Table 2 Comparison of the Fe(TDCPN5P)Cl-catalyzed oxidations of various substrates either with H2O2 or with PhIO
Substrate
Oxidant
Products (yield in %)d
Adamantanea
ol-1
62
26
1,2-Oxide
35
24
o-OH
< 1
11
ol-2
30
9
8,9-Oxide
18
9
p-OH
7
4
p-OH
8
5
one-2
2
< 1
ol-1/ol-2
2.1
2.9
PhIO
H2O2
Limoneneb
Anisolec
Oxide ratio Carveols
1.9
2.7
o/p
< 0.1
2.75
o/p
< 0.1
2.8
Carvone
4
< 1
PhIO
H2O2
8
2
Phenol
2
4
Phenol
2
7
PhIO
H2O2
Ethoxybenzenec
o-OH
< 1
14
PhIO
H2O2
a Conditions: Fe(TDCPN5P)Cl+oxidant+adamantane molar ratio = 1+40+300 in CH2Cl2–MeCN (1+1) for 2 h at 20 °C; [catalyst] = 2 mM; o1-1, ol-2 and
one-2 denote adamantan-1-ol, -2-ol and -2-one. b Conditions: Fe(TDCPN5P)Cl+oxidant+limonene = 1+40+800 in CH2Cl2–MeCN (1+1). Carveols and
carvone are the products resulting from allylic hydroxylation of limonene (at position 6).3a c Conditions: Fe(TDCPN5P)Cl+oxidant+alkoxybenzene =
1+20+3000 in CH2Cl2–MeCN (1+1). o-OH and p-OH are used for the ortho- and para-hydroxylation products. d Yields are based on starting H2O2 or PhIO.
All the experiments described in Tables 1 and 2 were usually performed under aerobic conditions. However, it is noteworthy that many experiments were
also done under anaerobic conditions (under Ar) and gave almost identical results, showing that the observed regioselectivities were not affected by the
presence of O2.
huge excess of substrate used in these reactions (catalyst+
H2O2+substrate = 1+20+3000). Moreover, identical experi-
ments performed under stoichiometric conditions (iron por-
phyrin+H2O2+substrate = 1+1+3000) led to almost identical
regioselectivities for anisole hydroxylation. It is noteworthy
that the Fe(TDCPNxP)Cl catalysts were stable under the
conditions described in Table 1. This was shown by their visible
spectra that were found to be unchanged at the end of the
reactions, and by their catalytic activity that remained almost
identical after a further addition of H2O2 in the reaction
medium.
The aforementioned results show the particular efficiency of
iron porphyrins bearing around five b-nitro substituents to
catalyze alkene epoxidation and alkane and aromatic hydroxy-
lations with H2O2 in the absence of a cocatalyst. In order to have
a first idea of the nature of the major active species responsible
for these oxidations, we compared the oxidations of several
substrates with H2O2 and Fe(TDCPN5P)Cl to those performed
with PhIO and the same catalyst under identical conditions.
Table 2 clearly shows marked differences in the chemo- and
regio-selectivities of the H2O2- and PhIO-dependent oxidations.
This changes from small significant albeit small differences in
the regioselectivity of adamantane hydroxylation (ol-1/ol-2
molar ratio of 2.9 with H2O2 cf. 2.1 with PhIO) and in the
regioselectivity of limonene epoxidation (1,2 oxide/8,9 oxide =
2.7 cf. 1.9) to more dramatic differences in the regioselectivity
of anisole and ethoxybenzene hydroxylations (major formation
of ortho-hydroxylated products with H2O2 instead of almost
exclusive para hydroxylation with PhIO).
Several years ago, a similar comparison between the chemo-
and regio-selectivities of the oxidations of alkenes and alkanes
with either PhIO or H2O2, in the presence of the Mn(TDCPP)Cl
catalyst and imidazole cocatalyst, led to the conclusion that both
systems involve the same active oxygen species, presumably a
high-valent MnNO complex.3a The results of Table 2 strongly
suggest that the Fe(TDCPN5P)Cl–H2O2 and Fe(TDCPN5P)Cl–
PhIO systems involve different active oxygen species. If one
assumes that the active species formed with PhIO is a
(por+·)Fe(IV)NO intermediate, as has been proposed for many
other iron porphyrins,1 one is led to the conclusion that different
active species are involved in the H2O2-dependent oxidations.
Several authors have already mentioned the possible formation
of a ferric–hydroperoxo complex as an intermediate towards the
high-valent iron–oxo species, in reactions of H2O2 with iron
porphyrins1,8 [eqn. (2)]. Ortho-hydroxylation of anisole from
transfer of the terminal oxygen atom of such a ferric–
hydroperoxo complex should be less sterically restricted than
the transfer of the oxygen atom of a high-valent (TDCPNxP)
iron–oxo intermediate.9 Preferential involvement of a Fe(III)–
OOH intermediate in reaction of H2O2 with iron–b-poly-
nitroporphyrins, which should be less prone to stabilize high-
valent iron–oxo species, is likely and could explain the
surprising regioselectivity observed in the hydroxylation of
anisole and ethoxybenzene. However, additional experiments
are necessary to determine the nature of the active inter-
mediate(s) involved in reactions using H2O2 and iron–b-
polynitroporphyrins. Nonetheless, the preliminary results re-
ported here indicate that the Fe(TDCPNxP)Cl complexes with x
! 4 are interesting catalysts for alkane and aromatic hydro-
carbon oxidation with H2O2 without need of a cocatalyst, that
lead to new regioselectivities, especially in the hydroxylation of
aromatic molecules.
Notes and references
1 For a recent review, see: B. Meunier, A. Robert, G. Pratviel and J.
Bernardou, in The Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith
and R. Guilard, Academic Press, New York, 1999, vol. 4, p. 119.
2 T. G. Traylor, C. Kim, K. L. Richards, F. Xu and C. L. Perrin, J. Am.
Chem. Soc., 1995, 117, 3468.
3 P. Battioni, J. P. Renaud, J. F. Bartoli, M. Reina-Artiles, M. Fort and D.
Mansuy, J. Am. Chem. Soc., 1988, 110, 8462.
4 (a) J. F. Bartoli, P. Battioni, W. R. De Foor and D. Mansuy, J. Chem. Soc.,
Chem. Commun., 1994, 23; (b) S. Tsuchiya and M. Seno, Chem. Lett.,
1989, 263; (c) W. Nam, Y. M. Goh, Y. J. Lee, M. H. Lim and C. Kim,
Inorg. Chem., 1999, 38, 3238; (d) E. Porhiel, A. Bondon and J. Leroy,
Tetrahedron Lett., 1998, 39, 4829.
5 M. Palacio, V. Mansuy-Mouries, G. Loire, K. Le Barch-Ozette, P. Leduc,
K. M. Barkigia, J. Fajer, P. Battioni and D. Mansuy, Chem.Commun.,
2000, 1907.
6 The only Fe(III)(b-polynitroporphyrin)complexes described so far4a have
been obtained from a nitration method that only led to a mixture of penta-
and hexa-nitroporphyrin derivatives.
7 TDCPP and TDCPNxP represent tetrameso(2,6-dichlorophenyl)por-
phyrin dianion and TDCPP bearing x b-NO2 substituents (1 @ x @ 8)
respectively; SCE = saturated calomel electrode.
8 W. Nam, M. H. Lim, H. J. Lee and C. Kim, J. Am. Chem. Soc., 2000, 122,
6641.
9 Previous literature data indicate that hydroxylation of anisole by
Fe(TDCPP)Cl-based systems, that are believed to involve iron–oxo
active species, mainly leads to para-hydroxyanisole: M. N. Carrier, C.
Scheer, P. Gouvine, J. F. Bartoli, P. Battioni and D. Mansuy, Tetrahedron
Lett., 1990, 31, 6645.
(2)
Chem. Commun., 2001, 1718–1719
1719