New manganese β-polynitroporphyrins as particularly efficient catalysts for biomimetic hydroxylation of aromatic compounds with H2O2

Article abstract of DOI:10.1039/b001776k

A series of Mn porphyrins bearing one to eight β-nitro substituents were synthesized in high yield in three steps by using a new general method for selective nitration of a Zn(meso-tetraarylporphyrin); this Mn porphyrin series exhibits a remarkably wide span of Mn(III)/Mn(II) redox potentials from -290 to + 1150 mV (vs. SCE), and the Mn porphyrins bearing one to five β-nitro groups are particularly good catalysts for hydroxylation of aromatic compounds with H₂O₂, with yields up to 98, 83, 80 and 12%, respectively for anisole, naphthalene, acetanilide and ethylbenzene.

Full text of DOI:10.1039/b001776k

New manganese b-polynitroporphyrins as particularly efficient catalysts for
biomimetic hydroxylation of aromatic compounds with H2O2
Jean-François Bartoli, Virginie Mouries-Mansuy, Karine Le Barch-Ozette, Magali Palacio, Pierrette Battioni
and Daniel Mansuy*
UMR 8601, Université Paris V, 45 Rue des Saints-Pères, 75270 Paris Cedex 06, France.
E-mail: Daniel.Mansuy@biomedicale.univ-paris5.fr
Received (in Basel, Switzerland) 6th March 2000, Accepted 7th April 2000
Published on the Web 27th April 2000
A series of Mn porphyrins bearing one to eight b-nitro
substituents were synthesized in high yield in three steps by
using a new general method for selective nitration of a
Zn(meso-tetraarylporphyrin); this Mn porphyrin series ex-
hibits a remarkably wide span of Mn(III)/Mn(II) redox
potentials from 2290 to + 1150 mV (vs. SCE), and the Mn
porphyrins bearing one to five b-nitro groups are partic-
ularly good catalysts for hydroxylation of aromatic com-
pounds with H2O2, with yields up to 98, 83, 80 and 12%,
respectively for anisole, naphthalene, acetanilide and
ethylbenzene.
spectrometry. Their detailed synthesis and characteristics will
be published elsewhere.
Preparation of the corresponding Mn complexes was per-
formed by reaction of Mn(OAc)2 with the free bases
TDCPNxPH2. Insertion of Mn(II) was increasingly easier as the
number of NO2 substituents increased. For instance, insertion of
Mn(II) in TDCPPH2 required several hours reaction at 153 °C in
DMF, whereas insertion in TDCPN5PH2 was complete within a
few minutes at 20 °C in CHCl3–CH3OH (80+20). After
purification by column chromatography and treatment with
gaseous HCl in aerobic CH2Cl2, the Mn-porphyrin complexes
bearing between one and four b-nitro groups exhibited UV-VIS
spectra characteristic of Mn(III)Cl porphyrins with a red-shifted
Soret peak around 490 nm (482, 487, 493 and 498 nm for x =
1, 2, 3 and 4, respectively) and a smaller peak around 400 nm.
Their mass spectra (MALDI) showed molecular peaks appear-
ing as isotopic clusters identical to those simulated for
Mn(TDCPNxP)Cl, and major peaks corresponding to
Mn(TDCPNxP). Above x = 4, the Mn(TDCPNxP) compounds
mainly exist as Mn(II) complexes. For x = 5, even treatment
Cytochrome P450-dependent monooxygenases very efficiently
catalyze the epoxidation of alkenes, the hydroxylation of
alkanes and of aromatic rings, as well as N- or S-oxidations by
O2 or oxygen atom donors.1 Three successive generations of
Fe(III) and Mn(III) porphyrins have been developed during the
last fifteen years as catalysts mimicking cytochrome P450
chemistry.2 Several systems based on such Fe(III) or Mn(III
)
porphyrins were found to be very efficient for alkene epoxida-
tion with quantitative yields and high turnover numbers, and for
alkane hydroxylation with satisfactory yields.2 Reproducing the
P450-dependent selective hydroxylation of aromatic rings
appears to be more difficult, presumably because of the very
easy further oxidation of the expected phenol products in the
medium. In fact, systems using strong oxidants such as PhIO
that directly react by themselves with phenols generally led to
low hydroxylation yields.3 The best reported aromatic hydrox-
ylation yields with these systems have been obtained with
H2O2, a milder oxidizing agent.3c,d,f However, even these H2O2-
dependent systems only led to hydroxylation yields of between
10 and 60% for the most reactive aromatic molecules. Some of
the best reported yields were obtained with H2O2 in the presence
of Mn(III) porphyrins bearing several electron-withdrawing
substituents.3d
Based on these data, it appeared to us that it would be
interesting to compare the abilities of Mn-porphyrins bearing
one to eight b-nitro substituents as catalysts for aromatic
hydroxylation with H2O2. We have recently reported4 the
synthesis of a Mn b-heptanitroporphyrin and showed that the
redox potential of this Mn-porphyrin is more than 1 V higher
than that of Mn(TDCPP).5 Much more recently, we have
succeeded in finding a new general method for selective
nitration of Zn(TDCPP) which affords a full series of Zn
porphyrins bearing one through to eight b-nitro groups in high
yield. Here we report that some of the corresponding Mn
complexes are particularly good catalysts for the hydroxylation
of aromatic compounds with H2O2.
with HCl in aerobic CH2Cl2 led to a mixture of Mn(II
)
(TDCPN5P) and Mn(III) (TDCPN5P)Cl. For x = 6, 7 and 8, it
was only possible to isolate the Mn(II) (TDCPNxP) complexes,
which showed a single Soret peak at 466, 467 and 468 nm,
respectively, and molecular peaks by mass spectrometry
(MALDI) corresponding to Mn(TDCPNxP). The existence of
these electron-poor Mn porphyrins as Mn(II) complexes is not
surprising; similar behavior was previously reported for b-
polyhalogenated Mn-porphyrins6 and for Mn(TDCPN7P) it-
self.4 This is also easily explained if one considers the high
redox potentials (vs. SCE) found for the Mn(III)/Mn(II) couple
which vary from +750 mV for x = 5 to + 1150 mV for x = 8.
In fact, the Mn(TDCPNxP) series with x = 1–8 exhibits a
remarkably wide span of Mn(III)/Mn(II) redox potentials that
range from 2290 to +1150 mV (vs. SCE in 0.1 M NBu4PF6–
CH2Cl2). The oxidation states of the Mn complexes were
clearly established on the basis of their UV-VIS and EPR
spectra and of their electrochemical properties.
The ability of these Mn-porphyrin complexes to act as
catalysts for the hydroxylation of aromatic compounds with
H2O2 was studied in the presence of a cocatalyst, ammonium
Scheme 1 shows the procedure used for preparing the eight
Mn-porphyrins, Mn(TDCPNxP) with (1 @ x @ 8), from
Zn(TDCPP). Titration of Zn(TDCPP) with increasing amounts
of HNO3–CF3SO3H–(CF3SO2)2O selectively afforded the eight
compounds of the Zn(TDCPNxP) series with yields between 50
and 95%. All compounds of the Zn(TDCPNxP) and
TDCPNxPH2 series were fully characterized by elemental
analysis and UV–VIS, 1H NMR spectroscopy and mass
Scheme 1
DOI: 10.1039/b001776k
Chem. Commun., 2000, 827–828
This journal is © The Royal Society of Chemistry 2000
827

Table 1 Hydroxylation of aromatic compounds with H2O2 in the presence of Mn b-polynitroporphyrins and ammonium mandelatea
Substrate
Anisole
Naphthalene
Ethylbenzene
Total
yield (%) a-OH
Total
yield (%) CH3
PhCHOH-
p-OH + arom./
Products
p-OH o-OH
b-OH
PhCOCH3 p-OH
o-OH
o-OH
benz. (%)
Mn(TDCPP)Cl
Mn(TDCPNP)Cl
Mn(TDCPN2P)Cl
Mn(TDCPN3P)Cl
Mn(TDCPN4P)Cl
Mn(TDCPN5P)
Mn(TDCPN6P)
Mn(TDCPN7P)
Mn(TDCPN8P)
67
90
88
88
71
32
7
6
7
8
10
8
5
1
< 1
< 1
73
97
96
98
79
37
8
62
72
75
61
43
39
20
18
1
5
7
8
8
6
5
3
3
< 1
67
79
83
69
49
44
23
21
1
20
23
27
21
26
22
7
22
15
15
14
15
5
12
24
5
< 1
2
2
6
8
< 1
1
1
3
4
< 2
3
3
< 5
8
7
26
29
37
9
12
10
< 2
< 2
< 2
6
4
< 1
< 1
< 1
< 1
< 1
< 1
5
< 1
5
< 2
4
1
a Conditions: Mn-porphyrin+H2O2+cocatalyst ratio = 1+50+20, [Mn-porphyrin] = 2 mM in CH2Cl2–CH3CN (1+1), 2 h at 20 °C in the presence of substrate
in excess; substrate/catalyst = 3000, 1600 and 500 for anisole, ethylbenzene and naphthalene, respectively. Yields (%) are based on H2O2; p-OH and o-OH
represent para- and ortho-hydroxylated products; arom./benz. is the ratio (%) between aromatic hydroxylation and benzylic oxidation products derived from
ethylbenzene.
acetate, that was previously described to give good results for
the Mn porphyrin-catalyzed transfer of an oxygen atom of H2O2
to various hydrocarbon substrates.7 Thus, Mn(TDCPP)Cl
catalyzed the hydroxylation of anisole to para- and ortho-
hydroxyanisole with 60 and 4% yields based on starting H2O2
when anisole was used in large excess relative to H2O2. From
various ammonium carboxylates that have been tested as
necessary cocatalysts for that reaction, ammonium mandelate
gave the best hydroxylation yields. The latter cocatalyst was
thus used in our comparison of the various Mn-porphyrins
(Table 1). Interestingly, Mn(TDCPN1P)Cl led to markedly
higher yields of anisole hydroxylation with an almost quantita-
tive use of H2O2 for formation of 90% and 7% para- and ortho-
aromatic ring with yields up to 12%. Another aromatic
compound bearing an electron-donating substituent, acet-
anilide, was hydroxylated with the Mn(TDCPN2P)Cl–H2O2–
ammonium mandelate system, affording p-hydroxyacetanilide
in 80% yield. Benzene itself or aromatic compounds bearing
electron-withdrawing substituents were much poorer substrates,
and lead to phenol yields < 5%.
The origin of the particular efficiency of Mn(TDCPNxP)Cl
catalysts, with x = 1–4, for hydroxylation of electron-rich
aromatic compounds with H2O2, and of the changes observed in
the regioselectivity of these hydroxylations as a function of x
(Table 1) is currently under investigation. It is likely that the
lower activities of Mn catalysts with x = 5–8 are due to their
pronounced tendency to exist in the Mn(II) state.
hydroxyanisole
respectively.
Mn(TDCPN2P)Cl
and
Mn(TDCPN3P)Cl also gave very high total yields based on
H2O2 and were recovered unchanged at the end of the reaction
(ca. 50 turnovers under the used conditions). With more b-nitro
substituents, the hydroxylation yields greatly decreased from
79% with Mn(TDCPN4P)Cl to < 2% with Mn(TDCPN8P).
Similar results were observed for hydroxylation of naphthalene,
the best total yield of formation of a- and b-naphthols being
maximum (83%) for Mn(TDCPN2P)Cl (Table 1). Interestingly,
the regioselectivity slightly changed upon increasing the
number of b-nitro substituents [a+b ratio from 12 with
Mn(TDCPP)Cl to ca. 6 for Mn(TDCPN7P)]. The most
spectacular results were observed in the case of ethylbenzene.
Its Mn(TDCPP)Cl catalyzed oxidation with H2O2 exclusively
led to products arising from hydroxylation of its very reactive
benzylic position, 1-phenylethanol and acetophenone. Upon
introduction of an increasing number of b-NO2 substituents on
the catalyst, para- and ortho-hydroxyethylbenzene were formed
in significant amounts and the aromatic hydroxylation yield
increased up to 12% for x = 4; then it markedly decreased for
x > 5. Thus, the aromatic hydroxylation/benzylic hydroxylation
ratio reaches maximum values ca. 30% for x = 3–5.
Notes and references
1 Cytochrome P450: Structure, Mechanism and Biochemistry, ed. P. Ortiz
de Montellano, Plenum Press, New York, 1995.
2 For general reviews, see: B. Meunier, Chem. Rev., 1992, 92, 1411; D.
Mansuy, Coord. Chem. Rev., 1993, 125, 129; R. A. Sheldon, Metal-
loporphyrins in Catalytic Oxidations, M. Dekker, New York, 1994; J. T.
Groves and Y. Z. Han, in Cytochrome P450: Structure, Mechanism and
Biochemistry, ed. P. Ortiz de Montellano, Plenum Press, New York,
1995, p. 3; D. Dolphin, T. G. Traylor and L. Xie, Acc. Chem. Res., 1998,
31, 155.
3 (a) C. K. Chang and F. Ebina, J. Chem. Soc., Chem. Commun., 1981, 778;
(b) J. R. Lindsay-Smith and P. R. Sleath, J. Chem. Soc., Perkin Trans. 2,
1982, 1009; (c) S. Tsuchiya and M. Seno, Chem. Lett., 1989, 236; (d)
M. N. Carrier, C. Scheer, P. Gouvine, J. F. Bartoli, P. Battioni and D.
Mansuy, Tetrahedron Lett., 1990, 31, 6645; (e) K. Ikida, M. Nango, K.
Okada, S. Matsumoto, M. Matsuura, K. Yamashita, K. Tsuda, Y. Kuruno
and Y. Kimura, Chem. Lett., 1994, 1307; (f) see also, as a recent review:
B. Meunier, A. Robert, G. Pratviel and J. Bernadou, in The Porphyrin
Handbook, ed. K. M. Kadish, K. M. Smith and R. Guilard, Academic
Press, New York, 2000, vol. 31, 119.
4 K. Ozette, P. Battioni, P. Leduc, J. F. Bartoli and D. Mansuy, Inorg.
Chim. Acta, 1998, 272, 4.
5 TDCPP and TDCPNxP represent meso-tetra(2,6-dichlorophenyl)por-
phyrin dianion and TDCPP bearing x b-NO2 substituents (1 @ x @ 8)
respectively.
6 P. Hoffmann, A. Robert and B. Meunier, Bull. Soc. Chim. Fr., 1992, 129,
85.
7 A. Thellend, P. Battioni and D. Mansuy, J. Chem. Soc., Chem. Commun.,
1994, 1035.
The above results illustrate the particular efficiency of Mn-
porphyrins bearing between one and four b-nitro substituents
for hydroxylation of aromatic molecules with H2O2 in the
presence of an NH4CO2R cocatalyst. By a proper choice of the
Mn catalyst it is possible to hydroxylate anisole and naph-
thalene, used in excess relative to H2O2, with yields (based on
H2O2) up to 98 and 83%, respectively. Even ethylbenzene, that
has a very reactive benzylic position, was hydroxylated at its
828
Chem. Commun., 2000, 827–828