Olefin oxygenation by the hydroperoxide adduct of a nonheme manganese(IV) complex: Epoxidations by a metallo-peracid produces gentle selective oxidations

Article abstract of DOI:10.1021/ja055413k

The reactive intermediates and mechanisms of oxygenation of olefins by manganese complexes were investigated by treating olefins with newly synthesized [Mn^IV(Me₂EBC)(OH)₂](PF₆)₂ in the presence and absence of peroxide and by studying its catalytic epoxidation reaction in normal aqueous solution and, individually, with isotopically labeled H₂¹⁸O, ¹⁸O₂, and H₂¹⁸O₂. The manganese oxo species is not the reactive intermediate for the oxygen transfer process mediated by this manganese complex. A novel manganese(IV) peroxide intermediate, Mn^IV(Me₂EBC)(O)(OOH)⁺, was captured by mass spectrometry and is proposed as the intermediate that oxygenates olefins in this catalytic system. Copyright

Full text of DOI:10.1021/ja055413k

Published on Web 11/17/2005
Olefin Oxygenation by the Hydroperoxide Adduct of a Nonheme
Manganese(IV) Complex: Epoxidations by a Metallo-Peracid Produces
Gentle Selective Oxidations
†
†
†
‡
‡
Guochuan Yin, Maria Buchalova, Andrew M. Danby, Chris M. Perkins, David Kitko,
‡
‡
,†
John D. Carter, William M. Scheper, and Daryle H. Busch*
Department of Chemistry, The UniVersity of Kansas, Lawrence, Kansas 66045, and
The Procter and Gamble Company, Cincinnati, Ohio 45202
Received August 8, 2005; E-mail: busch@ku.edu
In this communication, we report the first example in which a
high valent manganese compound, a pseudo-octahedral manganese-
IV) derivative, catalyzes epoxidation, not by way of the familiar
Scheme 1
(
manganese oxo intermediate, but by a Lewis acid pathway involving
the manganese/hydrogen peroxide complex. Reflecting on the
history of epoxidation catalysis by manganese, our results now
demonstrate that all three catalytic pathways well-known for
epoxidation reactions by high valent metal ions are utilized by
manganese, the metal atom redox pathway often called the rebound
mechanism, the peroxy radical pathway, and the Lewis acid
promoted oxygen atom transfer catalysis, most simply typified by
Ti(IV).
the MnIV complex alone is not capable of epoxidation of olefins
even though substantial amounts of the compound would be
IV
a 2
expected to exist as the oxo complex (pK of [Mn (Me EBC)-
2+
8b
2
(OH) ]
) 6.86).
Because the manganese(IV) complex is unstable in base, slowly
III
degrading to Mn (Me
2
EBC) species, a disproportionation mech-
anism for decomposition was suspected.8b
Epoxidation of olefins has long been important in commercial
applications, in addition to being of fundamental mechanistic
2
Mn(IV) T Mn(III) + Mn(V)
1
interest. The epoxide is commonly a minor product in relatively
nonselective radical pathways.2 For catalysis or stoichiometric
reactions based on compounds of iron, manganese, cobalt, and
IV
3c
V
3b,d,4b
Mn oxo and Mn oxo
are frequently proposed as active
intermediates in epoxidation reactions. Although we had shown that
3
chromium, metal oxo species are the established intermediates.
IV
2+
[
Mn (Me
2
EBC)(OH)
2
]
does not epoxidize olefins, the possibility
The metal ion undergoes two-electron reduction, and an oxygen
atom is transferred from the metal coordination sphere to the
substrate, a process strongly supported by studies using isotopic
oxygen labels. The epoxide may be the dominant but often not the
sole product. In contrast, hydrogen peroxide or alkyl peroxide
adducts of metal complexes have long been recognized as inter-
mediates in epoxidations mediated by high valent compounds of
titanium, vanadium, tungsten, molybdenum, and rhenium, in
processes in which the metal ion is assigned the role of a Lewis
V
remained that a Mn oxo derivative might be produced by dispro-
portionation in basic media, subsequently epoxidizing a substrate.
Despite numerous attempts, no epoxide product could be detected
and no conversion of olefin could be found upon degradation of
the manganese(IV) complex to the manganese(III) derivative in base
in the presence of olefins, such as norbornylene.
18
O tracer experiments have proven that the epoxide formed in
II
catalysis by [Mn (Me
2
EBC)Cl
2
does not come from a high valent
manganese complex; the rebound mechanism is not involved in
the process. First, it was necessary to show that the oxo and hydroxo
5
acid in a mechanism believed to proceed by concerted oxygen
6
atom transfer. Despite the association of specific mechanisms with
IV
+
2
groups of [Mn (Me EBC)(O)(OH)] equilibrate with the solvent
specific kinds of catalyst, a growing literature indicates that some
transition metal ions participate in two or more epoxidation
mechanisms.2 For example, studies indicate that, in addition to
the rebound mechanism, an iron adduct of hydrogen peroxide and
a manganese adduct of iodosylbenzene also serve as Lewis acids
in certain oxygen transfer processes, including epoxidation. Previous
to our work, peroxide adducts of high oxidation state manganese
have not been implicated in the Lewis acid mechanism of
18
4
IV
2+
18
H
2
O. When [Mn (Me
95% 18O), the original ms peak for [Mn (Me
2
EBC)(OH)
2
]
2
was dissolved in H O
IV
+
(
2
EBC)(O)(OH)] at
b,7
m/z ) 342, observed for normal aqueous solutions, disappeared,
IV
18
18
+
2
and a new ms peak for [Mn (Me EBC)( O)( OH)] appeared
immediately at m/z ) 346 (Scheme 1).
Second, it was necessary to show that the oxygen in the epoxide
18
comes from oxidant, not the catalyst or solvent. The O tracer
2 2
experiment for catalytic epoxidation of cis-stilbene used 50% H O
epoxidation.
The catalyst in this work is [Mn (Me
18
18
in 4:1 acetone:H
2
O (95% O atom). The product analysis by GC-
II
2 2
EBC)Cl ], the manganese-
18
MS shows that the amount of O introduced into the epoxide
(II) complex of a cross-bridged cyclam ligand, 4,11-dimethyl-1,4,8,-
18
product from H
2
O is identical to that obtained in the control
1
1-tetraazabicyclo[6.6.2]hexadecane. Its well characterized higher
16
experiments; therefore, all oxygen in the epoxide is O from H
O
2 2
III
IV
valent complexes, [Mn (Me
2
EBC)Cl
2
]PF
6
8
and [Mn (Me
2
EBC)-
18
18
(
1.7 ( 0.4% incorporation of O from H
2
O vs 1.7 ( 0.3%
(OH) ](PF , were synthesized as reported. Direct reaction between
2
)
6 2
18
incorporation of O from normal water in cis-stilbene oxide, and
olefins, such as norbornylene and cis-stilbene, and either the
manganese(III) or the manganese(IV) complex does not occur in
solution even after standing for days at room temperature. Clearly
3
.8 ( 0.4% vs 2.6 ( 1% in trans-stilbene oxide). It is concluded
that oxygen transfer from a manganese oxo species is not
responsible for epoxidation in this manganese complex catalyzed
reaction. This is consistent with the failure (above) of [Mn (Me -
2
EBC)(OH)
IV
†
The University of Kansas.
‡
2+
The Procter and Gamble Company.
2
]
to react directly with olefins and with the absence
17170
9
J. AM. CHEM. SOC. 2005, 127, 17170-17171
10.1021/ja055413k CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Epoxidation of Various Olefins by Mn(Me2EBC)Cl2 with
Scheme 2
H2O2a
substrate
product
yield (%)
cyclohexene
cyclohexene oxide
cyclohexen-1-one
styrene oxide
18.0
13.3
45.5
2.8
styrene
benzaldehyde
norbornylene
cis-stilbene
norbornylene oxide
cis-stilbene oxide
trans-stilbene oxide
benzaldehyde
32.0
17.5
2.0
high charge of manganese(IV) and the associated proton polarize
the O-O bond, jointly promoting the electrophilic epoxidation of
2.6
6
the olefin. As has been described by others, similar mechanisms
almost certainly apply in the epoxidations of olefins by organic
a
Reaction conditions: solvent, acetone/water (4:1), catalyst (1 mM),
olefin (0.1 M), 50% H2O2 (1 mL), added stepwise by 0.2 mL/0.5 h, rt,
yield determined by GC with internal standard.
peracids and such inorganic analogues (Scheme 2).5
IV
To capture the expected main reactive intermediate, [Mn (Me
2
-
+
of any products from a possible transient Mn(V) (from dispropor-
tionation of Mn(IV) in base, also above).
Catalytic epoxidation of various olefins by Mn (Me
using 50% H
Greater yield of cyclohexene oxide (18%) versus cyclohexen-1-
one (13%) supports multiple, or at least dual, mechanisms of
oxidation by this catalyst. Further, the dominant cis-stilbene oxide
EBC)(O)(OOH)] , for the epoxidation reaction described here, the
II
2
mass spectra were studied for solutions in which [Mn (Me EBC)-
II
2+
2
EBC)Cl
2
,
(OH
2
2
) ]
was actively catalyzing the oxidation reaction. Indeed,
-
2 2
O , gave substantial yields of epoxide (Table 1).
those mass spectra show the presence of the HO
2
complex,
IV
+
[Mn (Me
2
EBC)(O)(OOH)] , via a moderate ms peak at m/z )
+
358. Accurate mass measurement is definitive (M calcd 358.1777;
found 358.1761).
(
17.5%) (vs minor trans-stilbene oxide (2.0%)) could not be the
In conclusion, this is the first demonstration that the hydrogen
peroxide adduct of a high valent manganese complex serves as the
key active intermediate in an epoxidation reaction. This conclusion
extends the expectation of oxidations by manganese catalysts to
include Lewis acid catalyzed peroxide oxidations.
2a
product of oxidation by a peroxy radical. The similarly high yield
of styrene oxide (45.5%), with little benzaldehyde, also suggests a
nonradical mechanism.
To substantiate the origin of the oxygen in the epoxide, cis-
stilbene was oxidized catalytically with 2% H
2
O
2
under an
. For the cis-stilbene oxide product, the deviation
in the 16O content from 100% approximates experimental error (3.6
0.5 vs 1.7 ( 0.3% in the blank); essentially, all of the cis product
derives its oxygen from sources other than O , ruling out a peroxy
18
Acknowledgment. Support by the Procter and Gamble Com-
pany is deeply appreciated, and we also acknowledge the NSF/
ERC Grant (EEC-0310689) for partial support. At KU, mass
spectral measurements were performed by R. C. Drake.
atmosphere of
O
2
(
2
radical pathway for the dominant product. The small amount of
Supporting Information Available: Experimental procedures for
trans-stilbene oxide (∼2%) contained a substantial fraction (16 (
IV
2+
epoxidations; mass spectrum of [Mn (Me
2
EBC)(OH)
2
]
under oxida-
3
%) of oxygen from 18
O , implicating the expected radical pathway.
2
tive conditions; GC/MS graphs for olefin epoxidation reactions. This
material is available free of charge via the Internet at http://pubs.acs.org.
18
The small value of the fraction of O in the trans isomer may
reflect the abundance of hydrogen peroxide in this experiment.
In the complementary experiment in which the label resides on
the peroxide, epoxidation of cis-stilbene with 2% H O (90% O;
2 2
used as received due to cost) under air, incorporation of 18O in
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1
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IV
+
that a “peroxo manganic acid”, Mn L(O)(OOH) , is formed by
IV
+
ligand exchange between Mn (Me
2
EBC)(O)(OH) and H
2 2
O . The
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J. AM. CHEM. SOC.
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