Chemoselective aerobic oxidation of 4-allylanisol by Fe(III) porphyrins in an aqueous system

Article abstract of DOI:10.1016/j.tetlet.2011.03.050

The allyl moiety of 4-allyl-anisol was oxidized in the presence of a Fe(III) porphyrin derivative to the corresponding α,β-unsaturated aldehyde in an initial oxidation step with perfect chemoselectivity. Molecular oxygen was employed as the sole environmental innocuous oxidant. The reaction was performed in an aqueous buffer/CH₂Cl₂ mixture using the detergent Tween 80 to homogenize the system.

Full text of DOI:10.1016/j.tetlet.2011.03.050

Tetrahedron Letters 52 (2011) 3555–3557
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Tetrahedron Letters
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Chemoselective aerobic oxidation of 4-allylanisol by Fe(III) porphyrins
in an aqueous system
Olena Vakuliuk a, Francesco G. Mutti b, Miguel Lara b, Daniel T. Gryko a, , Wolfgang Kroutil b,
⇑
⇑
a Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, Warsaw 01-224, Poland
b Department of Chemistry, Organic and Bioorganic Chemistry, University of Graz, Heinrichstrasse 28, A-8010 Graz, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
The allyl moiety of 4-allyl-anisol was oxidized in the presence of a Fe(III) porphyrin derivative to the cor-
responding ,b-unsaturated aldehyde in an initial oxidation step with perfect chemoselectivity. Molecu-
lar oxygen was employed as the sole environmental innocuous oxidant. The reaction was performed in an
aqueous buffer/CH2Cl2 mixture using the detergent Tween 80 to homogenize the system.
Ó 2011 Published by Elsevier Ltd.
Received 31 January 2011
Revised 7 March 2011
Accepted 9 March 2011
Available online 17 March 2011
a
Keywords:
Oxidation
Porphyrin
Oxygen
Terminal alkene
Aldehyde
Metallo porphyrins are currently heavily investigated as biomi-
metic catalysts.1–3 They have been reported to catalyze various
oxidation reactions like epoxidation, hydroxylation, dehydrogena-
tion, dealkylation, and oxidation of sulfides, amines, alcohols and
aldehydes.2d,e The reactions were performed in general in organic
solvent using various oxidants like iodosylbenzene, 2,6-Cl2pyNO,
tBuOOH, oxone or very seldom H2O2 and O2.2 In a previous study
we observed the cleavage of selected 1-phenyl-1-propene deriva-
tives to yield benzaldehyde derivatives catalyzed by hemin at the
expense of molecular oxygen in aqueous solution.4
3%. Nevertheless, 5,15-dibromo-10,20-diphenylporphyrin iron(III)
chloride 3i led to improved conversion (9%) and was therefore em-
ployed for further optimisation study.
It has to be noted that the transformation of 1 to 2 by 3i was
highly chemoselective as judged by the GC, thus 1 was exclusively
transformed to aldehyde 2; neither epoxidation nor formal alkene
hydration were observed; epoxidation and alkene hydration are
generally reported to occur under the conditions employed.1 Fur-
thermore, no over-oxidation to the corresponding carboxylic acid
was detected.
We speculated that mono-substituted alkenes like 4-allylanisol
1 could be cleaved, too. However, in contrast to our expectation,
transformation of the terminal alkene 4-allylanisol 1 employing
hemin as catalyst in buffer (pH 7, 50 mM Bis–Tris) led exclusively
To improve the reaction, detergents were tested to get from the
bi-phasic system to a more homogeneous system (Fig. 2). Tween
80 turned to be the most suitable detergent for the transformation.
Again, the transformation displayed perfect chemoselectivity.
Since the detergent concentration employed in the first test was
much above the critical micelle concentration (CMC) for Tween 80
(CMC = 0.002 vol %), the conversion was measured at varied Tween
80 concentration (Fig. 3). Best conversion (59%) was achieved at
0.0025 vol % of Tween 80. At elevated conversion two further prod-
to the formation of a minimal amount of the a,b-unsaturated alde-
hyde 2 (3%) at 2 bar molecular oxygen and 25 °C (Scheme 1). To
test whether other porphyrins lead to higher conversion, a selec-
tion of Fe(III) porphyrins were tested. Porphyrins like Fe(III)-TPP
3a or 3b (Fig. 1) had to be pre-dissolved in an organic solvent, since
they were barely soluble in buffer. From all solvents tested CH2Cl2
turned out to be the most suitable. Employing Fe(III)-TPP 3a
(0.25 mol %) in a two phase system (buffer/CH2Cl2 = 95:15) led al-
ready to 6% conversion. Other iron porphyrin complexes (Fig. 1, 3
b–h, 3j) showed only poor activity as catalysts, giving 2 in up to
Fe3+-porphyrin derivative 3
buffer, 25°C
25°C
H
O
O
O
2
1
H2O
O2
2 bar
⇑
Corresponding authors. Tel.: +43 3163805350; fax: +43 3163809840 (W.K.).
E-mail addresses: danieltgryko@gmail.com (D.T. Gryko), wolfgang.kroutil@
Scheme 1. Oxidation of the terminal alkene of the allyl group yielding the
a,b-unsaturated aldehyde.
unigraz.at (W. Kroutil).
0040-4039/$ - see front matter Ó 2011 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2011.03.050

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O. Vakuliuk et al. / Tetrahedron Letters 52 (2011) 3555–3557
250
200
150
100
50
0
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
catalyst
Figure 4. Testing of porphyrin derivatives for the transformation of
1 to 2.
Conditions: 1 (42 mM), Tween 80 (0.0025 mol %), buffer (pH 7, 1 mM Bis–Tris)/
CH2Cl2 = 95:15, catalyst (0.25 mol %), 2 bar O2, 72 h, 25 °C.
aldehyde 2 (<1%) as well as cleavage of the C@C double bond4 of
2 giving p-methoxy benzaldehyde (4%). Nevertheless, the initial
oxidation step showed perfect chemoselectivity, thus 1 was exclu-
sively transformed to 2.
Testing the pH dependence of the transformation between pH
1–9 showed that the transformation is independent of the pH be-
tween pH 2–9; at pH 1 only 20% residual activity was found. For
convenience neutral pH was chosen for further studies. Addition-
ally, variation of the buffer salt concentration showed that just a
minimum buffer salt concentration (1 mM) is sufficient to achieve
comparable good conversion.
With optimized conditions a turnover number (TON) of 216 was
achieved with catalyst 3i. Consequently, all prior tested porphyrin
derivatives were retested under optimized reaction conditions
(Fig. 4). However, no other catalyst performed better than 3i; for
instance 3a and 3f reached TONs around 160. Employing 3i the
reaction was performed using 579 mg substrate 1 leading to 67%
conversion and 32% isolated yield of the pure product 2.5 Again
the excellent chemoselectivity for the first oxidation step could
be confirmed. Performing the same experiment in the absence of
Tween led to 22% conversion without any follow-up products.
Terminal alkenes have been reported previously to be trans-
formed by Ru-porphyrins6 to a ‘Wacker oxidation like product
Figure 1. Porphyrin derivatives used in this study.
10
8
6
4
2
0
(R–CH2–CHO)’ but not to
a,b-unsaturated aldehydes. The transfor-
mation of allyl moieties to
a
,b-unsaturated aldehyde has been
TritonX-100
Tween20
Tween80
Aliquat336
SDS
reported only by using 2,3-dichloro-5,6-dicyanoquinone (DDQ) as
oxidant,7 by the light-promoted oxidation employing Pd(II) and
oxygen,8 with a catalytic amount of [Rh3O(OAc)6(H2O)3]OAc in ace-
tic acid in the presence of tBuOOH,9 and SiO2/ZrO2/Cr (VI) employ-
detergents
Figure 2. Comparision of conversion in the presence of detergents; conditions:
substrate (42 mM), catalyst 3i (0.25 mol %), buffer (Bis Tris 1 mM, pH 7)/
CH2Cl2 = 95:15, 1 vol % detergent, 2 bars oxygen, 25 °C, 24 h.
1
t
ing BuOOH.10
In summary, the oxidation of the allyl moiety of 4-allyl-anisol 1,
60
50
40
a terminal alkene, led to an
a,b-unsaturated aldehyde at the ex-
60
50
pense of molecular oxygen using Fe(III) porphyrin 3i as catalyst.
The reaction was performed under oxygen pressure (2 bar) which
can be considered as safe under the conditions employed, since
the reaction was performed in 86% aqueous buffer. In contrast to
previously reported oxidations of alkenes leading to a product mix-
ture of epoxide, hydroxylation product and ketone formation in
allylic position after a initial first oxidation step, the here presented
oxidation reaction shows excellent chemoselectivity; neither
epoxide, nor alcohol formation was detected: exclusively the
40
30
20
30
20
10
10
0
0.0001
0.001
0.01
0.1
1
a
,b-unsaturated aldehyde was formed in an initial oxidative step.
Tween 80 (vol. %) log.
Acknowledgments
Figure 3. Conversion of
1 (42 mM) at varied detergent concentration (x-axis
logarithmic), buffer (pH 7, 50 mM Bis–Tris)/CH2Cl2 = 95:15, catalyst 3i (0.25 mol %),
2 bar O2, 72 h, 25 °C.
This study was financed by the European community [RTN Net-
work (R) Evolutionary Catalysis MRTN-CT-2006-035866]. We
thank Professor Albrecht Berkessel of the University of Cologne
for providing compounds 3g, 3h, 3j. Financial support by the
ucts were detected, which resulted from a subsequent oxidation of
the initial product 2, namely epoxidation of the
a,b-unsaturated

O. Vakuliuk et al. / Tetrahedron Letters 52 (2011) 3555–3557
3557
European community (THEME KBBE-2009-3-3-02, Project: AmBio-
Cas, Grant agreement no.: 245144) is acknowledged.
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.03.050.
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