In situ formation of peracetic acid in iron-catalyzed epoxidations by hydrogen peroxide in the presence of acetic acid

Article abstract of DOI:10.1002/adsc.200303204

Iron complexes (tpa)Fe( OTf)₂ (1) and (bpmen)Fe(OTf)₂ (2) [tpa = tris-(2-pyridylmethyl)-amine; bpmen = N,N′-bis-(2- pyridylmethyl)-N,N′-dimethyl-1,2-ethylenediamine] were found to catalyze the in situ formation of AcOOH from H₂O₂ and AcOH in the course of olefin oxidations. While oxidation of cyclooctene by H ₂O₂ catalyzed by 1 gives nearly equimolar epoxide and cis-diol products, introduction of AcOH to the reaction greatly enhances the selectivity for epoxidation. The resulting product distribution is nearly identical to that of cyclooctene oxidation by AcOOH catalyzed by 1. The in situ generation of AcOOH from H₂O₂ and AcOH in epoxidation catalyzed by some iron complexes opens a possibility to attain AcOOH-type efficiency of epoxidation using H₂O₂ as a terminal oxidant.

Full text of DOI:10.1002/adsc.200303204

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In situ Formation of Peracetic Acid in Iron-Catalyzed
Epoxidations by Hydrogen Peroxide in the Presence of Acetic
Acid
Megumi Fujita, Lawrence Que, Jr.*
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota
55455, USA
Fax: ( ‡ 1)-612-624-7029, e-mail: que@chem.umn.edu
Received: November 7, 2003; Accepted: February 9, 2004
decomposition. Development of catalytic epoxidation
Abstract: Iron complexes (tpa)Fe(OTf)2 (1) and
(bpmen)Fe(OTf)2 (2) [tpa ˆ tris-(2-pyridylmethyl)-
[4]
methods with H2O2 or AcOOH[5] thus stands as an
important objective.
amine; bpmen ˆ N,N'-bis-(2-pyridylmethyl)-N,N'-di-
Non-heme iron complexes have recently been inves-
methyl-1,2-ethylenediamine] were found to catalyze
tigated as oxidation catalysts for olefins.[5 8] We reported
the in situ formation of AcOOH from H2O2 and
that [(tpa)Fe(CH3CN)2]2‡ (1) and [(bpmen)Fe
AcOH in the course of olefin oxidations. While
(CH3CN)2]2‡ (2) (Scheme 1) catalyze epoxidation by
oxidation of cyclooctene by H2O2 catalyzed by 1
H2O2, accompanied by differing extents of cis-dihydrox-
gives nearly equimolar epoxide and cis-diol prod-
ylation.[7] Jacobsen found 2 with SbF6À counterions to be
ucts, introduction of AcOH to the reaction greatly
an efficient epoxidation catalyst with H2O2 in the
enhances the selectivity for epoxidation. The result-
presence of AcOH under synthetically more practical
ing product distribution is nearly identical to that of
conditions.[8] Similarly, Stack recently reported
cyclooctene oxidation by AcOOH catalyzed by 1.
[{(phen)2FeIII(OH2)}2(m-O)]4‡ (3) as a highly efficient
The in situ generation of AcOOH from H2O2 and
catalyst for epoxidation by AcOOH.[5b] Our interest in
AcOH in epoxidation catalyzed by some iron com-
developing bio-inspired epoxidation catalysts prompted
plexes opens a possibility to attain AcOOH-type
us to investigate how the reaction conditions utilized by
efficiency of epoxidation using H2O2 as a terminal
the three different groups may relate to each other. In
oxidant.
the course of this study, we have determined that 1 and 2
catalyze in situ formation of AcOOH from H2O2 and
AcOH, affording the possibility of developing an olefin
epoxidation system with high efficiency with the use of
Keywords: dihydroxylation; epoxidation; hydrogen
peroxide; iron; N ligands; peracetic acid
inexpensive components.
(tpa)FeII(OTf)2 (1): We have carried out the most
extensive investigation on 1, because the 1-catalyzed
Epoxidation of olefins is an important reaction in oxidation of cyclooctene with H2O2 gives an epoxide/cis-
organic synthesis. While organic peracids such as 3- diol ratio of close to 1, which serves as a sensitive
chloroperbenzoic acid are widely used as reagents, the indicator of mechanism.[7b d] The product distribution
major drawback is the toxicity of the organic acid was examined under oxidant-limiting conditions (sub-
byproduct.[1] Peracetic acid (AcOOH) and hydrogen strate:oxidant:catalyst ˆ 500:4.5~14.5:1, Conditions A)
peroxide (H2O2) are more desirable oxidants because of to obtain accurate product ratios and to minimize effects
the non-toxic byproducts (acetic acid and water, re- of possible alteration of a catalyst over time or over-
spectively). Although AcOOH by itself is capable of
epoxidation, it requires prolonged reaction time and
often elevated reaction temperature compared to 3-
chloroperbenzoic acid.[2] AcOOH is readily prepared by
the reaction of concentrated aqueous H2O2 and
AcOOH with a strong acid catalyst, typically sulfuric
acid.[2] On the other hand, H2O2 is a more challenging
oxidant to utilize in epoxidation. Unlike AcOOH, H2O2
by itself is a very poor epoxidation reagent (except
under alkaline conditions where it epoxidizes electron-
deficient olefins[3]), and it is prone to metal-based Scheme 1. Iron complexes studied.
190
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DOI: 10.1002/adsc.200303204
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In situ Formation of Peracetic Acid in Iron-Catalyzed Epoxidations
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oxidation of diol products. The effects of adding AcOH tries 1 5). The suppression of cis-diol formation with
on the oxidation of cyclooctene by H2O2 or AcOOH the addition of AcOH increases the potential utility of 1
catalyzed by 1 are summarized in Table 1 (entries 1 to as an olefin epoxidation catalyst.
11). When 14.5 equivs. (per Fe) H2O2 were gradually
Entry 6 shows the effect of using 10 equivs. AcOOH
added to the solution of cyclooctene and 1 in acetoni- as oxidant instead of H2O2. The amount of epoxide
trile, 4.3 equivs. epoxide and 5.9 equivs. cis-diol were obtained was over three-fold higher than that obtained
formed (entry 1), with overall yield of 70% based on the in the absence of a catalyst (entry C3, Table 1), suggest-
oxidant, consistent with our earlier reports. Notably, the ing that 1 catalyzes olefin epoxidation by AcOOH as
introduction of 2 100 equivs. AcOH into the reaction well. Interestingly, the reaction in entry 6 afforded
mixture significantly enhanced the selectivity for epox- 11.7 equivs. epoxide and 2.0 equivs. cis-diol, a combined
idation and resulted in near quantitative conversion of yield that exceeded the number of equivalents of
oxidant to desired products (Figure 1; Table 1, en- AcOOH added. This result may be rationalized by the
Table 1. Cyclooctene oxidation (Conditions A).
Entry Oxidant and Other Components[a]
Products[a]
Total Yield [%][d]
AcOOH
H2O2
AcOH
H2O
Epoxide[b] cis-Diol[c] Epoxide:Diol
[(tpa)FeII(OTf)2] (1)
1
2
3
4
5
14.5
14.5
14.5
14.5
14.5
4.5
4.5
4.5
4.5
4.5
4.3(5)
6.5
11.8
12.7
13.1
11.7(3)
1.5
5.9(8)
6.8
2.7
2.1
0.8
2.0(2)
1.9
0.8
1.6
1.5
0.7: 1
1: 1
4: 1
6: 1
16: 1
6: 1
1: 1
4: 1
8: 1
8: 1
8: 1
4: 1
70
92
100
102
96
94
76
87
94
2
17
29
100
17
6
10
7
8
17
34
100
200
17
3.1
9
10
10
10
10
12.0
12.7
12.8
10.5
10
11
12
98
99
93
4.5
4.5
1.5
3.0
840[e]
No catalyst
C1
C2
[f]
14.5
14.5
4.5
0.3(1)
0.3(1)
3.4(1)
0.0
0.0
0.0
Nodiol
Nodiol
Nodiol
2
2
23
[f]
17
17
[f]
C3
10
[(tpa)ZnII(OTf)2]
[f]
C4
C5
14.5
14.5
4.5
0.3(1)
0.5
2.8
0.0
0.0
0.0
Nodiol
Nodiol
Nodiol
2
3
19
[f]
29
17
[f]
C6
10
ZnII(OTf)2
[f]
C7
C8
14.5
14.5
4.5
0.4(2)
0.2
3.1
0.0
0.0
0.0
Nodiol
Nodiol
Nodiol
3
1
21
[f]
29
17
[f]
C9
10
tpa ligand
C10
C11
[f]
14.5
14.5
4.5
0.3(2)
0.1(0)
3.4(2)
0.0
0.0
0.0
Nodiol
Nodiol
Nodiol
2
17
17
< 1[f]
[f]
C12
10
23
[(bpmen)FeII(OTf)2] (2)
21
22
23
14.5
14.5
4.5
13.0(3)
14.8(2)
14.1(2)
0.7(1)
0.4(1)
0.3(0)
19: 1
34: 1
47: 1
94
105
99
34
17
10
[{(phen)2FeIII(OH2)}2(m-O)](ClO4)4 (3)
31
32
33
14.5
14.5
4.5
> 53
> 53
> 53
1.7(2)
1.0(0)
9.1(2)
0.2(0)
0.2(0)
0.0
11: 1
5: 1
No diol
13
8
63
29
17
10
[a]
[b]
[c]
[d]
Amounts expressed as equivalents formed per mol iron catalyst.
Retention of configuration was>94% for all cases except for C3 (75%) and C9 (67%).
Retention of configuration was > 99% for all cases.
Total yield of the epoxide and the diol based on the total peroxide content.
[e] 18O-Labeled water (95% 18O).
[f]
Various unidentified byproducts were observed.
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191

Megumi Fujita, Lawrence Que, Jr.
COMMUNICATIONS
C4 C9), but they were ineffective. These results imply
that the redox active Fe center is crucial in the catalytic
behavior of complex 1.
Lastly, an isotope labeling experiment using H218O
(95% labeled) was carried out to ascertain the source of
the epoxide oxygen in the oxidation by AcOOH. The
product distribution was only slightly affected by
addition of 840 equivs. of labeled water (see entry 12
vs. 6). There was no 18O incorporated into the epoxide
product from the labeled water under the conditions of
entry 12. Our previous labeling studies showed that
cyclooctene oxide formed from H2O2/1 in the presence
of excess H218O exhibited approximately 10% 18O
incorporation via a putative HO-FeV O intermedia-
ˆ
Figure 1. Effect of AcOH on epoxide and diol yields from
cyclooctene (from Table 1). Comparison of product yields
from 1-catalyzed oxidation of cyclooctene, expressed as
equivalents formed per mol iron catalyst. Light color: cis-
te.[7c, d] This lack of 18O incorporation from H218O in the
AcOOH reaction indicates minimal involvement of the
1/H2O2 pathway in epoxide formation and strongly
diol; dark color: epoxide. Entry numbers correspond to those suggests an alternative mechanism wherein the iron
in Table 1.
center activates the peracid for epoxidation.
(bpmen)FeII(OTf)2 (2): In contrast to 1, the selectivity
for epoxidation in the 2-catalyzed oxidation of cyclo-
fact that, in the commercial preparation we used, octene by H2O2 is already quite high in the absence of
10 equivs. AcOOH also contained 4.5 equivs. H2O2 AcOH (epoxide/diol ˆ 8) (entry 21, Table 1).[7c] Addi-
and ca. 17 equivs. AcOH, so there was clearly some tion of AcOH further enhanced the selectivity for
contribution of H2O2 to the oxidation yield. Assuming epoxidation and improved the overall yield (entry 22),
that the 2.0 equivs. of diol derived only from the residual and the cis-diol yield became nearly negligible ( < 3%).
H2O2, then 1.5 equivs. epoxide may be expected from These observations are consistent with those reported
the H2O2 component, as suggested by the experiment in by Jacobsen.[8] The effect of AcOH and the similarity
entry 7 where only 4.5 equivs. H2O2 were used as between the [H2O2 ‡ AcOH] run and the AcOOH run
oxidant. Another control experiment with 4.5 equivs. (entry 23) parallel the results from catalyst 1. Thus it
H2O2 and 17 equivs. AcOH (entry 8) afforded more appears that both 1 and 2 catalyze in situ AcOOH
epoxide and less diol. As already shown in entries 1 to 5, formation from H2O2 and AcOH as well as AcOOH
the addition of increasing amounts of AcOH to the epoxidation, but the AcOH effect on H2O2 oxidation is
oxidations by H2O2 resulted in a larger yield of epoxide much less dramatic with 2 than with 1.
and a smaller amount of diol. Similarly, adding more
[{(phen)2FeIII(OH2)}2(m-O)](ClO4)4 (3): Stack re-
AcOH to the oxidations by AcOOH (entries 9 11) ported that the oxo-bridged diion(III) complex 3 was
further enhanced epoxide formation and suppressed an excellent epoxidation catalyst with AcOOH as
diol formation (see also Figure 1). The near identity of oxidant.[5b] Indeed the epoxide yield in our experiment
the epoxide/diol ratio and total product yield in (entry 33) was quite high, 91% when calculated based
entries 3 6 strongly suggests the possibility that 1 can only on the number of equivalents of AcOOH intro-
catalyze the in situ formation of AcOOH from H2O2 and duced. However, the corresponding experiments with
AcOH and then utilize the incipient AcOOH for an equivalent amount of H2O2 (entry 31) or with H2O2
epoxidation. Such in situ conversion of H2O2 to AcOOH and AcOH (entry 32) showed that 3 is not a good
is not commonly observed in oxidation catalysis but has catalyst for epoxidation with H2O2 as oxidant nor does it
been reported for metal-free haloperoxidases in acetate catalyze in situ formation of AcOOH from H2O2 and
buffer.[9] However, to our best knowledge, 1 is the first AcOH.
iron catalyst shown to catalyze formation of AcOOH
from H2O2/AcOH and its subsequent activation for enhanced epoxidation catalysts 1 and 2 have been
substrate oxidations. examined for the oxidation of a terminal olefin, 1-
1-Octene Oxidation, Conditions B: The AcOH-
Control experiments supported this mechanistic octene, by H2O2 under more practical, substrate-limiting
hypothesis. Cyclooctene oxidation with AcOOH in the conditions (1-octene:H2O2:Fe ˆ 1:1.5:0.03, Conditions
absence of a catalyst (entry C3, Table 1) formed B), in the presence or absence of AcOH, following the
3.4 equivs. epoxide. (The epoxide yield in the presence conditions used for [(bpmen)Fe(NCCH3)2](SbF6)2 (2b)
of 1 (entry 1) is>3 times higher, vide supra.) Non-redox- by Jacobsen and coworkers.[8] We have previously
active Lewis acids such as (tpa)Zn(OTf)2 and Zn(OTf)2 reported H2O2-only oxidation under similar conditions
were tested as well for their ability to catalyze either in with [LFe(NCCH3)2](ClO4)2 series, including L ˆ tpa
situ AcOOH generation or olefin epoxidation (entries and bpmen.[7d] In this study we have examined the role of
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In situ Formation of Peracetic Acid in Iron-Catalyzed Epoxidations
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AcOH under these conditions and compared our results Optimization of the conditions to achieve greater
with those reported for 2b. efficiency and investigation of the range of applicable
Enhancement of epoxidation by AcOH was observed olefins are currently in progress. Attaining AcOOH-like
under these conditions as well, with both catalysts 1 and epoxidation character from H2O2 and AcOH by means
2. With 1, the epoxide/diol ratio favored epoxide with > of metal catalysis as found in this study may be
3 equivs. of AcOH per 1-octene (entries 1 4, Table 2). advantageous for developing H2O2-based epoxidation
The selectivity for epoxide increased from 16% to 46% catalysts.
upon addition 20 equivs. of AcOH per 1-octene,
although the degree of mass loss also became higher
(entry 4). The same trend was observed with catalyst 2,
Experimental Section
but a smaller amount of AcOH (0.5 equiv. per 1-octene)
was sufficient to suppress the diol formation to a
negligible level (entry 6), while the selectivity for
epoxide was enhanced from 52% to 64%. The only
difference between 2 and 2b was the counteranion
The AcOOH reagent (Aldrich, 32 wt % in dilute acetic acid)
contains < 6.0% H2O2 and approx. 40 45% acetic acid
according to the reagent description. Iodometric titration
showed that the peroxide content was 1.45 times higher than
the described AcOOH content; thus the 0.45 equiv. was
presumed to come from H2O2. This is equivalent to 6.4 wt %
of the reagent, reasonably matching the reagent description.
The AcOH content was calculated to be 1.6 1.8 equivs. of
AcOOH based on the reagent description, and found to be
1.9 equivs. based on the AcOH/AcOOH ratio in the 13C NMR
spectrum. In this study, the AcOH/AcOOH ratio of the
reagent was assumed to be 1.7. The H2O2 solutions in
acetonitrile were diluted from a 35 wt % aqueous solution
(Aldrich). Acetonitrile was distilled from CaH2 under argon.
Preparations of the iron complexes 1, 2, and 3 were described
previously.[5b,7c,10,11]
(TfOÀ vs. SbF6 ), which can be readily displaced by
À
solvent upon dissolution in CH3CN, but the epoxidation
yield with 2 was not as high as that of 2b with either H2O2
alone or with added AcOH (entries 5 9). Jacobsen also
noted that the SbF6À salt was a better catalyst than the
ClO4À salt, so the counteranions seem to have effects on
the catalyst efficiency.
Although Jacobsen proposed that the role of AcOH
was to generate the acetate-bridged diiron(III) complex
[(bpmen)FeIII(m-O)(m-k2-OAc)FeIII(bpmen)]3‡ in situ
that was responsible for epoxidation catalysis, our
current investigation suggests that iron-catalyzed in
situ generation of AcOOH from H2O2 and AcOH must
play a significant role in the epoxidation mechanism.
In conclusion, complexes 1 and 2 were found to
catalyze in situ generation ofAcOOH in the epoxidation
of olefins by H2O2 in the presence of AcOH. Addition of
AcOH to olefin oxidation by H2O2/1 greatly enhanced
Reaction Conditions A
Solvent acetonitrile and the oxidant solutions in acetonitrile
were dried over MgSO4 and filtered before use in order to
control the water content. The concentration of the oxidant
was not affected by this drying process, confirmed by iodo-
the selectivity for epoxidation over cis-dihydroxylation. metric titration. To
a stirred solution of cyclooctene
Table 2. 1-Octene oxidation (Conditions B)
Entry
AcOH/1-octene[a] /Fe[b]
Conversion
[%][c]
Epoxide
[%][d]
Diol
Selectivity for
Epoxide [%][e]
Mass loss
[%][f]
[%][d]
[(tpa)FeII(OTf)2] (1)
1
2
3
4
0
0.5
3
0
17
100
667
38(2)
31(4)
53(0)
74(0)
6(0)
9(1)
23(2)
34(0)
33(4)
27(7)
19(0)
18(0)
16
30
44
46
11
22
20
[(bpmen)FeII(OTf)2] (2)
5
6
0
0.5
0
17
94(1)
99(1)
49(1)
63(1)
7(0)
1(0)
52
64
38
35
[(bpmen)FeII(NCCH3)2](SbF6)2 (2b)[g]
7
8
9
0
0
1
9
98
> 99
> 99
70
85
82
71
85
82
28
14
17
0.055
0.5
[a]
[b]
[c]
[d]
[e]
[f]
Equivalents per 1-octene.
Equivalents per Fe.
Conversion [%] ˆ [1 (remaining substrate/initial amount of substrate)]  100.
Yields are based on the substrate.
Selectivity for epoxide [%] ˆ (Epoxide yield [%]/Conversion [%])  100.
Mass loss [%] ˆ Conversion [%] (Epoxide [%] ‡ Diol [%]).
From Ref.[8] with 1-decene and catalyst loading 5.5 mol %.
[g]
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Megumi Fujita, Lawrence Que, Jr.
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(1.05 mmol), a catalyst (2.1 mmol Fe) and additives (if appli-
cable) in 2.6 mL of acetonitrile was added a solution of either
H2O2 (70 mM, 0.435 mL for 14.5 equivs./Fe) or AcOOH
(70 mM of AcOOH content, 0.300 mL for 10 equivs./Fe
AcOOH) in acetonitrile at the rate of 14 mL/min via a syringe
pump at r.t. under air. After 5 min of additional stirring,
esterification reagents (acetic anhydride or propionic anhydride
0.5 1.0 mL and 1-methylimidazole, 0.1 0.3 mL) and an inter-
nal standard (naphthalene, 13.7 mM in acetonitrile, 0.50 mL)
were added.[12] After 15 min of stirring, chloroform and ice were
added, and the organic layer was washed with 1 N H2SO4,
saturated NaHCO3, and water, respectively, and dried over
MgSO4. The solution was analyzed by GC for product yields.
[2] D. Swern, Organic Peroxides, John Wiley & Sons, New
York, 1970, Vol. 1.
[3] J.-P. Schirmann, S. Y. Delavarenne, Hydrogen Peroxide
in Organic Chemistry, Edition et documentation indus-
trielle, Paris, 1979.
[4] Recent review, see: B. S. Lane, K. Burgess, Chem. Rev.
2003, 103, 2457 2473.
[5] a) A. Murphy, G. Dubois, T. D. P. Stack, J. Am. Chem.
Soc. 2003, 125, 5250 5251; b) G. Dubois, A. Murphy,
T. D. P. Stack, Org. Lett. 2003, 5, 2469 2472.
[6] W. Nam, R. Ho, J. S. Valentine, J. Am. Chem. Soc. 1991,
113, 7052 7054.
[7] a) C. Kim, K. Chen, J. Kim, L. Que, Jr. J. Am. Chem. Soc.
1997, 119, 3635 3636; b) K. Chen, L. Que, Jr. Angew.
Chem. Int. Ed. 1999, 38, 2227 2229. c) K. Chen, M.
Costas, J. Kim, A. K. Tipton, L. Que, Jr., J. Am. Chem.
Soc. 2002, 124, 3026 3035; d) K. Chen, M. Costas, L.
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e) J. Y. Ryu, J. Kim, M. Costas, K. Chen, W. Nam, L.
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M. Costas, L. Que, Jr., J. Am. Chem. Soc. 2003, 125,
9912 9913.
Reaction Conditions B
Solvent and oxidant solutions were not dried. To a solution of
1-octene (0.24 mmol) and a catalyst (3 mol % Fe) in 1.5 mL of
acetonitrile was added, if applicable, AcOH (7 mL, 0.13 mmol
for 0.5 equiv/1-octene) and the mixture was cooled in an ice/
water bath. To this stirred solution was added a solution of
H2O2 in acetonitrile (0.36 mmol, 1.5 equivs., 0.36 M, 1.0 mL),
precooled in an ice/water bath, via a syringe pump over 40 sec
(with AcOH) or 2.5 min (no AcOH). After 5 min, esterifica-
tion reagents and an internal standard were added and product
yields were analyzed in the same manner as for Conditions A.
[8] M. C. White, A. G. Doyle, E. N. Jacobsen, J. Am. Chem.
Soc. 2001, 123, 7194 7195.
[9] M. Picard, J. Gross, E. L¸bbert, S. Tˆlzer, S. Krauss, K.-
¬
H. van Pee, A. Berkessel, Angew. Chem. Int. Ed. Engl.
1997, 36, 1196 1199.
[10] M. H. Lim, J.-U. Rohde, A. Stubna, M. R. Bukowski, M.
Costas, R. Y. N. Ho, E. M¸nck, W. Nam, L. Que, Jr, Proc.
Natl. Acad. Sci. USA 2003, 100, 3665 3670.
Acknowledgements
This work was supported by the Petroleum Research Fund
administered by the American Chemical Society (38602-AC)
and Department of Energy (DE-FG02 03ER15455).
¬
[11] S. Menage, J. M. Vincent, C. Lambeaux, G. Chottard, A.
Grand, M. Fontecave, Inorg. Chem. 1993, 32, 4766 4773.
[12] This esterification procedure was implemented to facil-
itate the analysis of the diol products. It has been
confirmed that this step does not affect the yield of
epoxides.
References and Notes
[1] H. Heaney, Top. Curr. Chem. 1993, 164, 1 19.
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