An efficient synthesis of poly(ethylene glycol)-supported iron(II) porphyrin using a click reaction and its application for the catalytic olefination of aldehydes

Article abstract of DOI:10.1002/adsc.200800361

The first successful synthesis of poly(ethylene glycol) (PEG) immobilized iron(II) porphyrin using a copper-catalyzed azide-alkyne [3+2] cycloaddition is reported. In contrast to the existing methods for the synthesis of PEG-immobilized metalloporphyrins, the click method offers better catalyst loading under comparatively mild reaction conditions. The prepared complex 5 (PEG-C₅₁H₃₉FeN₇O) was found to be an efficient catalyst for the selective olefination of aldehydes with ethyl diazoacetate in the presence of triphenylphosphine and afforded excellent olefin yields with high (E) selectivities. The PEG-supported catalyst 5 was readily recovered by precipitation and filtration and was recycled for up to ten runs without significant loss of activity.

Full text of DOI:10.1002/adsc.200800361

RETRACTION
DOI: 10.1002/adsc.201180001
Retraction: An Efficient Synthesis of Poly(ethylene glycol)-
Supported Iron(II) Porphyrin using a Click Reaction and its
Application for the Catalytic Olefination of Aldehydes
Suman L. Jain,a,c Jomy K. Joseph,b,c Fritz E. Kꢀhn,b,* and Oliver Reisera,*
a
Institut fꢀr Organische Chemie, Universitꢁt Regensburg, Universitꢁtsstr. 31, 93053 Regensburg, Germany
Fax : (+49)-941-943-4121; e-mail: oliver.reiser@chemie.uni-regensburg.de
Catalysis Research Center (CRC) der Technischen Universitꢁt Mꢀnchen, Lichtenbergstrasse 4, 85747 Garching bei
b
Mꢀnchen, Germany
Fax : (+49)-89-289-13473; e-mail: fritz.kuehn@ch.tum.de
Chemical and Biotechnology Division, Indian Institute of Petroleum, Dehradun – 248005, India
c
Received: April 17, 2011; Published online: May 27, 2011
The following article from Advanced Synthesis & alytic reactions with a mixture of 2/3 under the identi-
Catalysis, “An Efficient Synthesis of Poly- cal concentrations and stoichiometries as reported in
(ethylene glycol)-Supported Iron(II) Porphyrin using the original paper and have obtained identical results,
a Click Reaction and its Application for the Catalytic i.e. that the olefination of aldehydes in the presence
Olefination of Aldehydes” by Suman L. Jain, Jomy K. of 2/3 proceeded in high yields and that this catalyst
Joseph, Fritz E. Kꢀhn and Oliver Reiser, published system can be reused in multiple cycles by simple pre-
online on January 12, 2009, in Wiley InterScience cipitation from diethyl ether and filtration. In addi-
(now: Wiley Online Library), and in print Volume tion, the authors have tested an equimolar mixture of
351, Issue 1+2, 2009, pages 230-234, has been retract- MeOPEG 2 and simple FeTPPCl as a catalyst mix-
ed by agreement between the authors, the Editor, Dr. ture. For the olefination of p-chlorobenzaldehyde on
Joe P. Richmond, and Wiley-VCH Verlag GmbH & a 5 mmol scale with an initial catalyst concentration
Co. KGaA. The retraction has been agreed because it of 1 mol% they observed in 10 runs quantitative
was discovered that the click reaction had not oc- product formation, and after the 10th run could still
curred and that the iron porphyrin was not covalently recover about 60% (by weight) of the catalyst. In
bound to the poly(ethylene glycol) support. The prin- comparison, analogous experiments performed with
cipal hypotheses and conclusions are therefore not FeTPPCl in the absence of MeOPEG 2 resulted in a
verified.
complete loss of the catalyst after the 3rd cycle. They
The procedure reported by the authors for the therefore conclude that MeOPEG is a relative effec-
propargylation of MeOPEG 3 did not lead to 4 but tive support for iron porphyrins without the need of
had only resulted in reisolation of starting material. covalent connection of both parts.
1
They inadvertently misassigned the signal in the H
Nevertheless, catalyst 5 has now been prepared by
NMR spectrum at 1.98 ppm for the propargylic a different procedure and is an efficient, reusable cat-
proton instead of residual water, being misled by the alyst for the catalytic olefination of aldehydes. The
considerable shift the water signal displays in the authors will report these results in due course.
presence of MeOPEG in CDCl3. Consequently, the
The authors are indebted to Dr. V. Rodionov, Uni-
synthesis of 5 was not successful as well, but only a versity of California, Berkeley, USA, for bringing this
mixture of azido ironporphyrine 2 and MeOPEG 3 matter to our attention.
has been obtained. The authors have repeated all cat-
Adv. Synth. Catal. 2011, 353, 1391
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1391

FULL PAPERS
DOI: 10.1002/adsc.200800361
An Efficient Synthesis of Poly(ethylene glycol)-Supported
Iron(II) Porphyrin using a Click Reaction and its Application
for the Catalytic Olefination of Aldehydes
Suman L. Jain,a,c Jomy K. Joseph,b,c Fritz E. Kꢀhn,b,* and Oliver Reisera,*
a
Institut fꢀr Organische Chemie, Universitꢁt Regensburg, Universitꢁtsstr. 31, 93053 Regensburg, Germany
Fax : (+49)-941-943-4121; e-mail: oliver.reiser@chemie.uni-regensburg.de
Catalysis Research Center (CRC) der Technischen Universitꢁt Mꢀnchen, Lichtenbergstrasse 4, 85747 Garching bei
b
Mꢀnchen, Germany
Fax : (+49)-89-289-13473; e-mail: fritz.kuehn@ch.tum.de
Chemical and Biotechnology Division, Indian Institute of Petroleum, Dehradun – 248005, India
c
Received: June 10, 2008; Revised: December 22, 2008; Published online: January 12, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800361.
Abstract: The first successful synthesis of poly(ethy- presence of triphenylphosphine and afforded excel-
lene glycol) (PEG) immobilized iron(II) porphyrin lent olefin yields with high (E) selectivities. The
using a copper-catalyzed azide-alkyne [3+2]cycload- PEG-supported catalyst 5 was readily recovered by
dition is reported. In contrast to the existing methods precipitation and filtration and was recycled for up
for the synthesis of PEG-immobilized metallopor- to ten runs without significant loss of activity.
phyrins, the click method offers better catalyst load-
ing under comparatively mild reaction conditions.
The prepared complex 5 (PEG-C51H39FeN7O) was Keywords: click chemistry; iron; olefination; poly-
found to be an efficient catalyst for the selective ole-
A C H T U N G T R E N N U N G (ethylene glycol) (PEG); porphyrins; s
fination of aldehydes with ethyl diazoacetate in the lysts
Introduction
ty of being soluble in most common organic solvents
but also being insoluble in some such as diethyl
The need to improve both the efficiency and environ- ether.[2]
mental acceptability of catalytic processes has created
Metalloporphyrins, owing to their high reactivity
considerable interest in the development of supported and remarkable stability, have been recognized as ef-
metal catalysts and reagents that maintain high selec- ficient yet versatile catalysts, which find a broad spec-
tivity and reactivity in organic synthesis.[1] Since tran- trum of applications in organic synthesis.[3] However,
sition metal complexes are often expensive and diffi- the homogeneous nature of these catalysts render
cult to prepare, immobilization on polymeric supports their recovery and recycling difficult, which can be
provides a means to handle them more easily and to overcome by anchoring them to polyA C H T U N G T R E N N U N G (
facilitate recycling from the reaction mixtures. Soluble via a covalent attachment.[4]
organic polymers have recently emerged as most
The copper-catalyzed [3+2][5] azide-alkyne cycload-
promising and convenient supports for the immobili- dition (CuAAC)[6] termed ꢂclick reactionꢃ is becoming
zation of transition metal complexes. Reactions pro- an indispensable tool for ligating functionalized mole-
moted by a soluble, polymer-supported catalyst can cule fragments and has been well acknowledged for
consequently be run under homogeneous conditions the preparation of functionalized polymers, dendrim-
while the catalyst itself can be easily recovered from ers, diverse scaffolds and immobilized catalysts and
the reaction mixture by precipitation and filtration reagents.[7] Apart from the few reports on the
upon completion. Among the various polymeric ma- CuAAC for directly immobilizing metal complexes on
trices, polyA C H T U N G T R E N N U N G
(ethylene) glycols pholydmaerprsoumpipnoerntst,[8p]osiit-s potential for immobilizing
tion due to their ease of accessibility, facile function- metalloporphyrins to polymer supports remains unex-
alization, inexpensive nature and their unique proper- plored.
230
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An Efficient Synthesis of Poly(ethylene glycol)-Supported Iron(II) Porphyrin
FULL PAPERS
Results and Discussion
PEG-immobilized iron(II) porphyrin 5 in a virtually
quantitative reaction as evidenced by complete disap-
We report here the first successful synthesis of a pearance of the typical azide band (2219 cmÀ1). The
PEG-immobilized iron(II) porphyrin using a copper- obtained black coloured, PEG-bound catalyst 5 was
catalyzed azide-alkyne [3+2]cycloaddition ꢂclick re- precipitated with diethyl ether and separated by filtra-
actionꢃ and its catalytic activity for the olefination of tion, washed with 2-propanol and diethyl ether, and
aldehydes using ethyl diazoacetate in the presence of dried under vacuum (96% yield). The complex load-
triphenylphosphine.
ing was found to be 0.15 mmolgÀ1 as estimated from
The unsymmetrically substituted azidoporphyrin 1, the nitrogen content, being determined by elemental
accompanied by some tetra-p-tert-butylphenylpro- analysis.[10]
phyrin, was readily prepared according to a literature
The catalytic potential of catalyst 5 was tested for
procedure.[9] As noted before,[9] the free porphyrin 1 the olefination of aldehydes with ethyl diazoacetate in
is only moderately stable and was therefore directly presence of triphenylphosphine as a reducing agent
metallated with ferrous acetate in DMF at 908C to (Scheme 2).[11]
give 2 in nearly quantitative yield (Scheme 1). Attach-
In a typical experiment, a 10-mL Schlenk tube
ment of iron porphyrin 2 to MeOPEG5000 3 could placed in a preheated oil bath at 808C was charged
easily be achieved in an easy two-step sequence: with benzaldehyde (1 mmol), ethyl diazoacetate
Propargylation of 3 gave rise to 4, which, on subse- (1.2 mmol), triphenylphosphine (1.2 mmol) and cata-
quent copper(I) iodide-catalyzed cycloaddition with lyst (1 mol%) in toluene under a nitrogen atmos-
azidoporphyrin 2 resulted in covalently attached, phere. The homogeneous mixture was vigorously
Scheme 1. Synthetic route to PEG-immobilized iron porphyrin using a click reaction.
Adv. Synth. Catal. 2009, 351, 230 – 234
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Suman L. Jain et al.
FULL PAPERS
Table 1. Olefination of aldehydes using PEG-immobilized
iron(II) porphyrin (PEG-C51H39FeN7O) 5 as catalyst.[a]
Entry Substrate
Catalyst Reaction Yield[b] E/Z
Loading Time [h] [%]
[mol%]
ratio[c]
Scheme 2. Olefination of aldehydes.
1
2
3
1
5
1
3.0
2.5
3.5
96
97
98
24
26
18
stirred under nitrogen at the same temperature. Com-
plete conversion was achieved within 3 h, upon which
the catalyst could easily be recovered from the reac-
tion mixture in quantitative yield by precipitation
with diethyl ether and filtration, and was then reusa-
ble as such for subsequent runs. The cinnamate was
obtained upon purification in 96% isolated yield with
an E/Z ratio of 24.
4
5
1
1
4.0
5.0
96
96
16
12.5
To evaluate the scope of this catalytic system, we
studied the olefination of a variety of aldehydes both
aromatic and aliphatic types under the described reac-
tion conditions (Table 1). In all cases, almost quantita-
tive conversion, high yields and (E) selectivity were
achieved, being comparable to those reported for the
homogeneous iron porphyrins with the added advan-
tages of facile recovery of the catalyst 5 from the re-
action mixture and its reusability.
Among the various aromatic aldehydes studied, in
general those substituted with electron-withdrawing
groups (Table 1, entries 8–11) were found to be more
reactive and required shorter reaction times than ben-
6
7
1
1
6.5
1.5
97
98
13.4
15
8
1
2.0
97
13
9
1
1
1
1.5
96
98
94
15
10
11
1.5
16
10.0
18.5
zaldehyde.
However,
p-methoxybenzaldehyde
(Table 1, entry 7) and p-
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(dimethyl)aminobenzalde-
12
1
8.0
85
10.5
hyde (Table 1, entry 12) were found to be the least re-
active and the reactions could be only completed
after prolonged reaction times. In case of the three
methylbenzaldehydes, the para-isomer was found to
react faster than the ortho- and meta-derivatives
(Table 1, entries 4–6).
13
14
1
1
2.5
3.5
97
97
12.5
15
To evaluate the effect of catalyst loading, we stud-
ied the olefination of benzaldehyde by using different
catalyst concentrations (Table 1, entries 1 and 2)
under similar reaction conditions. With increasing cat-
alyst concentration (from 1 to 5 mol%), there was a
marginal decrease in the reaction time although the
selectivity remained almost the same, indicating the
effectiveness of the PEG-supported catalyst even at
low concentrations.
To check the recyclability and reusability of the
prepared catalyst 5 we studied the olefination of ben-
zaldehyde as a representative example. After comple-
tion, the catalyst separated from the reaction mixture
15
1
1
6.0
7.5
92
85
10
16
8.5
[a]
Reaction conditions: aldehyde (1 mmol), ethyl diazoace-
tate (1.2 mmol), Ph3P (1.2 mmol), catalyst (1 mol%,
0.01 mmol), toluene (2 mL) at 808C under N2 atm.
Isolated yields.
[b]
[c]
E/Z ratio determined by 1 H NMR.
by precipitation with diethyl ether and subsequent fil- high and almost unchanged during these recyclability
tration was reused as such in up to ten runs under experiments.
identical reaction conditions (Table 2). In the recy-
The exact mechanism of the reaction is not clear at
cling experiments there was only a small decrease in this stage. The reaction mechanism for the olefination
catalytic efficiency in each cycle within ten runs, of aldehydes with diazoacetate in the presence of tri-
moreover, the selectivity of the product remained phenylphosphine by using Fe(II) porphyrin has been
232
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An Efficient Synthesis of Poly(ethylene glycol)-Supported Iron(II) Porphyrin
FULL PAPERS
Table 2. Results of recyclability experiments.[a]
cooled to room temperature, the solvent was removed under
reduced pressure and the obtained residue was dissolved in
ethyl acetate. The organic layer was washed with water (3
times), dried over anhydrous MgSO4 and concentrated
under reduced pressure to give propargylated MeOPEG5000
Run Substrate
Hours at r.t. Yield[b] [%] E/Z ratio
1
2
3
4
5
6
7
8
9
10
Benzaldehyde 3.0
96
96
95
95
95
92
85
80
78
75
24
24
24
24
23.5
23.5
23.5
23
23
23
Benzaldehyde 3.0
Benzaldehyde 3.0
Benzaldehyde 3.5
Benzaldehyde 3.5
Benzaldehyde 3.5
Benzaldehyde 4.5
Benzaldehyde 4.5
Benzaldehyde 5.5
Benzaldehyde 6.5
1
4; yield: 19.5 g (97%); H NMR (300 MHz, CDCl3): d=1.98
(s, 1H, CH), 3.42 (s, 3H, OMe).
Synthesis of azido iron(II) porphyrin: Into a stirred solu-
tion of azidoporphyrin (1 mmol, 0.853 g) in dry DMF
(25 mL) was added iron(II) acetate (1.2 mmol, 0.2 g) at
room temperature. The resulting solution was heated at
858C for 8 h under a nitrogen atmosphere. After comple-
tion, the solvent was evaporated under reduced pressure
and the resulting residue was dissolved in ethyl acetate,
washed with brine solution, dried and the solvent was
evaporated under reduced pressure to yield azido iron(II)
porphyrin; yield: 92%; MS: m/z= 863 (M+ÀN2); IR: n=
2933, 2870, 2220, 1981, 1638, 1492, 1231, 901 cmÀ1; anal.
calcd. for C56H51FeN7: C 76.61, H 5.86, N, 11.17; found: C
72.95, H 5.70, N 10.38.
[a]
Benzaldehyde (1 mmol), ethyl diazoacetate (1.2 mmol),
triphenylphosphine (1.2 mmol) and catalyst 5 (1 mol%),
808C in toluene. Recovery of the catalyst by precipita-
tion with diethyl ether.
[b]
Isolated yields.
Immobilization of azido iron(II) porphyrin 2 to PEG by
the click reaction: A solution of azidoiron(II) porphyrin
(1.2 mmol, 1.0 g), CuI (5 mol%), triethylamine (1 mL) and
propargylated MeOPEG5000 4 (1 mmol, 5.0 g) in dry CH2Cl2
(30 mL) was vigorously stirred under reflux for 12 h under a
nitrogen atmosphere. After completion, the reaction mixture
was concentrated under reduced pressure and the residue
obtained was treated with diethyl ether. The precipitated
black solid was isolated by filtration, washed thoroughly
with 2-propanol, diethyl ether and dried under vacuum to
afford PEG-immobilized Fe(II) porphyrin 5; yield: 5.5 g
(96%); UV-vis (CH3CN): lmax =355, 310, 240 nm. The load-
ing of the iron porphyrin compound 2 was found to be
0.15 mmolgÀ1 based on the nitrogen content (1.47% N)
being determined by elemental analysis.[10] IR: n=2883,
1622, 1540, 1466 cmÀ1.
studied in detail by Woo et al.[12] They reported that
iron(II) complexes catalytically convert diazo esters
in the presence of phosphines to phosphoranes which
in turn on reaction with aldehydes yield olefins and
phosphine oxide. We assume an analogous mechanism
for our catalytic system.
Conclusions
In summary, the first successful synthesis of a soluble
PEG-immobilized iron porphyrin by using a copper-
catalyzed azide-alkyne [3+2]cycloaddition ꢂclick re-
actionꢃ is reported. The click method offers the ad-
vantages of being simple as well as allowing a high
catalyst loading and mild reaction conditions, when
compared to the previously known methods for im-
mobilization of metalloporphyrins. The PEG-immobi-
lized iron porphyrin catalyst can be conveniently used
General Experimental Procedure for the Olefination
of Aldehydes
Into a 10-mL Schlenk tube placed in a preheated oil bath at
808C were added aldehyde (1 mmol), ethyl diazoacetate
as a recoverable and recyclable catalyst in the olefina- (1.2 mmol), triphenylphosphine (1.2 mmol) and catalyst 5 (1
mol%) in toluene (2 mL) under a nitrogen atmosphere. The
homogeneous mixture was stirred for the period indicated
in Table 1. After completion of the reaction, the resulting
mixture was cooled to room temperature and concentrated
under reduced pressure. The residue was treated with dieth-
yl ether, the precipitated PEG-supported iron porphyrin 5
was recovered by filtration and washed with Et2O. The fil-
trate was concentrated under reduced pressure and purified
by column chromatography using hexane/ethyl acetate
(15:1) to give the pure olefin.
tion of aldehydes with ethyldiazoacetate in the pres-
ence of triphenylphosphine. The PEG-immobilized
Fe(II) porphyrin catalyst afforded yields and selectivi-
ties comparable to those obtained with homogeneous
iron porphyrins for the olefination of aldehydes but
with the added advantages of facile recovery and re-
cyclability of the catalyst.
Experimental Section
Synthesis of PEG-Immobilized Iron(II) Porphyrin (5)
Acknowledgements
Propargylation of MeOPEG5000: Into a stirred solution of
MeOPEG5000 3 (4 mmol, 20.0 g) in acetone (60 mL), K2CO3
(7 mmol, 1.0 g) and KI (0.6 mmol, 0.1 equiv., 0.1 g) were
added at room temperature. Propargyl bromide (80 wt% so-
lution in toluene, 1.0 mL, 7.5 mmol) was added dropwise
and the resulting mixture was refluxed for 12 h. After being
This work was supported by DST, New Delhi (India) and
DAAD, Germany, as a Project based Personal Exchange
Programme (PPP), the Alexander von Humboldt Founda-
tion and the Bavarian Elite Network NANOCAT.
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Suman L. Jain et al.
FULL PAPERS
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