Rapid and efficient oxidative decarboxylation of carboxylic acids with sodium periodate catalyzed by manganese (III) Schiff base complexes

Article abstract of DOI:10.1016/j.bmc.2003.12.025

Rapid and efficient oxidative decarboxylatoin of α-aryl carboxylic acids was observed. In the chemical system containing Mn(III)-salophen complex as catalyst, carboxylic acids are converted efficiently to the corresponding carbonyl derivatives with sodium periodate. The ability of various Schiff base complexes in the oxidative decarboxylation of carboxylic acids was also investigated.

Full text of DOI:10.1016/j.bmc.2003.12.025

Bioorganic & Medicinal Chemistry 12 (2004) 903–906
Rapid and efficient oxidative decarboxylation of carboxylic acids
with sodium periodate catalyzed by manganese (III) Schiff
base complexes
Valiollah Mirkhani,a,* Shahram Tangestaninejad,a Majid Moghadamb
and Maryam Moghbela
aDepartment of Chemistry, Isfahan University, Isfahan 81746-73441, Iran
bDepartment of Chemistry, Yasouj University, Yasouj 75914-353, Iran
Received 8 September 2003;revised 20 December 2003;accepted 22 December 2003
Abstract—Rapid and efficient oxidative decarboxylatoin of a-aryl carboxylic acids was observed. In the chemical system containing
Mn(III)-salophen complex as catalyst, carboxylic acids are converted efficiently to the corresponding carbonyl derivatives with
sodium periodate. The ability of various Schiff base complexes in the oxidative decarboxylation of carboxylic acids was also
investigated.
# 2004 Elsevier Ltd. All rights reserved.
1. Introduction
We have chosen the salophen ligand because it is similar
to porphyrin and the electronic and steric nature of the
metal complex can be tuned by introducing electron-
withdrawing and electron-releasing substituents and
bulky groups in the ligand.
Many efficient biomimetic oxidation systems using iron
and manganese porphyrins as catalysts, and various
single oxygen atom donors such as PhIO, ClOꢀ, H2O2,
ROOH or IOꢀ4 have been reported.1ꢀ10 The high effi-
ciency of some of these systems makes them potentially
useful for preparative oxidations in organic synthesis.
Recently, mono- and binuclear transition metal com-
plexes derived from ligands other than porphyrins have
also been employed as catalysts. The use of metal Schiff
base complexes, that is, metal salen and metal salophen
to catalyze the oxidation of hydrocarbons by single oxygen
atom donors has received much attention. Manganese,
chromium, nickel and cobalt Schiff base complexes have
been used for these transformations.11ꢀ20
This catalytic system exhibits a high activity in the oxi-
dative decarboxylation of a-aryl carboxylic acids with
sodium periodate in 1:1 CH3CN/H2O mixture in the
presence of imidazole as axial ligand (Scheme 1).
The type of metal ion and Schiff base and the nature of
reactive intermediate were also investigated.
2. Results and discussion
In continuation of our research on the oxidative dec-
arboxylation of carboxylic acids,21ꢀ23 here we report the
rapid and efficient oxidative decarboxylation of a-aryl
carboxylic acids mediated by manganese (III) salophen
complex.
2.1. Oxidative decarboxylation of carboxylic acids with
different metal-Schiff base complexes
In a preliminary approach to the periodate anion acti-
vation by metal Schiff base complexes, we decided to
investigate the activity of different Schiff base complexes
of Fe, Mn, Co and Ni as metal ions. Table 1 sum-
marizes our trials on catalytic oxidative decarboxylation
of diphenylaceticacid with NaIO4 in the presence var-
ious metal Schiff base complexes (shown in Fig. 1). We
found that the nature of the metal ion has an important
Keywords: Biomimetic oxidation;Schiff base;Oxidative decarboxylation.
* Corresponding author. Tel.: +98-311-7932713;fax: +98-311-
6689732;e-mail: mirkhani@sci.ui.ac.ir
0968-0896/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2003.12.025

904
V. Mirkhani et al. / Bioorg. Med. Chem. 12 (2004) 903–906
Scheme 1.
role on the catalytic properties of Schiff base complexes.
The use of iron, cobalt and nickel complexes gives small
amount of corresponding carbonyl compound in the
oxidative decarboxylation of diphenylaceticacid. How-
ever, the use of manganese (III) complexes, Mn-BPB
[BPB=N,N0 -bis(2-pyridinecarboxamide)-1,2-benzene]
and Mn-salophen resulted in a significant amount of
diphenylketone after 5 min (31 and 100%).
situation have been observed with metalloporphyrin
complexes and NaOCl as oxygen donors.24
2.2. Effect of solvent on the oxidative decarboxylation of
diphenylaceticacid
Among the 1:1 mixture of methanol, ethanol, acetone,
acetonitrile (single phase systems), chloroform and car-
bontetrachloride (two phase systems with Bu4NBr as
phase transfer catalyst) with water, the 1:1 acetonitrile/
water mixture was chosen as the reaction medium,
because metal Schiff base complexes are highly soluble
in this solvent and higher ketone yields were observed.
The results were shown in Table 2.
In comparing the influence of Schiff base ligands on
catalytic activity, the hindered Schiff base ligand, salo-
phen, exhibits a significantly greater catalytic power
than the unhindered Schiff base ligand, salen. The similar
Table 1. Oxidative decarboxylation of diphenylaceticacid by various
metal Schiff base complexes with sodium periodate
2.3. Effect of axial ligand on the oxidative decarboxy-
lation of diphenylaceticacid
Schiff
base
Yield (%)a after
5 min.
Yield (%)a
after 10 min.
Yield (%)a
after 30 min.
One important aspect of this catalytic system is the
modification of the decarboxylation rate by addition of
small amount of imidazole to the reaction mixture. The
formation of diphenylketone in the absence of axial
ligand is very slow and the yields are always below 30%,
whereas the amount of diphenylketone reaches to 100%
in the catalyzed reaction with the imidazole as the axial
base.
1
2
3
100
36
4
100
41
5
100
45
6
4
4
4
4
5
4
5
5
6
7
8
5
10
8
11
9
8
9
31
4
35
4
39
4
The effect of different axial ligands upon the oxidation
rate decreased in the order: imidazole=1-methyl-
imidazole>4 - t - buthylpyridine>4 - methylpyridine>2-
methylpyridine>pyridine.
10
11
10
5
11
6
13
7
a GC yields.
These results may be explained in terms of the greater
stability of Mn(salophen)Cl achieved in the presence of
imidazole and 1-methyl imidazole. As a result, the main
effect of nitrogen donors of the catalytic activity of Mn-
catalyst does not seem to be directly related to the
basicity of the donors. Behavior of nitrogen donors in
catalytic reactions can be explained based on the steric
and electronic property of them for ligation to the
manganese salophen complexes.
Table 2. Effect of solvent on oxidative decarboxylation of diphenyl-
aceticacid with NaIO4/Mn(salophen) system
Solvent
Yield (%)a
after 5 min.
Yield (%)a
after 15 min.
CH3CN/H2O
CH3COCH3/H2O
CH3OH/H2O
CH3CH2OH/H2O
CHCl3/H2O
100
68
45
38
5
100
80
50
42
5
CCl4/H2O
5
5
Figure 1. Transition metal Schiff base complexes used in this study.
a GC Yields.

V. Mirkhani et al. / Bioorg. Med. Chem. 12 (2004) 903–906
905
Table 3. Oxidative decarboxylation of a-aryl carboxylic acids by
Mn-salophen/ NaIO4
2.4. Oxidative decarboxylation of carboxylic acids with
sodium periodate catalyzed by Mn (III)-salophen
Run
1
Substrate
Product
Ketone
Time
(min)
Yielda
(%)
The imidazole-modified Mn-salophen/NaIO4 oxidizing
system can be applied to decarboxylation of a large
number of aryl substituted acetic acids in high yields at
short times and room temperatures. All experiments
were carried out with 2 equiv of sodium periodate per 1
equiv of carboxylic acid. As shown in Table 3, the yields
are between 78–94% and the principal product was
carbonyl derivative (except for 1-Naphtoic acid) and
only small amount of the alcohol derivatives was
obtained.
PhCH2COOH
5
91
2
3
4
Ketone
Ketone
Ketone
5
94
5
5
93
93
Oxidative decarboxylation of anti-inflammatory drugs
such as Indomethacin and Ibuprophen (Run 14,15) at
room temperature afforded corresponding carbonyl
derivatives as the major product in 82 and 94% yields,
respectively. Such oxidative decarboxylation pathway has
been also observed during drug metabolism in vivo.25,26
5
6
Ketone
Ketone
5
5
91
89
Comparison of this homogeneous oxidizing system with
previously reported homogeneous metalloporphyrin
systems21,22 showed that in the Mn-salophen/NaIO4
system the yields are higher than that metalloporphyrin/
NaIO4 systems and the reaction times are shorter, and in
the case of heterogeneous metalloporphyrin system23 the
yields are similar whereas the reaction times are shorter.
7
Ketone
5
89
8
9
Ketone
Ketone
5
5
90
90
In the absence of manganese (III)-salophen catalyst,
NaIO4 has poor ability to decarboxylate aryl carboxylic
acids at room temperature (4–10% yields).
2.5. Oxo manganese (V) species as the reactive intermediate
10
Ketone
10
91
Although we have assumed the active manganese spe-
cies to be reactive (salophen)Mn (V) oxo intermediate,
[(salophen)MnV=O], by comparing the present spectral
observation with the previous reports,27 we could not
isolate this active species. It is pertinent to point out
that to date no (salophen)Mn (V) oxo species have
yielded to structural characterization, although Groves
et al. and others have characterize oxo manganese (V)
porphyrin complexes in recent years.28
11
12
13
Ketone
Alcohol
Ketone
15
15
30
90
84
78
When a clear brown solution of Mn (III)-salophen in
acetonitrile is treated with sodium periodate, it immedi-
ately turns dark. The appearance of a new absorption
band (with lmax=520–530 nm, Fig. 2) is strongly
reminiscent of the spectral change obtained during
the conversion of the Mn (III)-salophen cation to the
corresponding oxomanganese (V) species. Upon stand-
ing, the dark brown solution fades to the original light
brown within 10 min. When the same experiment is
carried out in the presence of diphenylaceticacid, the
dark brown solution immediately changed to its original
light brown color.
14
Ketone
Ketone
30
30
82
92
3. Experimental
15
Schiff base complexes 1–11 (Fig. 1), were prepared as
described by Boucher29 or by the more recently mod-
ified methods.11,30,31 Carboxylic acids were obtained
a Isolated yields.

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V. Mirkhani et al. / Bioorg. Med. Chem. 12 (2004) 903–906
The partial support of this work by Isfahan University
Council of Research is acknowledged.
References and notes
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Figure 2. Absorption spectra of [Mn(III)-salophen]Cl in CH3CN/
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with excess NaIO4.
4. Amatsu, H.;Miyamoto, T. K.;Sasaki, Y. Bull. Chem.Soc.
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3.1. General procedure for oxidative decarboxylation of
ꢀ-aryl substituted carboxylic acids
All of the reactions were carried out at room temperature
in a 25 mL flask equipped with a magnetic stirring bar.
A solution of sodium periodate (2 mmol in 5 mL H2O)
was added to a mixture of carboxylic acid (1 mmol),
Mn-salophen (0.067 mmol) and imidazole (0.067 mmol)
in CH3CN (5 mL). Progress of the reaction was mon-
itored by TLC. After the reaction was completed, the
reaction products were extracted with CH2Cl2 (20 mL)
and were purified by silica gel plate or silica gel column
(eluent: CCl4–Et2O). The identities of products were
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1
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acid (5 mmol) was also investigated. The results showed
that this compound decarboxylated with sodium periodate
in the presence of Mn(salophen)Cl in good yield (92%).
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4. Conclusions
Although many oxidation systems that mimic cyto-
chrome P-450 dependent monooxygenase have been
reported, Mn(III)salophen/NaIO4 catalytic system have
the following advantages: (i) short reaction time (ii) high
efficiency for oxidative decarboxylation of carboxylic
acids;(iii) ease of preparation of the catalyst.
26. Komuro, M.;Higuchi, T.;Hirobe, M.
Medicinal Chem. 1995, 3, 55.
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In comparison with other reported Schiff-base oxidation
systems, performance of the novel Mn(III)salophen/
NaIO4 system can be described as outstanding. There-
fore, in the list of suitable oxidants able to behave like
single oxygen donor to Schiff base complex, sodium
periodate is among the most efficient.