Trimanganese complexes bearing bidentate nitrogen ligands as a highly efficient catalyst precursor in the epoxidation of alkenes

Article abstract of DOI:10.1021/jo060709+

A series of trinuclear manganese complexes coordinated with neutral bidentate nitrogen ligands, [Mn₃L₂-(OAc)₆], were prepared from manganese acetate and the corresponding ligands. Using peracetic acid as the oxidant, the air- and moisture-stable manganese clusters exhibited excellent catalytic activity and selectivity in the epoxidation of olefins under mild conditions. The highest activity was observed with a trinuclear complex containing a 2-pyridylimino ligand, [Mn₃(ppei) ₂(OAc)₆] (ppei = 2-pyridinal-1-phenylethylimine). With this system, the substrate scope was extremely wide to include terminal and electron-deficient double bonds of both aliphatic and aromatic alkenes. The high activity was undiminished under the reaction conditions even directly using a mixture of the pyridylimino ligands and manganese acetates, making this process more convenient. It was also observed that analogous trinuclear complexes, such as [Mn₃(bipy)₂(OAc)₆] and [Mn ₃(phen)₂(OAc)₆], displayed excellent activities. While radical intermediacy was inferred from the product distribution, kinetic data revealed that the epoxidation is roughly first-order in manganese cluster precursor and oxidant, respectively, and zero-order in olefin. These results led us to propose that the trinuclear complexes [Mn ₃L₂(OAc)₆] serve as catalyst precursors that dissociate into monomeric species with the formulation of [MnL ₂(OAc)₂] under the reaction conditions.

Full text of DOI:10.1021/jo060709+

Trimanganese Complexes Bearing Bidentate Nitrogen Ligands as a
Highly Efficient Catalyst Precursor in the Epoxidation of Alkenes†
Byungman Kang, Min Kim, Junseong Lee, Youngkyu Do, and Sukbok Chang*
Center for Molecular Design and Synthesis (CMDS), Department of Chemistry and School of Molecular
Science (BK21), Korea AdVanced Institute of Science and Technology (KAIST), Daejon 305-701, Korea
sbchang@kaist.ac.kr
ReceiVed April 3, 2006
A series of trinuclear manganese complexes coordinated with neutral bidentate nitrogen ligands, [Mn3L2-
(OAc)6], were prepared from manganese acetate and the corresponding ligands. Using peracetic acid as
the oxidant, the air- and moisture-stable manganese clusters exhibited excellent catalytic activity and
selectivity in the epoxidation of olefins under mild conditions. The highest activity was observed with a
trinuclear complex containing a 2-pyridylimino ligand, [Mn3(ppei)2(OAc)6] (ppei ) 2-pyridinal-1-
phenylethylimine). With this system, the substrate scope was extremely wide to include terminal and
electron-deficient double bonds of both aliphatic and aromatic alkenes. The high activity was undiminished
under the reaction conditions even directly using a mixture of the pyridylimino ligands and manganese
acetates, making this process more convenient. It was also observed that analogous trinuclear complexes,
such as [Mn3(bipy)2(OAc)6] and [Mn3(phen)2(OAc)6], displayed excellent activities. While radical
intermediacy was inferred from the product distribution, kinetic data revealed that the epoxidation is
roughly first-order in manganese cluster precursor and oxidant, respectively, and zero-order in olefin.
These results led us to propose that the trinuclear complexes [Mn3L2(OAc)6] serve as catalyst precursors
that dissociate into monomeric species with the formulation of [MnL2(OAc)2] under the reaction conditions.
Introduction
olefin epoxidation has received considerable interests from both
academics and industry because epoxides are frequently em-
ployed as important building blocks in organic synthesis and
materials science.3 Although numerous procedures have been
developed,4 there is still an increasing demand for more efficient
Metal-catalyzed oxidation is one of the most fundamental
transformations in chemistry and biology.1 Detailed understand-
ing of oxidation pathways has led to the development of efficient
catalytic procedures potentially viable for large-scale production
of valuable compounds from raw materials.2 Among those,
(3) (a) Ko, S. Y.; Lee, A. W. M.; Masamune, S.; Reed, L. A., III;
Sharpless, K. B.; Walker, F. J. Science 1983, 220, 949. (b) Nicolaou, K.
C.; Winssinger, N.; Pastor, J.; Ninkovic, S.; Sarabia, F.; He, Y.; Vourloumis,
D.; Yang, Z.; Li, T.; Giannakakou, P.; Hamel, E. Nature 1997, 387, 268.
(c) Gagnon, S. D. In Encyclopedia of Polymer Science and Engineering,
2nd ed.; Mark, H. F., Bikales, N. M., Overberger, C. G., Menges, G.,
Kroschwitz, J. I., Eds.; John Wiley & Sons: New York, 1985; Vol. 6, pp
273-307. (d) Darensbourg, D. J.; Mackiewicz, R. M.; Phelps, A. L.;
Billodeaux, D. R. Acc. Chem. Res. 2004, 37, 836.
† Dedicated to Professor Sunggak Kim on the occasion of his 60th birthday.
(1) (a) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidation of Organic
Compounds; Academic Press: New York, 1981. (b) Sono, M.; Roach, M.
P.; Coulter, E. D.; Dawson, J. H. Chem. ReV. 1996, 96, 2841. (c) Gamez,
P.; Aubel, P. G.; Driessen, W. L.; Reedijk, J. Chem. Soc. ReV. 2001, 30,
376.
(2) (a) Meunier, B. Chem. ReV. 1992, 92, 1411. (b) Modern Oxidation
Methods; Ba¨ckvall, J.-E., Ed.; Wiley-VCH: Weinheim, Germany, 2004.
(c) Deubel, D. V.; Frenking, G.; Gisdakis, P.; Herrmann, W. A.; Ro¨sch,
N.; Sundermeyer, J. Acc. Chem. Res. 2004, 37, 645.
(4) (a) Jørgensen, K. A. Chem. ReV. 1989, 89, 431. (b) Katsuki, T. AdV.
Synth. Catal. 2002, 344, 131 (c) De Vos, D. E.; Sels, B. F.; Jacobs, P. A.
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10.1021/jo060709+ CCC: $33.50 © 2006 American Chemical Society
Published on Web 08/01/2006
J. Org. Chem. 2006, 71, 6721-6727
6721

Kang et al.
and selective routes using readily available and stable catalysts.
While these reactions are known to proceed via oxo-, peroxo-,
or peroxide metal intermediates, efficiency of the oxygen
transfer step is dramatically influenced in common by the
coordination environment on the metal center. We report herein
the utility of highly stable trinuclear manganese complexes5
bearing neutral bidentate nitrogen ligands (e.g., 2-pyridylimine,
bipyridine, and phenanthroline) in the olefin epoxidation reac-
tions using commercial peracetic acid. On the basis of kinetic
measurements and site-isolation studies of the catalyst system,
some mechanistic aspects and the nature of active catalyst
species are also proposed.
FIGURE 1. Structure of ppei and an ORTEP diagram of [Mn3(ppei)2-
(OAc)6] (1).
neutral manganese complexes in high yields. For example, a
manganese complex of 2-pyridinal-1-phenylethylimine (ppei)10
was obtained through a condensation of 2-pyridinecarboxalde-
hyde with R-methylbenzylamine followed by metal complex-
ation using Mn(OAc)2‚4H2O to afford the corresponding
manganese complex in 95% overall yield. Although this two-
step procedure provides the manganese complexes in satisfactory
yields, the same species can be also obtained in situ with similar
efficiency, in which the complexes are prepared by the tandem
reaction of primary amines with pyridinecarboxaldehyde fol-
lowed by complexation in one-pot without isolation of Schiff
base ligands (see the Experimental Section for detail). The
resultant manganese complexes are highly stable to air and
moisture such that those compounds can be stored more than a
few months in air without loss of catalytic activity.
The structure of the prepared complexes was characterized
unambiguously by a single-crystal X-ray diffraction study which
shows a configuration of trinuclear species bearing six acetates
and two ppei ligands (Figure 1).11 Each metal is positioned in
a center of an octahedral geometry, and two different coordina-
tion patterns exist with regard to six acetates. Specifically, the
four carboxylate groups span the central metal and terminal
manganese in a bidentate [Mn-O-C(Me)-O-Mn] fashion,
while the remaining two acetate groups involve a monatomic
oxygen [Mn-O-Mn] bridge. The separation between the
central Mn and neighboring Mn is 3.59 Å, which is within the
range of known manganese cluster complexes.12
Results and Discussion
Among various transition metal complexes, iron and man-
ganese species have been investigated most extensively as
catalysts in combination with suitable ligands in the epoxidation
reactions mainly due to the fact that the corresponding com-
plexes are easily prepared, generally stable to air and moisture,
inexpensive, and readily amenable to diverse reaction conditions.
It was shown that a cationic iron complex coordinated with a
tetradentate ligand, [FeII(mep)(CH3CN)2](SbF6)2 (mep ) N,N′-
dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminoethane), self-
assembles in situ to form a carboxylate-bridged µ-oxo diiron(III)
complex that carries out efficient epoxidation reactions using
50% aqueous H2O2.6 Although the catalyst system displayed
good activities on aliphatic olefins, reactions with electron-
deficient olefins as well as styrene derivatives turned out to be
rather inefficient. More recently, Stack and co-workers reported
a notable catalyst system, [MnII(R,R-mcp)(CF3SO3)2] (mcp )
N,N′-dimethyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminocyclohex-
ane),7 a chiral variant of the above-mentioned iron system. Using
peracetic acid, scope of olefin substrates was significantly
expanded to include terminal and electron-deficient double
bonds despite the fact that some sensitive olefins, such as styrene
derivatives, are still problematic. Later studies revealed that
monomeric manganese catalysts bearing more robust bidentate
nitrogen ligands, such as [MnII(bipy)2(CF3SO3)2], display even
higher activities than the initial [MnII(R,R-mcp)(CF3SO3)2]
system.8
Catalytic activity of the resulting manganese cluster was next
investigated in the epoxidation of representative terminal and
internal double bonds of aliphatic and aromatic alkenes (Table
1). A dramatic ligand effect was observed with regard to
catalytic activity using commercial peracetic acid (32%) as an
We envisioned that metal complexes of 2-pyridylalkylidine
ligands might afford notable benefits, such as ease of preparation
and facile tuning of both steric and electronic nature in the
complexes. Accordingly, a series of 2-pyridylimino derivatives
were prepared by the reaction of 2-pyridinecarboxaldehyde
derivatives with primary amines.9 Subsequent complexation of
these ligands with manganese acetates was carried out giving
(10) For representative examples of transition metal complexes bearing
the ppei ligand, see: (a) Lindoy, L. F.; Livingstone, S. E. Coord. Chem.
ReV. 1967, 2, 173. Mo: (b) Brunner, H.; Herrmann, W. A. Angew. Chem.,
Int. Ed. Engl. 1972, 11, 418. (c) La Placa, S. J.; Bernal, I.; Brunner, H.;
Herrmann, W. A. Angew. Chem., Int. Ed. Engl. 1975, 14, 353. Co: (d)
Brunner, H.; Rambold, W. J. Organomet. Chem. 1974, 64, 373. Pd: (e)
Mishnev, A.; Iovel, I.; Popelis, J.; Vosekalna, I.; Lukevics, E. J. Organomet.
Chem. 2000, 608, 1. Ru: (f) Moreau, C.; Frost, C. G.; Murrer, B.
Tetrahedron Lett. 1999, 40, 5617. Ir: (g) Zassinovich, G.; Bettella, R.;
Mestroni, G.; Bresciani-Pahor, N.; Geremia, S.; Randaccio, L. J. Organomet.
Chem. 1989, 370, 187. (h) Carmona, D.; Lahoz, F. J.; Elipe, S.; Oro, L. A.;
Lamata, M. P.; Viguri, F.; Mir, C.; Cativiela, C.; Lo´pez-Ram de V´ıu, M.
P. Organometallics 1998, 17, 2986. Rh: (i) Brunner, H.; Tracht, T.
Tetrahedron: Asymmetry 1998, 9, 3773. (j) Himeda, Y.; Onozawa-
Komatsuzaki, N.; Sugihara, H.; Arakawa, H.; Kasuga, K. J. Mol. Catal. A:
Chem. 2003, 195, 95.
(5) For some selected examples of multinuclear manganese catalysts in
oxidation reactions, see: (a) Taft, K. L.; Kulawiec, R. J.; Sarneski, J. E.;
Crabtree, R. H. Tetrahedron Lett. 1989, 30, 5689. (b) Sarneski, J. E.; Michos,
D.; Thorp, H. H.; Didiuk, M.; Poon, T.; Blewitt, J.; Brudvig, G. W.;
Crabtree, R. H. Tetrahedron Lett. 1991, 32, 1153. (c) Wang, K.; Mayer, J.
M. J. Am. Chem. Soc. 1997, 119, 1470. (d) Tembe, G. L.; Ganeshpure, P.
A.; Satish, S. J. Mol. Catal. A: Chem. 1997, 121, 17. (e) Barton, D. H. R.;
Li, W.; Smith, J. A. Tetrahedron Lett. 1998, 39, 7055. (f) Barton, D. H.
R.; Choi, S.-Y.; Hu, B.; Smith, J. A. Tetrahedron 1998, 54, 3367. (g) Cui,
Y.; Chen, C.-L.; Gratzl, J. S.; Patt, R. J. Mol. Catal. A: Chem. 1999, 144,
411.
(6) White, M. C.; Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2001,
123, 7194.
(7) Murphy, A.; Dubois, G.; Stack, T. D. P. J. Am. Chem. Soc. 2003,
125, 5250.
(8) Murphy, A.; Pace, A.; Stack, T. D. P. Org. Lett. 2004, 6, 3119.
(9) Savoia, D.; Trombini, C.; Umani-Ronchi, A. J. Org. Chem. 1978,
43, 2907.
(11) For selected reviews on the structure and reactivity of manganese
clusters and their model systems, see: (a) Christou, G. Acc. Chem. Res.
1989, 22, 328. (b) Dismukes, G. C. Chem. ReV. 1996, 96, 2909. (c)
Yachandra, V. K.; Sauer, K.; Klein, M. P. Chem. ReV. 1996, 96, 2927. (d)
Wu, A. J.; Penner-Hahn, J. E.; Pecoraro, V. L. Chem. ReV. 2004, 104, 903.
6722 J. Org. Chem., Vol. 71, No. 18, 2006

Trinuclear Manganese Species as an Epoxidation Catalyst Precursor
TABLE 1. Effects of the ppei Ligand on Olefin Epoxidationa
Mn(OAc)2•4H2O
with rac-ppei
Mn(OAc)3•2H2O
with rac-ppei
Mn(OAc)2•4H2O
1
alkene
1-octene
cyclohexene
styrene
conv (%)
conv (%)
yield (%)
conv (%)
yield (%)
conv (%)
yield (%)
<1
<5
0
>98
>98
>98
>98
96
95
93
91
96
96
>98
>98
96
89
92
93
96
>98
>98
>98
95
87
90
94
trans-â-methylstyrene
<1
a Reaction conditions: alkene (0.5 mmol), 1 (1.2 mol % with aliphatic and 2.0 mol % with aromatic olefins), Mn(OAc)2‚4H2O (2.0 mol %) or
Mn(OAc)3‚2H2O (2.0 mol %), 32% CH3CO3H (0.6 mmol), CH3CN (1.0 mL) either in the absence or presence of rac-ppei (5.0 mol %), 25 °C, 18 min.
Conversion and yield were determined by GC analyses using biphenyl as an internal standard on the basis of an average of three runs.
oxidant. While almost negligible conversion was observed with
simple manganese (II or III) complexes, such as Mn(OAc)2‚
4H2O or Mn(OAc)3‚2H2O (2 mol %), in the absence of external
ligands,13 the use of ligated Mn complex 1 (1.2 or 2 mol %)
led to almost complete conversion and selectivity in all cases
examined, usually achieving >95% conversion and >90%
selectivity for the formation of epoxides within 10 min at room
temperature.14 It should be mentioned that highly comparable
results could be obtained using in situ conditions, in which
simple manganese acetate precursors (2 mol %) are directly
employed in combination with rac-ppei (5 mol %). This offers
a notable and practical advantage in that no preformed metal
complex is required to achieVe the high efficiency in the
FIGURE 2. ORD spectrum of rac-1, (R,R)-1, and (S,S)-1 in methanol
epoxidation reactions. It turned out that the oxidation state of
(2.0 × 10-5 M).
the manganese acetate precursors has little effects on the
resulting catalysts’ activities, affording almost identical results
employed ligand: rac-ppei, R-ppei, or S-ppei. When optical
from either a Mn(II) or Mn(III) precursor. It is especially
rotatory dispersion (ORD) was measured for two complexes
noteworthy that styrene can also be employed as a high yielding
[Mn3(ppei)2(OAc)6] bearing either R-ppei or S-ppei, the pair
substrate with this catalyst system, which is markedly in contrast
appeared to be optical antipodes, while a complex of rac-ppei
to the [MnII(R,R-mcp)]2+ case that results in significant amounts
did not show any optical behavior as anticipated (Figure 2).16
of side products when styrene was examined.7 With the present
A manganese complex prepared from R-ppei displays a peak
catalyst system, the use of oxidants other than peracetic acid,
and a trough at wavelengths of 243 and 210 nm, respectively,
such as NaOCl, Oxone, N-methylmorpholine oxide (NMO), or
thus showing a positive Cotton effect.17
H2O2,15 gave much reduced yields of epoxide.
Under the optimized conditions, a wide range of double bonds
It was observed that handedness of the coordinating ppei
of internal, terminal, and conjugated aliphatic alkenes was
ligand has little effects on the enantio- and diastereoselectivities
cleanly epoxidized with the catalyst 1 (1.2-2 mol %) using
as well as reactivity in the epoxidation. In fact, all manganese
commercial peracetic acid (Table 2). In general, the epoxidation
complexes showed the same conversion and selectivity in the
reactions were completed within 30 min at room temperature,
epoxidation reactions irrespective of the stereochemistry of the
and selectivity for the formation of epoxides was generally
excellent enough to afford epoxides in high yields almost
irrespective of electronic and/or steric variation on substrates.
(12) (a) Kessissoglou, D. P.; Kirk, M. L.; Bender, C. A.; Lah, M. S.;
Pecoraro, V. L. J. Chem. Soc., Chem. Commun. 1989, 84. (b) Me´nage, S.;
Epoxidation of cis aliphatic olefins proceeds with a high
stereoselectivity to afford predominantly cis-epoxides (cis/trans,
>20:1) as demonstrated in entry 4. Labile functional groups,
such as acetate or a free hydroxyl group, were tolerated under
the reaction conditions (entries 9 and 10, respectively). In
addition, a conjugated enone was selectively epoxidized in high
yield (entry 11), albeit requiring slightly higher loading of
catalyst 1 (2 mol %). However, selectivity in a diene epoxidation
turned out to be rather low. For example, reaction of (R)-(-)-
carvone and 4-vinylcyclohexene resulted in an almost nonselec-
tive epoxidation (entries 13 and 14). The reaction was readily
applicable to a gram scale process giving isolated yields almost
comparable to those in the small scale reactions. It should be
Vitols, S. E.; Bergerat, P.; Codjovi, E.; Kahn, O.; Girerd, J.-J.; Guillot, M.;
Solans, X.; Calvet, T. Inorg. Chem. 1991, 30, 2666. (c) Rardin, R. L.;
Poganiuch, P.; Bino, A.; Goldberg, D. P.; Tolman, W. B.; Liu, S.; Lippard,
S. J. J. Am. Chem. Soc. 1992, 114, 5240. (d) Baldwin, M. J.; Kampf, J. W.;
Pecoraro, V. L. J. Chem. Soc., Chem. Commun. 1993, 1741. (e) Ferna´ndez,
G.; Corbella, M.; Mah´ıa, J.; Maestro, M. A. Eur. J. Inorg. Chem. 2002,
2502.
(13) It was reported that Mn(II)SO4 carries out an effective epoxidation
of various olefins (except aliphatic 1-alkenes) using hydrogen peroxide in
hydrogen carbonate buffer conditions. See: Lane, B. S.; Burgess, K. J.
Am. Chem. Soc. 2001, 123, 2933.
(14) It has been reported that certain types of dinuclear manganese
complexes bearing amino, pyridyl, or Schiff base ligands exhibit excellent
catalytic activity in the epoxidation of olefins. See: (a) Hage, R.; Iburg, J.
E.; Kerschner, J.; Koek, J. H.; Lempers, E. L. M.; Martens, R. J.; Racherla,
U. S.; Russell, S. W.; Swarthoff, T.; van Vliet, M. R. P.; Warnaar, J. B.;
van der Wolf, L.; Krijnen, B. Nature 1994, 369, 637. (b) Sureshan, C. A.;
Bhattacharya, P. K. J. Mol. Catal. A: Chem. 1998, 130, 73. (c) Brinksma,
J.; Hage, R.; Kerschner, J.; Feringa, B. L. Chem. Commun. 2000, 537.
(15) (a) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457. (b)
Grigoropoulou, G.; Clark, J. H.; Elings, J. A. Green Chem. 2003, 5, 1. (c)
Mizuno, N.; Yamaguchi, K.; Kamata, K. Coord. Chem. ReV. 2005, 249,
1944. (d) De Vos, D. E.; Sels, B. F.; Reynaers, M.; Rao, Y. V. S.; Jacobs,
P. A. Tetrahedron Lett. 1998, 39, 3221. (e) Berkessel, A.; Sklorz, C. A.
Tetrahedron Lett. 1999, 40, 7965.
(16) Interestingly, when rac-ppei was employed as a ligating ligand to
react with Mn(II) sources, homochiral manganese clusters of [Mn3(R-ppei)-
(R-ppei)(OAc)6] and [Mn3(S-ppei)(S-ppei)(OAc)6] were isolated separately
from the reaction mixture, whose structures were determined by X-ray
crystallographic analyses.
(17) For a previous example of CD spectra of chiral Pd(ppei) complexes,
see ref 10e.
J. Org. Chem, Vol. 71, No. 18, 2006 6723

Kang et al.
TABLE 2. Epoxidation of Aliphatic Olefins Using Isolated
Catalyst 1 and under in situ Conditionsa
GC yield (%)
Mn(OAc)2‚4H2O
entry
alkene
1-decene
1-octene
trans-2-octene
cis-2-octene
2-methyl-1-heptene
2-methyl-2-hexene
cyclohexene
cyclooctene
allyl acetate
9-decen-1-ol
2-cyclohexen-1-one
norbornene
R-(-)-carvone
4-vinylcyclohexene
condition (isolated yield)b
with rac-ppeic
1
2
3
4
5
6
7
8
9
10
11
12
13
14
A
A
A
A
A
A
A
A
B
A
B
B
A
A
96 (92)
96 (86)
94 (87)
97 (91)d
97 (90)
92 (83)
95 (79)
90 (81)
97
93
95
94
94
89 (82)
95 (85)
72e
+
FIGURE 3. Hammett plot of log krel versus σp for the epoxidation
of styrene and p-substituted styrenes.
93f
isomerization of double bonds takes place under the epoxidation
conditions.19 Conjugated aromatic olefins, such as cinnamic
ester, were also efficiently epoxidized to afford epoxy ester
(entry 8).
77g
a Condition A: alkene (0.5 mmol), 1 (1.2 mol %), 32% CH3CO3H (0.6
mmol), CH3CN (1.0 mL), 25 °C, 18 min. Condition B: alkene (0.5 mmol),
1 (2.0 mol %), 32% CH3CO3H (1.0 mmol, added in two portions), CH3CN
(1.0 mL), 25 °C, 28 min. b GC yields were determined using biphenyl as
an internal standard on the basis of an average value of three runs. Isolated
yields were based on the reactions performed on a 1.0 g scale. c 2.0 mol %
of Mn(OAc)2‚4H2O and 5.0 mol % of rac-ppei. d cis/trans-Isomer, >20:1.
e exo-2,3-Epoxynorbornane. f Mixture of 1,2-epoxide and 8,9-epoxide (1:
1.2). g Equal mixture of internal and terminal epoxide.
When competition experiments were carried out between
styrene and an equivalent amount of p-substituted styrene
derivatives, a significant electronic effect on the reaction rates
was observed to show that the more electron-rich olefins react
faster than electron-deficient substrates.20 Styrenes having a
substituent at the para-position reacted as the following order:
OMe (krel ) 6.35) > Me (2.27) > F (1.17) > H (1.0) > Cl
(0.87) > CF3 (0.42). A shown in Figure 3, a more precise linear
TABLE 3. Epoxidation of Aromatic Olefins Using Catalyst 1a
+
GC yield (%) Mn(OAc)2‚4H2O
correlation can be obtained when krel is plotted with σp (r )
entry
alkene
condition (isolated yield)
with rac-ppeic
0.97) rather than with σp (r ) 0.84). The small negative value
of the observed reaction constant (F ) -0.86) is highly
comparable to those in the previously reported epoxidation
procedures using porphyrin metal oxo systems.21
1
2
3
4
5c
6
7
8
styrene
4-chlorostyrene
A
A
A
A
A
A
B
B
93 (84)
94 (88)
90b
96
4-acetoxystyrene
trans-â-methylstyrene
trans-stilbene
cis-â-methylstyrene
cis-stilbene
85b
91
94 (91)
93
Dependence of the initial rates on the concentration of catalyst
1, styrene, and oxidant was next measured to reveal that the
reaction is roughly first-order in catalyst 1 (1.10) and the
oxidant’s concentration (1.24), respectively, and close to zero-
order in the olefin concentration (0.06).22,23 In addition, an
induction period was observed during the epoxidation process.20
In analogy to the previous reports,24 the present kinetic behavior
suggests that the rate-determining step is most likely related to
the formation of a putative manganese peroxo species rather
than an oxygen atom transfer step from the active manganese
intermediates to the double bonds.
80d
89 (86)e
94 (92)
ethyl-trans-cinnamate
a Same conditions as in Table 2 were used (2.0 mol % of 1 was used in
all cases except in entry 8, in which 3.0 mol % of 1 was employed). b 1H
NMR yield of the crude reaction mixture. c Reactions were carried out in
a cosolvent (CH2Cl2/CH3CN, 1:1). d cis/trans-Isomeric epoxides (4.1:1).e cis/
trans-Isomeric epoxides (3.2:1).
mentioned that, even under the in situ conditions, highly
comparable results were obtained with regard to reactivity and
selectivity, in which the epoxidation was carried out with a
manganese complex catalyst prepared in one-pot without using
the isolated trinuclear manganese species. In this procedure,
peracetic acid is added into a solution containing an olefin,
manganese(II) acetate (2 mol %), and the ppei ligand (5 mol
%) in acetonitrile.
In addition, we were interested in investigating some relevant
manganese clusters having certain bidentate ligands, such as
(19) (a) Srinivasan, K.; Michaud, P.; Kochi, J. K. J. Am. Chem. Soc.
1986, 108, 2309. (b) Groves, J. T.; Stern, M. K. J. Am. Chem. Soc. 1988,
110, 8628. (c) Zhang, W.; Lee, N. H.; Jacobsen, E. N. J. Am. Chem. Soc.
1994, 116, 425.
(20) For detailed competition experiments and data of the kinetic
measurements, radical inhibition experiments, and induction period observa-
tion, see the Supporting Information.
(21) (a) Bortolini, O.; Meunier, B. J. Chem. Soc., Perkin Trans. 2 1984,
1967. (b) Collman, J. P.; Hampton, P. D.; Brauman, J. I. J. Am. Chem.
Soc. 1990, 112, 2986. (c) Arasasingham, R. D.; He, G.-X.; Bruice, T. C. J.
Am. Chem. Soc. 1993, 115, 7985.
(22) For detailed kinetic data on the concentration of 1, styrene, and
oxidant, see the Supporting Information.
(23) For some reports on the saturation kinetics behavior in metal-
catalyzed oxygenations, see: (a) Bressan, M.; Morvillo, A. Inorg. Chem.
1989, 28, 950. (b) Amatsu, H.; Miyamoto, T. K.; Sasaki, Y. Bull. Chem.
Soc. Jpn. 1988, 61, 3193.
(24) For related mechanistic discussions on olefin epoxidation, see: (a)
Collman, J. P.; Brauman, J. I.; Meunier, B.; Hayashi, T.; Kodadek, T.;
Raybuck, S. A. J. Am. Chem. Soc. 1985, 107, 2000. (b) Collman, J. P.;
Brauman, J. I.; Hampton, P. D.; Tanaka, H.; Bohle, D. S.; Hembre, R. T.
J. Am. Chem. Soc. 1990, 112, 7980. (c) Norrby, P.-O.; Linde, C.; Akermark,
B. J. Am. Chem. Soc. 1995, 117, 11035. (d) Collman, J. P.; Zeng, L.;
Brauman, J. I. Inorg. Chem. 2004, 43, 2672.
We next turned our attention to aromatic olefins because
epoxidations of this type of substrate were known to be sluggish,
leading to significant amounts of side products especially when
the metal systems of Jacobsen6 or Stack7,8 were employed. It
was observed that epoxidation of styrene derivatives proceeds
quite efficiently with 1 (Table 3).18 Electronic variation on aryl
substituents did not affect the efficiency of the reaction (entries
2 and 3). In contrast to aliphatic cis olefins, epoxidation of
internal aromatic olefins gave the corresponding epoxides as a
mixture of cis- and trans-isomers with a ratio of 4.1:1 and 3.2:
1, respectively (entries 6 and 7). This implies that the epoxi-
dation proceeds via a radical pathway (vide infra) since no
(18) (a) Murray, R. W.; Singh, M. Org. Synth. 1996, 74, 91. (b) Rudolph,
J.; Reddy, K. L.; Chiang, J. P.; Sharpless, K. B. J. Am. Chem. Soc. 1997,
119, 6189.
6724 J. Org. Chem., Vol. 71, No. 18, 2006

Trinuclear Manganese Species as an Epoxidation Catalyst Precursor
TABLE 4. Epoxidations Catalyzed by [Mn3(bipy)2(OAc)6] (2) and
[Mn3(phen)2(OAc)6] (3)a
Attachment of metal complexes to a solid support has been
frequently utilized as an insightful tool to obtain information
on the reaction pathway as well as a means of extending
practical advantages of homogeneous catalysis. This approach
has turned out to be valid especially in a search for the existence
of putative internuclear interactions.32 With this context, syn-
thesis of an anchored pyridylimino ligand and its manganese(II)
complex (4) was successfully carried out (Scheme 1). In each
step, the pyridyl-containing compounds were detached from the
resin, and the chemical structure was fully characterized using
spectroscopic tools. In addition, elemental analyses of nitrogen
contents were satisfactorily carried out to confirm the validity
of each transformation (see the Supporting Information).
Inductively coupled plasma (ICP) analysis of 4 revealed that
manganese content in the supported complex was 0.329 mmol/g
(1.81 wt %), indicating that each synthetic step proceeded with
high efficiency (>80% yield).
When the supported manganese complex 4 (2.0 mol %) was
employed under the same conditions as with the manganese
clusters 1-3, the reactivity was shown to be significantly
attenuated, although selectivity remained still high (Table 5).
Considering that the polystyrene-bound pyridylimino group (L)
has little flexibility, it is reasonable to regard that a metal
coordination of two anchored bidentate ligands (L2) at the same
time giving the assumed active manganese species [MnL2-
(OAc)2] is not feasible under this environment. Therefore, the
decreased reactivity of the site-isolated catalyst can be ascribed
mainly to the geometric restraints present in the anchored ligand,
although the heterogenization itself may also contribute in part
to the decreased catalytic activity. We assume that this result
may support our proposal that the active species is the
monomeric moieties [MnL2(OAc)2] when we employ the
trinuclear complexes [Mn3L2(OAc)6]. On the other hand,
catalytic activity of the recovered resin-bound manganese
complex dropped sharply, especially more significant with the
slow reacting substrates.
2
3
entry
alkene
1-decene
9-decen-1-ol
styrene
conv (selec %)
conv (selec %)
1
2
3
4
88 (95)
82 (84)
>98 (88)
>98 (85)
76 (95)
81 (84)
96 (80)
ethyl-trans-cinnamate
>98 (74)
a Same conditions as in Table 2. In entry 1, 1.2 mol % of each catalyst
2 and 3 was used. In entries 2 and 3, 2.0 mol % of each catalyst 2 and 3
was used. In entry 4, 3.0 mol % of 2 and 3 was employed.
2,2′-bipyridine or 1,10-phenanthroline. The corresponding tri-
nuclear complexes, [Mn3(bipy)2(OAc)6]25 (2) and [Mn3(phen)2-
(OAc)6]26 (3), were similarly prepared, and their molecular
structures were unambiguously characterized by X-ray diffrac-
tion analyses.27 When the trinuclear manganese complex 2 or
3 was examined as a catalyst using peracetic acid, both species
displayed quite comparable reactivity and selectivity when
compared to those of the corresponding monomeric catalyst
[Mn(bipy)2(CF3SO3)2] or [Mn(phen)2(CF3SO3)2] as reported by
Stack and co-workers (Table 4).8
These results, in combination with the above-mentioned
kinetic measurements and induction period, led us to propose
that the trinuclear manganese complexes, [Mn3L2(OAc)6], work
as a catalyst precursor generating in situ an actiVe catalyst
[MnL2(OAc)2] and much less actiVe [Mn(OAc)2(solVent)n]
species under the reaction conditions. Indeed, we isolated from
the reaction mixture an insoluble manganese species, [Mn(OAc)2-
(solvent)n], that shows almost no catalytic reactivity in the
epoxidation. As a result, it can be assumed that the ppei-bound
monomeric species [Mn(ppei)2(OAc)2] is mainly responsible for
the observed catalytic activity of the trimanganese complex
precursor. If manganese chloride was employed as a Mn source,
the resultant unstable complex exhibited very similar catalytic
activity when compared to that of [Mn3(ppei)2(OAc)6].28
Although it has been known that dissociation of certain
manganese clusters into monometallic moieties readily occurs
by a ligand substitution or rearrangement processes,29 a pos-
sibility that the trinuclear species themselves participate in the
present catalysis in our system cannot be completely ruled out
at present.30 In fact, Crabtree and co-workers reported that a
manganese cluster, [Mn3O4(bipy)4(H2O)2](ClO4)4, shows excel-
lent epoxidation activity using Oxone and that the oxidation
takes place through an oxo transfer pathway without dissociation
into monomeric manganese moieties.5b,31
Conclusions
Trinuclear manganese complexes, [Mn3L2(OAc)6], bearing
certain bidentate nitrogen ligands L, such as 2-pyridylimino or
bipyridino groups, displayed excellent catalytic activity in the
epoxidation of olefins using commercial peracetic acid. Among
those complexes investigated, species bearing 2-pyridylimino
(ppei) exhibited especially high reactivity and selectivity. With
the present catalyst system, substrate scope is extremely wide
such that terminal and electron-deficient double bonds of both
aliphatic and aromatic alkenes become now viable substrates.
The catalytically active species can be also directly generated
under the in situ conditions, in which the use of a preassembled
trinuclear manganese complex is not required, thus making this
epoxidation procedure operationally attractive. Results from the
preliminary mechanistic studies and kinetic measurements led
us to propose that the trinuclear manganese clusters, upon
addition of peracetic acid oxidant, dissociate into two types of
metal species, [MnL2(OAc)2] and [Mn(OAc)2(solvent)n], or their
oligomeric species; the former L-bound species is assumed to
be mainly responsible for the observed activity. This assumption
was further supported both by the observation of an induction
(25) For the structural determination of 2, see ref 12b.
(26) For the structural determination of 3, see ref 12c.
(27) See the Supporting Information for the X-ray crystal structure of
complex 3.
(28) Although we could not characterize the generated complex, forma-
tion of a monometallic species can be envisioned as a major complex due
to the much less bridging ability of the chloride ligand.
(29) For example, see: Ma, C.; Chen, C.; Liu, Q.; Liao, D.; Li, L. Eur.
J. Inorg. Chem. 2003, 1227.
(30) For previous examples of multinuclear manganese complexes being
active species, see: (a) Mathur, P.; Crowder, M.; Dismukes, G. C. J. Am.
Chem. Soc. 1987, 109, 5227. (b) Chavan, S. A.; Halligudi, S. B.; Srinivas,
D.; Ratnasamy, P. J. Mol. Catal. A: Chem. 2000, 161, 49. (c) Carrell, T.
G.; Cohen, S.; Dismukes, G. C. J. Mol. Catal. A: Chem. 2002, 187, 3. (d)
Larsen, A. S.; Wang, K.; Lockwood, M. A.; Rice, G. L.; Won, T.-J.; Lovell,
S.; Sad´ılek, M.; Turee`ek, F.; Mayer, J. M. J. Am. Chem. Soc. 2002, 124,
10112.
(32) (a) Collman, J. P.; Belmont, J. A.; Brauman, J. I. J. Am. Chem.
Soc. 1983, 105, 7288. (b) To¨llner, K.; Popovitz-Biro, R.; Lahav, M.;
Milstein, D. Science 1997, 278, 2100. (c) Annis, D. A.; Jacobsen, E. N. J.
Am. Chem. Soc. 1999, 121, 4147.
(31) (a) Wessel, J.; Crabtree, R. H. J. Mol. Catal. A: Chem. 1996, 113,
13. (b) Sarneski, J. E.; Thorp, H. H.; Brudvig, G. W.; Crabtree, R. H.;
Schulte, G. K. J. Am. Chem. Soc. 1990, 112, 7255.
J. Org. Chem, Vol. 71, No. 18, 2006 6725

Kang et al.
SCHEME 1. Synthesis of the Polymer-Supported Ligand and Its Manganese Complex 4a
a Reagents and conditions: (a) 5-hydroxy-2-methylpyridine (4 equiv), NaI (4 equiv), Cs2CO3 (4 equiv), DMF, 80 °C, 12 h; (b) m-chloroperoxybenzoic
acid (4 equiv), CHCl3, room temperature, 24 h; (c) trifluoroacetic anhydride (5 equiv), CH2Cl2, room temperature, 24 h; (d) SeO2 (6 equiv), 1,4-dioxane, 90
°C, 20 h; (e) R-methylbenzylamine (6 equiv), EtOH, 80 °C, 12 h; (f) Mn(OAc)2‚4H2O (5 equiv), EtOH, 80 °C, 12 h.
TABLE 5. Effect of Polymer-Supported Mn (4) on the
Epoxidation of Olefinsa
Synthetic procedure for the preparation of analogous manganese
clusters 2 and 3 was identical to that for the synthesis of 1: [Mn3-
(bipy)2(OAc)6] (2). Yield was 68% based on Mn content. Anal.
Calcd for C32H34N4O12Mn3: C, 46.23; H, 4.12; N, 6.74. Found:
C, 46.16; H, 4.09; N, 6.73.
[Mn3(1,10-phenanthroline)2(OAc)6] (3). Yield was 70% based
on Mn content. Anal. Calcd for C36H34N4O12Mn3: C, 49.16; H,
3.90; N, 6.37. Found: C, 48.63; H, 3.94; N, 6.32. X-ray structure
and collection data of 3 are presented in Supporting Information.
1st Run
2nd Runb
alkene
cyclooctene
1-decene
trans-â-methylstyrene
conv (%)
selec (%)
conv (%)
selec (%)
99
74 (>98)
50 (>98)
47 (>98)
99 (90)
97 (96)
94 (91)
64
<1
29
90
a Reaction conditions: olefin (0.5 mmol), polymer-supported Mn
complex (30.4 mg, 2.0 mol %), and 32% CH3CO3H (0.6 mmol) in CH3CN
(1.0 mL), 25 °C, 18 min. Conversion was determined by GC using dodecane
as an internal standard. Selectivity indicates the formation of epoxide, and
the numbers in parentheses are the results of epoxidation using catalyst 1
(1.2 mol % with aliphatic and 2.0 mol % with aromatic olefin). b Reactions
were performed with the filtered resin from the first run.
Representative Procedure for the Epoxidation Reactions.
Condition A: To a solution of 1-decene (74.6 mg, 0.5 mmol) and
1 (5.6 mg, 1.2 mol %) in acetonitrile (1.0 mL) was added
commercial peracetic acid (32%, 0.13 mL, 0.6 mmol) dropwise
via syringe over 3 min, and the reaction mixture was stirred for 15
min at room temperature. After adding diethyl ether (5 mL) and
biphenyl (0.5 mmol, an internal standard), the reaction mixture was
passed through a pad of silica gel washing with diethyl ether. The
oven condition for the GC/MS analysis was 70 °C for 3 min, 10
°C/min to 90 °C holding for 1 min, and 5 °C/min to 150 °C holding
for 1.0 min on HP-5MS column, and the retention time of 1,2-
epoxydecane was 11.18 min. The reaction proceeded with >98%
conversion and in 96% GC yield on the basis of an average of
three runs. CAUTION! The reaction exotherms if peracetic acid
is added too quickly or if heat transfer from the reaction flask is
inadequate to maintain the desired temperature.
Gram Scale Epoxidation: To a solution of 1-decene (1.06 g,
7.13 mmol) and 1 (80.4 mg, 0.085 mmol, 1.2 mol %) in CH3CN
(17 mL) was added peracetic acid (32%, 1.80 mL, 8.55 mmol)
dropwise via syringe over 5 min. After stirring for 25 min at room
temperature, the reaction mixture was quenched with saturated
NaHCO3 solution and was subsequently extracted with n-pentane.
The organic layer was dried over MgSO4, filtered, and concentrated
under reduced pressure. The residue was purified by column
chromatography on silica gel (Et2O/n-pentane, 1:10) to afford 1,2-
epoxydecane as a colorless oil (1.02 g, 92%): 1H NMR (400 MHz,
CDCl3) δ 2.88 (m, 1H), 2.72 (dd, 1H, J ) 5.0, 4.0 Hz), 2.43 (dd,
1H, J ) 5.1, 2.7 Hz), 1.25-1.51 (m, 14H), 0.86 (t, 3H, J ) 6.7
Hz); 13C NMR (100 MHz, CDCl3) δ 52.4, 47.1, 32.5, 31.8, 29.5,
29.4, 29.2, 25.9, 22.6, 14.0.
Condition B: A 10 mL round-bottom flask equipped with a
stirring bar was charged with cis-stilbene (93.8 mg, 0.5 mmol), 1
(9.4 mg, 0.01 mmol, 2.0 mol %), and acetonitrile (1.0 mL). Peracetic
acid (32%, 0.10 mL, 0.5 mmol) was added dropwise via syringe
over 5 min at the start, and additional portion of peracetic acid
(0.5 mmol) was next added after 10 min over 3 min. The reaction
mixture was stirred for additional 10 min at room temperature. After
addition of diethyl ether (5 mL) and biphenyl (0.5 mmol, internal
standard), the reaction mixture was passed through a pad of silica
gel washing with diethyl ether. The oven condition for the GC/MS
analysis was 110 °C for 3 min, 15 °C/min to 180 °C holding for 1
min, and 5 °C/min to 210 °C holding for 1 min on a HP-5MS
column. The retention time of cis- and trans-stilbene oxide was
9.30 and 10.76 min, respectively. GC analysis indicates that
period in the epoxidation reaction and by the results with the
solid-supported catalyst system.
Experimental Section
Preparation of 2-Pyridinal-1-phenylethylimine (ppei) Ligand.
Anhydrous magnesium sulfate (4.81 g, 40.0 mmol) and R-meth-
ylbenzylamine (2.42 g, 20.0 mmol) were added at 0 °C to a solution
of 2-pyridinecarboxaldehyde (2.14 g, 20.0 mmol) in 20 mL of
diethyl ether. The reaction mixture was stirred at room temperature
for 12 h, and then it was filtered through a pad of Celite. The filtrate
was evaporated under reduced pressure to afford the desired ppei
ligand in 97% yield (4.08 g) as a yellow oil which was used without
further purification: 1H NMR (400 MHz, CDCl3) δ 8.61 (d, 1H, J
) 4.7 Hz), 8.44 (s, 1H), 8.07 (d, 1H, J ) 7.8 Hz), 7.71 (dt, 1H, J
) 7.6, 1.5 Hz), 7.42 (d, 2H, J ) 7.5 Hz), 7.32 (t, 2H, J ) 7.3 Hz),
7.29-7.21 (m, 2H), 4.62 (q, 1H, J ) 6.6 Hz), 1.60 (d, 3H, J ) 6.6
Hz); 13C NMR (100 MHz, CDCl3) δ 160.4, 154.7, 149.3, 144.5,
136.4, 128.4, 126.9, 126.6, 124.6, 121.4, 69.5, 24.5; IR (neat) 2972,
2862, 1645, 1587, 1567, 1493 cm-1; HRMS (EI) calcd for C14H14N2
210.1157 [M+], found 210.1159.
General Procedure for the Synthesis of [Mn3(ppei)2(OAc)6]
(1), [Mn3(2,2′-bipyridine)2(OAc)6] (2), and [Mn3(1,10-phenan-
throline)2(OAc)6] (3). R-Methylbenzylamine (1.21 g, 10.0 mmol)
was added to a solution of 2-pyridinecarboxaldehyde (1.07 g, 10.0
mmol) in absolute ethanol (50 mL). The mixture was heated to
reflux for 1 h, and then Mn(OAc)2‚4H2O (2.45 g, 10.0 mmol) was
added in one portion. The yellow solution immediately turned into
a pale brown solution, and then a solid began to appear. After 10
min of stirring under the same temperature, the reaction mixture
was cooled to 25 °C, and the resulting heterogeneous brown solution
was partially evaporated. The pale brown precipitate was isolated
by filtration washing with cold ethanol to afford 1 in 92% yield
(2.89 g based on Mn). X-ray quality crystals were grown by the
vapor diffusion method, where a small vial containing the reaction
mixture of 1 was sealed in a large vial containing excess diethyl
ether. Anal. Calcd for C40H46N4O12Mn3 (1): C, 51.13; H, 4.93; N,
5.96. Found: C, 50.63; H, 4.80; N, 6.01.
6726 J. Org. Chem., Vol. 71, No. 18, 2006

Trinuclear Manganese Species as an Epoxidation Catalyst Precursor
epoxidation proceeded with >98% conversion and in 89% GC yield
(cis/trans, 3.2:1) as an average of three runs.
through a pad of silica gel washing with diethyl ether. Conversion
and yield of epoxide were determined by a GC analysis as above.
Epoxidation Reactions under the in situ Conditions. A 10
mL round-bottom flask equipped with a stirring bar was charged
with olefin (0.5 mmol), ppei (0.025 mmol, 5.0 mol %), Mn(OAc)2‚
4H2O (2.0 mol %) or Mn(OAc)3‚2H2O (2.0 mol %), and acetonitrile
(1.0 mL). The mixture was stirred at room temperature for 6 min,
during which time a pale pink color of the mixture solution
containing Mn(OAc)2‚4H2O turned to pale brown, and a hetero-
geneous solution having Mn(OAc)3‚2H2O was changed to a
homogeneous brown solution. Subsequently, peracetic acid (32%,
0.6 mmol, 1.2 equiv to olefin) was added dropwise to the reaction
mixture via syringe over 3 min. After stirring the mixture for 15
min at room temperature, diethyl ether (5 mL) and biphenyl (internal
standard) were added. The crude reaction mixture was then passed
Acknowledgment. This research was supported by the
Center for Molecular Design and Synthesis (CMDS) at KAIST.
Supporting Information Available: X-ray crystallographic data
of 1 and 3, data of competition experiments, procedure of radical
inhibition experiments, spectroscopic data and GC detection condi-
tions of the produced epoxides, experimental details and data of
kinetic studies, procedure and data of induction period experiments,
and details for the synthesis of polymer-supported manganese
complex 4. This material is available free of charge via the Internet
at http://pubs.acs.org.
JO060709+
J. Org. Chem, Vol. 71, No. 18, 2006 6727