Temperature effect on the epoxidation of olefins by an iron(III) porphyrin complex and tert-alkyl hydroperoxides

Article abstract of DOI:10.1039/b005162o

An electron-deficient iron porphyrin complex catalyzes the epoxidation of olefins by tert-alkyl hydroperoxides via radical-free oxidation reactions in aprotic solvent; the epoxidation reactions were markedly influenced by reaction temperature and high yields of epoxide products were obtained with retention of stereospecificity at low temperature.

Full text of DOI:10.1039/b005162o

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Temperature effect on the epoxidation of olefins by an iron(III) porphyrin
complex and tert-alkyl hydroperoxides
Wonwoo Nam,* So-Young Oh, Mi Hee Lim, Mee-Hwa Choi, So-Yeop Han and Gil-Ja Jhon
Department of Chemistry and Division of Molecular Life Sciences, Ewha Womans University, Seoul 120-750, Korea.
E-mail: wwnam@mm.ewha.ac.kr
Received (in Cambridge, UK) 27th June 2000, Accepted 11th August 2000
First published as an Advance Article on the web 4th September 2000
An electron-deficient iron porphyrin complex catalyzes the
epoxidation of olefins by tert-alkyl hydroperoxides via
radical-free oxidation reactions in aprotic solvent; the
epoxidation reactions were markedly influenced by reaction
temperature and high yields of epoxide products were
obtained with retention of stereospecificity at low tem-
perature.
complex and tert-alkyl hydroperoxides such as tert-butyl
hydroperoxide (ButOOH) and MPPH. Here we report that the
epoxidation reactions were markedly influenced by reaction
temperature and high yields of epoxide products with retention
of stereospecificity were obtained at low temperature.
The catalytic epoxidation of cyclohexene by ButOOH was
carried out in the presence of Fe(TF4TMAP)5+ at various
reaction temperatures under argon atmosphere in MeCN. As
detailed in Table 1, the formation of cyclohexene oxide,
cyclohexenol and tert-butyl cyclohexenyl peroxide (c-
C6H11OOBut) was observed at room temperature, indicating
that the cyclohexene oxidation proceeds via free alkoxyl radical
chemistry (i.e. O–O bond homolysis of ButOOH).1,8 Inter-
estingly, we found that the product distribution varied depend-
ing on reaction temperature. As the reaction temperature was
lowered, the yields of cyclohexene oxide increased, while those
of allylic oxidation products (i.e. cyclohexenol and c-C6H11OO-
But) diminished (Table 1). These results indicate that the
reaction of iron(III) porphyrin complex with ButOOH is
sensitive to reaction temperature and that two-electron epoxida-
tion becomes a major pathway in the cyclohexene epoxidation
at low temperature [Scheme 1(A), pathway A]. Further
evidence that the epoxidation of olefins by Fe(TF4TMAP)5+ and
ButOOH at low temperature occurs via radical-free chemistry
was obtained by analyzing products obtained in cis-stilbene
epoxidation. cis-Stilbene was predominantly oxidized to cis-
stilbene oxide [eqn. (1)],‡ demonstrating that the ButOO·
radical was not involved as an epoxidizing agent in the
epoxidation reactions.2,8
The reactions of iron(III) complexes of porphyrin and non-
porphyrin ligands with alkyl hydroperoxides have been ex-
tensively studied as biomimetic models for heme- and non-
heme-containing enzymes.1 It has been shown in a number of
reports that oxidation of hydrocarbons by iron(III) porphyrin
complexes and tert-alkyl hydroperoxides proceeds via O–O
bond homolysis (i.e. free alkoxyl radical chemistry) in aprotic
solvents.1–3 In addition, Ingold and coworkers evidenced, using
2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH) as a mech-
anistic probe to distinguish between free alkoxyl radical
chemistry and radical-free chemistry, that O–O bond homolysis
is a predominant pathway in non-porphyrin iron complex-
catalyzed oxidation of hydrocarbons by tert-alkyl hydro-
peroxides.1,4 Despite intensive study for the last two decades, it
has rarely been observed that simple iron complexes of
porphyrin and non-porphyrin ligands are able to catalyze the
oxidation of hydrocarbons by alkyl hydroperoxides via radical-
free chemistry in aprotic solvents.5,6† Since we found recently
that a highly electron-deficient iron(III) porphyrin complex,
Fe(TF4TMAP)(CF3SO3)5 [TF4TMAP = meso-tetrakis(2,3,5,6-
tetrafluoro-N,N,N-trimethyl-4-aniliniumyl)porphinato
dia-
nion], catalyzes the epoxidation and hydroxylation of hydro-
carbons by H2O2 via radical-free oxidation reactions,7 we
attempted the epoxidation of olefins with the iron porphyrin
The temperature effect on the reaction of Fe(TF4TMAP)5+
with alkyl hydroperoxides in MeCN was also observed when
MPPH was used as an oxidant (Table 1). Only a small amount
Table 1 Epoxidation of cyclohexene by Fe(TF4TMAP)5+ and tert-alkyl hydroperoxides at various reaction temperatures in MeCN and in MeOH–
a,b
CH2Cl2
Yields (%) of products formed in ButOOH reactionsc,d
Cyclohexene
Yields (%) of products formed in MPPH reactionsc
Cyclohexene
T/°C
t/h
oxide
Cyclohexenol c-C6H11OOBut t/h
oxide
MPPOH
PhCH2OH
PhCHO
MeCN
20
10
0
210
220
230
2
2
4
4
4
4
20 ± 3
31 ± 3
45 ± 4
52 ± 4
70 ± 4
74 ± 4
29 ± 3
22 ± 3
18 ± 3
14 ± 2
9 ± 2
12 ± 5
8 ± 4
5 ± 3
< 2
< 2
0
2
2
4
4
5
6
7 ± 1
12 ± 2
17 ± 3
32 ± 3
38 ± 3
46 ± 4
20 ± 3
23 ± 3
29 ± 3
44 ± 4
46 ± 5
55 ± 6
63 ± 6
47 ± 5
33 ± 4
27 ± 3
18 ± 3
12 ± 3
8 ± 4
8 ± 3
8 ± 2
6 ± 2
12 ± 4
12 ± 4
7 ± 2
e
MeOH–CH2Cl2
20
10
0
210
220
2
2
4
4
4
59 ± 3
70 ± 5
68 ± 5
76 ± 5
75 ± 5
8 ± 2
5 ± 1
4 ± 2
< 2
Tracef
2
2
4
4
5
36 ± 2
42 ± 3
57 ± 4
63 ± 5
68 ± 5
47 ± 3
45 ± 3
62 ± 4
69 ± 5
79 ± 6
30 ± 3
20 ± 2
13 ± 2
< 4
Tracef
Tracef
Tracef
Tracef
Tracef
Tracef
Tracef
Tracef
Tracef
< 2
< 4
a MPPH4 and tert-butyl cyclohexenyl peroxide (c-C6H11OOBut)11 were prepared according to literature procedures. All reactions were run at least in
triplicate, and the data represent the average of these reactions. b In a typical reaction, oxidant (2 3 1022 mmol diluted in 80 mL of MeCN) was added to
a reaction solution containing Fe(TF4TMAP)(CF3SO3)5 (1 3 1023 mmol) and cyclohexene (0.3 mmol) in 0.3 mL MeCN at the given temperature under an
inert atmosphere. After the reaction mixture was stirred for the given time, the reaction solution was directly analyzed by GC or GC–MS with known authentic
sample. c Yields were based on oxidants added. d < 5% of cyclohexenone was formed in all of the reactions. e Since methanolysis of cyclohexene oxide took
place under the reaction conditions, the yields of cyclohexene oxide were determined by summing the amounts of cyclohexene oxide and
2-methoxycyclohexanol.12 f < 2% based on oxidant added was formed.
DOI: 10.1039/b005162o
Chem. Commun., 2000, 1787–1788
This journal is © The Royal Society of Chemistry 2000
1787

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alcohol products came from H218O demonstrates unambigu-
ously that cyclohexenyl and benzyl radicals were trapped by the
(TF4TMAP)FeIV–OH intermediate [pathway B in Scheme 1(A)
and pathway D in Scheme 1(B)].4b,9,10 The different and
relatively small amounts of 18O-incorporation into the cyclo-
hexenol and benzyl alcohol products are ascribed to the fact that
the oxygen exchange between the iron(IV)–OH intermediate and
labeled water [pathway C in Scheme 1(A) and pathway E in
Scheme 1(B)] is competing with the C–O bond-forming step
between the intermediate and alkyl radicals [pathway B in
Scheme 1(A) and pathway D in Scheme 1(B)].3,9b
In conclusion, we have shown here that high yields of oxide
products were formed via a radical-free mechanism in the
epoxidation of olefins by tert-alkyl hydroperoxides at low
temperature. The oxide yields were found to depend sig-
nificantly on reaction temperature. In addition, it has been
demonstrated unambiguously that alcohol products such as
cyclohexenol and benzyl alcohol were formed by the trapping
of alkyl radicals by (TF4TMAP)FeIV–OH species. Future
studies will focus on attempts to understand the temperature
effect on the mechanism of hydroperoxide O–O bond activation
by iron porphyrin complexes and to use the alkyl hydro-
peroxides in biomimetic alkane hydroxylation reactions.
This research was supported by KOSEF (1999-2-122-002-4
for W. N. and 981-0304-022-1 for G. J.), KRF (KRF-
99-042-D00068), the MOST through the Women’s University
Research Fund, Brain Korea 21 Project.
(1)
of cyclohexene oxide was formed and PhCH2OH was the major
product derived from MPPH decomposition at room tem-
perature. This result demonstrates that the reaction of Fe(TF4T-
MAP)5+ with MPPH occurs via O–O bond homolysis [Scheme
1(B), pathway A].4 As the reaction temperature was lowered,
the formation of cyclohexene oxide and PhCH2CMe2OH
(MPPOH) products increased, indicating that two-electron
epoxidation takes place in the reaction of Fe(TF4TMAP)5+ and
MPPH at low temperature [Scheme 1(B), pathway B]. When the
epoxidation of cis-stilbene by Fe(TF4TMAP)5+ and MPPH was
performed at low temperature, cis-stilbene oxide was obtained
as a major product [eqn. (1)].‡ Also, MPPOH was the
predominant product of MPPH decomposition (70% based on
MPPH used). These results demonstrate unambiguously that the
epoxidation of olefins by MPPH at low temperature occurs via
radical-free chemistry.
Since it is known that methanol is a better solvent for iron
porphyrin complex-catalyzed epoxidation of olefins by hydro-
peroxides,5 the epoxidation of cyclohexene by ButOOH and
MPPH was carried out in a solvent mixture of MeOH and
CH2Cl2 (3+1). As shown in Table 1, the yields of cyclohexene
oxide formed in MeOH–CH2Cl2 were higher than those
obtained in MeCN. In addition, as observed in the MeCN
reactions, the oxide yields in the MPPH reactions increased as
the reaction temperature was lowered. Interestingly, ca. 80% of
MPPH was converted to MPPOH below 220 °C, and, to the
best of our knowledge, this is the highest MPPOH formation in
iron complex-mediated O–O bond cleavage of MPPH.4
We then studied 18O-labeled water experiments in the
epoxidation of cyclohexene by ButOOH and MPPH,9 in order to
understand the source of oxygen atoms in cyclohexenol and
benzyl alcohol products formed in the reactions of ButOOH and
MPPH, respectively.4b,10 When the epoxidation reactions were
carried out in the presence of H218O at room temperature,§ the
percentages of 18O incorporated from the labeled water into
cyclohexenol and benzyl alcohol were 21 ± 2 and 14 ± 2%,
respectively. The observation that some of the oxygen in the
Notes and references
† Traylor et al. reported that epoxidation of olefins by electron-deficient
iron(III) porphyrin complexes and ButOOH gives high yields of oxide
products in protic solvents (i.e. MeOH).5 We have shown that the reactions
of water-soluble iron(III) porphyrins with tert-alkyl hydroperoxides epoxi-
dize olefins to give the corresponding oxide products in aqueous
solution.6
‡ Reaction conditions: oxidant (2 3 1022 mmol, diluted in 80 mL of MeCN)
was added to a reaction solution containing Fe(TF4TMAP)5+ (3 3 1023
mmol) and cis-stilbene (0.3 mmol) in a solvent mixture (1 mL) of MeCN
and CH2Cl2 (1+1) at 220 °C. After stirring for 4 h, the reaction solution was
directly analyzed by HPLC.
§
18O-labeled water experiments with ButOOH and MPPH were performed
at 20 °C under the same reaction conditions as described in footnote a of
Table 1 except that H218O (25 mL, 95% 18O enriched) was present in the
reaction media. The 16O and 18O compositions in cyclohexenol and benzyl
alcohol were determined by the relative abundances of mass peaks at m/z 83
and 85 for cyclohexenol and at m/z 108 and 110 for benzyl alcohol.
1 K. U. Ingold and P. A. MacFaul, in Biomimetic Oxidations Catalyzed by
Transition Metal Complexes, ed. B. Meunier, Imperial College Press,
London, 2000, pp. 45–89.
2 F. Minisci, F. Fontana, S. Araneo, F. Recupero and L. Zhao, Synlett,
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54, 7977 and refs. therein.
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Chem. Commun., 1999, 387.
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1999, 38, 3238; Y. J. Lee, Y. M. Goh, S.-Y. Han, C. Kim and W. Nam,
Chem. Lett., 1998, 837.
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Compounds, Academic Press, New York, 1981.
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Scheme 1
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1788
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