spectra of the iron complex have
been measured in dichloromethane in the range of 200–800
nm (Figure S3). The UV-visible spectrum shows that there are
two important absorption bands for this complex. The
absorption band in the higher energy region (250-350 nm)
resulted from π to π* of ligands. Another electronic absorption
band at 400 nm is assigned to ligand to-metal charge-transfer
of phenolate-to-iron (LMCT).
X-ray analysis
Red-brown crystals of the iron complex for X-ray analysis were
obtained from a methanol/dichloromethane (1:1) mixture.
Crystal data for (FeLAPIP)2O are shown in Table 1 and the
selected bond distances are listed in Table 2.
Table 1. Crystal data and structure refinement for (FeLAP)2O
The reported complex crystallizes in the monoclinic space
group C2/c. The asymmetric part of the structure consists of a
half of the (FeLAPIP)2Ocomplex, with the oxo bridge O2
positioned on the two-fold axis (Figure 1). As a consequence,
the other half of the dimmer is related by the [-x, y, -z+1/2]
two-fold symmetry. A notable modification in the ligand
framework of the single crystals obtained from
methanol/dichloromethane mixture (1:1) is nucleophilic
methoxylation of the imine C14 atom located between the
central phenyl moiety and the pyridine ring (Figure 1). The
addition of water or alcohol to Schiff base coordinated to
copper complexes has been studied previously.47,48
Identification code
Empirical formula
Formula weight
Temperature; K
Wavelength; Å
(FeLAP)2O
C28 H33 Fe N3 O2.50
507.42
293(2)
0.71073
Crystal system, space group
Unit cell dimensions; Å, °
Monoclinic, C2/c
a = 25.1440(14)
b = 13.5600(6)
c = 17.3063(9)
β = 112.785(7)
5440.1(5)
8, 1.239
0.584
2144
0.53 x 0.49 x 0.26
2.35 to 28.11
-32<=h<=31,
19<=l<=22
17578 / 5961 [R(int) = 0.0325]
25.00 99.9 %
Analytical
Volume; Å3
Z, Calculated density; Mg/m3
Absorption coefficient; mm-1
F(000)
Crystal size; mm
Theta range for data collection; º
Limiting indices
Electron density maps reveal no density peak that might
correspond to an H atom attached to N2. Therefore the central
atom seems to be Fe(III) and its charge is compensated by the
single negative charges supplied by phenolate O1, the oxo O22-
bridge shared by two Fe centres, and the negative charge of
amidate nitrogen moiety, N2 attached to C14. The
coordination sphere FeN3O2 has geometry of a distorted
square pyramid. It consists of the four-dentate LAPIP ligand
coordinating in a square-planar manner and axial oxo bridge.
-17<=k<=17,
-
Reflections collected / unique
Completeness to theta
Absorption correction
Max. and min. transmission
Refinement method
Data / restraints / parameters
Goodness-of-fit on F2
0.895 and 0.791
Full-matrix least-squares on F2
5962 / 6 / 312
0.998
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0416, wR2 = 0.1114
R1 = 0.0653, wR2 = 0.1234
0.373 and -0.223
Largest diff. peak and hole; e.Å-3
Table 2. Selected bond lengths [Å] and angles [°] for (FeLAP)2O
Bonds Angles
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processes in the solvent window (Figure 3, Table 3). (FeLAPIP)2O
displays two quasi-reversible anodic redox processes at 0.19 V
and 0.41 V, exhibiting the ligand-based oxidation to form two
phenoxyl radical species (see below). These values of oxidation
potential for (FeLAPIP)2O are less positive when compared with
those reported for iron-phenolate (0.63 V vs. Fc+/Fc) and Iron
salen complexes (0.85 V vs. Fc+/Fc ),57,58 indicating facile ligand
radical formation. Considering symmetric complex, the
difference in potential between sequential oxidations is
correlated to the extent of radical delocalization in the one-
electron oxidized complexes.59 The redox process at negative
potentials (–1.41 V) could be assigned to a FeIII/FeII reduction
process, based on previously reported data.51,58
Fe1-O2
Fe1-O1
Fe1-N2
Fe1-N1
Fe1-N3
O1-C1
1.7769(7)
1.9369(15)
2.0105(18)
2.0551(17)
2.1115(19)
1.323(3)
O2-Fe1-O1
O2-Fe1-N2
O1-Fe1-N2
O2-Fe1-N1
O1-Fe1-N1
O2-Fe1-N3
C3-C2-C1
106.90(5)
111.52(6)
141.51(7)
109.52(8)
80.51(7)
106.85(9)
116.8(2)
C6-N1
1.424(2)
N1-C7
1.284(3)
C3-C2-C20
C5-C6-N1
C1-C6-N1
C7-N1-C6
123.4(2)
C13-N2
N2-C14
C14-O3
1.374(3)
125.8(2)
1.434(3)
112.36(18)
124.11(18)
1.450(3)
The angles between the axial Fe1-O2 bond and those in the
pyramid base range from O2-Fe1-N3 106.85(9) to O2-Fe1-N2
111.52(6)°. Within the pyramid base, the cis angles range from
N2-Fe1-N3 79.40(8) to N2-Fe1-N1 88.90(7)° and trans angles
are O1-Fe1-N2 141.51(7) and N1-Fe1-N3 143.60(7)°. Geometry
of the LAPIP ligand is similar to that reported for a complex
copper containing similar ligand.52
Magnetic Susceptibility Measurements
Magnetic susceptibility (χ) was measured for polycrystalline
sample of (FeLAP)2Ocomplex in magnetic field of 1000 Oe as a
function of temperature in the range of 2-300 K. The measured
data were corrected for the temperature independent Larmor
diamagnetic susceptibility obtained from Pascal's tables53,54
and for sample holder contribution. A plot of the temperature
dependence of χT is shown in Figure 2. The decrease in χTwith
decreasing temperature is characteristic of anti ferromagnetic
exchange coupling. Such behaviour is expected for a FeIII-O-FeIII
compound, in which the coupling between two d5 (S = 5/2)
iron atoms occurs.55 As the temperature is lowered,
χTdecreases until 50 K, at which temperature a plateau is
observed. This plateau corresponds to the residual para-
magnetism. The downturn in χT at very low temperatures may
be due to a small amount of radical-radical anti ferromagnetic
exchange, between neighbouring molecules.56
Figure 3. Cyclic voltammogram of (FeLAPIP)2O. Conditions: 2.5 mM
complex, 0.1 M NBu4ClO4, scan rate 200 mV / s, CH2Cl2, 298 K.
Table 3. Redox potentials of (FeLAPIP)2O versus Fc+/Fc. Peak to peak
separation in brackets
Compound
(FeLAPIP)2O
E1/21/ V
E1/22/ V
E1/22/ V
-1.41 (0.27)
0.19 (0.06)
0.41 (0.11)
1.2
Catalytic application of (FeLAPIP)2O catalyst in synthesis of
benzoxazoles
1
0.8
0.6
0.4
0.2
0
To investigate the catalytic performance of (FeLAPIP)2O complex
as catalyst in synthesis of benzoxazoles, a reaction model
containing benzyl alcohol and 2-aminophenol was selected. To
obtain maximum yields of desired product, the effect of
different reaction parameters such as temperature, solvent,
catalyst loading, and reaction time were systematically
investigated. Results of optimization study are depicted in
Table 4.
As shown in Table 4, in the absence of both (FeLAPIP)2O catalyst
and oxidant in water as solvent, no product was observed after
24h (Table 4, entry 1). Using this condition and in the presence
of oxidant only, a trace amount of benzoxazole was observed
(Table 4, entry 2). Also, in the presence of (FeLAPIP)2O and
without use of oxidant only 9% of product was isolated (Table
4, entry 3). Interestingly, by use of (FeLAPIP)2O as catalyst and
TBHP (tert-Butyl hydroperoxide) in water as an oxidant, about
0
50
100
150
200
250
300
Temperature (K)
Figure 2. Temperature dependence of magnetic susceptibility XT of
(FeLAPIP)2Ocomplex.
Electrochemistry
The cyclic voltammetry experiment was conducted to study
the electrochemical behavior of this iron(III) complex. The iron
complex exhibits three quasi-reversible one-electron redox
4 | J. Name., 2012, 00, 1-3
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OH
1.2 mol% (FeLAPIP)2O
R
82% of product was observed after 8h in water (Table 4, entry
4). Some other solvents were also tested and no improvement
in reaction conditions and yield was observed (Table 4, entries
5-7). The catalyst loading was also investigated and 1.2 mol%
of catalyst was found to be efficient for promotion of reaction
(Table 4, entries 8-10). Other oxidants were tested and also
different amounts of TBHP were used but no superiority was
observed (Table 4, entries 11-13).
N
OH
+
R
TBHP in water (0.4 mL)
NH2
O
80 oC,
H O
2
(3a-l)
N
O
N
N
O
NO2
CN
O
3c: 5h, 91%
TOF: 15.2 h-1
3b:
10h, 90%
TOF: 6.8 h-1
3a: 8h, 85%
TOF: 8.9 h-1
Cl
OH
Thus optimal conditions for conversion of benzyl alcohols to
benzoxazoles using (FeLAPIP)2O correspond to entry 10 of Table
4.
N
O
N
O
N
O
Br
3d: 8h, 85%
3e: 8h, 83%
3f: 12h, 80%
TOF: 8.8 h-1
TOF: 8.6 h-1
TOF: 5.5 h-1
Table 4.
Synthesis of benzoxazoles using benzyl alcohol and 2-
H3C
N
Cl
N
aminophenolcatalyzed by (FeLAPIP)2Ocomplexunder various conditionsa
N
O
CH3
O
O
3g: 10h, 82%
TOF: 6.8 h-1
3i:
10h, 86%
3h: 12h, 80%
TOF: 5.5 h-1
TOF: 7.2 h-1
NH2
N
O
N
O
N
O
Entry
Catalyst (mol%)
Solvent
Time
(h)
24
24
24
8
Yield
(%)b
0c
Traced
9c
OSO2Me
OH
3k: 10h, 80%
TOF: 6.7 h-1
3l: 15h, 74%
TOF: 4.1 h-1
3j:
12h, 78%
TOF: 5.4 h-1
1
None
H2O
2
None
H2O
3
(FeLAP)2O (1)
(FeLAP)2O (1)
(FeLAP)2O (1)
(FeLAP)2O (1)
(FeLAP)2O (1)
(FeLAP)2O (1.5)
(FeLAP)2O (2)
(FeLAP)2O (1.2)
(FeLAP)2O (1.2)
(FeLAP)2O (1.2)
(FeLAP)2O (1.2)
H2O
Scheme 2. The synthesis of benzoxazoles in the presence of (FeLAPIP)2O
catalyst in water. Reaction conditions: benzyl alcohol (1.1 mmol), 2-
aminophenole (1.0 mmol), TBHP in water (0.4 mL), H2O (5 mL). All
yields are GC yield. TOF = (mol product/mol catalyst)h-1.
4
H2O
82
5
EtOH
EtOH: H2O(1:1)
CH3CN
H2O
8
72
6
8
75
7
8
45
8
8
85
Results show that efficiency of the new synthetic methodology
is general for the synthesis of diverse benzoxazoles using
benzyl alcohols and 2-aminophenoles. The reaction yield for
benzyl alcohols with electron donating group (methoxy) is less
than those for electron withdrawing groups (4-NO2 and 4-CN).
Also the reaction yields for benzyl alcohols functionalized in
ortho position is relatively lower; possibly due to the steric
hindrance. However, the steric effects of the substituents
seem to be of secondary importance, since results for meta-
substituted substrates are comparable. Moreover, 2-
aminopyridin-3-ol is less active than 2-aminophenol. Overall,
this catalyst system under optimized conditions is efficient for
synthesis of diverse benzoxazole derivative starting from
benzyl alcohols.
9
H2O
8
86
10
11
12
13
H2O
8
85
H2O
12
12
8
75e
72f
85g
H2O
H2O
aReaction conditions: 1a (1.1 mmol), 2a (1.0 mmol), TBHP in water (0.4
b
c
d
mL), solvent (5.0 mL). Isolated yield. no TBHP was used. Only TBPH
was used. eTBHP in dodecane was used as oxidant. fK2S2O8 was used as
oxidant. g0.5 mL of TBHP in water was used.
In order to show the generality and scope of this synthetic
methodology in synthesis of diverse benzoxazoles, different
benzyl alcohols were tested under optimized reaction
conditions and results are summarized in Scheme 2.
In this catalytic system it was considered that the in situ
generated aldehyde from benzyl alcohols do subsequent
condensation with 2-aminophenol to produced benzoxazole
(Scheme 3). In this catalytic system, the role of TBHP is the
production of radical species60 and the role of (FeLAPIP)2O is the
oxidation of radicals and reduction of radical-cations.61
According to the proposed reaction mechanism (Scheme 3),
first benzyl alcohol is converted to benzyl radical (I) which
could oxidized by Fe(III) to produce cation species which is an
activated intermediate (II
&
III) to be attacked by 2-
reaction between 2-
aminophenol.62 Condensation
aminophenol and in situ generated aldehyde resulted in the
production of IV and next V intermediates.63 The generated t-
BuO radical can convert intermediate V to radical-cation VI.
The produced Fe(II) in previous step can convert VI to VII
intermediate. Afterward, intermediate VII is converted to
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6 (66)a
γ-MnO2 (2 mg)
Toluene, 140 °C, 12h
97
radical VIII. In next step VIII lose an electron in the presence of
Fe(III) catalyst to produce cation intermediate IX. The t-BuO
radical can convert cation IX to radical-cation X which can
produce final imidazole product by capture of electron from
Fe(II) and produce Fe(III) to start next cycle.
a 2-nitrophenol was used instead of 2-aminophenol.
Conclusions
Inspired by proteins containing oxo-bridged dinuclear iron centres,
a new iron (III) complex of o-aminophenol-iminopyridine based
ligand, HLAPIP, was successfully synthesized and characterized. X-ray
crystal structure of (FeLAPIP)2O reveals that this complex has
symmetrical dimeric structure and the coordination sphere FeN3O2
has geometry of a distorted square pyramid. Variable temperature
magnetic susceptibility measurements of this complex displayed an
anti-ferromagnetic coupling between Fe(III) ions through oxo
bridges. It has been shown that electrochemical oxidations are
ligand-centered, i.e. formation of the phenoxyl radicals from the
coordinated phenolates, while reductions are metal-centered
related to FeIII/FeII couple. The catalytic activity of (FeLAPIP)2O
catalyst was evaluated in one-pot synthesis of benzoxazoles from
benzyl alcohols in water as green solvent. This catalyst system is
very efficient even at low loading of Fe catalyst (1.2 mol%) and in an
aqueous solution, providing the environment friendly method for
synthesis of benzoxazoles under mild conditions.
Appendix A. The structural data for (FeLAPIP)2O have been deposited
with the Cambridge Crystallographic Data Centre, the deposition
number being CCDC 1562642. These data can be obtained free of
charge viahttp://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
Scheme 3. Proposed catalytic cycle of the benzoxazole synthesis by use of
TBHP as oxidant in the presence of (FeLAPIP)2O catalyst.
To demonstrate the practicality and the value of the
introduced catalyst system in comparison with other reported
catalysts, we tabulated some of the results for synthesis of
benzoxazoles using benzyl alcohols in Table 5. As clearly
exposed in Table 5, (FeLAP)2O is better in comparison with
some other catalyst systems and reaction conditions, time and
yield were improved.
Acknowledgements
Authors are grateful to Shiraz University, Amirkabir University of
Technology, Institute for Advanced Studies in Basic Sciences
(IASBS), Nicolaus Copernicus and Ljubljana Universities for their
supports.
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Synthesis and characterization of a novel oxo-bridged binuclear iron(III)
complex: Its catalytic application in synthesis of benzoxazoles using benzyl
alcohol in water
Elham Safaei,*,a Zahra Alaji,b Farhad Panahi,*,a Andrzej Wojtczakc and Janez Zvonko Jagličićd
A novel oxo-bridged binuclear iron(III) complex was found to be an efficient catalyst in synthesis of
benzoxazoles from benzyl alcohols.