IronACHTREUNG(III)-Catalyzed Biomimetic Hydrocarbon Oxidation
FULL PAPER
Table 5. Yield of products derived from peroxyphenylacetic acid (PPAA) mediated by iron catalysts in the
presence of cyclohexene.[a]
ard Model 5989B mass spectrometer
or a Donam Systems 6200 gas chroma-
tograph equipped with an FID detec-
Entry
Catalyst
Heterolysis
Homolysis
Oxidation products[b]
tor using
a 30 m capillary column
8
10
11
12
oxide
ol
one
(Hewlett-Packard, HP-1, HP-5, and
Ultra 2). Elemental analyses (C, H, N)
were carried out on an EA1108 instru-
ment (Carlo Erba Instrument, Italy) in
the Organic Chemistry Research
Center of Sogang University, Korea.
IR spectra were measured on a BIO
RAD FTS 135 spectrometer as KBr
pellets.
1
2
3
4
3
3a
4
86.5Æ4.0
85.2Æ6.0
83.7Æ6.0
84.0Æ6.0
7.0Æ0.1
9.5Æ0.5
7.2Æ0.5
9.4Æ0.3
3.5Æ0.1
4.0Æ0.5
3.8Æ0.1
3.9Æ0.2
-
-
-
-
28.5Æ1.0
36.3Æ1.0
29.8Æ1.8
44.2Æ1.5
4.5Æ0.2
8.2Æ0.1
4.5Æ0.3
8.2Æ0.5
4.3Æ0.4
6.4Æ0.1
4.8Æ0.5
6.8Æ0.1
4a
[a] See Experimental Section for details. [b] oxide, ol, and one denote cyclohexene oxide, cyclohexenol, and
cyclohexenone, respectively.
produce high-valent FeV=O species in the reactions involv-
ing these iron complexes and peracids. To our knowledge,
this is only the second example of an FeV=O species that
can be generated in nonheme iron complexes by using a per-
acid as the oxidant.[6] Interestingly, the supporting chelating
ligand in these systems is dianionic, whereas in the previous-
ly reported complex generated from a peracid the FeV oxo
moiety is supported by a tetraanionic macrocyclic ligand.[6]
Our work provides evidence that complexes having a
mixed-donor environment comprising both neutral and
anionic nitrogen donors can afford FeV=O species that are
relevant to the proposed active oxidant in nonheme Rieske
dioxygenases.
Synthesis of iron complexes: [Et3NH][Fe
[Fe
(bpc)Cl2] (4) were obtained from a previous study.[10] For the synthesis
of [Fe(Me2bpb)Cl(H2O)] (3a), 3 (300 mg) was dissolved in methanol.
ACHTRE(UNG Me2bpb)Cl2] (3) and [Et3NH]
AHCTREUNG
G
ACHTREUNG
After the solution was stirred for 30 min, dark green microcrystals that
precipitated were collected by filtration, washed with methanol, and air-
dried. Yield: 219 mg (73%). IR (KBr): n˜ =1614 (n
tal analysis (%) calcd for 3a (C21H22ClFeN4O4, 485.73): C 51.92, H 4.57,
N 11.54; found: C 52.02, H 4.57, N 11.56. [Fe(bpc)Cl(H2O)] (4a) was ob-
(C=
(C=O)) cmÀ1. Elemen-
ACHTREUNG
A
ACHTREUNG
tained by a similar procedure. Yield: 126 mg (42%). n˜ =1638 (n
ACHTREUNG
O)) cmÀ1. Elemental analysis (%) calcd for 3a (C18H12Cl3FeN4O3,492.93):
C 43.72, H 2.45, N 11.33; found: C 43.66, H 2.59, N 11.53.
X-ray analysis: Dark brown plate crystals of 3a suitable for crystallo-
graphic analysis were obtained from CH3OH/diethyl ether. X-ray diffrac-
tion data were collected on an Enraf-Nonius CAD-4 Mach3 diffractome-
ter equipped with a monochromator in the MoKa (l=0.71073 ) incident
beam. The crystal was mounted on a glass fiber. Final cell parameters
were obtained from least-squares fit to 25 reflections in the range 9.40ꢀ
qꢀ12.328. Intensities were corrected for Lorentzian and polarization ef-
fects but not for absorption. The crystal structure was determined by
direct methods and Fourier techniques. All calculations were performed
on an IBM Pentium computer using SHELXS-97 and SHELXL-97,[18]
and atomic scattering factors for all non-hydrogen atoms were supplied
by SHELXS-97. All hydrogen atoms except for those of a water mole-
cule were placed in calculated positions. The crystallographic data are
listed in Table 1, and selected bond lengths and angles in Table 2.
Conclusion
We have found that mononuclear nonheme ironACHTER(UNG III) com-
plexes supported by chelate ligands having two deprotonat-
ed amide moieties are capable of catalyzing olefin epoxida-
tion and alcohol oxidation on treatment with peracids. The
active oxidant in these systems was proposed to be an FeV=
O species, as indicated by KIE (kH/kD) and H218O exchange
experiments, as well as the use of PPAA as an oxidant. This
FeV=O species is relevant to intermediates postulated in the
mechanisms of dioxygen-activating mononuclear nonheme
iron enzymes such as Rieske dioxygenases. Comparison of
the results presented herein with those recently reported by
others[5,6] reveals the important role that the structure of the
supporting chelating ligand plays in influencing the forma-
tion of high-valent iron oxo species. Our future work will
focus on 1) further exploring the biologically relevant reac-
CCDC-619186 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the Cambridge
Catalytic hydrocarbon oxidation on treatment of iron complexes with
MCPBA: MCPBA (0.02 mmol) was added to a mixture of substrate
(0.2 mmol), iron complex (0.001 mmol), and solvent (CH3CN/CH2Cl2 1/1,
1 mL). The mixture was stirred for 10 min at room temperature. Each re-
action was monitored by GC/MS analysis of 20 mL aliquots withdrawn
periodically from the reaction mixture. All reactions were run at least in
triplicate, and average product yields based on MCPBA are presented. In
the competitive reaction of cis-2-hexene and trans-2-hexene, the amount
of each substrate was 0.1 mmol.
KIE study on the oxidation of benzyl alcohol by iron complexes and
MCPBA: To improve the accuracy of measuring the amount of deuterat-
ed benzyl alcohol product, a 1:6 mixture of benzyl alcohol and deuterat-
ed benzyl alcohol was used. MCPBA (0.02 mmol) was added to a mixture
of benzyl alcohol (0.1 mmol), deuterated benzyl alcohol (0.6 mmol), iron
complex (0.001 mmol), and solvent (CH3CN/CH2Cl2 1/1, 1 mL). The mix-
ture was stirred for 10 min at room temperature. The reaction was moni-
tored by GC/MS analysis of 20 mL aliquots withdrawn periodically from
the reaction mixture. All reactions were run at least in triplicate, and the
average KIE values are presented.
H218O experiments: MCPBA (0.005–0.01 mmol) was added to a mixture
of cyclohexene (0.01–0.02 mmol), iron complex (0.001 mmol), and H218O
(10–20 mL, 95% 18O enriched, Aldrich Chemical Co.) in dried CH3CN/
CH2Cl2 (1/1, 1 mL). The reaction mixture was stirred for 3 min at room
temperature and then directly analyzed by GC/MS. The 16O and 18O com-
positions were determined by the relative abundance of mass peaks at m/
z 99 for [16O]cyclohexene oxide and m/z 101 for [18O]cyclohexene oxide.
tivity of the ironACHTREUNG(III) complex/MCPBA systems, and 2) ob-
taining spectroscopic evidence for the FeV=O species de-
scribed herein.
Experimental Section
Materials: Olefins, cyclohexanol, benzyl alcohol, [D7]benzyl alcohol, di-
chloromethane, acetonitrile, MCPBA (65%), and H218O (95% 18O en-
richment) were purchased from Aldrich Chemical Co. and were used
without further purification. Peroxyphenylacetic acid (PPAA) was syn-
thesized according to the literature method.[16]
Instrumentation: Products of olefin epoxidation, alcohol oxidation, and
18O incorporation by cyclohexene oxide were analyzed on a Hewlett-
Packard 5890 II Plus gas chromatograph interfaced with a Hewlett-Pack-
Chem. Eur. J. 2007, 13, 9393 – 9398
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
9397