Synthesis of poly(ethylene glycol)-supported manganese
porphyrins: efficient, recoverable and recyclable catalysts for
epoxidation of alkenes
Maurizio Benaglia,*a Tamara Danellia and Gianluca Pozzi*b
a Dipartimento di Chimica Organica e Industriale, Università degli Studi di Milano,
via Golgi 19, 20133 Milano, Italy
b CNR-Istituto di Scienze e Tecnologie Molecolari, via Golgi 19, 20133 Milano, Italy.
E-mail: Maurizio.Benaglia@unimi.it
Received 8th November 2002, Accepted 2nd December 2002
First published as an Advance Article on the web 2nd January 2003
Two new poly(ethylene glycol) supported manganese por-
phyrins have been prepared and their catalytic activity and
recyclability were investigated for the epoxidation of
alkenes using H2O2 and PhIO as stoichiometric oxidants.
polymeric starting material to which meso-tetraarylporphyrins
bearing free -OH substituents could be conveniently linked.
Mesylates 3a, 3b, 3c were prepared in three steps and 95%
overall yield starting from commercially available mono-
methyl ether of PEGs (MeOPEGs, MW 750, 2000 and 5000,
respectively) as previously described.10 Our first approach
involved the attachment of four poly(ethylene glycol) chains to
a single molecule of the commercially available, symmetrically
The development of catalysts anchored to solid supports has
been one of the areas of most intense research activity over the
past years. The possibility of recovering and recycling catalysts
which are often expensive has positive effects from the econom-
ical and environmental points of view. A further benefit is the
ease of product isolation and purification.1 Among the different
polymeric matrixes employed, poly(ethylene glycol)s (PEGs)
have recently emerged as very convenient supports for the
synthesis of a variety of small organic molecules, ligands and
catalysts.2 This inexpensive, readily functionalized class of
polymers provides a distinct advantage over other supports,
being soluble in most organic solvents but insoluble in a few
common ones, like Et2O. Thus a reaction catalysed by a PEG-
supported catalyst can be run under homogeneous (and likely
best performing) conditions while the catalyst itself can be
easily recovered as if it was bound to an insoluble polymer.3 We
have recently reported the synthesis of achiral phase transfer
catalysts,4 chiral organic catalysts5 and chiral ligands6 anchored
to PEGs modified with a linker and spacer with a suitable
functional group. In many cases the obtained systems showed
catalytic activity and stereocontrol ability similar to (and some-
times even higher than) those displayed by the corresponding
non-supported species. As a part of this project, we have also
investigated the immobilisation of metalloporphyrins. Indeed,
such compounds are known to be versatile catalysts for several
organic reactions, but their synthesis is often tedious and
low-yielding and their recovery from reaction mixtures is low.
Immobilisation of porphyrins and metalloporphyrins onto
insoluble organic and inorganic polymers has been investigated
for many years, but the advantages of these heterogeneous
systems are often counterbalanced by the limited accessibility
of active sites to organic substrates, especially in the case of
oxidation reactions.7
substituted 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin
(Scheme 1).
4
The reaction of 1 mol. equiv. of 4 with 4 mol. equiv. of the
mesylates 3a,b in DMF in the presence of 12 mol. equiv. of
Cs2CO3 afforded the desired supported porphyrin derivatives
5a,b, respectively. It is worth mentioning that 5a with
MeOPEGs750 substituents did not precipitate as a solid upon
addition of Et2O to the reaction mixture, but it was recovered
as a very thick oil (93% yield).11 However by employing the
mesylate 3b of MeOPEG2000 the expected PEG-supported
porphyrin 5b was obtained as a solid that precipitated out by
pouring the reaction mixture in Et2O (87% yield).12 The PEG-
supported porphyrin 5b was then reacted with Mn(OAc)2ؒ
4H2O in DMF at 160 ЊC for 18 hours; after elimination of the
solvent by distillation under vacuum, the crude mixture was
dissolved in CH2Cl2 and shaken with saturated aqueous NaCl.
Cold Et2O was added to the organic layer to induce the pre-
cipitation of the manganese porphyrin complex 1 bound to four
MeOPEGs700 chains as a green solid in 70% yield after filtration
and two washings with Et2O. TLC analysis clearly indicated the
presence of a single species with no trace of starting porphyrin.
Formation of the manganese complex 1 was confirmed by
UV–Vis spectroscopy, which showed a shift of the character-
istic 421 nm peak of 5b (Soret band, free-porphyrin) to 480 nm,
corresponding to that of a complexed porphyrin. In order to
increase the catalyst loading, the attachment of the unsym-
metrically substituted 5-(4-hydroxy-2,6-dichlorophenyl)-10,15,
20-tris(2,6-dichlorophenyl)porphyrin 6 to a single MeOPEG5000
residue was envisaged (Scheme 1).
Porphyrin 6 was prepared in 14% yield following the pro-
cedure previously described for the synthesis of its isomer 5-(3-
hydroxy-2,6-dichlorophenyl)-10,15,20-tris(2,6-dichlorophenyl)-
porphyrin.13 Reaction of mesylate 3c with 6 in DMF gave the
PEG-supported porphyrin 7 which was isolated after precipit-
ation with Et2O in 90% yield. Treatment of 7 with Mn(OAc)2ؒ
4H2O and anion exchange with saturated aqueous NaCl
afforded the PEG-supported manganese porphyrin 2 in 78%
yield. The UV–Vis spectra of 2 showed the diagnostic peak at
478 nm and the total absence of the Soret band of the free-
porphyrin 7 at 417 nm. The inverse reaction sequence (com-
plexation of porphyrin 6 followed by immobilisation of the
manganese complex onto PEG) was also feasible and it
afforded 2 in comparable overall yield (73% vs. 71%). The
A few metalloporphyrins linked to PEGs have been syn-
thesized, mainly to tune their solubility properties for bio-
medical applications,8 but recently the first example of the use
of similar complexes in catalysis has been reported.9 This
prompted us to disclose our preliminary results in the field.
Herein we describe the synthesis of two PEG-supported
manganese porphyrins 1 and 2 (Scheme 1) and the study of
their catalytic activity in the epoxidation of alkenes.
Results and discussion
On the basis of our experience in the PEG-supported synthesis
of small organic molecules,10 mesylate 3 was selected as the
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 4 5 4 – 4 5 6
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
454
Benaglia, Maurizio
