A new square planar mononuclear MnIII complex for catalytic epoxidation of stilbene

Article abstract of DOI:10.1016/j.jorganchem.2008.01.009

The manganese(III) complex (2) with a diamide ligand has been synthesized. This complex was found to catalyze both the epoxidation of (Z)- and (E)-stilbene with high conversion and the oxidation of benzyl alcohol to benzaldehyde.

Full text of DOI:10.1016/j.jorganchem.2008.01.009

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Journal of Organometallic Chemistry 693 (2008) 1150–1153
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Note
A new square planar mononuclear MnIII complex
for catalytic epoxidation of stilbene
a
b
c
a,
*
˚
Lien-Hoa Tran , Lars Eriksson , Licheng Sun , Bjorn Akermark
¨
a Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden
b Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, 106 91 Stockholm, Sweden
c Department of Chemistry, Organic Chemistry, Royal Institut Technology (KTH), Teknikringen 30, 100 44 Stockholm, Sweden
Received 11 October 2007; received in revised form 21 December 2007; accepted 6 January 2008
Available online 11 January 2008
Abstract
The manganese(III) complex (2) with a diamide ligand has been synthesized. This complex was found to catalyze both the epoxidation
of (Z)- and (E)-stilbene with high conversion and the oxidation of benzyl alcohol to benzaldehyde.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords: Manganese; Epoxidation; Oxidation of alcohols
1. Introduction
preparing a MnV oxo complex from 2, we have been able
to show that it catalyzes epoxidation of both (Z)- and
(E)-stilbene, in contrast to some related complexes [16–
18]. The reason could be that complex 2 is neutral and thus
more electrophilic than these negatively charged com-
plexes. It also catalyzes the oxidation of benzyl alcohol to
benzaldehyde, all using iodosobenzene as oxidant (see
Scheme 1).
Ligand 1 was prepared by reacting the acid chloride of
2-hydroxy-3,5-di-tert-butylbenzoic acid with the appropri-
ate amine. This ligand 1 was then stirred with manga-
nese(II) chloride and sodium methoxide in methanol
solution under air, to give the crude complex 2 as a green
powder. The green microcrystalline solid was dissolved in
methanol and hexane was slowly diffused into this solution
to give crystals, suitable for X-ray structure analysis (see
Fig. 1).
Manganese catalyzed epoxidation [1–5] and water oxi-
dation [6–12] both probably proceed via MnV oxo com-
plexes. These complexes have been found to be very
elusive and attempts to detect such species in salen-Mn cat-
alyzed epoxidations [5,13–15] suggest that they are unstable
and are readily transformed into MnIV species. However,
some time ago Collins and coworkers managed to isolate
a few MnV oxo species [16,17], in which deprotonated
amide functions have replaced the phenolic ligands in the
salen complexes. Perhaps because of the over all negative
charge, these complexes did not effect epoxidation unless
positive ions such as Zn2+ were added [18].
2. Results and discussion
We would now like to report on the preparation of the
MnIII complex 2, which is the potential precursor of a neu-
tral oxo complex, related to the anionic ones presented ear-
lier [16,17]. Although we have not yet succeeded in
Initial results of epoxidation of stilbene by using manga-
nese(III) catalyst 2 (5 mol%) and PhIO (5 equiv.) as oxi-
dant in either pure acetonitrile or a mixture of methylene
chloride and acetonitrile (1:5) gave the epoxide in high con-
version. By contrast, tert-butylhydroperoxide (TBHP) as
an oxidant gave low conversion to the epoxide (<1%),
and with hydrogen peroxide and sodium hypochlorite as
oxidants, no epoxide formation could be detected. The
*
Corresponding author. Tel.: +46 8 163223; fax: +46 8 154908.
˚
E-mail address: bjorn.akermark@organ.su.se (B. Akermark).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.01.009

L.-H. Tran et al. / Journal of Organometallic Chemistry 693 (2008) 1150–1153
1151
O
O
O
O
NH NH
HO
N
N
Mn Cl2
NaOMe / MeOH
Air, r.t
N
N
Mn
O
2
1
Scheme 1. Synthesis of manganese(III) complex 2.
Table 2
Conversion and relative yield of isomers on the epoxidation of (E)-stilbene
Entry
Mn (%)
Conversion (%)
Ratio (Z/E)
1
2
3
4
5
10
15
20
85
80
40
42
1:3
1:3
1:4
1:6
isomeric epoxide was formed. There are several possible
reasons why the epoxidation is not stereospecific (for a dis-
cussion see Ref. [19] and references cited therein).
Fig. 1. ORTEP diagram of a square planar MnIII amide complex 2.
O
Cat. 2
PhIO (5 equiv)
initial results also showed that substantial amounts of (E)-
stilbene oxide were formed, in addition to the expected (Z)-
stilbene oxide (Table 1). This cannot be explained by isom-
erization of (Z)-stilbene, which was less than 10% (Table
1). With a higher ratio of catalyst to substrate, the cis-spec-
ificity of the epoxidation increased (Table 1), perhaps
because the epoxidation can compete with the conversion
DCM / MeCN (1:5)
11 h
One reasonable explanation is that the MnV oxo com-
plex which is probably formed initially, is partly converted
to a MnIV oxo complex by the intramolecular oxidation of
the coordinated phenolic part of the ligand. This should
lead to the epoxidation via a diradical intermediate and
the formation of a mixture of (Z)- and (E)- products from
both (Z) and (E)-stilbene, as observed (cf. [19] and Tables 1
and 2).
of the manganese intermediate to a diradical
.
O
Cat. 2
PhIO (5 equiv)
In addition to the epoxidation, the high valent manga-
nese oxo complex formed from the addition of iodosoben-
zene to the manganese(III) complex 2 was shown to oxidize
benzyl alcohol to benzaldehyde and 1-methylbenzyl alco-
hol to acetophenone. The aliphatic alcohols 1-octanol
and 2-octanol were also oxidized to the corresponding car-
bonyl compounds, albeit in a fairly low conversion. How-
ever, the attempts to oxidize a hydrocarbon, cyclohexane,
failed.
MeCN
11 h
In contrast, when salen-type catalysts are used, also (E)-
stilbene could be epoxidized, using 2 as a catalyst (Table 2).
The reaction was less facile than the epoxidation of (Z)-stil-
bene. Also in this case, the major product was the result of
cis-addition of the oxygen to the alkene, but some of the
Although the epoxidation is not completely stereospe-
cific, our results open up a road to a new type of epoxida-
tion catalyst, which is related to the salen-based catalysts
but can be applied to both (Z)- and (E)-alkenes. The reason
for the moderate stereo-specificity may be the ease of oxi-
dation of the phenolic part of the ligand to give an interme-
diate with diradical character. The preparation of
complexes with a less readily oxidized ligand is therefore
in progress.
Table 1
Conversion and relative yield of isomers on the epoxidation of (Z)-stilbene
Entry
Mn (%)
Conversion (%)
Isomerizationa
Ratio (Z/E)
1
2
3
4
a
5
10
15
20
100
100
100
100
4
9
9
2:1
5:1
5:1
5:1
10
Isomerization of (Z)-stilbene (%).

1152
L.-H. Tran et al. / Journal of Organometallic Chemistry 693 (2008) 1150–1153
3. Experimental
3.1. General procedures
1H NMR (400 MHz, CDCl3): d 1.37 (s, 9H), 1.43 (s,
9H), 7.26 (m, 1H), 7.33 (m, 1H), 7.39 (m, 1H), 7.47 (d,
1H), 7.52 (m, 1H), 7.67 (d, 1H), 7.91 (m, 1H), 7.95 (m,
1H), 8.31 (m, 1H), 8.63 (m, 1H), 10.10 (s, 1H), 10.25 (s,
1H), 12.80 (s, 1H).
Solvents were dried by standard methods and the chemi-
cals were purchased from Aldrich or Lancaster and used as
received. Electrospray ionization mass spectra were deter-
mined, using a Bruker Daltonics BioApex-94c instrument
and yields were determined by HPLC, using a Waters 2695
instrument, equipped with a Daicol–Chiralcol OD-H col-
umn, (0.46 mm  25 cm), UV-detector, eluent isohexane/
isopropanol 9/1. N-(2-amino-phenyl)pyridine-2-carboxam-
ide was prepared according to a published procedure [20].
Single crystal X-ray diffraction data were recorded with
13C NMR (400 MHz, CDCl3): d 29.44, 29,45, 31.55,
31.56, 34.47, 35.24, 113.45, 120.72, 122.69, 124.47,
126.05, 126.78, 126.99, 127.05, 128.91, 129.40, 131.03,
137.73, 137.90, 139.95, 148.43, 148.56, 159.49, 163.53,
169.84.
HRMS-ESI (M+H) calcd for C27H31N3O3Na+:
468.2258, found: 468.2251.
Elemental Anal. Calc. for C27H31N3O3: C, 72.78; H,
7.01; N, 9.43. Found: C, 72.88; H, 7.17; N, 9.27%.
˚
a STOE IPDS, using Mo-radiation source (k = 0.71073 A).
Data were processed by the diffractometer software [21].
The structure was solved by conventional direct methods
using SHELXS97 [22] giving electron density maps where
most of the non-hydrogen atoms could be resolved. The
rest of the non-hydrogen atoms were located at different
electron density maps and the structure model was refined
with full matrix least square calculations on F2 using the
program SHELXL97-2 [23]. All non-hydrogen atoms were
refined with anisotropic displacement parameters and the
hydrogen atoms, which were placed at geometrically calcu-
lated positions and let to ride on the atoms they were
bonded to, were given isotropic displacement parameters
calculated as n Á Ueq for the non-hydrogen atoms with
n = 1.5 for methyl hydrogen atoms (–CH3) and n = 1.2
for methylenic (–CH2–) and aromatic (–CH) hydrogen
atoms. The picture is made with the program DIAMOND
[24] with ellipsoids drawn at the 50% probability level.
More details about the crystal structure can be found in
the cif-file available as supplementary material.
3.3. Synthesis of complex 2
To a solution of 1 (0.5 g, 1.12 mmol) in methanol
(10 ml) was added NaOMe in one portion, followed by
manganese(II) chloride (0.141 g, 1.12 mmol) and the color
immediately turned dark brown. The mixture was then
allowed to stir in aerobic atmosphere at ambient tempera-
ture for 1 h and a precipitate was formed. This was isolated
by filtration through a glassfilter, washed with ice-cold
chloroform, ice-cold pentane, and finally with ice-cold
diethyl ether. After drying overnight under vacuum,
2(0.486 g, 87% yield) was obtained as a light green micro-
crystalline solid.
Elemental Anal. Calc. for C27H28 MnN3O3–MnCl2 3:2:
C, 55.8; H, 4.8; N, 7.2; Mn, 15.8. Found: C, 54.0; H, 5.0; N,
6.6; Mn, 13.7%.
The green microcrystalline solid was dissolved in meth-
anol and hexane was slowly diffused into this solution to
give a single crystal, suitable for an X-ray crystal structure
determination.
3.2. Synthesis of ligand 1
3.4. General procedure for epoxidation
3,5-Di-tert-butylsalicylic acid hydrate (0.5 g, 2 mmol)
was added in portions to a stirred solution of SOCl2(2 ml).
This reaction was heated at 60 °C for 15 h. Excess thionyl-
chloride was removed in vacuo and then co-evaporated six
times with dry chloroform to obtain a yellow-brown
residue.
This yellow-brown residue was dissolved in dry CH2Cl2
and then triethylamine (1.4 ml) was added dropwise. To
this stirred mixture, a solution of N-(2-amino-phenyl)pyri-
dine-2-carboxamide (0.426 g, 2 mmol) in dichloromethane
was added. This reaction mixture was stirred overnight at
ambient temperature. After the addition of water (15 ml),
it was extracted with dichloromethane (3 Â 10 ml). The
combined organic extracts were then washed with water
(3 Â 25 ml) and finally with brine (3 Â 25 ml). After drying
over night with Na2SO4, filtration and removal of the sol-
vent by a rotavap, a yellow solid was obtained. This was
crystallized from chloroform/pentane and the crystals were
washed with ice-cold methanol to give white crystals
(0.696 g, 78% yield).
PhIO (0.110 g, 0.50 mmol) was added in one portion to
a stirred suspension of stilbene (0.090 g, 0.50 mmol) and
catalyst 2 (0.0124 g, 0.025 mmol) in CH3CN (2 ml). This
reaction mixture was then stirred at rt for 11 h. The reac-
tion was quenched by filtering off catalyst 2 through a silica
pad and the crude product was analyzed by HPLC. With
(Z)-stilbene, the conversion to epoxide was 100% and with
(E)-stilbene ca. 85%.
3.5. General procedure for the oxidation of alcohols
PhIO (0.220 g, 1 mmol) was added in one portion to a
stirred suspension of benzyl alcohol (0.108 g, 1 mmol) and
catalyst 2 (0.025 g, 0.050 mmol) in CH3CN (2 ml). The reac-
tion mixture was stirred at rt for 5 h. The reaction was then
quenched by filtering off catalyst 2 through a silica pad and
the crude product was analyzed by HPLC, showing essen-
tially 100% conversion to benzaldehyde. The oxidation of
1-phenylethanol similarly gave acetophenone.

L.-H. Tran et al. / Journal of Organometallic Chemistry 693 (2008) 1150–1153
1153
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Appendix A. Supplementary material
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2008.01.009.
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