8
48
Chemistry Letters Vol.36, No.7 (2007)
Mild Oxidation of Styrene and Its Derivatives with Ionic Manganese Porphyrin
Immobilized in the Similarly Structured Ionic Liquid
ꢀ
Ye Liu, Hong-Jiao Zhang, Yue-Qin Cai, Hai-Hong Wu, Xiu-Li Liu, and Yong Lu
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Chemistry Department of East China Normal University, Shanghai 200062, P. R. China
(Received March 30, 2007; CL-070343; E-mail: ylu@chem.ecnu.edu.cn)
Without the auxiliary involvement of axial ligands and
Me
+
N
organic solvents, the ionic manganese porphyrin 1c immobilized
in [BPy][BF4] efficiently catalyzed the oxidation of styrene and
its derivatives under mild conditions, affording high activity/
oxide selectivity and good stability even after 5 recycling uses.
N
N
PF6-N
Mn(III)
N
+
+
Me
N
N
Me
+
-
-
N
n-Bu BF4-
(
4
[BPy][BF ])
Various oxidations such as epoxidation of olefins and hy-
droxylation of hydrocarbons with oxygen donors (iodosylarenes,
alkylhydroperoxides, hydrogen peroxide, hypochlorites, period-
ates, etc.) have been catalyzed by metalloporphyrins as model
catalysts of cytochrome P450 in homogeneous organic media,
in which axial ligands like imidazole or pyridine (derivatives)
are required necessarily to activate and stabilize active metal
porphyrin species.1 Anyway, the intermolecular self-aggrega-
tion due to ꢀ–ꢀ stacking and the propensity for oxidation led to
destruction and difficult recovery of metalloporphyrin in such
systems. In order to solve these problems, many heterogeneous
methods have been developed to improve metalloporphyrin
4
6
PF -
+
N
Me
(
1c)
Scheme 1. Multi-component IL system: 1c immobilized in
BPy][BF4].
[
–5
group in para-position of styrene, the selectivities for oxides
were remarkable (100%). If the substituting groups possess elec-
tron-donor nature (–Cl, –Br, –CH3), the increased electron-den-
sity at the ꢁ-C of C=C bond resulted in the formation of benzal-
dehyde through the oxidation of side-chain cleavage. While
for the substrates (Entries 7 and 8) with an internal C=C bond,
the epoxides were the only products, implicating the facial ac-
cess of the substrate to oxomanganese(V) porphyrin domain.13
For the olefins with saturated alkyl group, such as 1-hexene,
1-chloropropene, cyclohexene, etc., the poor conversions were
observed mainly because of the mass-transfer limitation, ascrib-
ing to the immiscibility between the substrates and the conjugat-
ed multi-component IL of 1c–[BPy][BF4].
Upon completion of the reaction, the solution was extracted
with diethyl ether leaving behind the light purple IL which could
be reused for next pass (The leaching of Mn in organic phase was
beyond the limit of ICP detection.) The results in Table 2 indi-
cated that the activity and the stability maintained quite well
even after 5 runs (the fresh, 96% conv.; the fifth run, 71% conv.).
Anyway in the third run, the selectivity of styrene oxide and
phenylacetaldehyde varied greatly. In order to clarify the deriva-
tion of the various oxidized products, the purchased (ꢃ)-styrene
oxide (Johnson Matthery, 98%) was used to replace styrene in
the fresh 1c–[BPy][BF4] system and the used one in the presence
6
–8
dispersion, stability and recovery.
Ambient ionic liquids (ILs) are not only environmentally
benign solvents with unique physical properties, but also liquid
and flexible ‘‘carriers’’ of functional units (such as metal com-
plexes, phosphines etc.) through the formation of covalent
bond.9 For example, to avoid metal catalyst leaching out of
IL system, the efforts have been made to enhance the solubili-
ty/miscibility of the metal catalysts through grafting electron-
donor ligands into ILs which can coordinate with metal cen-
ters,11 or incorporating imidazolium/pyridinium tags into a met-
al complex.12 Herein, we report the oxidation of styrene and its
derivatives with iodosylbenzene (PhIO) oxidant catalyzed by
Mn porphyrin with pyridinium tag 1c which was immobilized
in ambient IL of [BPy][BF4], without auxiliary involvement of
the axial ligand and the volatile organic solvent.
,10
The catalyst of manganese tetrakis-(N-methyl-4-pyridi-
III
nium)porphyrin hexafluorophosphate ([Mn T(N-Me-4-Py)P]-
[
PF6]5, 1c) was miscible in N-butylpyridinium tetrafluoroborate
[BPy][BF4]) due to the ionic nature and the similar skeletons,
(
building up the multi-component IL system which was com-
1
6
of PhIO, respectively (see Supporting Information). It was
found that there was no any conversion of (ꢃ)-styrene oxide
to phenylacetaldehyde, ruling out the possibility of the subse-
quent isomerization of styrene oxide to phenylacetaldehyde.
The formation of phenylacetaldehyde maybe resulted from the
III
5þ
þ
posed of four types of ions ([Mn T(N-Me-4-Py)P] , [BPy] ,
ꢁ ꢁ
[
and its derivatives catalyzed by 1c with PhIO was carried out
PF6] , [BF4] ) (Scheme 1). In open air, the oxidation of styrene
ꢂ
(
Table 1). Under mild reaction conditions (30 C) and without
IV
auxiliary involvement of axial ligands, the isoelectronic products
of styrene oxide and phenylacetaldehyde were obtained in 86%
and 10% yield, respectively, without C=C bond cleavage. Even
the concentration of catalyst 1c was decreased to 0.005 mol %,
the styrene conversion of 41% was obtained, with turnover rate
induced Mn –oxo derivative of 1c which was considered to
react with the substrate by a radical mechanism giving rise to
increased the amount of aldehydes relative to that of oxide.
The decreased catalytic activity of 1c was mainly attributed
to the degradation of the porphyrino ring, but not from the
ꢁ1
16
V
III,IV,V
of 8200/h (see Supporting Information). The oxidation of
styrene derivatives showed that with the electron-withdrawing
derivation of Mn =O species to other Mn –oxo species,
which was only responsible for the selective distribution of
Copyright ꢀ 2007 The Chemical Society of Japan
Liu, Ye
