20
M. Moshref Javadi et al. / Polyhedron 72 (2014) 19–26
Scheme 1. Oxidation of propylene using alkyl hydroperoxides as oxidants, catalyzed by a homogeneous Mo(VI).
O
O
O
N
O
O
O
Mo
O
Mo
O
O
O
O
N
+ Oxidation products
or
O
R
O
or
S
O
S
O
O
TBHP, 1,2-DCE
S
+
R
R'
R
R'
R'
Scheme 2. Oxidation of alkenes and sulfides with TBHP catalyzed by [Mo2(O)4BOX(acac)2].
In this article, a novel binuclear molybdenum oxazoline com-
pound was synthesized and used as a homogeneous and efficient
catalyst for the epoxidation of olefins and oxidation of sulfides
with tert-BuOOH (Scheme 2). The effect of different solvents,
oxidants, and temperatures on the activity and selectivity of the
catalyst was also studied.
NC
HO
CN
OH
HO
OH
SSA
N
N
+
.
Reflux or
)
)
)
)
O
O
NH2
Mo(O)2(acac)2
Ethanol
2. Experimental
Reflux
All materials and chemicals were of commercial reagent grade
and prepared from Merck, Aldrich or Fluka chemical companies.
Alkenes were obtained from Merck or Fluka and were passed
through a column containing active alumina and/or silica to
remove peroxide impurities. 1H NMR spectra were recorded on a
Bruker Avance 400 MHz spectrometer using DMSO-d6 as solvent.
Elemental analysis was performed on a LECO, CHNS-932 analyzer.
Thermogravimetric analysis (TGA) were carried out on a Mettler
TG50 instrument under air flow at a uniform heating rate of
5 °Cminꢀ1 in the range 25–800 °C. FT-IR spectra were obtained as
potassium bromide pellets in the range of 400–4000 cmꢀ1 by a
JASCO 6300 spectrophotometer. Gas chromatography experiments
(GC) were performed on a Shimadzu GC-16A instrument using a
2 m column packed with silicon DC-200 or Carbowax 20 m.
UV–Vis spectra were obtained on a Shimadzu UV 265 spectrometer.
The Mo content of the catalyst was determined by a Jarrell-Ash
1100 ICP analysis. Mass spectra were recorded by a Platform II
spectrometer from Micromass, EI mode at 70 eV.
O
O
O
O
O
O
O-
Mo
O
Mo
O
=
O
O
O
O
O
O
N
N
O
Scheme 3. The synthesis route for BOX ligand and its Mo complex.
bis-oxazoline crystals were obtained as described above. Yield
91%; Mp 198 °C; Elemental Anal. Calc. for C14H16N2O4
(MW = 276.29) C, 60.86; H, 5.84; N, 10.14. Found: C, 58.69; H,
6.23; N, 10.16%. Exact Mass: 276.11; m/z: 276.11 (100.0%),
277.11 (15.9%), 278.12 (1.9%).
2.2. General procedure for synthesis of Mo complex
The bis-oxazoline was metallated as follows: a degassed ethanol
solution (40 ml) of the corresponding bis-oxazoline (0.276 g,
1 mmol) was added to a solution of Mo(O)2(acac)2 (1 mmol,
0.326 g) in absolute ethanol (20 ml) under argon atmosphere.
The reaction was stirred under reflux conditions for 24 h. The solu-
tion was then cooled, filtered and evaporated under reduced pres-
sure. Dissolving this crude product in methanol and evaporation
under reduced pressure for a second time, gave the pure product.
Yield: (0.257 g, 35%, light green powder). Elemental analysis: Anal.
Calc. for C24H28Mo2N2O12 (MW = 728.41): C, 39.57; H, 3.87; N, 3.85.
Found: C, 39.45; H, 3.76; N, 4.15%.
2.1. General procedure for ligand (BOX) synthesis
The novel bis-oxazoline ligand, BOX, was synthesized using our
previously reported methods with slight modifications [28,29]. A
mixture of 1,3-dicyanobenzene (4 mmol), 2-amino-1,3-propane-
diol (16 mmol) and silica sulphoric acid, SSA, (200 mg) was stirred
at 100 °C for 3 days. The progress of the reaction was monitored by
TLC (eluent:n-hexane/EtOAc, 2:1). After completion of the reaction,
the mixture was cooled to room temperature and dissolved in
methanol to remove the unreacted aminoalcohol. The mixture
was filtered and the solid material was dissolved in hot methanol,
and the bis-oxazoline light pink needle-like crystals were obtained
by the slow evaporation of methanol. The same reaction was
also carried out under ultrasonic irradiation as following: a mix-
ture of 1,3-dicyanobenzene (4 mmol), 2-amino-1,3-propanediol
(16 mmol) and SSA (200 mg) was exposed to ultrasonic irradiation
for 3 h (6 discontinuous 30 min. exposures). The reaction pro-
gress was monitored by TLC (eluent:n-hexane/EtOAc, 2:1). The
2.3. General procedure for oxidation of alkenes with TBHP catalyzed by
Mo BOX complex
In a round-bottom flask (25 mL) equipped with a reflux con-
denser, a gas inlet and a magnetic stirrer, a solution of alkene
(1 mmol) in 1,2-dichloroethane (4 mL) was prepared. The molyb-
denum BOX complex (8 mg, 0.01 mmol, 0.02 mmol Mo) and TBHP
(2 mmol) was added to this solution and the reaction mixture was