Nature of the Metal-Based Oxidant
A R T I C L E S
Chart 1. Representation of the Structures of [LFe (NCCH3)x]2+
II
performed on a HP 5898 GC (DB-5 column, 60 m) with a Finnigan
MAT 95 mass detector or a HP 6890 GC (HP-5 column, 30 m) with
Complexes Used in This Study
3 4
an Agilent 5973 mass detector. A 4% NH /CH mix was used as the
ionization gas for chemical ionization analyses. Electronic absorption
spectra were recorded on a Hewlett-Packard (Agilent) 8452 diode array
spectrophotometer over a 190-1100 nm range in quartz cuvettes cooled
to the desired temperature by liquid nitrogen in a Unisoku cryostat.
X-band EPR spectra were obtained at 2 K on a Bruker E-500
spectrometer equipped with an Oxford ESR-10 liquid-helium cryostat.
Reaction Conditions for Catalysis Experiments with Limiting
Oxidant. A total of 0.287 mL of a 70 mM H
a 35% H solution) in CH CN was delivered by syringe pump over
5 min at 25 °C in air to a CH CN solution (2.0 mL) containing iron
mM catalyst and 200 mM olefin substrate. In various experiments,
acetic acid was added to the initial solution such that the final
concentration of acid was 85 mM. The solution was stirred for another
2 2
O solution (diluted from
2
O
2
3
2
1
3
epoxidation catalyst with peracetic acid as the oxidant.13 This
observation raised the possibility that iron-catalyzed in situ
formation of peracetic acid from CH3COOH and H2O2 could
rationalize the effect of acetic acid on olefin oxidation catalyzed
by complexes 1 or 2.12 Interestingly, results reported for 1
suggest that the presence of acetic acid sufficiently enhances
5
min after syringe pump addition was complete. In all cases, the
resulting solutions were treated with acetic anhydride (1 mL) together
with 1-methylimidazole (0.1 mL) to esterify the diol products.
Naphthalene was added as an internal standard. Organic products were
extracted with CHCl and the solution was subjected to GC analysis.
3
All experiments were run at least in duplicate, the reported data being
the average of these reactions.
its catalytic activity to envisage synthetic applications of these
bioinspired systems.11 However, despite the potential practical
implications of these observations, the mechanism of olefin
epoxidation catalyzed by non-heme iron complexes in the
presence of acetic acid remains unexplored. To fill this gap,
identification of intermediate species generated under catalysis
conditions would be very helpful. In the past decade, a large
amount of data has been accumulated concerning the spectro-
scopic characterization of Fe(TPA)-derived intermediates such
Reaction Conditions for Catalysis Experiments with Limiting
Olefin Substrate. A total of 0.3 mL of a 2.0 M H
equiv) was delivered by syringe pump over 60 min or added all at
once to a CH CN solution (2.0 mL) containing iron 1 mM catalyst and
00 mM olefin substrate. The solution was stirred for another 30 min
after H addition was complete by syringe pump or for 90 min if
the H was added all at once. These experiments were carried out
either at room temperature or at 0 °C (in ice/water bath) with variable
amounts of acetic acid or HClO . Samples were analyzed by GC
2 2
O solution (300
3
2
2 2
O
2 2
O
III
5,14
III
15
IV
as Fe OOH
and Fe OOR complexes as well as Fe O
1
6
species. However, less is known about Fe(BPMEN)-derived
intermediates, but some information has recently become
available. Taking advantage of the knowledge accumulated
4
following the acetylation work up explained above. Epoxide production
was monitored by adding all at once the H O solution to a pure CH -
1
7
2
2
3
II
CN solvent, a 1:2 mixture CH
3 3
CN/CH COOH, or a 1 mM solution of
on the oxidation chemistry exhibited by Fe TPA, this work
HClO in CH CN at room temperature containing iron catalyst and
4
3
explores the mechanistic implications of adding acetic acid to
cyclooctene. To aliquots extracted from reaction mixtures at different
reaction times were added a solution containing 1-methylimidazole (0.1
mL) and naphthalene and passed through a silica gel column before
GC analysis. The products were identified by their GC retention times.
1
and 2. Furthermore, this work reports experimental conditions
where the presence of acetic acid allows nearly 100% conversion
of olefin to epoxide using 0.5 mol % catalyst.
3 3
Analogous experiments were carried out at 0 °C in a 1:2 CH CN/CH -
COOH mixture.
Experimental Section
Materials and Methods. All reagents were purchased from Aldrich
18
O-Labeling Experiments. For experiments with labeled water,
and used as received unless noted otherwise. CH
3
CN solvent was
(90% 18O-enriched, 2% or 10%
O) was obtained from Cambridge Isotope Laboratories
8O (95% 18O-enriched) was purchased from ICON isotopes.
18
18
H
2
O, 42 µL of H
containing 1000 equiv of CH
the comparable experiment with labeled hydrogen peroxide, 70 mM
2
O (1000 equiv) were added to the catalyst solution
1
8
2 2 2
distilled over CaH before use. H O
3
COOH prior to the injection of H . In
2 2
O
16
solutions in H
2
1
Inc., and H
2
18
18
H
2
O
2
(diluted in CH
instead of H , to a solution containing the iron complex and CH
COOH (1000 equiv). In a subsequent experiment carried out with the
goal of determining if the label from H was incorporated to a fraction
(diluted in CH CN from a 10%
O solution) was added, instead of H , to a solution
containing the iron complex and CH COOH (50 equiv). In all cases,
3 2 2 2
CN from a 2% H O /H O solution) was added,
The olefin substrates were purified by passing through basic alumina
immediately before the reactions. The syntheses of complexes 1 and 2
were carried out in a glove box under a dry N
procedures previously reported.
experiments were carried out on a Bruker BioTOF II mass spectrometer
using the following conditions: spray chamber voltage ) 4000 V; gas
carrier temperature ) 200 °C. GC product analyses were performed
on a Perkin-Elmer Sigma 3 gas chromatograph (AT-1701 column, 30
m) with a flame ionization detector. GC mass spectral analyses were
O
2 2
3
-
2
atmosphere following
Electrospray ionization mass spectral
2 2
O
16,18
18
of the acetic acid, 70 mM H
2
O
2
3
18
H
2
O
2
/H
2
2 2
O
3
the organic products were subjected to GC/CI-MS analyses.
Spectroscopic Experiments. Spectroscopic experiments were carried
out using 1 mM solutions of [Fe(TPA)(OTf)
CN in a 1 cm UV-visible cuvette precooled to -20 °C. A total of 43
µL of 0.7 M H (15 equiv) was added to a solution containing the
iron(II) complex and variable amounts of CH COOH. In another set
of experiments the metastable Fe OOH species was first generated by
adding 43 µL of H 0.7 M (15 equiv) to a 1 mM solution of [Fe-
TPA)(OTf) ] at -30 °C. After maximal formation of the Fe OOH
species, variable amounts of CH COOH or CH COOD were added.
2 3
] (2) in 2.0 mL of CH -
(
13) Dubois, G.; Murphy, A.; Stack, T. D. P. Org. Lett. 2003, 5, 2469.
14) Kim, C.; Chen, K.; Kim, J.; Que, L., Jr. J. Am. Chem. Soc. 1997, 119,
(
2 2
O
5
964.
3
(
(
(
(
15) (a) Zang, Y.; Kim, J.; Dong, Y.; Wilkinson, E. C.; Appelman, E. H.; Que,
L., Jr. J. Am. Chem. Soc. 1997, 119, 4197. (b) Lehnert, N.; Ho, R. Y. N.;
Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc. 2001, 123, 8271.
III
2 2
O
16) Lim, M. H.; Rohde, J.-U.; Stubna, A.; Bukowski, M. R.; Costas, M.; Ho,
R. Y. N.; M u¨ nck, E.; Nam, W.; Que, L., Jr. Proc. Natl. Acad. Sci. U.S.A.
III
(
2
2
003, 100, 3665.
3
3
17) (a) Duban, E. A.; Bryliakov, K. P.; Talsi, E. P. Eur. J. Inorg. Chem. 2007,
Both procedures resulted in the formation of the Fe O intermediate
IV
8
52. (b) Duban, E. A.; Bryliakov, K. P.; Talsi, E. P. MendeleeV Commun.
IV
with yields up to 65%. Quantitative obtention of Fe O species from 2
2
005, 15, 12.
18) Chen, K.; Que, L., Jr. Chem. Commun. 1999, 1375.
was achieved by addition of 1 equiv of AcO
2
H (0.004 mmol, 32 wt %
J. AM. CHEM. SOC. VOL. 129, NO. 51, 2007 15965
9