Manganese(III) porphyrin covalently bound to sol-gel derived silica (Mn(III) porphyrinosilica): A reusable and green heterogeneous photocatalyst for oxidative decarboxylation of &α-arylacetic acids with H 2O2

Article abstract of DOI:10.3184/174751911X12983110046865

Manganese(III) (5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin)chloride, Mn^III(TDCPP)Cl, was chemically (covalently) bound to a silica matrix by a sol-gel method and used as a new reusable heterogeneous photocatalyt for the selective and efficient oxidative decarboxylation of α-arylacetic acids with H₂O₂ as a mild and clean oxidant at room temperature. The activity of this heterogeneous photocatalytic system is higher than that of a corresponding homogeneous systems and the catalyst can be reused several times without loss of its activity and selectivity.

Full text of DOI:10.3184/174751911X12983110046865

JOURNAL OF CHEMICAL RESEARCH 2011
RESEARCH PAPER 157
MARCH, 157–160
Manganese(III) porphyrin covalently bound to sol-gel derived silica
(Mn(III) porphyrinosilica): a reusable and green heterogeneous
photocatalyst for oxidative decarboxylation of α-arylacetic acids
with H2O2
Saeid Farhadia*, Abedien Zabardastia and Mohammad Hosien Rahmatib
aDepartment of Chemistry, Lorestan University, Lorestan, Khoramabad, Iran
bEducational andTraining Department, Qom, Iran
Manganese(III) (5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin)chloride, MnIII(TDCPP)Cl, was chemically (cova-
lently) bound to a silica matrix by a sol-gel method and used as a new reusable heterogeneous photocatalyt for
the selective and efficient oxidative decarboxylation of α-arylacetic acids with H2O2 as a mild and clean oxidant at
room temperature. The activity of this heterogeneous photocatalytic system is higher than that of a corresponding
homogeneous systems and the catalyst can be reused several times without loss of its activity and selectivity.
Keywords: manganese(III) porphyrinosilica, α-arylacetic acids, photocatalyst, oxidative decarboxylation, hydrogen peroxide
Metalloporphyrins (MPs) have been widely used as active
homogeneous catalysts in the oxidation of organic substrates
in recent years (for a recent review see ref.1 and references
cited therein).2,3 They are model systems for studying the
mechanism of mild oxidation of organic compounds in the
liquid phase by biocatalysts such as monooxygenase cyto-
chrome P-450.4 Furthermore, the photoredox chemistry of
MPs in the presence of organic compounds, which resulted in
reduction of the former and oxidation of the latter, have been
extensively studied.5 It is well known that the photocatalytic
efficiency of MPs is comparable to that of the TiO2 semicon-
ductor and new oxidation catalysts based on photoexcited MPs
have been developed in homogeneous solutions.6,7 However, it
is difficult to separate these expensive and valuable complexes
from the reaction mixture mainly due to their high solubility
in polar media which impedes their ready recovery and reuse.
For this reason, heterogenisation of MPs for catalytic purposes
has attracted particular attention,8–10 since the solid supports:
(i) make the handling and recycling of the system easier;
(ii) allow a freer choice of the reaction medium; (iii) can
sometimes control the efficiency and selectivity of the photo-
catalytic processes; (iv) stabilise unhindered MPs against
intermolecular self-oxidation or µ-oxo-dimerization; and (iv)
minimise the problem of industrial waste treatment and dis-
posal. A variety of solids such as molecular sieves,11 silica,12
zeolites,13 clays,14 as well as organic polymers15,16 and resins17
have been tested as the support of MPs. Among them, sol-gel
derived silica provides a good host matrix for the immobilisa-
tion of enzymes and MPs due to its inertnes and stability even
under drastic conditions.18,19 In this context, metalloporphyri-
nosilicas in which the MP molecules were covalently bound to
silica, have been prepared by the sol-gel process as a new class
of hybrid organic-inorganic materials.20–25
to their corresponding aldehydes and ketones.30–35 Photode-
carboxylation of these acids has also been reported in the pres-
ence of various electron acceptors or photosensetizers.36–42
Most of reported methods produce a large amount of metal-
containing waste, for example Cr, Mn and Hg derivatives and
thus cause environmental problems. Therefore, the develop-
ment of catalytic methods involving clean oxidants to carry out
selective oxidative decarboxylation of arylacetic acids is still
an active area of research.
Here, we wish to report on the oxidative decarboxylation of
various arylacetic acids with H2O2 over a Mn(III) porphyrin
covalently bound to sol-gel derived silica as a reusable hetero-
geneous photocatalyst which resulted in the formation of the
corresponding aldehydes and ketones in high yields. To the
best of our knowledge, this is the first report of photocatalytic
decarboxylation of arylacetic acids with H2O2 in the presence
of an MP covalently bonded to silica as the heterogeneous
photocatalyst.
Experimental
Materials and methods
All chemicals were used as received from different commercial
sources (Merck, Aldrich, Fluka) without further purification. UV-Vis
spectra were recorded on a SINCO S-2100 UV-Vis spectrophotome-
ter. Photocatalytic reactions were performed with an Osram 400W
high-pressure mercury lamp equipped with a cool water circulating
filter to absorb the near IR and a 320 nm cut-off filter in order to avoid
direct photolysis of organic substrates. The distance between the light
source and reaction vessel was 20 cm. The total incident light flux was
6.88 × 10−4 Einstein cm−2 min−1 as determined by ferrioxalate acti-
nometry. GC-MS analyses of organic products of acids were carried
out on a Shimadzu QP5050 GC-MS instrument. Quantitative analysis
to determine the GC-yields has been carried out with calibration
curves obtained with authentic samples. Identification of the GC-MS
spectrometric features was achieved with the use of the Wiley library.
All library-matched species exhibited the degree of match better
than 95%. The 1H NMR spectra were recorded on a Bruker 300 MHz
spectrometer in CDCl3. Elemental analysis was performed using a
Carlo Erba 1106 instrument. Mass spectra of porphyrins and metalo-
porphyrins were measured on a Finnigan Mat 251 mass spectrometer
using the fast atom bombardment (FAB) method.
Preparation of 5,10,15,20-tetrakis(2,6-dichlorophenyl)porphyrin,
H2(TDC)PP:43 This porphyrin was prepared by adding aldehyde
(10 mmol) and pyrrole (10 mmol) to dry CH2Cl2 (500 mL) in a
1000 mL three-necked flask. The solution was stirred magnetically
at room temperature under a nitrogen atmosphere. After purging the
solution for 5 min, the appropriate amount of BF3 etherate (1.32 mL
of a 2.5 M solution in CH2Cl2, 3.3 mmol), was added. The reaction
vessel was shielded from ambient lighting. A sample of p-chloranil
(1.84 g, 7.5 mmol) in powder form was added at once and the flask
One of the best known chemical reactions of carboxylic
acids and their derivatives is decarboxylation. Among the vari-
ous carboxylic acids, oxidative decarboxylation of arylacetic
acids has been given considerable attention due to its impor-
tance in biology and medicine.26–29 As an example, most
non-steroidal anti-inflammatory drugs (NSAIDs) such as
Ketoprofen, Indomethacin and Ibuprofen, have an α-arylacetic
acid or α-arylpropionic acid structure. It seems that these drugs
are metabolised in vivo through an oxidative decarboxylation
pathway. Study of the mode of metabolism of NSAIDs is
one of the main reasons for the attention to decarboxylation of
arylacetic acids. A variety of stoichiometric methods has been
reported for the oxidative decarboxylation of arylacetic acids
* Correspondent. E-mail: sfarhad2001@yahoo.com

158 JOURNAL OF CHEMICAL RESEARCH 2011
was immersed in a water bath preheated to 45 °C. The solution was
refluxed for 1 hour and then concentrated by rotary evaporation to
a volume of approximately 100 mL. The solution WAS poured into
a 250 mL, round-bottomed flask containing 10 g of Florisil and the
resulting slurry was evaporated to dryness. The powder was poured on
top of a column (2 cm diameter) packed with a bed of Florisil, 25 cm
in height. Flash chromatography with CH2Cl2/petroleum ether (9:1)
eluted both porphyrin and polypyrrylmethenes. The eluant was
concentrated, diethyl ether was added, and the solution was slowly
concentrated again. Filtration afforded crystals of the porphyrin in
22% overall yield. Anal. Calcd for C44H22N4Cl8: C, 59.3; H, 2.5;
N, 6.3. Found: C, 59.3; H, 2.5; N, 6.3%. MS (FAB), [M+H]+,
m/z 887–905; UV-Vis (CHCl3; log ε): 418 nm (100%), 512 (4.5),
588 (4.1), 611 (3.0), 657 (3.1); 1H NMR (CDCl3), δ: 8.62 (s, 8H), 7.68
(m, 12H), –2.59 (s, 2H).
reflux overnight and the latter complex was purified in a silica column
using dichloromethane as eluent. MnTDC(SO2Cl)PPCl (Yield: 85%);
UV-Vis [CH2Cl2, {λ(ε mol−1 l cm−1)}] = 378, 400 (shoulder) 480
(9.8 × 103, Soret band), 580 nm.
Preparation of Mn(III) porphyrinosilica: This material was
prepared according to the reported method as follows:46 To a round
bottom flask containing MnTDC(SO2Cl)PP (0.042 mmol), dry
dichloromethane (8 mL) and pyridine (0.500 mmol) was added 3-
aminopropyltriethoxysilane (APTES, 0.017 mmol). The mixture was
refluxed under an argon atmosphere for 6 h. Then, pyridine (0.5 mmol)
as a template, tetraethoxysilane (TEOS, 3 mL), ethanol (3 mL) and
water (0.5 mL) were added to the flask. The reaction vessel was open
to air and left to stand for 9 days at room temperature. The resulting
solid was washed with water (20 mL), methanol (10 mL), acetone
(10 mL) and dichloromethane (10 mL), successively. The solid was
ground and washed with dichloromethane (25 mL), methanol (25 mL)
and acetonitrile (25 mL) in a Soxhlet extractor during 3, 2 and 2 days,
respectively. UV/Vis spectroscopy of the combined washing liquids
was performed to determine the amount of Mn(III) porphyrin that
did not polymerise, which was less than 1%. The Mn (III) porphyrin
loading onto silica was 18 wt.%. The solid material had the Soret
band at 470 nm which has blue shifted when compared with Mn(III)
porphyrin spectrum (Soret band at 480 nm). Finally, the Mn(III)
porphyrinosilica without the template molecule was obtained through
Soxhlet extraction in dichloromethane (50 mL) acidified with 10%
HCl (5 mL) according to Scheme 1.
Preparation of 5,10,15,20-tetrakis(2,6-dichlorophenyl-3-sulfonyl-
phenyl) porphyrin, H2TDC(SO3H)PP:44 5,10,15,20-tetrakis(2,6-
dichlorophenyl)porphyrin (100 mg, 0.11 mmol) was stirred with neat
chlorosulfonic acid (6 mL, 90 mmol) at 100 °C for 3 h. After cooling
the solution to room temperature, ice/water was added slowly (30–
40 g) to give a green crystalline precipitate of diprotonated 5,10,15,20-
tetrakis(2,6-dichlorophenyl-3-sulfonylphenyl)porphyrin. The required
compound was extracted immediately into chloroform, which was
then dried (MgSO4) and evaporated to give the solid H2TDC(SO3H)
PP (115 mg, 89%). Anal. Calcd for C44H18N4O8Cl12S4: C, 41.1; H, 1.4;
N, 4.4. Found: C, 40.6; H, 1.7; N, 4.1%. MS (FAB), [M+H]+, m/z
1279; UV-Vis (CHCl3; log ε): 422 (100%), 514 (6.2), 548 (2.9), 592
Photocatalytic oxidative decarboxylation of α-arylacetic acids with
H2O2 over Mn(III) porphyrinosilica
1
(1.7), 658 nm (1.6); H-nmr (CDCl3), δ: 8.65 (s, 8H), 8.60 (d, 4H),
8.05 (d, 4H), -2.40 (s, 2H).
To a solution of α-arylacetic acid (1 mmol) in acetonitrile (20 mL) in
a 50 mL Pyrex cell containing a Teflon-coated magnetic stirring bar,
was added hydrogen peroxide (0.5 mL, 30%) and Mn(III) porphyrino-
silica (0.1 g). The reaction mixture was then placed in a water bath
with its temperature adjusted to 25 2 °C. It was irradiated with a
high-pressure Mercury lamp (HPML, λ > 320 nm) while every 2 h
was added hydrogen peroxide (0.2 mL, 30%). The progress of reac-
tion was followed by TLC and GC-MS. After an appropriate time, the
irradiation was stopped and the Mn(III) porphyrinosilica photocata-
lyst was separated via filtration. The filtrate was concentrated and
chromatographed on a silica-gel plate or column with n-hexane–
EtOAc as the eluting solvent to give the pure product. The results are
shown in Table 1. All products are commercially available and were
identified by comparison of their physical and spectroscopic data
(m.p., TLC, IR, GC-MS) with those of authentic samples. Decarbox-
ylation of arylacetic acids was also investigated in the presence
of homogeneous Mn(III) porphyrin (MnTDCPPCl, 0.025 mmol) in a
similar manner above. After 24 h, the reaction mixture was analysed
by GC-MS. The results obtained are compared with the heterogeneous
system in Table 1.
Metallation of H2(TDC)PP and H2TDC(SO2Cl)PP:45 The porphyrin
(100 mg) was added to a 25 mL flask containing N,N-dimethylfor-
mamide (10 mL), 1 minute was allowed for complete solution to
occur, and then a stoichiometric amount of manganese(II) chloride
(0.5 g) was added and reaction was allowed to proceed under reflux on
a hot plate. After a few minutes completion of the reaction was
checked by the loss of the free porphyrin red fluorescence under long
wave UV light or spectrophotometrically by withdrawing a small ali-
quot. The reaction vessel was removed from the hot plate and cooled
in an ice-water bath for 15 minute. Chilled distilled water (1 L) was
then added and the resulting partially crystalline precipitate was
filtered in a Buchner funnel until the filtrate was clear. The filtered
material was washed with water and then air dried. MnTDCPPCl
(Yield: 98%): UV-Vis [CH3OH,{λ(ε mol−1 l cm−1)}] = 372, 398
(shoulder), 466 (7.9 ×104, Soret band), 506 (1.1×104), 576, 616 nm.
MnTDC(SO3H)PPCl (Yield: 93%): UV-Vis [CH3OH,{λ(ε mol−1
l
cm−1)}] = 372, 398 (shoulder), 466 (7.9×104, Soret band), 506
(1.1 × 104), 576, 616 nm. Then, MnTDC(SO3H)PPCl was converted
into MnTDC(SO2Cl)PPCl byreaction with 10 mL thionyl chloride at
Scheme 1 Preparation of Mn(III) porphyrinosilica.

JOURNAL OF CHEMICAL RESEARCH 2011 159
Table 1 Results of the oxidative decarboxylation of arylacetic acids with H2O2 over Mn(III) porphyrinosilicaa
Entry
Ar
R
Heterogeneous photocatalyst
Homogeneous photocatalyst
Time /h
Yieldb,c /%
Time /h
Yieldb /%
1
2
C6H5
H
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
A: 100 (93)
A: 100 (95)
A: 100 (92)
A: 100 (92)
A: 100 (96)
A: 100 (93)
A: 100 (95)
A: 100 (96)
A: 100 (94)
A: 100 (94)
A: 12.5
A: 32.5; C: 2
A: 55; C: 45
C: 100 (92)
A: 85 (79); B: 15
A: 22; B: 10
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
A: 18; B: 1; C: 5
4-MeC6H4
3-MeC6H4
2-MeC6H4
4-MeOC6H4
2-MeOC6H4
2,4-(MeO)2C6H3
4-ClC6H4
3-ClC6H4
2-ClC6H4
2,6-(Cl)2C6H3
4-FC6H4
H
A: 9.5; B: 1.5; C: 2
A: 4.5; B: 1.5; C: 0.5
A: 7; B: 1.5; C: 1
3
H
4
H
5
H
A: 20.5; B: 7.5; C: 3.5
A: 9.5; B: 2.2
A: 27; B: 14
A: 19; B: 2.2; C: 6.3
A: 15; B: 2.2; C: 4.3
A: 18.5; B: 2.2; C: 2.8
A: 8; B: 2.6; C: 0.6
A: 22; B: 3.2; C: 6.5
A: 22.5; B: 3.2; C: 5.5
B: 9.4; C: 15.5
6
H
7
H
8
H
9
H
10
11
12
13
14
15
16
H
H
H
4-NO2C6H4
1-Naphthyl
C6H5
H
H
C2H5
C6H5
A: 3; B: 2.2; C: 6.3
A: 46.25; B: 2; C: 6.3
C6H5
Conditions: substrate (1 mmol), oxidant (0.5 mL) in CH3CN (20 m), r.t. b GC-yields. c The numbers in parenthesis are isolated yield of
products.
Table 2 Reusability of the Mn(III) porphyrinosilica
conditions (Table 1, entries 2-13). Phenylacetic acid (Table 1, entry 1)
and acids having electron-donating groups e.g. –Me and –OMe on
their aromatic ring, (Table 1, entries 2–11), were oxidised into the
corresponding aldehydes in excellent isolated yields (≥ 90%) while
systems with electron-withdrawing substituents, e.g. –F and –NO2
(Table 1, entries 12–13) gave lower yields and increasing the irradia-
tion time did not improve the yields. In contrast to ring-substituted
phenylacetic acids, 1-naphthylacetic acid gave 1-naphthycarboxylic
acid as the sole product in high yield (Table 1, entry 14). Under the
same conditions, a secondary α-aryl– and an α,α–diaryl-carboxylic
acid were also converted into ketone and alcohol in low to moderate
yields (Table 1, entries 15–16). Note that sterically hindered arylacetic
acids such as 2,6-dichlorophenylacetic acid (Table 1, entry 11) and
diphenylacetic acid (Table 1, entry 16) have been decarboxylated with
very low yields under the reaction conditions, confirming that steric
effects play an important role in this photoreaction.
photocatalyst.a
Run
1
2
3
4
Yield (%)b,c 100 (93)
100 (91)
100 (92)
100 (91)
a Conditions: substrate (1 mmol), oxidant (0.5 mL) in CH3CN
(20 m), r.t.
b GC-yields.
c The numbers in parenthesis are isolated yield of products.
Reuseability of the photocatalyst
After the decarboxylation of phenylacetic acid, the reaction mixture
was filtered and the heterogeneous photocatalyst was separated.
The recovered catalyst was washed, dried and then reused for four
further runs. The results presented in Table 2.
In all cases, results obtained over the heterogeneous photocatalytic
system were compared with the homogeneouse system under similar
conditions. As shown in Table 1, the oxidative decarboxylation of
arylacetic acids in homogeneous solution leads to the formation of
various products including aldehydes, ketones and acids while hetero-
geneous oxidation leads to the formation of aldehydes as the sole
product. The results clearly indicate that the Mn(III) porphyrinosilica
is much more active than the parent homogeneous system, so that it
took shorter reaction times. Higher selectivity was observed for the
heterogeneous system. As expected, the immobilisation of Mn(III)
porphyrin on a solid matrix strongly affects the chemoselectivity of
the oxidation process and increases both the photochemical efficiency
and the stability of the Mn(III) porphyrin, mainly due to uniform
dispersion and site isolation of the MP covalently bonded to the
silica.
In order to establish the reusability and stability of the Mn(III)
porphyrinosilica photocatalyst, it was separated from the reaction
mixture after its first use in the decarboxylation of phenylacetic acid.
The recovered catalyst was found to be reusable for four runs without
significant loss in activity (Table 2). At the same time, the concentra-
tion of Mn in the filtrate was determined to be less than 0.1 % by
ICP-AES. These observations confirm that the Mn(III) porphyrino-
silica is stable under the reaction conditions and was not affected by
the reactants. The high stability of Mn(III) porphyrin chemically
bound to the sol-gel derived silica, prevents both fast oxidative
degradation of the porphyrin ring and the formation of µ-oxo-dimers.
Although the exact mechanism for this reaction over Mn(III) por-
phyrinosilica is not very clear, on the basis of the above observations
and reported mechanisms in the literature,5,47 a plausible mechanism is
suggested. As shown in Scheme 3, catalytic reduction is initiated by
coordinating the carboxyl group of the acid to Mn(III) of MP (a). One
Results and discussion
In initial experiments when a mixture of phenylacetic acid (1,
1 mmol), H2O2 (0.5 mL) and manganese(III) porphyrinosilica (0.1 g)
in acetonitrile solution (20 mL) was irradiated with a high pressure
mercury lamp (HPML, λ > 320 nm), benzaldehyde (2) was formed as
the sole product in 100 % GC-yield (93 % isolated yield) after 10 h
(Scheme 2).
Blank experiments showed that no decarboxylation occurs when
Mn(III) porphyrinosilica or irradiation were omitted from the system.
Also, the reaction did not take place when the metal-free porphyrin
was used revealing that the Mn(III) ion in the Mn(III) porphyrinosilica
plays an important role in the oxidation process.
To evaluate the scope and limitations of this method, the oxidative
decarboxylation of various α-arylacetic acids was studied under the
same reaction conditions. The results are listed in Table 1. As can be
seen, a variety of ring-substituted phenylacetic acids having various
electron-donating and electron-withdrawing groups were converted
into the corresponding aldehydes in moderate to excellent yields
without over-oxidation to the carboxylic acids under the reaction
Scheme 2

160 JOURNAL OF CHEMICAL RESEARCH 2011
Scheme 3 The proposed photocatalytic pathway for the oxidative decarboxylation of α-arylacetic acids with H2O2 over
Mn(III) porphyrinosilica.
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Conclusions
In conclusion, a new and green method with high efficiency
and selectivity has been presented for the oxidative decarbox-
ylation of arylacetic acids by using H2O2 as an inexpensive,
readily available clean oxidant in the presence of Mn(III)
porphyrin covalently bonded to silica as a heterogeneous pho-
tocatalyst. The activity of this heterogeneous photocatalytic
system was higher than homogeneous system and can be
reused several times, without loss of its activity and selectivity.
This work should be of interest to organic and inorganic
photochemists, and biologists.
Received 23 November 2010; accepted 30 January 2011
Paper 1000443 doi: 10.3184/174751911X12983110046865
Published online: 23 March 2011
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