Effect of added donor ligands on the selective oxygenation of organic sulfides by oxo(salen)chromium(V) complexes

Article abstract of DOI:10.1016/j.tet.2006.03.103

Oxo(salen)chromium(V) complexes, [(salen)Cr^V{double bond, long}O]⁺, oxidize organic sulfides selectively to sulfoxides in high yield. This oxygenation reaction is catalyzed by ligand oxides (LO's), pyridine N-oxide, 4-picoline N-oxid

Full text of DOI:10.1016/j.tet.2006.03.103

Tetrahedron 62 (2006) 5645–5651
Effect of added donor ligands on the selective oxygenation of
organic sulfides by oxo(salen)chromium(V) complexes
Natarajan Sathiyamoorthy Venkataramanana,b and Seenivasan Rajagopalb,
*
aNational Institute of Advanced Industrial Science and Technology, 4-2-1, Nigatake, Miyagino-ku, Sendai 983-8551, Japan
bSchool of Chemistry, Madurai Kamaraj University, Madurai 625-021, India
Received 28 December 2005; revised 18 March 2006; accepted 30 March 2006
Available online 2 May 2006
Abstract—Oxo(salen)chromium(V) complexes, [(salen)CrV]O]+, oxidize organic sulfides selectively to sulfoxides in high yield. This
oxygenation reaction is catalyzed by ligand oxides (LO’s), pyridine N-oxide, 4-picoline N-oxide, 4-phenyl pyridine N-oxide and triphenyl-
phosphine oxide. The rate is accelerated by 10–20 times with an increase in yield of sulfoxide in less reaction time. This catalytic activity is
highly sensitive to the nature of the substituent in the phenyl ring of ArSMe and in the 3- and 5-position of the salen ligand. The reaction
constant (r) value obtained with the ligand oxide catalyzed reaction is low compared to the value in the absence of LO. The strong binding
and catalytic activity of ligand oxides on the oxo(salen)chromium(V) ion oxygenation is explained in terms of binding constants and a
mechanism involving the electrophilic attack of [(salen)CrV]O]+–LO adduct on the sulfur centre of phenyl methyl sulfide.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
and of the substrate. In recent years, Gilheany and co-
workers14–17 in a series of publications have shown that
the stereoselectivity in the epoxidation of alkenes with
oxo(salen)chromium(V) complexes is substantially changed
on the addition of certain oxygen containing donor ligands
such as phosphine oxide and amine N-oxides. In most cases
the presence of ligand oxides (LO’s) has a significant effect,
increasing the ee by 30%. It is known that such additives are
coordinated to the metal atom at the axial position through
the oxygen atom of LO’s, which weakens the Cr]O bond
in [(salen)CrV]O]+ ion and thereby increases the reactiv-
ity.1,14–17 Though the effect of such additives on the reactiv-
ity of oxo(salen)metal complexes has been studied on the
epoxidation reaction, no attempt has been made to look at
such an effect on the sulfoxidation reaction. Moreover, the
addition of ligand oxides was found to change the reaction
mechanism.18 To get a clear picture we have attempted a
study, on the oxidation of sulfides with several oxo(salen)-
chromium(V) in the presence of donor ligands, like tri-
phenylphosphine oxide (TPPO), pyridine N-oxide (PyO),
4-picoline N-oxide (PicNO) and 4-phenyl pyridine N-oxide
(PPNO). These were studied using spectrophotometric and
EPR techniques and the observed results are presented in
this article.
The use of metal–salen complexes as efficient catalysts in
oxygenation reactions, particularly epoxidation and sulfoxi-
dation, has been widespread in recent years.1–10 Though the
catalytic role of many metal–salen complexes is reported in
the literature, most of the work is concerned with Fe(III)–
salen, Mn(III)–salen and Cr(III)–salen complexes. Among
these metal complexes, Cr(III)–salen has the advantage
that the oxo(salen)chromium(V) ion, [(salen)CrV]O]+,
generated from Cr(III)–salen complex and PhIO, is stable
and can be isolated unlike the oxo(salen)manganese and
oxo(salen)Iron ion, which has a fleeting existence.1,2,8,9,11
The [(salen)CrV]O]+ ion has been characterized definitely
by spectroscopic techniques and by single crystal X-ray
structure determination. Consequently, there is no ambiguity
concerning the identity of the active oxidizing agent. Thus
the use of the isolated oxo(salen)chromium(V) complex
allows reactions to be carried out stoichiometrically, in the
absence of co-oxidants, facilitating kinetic studies and
mechanistic evaluation. Recently Venkataramanan et al.13
have studied the oxygenation reaction of organic sulfides
and sulfoxides with 10 oxo(salen)chromium(V) complexes
by varying the substituents on the salen ligand electronically
and sterically. This redox reaction is highly sensitive to the
change in structure of the ligand in the Cr(V)–salen complex
2. Results
2.1. Spectral studies
Keywords: Cr(III)–salen; Sulfide; Sulfoxide oxygenation; Ligand oxides;
Rate enhancement.
The selective oxidation of several substituted phenyl methyl
sulfides with 10 oxo(salen)chromium(V) complexes was
* Corresponding author. Tel.: +91 452 2459084; fax: +91 452 2459139;
e-mail addresses: ns-venkataramanan@aist.go.jp; seenirajan@yahoo.com
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.03.103

5646
N. S. Venkataramanan, S. Rajagopal / Tetrahedron 62 (2006) 5645–5651
Z
Z
Z
Z
X
O
N
O
N
X
O
Cr
X
O
N
O
N
X
+
PhI
+
PhIO
Cr
II
I
Ic, IIc: X = Cl, Z = H
Ib, IIb: X = Me, Z = H
Ie, IIe: X = tBu, Z = tBu
+
Ia, IIa: X = H, Z = H
Id, IId: X = Br, Z = H
+
Z
Z
Z
Z
X
O
N
O
N
X
O
Cr
X
O
N
O
N
X
+
PhI
+
PhIO
Cr
I
II
Ij, IIj: X = tBu, Z = tBu
Ii, IIi: X = Z = H
TPPO
PyO
N
N
P
O
O
O
PPNO
PicNO
H3C
N
O
Scheme 1. Structure of complexes IIa–IIg and the ligand oxides (LO’s).
well studied.10b,13 A mechanism involving an electrophilic
attack of the oxygen of the oxidant at the electron-rich sulfur
centre of the substrate has been proposed. Realizing the
importance of donor ligands on the oxygenation reaction
with oxo-metal complexes10b,14–19 we have investigated
the effect of LO’s on the sulfoxidation reaction with oxo
(salen)chromium(V) complexes and the structures of Cr(III)
and Cr(V) complexes and the donor ligands used in the pres-
ent study are shown in Scheme 1.
concentration of additive increases the absorbance of Cr(V)–
LO adduct. A sample spectrum to show the increase of
absorption with [LO] is given in Figure 1.
0.8
0.6
0.4
0.2
0.0
The parent oxo(salen)chromium(V) ion has an absorption
maximum (lmax) at 560 nm in CH3CN. When the ligand
oxide, LO, is added to the Cr(V) ion a substantial red shift
is noticed in the absorption maximum. This shift in the
lmax depends on the nature of the LO and is in the range
of 15–100 nm. The lmax values of all substituted oxo
(salen)chromium(V) complexes in the absence and presence
of additives are given in Table 1. Further, an increase in the
Table 1. Absorption maxima, lmax, of [(salen)CrV]O]+ complexes in the
presence and absence of LO
Serial Complex lmax (nm)
number
lmax (nm) in the presence of LO
in the absence
of LO
TTPO
PyO
PicNO
PPNO
1.
2.
3.
4.
5.
6.
7.
IIa
IIb
IIc
IId
IIe
IIf
560
557
584
590
595
600
610
627
640
645
645
610
620
630
612
620
630
632
671
615
685
608
642
630
630
650
610
664
615
655
630
625
654
615
665
500
600
700
800
900
Wavelength, nm
Figure 1. Absorption spectra of IIa in the absence of PyO and at 0.025 M,
0.05 M, 0.075 M, 0.1 M and 0.125 M PyO. [IIa]¼3ꢀ10ꢁ4 M.
IIg

N. S. Venkataramanan, S. Rajagopal / Tetrahedron 62 (2006) 5645–5651
5647
Table 2. Binding constants for the complex formed between IIa–IIg and
ligand oxides
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Serial
number
Complex
Ligand oxides (LO’s)
TPPO
PyO
PicNO
PPNO
1.
2.
3.
4.
5.
6.
7.
IIa
IIb
IIc
IId
IIe
IIf
44ꢂ11 125ꢂ21
32ꢂ11 140ꢂ14
165ꢂ32
183ꢂ41
146ꢂ42.0 150ꢂ31.5
71ꢂ19 123ꢂ24.2 241ꢂ34.1 192ꢂ21.4
84ꢂ31 184ꢂ44
24ꢂ9 73ꢂ14.4 112ꢂ21.4 156ꢂ15
41ꢂ10 101ꢂ12.2 185ꢂ16.4 646ꢂ51
27ꢂ11 69ꢂ18.8 142ꢂ34.8 150ꢂ29
321ꢂ48
230ꢂ36
IIg
From these spectral changes we have estimated the binding
constants by using the Benesi–Hildebrand equation and the
values of binding constants are given in Table 2.
The binding constant depends on the nature of the LO’s as
well as the substituents in the salen ligand. Furthermore,
the metal ion containing electron donating groups in the
5-position of the salen ligands has a low binding constant
value compared to those carrying electron withdrawing
groups. This is expected as the electrophilicity of metal cen-
tre is decided by the nature of the substituents in the salen
ligand. The low binding constant values for the complexes
IIe and IIg can be accounted for on the basis of the presence
of the bulky tert-butyl group at the 3-position, which may
hinder the binding of the LO’s to the metal centre.
400
500
600
700
800
900
1000
Wavelength nm
Figure 2. Sample run showing the change in OD of IIc–TPPO adduct with
time in the presence of PhSMe. [IIc]¼5ꢀ10ꢁ4 M, [MPS]¼0.01 M and
[TPPO]¼0.01 M.
chromium(V) complexes (IIa–IIg) in the absence and pres-
ence of donor ligands are presented in Tables 3–7. These
kinetic data show that the reaction is highly sensitive to
the change of substituents in the phenyl ring of ArSMe,
and in the 5,50-positions of the salen ligand. The kinetic
data in Tables 3–7 have been analyzed using the Hammett
equation, and the reaction constant (r) values obtained for
the substituent variation in ArSMe are given at the bottom
of the tables and for the substituent variation in the salen
ligand of oxo(salen)chromium(V) complexes in the 7th col-
umn of the tables. Though the r value is highly sensitive to
the structure of the salen ligand and nature of the LO it is not
very sensitive to the change of the substituent in the aryl
moiety of ArSMe. The important point to be noted regarding
r value is that the r value is always small in the presence of
LO’s compared to the r value in the absence of LO.
Kochi and co-workers1 have isolated the oxo(salen)chro-
mium(V) ion and its adduct with PyO and determined their
structure by X-ray analysis. The authors have shown that the
Cr(V) ion and the Cr(V)–LO adduct have roughly the square
pyramidal and octahedral geometries in which the Cr atom is
˚
displaced 0.53 and 0.26 A units above the mean-salen plane,
respectively. Thus the LO binds strongly with metal centre
and affects the strength of Cr–O bond. In the present study
the enormous shift observed in the lmax value of [(salen)
CrV]O]+ on the addition of LO and large binding constants
support the strong binding of LO to the metal resulting in the
formation of O]Cr–O–L.
2.2. Kinetics
The progress of the title reaction is followed by measuring
the change in OD of the Cr(V)–LO adduct at the wavelength
given in Table 1. A sample run to show the decrease in OD of
Cr(V)–LO adduct with time is shown in Figure 2.
1400
H
OMe
1200
Me
The reaction is of first order for both the oxidant and the sub-
strate. The first order dependence in the oxidant is evident
from the linear log OD versus time plot (figure not shown)
and in the substrate is confirmed from the linear k1 versus
[substrate] plot (Fig. 3).
F
Cl
1000
800
600
400
200
0
Br
COCH3
To know the effect of LO’s on reaction rates, experiments
were carried out with different [LO]. At low concentration,
the k1 values vary linearly and at high [LO] the rate reaches
the maximum. Further increase in [LO] has no profound in-
fluence on the rate of the reaction. Thus the reaction follows
saturation kinetics with respect to the added LO’s.
0.00
0.02
0.04
0.06
[sulfide] M
0.08
0.10
The rate constants for the oxygenation of several para-
substituted phenyl methyl sulfides with oxo(salen)
Figure 3. Plot of k1 versus [sulfide] for the oxidation of sulfides by IIa in the
presence of TPPO at 298 K. [TPPO]¼0.01 M and [IIa]¼5ꢀ10ꢁ4 M.

5648
N. S. Venkataramanan, S. Rajagopal / Tetrahedron 62 (2006) 5645–5651
Table 3. Second-order rate constant (k2) and reaction constant (r) values for the IIa–IIg oxygenation of X–C6H4SMe in the absence of LO
X–C6H4SCH3 (p-X¼)
k2ꢀ103 Mꢁ1 sꢁ1
IIe
IIa
IIb
IIc
IId
r
IIf
IIg
H
OMe
Me
F
Cl
Br
COCH3
r
1.31ꢂ0.03
25.9ꢂ0.60
11.4ꢂ0.23
1.58ꢂ0.08
0.93ꢂ0.01
0.93ꢂ0.01
0.16ꢂ0.01
ꢁ2.8
1.15ꢂ0.04
4.60ꢂ0.13
2.60ꢂ0.52
0.59ꢂ0.10
0.32ꢂ0.01
0.28ꢂ0.06
0.23ꢂ0.01
ꢁ1.9
36.4ꢂ0.9
320ꢂ9.6
114ꢂ2.4
26.9ꢂ0.4
21.6ꢂ0.4
18.8ꢂ0.8
7.40ꢂ0.22
ꢁ2.0
45.8ꢂ1.4
526ꢂ14
140ꢂ4.2
61.0ꢂ1.3
43.8ꢂ1.0
24.2ꢂ0.4
8.80ꢂ0.2
ꢁ2.2
0.11ꢂ0.00
0.36ꢂ0.01
0.17ꢂ0.00
0.06ꢂ0.00
0.06ꢂ0.00
0.04ꢂ0.00
0.02ꢂ0.00
ꢁ1.8
2.2(0.980)
2.6(0.995)
2.4(0.988)
2.4(0.995)
2.4(0.995)
2.4(0.998)
2.3(0.965)
1.20ꢂ0.03
5.10ꢂ0.15
2.20ꢂ0.06
1.10ꢂ0.04
0.65ꢂ0.02
0.42ꢂ0.01
0.22ꢂ0.00
ꢁ1.8
0.19ꢂ0.01
0.37ꢂ0.01
0.28ꢂ0.00
0.15ꢂ0.00
0.13ꢂ0.00
0.09ꢂ0.00
0.06ꢂ0.00
ꢁ1.1
r
0.965
0.980
0.972
0.953
0.970
0.988
0.990
Table 4. Second-order rate constant (k2) and reaction constant (r) values for the IIa–IIg oxygenation of X–C6H4SMe in the presence of TPPO
X–C6H4SCH3 (p-X¼)
k2ꢀ103 Mꢁ1 sꢁ1
IIe
IIa
IIb
IIc
IId
r
IIf
IIg
H
OMe
Me
F
Cl
Br
COCH3
r
48.8ꢂ1.5
133ꢂ4.1
105ꢂ3.3
27.4ꢂ0.8
18.5ꢂ0.4
18.1ꢂ0.3
7.21ꢂ0.1
ꢁ1.8
10.4ꢂ0.4
24.2ꢂ0.8
16.4ꢂ0.5
5.75ꢂ0.1
3.92ꢂ0.0
3.22ꢂ0.0
2.76ꢂ0.0
ꢁ1.4
184ꢂ5.4
561ꢂ17
499ꢂ14
190ꢂ4.2
142ꢂ2.4
137ꢂ1.5
72.8ꢂ0.7
ꢁ1.2
207ꢂ7.4
744ꢂ27
702ꢂ23
145ꢂ4.5
147ꢂ3.4
122ꢂ2.1
69.4ꢂ1.0
ꢁ1.5
0.20ꢂ0.01
0.60ꢂ0.02
0.35ꢂ0.01
0.13ꢂ0.00
0.11ꢂ0.00
0.08ꢂ0.00
0.05ꢂ0.00
ꢁ1.5
2.3(0.934)
2.2(0.979)
2.4(0.981)
2.2(0.980)
2.3(0.987)
2.3(0.985)
2.2(0.983)
8.98ꢂ0.3
21.9ꢂ0.7
18.9ꢂ0.6
6.91ꢂ0.2
5.30ꢂ0.1
4.63ꢂ0.08
3.68ꢂ0.00
ꢁ1.2
0.23ꢂ0.01
0.60ꢂ0.02
0.37ꢂ0.01
0.18ꢂ0.01
0.13ꢂ0.00
0.10ꢂ0.00
0.07ꢂ0.00
ꢁ1.3
r
0.994
0.989
0.965
0.967
0.996
0.984
0.991
Table 5. Second-order rate constant (k2) and reaction constant (r) values for the IIa–IIg oxygenation of X–C6H4SMe in the presence of PyO
X–C6H4SCH3 (p-X¼)
k2ꢀ103 Mꢁ1 sꢁ1
IIe
IIa
IIb
IIc
IId
r
IIf
IIg
H
OMe
Me
F
Cl
Br
COCH3
r
37.3ꢂ1.2
90.9ꢂ2.7
38.5ꢂ1.5
17.2ꢂ0.35
10.4ꢂ0.17
9.77ꢂ0.14
4.69ꢂ0.06
ꢁ1.7
5.6ꢂ0.21
64.9ꢂ2.5
334ꢂ11
230ꢂ9
140ꢂ5.5
677ꢂ23
527ꢂ15
120ꢂ2.7
93.5ꢂ1.8
86.1ꢂ1.2
38.0ꢂ0.40
ꢁ1.7
0.48ꢂ0.02
0.88ꢂ0.02
0.65ꢂ0.01
0.42ꢂ0.01
0.29ꢂ0.00
0.25ꢂ0.00
0.20ꢂ0.00
ꢁ0.90
1.9(0.950)
2.2(0.942)
2.2(0.958)
1.8(0.965)
1.9(0.967)
1.9(0.966)
1.7(0.974)
18.9ꢂ0.72
31.6ꢂ1.23
26.7ꢂ0.65
12.2ꢂ0.26
8.5ꢂ0.12
8.2ꢂ0.09
3.2ꢂ0.01
ꢁ1.3
0.30ꢂ0.02
0.55ꢂ0.02
0.37ꢂ0.02
0.19ꢂ0.01
0.14ꢂ0.00
0.12ꢂ0.00
0.07ꢂ0.00
ꢁ1.2
35.5ꢂ1.4
21.2ꢂ0.72
4.3ꢂ0.13
3.3ꢂ0.08
3.1ꢂ0.05
1.6ꢂ0.02
ꢁ1.8
39.6ꢂ1.6
30.2ꢂ0.91
27.4ꢂ0.55
13.1ꢂ0.14
ꢁ2.0
r
0.989
0.975
0.988
0.972
0.990
0.980
0.994
Table 6. Second-order rate constant (k2) and reaction constant (r) values for the IIa–IIg oxygenation of X–C6H4SMe in the presence of PicNO
X–C6H4SCH3 (p-X¼)
k2ꢀ103 Mꢁ1 sꢁ1
IIe
IIa
IIb
IIc
IId
r
IIf
IIg
H
OMe
Me
F
Cl
Br
COCH3
r
16.6ꢂ0.7
45.2ꢂ1.5
27.2ꢂ0.5
15.3ꢂ0.3
14.3ꢂ0.3
13.8ꢂ0.2
7.27ꢂ0.08
ꢁ0.95
10.4ꢂ0.3
20.5ꢂ0.5
15.9ꢂ0.4
6.72ꢂ0.1
3.45ꢂ0.07
2.76ꢂ0.03
1.61ꢂ0.02
ꢁ1.6
99.72ꢂ3.1
338.6ꢂ11
192.3ꢂ7
64.3ꢂ1.8
38.5ꢂ1.1
37.5ꢂ0.53
21.2ꢂ0.30
ꢁ1.7
146ꢂ5.5
0.63ꢂ0.02
0.86ꢂ0.03
0.75ꢂ0.03
0.44ꢂ0.01
0.33ꢂ0.01
0.32ꢂ0.01
0.22ꢂ0.00
ꢁ0.89
1.8(0.966)
2.1(0.971)
2.0(0.967)
1.8(0.966)
1.8(0.968)
1.8(0.973)
1.8(0.979)
3.91ꢂ0.12
14.5ꢂ0.28
9.44ꢂ0.29
3.68ꢂ0.10
3.22ꢂ0.07
2.99ꢂ0.06
2.19ꢂ0.03
ꢁ1.1
0.25ꢂ0.01
0.48ꢂ0.02
0.37ꢂ0.01
0.23ꢂ0.00
0.19ꢂ0.00
0.19ꢂ0.00
0.17ꢂ0.00
ꢁ0.65
518ꢂ20
304ꢂ9.2
103.6ꢂ3.0
72.8ꢂ1.8
69.8ꢂ1.3
40.5ꢂ0.45
ꢁ1.5
r
0.963
0.984
0.997
0.994
0.992
0.960
0.977
Table 7. Second-order rate constant (k2) and reaction constant (r) values for the IIa–IIg oxygenation of X–C6H4SMe in the presence of PPNO
X–C6H4SCH3 (p-X¼)
k2ꢀ103 Mꢁ1 sꢁ1
IIe
IIa
IIb
IIc
IId
r
IIf
IIg
H
OMe
Me
F
Cl
Br
COCH3
r
12.9ꢂ0.4
35.2ꢂ1.3
24.0ꢂ0.9
9.93ꢂ0.3
7.60ꢂ0.16
6.68ꢂ0.08
4.38ꢂ0.05
ꢁ1.3
12.6ꢂ0.5
26.1ꢂ0.8
23.6ꢂ0.5
9.2ꢂ0.3
8.7ꢂ0.2
8.4ꢂ0.2
3.7ꢂ0.01
ꢁ1.1
152ꢂ5.3
522ꢂ18
310ꢂ12
94.9ꢂ2.9
68.4ꢂ2.0
65.8ꢂ1.4
29.9ꢂ0.5
ꢁ1.7
186ꢂ7.4
488ꢂ17
304ꢂ11
129ꢂ4.8
88.2ꢂ2.7
63.8ꢂ1.3
28.5ꢂ1.0
ꢁ1.6
0.65ꢂ0.01
0.99ꢂ0.03
0.71ꢂ0.02
0.58ꢂ0.01
0.46ꢂ0.01
0.45ꢂ0.01
0.35ꢂ0.00
ꢁ0.59
1.9(0.964)
2.2(0.962)
2.1(0.952)
1.9(0.966)
1.8(0.953)
1.7(0.945)
1.6(0.964)
29.4ꢂ1.0
111ꢂ3.9
56.1ꢂ2.3
31.6ꢂ1.0
20.6ꢂ0.7
20.5ꢂ0.6
12.0ꢂ0.2
ꢁ1.2
0.51ꢂ0.02
0.99ꢂ0.04
0.78ꢂ0.02
0.45ꢂ0.01
0.39ꢂ0.00
0.35ꢂ0.00
0.23ꢂ0.00
ꢁ0.85
r
0.995
0.972
0.997
0.989
ꢁ0.986
0.976
0.989

N. S. Venkataramanan, S. Rajagopal / Tetrahedron 62 (2006) 5645–5651
5649
This trend is expected based on the reactivity–selectivity
principle (RSP).10,11,21–23,25 As the reactivity in the presence
of LO is more than that in the absence of LO, we expect that
the r value, an empirical term representing the selectivity of
the reaction, should be less than what is actually observed.
organic sulfides. From the data given in Table 8 we can under-
stand that the added LO’s have a small effect on the product
yield. All the LO’s increase the percentage of product formed
with less reaction time. Thus the data indicate that in the pres-
ence of ligand oxides, product yield is improved along with
the increase in the rate of the reaction. When we compare
the analogous epoxidation reactions with similar complexes,
Gilheany and co-workers found that yields did not exceed
50%. Furthermore the rates of oxidation in the presence
LO’s for sulfides are in the range of 10ꢁ3 Mꢁ1 sꢁ1 and that
2.3. EPR studies
The added advantage of Cr(V) complexes is that they are
EPR active and hence EPR spectra were recorded at room
temperature for the complexes and they show a strong signal
at ca. CgD¼1.987 and possess nitrogen and 53Cr hyper-
fine splitting. Upon addition of LO to the oxo(salen)
chromium(V) ion, the g value shifts from 1.987 to 1.975
with the disappearance of the hyperfine splitting, indicating
the binding of the LO to the metal centre.1 The kinetics of the
reaction can also be followed by this EPR technique. The
change in the intensity of the EPR signal of [O]Cr(V)
salen]+ ion with time is used to follow the kinetics of the re-
action. A sample run to show the change in the EPR signal
intensity with time obtained during the reaction of IIc with
MPS in the presence of TPPO as the additive is shown in
Figure 4. The rate constant obtained by EPR technique is
found to be in close agreement with the values obtained by
the spectrophotometric method.
of epoxidation are in 10ꢁ2 Mꢁ1 sꢁ1 1b Thus both the kinetic
.
data and percentage of sulfoxide formed support the catalytic
role of all LO’s used in the reaction.
3. Discussion
The spectral and kinetic data given in Figures 1–4 and Tables
1–8 show the catalytic role of the added ligand oxide, LO.
The addition of LO shifts the lmax of IIa–IIg enormously
and accelerates the reaction rate by 10–20 times. The coor-
dination of the LO occurs at the axial position to complete
the octahedral coordination of chromium. The infrared spec-
trum of the LO bound chromium(V) complex indicates that
significant bond weakening occurs upon axial coordina-
tion.1a,2b Among the five LO’s used, triphenylphosphine ox-
ide was known to have a better binding ability, but 4-phenyl
pyridine N-oxide was found to have highest binding constant
and the maximum rate. This unusual behaviour may be
attributed to the steric hinderance created by TPPO during
the binding with the metal centre.12a
The spectral, kinetic and product yield data presented above
confirm the formation of oxidant–ligand oxide adduct as the
first step, which in turn acts as a more powerful oxidant than
the oxo(salen)chromium(V) ion. At least two different
modes of oxidation may be envisaged to account for the
type of reactivity observed: (i) a bimolecular electrophilic
oxygen transfer from [O]CrV(salen)]+ to the substrate
and (ii) a bimolecular electrophilic oxidation performed by
[O]CrV(salen)–LO]+. As the binding constant of LO with
Cr(V) ion is high and the rate of oxidation in the presence
of LO is 10–20 times more than that in the absence of LO;
the bimolecular electrophilic oxidation of the complex alone
is less important compared to bimolecular electrophilic
oxidation of the adduct complex under the present experi-
mental conditions. Hence a mechanism involving electro-
philic attack of [(salen)CrV]O]+–LO adduct on the sulfur
centre of ArSMe (Scheme 2) can be proposed.
Figure 4. EPR spectra showing the change in the intensity with time for the
oxidation of MPS by IIc in the presence of TPPO as ligand oxide.
2.4. Effect of donor ligand on product yield
Table 8 shows the effect of added donor ligands on the yield
of sulfoxide formed in the stoichiometric oxidation of
Table 8. Percentage of sulfoxide formed from the selective oxidation of p-X–C6H4SCH3 with IIa in CH3CN in the presence and absence of ligand oxides at
298 K
X–C6H4SCH3
X¼
Without LO
TTPO
PyO
PicNO
Reaction time (min)
% of sulfoxide
Reaction time (min)
% of sulfoxide
% of sulfoxide
% of sulfoxide
H
240
60
75
200
240
240
240
96
95
93
78
60
61
40
90
60
60
90
90
90
90
97
99
94
81
76
56
55
96
98
98
72
68
65
48
94
97
98
76
72
64
49
OMe
Me
F
Cl
Br
COCH3

5650
N. S. Venkataramanan, S. Rajagopal / Tetrahedron 62 (2006) 5645–5651
to CrIII–salen complex dissolved inw25 mL of CH3CN. The
O
+
O
K
(salen)CrV
colour of the reaction mixture has turned from orange to dark
green. This slurry is stirred for 20 min and then filtered
to remove the unreacted iodosylbenzene. Ether is slowly
added to the dark filtrate in order to precipitate crystals of
oxo(salen)chromium(V) salts. The ligand oxides used as
donor ligands are commercial samples from Aldrich and
they were used as such without further purification. Various
substituted phenyl methyl sulfides were prepared29,30 and
their purity checked by established methods. The solvents
used in the study were of HPLC grade.
(salen)CrV
+ LO
LO
+
+
O
Ar
Slow
k2
(salen)CrV
+
ArSMe
(salen)CrIII
O S
Me
LO
LO
Fast
+
4.2. Estimation of binding constants
(salen)CrIII
LO
+
ArS(O)Me
The complexation/adduct formation of LO with the oxo
(salen)chromium(V) cation was monitored by UV–vis spec-
troscopy. A 5ꢀ10ꢁ4 M solution of oxo(salen)chromium(V)
ion in acetonitrile, taken in a quartz cuvette with 1 cm path
length, was treated with successive aliquots of concentrated
solution of LO in the same solvent. The absorption spectrum
of the oxo(salen)chromium(V) ion exhibited a red shift upon
the addition of LO. The formation of complex/adduct was
evident from the colour change (dark green to emerald green)
and the shift in the lmax. By measuring the OD at different
Scheme 2.
The ligand oxide binds strongly with the oxo(salen)chro-
mium(V) ion via oxygen atom of the ligand oxide(LO) to
chromium. The binding of the oxygen of the LO to the metal
weakens the Cr]O bond in the oxo(salen)chromium(V)
complex and facilitates the removal of oxygen atom from
the oxo complex to form sulfoxide as the product. When
the Cr(III) complex is formed as the product, the ligand
oxide occupies two coordination sites making the product
octahedral. It is pertinent to recall that Kochi and co-
workers1 have isolated the Cr(III) complex, [CrIII(salen)
(OPEt3)2]+, which indicates that the one molecule of ligand
oxide entered the coordination sphere of oxo(salen)chro-
mium(V) ion prior to oxygen transfer to the substrate since
the rate of incorporation of ligand oxide into the substitu-
tionally inert Cr(III)–salen is slow.1
concentration of a particular LO at the appropriate lmax
the binding constant has been calculated by using the modi-
,
fied Benesi–Hildebrand (double reciprocal plot) equation.24
½oxidꢃ½LOꢃ ½oxidꢃ þ ½LOꢃ
1
¼
þ
DOD
D3
KfD3
In the above equation DOD is the difference in the absorp-
tion intensities of [(salen)CrV]O]+ in the presence and
absence of LO and Kf is the equilibrium constant for the for-
mation of adduct or complex. The plot of [oxid][LO]/DOD
versus [oxid]+[LO] is linear and from the values of slope
and intercept, Kf values for the complex formation with
LO have been calculated.
When we compare the rate constant and product yield data
carefully, we realize that the rate constant in the presence
of ligand oxides is 10–20 times more than that in the absence
of ligand oxides. An increase in the product yield in the pres-
ence of LO is also observed. This was because the rates of
oxidation of sulfides are much higher compared to that of
epoxides. Therefore the formation of an unreactive m-oxo
dimer by the reaction between [O]CrV(salen)]+ and
[CrIII(salen)]+ in sulfoxidation has less effect compared to
the epoxidation reaction.16,17 On the other hand, in the
case of oxo(salen)manganese complexes the m-oxo dimer,
Mn(IV)–O–Mn(IV) formed is an alternative source of
[(salen)MnV]O]+ for the epoxidation and sulfoxidation
reactions.11,26–28
4.3. Kinetic measurements
An analytik-jena Specrod-diode array photometer (Specord
S100) was employed to record the absorption spectra of
Cr(III) and oxo(salen)chromium(V) complexes used in the
present study and to follow the kinetics.
The kinetic studies of oxidation of para-substituted phenyl
methyl sulfides with oxo(salen)chromium(V) complexes
were carried out in acetonitrile under pseudo first order con-
ditions using excess of substrate over the oxidant (10:1). The
rate of oxygenation of organic sulfides in the presence of
donor ligands was followed by measuring the change in
the absorbance of CrV–ligand oxide adduct at the wave-
length given in Table 1. The concentration of the ligand
oxides was chosen in such a way that the OD of the
oxo(salen)chromium(V) complexes was unaffected by the
further addition of the donor ligand.
4. Experimental
4.1. Materials
Chromium(III)–salen [salen¼N,N-bis(salicylidine) ethylene-
diaminato] complex was synthesized1,2 using the estab-
lished procedure. Iodosylbenzene was prepared by alkaline
hydrolysis of iodosobenzene diacetate according to the re-
ported method.20 The purity of ligands was checked by
1H NMR spectroscopy and that of Cr(III) complexes was
checked by IR and ESI-MS spectroscopy. The oxo(salen)-
chromium(V) complexes IIa–IIg were obtained from
chromium(III) complexes Ia–Ig by the general procedure
described below. A slight excess of iodosylbenzene is added
4.4. Product analysis
In a typical experiment involving the selective oxidation
of sulfides to sulfoxides, 2 mM oxo(salen)chromium(V)

N. S. Venkataramanan, S. Rajagopal / Tetrahedron 62 (2006) 5645–5651
5651
11. (a) Chellamani, A.; Alhaji, N. M. I.; Rajagopal, S. J. Chem.
Soc., Perkin Trans. 2 1997, 299–302; (b) Chellamani, A.;
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phenylphosphine oxide, pyridine oxide, 4-picoline N-oxide
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To this is added the 2 mM p-substituted phenyl methyl
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The resulting residue was analyzed by GC. Product yields
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and sulfoxide was the only product under the present exper-
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We thank Professor P. Sambasiva Rao and Dr. M. Vairamani,
for recording the EPR and ESI-MS spectra, respectively. The
authors gratefully acknowledge the referee for his detailed
comment and helpful suggestions. S.R. thanks CSIR for pro-
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thanks CSIR for an SRF fellowship.
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