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Homogeneous catalysis in the Cr(V) oxidation of organic sulfides
Terml~edmn, Vol. 53. No. 3. pp. 1131-I 144, 1997 Copyright 0 1996 Elaevw Sc~cnce Ltd Printed in Great Britam All rlphts reserved W40-4020/97 4 I7 00 + 0.00
Pergamon PII: SOO40-4020(96)01043-S
Homogeneous
JesurajjaBosco
Catalysis in the Cr(V) Oxidation Organic Sulfides
Bharathy,” Th:wumeya Kuppuwmy
of
Gnnes:m,”
Ibrshim Ali Mohammed Sheriff” and Seenivasan K;kj:agopl *b
Ambahm
Introduction
The relatively interest
because
compounds attention
less stable oxidation
of their
important
I-7 Even though
the chromium
we have recently
sulfide ’ In this
report
proposed we
oxidation
of organic
sulfides
alcohols
and organic
sulfides
formation
of a more reactive
test the catalytic present
have
a mechanism established
Picolinic Cr(VI)-PA
system
involving that
complex
systems
Cr(V) behaves
differently of
we thought
oxidation
oforganic Cr(V)
from Cr(VI)”
PA
is attributed
it would of organic
of organic
imidazole(lm)
between
little
of
to the
be interesting sulfides
and
in the
for the Cr(V1) oxidation
in the presence
“5’ ’ Therefore
oxochromium(V)
in several
For the Cr(V) oxidation
catalyst
~tl~ent
of chromium
formation
reactivity
1, IO-phenanthroline(Phen),
have received
activities
complex
pyridine bases on the Cr(V) oxidation
here our results on the catalysed 2,2’-bipyl-idyl(Bpy),
ofthis
is important
acid (PA) is an efficient
and the enhanced
role of PA and other
Cl-(V) and Cr(lV). and carcinogenic
- sulfur interaction
has been paid to the study of the chemistry
sulfides
acid(PA),
states ofchromium,
role in the biological
to
and we
sulfides taking picolinic
and N-~nethylilnidazole(Nl~~~)
as the catalysts
EXPERIMENTAL
Sodium hydroxybutanoic
bis(2-ethyl-Z-hydroxybutyrato) acid (HEBA)
(t\ldrich
DETAILS
oxochromate(V)
99oi,) and sodium dichromate II31
J_ was prepared (Merck)
from
in acetone
2-ethyl-L?(Merck.
AK
J. B. BHARATHY el al.
1132
0
Et
0
\
O\ CfII /
C-
l
EtA&-
0
C ‘Et
0
C
I
\
0'
%
Et’
0
Grade)
as described
hexane
over a period
procedures distilled
and water
1
earlier
“-I’
the purity
phenanthroline,
checked
used
of the product
All other reagents
by dropwise
were synthesized
HPLC grade
methods.‘,“,”
kinetic
runs performed
and N- methylimidazole
was induced
sulfides(ArSMe)
by spectral
in all the
imidazole
used as received
Crystallisation
of IO-15 min. The aryl methyl
were
1
/
Picolinic
were purchased
addition
acetonitrile
acid,
and doubly
2,2’-bipyridyl,
ftrom Aldrich
of
by standard
(99%
I, IU-
purity)
and
were of AnalaR grade
Stoichiometry The reaction (MPS)
(1x10.‘M),
similar to kinetic ratio
was carried where
studies
of the oxidant
out between
[Cr(V)]
> [MPS],
After completion
versus
sulfide
oxochromium(V)
1 (5x 10.‘M)
and methyl phenyl sulfide
in the
of catalysts
keeping
presence
of the reactionexamination
spectroscopically
mole of Cr(V) i.e, the stoichiometry
showed
that one
is I I Thus the net reaction catalyst ----------a
Cr(V) =0 + A&Me
Cr(ll1)
other conditions
of the products
and the mole
mole of MPS consumes
+ ArSOMe
one
by eq (I ).
can be represented
(1)
Product Annlysis A mixture
of methyl phenyl sulfide (0 OIM), picolinic
aqueous
acetonitrile
at pH 2 was allowed
mixture
was extracted
with chloroform
removed
under reduced
Shimadzu
408 IR Spectrometer.
pressure.
and also for a negligible chloroform absorption
spectrum
bis-chelated
Cr(III)
and dried
The residue
ofthe
product
over anhydrous
was analysed
The spectrum
amount of sulfone,
the chromium(lI1)
acid(O.OlM)
to stand until completion
and Cr(V) I (0 OIM) in 50% of the reaction.
sodium
sulfate
by recording
MPSO,
After
the organic
was analysed
sample showed the absorption
maximum
solvent
the IR spectrum
showed the peaks corresponding
left behind
The reaction
The
was
using
a
to the sulfoxide,MPSO,
product
was extracted
with
by absorption
spectroscopy
for the product
at 420nm indicating
The
product
Kinetic Mensurenzent.s The kinetics nm of Cr(V) compartment the
Cr(V)
acetonitrile
of the reaction
employing attached complex
was followed
a Hitachi-200 to a circulating
1 in aqueous
the ?Lmexis shifted
at pH 2 by measuring
UV- visible constant solution
spectrophotometer
temperature is 5lOnm
to 540 nm A similar
(E
shift
bath =
the absorbance having
at 540 cell
The absorption
I68 M-‘cm-l)
in h,,,,
changes
a thermostated
”
maximum In
the
due to the change
h,,,;,, foi-
presence
of
of the medium
1133
Catalysed Cr(V) oxidation of organic sulfides
from water to acetonitrile thiolactic
acids.16
oxidant
was noticed
In all reactions,
was used to ensure Rate
constants
versus reaction
evaluated
out that the reaction
by using the semilogarithmic computer
excess
A typical kinetic
conditions
program.
oflactic and
in the Cr(V) oxidation
stoichiometric
plots
The rate constant
of substrate
over the
trace is shown of absorbance
I
in Fig differences
for the self decomposition
as 2~1O-~s~’ at 25°C under similar conditions.’
It is pertinent
to point
of Cr(V) at pH l-2 has been studied by Ghosh and Gould. I7 The rate constants,
in Tables
l-5 were computed
rate and k, is the specific in the absence
IO-fold
a
first-order
time by least squares
1 (kd) was measured
of complex collected
pseudo
were
by Rajavelu and Srinivasan
at least
where kexpt is the experimentally
from eq.(2)
rate for the decomposition
of complex
1, measured
observed
k,
specific
under similar conditions,
of substrate. k,
In the presence
=
kerpt -
k#l
of PA, the value of k, is found to be 3x10-
that the rate constants
were reproducible
to within *5%
study we noticed
an initial increase
4-5. ’
Duplicate
kinetic
runs showed
Spectrfll Studies In the present 540nm.
During the course of the reaction
formation
of a Cr(lV)
reports.‘*
The disappearance
Cr(IV)
leading
Cr(V) complex (a=1 500M“ evidence
complex.
to the
in the
of Cr(V)-PA
we
the
in absorbance
end
of the reaction
the (Fig
4. It has been established
indicates of the
the
in earlier
the decay of spectrum
of
peak at 350 nm
2 a). We take this spectra1
change
as
when the absorption
spectrum
the peak at 350nm was shifted
to 400nm
The sharp peak at 400nm may be taken complex
species
the absorption
sharpening
3). However
PA and sulfide,
by slow decay at
to pink indicating
was taken as Cr(IV)
When we recorded observed
followed
from dark brown
intermediate
complex (complex
of oxidant,
in absorbance
changes
at
the products.
of PA alone
presence
the Cr(V)-PA-MPS
corresponds
of
and again an increase
(a=1600 M-’ c m-l) Fig.2.b of
of the pink colour
formation
for the formation
was recorded
This pink colored
in the presence
c m-1)
the colour
as spectral evidence
recently
for the formation
that the peak at 400 - 420 nm
to Cr-S bond formation.”
Isolation ofthe intermedinte complexes : During our kinetic studies we realised the possibility of the existence oftwo intermediate complexes 3 and 4 (see scheme III). Our attempts to isolate complexes rapid decomposition
3 and 4 resulted in
of these complexes
RESULTS AND DISCUSSION
In order to understand essential
to understand
order with respect
the role of catalysts
the mechanism
on the Cr(V) oxidation
of uncatalysed
to the Cr(V) and fractional
reaction.
order with respect
The
of organic
uncatalysed
to the substrates
sulfides,
reaction (Table
it is is first
I)
J. B. BHARATHY
1134
et&
3.000
Fig. 1. Curves a-e represent the change of the absorption spectrum ofCr(V) at one minute time intervals, for the reaction mixture containing O.OlM MPS, O.OOlM PA, O.OOlM Cr(V)(l) at pH2 in 50% (v/v) CH, CN
0.000 300
400
500 Wavclr~,@h,
600 nm
Fig.2 The absorption spectra ofCr(V) at pH2 in 50% (v/v) CH, CN H,O recorded under different conditions a)Cr(V)(O001M)andPA(0.00lM)only b) Cr(V) (O.OOlM), PA (O.OOlM) and MI’S (O.OlM) c) Ten times expanded spectrum for (b) Inset : Absorption spectrum for Cr( V) only.
H,O
1135
Catalysed Cr(V) oxidation of organic sulfides
Table 1. oxidation
Pseudo
of MPS
first order,
in 5O%(v/v)
k, and
aqueous
order
rate
constant,
k, ,values
kJlO‘* M-’ s-’
0.01
1 02
I 02
0.02
1.67
0.84
0.03
2.77
0.92
0.04
3 71
0.93
0.05
4.09
0.82
0.06
4.28
0.72
0.10
5 79
0.58
= 0.001 M ; [H’] = k,
for the Cr(V)
at 25°C.a
k,/10-4 s-’
W’SJ,M
[Oxidant] from k,
second
acetonitrile
= 0 01 M. a) Second
order
rate
constants
listed
were
calculated
/ [MPS]
This saturation kinetics was explained in terms of complex formation between Cr(V) and sulfide in the uncatalysed reaction.
The mechanism proposed for the uncatalysed
d+A
151.-. ,R
K
2s
reaction is shown in Scheme 1.
Cr”.j..s
,--’
‘R
2
Llgand
k
coupling
I III
Cr+ Rp
so
Scheme I According product,
in the complex from sulfide radical.
to Scheme
sulfoxide,
I, the oxidant
can be visualised
2.’
However
the alternative
to oxochromium(V)
Thus equations
forms a complex
as due to the ligand formulation
in complex
(3)-(6)
represent
with
the
coupling
the oxygen
is in terms of inner sphere
2, leading to the formation
the alternative
The formation
sulfide
between
mechanism2”
ofCr(lV) (Scheme
electron
=
(3)
Cr(V)....r( Me Complex J
k’l Complex 2
t.
+
Cr(IV) + Ad-Me
Cr(lV) + Ad-Me I %
Cr(V)/Cr(lV) + Ar -i- Me&
Cr(II1) + Ar-i:Me Cr(II1) + ArSOMe Scheme II
tr-anrfer
and sulfide cation II)
Ar Cr(J’) f Ar-S-Me
of
and sulfide
(4) (5) (6)
1136
J. B. BHARATHY et al.
A similar successfully
mechanism
to the
The Cr(V) and NIm,
and
Cr(V)
has been postulated oxidation
oxidation
the
and the Marcus
theory
of electron
transfer
applied
of dialkyl sulfides.2’
of organic
rate constant
sulfides
values
is catalysed
obtained
by the pyridine
for the
bases
ofMPS
oxidation
PA, Bpy, Phen, Im
at
different
[catalyst]
are given in Table 2 The carboxylato G(V)
in different
to the redox
reaction
been measured (Tables
bound Cr(V)
pH can be studied
I3 In the present
and this
l-4).
Though
the catalyst
the
and Phen it is about parent
reaction
sultide.Since
is
30 times Im and
the oxidant
ligands when the reaction oxidation
NIm This
of organic
order
versus
catalysts
the rate
was
ligands
confirmed
catalyst sulfides
our
1 at pH 2, has When
is 40 times with PA, whereas
in Bpy
rate enhancement
with
to complex
the least reactivity I exhibits
maximum
stability
is found to be unity from the
experimental
conditions kinetics
order in the catalyst
all could
linearity
of the
the substrates be explained
plots
exhibit only
400
200 I
I
30
40
50
60
7'0
80
1 /[Substrate] -
*p
Ii
-
OMe
*P
-Cl
*p-Me
*p
-
+p
Br
Fig.3. Michaelis - Menten
*p -
COMe
*m
-
cool-l -
Cl
Plot for Sulfides
90
of
for all the
600
I
of log
in terms
is also found to be fractional
l/k,
20
i’ in
fractional
study (Table 2 and Fig.4).
10
1 and
with these
BOO
0
the
observed
1,000
0’
values
varies
six fold
efficiency
“,22
type mechanism.The
used in the present
of complex
they tend to coordinate by
of
among these, which is similar to our earlier observations
(Table 3 and Fig 3) The complex
- Menten
show
the reactions
is slow compared
first order rate constant
the catalytic
enhancement
Im and Nlm
to oxidant
time.Under
dependence
a Michaelis
by all catalysts,
are unidentate fact
pseudo
was carried out in the pH 3 at which complex
The order with respect absorbance
is catalysed
However
of the oxidant
self decomposition
from the observed
However
Thus PA seems to be the superior the Cr(VI)
case also the
Ix~O-~M,
3-4.5
the self decomposition
value is substracted
concentration
try to stabilise
is highly stable in the pH range provided
100
Catalysed Cr(V) oxidation of organic sulfides
Table 2. Pseudo Zr(V) in 5O%(v/v)
first-order
aqueous
rate constant
acetonitrile
k, , values for the
[BPYI
0 000
0 102
0.001
I 03
0 002
I 78
0 003
261
0 005 001
3 89 4.54
0.02
8 64
0.000 0001
0.102 0 734
0.002
1 05 I 10
0 005
1 49
001
3 41
0 02 0 000
4 28 0 102
0 001
I 02 1 46 I 82
0 002 0 003
WI
[NW
a) [oxidant]
The substrate Table 3
0 005
2 45
0.01
3 37
0 02
4 43
0 000
0 102
0001
0 328
0 002
0 360
0 003
0 400
0 005
0 496
0.01
0.571
0.02
0 296
0 000
0 102
0001 0 002
0.411 0 436
0 003 0 005
0 449 0 533
0.01
0 593
0.02
0.401
= O.OOlM,
dependence
[MPS]
oxidation
of MPS by
k,/lO” s-l
0 003
[Phen]
catalytic
at 25’Ca.
[Catalyst],M WI
1137
= 0 OlM,
of PA catalysed
and
Cr(V)
[H+]
J
= 0 OIM
oxidation
of aryl methyl
sulfides
is shown
in
1138
J. B. BHARATHY et al.
Table 3. Pseudo Cr(V) oxidation
first-order,k,,
of substituted
phenyl
and second-order methyl
rate constant,kz,
sulfides (XC,H,SMe)
aqueous
acetonitrile
k 2/IO-*M-‘s-’
[XC,H4SMe] H
values for the PA catalysed
in 5O%(v/v)
001 0 02 0.03 0 OS
1 03 I 83
9 Ii
2 Ii 3 13
7 II 6 26
1 24 I 70
IO3
p- Cl 001 0 02 0 03 0 OS
2 69 3 80
124 8 52 8 97 7 60
001 0 02 0 03 0 05
I 2 3 4
84 44 32 3s
184 122 II I 8 7s
001 0 02 0 03 0 05
2 88 3 34 4 00 481
28 8 16 7 I3 3 9 63
0 0 0 0
01 02 03 05
3 02 3 65 4 39 S2l
30 2 I8 3 1-I 7 103
001 0 02 0 03 0 05
I 20 2 I2 2 95 4 53
120 IO 6 9 84 9 05
001 0 02 0 03 0.05
2 42 3 31 3 98 5 22
24 3 166 I3 3 10s
001 0 02 0 03 0.05
I I 2 3
I1 4 9 33 8 30 7.18
p-Me
P-CO,H
p-OMe
p-Br
p-COMe
m-C1
[PA]
a) Second
order
=
[Cr(V)]
0 OOIM,
rate
constants
=
OOOIM,
I4 87 49 59
listed [H+]
were =
OOIM
calculated
from
k,
-
k, : [MI’S]
Catalysed
1139
Cr(V) oxidation of organic sulfides
500
-
0 0
loo
50
150
200
250
350
300
400
450
500
1 /[CaWstl
PA --k
-
the
formation spectral
-
order
dependence
of catalyst
oxidant
and the
of more reactive evidence
formation
catalyst
involving
of complexes
mechanism,
Scheme
mechanism
may
Menten
Thus
Cr(V)-catalyst
for the formation
the other complex
Phen *
Michaelis
This fractional between
Bpy *
Fig-Q.
complex catalyst
3 and 4 have been
III, for the picolinic
operate
activity
If all these between
r-eversible
assumptions
Cr(V) oxidation
we
are valid one expects
oforganic
PA
=
Cr(V)
and
also
evidences
propose
for
for the
the following A similar
sulfides
also
6
Cr(V)+
formation
in terms of the
and the catalyst
Indeed the spectral
(see Fig 7) Thus
of other catalysts
complex
can be explained
the oxidant
and substrate
observed
Nlm Catalysta
the
indicates
acid catalysed
in the presence
for
the catalytic
of a complex
the oxidant,
Im *
Plot
(7)
- PA
Complex 3 K2
Complex 3 + MPS =
(W
MPS - Cr(V) - PA Complex 4
Complex
4
MPSO + Cr(III) - PA
-$
(9)
llgalld couplII,g or 0 and s
Scheme III Thus the reaction reactive
oxidant
shown
in Scheme
visualised
as due
is initiated
than complex Ill.
The formation
to ligand
by
1. This
coupling
the
formation
complex
3 forms
of the products, between
of a Cr(V)-PA
0 and S
another sulfoxide
in the
complex.3.
complex
is a more
4 with the substrate
and Cr(lll)-PA
hypervalent
which complex,
intermediate
as
can be
complex
4
1140
J. B. BHARATHY et al.
Such a ligand coupling of organic of
sulfides
ligand
recently
reviewed
alternative methods.
The
in the
involving
been proposed
oxidation
hypervalent
involving
However
importance
been highlighted
inner
sphere
it is difficult
by us in the PA catalysed
of organic
sulfur compounds
intermediates
by Oae and Uchida. 24 The formation
mechanism (10-12).
has already
and by others
coupling
equations has
mechanism
electron
been
of products transfer
to distinguish
of these two alternative
has
“.23
The
concept
extensively
applied
and
may be formulated
in terms
of an
within
the complex
these two alternative
mechanisms
Cr(V1) oxidation
in
the
4 as shown
mechanisms
reactions
of
in
by kinetic nucleophiles
recently.20,25
k, IET ->
Complex 4
Cr(IV)-PA + Ar-S-Me -> Cr(IV) / Cr(V)-PA + Ar-.?Me
The rate ofthe On the other (Table.4
reaction
is not appreciably
hand the reaction
is highly
-
affected
influenced
Cr(IV)-PA + Ar-:-Me
(10)
Cr(Ill)-PA + Ar-S-Me
(11)
Cr(II1) + ArSOMe
(12)
by the change in ionic strength
by the change
of the medium
of [H+] and solvent
composition
) Table
4. Effect
of varying
[H+] and solvent composition
on PA catalysed
Cr(V) (I) oxidation
of
MPS.
s-l
kl/lO"
* WI
CHsCN-H,O “h (v/v)
0001
0 4s
25.75
0 005
0.80
50-50
I 03
001
I 03
60-40
3 71
0.04
3 88
75-25
4 54
0.10
12 3
SO-20
7 22 IO.8 H,O , ii [H+] = 0.01~
90-10 [Cr(V)] = O.OOlM ; [MPS] = O.OlM ; * Solvent = 50% A plot oflog a positive reaction
l/D is fairly linear with a positive molecule
may be ascribed
to facile
electrophilicity
of
increased offered
k, versus
ion and a neutral
in the rate determining formaion the
by us and Rocek and co-workers
alcohols.8~t’
(V/V)
of Cr(V)-PA
protonated
CH,CN
slope pointing step
to an interaction
The substantial
complex
oxidant.
for the PA catalysed
0 78
in the presence
Similar
of the
of H’ and the
explanations
Cr(VI) oxidation
between
acid catalysis
of organic
have sulfides
been and
Catalysed
Both the electron phenyl ring of ArSCH3 The catalysed parameters isokinetic
donating accelerate
oxidation
evaluated
Cr(V) oxidation of organic sulfides
and electron
withdrawing
the catalysed
reaction
was carried
using the Eyring
plot of AH’ versus
substituents
in the para position
similar to the uncatalysed
out at four different
equation
1141
temperatures
along with k, values
AS # is linear and the slope ofthe
plot
of the
oxidation.’
and the thermodynamic
are collected
in Table 5 The
yields the isokinetic
temperature,
l3 , which is found to be 3 17K (r = 0.998). Table 5. Pseudo entropies
first-order
rate constants (AG#)
(AS’ ) and free energies
XC6H4SMe
in 5O%(v/v)
aqueous
x
at four different
of activation
p-CO,H p-Me p-Br p-COMe m-Cl
298K
308K
313K
0 73 0.91 2.89 2.46 1.51 0.85 2.05 0.83
I 03 I 24
2.40 2.26 3.34 3.74 2.97 2.59 3.54 2.25
3.48 3.46 3.52 4.72 3.66 3.50 4.44 3.37
* at 303 K conditions,
AH’
293K
3.02 2.88 1.84 I .20 2.42 I.14
[PA]=O.OOlM,
[Cr(V)]=O
Cr(V)
AS’
(AH’),
oxidation
of
AG#* kJ mol-’
57.5(129)’ 47.0(64.7) 5 27(5.71) 21.3(102) 31.4(31 4) 5 1.7(68 8) 26.6(133) 50.2(79.8)
OOlM: [Substrate]=O.OlM.
“The values in parentheses
and are collected
and enthalpies
catalysed
acetonitrile
k 1/10-3s-1
H p-Cl p-OMe
temperatures
for the PA
correspond
-109(+112) -143(-90 7) -275(-279)
90.4(95
-222(+25.8) -191(-200)
X8.6(94 2) 89.3(92.0) 90.2(92 X)
-127(-79.3) -206( 122) -133(-43.4)
I)
90.3(92 2) 88.7(90 2)
88 9(96 5) 90 4(92 9)
[H+]=O 01M to the
uncatalysed
Ireaction under blmilal-
from ref 8
The thermodynamic parameters given in Table 5 follow the trend expected for the catalysed reaction i e AH” is small and AS’ is mote negative compared to the uncatalysed reaction By applying Michaelis-Menten
kinetics to Scheme 111,we have derived an expression for k, (eq I-3) and
from the values of k, at different [substrate], the formation constant, for complex decomposition
of complex 4, k, for various substituted
Table 6. The equilibrium (k3) values for XC6H,SMe (X) Substitutent
constant
sulfides
4, K, and rate constant, for the
have been estimated and given in Table 6.
(K2) and the rate constant
for the decomposition
at 25°C K,,M’
k3 /IO-‘s-’ 4.41
H
21.9
p-Cl
25.3
4 56
p-OMe
102
4.49
P-CO:H p-Me
I17
3.99
46 3
4.25
m-Cl p-Br
20 0 10 8
9.40
The K, value is found to be 130 Me’ from equilibrium
5.21
measurements
of complex
4
1142
J. B. BHARATHY et
al.
Rate Benefit The rate benefit and is consistent substituent,
p-OMe,
COMe, p- CO,H groups
measured
showed
oxidation.
However
Table 7. aqueous
and PA catalysed
the rate benefit
I
It
of ArSMe
is found
This behaviour Cr(V)
oxidation
in Cr(V) oxidations
is given
in Table 7
that the electron
withdrawing
donating
substituents,
of electron
releasing
of ArSMe
is very similar
p-
and withdrawing to Cr(VI)
is high in the presence
of electi-on
in the substrate.
Rate benefit
acetonitrile
achieved
oxidation
and both the electron
the highest rate benefit.
substituents
Cr(V)
principle.”
has the least rate benefit
in the uncatalysed
withdrawing
in the PA catalysed
with the reactivity-selectivity
in
the
PA Catalysed
Cr(V)
oxidation
of
XC6H,SMe
in 5O%(v/v)
at 20°C Rate benefit’
Substituent Cr(V)
Cr(Vl)*
H p-Cl p-OMe p-CO,H p-Me p-COMe m-Cl
17.2 2 09 091 60 5 I 65 69.8 2.87
7.71 7 44 2.75 991 6 37
p-Br
2 50
8.38
149
*data taken from Ref. I I. ’ rate benefit
=
k,(cat) -- k,(uncat) -------------------------
where k,(cat) and k,(uncat) represent the rate constants for the catalysed and uncatalysed reactions respectively
k,(uncat)
under similar conditions
”
Rate ho Our kinetic
and spectral
observations
Cr(V)-PA
complexes
as intermediates
in Scheme
III lead to the rate law (13)
-d[Cr(V)]
1
k, = _____________ x
-------
dt
and catalyst
oxidant estimated
vs Ii[cat] substituents
transfer
irrespective through
the first order
and l/k,
dependence
vs l/[sub]
respectively
in the aryl ring of ArSMe
of the nature of the substituent
ligand coupling
to the nature of electron
Cr(V)-PA
and
ArSMe-
to these complexes
k, K, K2 [ Sullide] [catalyst] __________-_________~~~~~~______~~~~~~~~~_____
of the
of plot of l/k, k, value
of
The values of K, ,K2 and k, have been
influence
of the
out the formation
ofa steady state approximation
(13)
(K, [catalyst] +I) (K, [sulfide] +l)
IWW
This rate law explains substrate
=
point
Application
and withdrawing
order
explanation.
of k: and the
The constancy
in the aryl moiety may be taken as evidence transfer
substituents
However
in the
from the slope and intercept
(See Table 6) The constancy
on K2 deserve
rather than via electron
releasing
and fractional
the K,
value
of
for 0x0
is sensitive
in the phenyl ring of ArSMe
These
Catalysed Cr(V) oxidation of organic sulfides
results
point
out that the reversible
fractional
order
Hammett
equation
with respect
or other structure-reactivity
Since the reaction
step in eq (8) may be rate controlling
to substrate.
Thus the authors
I143
pseudo
first order
have not attempted
rate constants
cannot
be used for
is the
to analyse the kinetic data in terms of Hammett
relationships
Acknowledgnzent SR thanks University,
Prof
C
Srinivasan,
for his constant
Head,
encouragement
well as the research
and development
funds and necessary
facilities.
Department
of Materials
Science,
Madurai
Kamar-aj
and JRBB thanks the UGC, New Delhi and the Principal
committee
of The American
College,
Madurai
as
for providing
References
I
Mitewa,M , B0ntchev.P.R
2
Gould&S references
3.
Act
(‘hem.
(‘oo~~..(‘henl.Hev.1985, 61, 241.
lies
1986, 19, 66, Ghosh,M C ,Gould,E S. hrorg. (‘hem.
1991. 30, 483 and the
cited therein Judd R.J , Lay,P A Dixon,N E , Baker,R S U . Benin, A.M ~‘henr.Ke.~. 7ix~ol
Farrell&P.,
1rwin.J A., Judd,R J , Lay,PA.; Martin,B.D.,Bramley,R,
David,G.B , Hambley,TW;
1989,
2. 227;
Dixon,N E , Hendry,]‘,
Ji,J Y, Baker.R SU ~B0nin.A.M ltrorg (‘hem 1992, 3 1, 4906; Srinivasan, K , Kochi,J K Ittoq. (‘hem 1985,
24, 4670.
Scott. S.L
Bakac,A, Espenson,J H .1. Anz. C’hcnt. Sot , 1992, 114, 4205, Branca,M ,
Micera,G , Segre,U ; Dessi, A Itrorg. C’hem , 1992, 3 I, 2404; Rihter,B , Sri Hari,S., Hunter,S, Masnovi,J. J. Am. I’hent. 4
SW..
1993.
115. 3918; Ghosh,M.C ; Gould,E S. 1. Am. (‘hem. Sbc.,1993,
Borges, K.M, Wetterhahn, K E (‘~r-crttr~~~tr~sts, 1989, IO, 2 165, Kortenkamp, Mrrtnl. lies
1990, 232, 155; Casadevall, M, Kortenkamp, ~‘nr~ttto~~ttests.
115, 3167
A, Oetken. G; Beyersmann,
1994,
15. 407.
5
Sugden, K D, Wetterhahn, K E /ttotx. C‘Item 1996, 35, 65 I
6.
Farrell, R.P., Lay, P.A. ( ‘omntett/.c. Ittorg.
7.
Gould, E S ~‘ourd. (‘hem.
8
Ganesan,T.K
, Bharathy,J R.B , Sheriff,A I.Md , Rajagopal,S. fndrcrtr./.(‘hem.
9.
Srinivasan,C,
Chellamani,A, Rajagopal,S. ./.0rgC(‘henz.
IO.
Peng,T.Y., Rocek,J .I. Am. C‘hem. Sot., 1976, 98, IO26 , Rocek,J . Peng,T.Y J. Am. Clhem. SW. ~1977, 99,
Ker!,
1994,
C‘hem. 1992, 135, 65
13. 133.
I 1995. 34A, 522
1985, 50. 1201.
7622.
II
Srinivasan,C.,
Rajagopal,S.,
Chellamani,A. ./. (‘hem.
SK.,
P’rrkitr
7Putt.s. 2, 1990,
1839.
J. B. BHARATHY et al.
1144
12.
Krumpolc,M.,
Rocek,J. J. Am. (hem. Sot., 1979, 101, 3206
13.
KrumpolqM.;
Rocek,J. Znorg. Chem., 1985, 24, 6 17
14.
Fanchang,Y.T.; Bose,R.N.;
15.
Gnanaraj,G.A;
16.
Rajavelu,A.;
17.
GottIdES.
18.
Ghosh,M.C.;
19.
Brauer. S.L.. Hneihen, A S.; McBride. J S Wetterhahn, K E 6qq.
20
Lund,H., Daasbjerg,K.,
21
Ganesan,T K , Rajagopal,S.
Rajagopal,S; Srinivasan,V.S
Gelerinter,E ; Gould&S.
Ittorg. Chem. 1985, 24,4679.
Srinivasan,C; Pitchumani,K. Proc. htdturt.AcadSci.
Teirahedrotz, 1993,49,4721
((‘hem. Sk.)
1989,
10 I, 472 1
, Inorg. Chem 1985, 24, 2645. Gould,E.S. hzorg.Chem.l990,29,4258
, 1991,30,491
, 1992, 3 I, 702 (‘hrlrz. 1996.35,373
Lund,T., Pedersen,S U Au. (‘hem. Kes.1995,
28. 3 13
Bharathy,J R B , SheriffA I.Md. (Submitted)
and Ganesan, T K l’h.1) 7Rr.u.\.
Madurai Kamaraj University, 1996. 22.
Manohar,T C. Ph.D. Thesis, Madurai Kamaraj University, 1991.
23
Roh,K.R., Kim,K.S : Kim,Y H E/rahea’rott Lett, 1991,32,
24.
Oae,S., Uchida. Y. Acc.C’hem. Kes
25.
Balakumar,S
, ThanasekaranP;
Alhaji,N.M.I
; Rajagopal,S , Sewel,R ; Srinivasan.C. E~rahekm,
26.
Exner,O. J.(‘hem.Soc.Perkttt
(Received
in UK 13 June
1991, 24, 202
RajagopalS;
793
and the references cited therein.
Ramaraj,R
Efraheu’ron
7kttt.s.2 1993, 973 and the references
1996; revised 8 November
1996; accepted
1995,
5 1. 480
1995, 5 1, 12677
cited therein.
14 November
1996)
1, Chellamani,A ,
Pergamon PII: SOO40-4020(96)01043-S
Homogeneous
JesurajjaBosco
Catalysis in the Cr(V) Oxidation Organic Sulfides
Bharathy,” Th:wumeya Kuppuwmy
of
Gnnes:m,”
Ibrshim Ali Mohammed Sheriff” and Seenivasan K;kj:agopl *b
Ambahm
Introduction
The relatively interest
because
compounds attention
less stable oxidation
of their
important
I-7 Even though
the chromium
we have recently
sulfide ’ In this
report
proposed we
oxidation
of organic
sulfides
alcohols
and organic
sulfides
formation
of a more reactive
test the catalytic present
have
a mechanism established
Picolinic Cr(VI)-PA
system
involving that
complex
systems
Cr(V) behaves
differently of
we thought
oxidation
oforganic Cr(V)
from Cr(VI)”
PA
is attributed
it would of organic
of organic
imidazole(lm)
between
little
of
to the
be interesting sulfides
and
in the
for the Cr(V1) oxidation
in the presence
“5’ ’ Therefore
oxochromium(V)
in several
For the Cr(V) oxidation
catalyst
~tl~ent
of chromium
formation
reactivity
1, IO-phenanthroline(Phen),
have received
activities
complex
pyridine bases on the Cr(V) oxidation
here our results on the catalysed 2,2’-bipyl-idyl(Bpy),
ofthis
is important
acid (PA) is an efficient
and the enhanced
role of PA and other
Cl-(V) and Cr(lV). and carcinogenic
- sulfur interaction
has been paid to the study of the chemistry
sulfides
acid(PA),
states ofchromium,
role in the biological
to
and we
sulfides taking picolinic
and N-~nethylilnidazole(Nl~~~)
as the catalysts
EXPERIMENTAL
Sodium hydroxybutanoic
bis(2-ethyl-Z-hydroxybutyrato) acid (HEBA)
(t\ldrich
DETAILS
oxochromate(V)
99oi,) and sodium dichromate II31
J_ was prepared (Merck)
from
in acetone
2-ethyl-L?(Merck.
AK
J. B. BHARATHY el al.
1132
0
Et
0
\
O\ CfII /
C-
l
EtA&-
0
C ‘Et
0
C
I
\
0'
%
Et’
0
Grade)
as described
hexane
over a period
procedures distilled
and water
1
earlier
“-I’
the purity
phenanthroline,
checked
used
of the product
All other reagents
by dropwise
were synthesized
HPLC grade
methods.‘,“,”
kinetic
runs performed
and N- methylimidazole
was induced
sulfides(ArSMe)
by spectral
in all the
imidazole
used as received
Crystallisation
of IO-15 min. The aryl methyl
were
1
/
Picolinic
were purchased
addition
acetonitrile
acid,
and doubly
2,2’-bipyridyl,
ftrom Aldrich
of
by standard
(99%
I, IU-
purity)
and
were of AnalaR grade
Stoichiometry The reaction (MPS)
(1x10.‘M),
similar to kinetic ratio
was carried where
studies
of the oxidant
out between
[Cr(V)]
> [MPS],
After completion
versus
sulfide
oxochromium(V)
1 (5x 10.‘M)
and methyl phenyl sulfide
in the
of catalysts
keeping
presence
of the reactionexamination
spectroscopically
mole of Cr(V) i.e, the stoichiometry
showed
that one
is I I Thus the net reaction catalyst ----------a
Cr(V) =0 + A&Me
Cr(ll1)
other conditions
of the products
and the mole
mole of MPS consumes
+ ArSOMe
one
by eq (I ).
can be represented
(1)
Product Annlysis A mixture
of methyl phenyl sulfide (0 OIM), picolinic
aqueous
acetonitrile
at pH 2 was allowed
mixture
was extracted
with chloroform
removed
under reduced
Shimadzu
408 IR Spectrometer.
pressure.
and also for a negligible chloroform absorption
spectrum
bis-chelated
Cr(III)
and dried
The residue
ofthe
product
over anhydrous
was analysed
The spectrum
amount of sulfone,
the chromium(lI1)
acid(O.OlM)
to stand until completion
and Cr(V) I (0 OIM) in 50% of the reaction.
sodium
sulfate
by recording
MPSO,
After
the organic
was analysed
sample showed the absorption
maximum
solvent
the IR spectrum
showed the peaks corresponding
left behind
The reaction
The
was
using
a
to the sulfoxide,MPSO,
product
was extracted
with
by absorption
spectroscopy
for the product
at 420nm indicating
The
product
Kinetic Mensurenzent.s The kinetics nm of Cr(V) compartment the
Cr(V)
acetonitrile
of the reaction
employing attached complex
was followed
a Hitachi-200 to a circulating
1 in aqueous
the ?Lmexis shifted
at pH 2 by measuring
UV- visible constant solution
spectrophotometer
temperature is 5lOnm
to 540 nm A similar
(E
shift
bath =
the absorbance having
at 540 cell
The absorption
I68 M-‘cm-l)
in h,,,,
changes
a thermostated
”
maximum In
the
due to the change
h,,,;,, foi-
presence
of
of the medium
1133
Catalysed Cr(V) oxidation of organic sulfides
from water to acetonitrile thiolactic
acids.16
oxidant
was noticed
In all reactions,
was used to ensure Rate
constants
versus reaction
evaluated
out that the reaction
by using the semilogarithmic computer
excess
A typical kinetic
conditions
program.
oflactic and
in the Cr(V) oxidation
stoichiometric
plots
The rate constant
of substrate
over the
trace is shown of absorbance
I
in Fig differences
for the self decomposition
as 2~1O-~s~’ at 25°C under similar conditions.’
It is pertinent
to point
of Cr(V) at pH l-2 has been studied by Ghosh and Gould. I7 The rate constants,
in Tables
l-5 were computed
rate and k, is the specific in the absence
IO-fold
a
first-order
time by least squares
1 (kd) was measured
of complex collected
pseudo
were
by Rajavelu and Srinivasan
at least
where kexpt is the experimentally
from eq.(2)
rate for the decomposition
of complex
1, measured
observed
k,
specific
under similar conditions,
of substrate. k,
In the presence
=
kerpt -
k#l
of PA, the value of k, is found to be 3x10-
that the rate constants
were reproducible
to within *5%
study we noticed
an initial increase
4-5. ’
Duplicate
kinetic
runs showed
Spectrfll Studies In the present 540nm.
During the course of the reaction
formation
of a Cr(lV)
reports.‘*
The disappearance
Cr(IV)
leading
Cr(V) complex (a=1 500M“ evidence
complex.
to the
in the
of Cr(V)-PA
we
the
in absorbance
end
of the reaction
the (Fig
4. It has been established
indicates of the
the
in earlier
the decay of spectrum
of
peak at 350 nm
2 a). We take this spectra1
change
as
when the absorption
spectrum
the peak at 350nm was shifted
to 400nm
The sharp peak at 400nm may be taken complex
species
the absorption
sharpening
3). However
PA and sulfide,
by slow decay at
to pink indicating
was taken as Cr(IV)
When we recorded observed
followed
from dark brown
intermediate
complex (complex
of oxidant,
in absorbance
changes
at
the products.
of PA alone
presence
the Cr(V)-PA-MPS
corresponds
of
and again an increase
(a=1600 M-’ c m-l) Fig.2.b of
of the pink colour
formation
for the formation
was recorded
This pink colored
in the presence
c m-1)
the colour
as spectral evidence
recently
for the formation
that the peak at 400 - 420 nm
to Cr-S bond formation.”
Isolation ofthe intermedinte complexes : During our kinetic studies we realised the possibility of the existence oftwo intermediate complexes 3 and 4 (see scheme III). Our attempts to isolate complexes rapid decomposition
3 and 4 resulted in
of these complexes
RESULTS AND DISCUSSION
In order to understand essential
to understand
order with respect
the role of catalysts
the mechanism
on the Cr(V) oxidation
of uncatalysed
to the Cr(V) and fractional
reaction.
order with respect
The
of organic
uncatalysed
to the substrates
sulfides,
reaction (Table
it is is first
I)
J. B. BHARATHY
1134
et&
3.000
Fig. 1. Curves a-e represent the change of the absorption spectrum ofCr(V) at one minute time intervals, for the reaction mixture containing O.OlM MPS, O.OOlM PA, O.OOlM Cr(V)(l) at pH2 in 50% (v/v) CH, CN
0.000 300
400
500 Wavclr~,@h,
600 nm
Fig.2 The absorption spectra ofCr(V) at pH2 in 50% (v/v) CH, CN H,O recorded under different conditions a)Cr(V)(O001M)andPA(0.00lM)only b) Cr(V) (O.OOlM), PA (O.OOlM) and MI’S (O.OlM) c) Ten times expanded spectrum for (b) Inset : Absorption spectrum for Cr( V) only.
H,O
1135
Catalysed Cr(V) oxidation of organic sulfides
Table 1. oxidation
Pseudo
of MPS
first order,
in 5O%(v/v)
k, and
aqueous
order
rate
constant,
k, ,values
kJlO‘* M-’ s-’
0.01
1 02
I 02
0.02
1.67
0.84
0.03
2.77
0.92
0.04
3 71
0.93
0.05
4.09
0.82
0.06
4.28
0.72
0.10
5 79
0.58
= 0.001 M ; [H’] = k,
for the Cr(V)
at 25°C.a
k,/10-4 s-’
W’SJ,M
[Oxidant] from k,
second
acetonitrile
= 0 01 M. a) Second
order
rate
constants
listed
were
calculated
/ [MPS]
This saturation kinetics was explained in terms of complex formation between Cr(V) and sulfide in the uncatalysed reaction.
The mechanism proposed for the uncatalysed
d+A
151.-. ,R
K
2s
reaction is shown in Scheme 1.
Cr”.j..s
,--’
‘R
2
Llgand
k
coupling
I III
Cr+ Rp
so
Scheme I According product,
in the complex from sulfide radical.
to Scheme
sulfoxide,
I, the oxidant
can be visualised
2.’
However
the alternative
to oxochromium(V)
Thus equations
forms a complex
as due to the ligand formulation
in complex
(3)-(6)
represent
with
the
coupling
the oxygen
is in terms of inner sphere
2, leading to the formation
the alternative
The formation
sulfide
between
mechanism2”
ofCr(lV) (Scheme
electron
=
(3)
Cr(V)....r( Me Complex J
k’l Complex 2
t.
+
Cr(IV) + Ad-Me
Cr(lV) + Ad-Me I %
Cr(V)/Cr(lV) + Ar -i- Me&
Cr(II1) + Ar-i:Me Cr(II1) + ArSOMe Scheme II
tr-anrfer
and sulfide cation II)
Ar Cr(J’) f Ar-S-Me
of
and sulfide
(4) (5) (6)
1136
J. B. BHARATHY et al.
A similar successfully
mechanism
to the
The Cr(V) and NIm,
and
Cr(V)
has been postulated oxidation
oxidation
the
and the Marcus
theory
of electron
transfer
applied
of dialkyl sulfides.2’
of organic
rate constant
sulfides
values
is catalysed
obtained
by the pyridine
for the
bases
ofMPS
oxidation
PA, Bpy, Phen, Im
at
different
[catalyst]
are given in Table 2 The carboxylato G(V)
in different
to the redox
reaction
been measured (Tables
bound Cr(V)
pH can be studied
I3 In the present
and this
l-4).
Though
the catalyst
the
and Phen it is about parent
reaction
sultide.Since
is
30 times Im and
the oxidant
ligands when the reaction oxidation
NIm This
of organic
order
versus
catalysts
the rate
was
ligands
confirmed
catalyst sulfides
our
1 at pH 2, has When
is 40 times with PA, whereas
in Bpy
rate enhancement
with
to complex
the least reactivity I exhibits
maximum
stability
is found to be unity from the
experimental
conditions kinetics
order in the catalyst
all could
linearity
of the
the substrates be explained
plots
exhibit only
400
200 I
I
30
40
50
60
7'0
80
1 /[Substrate] -
*p
Ii
-
OMe
*P
-Cl
*p-Me
*p
-
+p
Br
Fig.3. Michaelis - Menten
*p -
COMe
*m
-
cool-l -
Cl
Plot for Sulfides
90
of
for all the
600
I
of log
in terms
is also found to be fractional
l/k,
20
i’ in
fractional
study (Table 2 and Fig.4).
10
1 and
with these
BOO
0
the
observed
1,000
0’
values
varies
six fold
efficiency
“,22
type mechanism.The
used in the present
of complex
they tend to coordinate by
of
among these, which is similar to our earlier observations
(Table 3 and Fig 3) The complex
- Menten
show
the reactions
is slow compared
first order rate constant
the catalytic
enhancement
Im and Nlm
to oxidant
time.Under
dependence
a Michaelis
by all catalysts,
are unidentate fact
pseudo
was carried out in the pH 3 at which complex
The order with respect absorbance
is catalysed
However
of the oxidant
self decomposition
from the observed
However
Thus PA seems to be the superior the Cr(VI)
case also the
Ix~O-~M,
3-4.5
the self decomposition
value is substracted
concentration
try to stabilise
is highly stable in the pH range provided
100
Catalysed Cr(V) oxidation of organic sulfides
Table 2. Pseudo Zr(V) in 5O%(v/v)
first-order
aqueous
rate constant
acetonitrile
k, , values for the
[BPYI
0 000
0 102
0.001
I 03
0 002
I 78
0 003
261
0 005 001
3 89 4.54
0.02
8 64
0.000 0001
0.102 0 734
0.002
1 05 I 10
0 005
1 49
001
3 41
0 02 0 000
4 28 0 102
0 001
I 02 1 46 I 82
0 002 0 003
WI
[NW
a) [oxidant]
The substrate Table 3
0 005
2 45
0.01
3 37
0 02
4 43
0 000
0 102
0001
0 328
0 002
0 360
0 003
0 400
0 005
0 496
0.01
0.571
0.02
0 296
0 000
0 102
0001 0 002
0.411 0 436
0 003 0 005
0 449 0 533
0.01
0 593
0.02
0.401
= O.OOlM,
dependence
[MPS]
oxidation
of MPS by
k,/lO” s-l
0 003
[Phen]
catalytic
at 25’Ca.
[Catalyst],M WI
1137
= 0 OlM,
of PA catalysed
and
Cr(V)
[H+]
J
= 0 OIM
oxidation
of aryl methyl
sulfides
is shown
in
1138
J. B. BHARATHY et al.
Table 3. Pseudo Cr(V) oxidation
first-order,k,,
of substituted
phenyl
and second-order methyl
rate constant,kz,
sulfides (XC,H,SMe)
aqueous
acetonitrile
k 2/IO-*M-‘s-’
[XC,H4SMe] H
values for the PA catalysed
in 5O%(v/v)
001 0 02 0.03 0 OS
1 03 I 83
9 Ii
2 Ii 3 13
7 II 6 26
1 24 I 70
IO3
p- Cl 001 0 02 0 03 0 OS
2 69 3 80
124 8 52 8 97 7 60
001 0 02 0 03 0 05
I 2 3 4
84 44 32 3s
184 122 II I 8 7s
001 0 02 0 03 0 05
2 88 3 34 4 00 481
28 8 16 7 I3 3 9 63
0 0 0 0
01 02 03 05
3 02 3 65 4 39 S2l
30 2 I8 3 1-I 7 103
001 0 02 0 03 0 05
I 20 2 I2 2 95 4 53
120 IO 6 9 84 9 05
001 0 02 0 03 0.05
2 42 3 31 3 98 5 22
24 3 166 I3 3 10s
001 0 02 0 03 0.05
I I 2 3
I1 4 9 33 8 30 7.18
p-Me
P-CO,H
p-OMe
p-Br
p-COMe
m-C1
[PA]
a) Second
order
=
[Cr(V)]
0 OOIM,
rate
constants
=
OOOIM,
I4 87 49 59
listed [H+]
were =
OOIM
calculated
from
k,
-
k, : [MI’S]
Catalysed
1139
Cr(V) oxidation of organic sulfides
500
-
0 0
loo
50
150
200
250
350
300
400
450
500
1 /[CaWstl
PA --k
-
the
formation spectral
-
order
dependence
of catalyst
oxidant
and the
of more reactive evidence
formation
catalyst
involving
of complexes
mechanism,
Scheme
mechanism
may
Menten
Thus
Cr(V)-catalyst
for the formation
the other complex
Phen *
Michaelis
This fractional between
Bpy *
Fig-Q.
complex catalyst
3 and 4 have been
III, for the picolinic
operate
activity
If all these between
r-eversible
assumptions
Cr(V) oxidation
we
are valid one expects
oforganic
PA
=
Cr(V)
and
also
evidences
propose
for
for the
the following A similar
sulfides
also
6
Cr(V)+
formation
in terms of the
and the catalyst
Indeed the spectral
(see Fig 7) Thus
of other catalysts
complex
can be explained
the oxidant
and substrate
observed
Nlm Catalysta
the
indicates
acid catalysed
in the presence
for
the catalytic
of a complex
the oxidant,
Im *
Plot
(7)
- PA
Complex 3 K2
Complex 3 + MPS =
(W
MPS - Cr(V) - PA Complex 4
Complex
4
MPSO + Cr(III) - PA
-$
(9)
llgalld couplII,g or 0 and s
Scheme III Thus the reaction reactive
oxidant
shown
in Scheme
visualised
as due
is initiated
than complex Ill.
The formation
to ligand
by
1. This
coupling
the
formation
complex
3 forms
of the products, between
of a Cr(V)-PA
0 and S
another sulfoxide
in the
complex.3.
complex
is a more
4 with the substrate
and Cr(lll)-PA
hypervalent
which complex,
intermediate
as
can be
complex
4
1140
J. B. BHARATHY et al.
Such a ligand coupling of organic of
sulfides
ligand
recently
reviewed
alternative methods.
The
in the
involving
been proposed
oxidation
hypervalent
involving
However
importance
been highlighted
inner
sphere
it is difficult
by us in the PA catalysed
of organic
sulfur compounds
intermediates
by Oae and Uchida. 24 The formation
mechanism (10-12).
has already
and by others
coupling
equations has
mechanism
electron
been
of products transfer
to distinguish
of these two alternative
has
“.23
The
concept
extensively
applied
and
may be formulated
in terms
of an
within
the complex
these two alternative
mechanisms
Cr(V1) oxidation
in
the
4 as shown
mechanisms
reactions
of
in
by kinetic nucleophiles
recently.20,25
k, IET ->
Complex 4
Cr(IV)-PA + Ar-S-Me -> Cr(IV) / Cr(V)-PA + Ar-.?Me
The rate ofthe On the other (Table.4
reaction
is not appreciably
hand the reaction
is highly
-
affected
influenced
Cr(IV)-PA + Ar-:-Me
(10)
Cr(Ill)-PA + Ar-S-Me
(11)
Cr(II1) + ArSOMe
(12)
by the change in ionic strength
by the change
of the medium
of [H+] and solvent
composition
) Table
4. Effect
of varying
[H+] and solvent composition
on PA catalysed
Cr(V) (I) oxidation
of
MPS.
s-l
kl/lO"
* WI
CHsCN-H,O “h (v/v)
0001
0 4s
25.75
0 005
0.80
50-50
I 03
001
I 03
60-40
3 71
0.04
3 88
75-25
4 54
0.10
12 3
SO-20
7 22 IO.8 H,O , ii [H+] = 0.01~
90-10 [Cr(V)] = O.OOlM ; [MPS] = O.OlM ; * Solvent = 50% A plot oflog a positive reaction
l/D is fairly linear with a positive molecule
may be ascribed
to facile
electrophilicity
of
increased offered
k, versus
ion and a neutral
in the rate determining formaion the
by us and Rocek and co-workers
alcohols.8~t’
(V/V)
of Cr(V)-PA
protonated
CH,CN
slope pointing step
to an interaction
The substantial
complex
oxidant.
for the PA catalysed
0 78
in the presence
Similar
of the
of H’ and the
explanations
Cr(VI) oxidation
between
acid catalysis
of organic
have sulfides
been and
Catalysed
Both the electron phenyl ring of ArSCH3 The catalysed parameters isokinetic
donating accelerate
oxidation
evaluated
Cr(V) oxidation of organic sulfides
and electron
withdrawing
the catalysed
reaction
was carried
using the Eyring
plot of AH’ versus
substituents
in the para position
similar to the uncatalysed
out at four different
equation
1141
temperatures
along with k, values
AS # is linear and the slope ofthe
plot
of the
oxidation.’
and the thermodynamic
are collected
in Table 5 The
yields the isokinetic
temperature,
l3 , which is found to be 3 17K (r = 0.998). Table 5. Pseudo entropies
first-order
rate constants (AG#)
(AS’ ) and free energies
XC6H4SMe
in 5O%(v/v)
aqueous
x
at four different
of activation
p-CO,H p-Me p-Br p-COMe m-Cl
298K
308K
313K
0 73 0.91 2.89 2.46 1.51 0.85 2.05 0.83
I 03 I 24
2.40 2.26 3.34 3.74 2.97 2.59 3.54 2.25
3.48 3.46 3.52 4.72 3.66 3.50 4.44 3.37
* at 303 K conditions,
AH’
293K
3.02 2.88 1.84 I .20 2.42 I.14
[PA]=O.OOlM,
[Cr(V)]=O
Cr(V)
AS’
(AH’),
oxidation
of
AG#* kJ mol-’
57.5(129)’ 47.0(64.7) 5 27(5.71) 21.3(102) 31.4(31 4) 5 1.7(68 8) 26.6(133) 50.2(79.8)
OOlM: [Substrate]=O.OlM.
“The values in parentheses
and are collected
and enthalpies
catalysed
acetonitrile
k 1/10-3s-1
H p-Cl p-OMe
temperatures
for the PA
correspond
-109(+112) -143(-90 7) -275(-279)
90.4(95
-222(+25.8) -191(-200)
X8.6(94 2) 89.3(92.0) 90.2(92 X)
-127(-79.3) -206( 122) -133(-43.4)
I)
90.3(92 2) 88.7(90 2)
88 9(96 5) 90 4(92 9)
[H+]=O 01M to the
uncatalysed
Ireaction under blmilal-
from ref 8
The thermodynamic parameters given in Table 5 follow the trend expected for the catalysed reaction i e AH” is small and AS’ is mote negative compared to the uncatalysed reaction By applying Michaelis-Menten
kinetics to Scheme 111,we have derived an expression for k, (eq I-3) and
from the values of k, at different [substrate], the formation constant, for complex decomposition
of complex 4, k, for various substituted
Table 6. The equilibrium (k3) values for XC6H,SMe (X) Substitutent
constant
sulfides
4, K, and rate constant, for the
have been estimated and given in Table 6.
(K2) and the rate constant
for the decomposition
at 25°C K,,M’
k3 /IO-‘s-’ 4.41
H
21.9
p-Cl
25.3
4 56
p-OMe
102
4.49
P-CO:H p-Me
I17
3.99
46 3
4.25
m-Cl p-Br
20 0 10 8
9.40
The K, value is found to be 130 Me’ from equilibrium
5.21
measurements
of complex
4
1142
J. B. BHARATHY et
al.
Rate Benefit The rate benefit and is consistent substituent,
p-OMe,
COMe, p- CO,H groups
measured
showed
oxidation.
However
Table 7. aqueous
and PA catalysed
the rate benefit
I
It
of ArSMe
is found
This behaviour Cr(V)
oxidation
in Cr(V) oxidations
is given
in Table 7
that the electron
withdrawing
donating
substituents,
of electron
releasing
of ArSMe
is very similar
p-
and withdrawing to Cr(VI)
is high in the presence
of electi-on
in the substrate.
Rate benefit
acetonitrile
achieved
oxidation
and both the electron
the highest rate benefit.
substituents
Cr(V)
principle.”
has the least rate benefit
in the uncatalysed
withdrawing
in the PA catalysed
with the reactivity-selectivity
in
the
PA Catalysed
Cr(V)
oxidation
of
XC6H,SMe
in 5O%(v/v)
at 20°C Rate benefit’
Substituent Cr(V)
Cr(Vl)*
H p-Cl p-OMe p-CO,H p-Me p-COMe m-Cl
17.2 2 09 091 60 5 I 65 69.8 2.87
7.71 7 44 2.75 991 6 37
p-Br
2 50
8.38
149
*data taken from Ref. I I. ’ rate benefit
=
k,(cat) -- k,(uncat) -------------------------
where k,(cat) and k,(uncat) represent the rate constants for the catalysed and uncatalysed reactions respectively
k,(uncat)
under similar conditions
”
Rate ho Our kinetic
and spectral
observations
Cr(V)-PA
complexes
as intermediates
in Scheme
III lead to the rate law (13)
-d[Cr(V)]
1
k, = _____________ x
-------
dt
and catalyst
oxidant estimated
vs Ii[cat] substituents
transfer
irrespective through
the first order
and l/k,
dependence
vs l/[sub]
respectively
in the aryl ring of ArSMe
of the nature of the substituent
ligand coupling
to the nature of electron
Cr(V)-PA
and
ArSMe-
to these complexes
k, K, K2 [ Sullide] [catalyst] __________-_________~~~~~~______~~~~~~~~~_____
of the
of plot of l/k, k, value
of
The values of K, ,K2 and k, have been
influence
of the
out the formation
ofa steady state approximation
(13)
(K, [catalyst] +I) (K, [sulfide] +l)
IWW
This rate law explains substrate
=
point
Application
and withdrawing
order
explanation.
of k: and the
The constancy
in the aryl moiety may be taken as evidence transfer
substituents
However
in the
from the slope and intercept
(See Table 6) The constancy
on K2 deserve
rather than via electron
releasing
and fractional
the K,
value
of
for 0x0
is sensitive
in the phenyl ring of ArSMe
These
Catalysed Cr(V) oxidation of organic sulfides
results
point
out that the reversible
fractional
order
Hammett
equation
with respect
or other structure-reactivity
Since the reaction
step in eq (8) may be rate controlling
to substrate.
Thus the authors
I143
pseudo
first order
have not attempted
rate constants
cannot
be used for
is the
to analyse the kinetic data in terms of Hammett
relationships
Acknowledgnzent SR thanks University,
Prof
C
Srinivasan,
for his constant
Head,
encouragement
well as the research
and development
funds and necessary
facilities.
Department
of Materials
Science,
Madurai
Kamar-aj
and JRBB thanks the UGC, New Delhi and the Principal
committee
of The American
College,
Madurai
as
for providing
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