Highly efficient aerobic epoxidation of cyclic olefins in mild conditions by a novel binuclear manganese(II) complex containing N-(4-nitrophenyl)picolinamide ligand

Article abstract of DOI:10.1016/j.catcom.2015.02.023

A novel binuclear Mn(II) complex based on a para-nitro substituted amidic ligand (N-(4-nitrophenyl)picolinamide) has been synthesized and characterized by X-ray crystallography. This complex shows a high degree of conversion and epoxide selectivity in Mukaiyama aerobic epoxidation reactions of cyclic olefins in mild conditions.

Full text of DOI:10.1016/j.catcom.2015.02.023

Highly efficient aerobic epoxidation of cyclic olefins in mild condi-
tions by a novel binuclear manganese(II) complex containing N-(4-
nitrophenyl)picolinamide ligand
Mojtaba Bagherzadeh, Alireza Ghanbarpour, Hamid Reza Khavasi
PII:
DOI:
Reference:
S1566-7367(15)00082-5
doi: 10.1016/j.catcom.2015.02.023
CATCOM 4240
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
17 December 2014
17 February 2015
18 February 2015
Please cite this article as: Mojtaba Bagherzadeh, Alireza Ghanbarpour, Hamid Reza
Khavasi, Highly efficient aerobic epoxidation of cyclic olefins in mild conditions by a
novel binuclear manganese(II) complex containing N-(4-nitrophenyl)picolinamide ligand,
Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.02.023
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ACCEPTED MANUSCRIPT
Highly efficient aerobic epoxidation of cyclic olefins in mild conditions by
a novel binuclear manganese(II) complex containing N-(4-
nitrophenyl)picolinamide ligand
a
b
Mojtaba Bagherzadeh *, Alireza Ghanbarpour, and Hamid Reza Khavasi *
Abstract
A novel binuclear Mn(II) complex based on a para-nitro substituted amidic ligand (
N
-(4-nitrophenyl)picolinamide) has
been synthesized and characterized by x-ray crystallography. This complex shows a high degree of conversion and
epoxide selectivity in Mukaiyama aerobic epoxidation reactions of cyclic olefins in mild conditions.
1
-Introduction
Catalytic epoxidation reactions based on transition metal
complexes have played an important role in organic synthesis
and chemical industries since these reactions may provide
epoxides as intermediates to generate useful chemicals [1,2].
Among catalytic epoxidation reactions, those which employ
dioxygen as an oxidant have attracted great attention due to
such advantages as having green nature and being
inexpensive [3a]. However, using dioxygen, as an oxidant,
usually has some disadvantages such as the low selectivity of
products, the slow rate of reactions and production of by-
products owing to overreacting [3]. Hence, to address these
problems some co-reagents are used. Aldehydes are one of
these co-reagent which may be utilized in the reactions
referred to as Mukaiyama-Yamada epoxidation [4,5]. These
reactions proceed through formation of acylperoxy radicals
that preferentially react with double bonds of olefins to elicit
epoxides. The role of metal catalyst, in these reactions, is not
obvious completely; nevertheless, it has been proven that the
metal catalyst can affect the initiation and the rate of
reactions, as well as the selectivity of epoxide against allylic
oxidation products [6,7]. Manganese complexes based on
amidic ligands previously were used as efficient catalysts in
these typical reactions [8-10]. Furthermore, over the past two
decades, there has been such a great interest to study
dimeric manganese(II) complex based on a para-nitro
substituted amidic ligand has been synthesized and
characterized by x-ray crystallography to evaluate its catalytic
activity, in Mukaiyama epoxidation reactions, in comparison
to its peers.
2
-Experimental:
2
.1. Catalyst preparation
As shown in scheme 1, complex (1) can easily be prepared
from the direct reaction of N-(4-nitrophenyl)picolinamide
o
molecule (HL) and MnCl . 4H O in acetonitrile in 60 C in a
2
2
convection tube (detailed procedure provided in supporting
manganese redox enzymes, especially those which involve Scheme 1. Synthetic procedure of the amidic ligand and complex 1.
binuclear active sites [13]. Because of the paramount
importance of manganese(II) complexes based on amidic information file). HL can be synthesized by using the
ligands that can be used as model catalysts [13b] and the synthetic route we reported earlier and as it has been shown in
efficiency of them in Mukaiyama epoxidations, here, a new
Scheme 1 [14].
1

ACCEPTED MANUSCRIPT
2
.2. Catalyst characterization
The suitable single crystal of complex 1 was also prepared
through using the convection tube method (see supporting
information file). The x-ray diffraction measurement was
made on a STOE IPDS-II diffractometer with graphite
monochromated Mo-Kα radiation. Suitable single crystal was
mounted on a glass fiber and used for data collection. Cell
constants and an orientation matrix for data collection were
obtained by least-squares refinement of diffraction data. The
structural data and refinement for this compound is provided Figure 2. Representation of a zigzag-like strand which is made by
π
ring#1…π
ring#2
and other weak interactions. The Nitro substituents
in Table S1.
hydrogen bonds with phenyl ring of adjacent molecule to connect
each strand to another one. The manganese (II) has been shown by
purple, in Ball and Stick form. Acetonitrile solvent molecules have
been illustrated by pink colour, in Ball and Stick representation.
3
- Results and discussion
3
.1. Crystal packing analysis
X-ray crystallographic data show that this compound
crystalized in a triclinic system and the space group of Pī,
Table S1. The crystal structure shows two manganese(II)
centres with the distance of 3.642 Å which are connected to
each other via two chloride bridged ions. Each Manganese
centre has distorted octahedral geometry containing two
oxygen atoms from two molecules of water, two bridged
chloride ions, one chloride ion, one nitrogen and one oxygen
atom from N-(4-nitrophenyl)picolinamide ligand, Figure 1.
The bond distances and the angles of the coordination region
has been provided in Table S2.The asymmetric unit of this
Figure 3. Trifurcated hydrogen bonds with three donors and one
compound involves one manganese atom, two chloride ions, a acceptor. The manganese (II) has been shown by purple, in ball and
water molecule, a N-(4-nitrophenyl)picolinamide molecule, stick form.
all of which are located in the coordination region of
manganese, and an acetonitrile molecule which exists in its
crystal packing (Z'=0.5), Figure 1.
Moreover, in the crystal packing, along b-axis, interactions
such as πring#1…πring#2 (with the centroid to centroid distance
of 3.870), in cooperation with other weak interactions,
generates a zigzag-like strand, each of which connects to its
neighbour through bifurcated hydrogen bonds provided by the
nitro substituent and a hydrogen atom of
the phenyl ring of adjacent molecules (C2-H2…O2, C2-
H2…O3, sym. code: -1+x, y, 1+z, Table S3). In addition, the
water molecule coordinated to the metal centre can form
hydrogen bond with acetonitrile molecule and traps it in the
crystal packing of complex 1 (O4-H4B…N4, sym. code: 2-x,
2
-y, -z, Table S3), Figure 2. There are also trifurcated
hydrogen bonds (C12-H12….Cl1, N2-H2A…Cl1, C4-
H4…Cl1, sym. code: 1-x, 3-y, 2-z, Table S3) giving a 2D-
sheet in the represented direction, Figure 3.
3
.2. Evaluation of catalyst
Based on our previous work in Mukaiyama epoxidation
reactions [8] and noticing other reported results [9-12], this
binuclear complex was designed and examined in aerobic
epoxidation reactions with isobutyraldehyde as a co-reductant
and different substrates. As illustrated in Table 1, complex 1
shows a significant conversion to product with the highest
selectivity towards epoxide products. There is a general
consensus that cyclohexene is the best substrate to evaluate
the selectivity for epoxide versus allylic products since this
Figure 1. The ORTEP diagram of compound 1. Ellipsoids at 50% substrate is very sensitive to the mechanism proceeding
probability level. Hydrogen atoms have been omitted for clarity.
Symmetry code; i: 1+x,-1+y,z
towards an allylic pathway, which can affect the ratio of
epoxide to allylic products. Hence, this substrate was used for
optimizing and studying the different conditions of catalytic
reaction, Table 1. All of the reactions were monitored by gas
chromatograph (Agilent Technologies 6890N Instrument,
equipped with a capillary column 19019 J-413 HP-5, 5%
phenyl methyl siloxane, capillary 60.0 m ×250µm ×1.00 µm,
and a flame ionization detector).
2

ACCEPTED MANUSCRIPT
Our results show that when reactions are catalysed by metal Table 1. Using 1 as a catalyst in the aerobic epoxidation of
a
cyclohexene .
catalyst, the highest conversion and highest selectivity are
achieved, Table 1 (entry1-4). In comparison to other similar
amidic Mn(II) complexes, catalyst 1 is superior, according to
its higher conversion and selectivity at room temperature. For
example, in our previous work [8], when a binuclear Mn(II)
complex based on N-((pyridin-2-yl)methyl)pyrazine-2-
carboxamide ligand as a catalyst, isobutyraldehyde as a co-
reagent, and DMF as a solvent, by the same stoichiometric
amounts as this paper and in its optimum conditions , after 1
entr
y
t(h)/
T( C)
Catalyst
(mmol)
Aldehyde
(mmol)
Conversion
(%)
Epoxide
selectivity (%)
TOF
(min )
o
-1
-
3
3
3
1
2
3
4
5
6
7
8
1/25 1.76×10-
2/25 1.76×10-
3/25 1.76×10
1/25 3.52×10
1/0 1.76×10
1.76
1.76
1.76
1.76
1.76
1.76
0.88
1.76
-
80%
96%
>99%
95%
69%
85%
63%
55%
10%
40%
>99%
>99%
>99%
>99%
>85%
>99%
95%
75%
10%
70%
6.7
4.0
2.8
3.9
5.7
7.1
5.2
-
-3
-3
-3
1/35 1.76×10-
o
3
hour, the conversion of 24% was obtained in 60 C, in the
1/25 1.76×10
aerobic epoxidation of cyclohexene. Furthermore, in another
similar work performed by Qi et al [10], when a mononuclear
complex of Mn(II) based on N-(4-methyl phenyl)
picolinamide ligand as a catalyst, isobutyraldehyde as a co-
reagent, and 1,2-dichlroethane as a solvent were utilized,
6/25
6/25 1.76×10-
/25 6.40×10
-
-
3
3
9
10
0.1
0.2
6
1.76
(
MnCl2)
-
3
1
1
1
2
6/25 6.40×10
free
1.76
1.76
58%
1%
77%
0.8
0
(
5
9.6% of conversion and 92.2% of epoxide selectivity in
ligand)
room temperature were obtained in optimum conditions and
the reaction completed after 13 hours. However, complex 1 is
able to catalyse the aerobic epoxidation of cyclohexene, with
isobutyraldehyde as a co-reagent, and 1,2-dichlroethane as a
solvent, with conversion percentage of 80% in room
b
-3
6/25 1.76×10
10%
-3
a: Substrates (0.88 mmol), the catalyst (1.76×10
mmol),
isobutyraldehyde (1.76 mmol), and pure O (1 atm) in 2 mL of 1,2-
dichlroethane as a solvent. Gas chromatography determination based
2
temperature, and epoxide selectivity of >99%. Furthermore, on chlorobenzene as an internal standard. The main byproduct is 2-
-
3
the reaction approximately completes over 3 hours. Although cyclohexen-1-one (from GC analysis). b: using 6.40×10 mmol of
2
,6 -di-tert-butyl-4-methylphenol as a radical trapping compound.
the mechanism of metal complexes in Mukaiyama
epoxidation reactions have not been understood thoroughly,
by comparing the catalytic activity of catalyst 1 with other
similar Mn(II) complexes based on amidic ligands, it is found
that the efficiency of these reactions can vary with the
electronic effect of ligands and coordination region of metal
catalysts.
o
In addition, decreasing the temperature from room to 0 C
results in decreasing conversion (69%) and increasing
temperature to 35 C leads to increasing conversion (85%).
o
Therefore, it can be concluded that this catalytic system is
sensitive towards reaction temperature and these results are in
agreement with free radical chain mechanism [15b]. For
further examining this catalytic system, the aerobic
epoxidation four cyclic substrates have been studied, Table2.
The results show a high amount of conversion to epoxide
products; however, the more conversion percentage has been
obtained for cyclohexene and its derivative (1-methyl-
cyclohexne), which seems that catalyst 1 is more efficient for
cyclohexenes in comparison to other cyclic olefins.
3
.3. Optimization results of Catalytic system
To gain insight into the effect of other factors on the catalytic
system, the effects of isobutyraldehyde concentration, catalyst
amount, temperature effects, and radical scavenger were
investigated, Table 1.
By doubling the amount of catalyst, the conversion increases
from 80% to 95%, over 1 hour, entries1,4. Moreover, when
the reaction is carried out without adding any catalyst, the
conversion and selectivity decreases significantly, entry 8.
4
-Conclusion
Hence, it shows that the existence of metal catalyst may In conclusion, a dimeric manganese(II) complex based
enhance conversion and epoxide selectivity. These results are on N-(4-nitrophenyl) picolinamide ligand has been
consistent with the results of Wentzel and Mizuno et al about prepared and characterized by x-ray crystallography. The
the role of metal catalysts in Mukaiyama epoxidation significant catalytic activities of this complex were
reactions [6]. Furthermore, this type of reaction, as reported obtained in aerobic Mukaiyama epoxidation of cyclic
earlier [6b,15], is sensitive to the amount of isobutyraldehyde. olefins under mild conditions. To the best of our
For instance, when isobutyraldehyde’s amount is reduced to
half of its initial concentration, the conversion reduces from
knowledge, this amidic Mn(II) complex has a superior
selectivity and conversion to epoxide products in
comparison to other similar Mn(II) complexes based on
amide ligands and especially can be a suitable candidate
for aerobic epoxidation of cyclohexene in room
temperature. These results may highlight the role of
metal catalyst to elicit higher conversion in Mukaiyama
epoxidation reactions.
8
0% to 63% and if the reaction is performed without adding
isobotyraldehyde, the conversion reduces significantly to 10%
after hours, entry 9. Furthermore, removing
6
isobutyraldehyde leads to decreased selectivity for epoxide
products. Moreover, using radical scavenger supports a fact
that the reaction proceeds through radical mechanism since
adding 2,6-di-tert-butyl-4-methylphenol, as
scavenger, the conversion decreases greatly, entry 12 .
a
radical
b
3

ACCEPTED MANUSCRIPT
Table 2. Conversion percentages of some cyclic olefins in
Mukaiyama epoxidation reaction after 3 hours.
Weiner, R. G. Finke, J. Mol. Catal. A: Chem. 1996, 114, 15.
C) B. B Wentzel, P.A. Gosling, M. C. Fieters, R. J. M Nolte,
(
Substrate
Conversion(%)
Epoxide
selectivity(%)
J. Chem. Soc., Dalton Trans, 1998, 13, 2241.
7] W. Nam, H. J. Kim, S. H. Kim, R. Y. N. Ho, and J. S.
Valentine, Inorg. Chem., 1996, 35, 1045.
8] H. R. Khavasi, K. Sasan, M. Pirouzmand, and S. N.
Ebrahimi, Inorg. Chem. 2009, 48, 5593.
9] C-M. Che and W-K. Cheng, J. Chem. Soc., Chem. Commun.,
[
[
[
[
[
[
[
Cyclopentene
-methyl-cyclohexene
Cycloheptene
90%
98%
84%
80%
>99%
>99%
>99%
>99%
1
Cyclooctene
1
986,1443.
10] J.-Y. Qi, Y.M Li, Z. Y Zhou, C. M. Che, C. H. Yeung, A. S.
The condition of reaction is: Substrate (0.88 mmol), catalyst
C. Chan, Adv. Synth. Catal. 2005, 347, 45.
-
3
(
(
1.76×10 mmol), isobutyraldehyde (1.76 mmol), and pure O2
1 atm) in 2 mL of 1,2-dichlroethane as a solvent, in room
11] Z. Li, C. Liu, R. Tang, and G. Liu, RSC. Adv., 2013,
745.
12] J. Y. Qi, L. Q. Qiu, K. H. Lam, C. W. Yip, Z. Y. Zhou, A. S.
C. Chan, Chem. Commun 2003, 1058.
13] (a) A. Willing, H. Follmann, G. Auling, Eur. J. Biochem.,
988, 178, 603. (b) K. Wieghardt, Angew. Chem. Int. Ed.
3,
9
temperature. Gas chromatography determination based on
chlorobenzene as an internal standard. The main byproduct is 2-
cyclohexen-1-one (from GC analysis)
,
1
Notes and references
Department of Chemistry, Sharif University of Technology,
Tehran 11155-3615, Iran, e-mail: bagherzadeh@sharif.edu
Engl., 1989, 28, 1153.
a
[
14] H. R. Khavasi, A. Ghanbarpour, A. A. Tehrani,
CrystEngComm. 2014, 16, 749.
b
[15] (a) T.V. Filippova, E.A. Blyumberg, Russ. Chem. Rev
982, 51, 582. (b) C. L. Hill, Nature, 1999, 401, 436.
,
Faculty of Chemistry, Shahid Beheshti University, G. C.,
1
Evin, Tehran 1983963113, Iran, e-mail: h-khavasi@sbu.ac.ir
Electronic Supplementary Information (ESI) available:
Experimental details for synthesized compounds (CCDC
No. 1008240), full crystallographic data, selected bond
distances and bond angles and hydrogen bonding
parameters.
References
[
1] (a) L. Haughton and J. M. J Williams, J. Chem. Soc., Perkin
Trans. 1, 1999, 2645. b) K. A. Jorgenson, Chem. Rev. 2004,
1
04, 1047.
[
2] (a) M. Amini, M. M. Haghdoost, M Bagherzadeh, Coord.
Chem. Rev., 2014, 268, 83. (b) M. Bagherzadeh, M. M.
Haghdoost, A. Ghanbarpour, M. Amini, H. R. Khavasi, E.
Payab, A. Ellern, L. K. Woo, Inorg. Chim. Acta, 2014, 411
1. (c) M. Amini, M. M. Haghdoost, M Bagherzadeh, Coord.
Chem. Rev., 2013, 257, 1039. (d) M. Bagherzadeh, L.
Tahsini, R. Latifi, Catal. Commun, 2008, , 1600.
3] (a) D. H. R. Barton, A. E Martell, D. T. Sawyer, The
Activation of Dioxygen and Homogeneous Catalytic
,
6
9
[
[
Oxidation; Plenum: New York, 1993. (b) J. Piera, J-E.
Bäckvall, Angew. Chem. Int. Ed., 2008, 47, 3506. (c) T.
Punniyamurthy, S. Vellusamy, and J. Iqbal, Chem. Rev.,
2
005, 105, 2329.
4] (a) T. Yamada and T. Mukaiyama, Chem. Lett., 1989, 18
19. (b) T. Yamada, K. Imagawa, and T. Mukaiyama, Chem.
Lett. 1992, 21, 2109.
,
5
[
[
5] K. R. Rodgers, I. M. Arafa, and H. M. Goff, J. Chem. Soc.,
Chem. Commun., 1990, 19, 1323.
6] (a) B. B. Wentzel, P. L. Alsters, M. C. Feiters, and R. J. M.
Nolte, J. Org. Chem., 2004, 69, 3453. (b) N. Mizuno, H.
4

ACCEPTED MANUSCRIPT
Highlights
•
•
•
Structural study of a novel binuclear Mn(II) complex
Evaluation of catalyst in Mukaiyama Epoxidation reaction
Eliciting the higher efficiency in aerobic epoxidation of cyclic olefins
5