Manganese-catalyzed epoxidations of alkenes in bicarbonate solutions

Article abstract of DOI:10.1021/ja025956j

This paper describes a method, discovered and refined by parallel screening, for the epoxidation of alkenes. It uses hydrogen peroxide as the terminal oxidant, is promoted by catalytic amounts (1.0-0.1 mol %) of manganese(2+) salts, and must be performed using at least catalytic amounts of bicarbonate buffer. Peroxymonocarbonate, HCO4^-, forms in the reaction, but without manganese, minimal epoxidation activity is observed in the solvents used for this research, that is, DMF and ^tBUOH. More than 30 d-block and f-block transition metal salts were screened for epoxidation activity under similar conditions, but the best catalyst found was MnSO₄. EPR studies show that Mn²⁺ is initially consumed in the catalytic reaction but is regenerated toward the end of the process when presumably the hydrogen peroxide is spent. A variety of aryl-substituted, cyclic, and trialkyl-substituted alkenes were epoxidized under these conditions using 10 equiv of hydrogen peroxide, but monoalkyl-alkenes were not. To improve the substrate scope, and to increase the efficiency of hydrogen peroxide consumption, 68 diverse compounds were screened to find additives that would enhance the rate of the epoxidation reaction relative to a competing disproportionation of hydrogen peroxide. Successful additives were 6 mol % sodium acetate in the ^tBUOH system and 4 mol % salicylic acid in the DMF system. These additives enhanced the rate of the desired epoxidation reaction by 2-3 times. Reactions performed in the presence of these additives require less hydrogen peroxide and shorter reaction times, and they enhance the yields obtained from less reactive alkene substrates. Possible mechanisms for the reaction are discussed.

Full text of DOI:10.1021/ja025956j

Published on Web 09/13/2002
Manganese-Catalyzed Epoxidations of Alkenes in Bicarbonate
Solutions
Benjamin S. Lane, Matthew Vogt, Victoria J. DeRose, and Kevin Burgess*
Contribution from Department of Chemistry, Texas A & M UniVersity, P.O. Box 30012,
College Station, Texas 77842-3012
Received February 17, 2002
Abstract: This paper describes a method, discovered and refined by parallel screening, for the epoxidation
of alkenes. It uses hydrogen peroxide as the terminal oxidant, is promoted by catalytic amounts (1.0-0.1
mol %) of manganese(2+) salts, and must be performed using at least catalytic amounts of bicarbonate
buffer. Peroxymonocarbonate, HCO4-, forms in the reaction, but without manganese, minimal epoxidation
t
activity is observed in the solvents used for this research, that is, DMF and BuOH. More than 30 d-block
and f-block transition metal salts were screened for epoxidation activity under similar conditions, but the
best catalyst found was MnSO4. EPR studies show that Mn2+ is initially consumed in the catalytic reaction
but is regenerated toward the end of the process when presumably the hydrogen peroxide is spent. A
variety of aryl-substituted, cyclic, and trialkyl-substituted alkenes were epoxidized under these conditions
using 10 equiv of hydrogen peroxide, but monoalkyl-alkenes were not. To improve the substrate scope,
and to increase the efficiency of hydrogen peroxide consumption, 68 diverse compounds were screened
to find additives that would enhance the rate of the epoxidation reaction relative to a competing
disproportionation of hydrogen peroxide. Successful additives were 6 mol % sodium acetate in the tBuOH
system and 4 mol % salicylic acid in the DMF system. These additives enhanced the rate of the desired
epoxidation reaction by 2-3 times. Reactions performed in the presence of these additives require less
hydrogen peroxide and shorter reaction times, and they enhance the yields obtained from less reactive
alkene substrates. Possible mechanisms for the reaction are discussed.
complexes that are active. These include porphyrins,10-22
salens,23-31 1,4,7-triazacyclononane (tacn) derived catalysts,32-41
and iron complexes from tetradentate diamine-dipyridine
Introduction
Aside from dioxygen, hydrogen peroxide is probably the
terminal oxidant of choice with respect to environmental and
economic considerations.1-4 Systems for epoxidation that use
hydrogen peroxide in conjunction with catalytic amounts of
cheap, relatively nontoxic metals, therefore, have the potential
to be viable for the large scale production of inexpensive
products, as well as for more specialized applications in
development, process, and research.
Some of the catalysts developed for the epoxidation of alkenes
with hydrogen peroxide are heterogeneous, for example, zeo-
lites5,6 or hydrotalcites.7 Others are polyoxometalate salts
composed of complex anions often incorporating two or more
metals,8 for example, (R4N)6SiW10Fe(OH2)2O38 (where R )
alkyl).9 There are also several homogeneous coordination
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* Address correspondence to this author. E-mail: burgess@tamu.edu.
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10.1021/ja025956j CCC: $22.00 © 2002 American Chemical Society

Manganese-Catalyzed Epoxidations
A R T I C L E S
ligands.42-44 Organic catalysts for epoxidation have also been
developed, mainly for asymmetric transformations.45,46 Some
selenium,47-49 arsenic,50 and organofluorine51 compounds are
surprisingly active and selective catalysts, but they have obvious
limitations with respect to large-scale “green” processes.
However, the most pertinent category of epoxidation catalysts
here are the “soluble metal oxides”.
accelerate the reaction and protect acid sensitive epoxides from
ring opening. The solvents used (MeNO2 or chlorocarbons) are
not ideal for process reactions, and there may be problems
separating the product from the additive. MTO and tungsten-
based catalysts are suitable for large scale epoxidations, but their
use tends to be constrained by metal toxicity issues. Other
soluble metal oxides have reactivity profiles characteristic of
reactions mediated by free hydroxy radicals, that is, “Fenton
chemistry”.10,72
This paper describes the epoxidations of alkenes via a method
(reaction 1) that has several attributes. It involves reagents and
solvents that have manageable levels of toxicity and proceeds
at room temperature with high selectivity for the epoxide product
and with catalytic turnovers as high as 6700. Several manganese-
Prominent among simple, soluble metal oxide epoxidation
catalysts are systems derived from tungstic acid (H2WO4),
phosphate, and ammonium or phosphonium counterions to act
as phase transfer agents.52-61 Typically, these are formed in situ,
but catalytically active complexes such as (R4N)3{PO4(W(O)-
(O2)2)4} have been isolated and even characterized crystallo-
62
graphically. Noyori showed that terminal aliphatic alkenes
could be epoxidized by this system at 90 °C without organic
solvent by rapid stirring.63 Acid sensitive epoxides such as
phenyl oxirane are not stable to these conditions, however.63
Herrmann and co-workers found 0.1-1.0 mol % methyltrioxo-
rhenium (MeReO3 or MTO) is an epoxidation catalyst that
works in tert-butyl alcohol using 30% H2O2 at room temperature
or below.64,65 The Lewis acidity of the catalyst tends to mediate
the ring opening of sensitive epoxides to diols, but Sharpless’
group found that pyridine66 and other basic additives67-71
(2+) salts can be used, and the cosolvent can be selected from
DMF and tert-butyl alcohol, whichever is more suitable for the
substrate to be epoxidized.
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73
Critical Role of Bicarbonate in the Reaction System.
Control experiments indicate the desired epoxidation reaction
only occurs in the presence of bicarbonate buffer. Attempts to
epoxidize 4-vinylbenzoic acid over a period of more than 24 h,
with and without 1 mol % MnSO4, in phosphate, triethanol-
amine, borate, or MOPS (3-{n-morpholino}propanesulfonic acid
buffer systems, all failed. Further investigations revealed that
the epoxidation is catalytic in bicarbonate. Table 1 shows
reactions for which good yields of epoxides were obtained using
only 0.25 equiv of NaHCO3.
The epoxidation of alkenes in the presence of bicarbonate
alone is known.74,75 Richardson76 has shown that a key aspect
of such reactions is that hydrogen peroxide and bicarbonate
combine in an equilibrium process to produce peroxymonocarbon-
ate77-79 (Figure 1). This entity should not be confused with
sodium percarbonate, the simple cocrystallite of sodium carbon-
ate and hydrogen peroxide,80 that does not epoxidize nonacti-
vated alkenes. The equilibrium that results in the formation of
peroxymonocarbonate is established in minutes, but epoxidation
reactions that rely on this species alone require reactions times
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J. AM. CHEM. SOC. VOL. 124, NO. 40, 2002 11947

A R T I C L E S
Lane et al.
Table 1. Epoxidations of Alkenes Using a Catalytic Amount (25
Mol %) of Sodium Bicarbonate
a Not determined.
Figure 2. Epoxide yield as a function of pH. 1 mol % MnSO4, 10 equiv
of hydrogen peroxide, tert-butyl alcohol/water (1:2), reaction time 1 h. Yield
determined by HPLC verses an internal standard.
Figure 1. Peroxymonocarbonate and sodium percarbonate.
of 1-2 d. For instance, when 4-vinylbenzoic acid was treated
with 10 equiv of H2O2 in tBuOH and 0.2 M NaHCO3, after 24
h, only a 37% yield of epoxide was obtained. Thus, in the
absence of a catalyst, it is epoxidation by peroxymonocarbonate
that is rate limiting, not its formation.
Figure 3. Attempted epoxidation reactions using NaHCO3/tBuO2H.
in minutes. Saturation transfer experiments showed the bicar-
The deprotonation of bicarbonate (pKa ) 10.3) to carbonate
becomes significant at pH values above ∼8-9. This deproto-
nation process should decrease the concentration of bicarbonate
in the equilibrium shown in Figure 1, and this reduces the
amount of peroxymonocarbonate present. The situation may be
further complicated by the deprotonation of the HCO4-, but
this is unlikely to increase the efficiency of the reaction.
Consequently, epoxide yields in the manganese-catalyzed reac-
tions would be expected to decrease at elevated pH values if
peroxymonocarbonate plays a key role in the process. Figure 2
shows that the epoxide yield does, in fact, decrease from around
95% at pH values less than 8.2 to less than 50% at pH 9.0.
Peroxymonocarbonate in equilibrium with NaH13CO3 and
hydrogen peroxide can be observed by 13C NMR.76 In the
current study, when NaH13CO3 and H2O2 were here mixed in
tBuOH, a new 13C NMR signal was observed at δ 158.8 ppm
that we attribute to peroxymonocarbonate. A one-dimensional
saturation transfer NMR experiment81 was used to prove that
the chemical entity responsible for the peak at δ 158.8 ppm
exchanged with the bicarbonate.
bonate resonance exchanged magnetization with the new
resonance at 158.3 ppm, consistent with the formation of
equilibrium quantities of tert-butyl peroxymonocarbonate (reac-
tion 2). Experiments a-d (Figure 3) were used to test combina-
tions of NaHCO3/tBuO2H in potential epoxidation systems. No
reaction was observed with or without manganese. It appears
from these observations that peroxymonocarbonate esters are
not active in the epoxidation reaction.
Special Character of Mn(2+) in the Catalysis
Parallel screens in a 128-well block were used to screen metal
salts for similar epoxidation reactivities to Mn(2+). Figure 4
shows data for the metals in the first transition and some likely
candidates in the second and third. Some metal salts gave less
epoxide than the background epoxidation by NaHCO3/H2O2
without catalyst, presumably because of the relatively rapid
decomposition of the hydrogen peroxide. Several gave epoxide
yields that are comparable to the background level. The third,
and most interesting, group, CrCl3, MnSO4, Mn(OAc)3, and Fe2-
(SO4)3, gave between 27 and 94% epoxide yields, but the best
of these catalysts was MnSO4. A similar screen using MnCl2,
Mn(OAc)2, MnSO4, and Mn(acac)2 showed that there was a
negligible difference in the performances of these salts in the
The reaction of NaH13CO3 with tBuO2H in tBuOH was also
studied by 13C NMR to explore the possibility that organic
peroxides might enter into a similar equilibrium with bicarbonate
and, if they did, that they might be viable epoxidation agents
with or without a manganese catalyst. Just as for the formation
of peroxymonocarbonate, a new resonance (158.3 ppm) slightly
upfield of the bicarbonate signal (160.4 ppm) was established
(81) Claridge, T. D. W. High-Resolution NMR Techniques in Organic Chemistry;
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Manganese-Catalyzed Epoxidations
A R T I C L E S
of substrate alkene causes an increase in line width but no
change in hyperfine splittings in the six-line pattern. When the
initiation of the reaction by the addition of H2O2 has occurred,
the g ) 2 signal immediately diminishes in amplitude and a
broad signal at g ≈ 4 grows in. This new g ≈ 4 signal persists
as the reaction progresses (Figure 5, inset), while, at the 12-
minute time point, the six-line g ≈ 2 EPR signal from Mn(2+)
is no longer visible. At the end of the reaction (∼24 min), the
g ≈ 4 EPR signal is not detected and the six-line g ≈ 2 EPR
signal from Mn(2+) reappears.
EPR signals at g ≈ 4 are characteristic of high-spin Mn(4+)
3
S )
/ species having D > hν, which also have a lower
2
amplitude feature at g ≈ 2.82 In addition to the g ≈ 4 signal,
the EPR spectrum from the 12 min time point has a feature at
g ≈ 2 which lacks the six-line hyperfine splitting characteristic
of Mn(2+) and may arise in part from the S ) 3/2 species. The
presence of Mn(4+) suggests a two-electron oxidation of
Mn(2+) to a high-valent Mn(4+)-oxo species as a potential
epoxidation catalyst in the system. Broad signals at low magnetic
fields also have been associated with Mn(3+) S ) 2 states, such
as in Jacobsen’s Mn-salen catalyst.83 These two possibilities
can be distinguished using parallel-mode EPR spectroscopy (Bext
|| Bmw), since the even-spin Mn(3+) systems can give strong
signals under those conditions.84 By contrast with the Mn(3+)
catalysts, the Mn(2+)-initiated reaction mixture showed no
evidence for a Mn(3+) species on the basis of parallel mode
EPR spectroscopy (Supporting Information), providing further
evidence that Mn(4+) predominates in the reaction.
Figure 4. Screen for epoxidation activity with various metal salts. Yield
determined by HPLC verses an internal standard (benzoic acid).
Solvent Effects
All the experiments described so far involve a slightly water-
soluble substrate. To accommodate lipophilic substrates, the
aqueous components were added slowly, and the buffer
concentration was set at 0.2 M NaHCO3 or less. DMF and
tBuOH proved to be the best cosolvents. Other solvents such
as acetone, acetonitrile, methanol, ethanol and DMSO were
inferior, in some cases because they were unstable to the reaction
conditions. Several attempts were also made to effect the
epoxidation reaction in a biphasic solvent system. Mixtures of
aqueous bicarbonate buffer with methylbenzene, dichlo-
romethane, diethyl ether, ethyl acetate, or pentane gave little
or no reaction. In separate experiments, a surfactant, Tween 20,
was added to these reactions, but this had no significant effect.
A phase transfer catalyst, (n-Bu)4NCl, was also added to the
methylbenzene, dichloromethane, and ethyl acetate systems, but
no epoxidation was observed.
Figure 5. EPR spectra of the reaction mixture prior to the addition of H2O2
(top) and at times following the addition. Reaction aliquots were also
measured for epoxide product (inset). Conditions: 0.2 mM MnSO4, 20 mM
4-vinylbenzoic acid, 100 mM carbonate in 1:2 BuOH/H2O, 10 equiv of
H2O2. EPR spectra: 9.4 GHz, 7K, 2mW microwave power.
Additives to Enhance the Reaction Efficiency
Preliminary studies had revealed two disadvantages of the
manganese-mediated epoxidation reaction that could be impor-
tant under some circumstances. First, terminal, monosubstituted,
aliphatic alkenes were unreactive. This may be an adVantage
in cases, where selectivity for an internal alkene was required,
but that is not always the case. Moreover, our preliminary studies
did not encompass 1,1-dialkyalkenes or acyclic 1,2-dialkylalk-
enes, so it was unclear whether these would react or not. Second,
t
epoxidation reaction. Fourteen different lanthanide(3+) sulfates
(Ln2(SO4)3; Ln ) La, Nd, Gd, Ho, Yb, Ce, Sm, Tb, Er, Lu, Pr,
Eu, Dy, Tm) were also screened as a potential epoxidation
catalyst in the reaction, but none of these were active.
EPR spectroscopy was performed to observe the Mn com-
ponent during the epoxidation reaction (Figure 5). The initial
solution containing Mn(2+) and bicarbonate buffer gave a six-
line pattern centered around g ) 2, consistent with high-spin S
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Britt, R. D. J. Am. Chem. Soc. 1999, 121, 4714-4715.
5
) /2 Mn(2+) (see the Supporting Information). The addition
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A R T I C L E S
Lane et al.
Figure 6. Effects of additives on epoxide yields relative to the process
without the additive that gave 85% of the epoxide when 2 equiv of hydrogen
peroxide were used. All yields were measured by HPLC relative to benzoic
acid as an internal standard.
Figure 7. Effects of additives (in triplicate, error bars represent one standard
deviation from the mean) relative to the process without the additive that
gave 22% when 2 eq of hydrogen peroxide were used. All yields were
measured by HPLC relative to benzoic acid as an internal standard.
10 equiv of hydrogen peroxide were required. Hydrogen
peroxide is a relatively cheap bulk chemical, but it would be
useful to reduce the excess of this reagent in very large-scale
reactions from a cost and safety perspective. For these reasons,
a series of catalytic additives were screened to test for possible
beneficial effects.
only 2 equiv of hydrogen peroxide; the yield in the reaction
without additive was then 47%, giving more scope for improve-
ment. The averaged data generated from this screen are
presented in Figure 6, with error bars representing one standard
deviation from the mean. The comparison of Figure 6 with data
from the initial screen (Supporting Information) shows that the
initial screen was quite reliable, since the absolute values did
not change significantly and the relative rankings of the additive
effects were much the same in the two sets of experiments. Of
these 22 additives, 13 were carboxylic acids or carboxylates.
Three of the other nine included a diaryl phosphate, phenyl
phosphonic acid, and a sulfonic acid. The more basic additives
tended to decrease the yields. The data in Figure 6 show that
there is no one particular additive that enhances the epoxide
yield more than the others.
Initially, 68 different additives were screened under the
t
reaction conditions indicated in Figure 6 (for BuOH), and the
same ones were also tested for DMF as cosolvent (Figure 7). A
128-well screening block was used throughout. The additives
were selected to include both organic and inorganic compounds
with a diversity of potentially relevant characteristics. These
included acidic and basic functionalities, compounds that could
coordinate Mn2+ very well (like EDTA) and others that cannot,
carboxylates that can bridge metal centers, and compounds with
activated carbonyl groups that may react with peroxide anions
in situ. In the initial screens (Supporting Information), 6 equiv
of hydrogen peroxide were used and the reactions were
performed only once. From these screens, a set of additives that
gave positive effects on the yields were identified. These were
then re-tested, in triplicate, using only 2 equiv of hydrogen
peroxide. The purpose of repeating the experiments in triplicate
was just to gauge the accuracy of the screen, and the motivation
for reducing the amount of hydrogen peroxide was to expose
additives that could increase yields significantly from relatively
low levels (i.e., from the relatively low yields obtained using
only 2 equiv of hydrogen peroxide and no additive). If the
additive reduced the yields relative to that of the reaction with
no additive, that is a negative relative yield, but if an additive
increased the yield that is shown as a positive relative yield.
t
The screening strategy described above for BuOH was
repeated using DMF as a cosolvent to test if the effects of
additives were similar in this solvent. It became evident early
in this phase of the work that they were not. In DMF, salicylic
acid caused the maximum yield enhancement (Figure 7).
However, we were unable to identify any recurring molecular
similarities between the additives that gave yield enhance-
ments: some are acids, some are bases, and some are inorganic
compounds. In fact, there were clearer similarities between
compounds that inhibit the reaction. Some chelating ligands such
as EDTA, bipyridine, and catechol diminished the yields
(Supporting Information).
At this stage, there was no basis to suggest that 10 mol %
additive (as used in the experiments outlined in Figures 6 and
7) was optimal. This is a typical multivariable problem in
catalysis. The ideal conditions cannot be determined with
certainty unless all the additives are tested at a comprehensive
range of relative concentrations. However, an indication of the
t
Of the 68 additives screened using BuOH as cosolvent, 22
resulted in a yield increase relative to that of the reaction without
additive. The 22 additives that increased the yield in the
preliminary assay were then screened three more times, but using
9
11950 J. AM. CHEM. SOC. VOL. 124, NO. 40, 2002

Manganese-Catalyzed Epoxidations
A R T I C L E S
importance of additive concentrations was obtained by retesting
the additives that gave positive yield enhancements, but this
time, at 0.1, 0.5, 1.0, 2.0, 4.0, 6.0, 10, and 30 mol % levels.
These screens were done in triplicate; this resulted in too much
data to present effectively in one plot. Instead, the data sets for
the best four additives in each cosolvent are shown here in
t
Figure 8a and b (corresponding to BuOH and to DMF,
respectively). Standard deviations for these data were calculated
and are given in the Supporting Information.
Of the additive mol % levels tested, 10 mol % was not the
t
best for any of the reactions in BuOH as cosolvent. The ideal
levels were in the 2-6 mol % range, that is, of the same order
of magnitude as the catalysts loading. High concentrations of
the diacid adipic acid did not increase but actually depressed
the epoxide yield. Thus, the mol % additive level is critical in
some cases. Sodium acetate (6 mol %) was the best additive/
t
loading identified for the BuOH cosolvent using 2 equiv of
hydrogen peroxide. In DMF, similar trends were observed. Even
though the additives were different, the optimal levels in most
cases were shown to be in the 4-6 mol % range. In this solvent,
the best additive identified was salicylic acid at 4 mol % loading.
The goal of screening additives was to increase the epoxide
yields, while decreasing the amount of hydrogen peroxide used.
Clearly, these screens enabled that. However, it was also of
interest to study, at least in one case per solvent system, the
effect of an influential additive on the rate of a typical
epoxidation reaction. A kinetic study was, therefore, performed
using UV to monitor the progress of the reaction of 4-vinyl-
benzoic acid in the presence of 1 mol % MnSO4, 6 mol %
sodium acetate in 0.2 M NaHCO3/tBuOH. An identical study
was performed using 4 mol % salicylic acid in DMF as
cosolvent. In both cases, a large excess of hydrogen peroxide
was used (750 equiv) to attempt to impose pseudo-first-order
t
kinetics with respect to the substrate. In the BuOH case, the
observed rate constants were 4 × 10-4 mol s-1 without additive
but 8 × 10-4 mol s-1 in the presence of 6 mol % NaOAc. Thus,
the additive approximately doubled the rate of the epoxidation
reaction. For DMF without additive, kobs was 7 × 10-4 mol
s-1, but in the presence of 4 mol % salicylic acid, it increased
to 2 × 10-3 mol s-1, that is, just under a 3-fold increase.
Scope of the Epoxidation Reaction
Seventeen alkenes were studied as epoxidation substrates in
the preliminary communication of this work. Data for 10 of
these alkenes are shown again in Table 2. The epoxide yields
for these particular reactions were very high in most cases.
Table 3 presents a comparison of the epoxidation reactions
without and with salicylic acid. The first six alkenes shown are
all ones that had been epoxidized in the preliminary study, and
the data for those experiments are presented in the “no additive”
column. A 10-equiv amount of hydrogen peroxide was used
for those experiments. In the presence of 4 mol % salicylic acid,
however, these same substrates were epoxidized in higher yields
and with only 5 equiv of hydrogen peroxide. In the first entry,
even less hydrogen peroxide was used (2.8 equiv), and this
seemed to be beneficial.
Figure 8. Effects of additives at a range of mol % values in the reactions
indicated: (a) for BuOH as cosolvent and (b) for DMF as cosolvent. A
2-equiv amount of H2O2 was used. All reactions were repeated in triplicate,
and the values shown are the average.
t
The last five entries in Table 3 address issues that were not
explored previously. For instance, 2,3-dimethylbut-2-ene is the
first tetraalkyl-substituted alkene to be studied in this reaction.
It was shown that this material is epoxidized in high yields using
only 5 equiv of hydrogen peroxide in the presence of salicylic
acid additive. Dialkyl-substituted alkenes reacted less readily
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A R T I C L E S
Lane et al.
Table 2. Epoxidations of Representative Alkenes without
Additives
Table 3. Comparison of Epoxidations of Representative Alkenes
without and with Catalytic Salicylic Acid
a Determined by GC versus dodecane internal standard; yields in
parentheses were isolated. b Isolated as a 1.00:1.45 cis/trans mixture.
c Isolated as a 1.00:1.10 cis/trans mixture.
a Unless otherwise specified, the reactions were performed using 10
equiv. of hydrogen peroxide, 0.01 equiv. of MnSO4 on a 1-mmol scale;
yields determined by NMR or GC versus an internal standard. b The
corresponding anthraquinone (35%) was also observed. c Trans-3-phenyl-
propenal was also observed (16%). d In place of DMF, tBuOH used.
e Isolated as the methyl ester. f 0.001 equiv. of MnSO4 was used. g 1-mol
scale.
Conclusions
The work described in this paper has led to a cheap and
environmentally compassionate method for epoxidation. High
throughput screening enabled us to identify additives that
increase the efficiency of the reaction with respect to hydrogen
peroxide consumption. Screening and spectroscopic techniques
have also allowed us to identify some key features of the
reaction that reflect upon its mechanism. Bicarbonate buffer
seems to be essential for the epoxidation process, and this led
to the implication of peroxymonocarbonate as a key molecular
entity. This assertion is supported by the NMR experiments with
NaH13CO3 and by the pH dependence of the reaction. The fact
in these epoxidation processes, so more hydrogen peroxide was
used to force the reaction. Trans- and cis-oct-4-ene were
epoxidized to give 75% yields of product when an additive was
used. In the absence of additive, the yields were less. For the
Z-isomer, the reaction was not stereospecific, and near equimolar
amounts of the corresponding cis- and trans-epoxide were
formed. The compound 2-ethylbut-1-ene was also epoxidized,
though even with additive the product yield was moderate
(51%). The terminal alkyl-substituted alkene 1-decene did not
react to give epoxide, even in the presence of the additive.
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Manganese-Catalyzed Epoxidations
A R T I C L E S
Scheme 1. Epoxidation Mechanism Relying on Mn2+ Acting as a
Lewis Acid
Scheme 2. Formation of the Peroxycarbonate Complex B (a) by
the Direct Reaction of Peroxymonocarbonate and (b) by the
Reaction of a Peroxy Complex with Bicarbonate
that tert-butyl peroxymonocarbonate is inactiVe in the reaction
implies that the alkyl substituent somehow blocks the reaction
pathway.
Throughout this study, the epoxidation reactions were carried
out in open vessels. In the early work, transformations performed
under an inert atmosphere were shown to give almost identical
product profiles, and in all cases, the reactions tend to be quite
clean. This is indicative of a process that does not involve a
high concentration of free radicals, particularly not reactive ones
such as HO• or HOO•. Radical intermediates are implicated in
the loss of stereochemistry seen in the epoxidations of Z-1,2-
diphenylethene and of Z-oct-4-ene. However, the radicals
involved in these reactions are almost certainly carbon-based
ones involved in a stepwise oxygen transfer pathway. For these
reasons, we conclude that epoxidation reactions that proceed
via HO• or HOO• radicals are not prevalent in this reaction.
Scheme 1 shows one of the simplest mechanisms that can
be drawn for the epoxidation process. In this, the manganese
acts as a Lewis acid to facilitate cleavage of the O-O bond by
stabilizing the carbonate leaving group. However, if it is the
prevalent mode of epoxide formation, then the EPR-active
species that are generated as the reaction proceeds must be
largely irrelevant because the metal is not required to undergo
redox processes. Moreover, if it were the predominant pathway,
then it is surprising that most of the metal salts that were
screened did not have similar activities, because many of them
would have comparable Lewis acidities. While this mechanism
cannot be excluded, it is less consistent with the experimental
data than some other possibilities.
Scheme 3. (a) Three Molecular Species Postulated for Oxygen
Transfer, (b) Transfer without Redox at the Mn(4+) Center, and
(c) Concerted and Stepwise Processes for O-transfer with Redox
Chemistry at the Manganese Center
The mechanistic pathways that we regard as most likely all
involve manganese η2-peroxycarbonate complexes. Peroxycar-
bonate complexes are not new. Rhodium85 and platinum86
peroxycarbonate complexes have been formed and characterized
primarily by vibrational spectroscopy. Very recently, the iron-
above -60 °C. At that temperature or below, it did not epoxidize
alkenes, but it would be unwise to infer from this that similar
complexes would not do so.
Scheme 2 outlines routes by which a manganese peroxycar-
bonate complex B could form. One possibility involves the
generation of an equilibrium concentration of peroxymonocar-
bonate, and then coordination of this to Mn(2+). Alternatively,
the coordination of peroxide anion to Mn(2+), deprotonation,
and reaction of this with bicarbonate would lead to the same
intermediate, as indicated. Both routes seem plausible, and they
might be operative simultaneously.
The most critical issue of the epoxidation process is the
oxygen delivery step. Scheme 3a shows species that could
potentially deliver oxygen to the alkene; equilibria of the type
shown would account for the presence of Mn(4+) in the
reaction. Intermediate B might directly epoxidize alkenes, as
shown in Scheme 3b. Alternatively, O-transfer from the
(3+) peroxycarbonate complex A was isolated and characterized
via X-ray crystallography.87 This complex is unstable in solution
(85) Aresta, M.; Tommasi, I.; Quaranta, E.; Fragale, C.; Mascetti, J.; Tranquille,
M.; Galan, F.; Fouassier, M. Inorg. Chem. 1996, 35, 4254-4260.
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Int. Ed. 2002, 41, 1202-1205.
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A R T I C L E S
Lane et al.
carbonate complex C or the free Mn(4+) oxospecies D could
take place, as shown in Scheme 3c. Manganese(4+) complexes
have been shown to be viable epoxidation intermediates, and a
simple interpretation of the mechanism of these reactions is that
a Mn(4+) oxo complex gives the epoxidation reaction.88-91
They might be presumed to act via the direct addition of the
alkene substrate to the oxo ligand with the concerted or
sequential formation of the C-O bonds. The latter is similar to
the Mn(3+)/Mn(5+) catalytic cycles widely accepted for
epoxidations for Mn-porphyrin92-94 and salen95,96 complexes.
The formation of intermediates B-D would not be possible if
tert-butyl peroxycarbonate were used (as observed, vide supra).
None of the mechanisms shown in Scheme 3 can be
eliminated. Moreover, the mechanisms outlined above are
oversimplifications if the active catalyst is di- or oligo-nuclear
(attempts to detect higher molecular mass species in ESI-MS
of the reactions mixtures, however, were negative). The acquisi-
tion of more data to exclude some of these possibilities will be
difficult. The system is hard to study because the active catalyst
forms in situ, it is not stabilized by organic ligands, kinetic
investigations are complicated by the disproportionation of
hydrogen peroxide and the possibility of competing pathways,
and the paramagnetic properties of some of the compounds in
the reaction mixture preclude NMR analyses. However, a
stepwise oxygen delivery via intermediates C or D as implied
in Scheme 3c seems the least speculative mechanistic proposal
based on similarities with Mn-porphyrin92-94 and -salen95,96
mediated epoxidations. Stepwise oxygen delivery via C-radical
intermediates is consistent with cis-trans isomerization, as
observed.
Additives are most useful for the epoxidation of less reactive
alkenes. The data collected in this study indicate that electron-
rich alkenes are most reactive. Thus, the yields of epoxide are
high for aryl-substituted alkenes, even without an additive.
Higher hydrogen peroxide concentrations and special additives
are required to obtain good yields from less reactive substrates
such as dialkyl-substituted alkenes. However, the modified
procedure, with additive, has other advantages that may justify
its use, even with reactive alkenes. The procedure with the
additive uses catalytic bicarbonate, less hydrogen peroxide,
shorter addition times, and the overall reaction is faster.
The main objective of the work presented here was to show
that screening could facilitate the discovery of an experimentally
useful epoxidation system. This has been achieved. The cost
and the lack of toxicity of Mn(2+) salts compares well with
rhenium and tungsten-based catalysts such as MeReO3 and
WO42-. Our catalyst is “ligand free”, and this further reduces
costs and increases experimental convenience relative to the case
of some other reaction types. The solvent systems used (H2O
t
and DMF or BuOH) are appropriate for process chemistry,
being relatively safe, nonhalogenated, and inexpensive. A major
advantage of the featured epoxidation method over virtually all
others is that acid sensitive epoxides are not only stable under
the reaction conditions but are also easily isolated. Methyltri-
oxorhenium is also a viable catalyst for preparations of acid
sensitive epoxides, provided pyridine or other basic additives
are included. However, if the only way to isolate the epoxide
from these basic additives is via an acidic workup, then the
practical value of that approach is diminished. Conversely, in
the work described here, most epoxides are easily isolated via
a simple extraction into an apolar solvent (e.g., pentane or
diethyl ether) and, in many cases, after removal of the solvent,
further purification is unnecessary. It is not essential to use
additives in this process, but if they are used, then the favored
Our efforts to increase the efficiency of hydrogen peroxide
usage in these reactions were successful. Screens with additives
led to the discovery of simple compounds such as sodium acetate
t
(in BuOH) and salicylic acid (in DMF) that enhance the rate
t
of the epoxidation reaction. Without a screening process, it
would have been extremely difficult to identify beneficial
additives. The additives studied could affect the epoxidation
process in different ways that this study did not attempt to
address. However, it does seem plausible that since two of the
most active ones {sodium acetate (in tBuOH) and salicylic acid
(in DMF)} are carboxylates, then the catalytically active species
might be bridged dimers or oligomers.97
ones (NaOAc for BuOH and salicylic acid for DMF) remain
in the basic aqueous layer during the extraction.
The system reported here is experimentally and mechanisti-
cally distinct from any of the well-known epoxidation systems
reported to date. We suspect it will find applications in both
research and process chemistry.
Acknowledgment. We wish to thank Mr. Steve Silber for
help with the presaturation NMR experiments and Dr. Huay-
Keng Loke for assistance with the preliminary EPR studies.
Financial support for this work was provided by The Robert
Welch Foundation. TAMU EPR facilities are supported by the
NSF (CHE-0092010). Use of the TAMU/LBMS-Applications
Laboratory and Dr. Shane Tichy are acknowledged.
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Supporting Information Available: NMR spectra for the
saturation transfer experiments, additional EPR spectra including
control- and parallel-mode experiments, details of the screens
using the additives, and full experimental details. This material
is available free of charge via the Internet at http://pubs.acs.org.
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