Efficient epoxidation of electron-deficient olefins with a cationic manganese complex

Article abstract of DOI:10.1021/ja029962r

The complex [Mn^II(R,R-mcp)(CF₃SO₃)₂] is an efficient and practical catalyst for the epoxidation of electron-deficient olefins. This catalyst is capable of epoxidizing olefins with as little as 0.1 mol % catalyst in under 5 min using 1.2 equiv of peracetic acid as the terminal oxidant. A wide scope of substrates are epoxidized including terminal, tertiary, cis and trans internal, enones, and methacrylates with >85% isolated yields. Copyright

Full text of DOI:10.1021/ja029962r

Published on Web 04/15/2003
Efficient Epoxidation of Electron-Deficient Olefins with a Cationic Manganese
Complex
Andrew Murphy, Geraud Dubois, and T. D. P. Stack*
Department of Chemistry, Stanford UniVersity, Stanford, California 94305
Received December 30, 2002; E-mail: stack@stanford.edu
The catalytic epoxidation of terminal and electron-deficient
olefins remains a challenging task in organic chemistry. Few
catalytic methods exist that are operationally efficient and readily
scaled using simple oxidants. Recent reports have detailed several
impressive catalytic systems for such olefins, but each system
has limitations that include expensive catalysts, long reaction
times, high catalyst loadings, or limited substrate scope.1-10 Here
we report a highly selective and efficient manganese complex, [MnII-
(R,R-mcp)]2+ (Figure 1), that can rapidly (<5 min) epoxidize a
wide range of olefins at room temperature with low catalyst loadings
using 1.2 equiv of commercially available peracetic acid. The
reactions are readily scaled from 5 mg to 25 g within minimal
solvent, and the isolated yields are generally >85%.11
Figure 2. ORTEP drawing [MnII(R,R-mcp)(CF3SO3)2] (50% thermal
ellipsoids). The average Mn-N and Mn-O distances are 2.25 and 2.17 Å,
respectively. The N(2)-Mn(1)-N(4) angle is 172°.
products. Metal-based catalysts may potentially maximize the
efficiency of this oxidant by minimizing unproductive chemistry.
[MnII(mep)(CF3SO3)2] (2) and commercially available peracetic
acid create a powerful epoxidation agent in MeCN at ambient
temperatures. Terminal olefins are rapidly and efficiently epoxidized
(vinyl cyclohexane; 1.2 equiv of CH3CO3H, <5 min, >90%), but
Figure 1. Tetradentate ligands mep and R,R-mcp.
complete conversion generally requires 5 mol % catalyst loading.
Control experiments, including one that shows that simple man-
ganese salts are ineffective as catalysts under these conditions,
The coordination geometries of the iron sites in heme and non-
heme iron enzymes that epoxidize olefins provide inspiration for
small-molecule catalyst design.12-14 Notable in the non-heme iron
suggested that catalyst degradation was the limiting factor with 2.
area is the recent report of [FeII(mep)]2+, that performs rapid,
[MnII(R,R-mcp)(CF3SO3)2] (1) is a more kinetically and ther-
efficient epoxidations of terminal olefins using H2O2,1 albeit with
modynamically stable complex under oxidative, acidic conditions
moderate catalyst loading (∼3%) and somewhat capricious selectiv-
(Figure 2).29 Under conditions described above, 1 efficiently
epoxidizes vinyl cyclohexane with 0.1 mol % of catalyst at a rate
greater than 250 turnovers min-1 (Scheme 1) at ambient temper-
atures.30 This catalyst also rapidly epoxidizes electron-rich di- and
trisubstituted olefins in addition to the electron-poor olefins of allyl
acetate, methacrylate, and 2-cyclohexen-1-one.31,32 The more
electron-deficient olefins (entries 9, 10, 11, 15, Table 1) require
slight adjustments of the reaction conditions; a slight increase in
either the catalyst loading or the amount of peracetic acid is
generally sufficient to achieve high conversion and yields.
ity. Curiously, a near identical chiral variant, [FeII(R,R-mcp)]2+
,
is a poor epoxidation catalyst under similar conditions even though
the iron centers in each complex are ligated in a cis-R coordination
geometry.15 The manganese analogues of these complexes, [MnII-
(R,R-mcp)(CF3SO3)2] (1) and [MnII(mep)(CF3SO3)2] (2), are ef-
fectively isostructural to the iron complexes with two cis-bound
CF3SO3- anions completing the manganese octahedral coordination
(Figure 1).16 These labile-ligand cis-sites allow the formation of
dimeric MnII and MnIII complexes under aqueous conditions,17-20
and such complexes are generally good H2O2 disproportionation
catalysts. Indeed, the efficient H2O2 disproportionation by 1 and 2
precludes its use as an effective oxidant. However, other oxidants
such as peracids, alkylperoxides, or iodosobenzene are effective
oxidants with other manganese catalysts.8,21-26 Particularly relevant
is the epoxidation of terminal olefins with specialized Mn-
porphyrin complexes and specially prepared peracetic acid.27
Commercially available peracetic is an inexpensive, readily
available oxidant that is generally overlooked for epoxidation
chemistry due to its strongly acidic composition (1% H2SO4).28 Yet,
peracetic acid is capable of epoxidizing terminal olefins without a
catalyst at elevated temperatures (>60 °C) over hours. Such
conditions often lead to significant amounts of epoxide ring-opened
Scheme 1. Epoxidation of Olefins with 1
As with many oxidation catalysts, the nature of the active oxidant
is difficult to assess. The addition of 3 equiv of peracetic acid to a
1 mM solution of high-spin 1 (µeff ) 5.6 µB)33 in MeCN induces
a slight color change from pale-tan to a light-brown at 238 K, but
neither the broad isotropic g ) 2.0 MnII EPR signal (77 K) nor the
magnetic susceptibility of this mixture is significantly altered from
a solution of pure 1. Under these lower-temperature conditions,
epoxidation of cyclooctene efficiently occurs. Together, the data
9
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J. AM. CHEM. SOC. 2003, 125, 5250-5251
10.1021/ja029962r CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Epoxidations Catalyzed by 1 with CH3CO3Ha
analogues are in progress to understand the nature of the active
oxidant and improve on the low observed enantio- and diastereo-
selectivities.34
oxidant
(equiv)
GC
yieldb
isolated
yieldc
alkene
mol % 1
1
2
3
4
5
6
7
8
cyclooctene
cyclohexene
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.2
0.5
0.1
1.0
1.0
0.5
0.5
1.2
1.2
2
1.2
1.2
1.2
1.2
1.2
2
1.2
1.2
1.2
1.2
1.2
3
99 (1)
98 (2)
92 (3)
99 (1)e
99 (1)f
97 (1)
95 (3)
99 (1)
89 (3)
98 (1)
97 (4)
94 (4)h
90 (1)
97 (1)
98 (1)i
97 (2)j
90 (4)d
85 (2)
Acknowledgment. We are grateful to the National Institutes
of Health (Grant GM50730) for financial support of this work.
1-methyl-cyclohexene
cis-2-heptene
trans-2-heptene
2-methyl-1-pentene
1-heptene
vinyl cyclohexane
allyl acetate
methyl methacrylate
2-cyclohexen-1-one
ethyl sorbateg
Supporting Information Available: Experimental procedures for
the synthesis of 1 and 2, and for the epoxidation of olefins with 2 (PDF).
Crystallographic data of 1 and 2 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
89 (3)
90 (2)
9
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11
12
13
14
15a
15b
86 (6)
88 (2)
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1
a Olefin (0.5 M in CH3CN), 32% CH3CO3H in acetic acid/water,
25 °C, 5 min. b Yields determined by GC versus internal standard, average
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epoxide, 2% trans-epoxide. 97% trans-epoxide, 3% cis-epoxide. g trans,trans-
f
CH3CH2dCH2CH2dCH2-CO2CH2CH3. h 4:1 mixture of 4,5-monoepoxide
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suggests the resting form of the catalyst is MnII and that the optical
changes are associated with catalyst decomposition. A Lewis-acid
assisted oxidation is possible, but [ZnII(R,R-mcp)]2+, a redox-
innocent complex, is unable to epoxidize olefins under identical
conditions. Thus, a higher-valent manganese species is suggested.
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e.g. allyl acetate < vinyl cyclohexane < cyclooctene (1:15:120).
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oxidant in the solution.
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further supports an electrophilic oxidant with 1. Ethyl sorbate yields
a 4:1 mixture of the 4,5- and 2,3-monoepoxide, respectively. High
regioselectivity is possible if the olefin groups are electronically
distinct, and the reaction temperature is reduced. The terminal olefin
of R-carvone is selectively epoxidized at -20 °C in 15 min with 1
equiv of peracetic acid (Scheme 2). Three equivalents of oxidant
at 0 °C gives the diepoxide of R-carvone in high yield. In both
cases, only a modest disastereoselectivity (de) of the terminal olefin
epoxidation is observed (∼20%).
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monoclinic P21, a ) 9.1024(5) Å, b ) 9.7740(6) Å, c ) 16.2918(8) Å,
â) 101.871(2), V ) 1418.4(1) Å3, 2230 reflections (4° < 2θ < 49.42°),
R(Rw) ) 0.045(0.108). Crystal Data for 2: a colorless rhombic crystal
from acetonitrile/ether; orthorhomic Pbca, a ) 9.4259(6) Å, b )
17.4826(9) Å, c ) 30.462(2) Å, V ) 5019.6(4) Å3, 3401 reflections
(4° < 2θ < 43.94°), R(Rw) ) 0.036 (0.095).
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(27) The peracetic acid used in ref 24 is prepared to be sulfuric acid and H2O2
free.
(28) Peracetic acid solutions are generally prepared from H2O2 and HOAc using
1% sulfuric acid catalyst.
(29) The formation constant for [MnIICDTA]2- is ∼4 orders of magnitude larger
than [MnIIEDTA]2-. CDTA ) trans-cyclohexyl-1,2-diamine tetraacetic
acid.
(30) The observed ee’s are small. At room temperature 1/CH3CO3H only gives
a 10% ee for vinyl cyclohexane. [MnII(6-methyl-R,R-mcp)](CF3SO3)2
shows no reactivity under these conditions.
(31) This system does not exclusively provide the epoxide product for sensitive
olefins such as styrene, ethyl acrylate, or R-pinene.
Scheme 2. Epoxidation of R-(-)-Carvone
(32) The dependence of the epoxidation reaction on the counterion is minimal.
-
-
Both the CF3SO3 and CH3CO2 complexes have identical reactivity.
However, the chloride complex provides only 25% conversion of vinyl
cyclohexane under the same conditions and time. Consequently, the
catalyst can be formed in situ by combining the available Mn(OAc)2 salt
and R,R-mcp. In addition, these reactions can be run in an identical manner
when glacial acetic acid is used as the solvent.
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(34) [FeIImep]2+ and [FeIIR,R-mcp]2+ have a similar scope of reactivity but
require higher catalyst loadings. P. Hung, unpublished results.
This system provides an operationally simple and rapid way to
epoxidize a wide scope of olefins using low catalyst loadings.
Mechanistic studies as well as comparative studies of the iron
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