Proximal effect of the nitrogen bases in the oxidative decarboxylation of phenylsulfinylacetic acids by oxo(salen)chromium(V) complexes

Article abstract of DOI:10.1016/j.molcata.2014.03.021

Oxidative decarboxylation of phenylsulfinylacetic acid (PSAA) and several substituted PSAAs with three oxo(salen)chromium(V) complexes in the presence of nitrogen bases are investigated in 100% acetonitrile medium using spectrophotometric technique. The nitrogenous bases such as imidazole, 1-methyl imidazole and pyridine catalyse the reaction and Michaelis-Menten kinetics is observed with respect to these bases. Among the various bases employed, imidazole with strong π donating ability shows the least accelerating effect and the maximum catalytic activity is observed with pyridine. Both the electron donating and electron withdrawing substituents in PSAA accelerate the reaction rate. The Hammett plots for the three oxo(salen)chromium(V) complexes with three nitrogen bases display a nonlinear upward curvature. The Hammett parameter ρ, changes from large negative to small positive values as the substituents are changed from electron donating to electron withdrawing groups. A mechanism involving direct oxygen transfer from oxo(salen)chromium(V)-nitrogen base adduct to PSAA with simultaneous decarboxylation to yield sulfone is proposed. The nonlinear Hammett plot has been ascribed to a shift in the rate determining step caused by the formation and decomposition of the sulfonium ion intermediate which is susceptible to substituent effect.

Full text of DOI:10.1016/j.molcata.2014.03.021

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Proximal effect of the nitrogen bases in the oxidative decarboxylation
of phenylsulfinylacetic acids by oxo(salen)chromium(V) complexes
P. Subramaniama,∗, S. Sugirtha Devib, S. Anbarasana
a Research Department of Chemistry, Aditanar College of Arts and Science, Tiruchendur 628 216, Tamil Nadu, India
b Department of Chemistry, Kamaraj College, Thoothukudi 628 003, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Oxidative decarboxylation of phenylsulfinylacetic acid (PSAA) and several substituted PSAAs with three
oxo(salen)chromium(V) complexes in the presence of nitrogen bases are investigated in 100% acetoni-
trile medium using spectrophotometric technique. The nitrogenous bases such as imidazole, 1-methyl
imidazole and pyridine catalyse the reaction and Michaelis–Menten kinetics is observed with respect
to these bases. Among the various bases employed, imidazole with strong ␲ donating ability shows the
least accelerating effect and the maximum catalytic activity is observed with pyridine. Both the electron
donating and electron withdrawing substituents in PSAA accelerate the reaction rate. The Hammett plots
for the three oxo(salen)chromium(V) complexes with three nitrogen bases display a nonlinear upward
curvature. The Hammett parameter ꢀ, changes from large negative to small positive values as the sub-
stituents are changed from electron donating to electron withdrawing groups. A mechanism involving
direct oxygen transfer from oxo(salen)chromium(V)-nitrogen base adduct to PSAA with simultaneous
decarboxylation to yield sulfone is proposed. The nonlinear Hammett plot has been ascribed to a shift in
the rate determining step caused by the formation and decomposition of the sulfonium ion intermediate
which is susceptible to substituent effect.
Received 22 November 2013
Received in revised form 19 March 2014
Accepted 21 March 2014
Available online xxx
Keywords:
Oxo(salen)chromium(V)
Phenylsulfinylacetic acid
Oxidative decarboxylation
Oxygen transfer reaction
Effect of nitrogen bases
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
molecules [10–13]. Hence oxidation of sulfoxides to sulfones
has been the subject of extensive studies. Phenylsulfinylacetic
Decarboxylation process plays an important role in the dif-
ferent areas of organic synthesis, biochemical reactions and in
biological systems. Several works on decarboxylation by thermal
[1,2], photochemical [3,4] and catalytic methods [5,6] have been
reported. Drug metabolism taking place in presence of substances
like indomethacin and ibuprofen are reported to act through a
cytochrome p-450 induced decarboxylation process leading to the
release of carbon dioxide that mitigates pain effects [7,8]. Many
drugs, herbicides and pesticides containing a carboxyl group pro-
duce radical intermediates via decarboxylation [9].
acid (PSAA) is an ambidentate ligand and with its three donor
atoms acts as a good chelating agent. It is used in the prepa-
ration of many cephalosporin derivatives, the famous broad
spectrum antibiotic. Like cytochrome p-450 and metallopor-
phyrin complexes many metal(salen) complexes (salen = N,Nꢀ-
bis(salicylidene)ethylenediaminato) act as efficient catalysts in
the epoxidation [14–17] and sulfoxidation [18–21] reactions of
organic substrates with hydrogen peroxide, periodates, hypochlo-
rites and iodosylarenes. Among the various salen complexes,
the oxo(salen)chromium(V−) species ([(salen)CrV = O]+) gener-
ated from [CrIII(salen)]+PF6 complex and PhIO is more stable
in non-aqueous solvents and thus facilitates kinetic studies.
Oxo(salen)chromium(V) ion mimics Cr(V) peptide complexes
which are formed during the intracellular reduction of Cr(VI) by
virtue of mixed N and O ligand chelation.
Sulfoxides as synthetic intermediates find wider application
in the construction of chemically and biologically important
Abbreviations: PSAA, phenylsulfinylacetic acid; salen, N,Nꢀ-bis(salicylidene)-
ethylenediaminato; Im, imidazole; MeIm, 1-methyl imidazole; Py, pyridine; GC–MS,
gas chromatography–mass spectrometry; LC–MS, liquid chromatography–mass
spectrometry; OD, optical density; r, correlation coefficient.
The reactivity of metal(salen) complexes is tuned [22–24] by
adding donor ligands in the reaction mixture. Addition of imidazole,
1-methyl imidazole, pyridine or pyridine-N-oxide to metal(salen)
complexes significantly lowers the oxidation potential by co-
ordination with the metal and results in altering their electro-
chemical properties and reactivity [25–29]. The added nitrogen
∗
Corresponding author. Tel.: +91 9443500381.
E-mail addresses: subramaniam.perumal@gmail.com (P. Subramaniam),
sugirthadevi.s@gmail.com (S. Sugirtha Devi), sanbarasan62@gmail.com
(S. Anbarasan).
http://dx.doi.org/10.1016/j.molcata.2014.03.021
1381-1169/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Subramaniam, et al., J. Mol. Catal. A: Chem. (2014), http://dx.doi.org/10.1016/j.molcata.2014.03.021

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bases alter the selectivity of products in many oxygenation reac-
tions. Realizing the importance of Cr(V) species in biological studies
it is considered worthwhile to study the influence of nitrogen bases
on the electronic structure of oxo(salen)chromium(V) complexes
and also their influence on the reaction rate towards oxidative
decarboxylation. Further, no systematic kinetic study is reported
so far in the literature on PSAA except our recent works on co-
oxidative decarboxylation of PSAA-oxalic acid mixture by Cr(VI)
[30] and oxidative decarboxylation of PSAA with Cr(VI) [31]. This
paper attempts to unravel the effect of substituents in the salen
moiety of oxo(salen)chromium(V) and phenyl ring of PSAA and that
of nitrogen bases like imidazole (Im), 1-methylimidazole (MeIm)
and pyridine (Py) on the oxidative decarboxylation of PSAA. The
overall reaction scheme is represented as
2.3. Kinetic studies
The kinetic study of PSAA and substituted PSAAs with the
oxo(salen)chromium(V) ion [O = CrV(salen)]+, in the presence of
nitrogen bases were carried out in acetonitrile medium under
pseudo first order condition with excess of PSAA concentra-
tion over the [O = CrV(salen)]+. The progress of the reaction
was followed by monitoring the decrease in absorbance of
oxo(salen)Cr(V)–PSAA–nitrogen base adduct at the appropriate
wavelength. The absorption spectral change for the reaction
between PSAA and complex I in the presence of pyridine is shown
in Fig. 1 as a representative plot. The Fig. 1 clearly shows the exist-
ence of isobestic point at the ꢁmax of 421 nm. A double beam BL
222 Elico UV–vis bio spectrophotometer with an inbuilt thermo-
stat was employed to record the absorption spectra to measure
the absorbance of the adduct and also to follow the kinetics of
decarboxylation reaction.
2. Experimental
2.1. Preparation of oxo(salen)chromium(V) complexes
The pseudo first order rate constants were calculated from the
slope of linear plots of log OD vs. time. The second order rate
constants were calculated by dividing the pseudo first order rate
constants with the concentration of the substrate. The error in the
rate constants were given according to 95% of the student’s t-test.
Thermodynamic parameters – enthalpy of activation (ꢂ‡H) and
entropy of activation (ꢂ‡S) were evaluated from the linear Eyring’s
plots of log (k2/T) vs. 1/T by the least square method.
Oxo(salen)chromium(V) (I), oxo(5,5ꢀ-dimethylsalen)chromium
(V) (II) and oxo(5,5ꢀ-dichlorosalen)chromium(V) (III) com-
plexes were p−repared by the oxidation of corresponding
[CrIII(salen)]+PF6 complexes with slight excess of iodosobenzene
in acetonitrile. The dark green coloured oxo(salen)chromium(V)
complexes formed were filtered and used for the kinetic study
after appropriate dilution. [CrIII(salen)]+PF6− complexes were syn-
thesised by the reaction between chromium(III) chloride and the
corresponding salen ligands using −established procedure [32,33].
The purity of the [CrIII(salen)]+PF6 and oxo(salen)chromium(V)
complexes were checked by comparing their absorption spectra
with the previous reports [34].
2.2. Preparation of phenylsulfinylacetic acids
Phenylsulfinylacetic acid and its meta- and para-substituted
acids were prepared from the corresponding phenylmercap-
toacetic acids by the controlled oxidation with equimolar hydrogen
peroxide [35]. They were purified by recrystallisation from suitable
solvents several times and their purities were checked by melt-
ing point [35,36] and LC–MS. The recrystallised PSAAs were kept
inside a vacuum desiccator in order to avoid contact with moisture.
Phenylmercaptoacetic acids were prepared by the condensation
of the corresponding thiophenols with chloroacetic acid in alka-
line medium [36]. p-Nitrophenylmercaptoacetic acid was prepared
from p-chloronitrobenzene and thioglycollic acid [37].
Salicylaldehyde, 5-methyl and 5-chloro salicylaldehydes (Alfa
Aeser), CrCl3·6H2O (Sigma–Aldrich), imidazole (GR, Merck), 1-
methyl imidazole (Sigma–Aldrich), pyridine (SD fine), thiophenol
(SD fine) and substituted thiophenols (Sigma–Aldrich) were pur-
chased and used as such. H2O2 (GR, Merck) and acetonitrile (HPLC
grade, Merck) were used as received.
Fig. 1. The absorption spectral change for the reaction between PSAA and complex
I in the presence of pyridine. [PSAA] = 5 × 10−2 M; [I] = 5 × 10−4 M; [Py] = 3 × 10−4 M;
temp. = 303 K; solvent = 100% CH3CN.
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3
2.4. Determination of binding constant between I and nitrogen
bases
binding constant values for Im, MeIm and Py with I are 1810, 1277
and 1012 M−1 respectively which is in the same order to that of
their ␲ donating ability.
The binding constants for the formation of adduct between
nitrogen ligands viz., Im, MeIm, Py and complex I were deter-
mined by absorption spectroscopy. Absorption spectral titration
experiments were carried out by keeping the concentration of
oxo(salen)chromium(V) complex I as constant (5 × 10−4 M) and
varying the nitrogen base concentration while maintaining the
total volume of the reaction mixture constant. Increase in con-
centration of nitrogen bases to the complex I showed both
hyperchromicity and red shift in the absorption maxima in the
spectra of I. The absorption spectral changes observed during the
titration processes are shown in Fig. 2.
The existence of isobestic point in the titration spectral curves
for all nitrogen bases give a direct evidence for the binding of
one equivalent of nitrogen base with the oxo(salen) complex. The
absorbance reading (OD) with increase in [nitrogen base] at the
maximum absorption were recorded and the binding constants
for the adduct formation were evaluated by using the modified
Benesi–Hildebrand equation [38].
3. Results
A substantial bathochromic shift in the absorption maxima of
oxo(salen)chromium(V) ions is observed when PSAA, a nitrogen
base, or both are added to Cr(V)salen complexes. The shift in the
ꢁmax is sensitive to the nature of nitrogen base and [O = CrV(salen)]+
and the change in ꢁmax is in the range of 6–64 nm. The ꢁmax values
of different Cr(V)salen complexes used in the present study, in the
absence and presence of PSAA and nitrogen bases are shown in
Supporting information (Table S1). It is also observed that there is
an increase in the absorbance of the [O = CrV(salen)]+ with increase
in the concentration of the nitrogen bases. Similarly, at constant
nitrogen base concentration, a substantial increase in absorbance
is noticed with increase in [PSAA].
The reaction is found to show first order dependence on
oxo(salen)chromium(V) complexes as evidenced from the excel-
lent linear log OD vs. time plots even beyond 60% completion
of the reaction. However, it is observed that the pseudo first
order rate constants decrease with increase in concentration of
oxo(salen)chromium(V) complexes. The values of pseudo first
order and second order rate constants, as a function of concen-
trations of I–III and PSAA are collected in Supporting information
(Table S2). The almost constant second order rate constants at
different [PSAA], unit slope values obtained from the plots of
log k1 vs. log [PSAA] and the observed linear plots between
[I][Nitrogen base]
ꢂ (OD)
[I] + [nitrogen base]
1
=
+
(1)
ꢂε
Kf ꢂε
Where ꢂ (OD) and ꢂε are the difference in the absorption inten-
sities and molar extinction coefficients of complex I in the presence
and absence of nitrogen base. The binding constant (Kf) was deter-
mined from the ratio of the slope to y-intercept in the liner plot of
[I] [nitrogen base]/ꢂ (OD) vs. [I] + [nitrogen base]. The calculated
Table 1
Influence of nitrogen bases on the oxidative decarboxylation of PSAA.
Oxo (salen)
104 [base] (M)
104 k1 (s−1
)
104 k2 (M−1s−1
)
104 k1 (s−1
)
104 k2 (M−1 s−1
)
104 k1 (s−1
)
104 k2 (M−1 s−1
)
(I)
aIm
aMeIm
bPy
0
0.677 0.02
3.39 0.11
0.677 0.02
3.39 0.11
0.368 0.01
3.68 0.10
0.10
0.30
0.50
0.80
1.0
3.0
5.0
8.0
10.0
2.02 0.08
3.02 0.12
3.66 0.16
4.20 0.21
10.1 0.40
15.1 0.60
18.3 0.80
21.0 1.1
–
–
–
–
–
–
–
–
–
–
–
–
4.00 0.05
–
5.04 0.04
8.20 0.08
11.6 0.04
–
20.0 0.25
–
25.2 0.20
41.0 0.40
58.0 0.22
–
4.80 0.18
24.0 0.92
4.23 0.13
7.12 0.14
9.36 0.16
11.4 0.21
14.0 0.13
42.3 1.3
71.2 1.4
93.6 1.6
114 2.1
140 1.3
–
–
–
–
–
–
–
–
23.7 0.57
119 1.8
(II)
aIm
bMeIm
bPy
0
1.25 0.05
6.25 0.26
31.6 1.5
41.4 1.2
55.5 1.5
97.5 1.8
129 2.0
0.633 0.02
5.60 0.12
7.42 0.10
8.48 0.11
10.4 0.26
15.4 0.10
6.33 0.20
56.0 1.2
74.2 1.0
84.8 1.1
104 2.6
154 1.0
0.633 0.02
6.01 0.01
7.96 0.02
9.02 0.02
12.0 0.16
18.4 0.19
6.33 0.20
60.1 0.10
79.6 0.21
90.2 0.24
120 1.6
0.50
0.80
1.0
3.0
5.0
6.31 0.30
8.28 0.24
11.1 0.30
19.5 0.37
25.7 0.40
184 1.9
(III)
bIm
bMeIm
bPy
0
0.10
0.30
0.50
0.80
1.0
0.452 0.03
4.52 0.30
17.7 0.10
28.5 0.30
36.6 0.70
44.7 0.60
57.0 1.6
–
0.452 0.03
–
4.52 0.30
–
0.452 0.03
4.52 0.30
–
1.77 0.01
2.85 0.03
3.66 0.07
4.47 0.06
5.70 0.16
–
–
–
–
5.61 0.08
7.10 0.09
9.20 0.10
11.2 0.15
19.4 0.12
–
56.1 0.80
71.0 0.89
92.0 1.0
112 1.5
194 1.2
–
5.64 0.15
8.43 0.20
10.1 0.31
17.5 0.32
26.5 0.25
56.4 1.5
84.3 2.0
101 3.1
175 3.2
265 2.5
3.0
5.0
–
–
[I] = [II] = [III] = 5.0 × 10−4 M; temp. = 303 K; solvent = 100% CH3CN.
a
[PSAA] = 0.20 M.
[PSAA] = 0.10 M.
b
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Fig. 2. The absorption spectral change for the titration experiments of the nitrogen base with I.
k1 and [PSAA] (Supporting information SI 1) for all the three
oxo(salen)chromium(V) complexes confirm the first order depend-
ence on PSAA. The data, in Table S2, show that the introduction
of both electron donating and electron withdrawing groups in the
salen moiety enhance the reaction rate and the observed order of
reactivity is III > II > I.
The order with respect to nitrogen base is found to be fractional
as evidenced from the non-integral slope values noticed in the dou-
ble logarithmic plots of rate constant vs. [nitrogen base]. Further the
plots of 1/k1 vs. 1/[nitrogen base] are linear with an intercept on the
rate coordinate showing the Michaelis–Menten type kinetics with
respect to nitrogen base. The values of Michaelis–Menten constant
(KM) evaluated from the double reciprocal plot of k1 vs. [nitrogen
base] are presented in Table 2. The rate benefits for the nitrogen
base catalysis are calculated using Eq. (2) and are given in Table 2.
To evaluate the effect of nitrogen bases on the reaction rate, the
reactions of PSAA with I, II and III are carried out in the presence
of nitrogen bases viz., Im, MeIm and Py at different concentra-
tions and the observed pseudo first order and second order rate
constants are presented in Table 1. The dependence of pseudo first
order rate constant on the concentration of the nitrogen bases for
complexes I–III is shown in Fig. 3. Examination of the kinetic results
reveals that the reaction is highly sensitive to the nature of nitro-
gen base and change of substituents in the 5,5ꢀ-positions of the
oxo(salen)complex. It is observed that the rate constant increases
with the increase in nitrogen base concentration and no saturation
kinetics is observed. The determination of rate constant is restricted
only to the concentration range of nitrogen bases mentioned in
Table 1 because above this range, the reaction is too fast to measure
or a drastic discharge of green colour of oxo(salen)chromium(V)
complexes with decrease in OD is observed even in the absence of
PSAA. This may be due to the oxidation of nitrogen bases [39–41].
k2(b) − k2(a)
Rate benefit =
(2)
k2(a)
Table 2
Michaelis–Menten constants (KM) and rate benefit values for the reactions.
Oxo(salen)
104 KM (M)
aRate benefit
Im MeIm
6.08 6.43
7.88 12.6
11.6 21.3
Im
MeIm
Py
Py
I
II
III
0.147
1.07
0.226
1.50
0.892
2.83
2.56
2.42
0.863
10.5
13.4
23.8
[I] = [II] = [III] = 5.0 × 10−4 M; temp. = 303 K; solvent = 100% CH3CN.
a
[nitrogen base] = 1.0 × 10−4 M.
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