lytically and homolytically,11 depending on the reaction
conditions such as pH and the electronic nature of the porphyrin
and axial ligands. These results rationalize the long-standing
dichotomy of the interpretations for the O–O bond cleavage
mechanism of ROOH by iron(iii) porphyrin complexes, mainly
suggested by Traylor2 and Bruice3 and co-workers.
This research was supported by the Korea Science and
Engineering Foundation (96-0501-01-01-3), the MOST through
the Women’s University Research Fund, and Ewha Womans
University (1998).
Notes and references
† MPPH was prepared according to literature procedures5 and the purity of
MPPH was determined to be 100% by NMR. In a typical reaction, MPPH
(4 mM, introduced as a 0.2 M solution in MeOH) was added to a reaction
solution containing Fe(TDFPPS)32 (0.04 mM, introduced as 0.01 M
solution in H2O) and (Z)-stilbene (6 mM, introduced as 0.3 M solution in
MeOH) in a solvent mixture (5 mL) of buffered H2O (2.5 mL)–MeOH (1.0
mL)–MeCN (1.5 mL) in order to make the reaction mixture homogeneous.
Reactions at pH 3 were performed in formate buffer (0.1 M), at pH 4–5 in
acetate buffer (0.1 M), and at pH 6–8 in phosphate buffer (0.1 M), and the
pH was adjusted by adding either HCl (3 M) or NaOH (3 M) solutions as
necessary. The reaction mixture was stirred in air for 4 h at 25 °C, and then
Fig. 2 Plot of the percent yield of products (5, cis-stilbene oxide; «,
MPPOH; :, PhCH2OH; 2, PhCHO) vs. pH of reaction solutions for the
catalytic epoxidation of (Z)-stilbene by Fe(TDFPPS)32 and MPPH. The
percent yields are calculated on the basis of MPPH used. See footnote † for
detailed experimental procedures.
homolysis of MPPH took place concurrently even at low pH
values, as demonstrated by the observation of the formation of
PhCHO and PhCH2OH [eqn. (2)]. As the pH of the reaction
solution increased, the yields of cis-stilbene oxide and MPPOH
products decreased and the amounts of the PhCHO and
PhCH2OH increased. These results indicate that the O–O bond
cleavage of MPPH was shifted from heterolysis to homolysis as
the pH of the reaction solutions increased.
analyzed by Orom Vintage 2000 HPLC equipped with
a variable
wavelength UV-200 detector. Detection was made at 215 and 254 nm.
‡ All iron(iii) porphyrin complexes used in this study were obtained from
Mid-Century Chemical. Abbreviations used: TDFPPS, [meso-tetrakis(2,6-
difluoro-3-sulfonatophenyl)porphyrin; TDCPPS, meso-tetrakis(2,6-di-
chloro-3-sulfonatophenyl)porphyrin; TMPyP, meso-tetrakis(N-methylpyr-
idin-4-ylium)porphyrin; TMPS, meso-tetrakis(2,5-disulfonatomesityl)-
porphyrin.
In addition to the pH effect on hydroperoxide O–O bond
cleavage, we found that there are other important factors that
control the type of O–O bond cleavage of tert-alkyl hydro-
peroxides. As shown in Table 1, the O–O bond cleavage was
significantly affected by the porphyrin ligands bound to iron
and the general trend appeared to be that more electro-
negatively-substituted iron porphyrins gave a high percentage
of heterolysis, whereas homolysis prevailed in the reactions
with less electronegatively-substituted iron porphyrins. This
result is consistent with the observation that electron-deficient
iron porphyrins are effective catalysts in the epoxidation of
olefins by H2O2 and ROOH.10 We also found, by studying the
epoxidation of (Z)-stilbene with Fe(TDFPPS)32 and MPPH in
the presence of imidazoles, that there is a significant axial ligand
effect on the ratio of the heterolytic and homolytic O–O bond
cleavage of tert-alkyl hydroperoxides.4b,11,12 Interestingly, the
presence of imidazoles such as 5-chloro-1-methylimidazole and
1-phenylimidazole increased the yields of cis-stilbene oxide and
MPPOH products, whereas 1-methylimidazole and 1,2-di-
methylimidazole did not alter the ratio of heterolysis to
homolysis significantly (data not shown), indicating that the
nature of the axial ligand bound to iron is another important
factor determining the type of the hydroperoxide O–O bond
cleavage.11–13
1 M. Sono, M. P. Roach, E. D. Coulter and J. H. Dawson, Chem. Rev.,
1996, 96, 2841; T. G. Traylor and P. S. Traylor, in Active Oxygen in
Biochemistry, ed. J. S. Valentine, C. S. Foote, A. Greenberg and J. F.
Liebman, Blackie Academic and Professional, Chapman and Hall,
London, 1995, pp. 84–187; B. Meunier, Chem. Rev., 1992, 92, 1411.
2 T. G. Traylor, C. Kim, W.-P. Fann and C. L. Perrin, Tetrahedron, 1998,
54, 7977 and refs. therein.
3 O. Almarsson and T. C. Bruice, J. Am. Chem. Soc., 1995, 117, 4533 and
refs. therein.
4 (a) F. Minisci, F. Fontana, S. Araneo, F. Recupero, S. Banfi and S.
Quici, J. Am. Chem. Soc., 1995, 117, 226; (b) S. E. J. Bell, P. R. Cooke,
P. Inchley, D. R. Leanord, J. R. Lindsay Smith and A. Robbins, J. Chem.
Soc., Perkin Trans. 2, 1991, 549; (c) R. Labeque and L. J. Marnett,
J. Am. Chem. Soc., 1989, 111, 6621.
5 P. A. MacFaul, D. D. M. Wayner and K. U. Ingold, Acc. Chem. Res.,
1998, 31, 159; P. A. MacFaul, K. U. Ingold, D. D. M. Wayner and L.
Que, Jr., J. Am. Chem. Soc., 1997, 119, 10594; P. A. MacFaul,
I. W. C. E. Arends, K. U. Ingold and D. D. M. Wayner, J. Chem. Soc.,
Perkin Trans. 2, 1997, 135; I. W. C. E. Arends, K. U. Ingold and
D. D. M. Wayner, J. Am. Chem. Soc., 1995, 117, 4710.
6 G.-X. He and T. C. Bruice, J. Am. Chem. Soc., 1991, 113, 2747; A. J.
Castellino and T. C. Bruice, J. Am. Chem. Soc., 1988, 110, 158.
7 J. T. Groves, Z. Gross and M. K. Stern, Inorg. Chem., 1994, 33,
5065.
8 A. Hadasch, A. Sorokin, A. Rabion and B. Meunier, New J. Chem.,
1998, 45; R. Song, A. Sorokin, J. Bernadou and B. Meunier, J. Org.
Chem., 1997, 62, 673; S. M. S. Chauhan, P. P. Mohapatra, B. Kalra,
T. S. Kohli and S. Satapathy, J. Mol. Catal. A: Chem., 1996, 113,
239.
In summary, we demonstrated unambiguously that the O–O
bond of tert-alkyl hydroperoxides is cleaved both hetero-
Table 1 Product yields formed in the epoxidation of (Z)-stilbene by MPPH
catalyzed by iron porphyrin complexes at pH 3.2a
9 S. J. Yang and W. Nam, Inorg. Chem., 1998, 37, 606 and refs.
therein.
Yields (%)b
10 Y. J. Lee, Y. M. Goh, S.-Y. Han, C. Kim and W. Nam, Chem. Lett.,
1998, 837; T. G. Traylor, C. Kim, J. L. Richards, F. Xu and C. L. Perrin,
J. Am. Chem. Soc., 1995, 117, 3468; J. F. Bartoli, P. Battioni, W. R. De
Foor and D. Mansuy, J. Chem. Soc., Chem. Commun., 1994, 23.
11 A. J. Allentoff, J. L. Bolton, A. Wilks, J. A. Thompson and P. R. Ortiz
de Montellano, J. Am. Chem. Soc., 1992, 114, 9744.
12 D. Mansuy, P. Battioni and J.-P. Renaud, J. Chem. Soc., Chem.
Commun., 1984, 1255.
13 K. Yamaguchi, Y. Watanabe and I. Morishima, J. Am. Chem. Soc.,
1993, 115, 4058.
Iron
cis-Stilbene
porphyrins
oxide
MPPOH
PhCH2OH
PhCHO
Fe(TDFPPS)32 51
Fe(TDCPPS)32c 33
54
38
7
8
12
14
18
b
21
41
59
44
Fe(TMPyP)5+
Fe(TMPS)72c
a
2
12
19
See footnote † for detailed reaction procedures. Based on MPPH
used. c Reactions were run for 8 h.
Communication 8/09876J
388
Chem. Commun., 1999, 387–388