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.
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6 W. Nam, H. J. Choi, H. J. Han, S. H. Cho, H. J. Lee and S.-Y. Han,
Chem. Commun., 1999, 387.
7 W. Nam, Y. M. Goh, Y. J. Lee, M. H. Lim and C. Kim, Inorg. Chem.,
1999, 38, 3238; Y. J. Lee, Y. M. Goh, S.-Y. Han, C. Kim and W. Nam,
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8 R. A. Sheldon and J. K. Kochi, Metal Catalyzed Oxidations of Organic
Compounds, Academic Press, New York, 1981.
9 (a) J. Bernadou and B. Meunier, Chem. Commun., 1998, 2167; (b) K. A.
Lee and W. Nam, J. Am. Chem. Soc., 1997, 119, 1916.
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11 T. W. Campbell and G. M. Coppinger, J. Am. Chem. Soc., 1951, 73,
1788.
Scheme 1
12 G. H. Posner and D. Z. Rogers, J. Am. Chem.Soc., 1977, 99, 8208.
1788
Chem. Commun., 2000, 1787–1788