A cheap, catalytic, scalable, and environmentally benign method for alkene epoxidations

Full text of DOI:10.1021/ja004000a

J. Am. Chem. Soc. 2001, 123, 2933-2934
2933
A Cheap, Catalytic, Scalable, and Environmentally
Benign Method for Alkene Epoxidations
Benjamin S. Lane and Kevin Burgess*
Department of Chemistry, Texas A & M UniVersity
PO Box 30012, College Station, Texas 77842-3012
ReceiVed NoVember 17, 2000
This paper reports a simple method wherein manganese (2+)
salts, for example, MnSO4, catalyze epoxidation of alkenes using
30% aqueous hydrogen peroxide as the terminal oxidant. The
reactions are performed by dissolving the substrate and catalyst
in DMF or tert-butyl alcohol and then slowly adding a mixture
of 30% hydrogen peroxide and aqueous 0.2 M sodium hydrogen
carbonate buffer. This method has several desirable attributes with
respect to cost, simplicity, and environmental factors.
This project emerged from a control experiment performed
while screening new, chiral, 1,4,7-triazacyclononane (TACN)
complexes as potential asymmetric epoxidation catalysts. High
throughput screens in a simple plate apparatus1 indicated simple
manganese (2+) salts, without any organic ligand, mediated the
epoxidation but only in hydrogen carbonate buffer. There was
no epoxidation in buffers based on triethanolamine, 3-[N-
morpholino]propanesulfonic acid (MOPS), phosphate, or borate.
Alkenes are epoxidized by hydrogen peroxide/NaHCO3 in H2O
(for water soluble alkenes) or in acetonitrile/water mixtures.2,3
We suspected that the transformations in the presence of
manganese (2+) salts were fundamentally different because the
reaction times reported for the metal-free system3 were signifi-
cantly longer than those required in the current study. Moreover,
the rates of epoxidation in the metal-free system were known to
be significantly slower when tert-butyl alcohol was used as the
solvent rather than acetonitrile; however, the former solvent was
effective in the manganese-containing system.
A set of experiments was performed to test for differences
between the metal-free and manganese-containing systems. Figure
1 shows a direct comparison of epoxidation of 4-vinylbenzoic
acid under exploratory, unrefined conditions (i.e., hydrogen per-
oxide added all at once at the beginning of the reaction; tert-
butyl alcohol solvent). These data showed that the extent of con-
version of alkene to epoxide was comparable when 0.1 and 1.0
mol % of manganese sulfate were used. It is less for 0.01 mol %
Mn2+, but still much greater than the background conversion that
occurred when no metal salt was used.
Epoxidation of trans-1,2-diphenylethene was chosen as a model
to optimize the conditions. This lypophilic substrate was selected
so that solubility issues could be addressed using a relatively
difficult case. When the substrate, 10 equiv of 30% hydrogen
peroxide, and 1 mol % MnSO4, were mixed in 0.2 M NaHCO3
(pH 8.0) and DMF (1.0:1.4) and the reaction was stirred for 24
h, the yield of the epoxide was only 20%. Precipitation was
observed in this experiment, indicating solubility prob-
lems. Consequently, slow addition of the aqueous components
was investigated to minimize the precipitation, and the yield of
product increased. Conversely, increasing the buffer concentration
above 0.2 M would be expected to accentuate the insolubility
problem, and indeed lower yields were obtained when higher
buffer concentrations were used. Finally, a set of conditions were
developed wherein a mixture of the buffer and 10 equiv of the
peroxide were gradually added over 16 h to a solution of the
substrate and catalyst in DMF. These reactions gave 1,2-diphen-
ylethene oxide in 92% isolated yield.
Figure 1. Opimization of the number of the hydrogen peroxide/catalyst
stoichiometry. Yield determined by HPLC versus an internal standard.
Error bars represent the standard deviation of two trials
Entries 2, 3, and 16 illustrate that disubstituted aliphatic alkenes
were reactive, and an excellent yield of cyclohexene oxide was
obtained. Oxidation of the tetrahydroanthraquinone (entry 3) gave
a significant amount of the corresponding quinone as a major
byproduct. No Baeyer-Villager oxidation was observed for this
material or in a control experiment using benzophenone as a
substrate (no reaction occurred, data not shown). Entries 4-8
illustrate epoxidations of trisubstituted alkenes. R-Pinene reacted
without cyclobutane rupture (entry 4), and citronellal was
epoxidized without oxidation of the aldehyde functionality (entry
5; NMR). Similarly, the alcohol functionality of 3-methyl-2-buten-
1-ol was preserved in the epoxidation process, and no Payne
rearrangement product was observed either (entry 6). Entry 7
tested for the generation of radical character adjacent the
cyclopropane in the epoxidation, but no cyclopropane opening
was observed. Epoxidation of linalool (entry 8) demonstrated that
trisubstituted aliphatic alkenes can be selectively epoxidized in
the presence of terminal alkenes. This experiment also implies
that the allylic hydroxyl does not activate the terminal alkene via
a directing effect. Entries 9-12 illustrate that epoxidations of aryl-
substituted alkenes proceed smoothly; qualitatively, the rates of
these reactions were observed to be appreciably faster than for
aliphatic alkenes. The only complication was that a sig-
nificant amount of trans-3-phenylpropenal was formed in entry
11. Epoxidation of the acid shown in entry 12 was not ac-
companied by decarboxylation or double bond migration. Some
reactions with less catalyst were then attempted since it was
evident that aryl alkenes were more reactive than aliphatic ones.
Only 0.1 mol % of manganese sulfate was used for the reactions
depicted in entries 13-15, and these epoxidations proceeded
smoothly. Entries 14 and 15 illustrate that even extremely acid-
sensitive epoxides can be formed, and the products are stable
under the reaction conditions. The last entry in the table was
performed on a 1 mol scale; a detailed procedure for preparation
and isolation of 84.5 g of cyclooctene oxide is provided here.4
The featured catalytic epoxidation method has numerous attri-
butes. Manganese (2+) salts are cheap, readily available, and rela-
tively nontoxic, and only small amounts (1.0-0.1 mol %) are
required. Hydrogen peroxide and sodium hydrogen carbonate are
widely used in large-scale production of other chemicals. No
halide is involved in the transformation. Slow addition reduces
the effective concentration of peroxide and the corresponding risk
of explosion. The reaction is run at room temperature in solvents
Table 1 summarizes the data obtained using various alkenes.
1-Decene was unreactive under these conditions (GC; entry 1).
(1) Porte, A. M.; Reibenspies, J.; Burgess, K. J. Am. Chem. Soc. 1998,
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(3) Yao, H.; Richardson, D. E. J. Am. Chem. Soc. 2000, 122, 3220-3221.
10.1021/ja004000a CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/02/2001

2934 J. Am. Chem. Soc., Vol. 123, No. 12, 2001
Communications to the Editor
Table 1. Epoxidations of Representative Alkenes
that are amenable to process chemistry, and no halogenated liquids
or ones with low flash points are required. No organic ligands or
additives are used, and this facilitates isolation of the desired
epoxide.
It is difficult to be certain that the process reported here is
completely unprecedented because studies involving transition-
metal salts and hydrogen peroxide are so ubiquitous.5 However,
it is clear that these Mn-catalyzed reactions are much cleaner than
Fenton’s6 and related systems that generate hydroxyl radicals.7
Several groups have reported epoxidation reactions using H2O2
mediated by TACN-manganese complexes,8,9 but the catalysts
tend to be relatively inaccessible or require larger excesses of
hydrogen peroxide.9-11 One of these studies mentions MnCl2 as
a control, and epoxidation activity was detected, but this finding
was not exploited.10 Most importantly, in prior studies of metal-
catalyzed epoxidations the special importance of bicarbonate in
the media has either not been investigated, realized, or empha-
sized. Investigations to elucidate the role of bicarbonate are in
-
progress. Our working hypothesis is that percarbonate (HCO4
)
formed in situ12 combines with the manganese to give the active
intermediate.
Nearly all of the existing methods for using hydrogen peroxide
as an epoxidation reagent have clear disadvantages compared with
the one reported here. For instance, most of them involve acidic
reagents that tend to decompose the epoxide products.13 One not-
able exception is catalytic methyltrioxorhenium (MTO)14 buffered
with pyridine.15 However, that procedure features a much more
expensive catalyst and media that are explosive or environmentally
hazardous, that is, nitromethane or dichloromethane. Moreover,
separation of acid-sensitive epoxides from pyridine is likely to
be inconvenient for many substrates. On the other hand, the
protocol reported here requires 10 equiv of H2O2 to drive the
reaction to completion, whereas near stoichiometric amounts are
used in the MTO/pyridine method. The two procedures are
(4) Large-Scale Synthesis of Cyclooctene Oxide: DMF (1.68 L) and MnSO4
(1.69 g, 0.01 mol) were placed in a 12 L three-neck flask, equipped with a
mechanical stirrer and a vent to an oil bubbler. Cyclooctene (110 g, 1.00 mol)
was added all at once. The flask was then placed in a water bath at 20 °C
(cryocooler). A 3 L two-neck flask equipped with a magnetic stirrer, was
charged with 20.6 g of NaHCO3, 0.123 g of Na2CO3, and 1.2 L of H2O, and
the pH of the resulting solution was adjusted to 8.0 with 1 M HCl. The flask
was then placed into a water bath maintained at 1 °C and then 1.1 L of 30%
H2O2 was added all at once. The aqueous solution of buffer/peroxide was
then added dropwise to the DMF solution over a period of 36 h via a cannula.
CAUTION! The reaction exotherms if the buffer/peroxide solution is added
too quickly or if heat transfer from the receiving flask is inadequate to maintain
the desired temperature. The reaction mixture was extracted into Et2O (900
mL × 4), washed with brine (900 mL), and dried (Na2SO4). The organic
fraction was concentrated, and residual DMF was fractionally distilled from
it at 5 mmHg. The final product was purified via bulb-to-bulb distillation at
5 mmHg and 57 °C oven temperature, 84.5 g, mp ) 53-55 °C. Small-Scale
Epoxidation Procedure: Similar to the above except that 23 mL of DMF and
17 mL of 0.2 M NaHCO3(aq) were used per 1 mmol of substrate. The aqueous
mixture of H2O2 and NaHCO3 was added dropwise over a period of 16 h.
(5) Jones, C. W. Applications of Hydrogen Peroxide and DeriVatiVes; MPG
Books Ltd.: Cornwall, UK, 1999.
a Unless otherwise specified, the reactions were performed using 0.01
equiv of MnSO4 on a 1 mmol scale; yields determined by NMR or GC
versus an internal standard. b 0.1 mol scale. c The corresponding
anthraquinone (35%) was also observed. d trans-3-Phenylpropenal was
also observed (16%). et BuOH used in place of DMF. f Isolated as the
methyl ester. g 0.001 equiv of MnSO4 were used. h 1 mol scale.
(6) Boguslavskaya, L. S. Russ. Chem. ReV. 1965, 34, 503-15.
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Compounds; Academic Press: New York, 1981.
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V. S.; Jacobs, P. A. Tetrahedron Lett. 1998, 39, 3221-4.
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complementary insofar as aliphatic terminal alkenes are epoxi-
dized by MTO/pyridine, whereas selective epoxidation of the
nonterminal alkenes in the presence of monosubstituted, aliphatic
alkenes is possible in the Mn-catalyzed reactions. In summary,
the epoxidation protocol presented here has the potential to fulfill
unmet needs in exploratory syntheses and large-scale reactions.
Acknowledgment. We thank Dr. D. E. Richardson, University of
Florida, for helpful discussions. This research was supported by The
Robert A. Welch Foundation.
Supporting Information Available: Outlines of optimization pro-
cedures, pilot kinetic study to show dependence of reaction rate on
manganese concentration (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
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v.; Warnaar, J. B.; Wolf, L. v. d.; Krijnen, B. Nature 1994, 369, 637-9.
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Hage, R. Inorg. Chem. 1996, 35, 6461-5; Vos, D. D.; Bein, T. Chem.
Commun. 1996, 917-8; Vos, D. E. D.; Wildeman, S. d.; Sels, B. F.; Grobet,
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JA004000A