Mechanism of selective oxidation of organic sulfides with oxo(salen)chromium(V) complexes

Article abstract of DOI:10.1021/jo9916380

The selective oxidation of organic sulfides to sulfoxides by oxo(salen)chromium(V) complexes in acetonitrile is overall second-order, first-order each in the oxidant and the substrate. The rate constant, k₂, values of several para-substituted p

Full text of DOI:10.1021/jo9916380

3334
J . Org. Chem. 2000, 65, 3334-3340
Mech a n ism of Selective Oxid a tion of Or ga n ic Su lfid es w ith
Oxo(sa len )ch r om iu m (V) Com p lexes
Ranganathan Sevvel,† Seenivasan Rajagopal,*,‡ Chockalingam Srinivasan,*,†
Nainamohamed Ismail Alhaji,§ and Arunachalam Chellamani§
Department of Materials Science and School of Chemistry, Madurai Kamaraj University,
Madurai - 625 021, India, and Department of Chemistry, Manonmaniam Sundaranar University,
Tirunelveli - 627 012, India
Received October 19, 1999
The selective oxidation of organic sulfides to sulfoxides by oxo(salen)chromium(V) complexes in
acetonitrile is overall second-order, first-order each in the oxidant and the substrate. The rate
constant, k2, values of several para-substituted phenyl methyl sulfides correlate linearly with
Hammett σ constants and the F values are in the range of -1.3 to -2.7 with different substituted
oxo(salen)chromium(V) complexes. The reactivity of different alkyl sulfides is in accordance with
Taft’s steric substituent constant, ES. A mechanism involving direct oxygen atom transfer from
the oxidant to the substrate rather than electron transfer is envisaged. Correlation analyses show
the presence of an inverse relationship between reactivity and selectivity in the reaction of various
sulfides with a given oxo(salen)chromium(V) complex and vice versa. Mathematical treatment of
the results shows that this redox system falls under strong reactivity-selectivity principle (RSP).
Sch em e 1
In tr od u ction
To mimic active intermediates in enzyme-catalyzed
oxidation reactions, metalloporphyrin and metallosalen
complexes [salen ) N,N′-bis(salicylidine)ethylenediami-
nato)] of iron, manganese, chromium, and ruthenium
have been used as model compounds and have been
shown to be capable of catalyzing oxygen atom transfer
from monooxygen sources such as iodosylbenzene, H2O2,
peracids, hypochlorite, etc. to saturated and unsaturated
hydrocarbons and other organic substrates.1-14 Although
participation of discrete, high-valent oxometal intermedi-
ates in the oxidation of organic substrates by heme
proteins and related model systems is widely accepted,
unambiguous assignment of the electronic configuration
of the metal center in these intermediates has presented
a difficult challenge. Despite the synthetic versatility of
these systems, controversy exists on the mechanistic
details of these oxygen atom transfer reactions.10,11 It has
been proposed that the sulfoxidation of sulfides by
hemoproteins is initiated by one electron transfer from
the sulfide to the iron(IV) oxoporphyrin radical cation15a
(Por•+-Fe(IV)dO), which is suggested to be the active
species in these reactions (Scheme 1)
† Department of Materials Science, Madurai Kamaraj University.
‡ School of Chemistry, Madurai Kamaraj University.
§ Department of Chemistry, Manonmaniam Sundaranar University.
(1) Sheddon, R. A.; Kochi, J . K. Metal Catalysed Oxidations of
Organic Compounds; Academic Press: New York, 1981.
(2) Oritz de Montellano, P. R., Ed. Cytochrome P 450. Structure,
Mechanism and Biochemistry; Plenum Press: New York, 1995.
(3) J orgensen, K. A. Chem. Rev. 1989, 89, 431. J orgensen, K. A.;
Schiott, B. Chem. Rev. 1990, 90, 1483.
(4) Holm, R. H. Chem. Rev. 1987, 87, 1401. Drago, R. S. Coord.
Chem. Rev. 1992, 117, 185.
(5) Meunier, B. Chem. Rev. 1992, 92, 1411. Ostovic D.; Bruice, T.
C. Acc. Chem. Res. 1992, 25, 314. Canali. L.; Sherrington, D. C. Chem.
Soc. Rev. 1999, 28, 85.
(6) Samsel, E. G.; Srinivasan, K.; Kochi, J . K. J . Am. Chem. Soc.
1985, 107, 7606. Srinivasan, K.; Kochi, J . K. Inorg. Chem. 1985, 24,
467.
(7) Srinivasan, K.; Michund P.; Kochi, J . K. J . Am. Chem. Soc. 1986,
108, 2309. Leung W. H.; Che, C. M. Inorg. Chem. 1989, 28, 4619.
(8) Rihter, B.; Srihari, S.; Hunter S.; Masnovi, J . J . Am. Chem. Soc.
1993, 115, 3918. Bousquet C.; Gilheany, D. G. Tetrahedron Lett. 1995,
36, 7739.
(9) J acobsen, E. N. In Comprehensive Organometallic Chemistry II;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Hegedus, L. S., Eds.;
Pergamon: New York, 1995; Vol. 12, Chapter II.1.
(10) Finney, N. S.; Pospisil, P. J .; Chang, S.; Palucki, M.; Konsler,
R. G.; Hansen K. B.; J acobsen, E. N. Angew. Chem., Int. Ed. Engl.
1997, 36, 1720 and references therein.
As illustrated in Scheme 1 (paths a and b), the
sulfoxide is then formed by reaction of the radical cation
with Por-Fe(IV)dO (oxygen rebound). Recently, Baciocchi
et al.15b found clear evidence for the formation of sulfide
radical cation in the oxidation of sulfides by horseradish
peroxidase. On the other hand, the mechanism appears
to be less defined for the cytochrome P-450 oxidation of
organic sulfides. Though an electron-transfer mechanism
is possible, a direct oxygen transfer from the iron oxo-
porphyrin radical cation to the substrate is also a viable
mechanism as shown in Scheme 1, path c. Thus, the
(13) (a) Chellamani, A.; Alhaji, N. M. I.; Rajagopal, S.; Sevvel R.;
Srinivasan, C. Tetrahedron 1995, 51, 12677. (b) Chellamani, A.; Alhaji
N. M. I.; Rajagopal, S. J . Chem. Soc., Perkin Trans. 2 1997, 299. (c)
Chellamani, A., Kulandaipandi, P.; Rajagopal, S. J . Org. Chem. 1999,
64, 2232.
(14) Chellamani, A.; Kulandaipandi, P.; Rajagopal S.; Srinivasan
C. Submitted to Tetrahedron.
(11) Linde, C.; Arnold, M.; Norrby P.; Akermark, B. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 1723 and references therein.
(12) J acobsen, E. N. In Catalytic Asymmetric Synthesis; Ojima, I.,
Ed.; VCH: Weinheim, 1993.
(15) (a) Baciocchi, E.; Lanzalunga, O.; Marconi, F. Tetrahedron Lett.
1994, 35, 9771. (b) Baciocchi, E.; Lanzalunga, O.; Malandrucco, S. J .
Am. chem. Soc. 1996, 118, 8973. Baciocchl, E.; Lanzalunga, O.; Pirozzi,
B. Tetrahedron 1997, 53, 12287.
10.1021/jo9916380 CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/05/2000

Selective Oxidation of Organic Sulfides
J . Org. Chem., Vol. 65, No. 11, 2000 3335
servations have been reported previously by us and also
by others.17,18
The oxidation reaction is overall second-order, first-
order with respect to the oxidant and the substrate. The
first-order in [OdCrV(salen)]+ is evident from the linear
log o.d. vs time plots and in the substrate from the linear
k1 vs [sulfide] plots shown in Figure 2 for different
sulfides. To understand the effect of acid on the kinetics
of this reaction, the rates at different concentrations of
trichloroacetic acid were measured, and the data are
collected in Table 1. The k1 value increases with increase
in [acid], confirming the catalytic role of acid in this
reaction. The addition of acid shifts the λmax of [OdCrV-
(salen)]+ from 560 to 570 nm, pointing out that carboxy-
late group may be coordinated to the metal thereby
enhancing the reactivity of the oxidant. We have observed
that addition of ligands such as imidazoles, pyridine,
heteroaromatic oxides such as pyridine N-oxide, tri-
phenylphosphine oxide, etc. to the [OdCrV(salen)]+ and
sulfide system enhances the rate, and in all these cases
the ligands are coordinated to the metal (the details will
be reported separately). Therefore, at this stage we
conclude that the catalytic activity of trichloroacetic acid
is due to the coordination of carboxylate ion to the metal.
Kinetic studies in different ratios of solvent systems
(acetonitrile-H2O; Table 1) revealed that the rate of
oxidation is facilitated with the increase in water content,
indicating charge separation in the transition state. Since
the influence of solvent on the rate is very large, this
increase may be due not only to a change in polarity but
also due to other effects. We have observed that the
addition of H2O shifts λmax of [OdCrV(salen)]+ from 560
to 600 nm. This leads us to believe that H2O is coordi-
nated to the metal (like triphenylphosphine oxide) and
the H2O coordinated oxidant enhances the rate to a larger
extent. H2O is likely to act as the axial ligand.
Su bstitu en t Effects. The data provided in Table 2
point out that the redox reaction between [OdCrV-
(salen)]+ and organic sulfides is highly sensitive to the
nature of the substrate and the structure of the ligand
of CrV complex. The reactivity is in the order dialkyl
sulfides > aryl methyl sulfides. The introduction of
substituents in the para position of the phenyl ring of
PhSMe alters the rate appreciably, i.e., the electron-
releasing substituents accelerate the rate and the electron-
withdrawing groups decelerate it. The kinetic data have
been analyzed with the Hammett equation, and the plot
of log k2 vs Hammett σ constants is linear in all four
complexes 1a -d , giving the F values -2.68 (r ) 0.973),
-1.14 (r ) 0.970), -2.15 (r ) 0.989), and -2.02 (r )
0.987),respectively, at 298 K (a representative plot is
shown in Figure 3). The correlation is not improved if
σ+/σ- values are used instead of Hammett’s σ constants.
Substantial change in the F value is observed with the
change of structure of the CrV complex. If we compare
the reactivity of four Cr complexes with the F values
(selectivity), a reverse trend is noticed between the
reactivity and selectivity; i.e., this redox system is in
accordance with the reactivity-selectivity principle (RSP).
We have also observed recently the operation of RSP in
F igu r e 1.
situation is not clear on the mechanism of oxidation of
sulfides with several oxometal ions.
Furthermore, in the oxygentation reactions of some
organic substrates catalyzed by metal-salen complexes,
the formation of oxometal species is rate determining
rather than the oxygen atom transfer step.10 Thus, it
becomes difficult to understand the mechanism of oxygen
transfer from oxometal ion to the organic substrate.
Recently, we have proposed single electron transfer
from organic sulfide to the oxometal complex as the rate-
controlling step in the oxygen atom transfer reaction from
several cationic oxo(salen)manganese(V) and oxo(salen)-
ruthenium(V) complexes to sulfides.13,14 A major disad-
vantage of manganese porphyrin and salen complexes is
that manganese complexes catalyzed oxidation reactions
display considerable radical character leading to radical
byproducts and losses in selectivities due to the formation
of radicaloid intermediates. Kochi and co-workers have
concluded that the epoxidation of alkenes with oxoman-
ganese(V) complexes favored a radical pathway, whereas
oxochromium(V) favored the SN2 mechanism.6,7 To learn
the mechanism and synthetic utility, the oxidation of
organic sulfides to sulfoxides with four oxo(salen)chro-
mium(V) [OdCrV(salen)]+ complexes (Figure 1) has been
studied and the results are discussed in this paper.
Resu lts a n d Discu ssion
In the present study, [OdCrV(salen)]+ complexes 1a -d
have been prepared from CrIII-salen complexes and
iodosylbenzene and then used for the oxidation of organic
sulfides (eq 1)
[CrIII(salen)]+ + PhIO f [OdCrV(salen)]+ + PhI (1)
The formation of oxometal complexes is confirmed by
their absorption spectra, which are similar to the previ-
ous reports.6,8,16 The kinetics of oxidation of several aryl
methyl and dialkyl sulfides with [OdCrV(salen)]+ com-
plexes have been studied in acetonitrile by spectropho-
tometric techniques (details are given in the Experimen-
tal Section). The rate of PhIO oxidation of these organic
sulfides has been checked and found to be very slow
under the present experimental conditions. Similar ob-
(17) Kannan, P.; Sevvel, R.; Rajagopal, S.; Pitchumani K.; Srini-
vasan, C. Tetrahedron 1997, 53, 7635.
(18) Ando, W.; Tajima R.; Takata, T. Tetrahedron Lett. 1982, 23,
1685. Takata, T.; Tajima R.; Ando, W. Phosphorous, Sulfur, Silicon
1983, 16, 67. Naruta, Y.; Tani, F.; Muruyama, K. Tetrahedron:
Assymetry 1991, 2, 553.
(16) Pfeiffer, P.; Breith, E.; Lubbe, E.; Tsumaki, T. Leibigs. Ann.
1933, 50384. Coggon, P.; McPhail, A. T.; Mabbs, F. E.; Richards, A.;
Thornley, A. S. J . Chem. Soc. A 1970, 3296.

3336 J . Org. Chem., Vol. 65, No. 11, 2000
Sevvel et al.
F igu r e 2. Plots of k1 versus [sulfide] for the oxidation of 1a with p-XC6H4SMe and R2S: [, X ) H; 9, X ) OMe; 2, X ) Cl; ×,
X ) COCH3; *, DES; b, DTBS.
Ta ble 1. Effect of Ch a n gin g th e [Cl3CCO2H] a n d th e
Solven t Com p osition on th e Ra te of Oxid a tion of Meth yl
P h en yl Su lfid e by 1a a t 298 Ka
Ta ble 2. Secon d -Or d er Ra te Con sta n t, k2, Va lu es for
Oxo(sa len )Ch r om iu m (V) Com p lexes 1a -d Oxid a tion of
Ar SR a n d R2S in CH3CN a t 298 Ka ,b
[Cl3CCO2H], M
105k1,b s-1
AN/H2O
100
99.2-0.8
90-10
75-25
60-40
50-50
105k1, s-1
k2 × 103 M-1 s-1
1a
1.31 ( 0.03 29.4 ( 0.51 9.30 ( 0.20 1.13 ( 0.21
25.9 ( 0.03 111 ( 4.0 62 ( 2.9 6.72 ( 0.62
11.4 ( 0.23 56.1 ( 0.10 19.9 ( 0.50 2.31 ( 0.25
0.93 ( 0.01 20.5 ( 0.50 3.25 ( 0.11 0.56 ( 0.23
0.93 ( 0.01 20.6 ( 0.60 3.33 ( 0.50 0.58 ( 0.25
0.24 ( 0.02 13.0 ( 0.40 1.26 ( 0.04 0.17 ( 0.03
0.16 ( 0.01 12.0 ( 0.10 1.06 ( 0.06 0.13 ( 0.03
XC6H4SCH3,
2.62
8.89
16.3
22.4
30.7
37.7
70.5
2.62
6.30
13.4
26.2
29.5
56.5
X )
1b
1c
1d
0.01
0.02
0.05
0.10
0.20
0.40
H
p-OMe
p-Me
p-Cl
p-Br
p-CO2H
p-COCH3
a
General conditions: [1a ] ) 0.001 M, [MPS] ) 0.02 M. b Solvent
CH3CN (AN).
k2 × 102M-1s-1
R2S, R )
1a
1b
1c
the oxo(salen)managanese(V) oxidation of organic sul-
fides and sulfoxides.13
Et
4.57 ( 0.11
3.55 ( 0.06
1.35 ( 0.03
1.63 ( 0.03
0.87 ( 0.02
0.22 ( 0.05
115 ( 2.12
54.0 ( 1.21
31.0 ( 0.51
32.3 ( 0.14
10.4 ( 2.21
4.2 ( 1.01
5.03 ( 0.12
4.03 ( 0.32
2.53 ( 0.31
2.73 ( 0.62
1.33 ( 0.11
0.76 ( 0.11
n-Pr
i-Pr
n-Bu
s-Bu
t-Bu
A useful clue on the mechanism of [OdCrV(salen)]+
oxidation of organic sulfides can be derived by comparing
the F values observed in the present study with the F
values obtained on other metal ions oxidation of similar
substrates. In the uncatalyzed CrVI (HCrO4-) oxidation
of aryl methyl sulfides, the F value is -1.80 in 50% (v/v)
aqueous acetonitrile, and in the picolinic acid catalyzed
CrVI oxidation, the F value is -1.07.19,20 These results led
the authors to postulate a single electron transfer from
sulfide to CrVI in the rate-controlling step of uncatalyzed
oxidation. On the other hand, two alternate mechanisms
viz., one-electron transfer or nucleophilic attack of sulfide
General conditions: [OdCrV(salen)]+ClO4 ) 0.001 M; [sub-
a
-
b
strate] ) 0.02 M. The error quoted in k2 is the 95% confidence
limit of student’s t test.
on the chromium of CrVI-picolinic acid complex in the
rate-determining step have been proposed in the cata-
lyzed oxidation. Ganesan et al.21 observed a F value of
-1.89 in the carboxylato-bound oxochromium(V) oxida-
tion of aryl methyl sulfides. The F values of -0.20 and
-3.21, respectively, have been observed in the cyto-
(19) Srinivasan, C.; Chellamani A.; Rajagopal, S. J . Org. Chem.
1985, 50, 1201. Srinivasan, C.; Rajagopal S.; Chellamani, A. J . Chem.
Soc., Perkin Trans. 2 1990, 1839.
(20) Manohar, T. C. Ph.D. Thesis, Madurai Kamaraj University,
1991.
(21) Ganesan, T. K.; Rajagopal, S.; Bosco Bharathy J . R.; Md. Sheriff,
A. I. J . Org. Chem. 1998, 63, 21.

Selective Oxidation of Organic Sulfides
J . Org. Chem., Vol. 65, No. 11, 2000 3337
F igu r e 3. Hammett plot for the oxidation of aryl methyl sulfides by 1a at 298 K.
chrome P450 and Fe(NN)33+ (NN ) 2,2′-bipyridyl or 1,10-
phenanthroline) oxidation of ArSMe.22,23 Single electron
transfer in the rate-controlling step has been postulated
in the above oxidation reactions. For the oxidation of
thioanisoles by oxo(phosphine)ruthenium(IV) complexes,
Takeuchi and co-workers24 have suggested a single
electron-transfer mechanism based on the excellent cor-
relation between log kx/kH and σ- values (F ) -1.5).
However, nucleophilic attack of sulfide on the metal
center has been postulated in the permanganate ion (F
) -1.52),25 lead tetraacetate (F ) -2.1),26 pyridinium
halochromates (F ) -2.1),27 and ruthenate (F ) -0.66)28
oxidation of ArSMe. These examples show that the
magnitude (high or low) of F value cannot be taken as
evidence in favor of SN2 or SET mechanism in a particu-
lar reaction.
Oxid a tion of Dia lk yl Su lfid es. To understand the
reactivity of alkyl sulfides toward CrV(salen) complexes
and the role of steric effect in this reaction, the kinetics
of oxidation of several dialkyl sulfides with the CrV
complexes 1a -c have been studied, and the relevant data
are collected in Table 2. The observed kinetic data point
out that alkyl sulfides are more reactive than aryl
sulfides and the reaction is sensitive to steric effect. To
understand the contribution of polar and steric effects
on this oxidation, we have analyzed the data in terms of
Taft’s equations (eqs 2-4)
log k ) log k0 + F* σ*
log k ) log k0 + δES
(2)
(3)
log k ) log k0 + F* σ* + δES
(4)
where σ* and ES are the polar and steric substituent
constants and F* and δ are the corresponding susceptibil-
ity constants. Though the correlation of log k with σ* is
not satisfactory, the kinetic data are better correlated
with ES values (eq 3) and the δ values are in the range
of 0.6-1.3 for three oxochromium(V) complexes. The use
of multiparmeter (eq 4) does not improve the correlation.
These arguments point out the predominant role of steric
effect over polar effect in this reaction.
Mech a n ism of [OdCr V(sa len )]+ Oxid a tion of Or -
ga n ic Su lfid es. From the recent literature, it is known
that oxygen atom transfer to organic sulfides from
oxometal complexes proceeds generally by two different
mechanisms. Single electron transfer from the substrate
to the oxidant is the rate-determining process for the
oxidation of ArSR with peroxy benzoate,29 hydroperoxi-
dase,30 CeIV 31 and oxoruthenium(IV)24 complexes. On the
,
other hand, the oxidation of sulfides by peroxoanions,32
(22) Watanabe, Y.; Iyanagi T.; Oae, S. Tetrahedron Lett. 1980, 21,
3685.
(23) Balakumar, S.; Thanasekaran, P.; Rajagopal S.; Ramaraj, R.
Tetrahedron 1995, 51, 4801.
(24) Acquaze, J . H.; Miller J . G.; Takeuchi, K. J . Inorg. Chem. 1993,
32, 160.
(25) Banerji, K. K. Tetrahedron 1988, 44, 2969. Lee D. G.; Chen, T.
J . Org. Chem. 1991, 53, 5346.
(26) Banerji, K. K. J . Chem. Soc., Perkin Trans. 2 1991, 759.
(27) Banerji, K. K. J . Chem. Soc., Perkin Trans. 2 1988, 2065.
Rajasekaran, K.; Baskaran T.; Gnanasekaran, C. J . Chem. Soc., Perkin
Trans. 2 1984, 1183.
(29) Pryor W. A.; Hendrichson,, W. H., J r. J . Am. Chem. Soc. 1983,
105, 7114.
(30) Miller, A. E.; Bischoff, J . J .; Bizub, C.; Luminoso, P.; Smiley,
S. J . Am. Chem. Soc. 1986, 108, 7773.
(31) Baciocchi, E.; Intiri, D.; Piermattei, A.; Roh C.; Ruzziconi, R.
Gazz. Chim. Ital. 1989, 119, 649.
(32) Arumugam, N.; Srinivasan C.; Kuthalingam, P. Indian J . Chem.
1978, 16A, 478. Srinivasan, C.; Kuthalingam P.; Arumugam, N. Can.
J . Chem. 1978, 56, 3043. Srinivasan, C.; Kuthalingam P.; Arumugam,
N. J . Chem. Soc., Perkin Trans. 2 1980, 170.
(28) Lee, D. G.; Gai, H. Can. J . Chem. 1995, 73, 49.

3338 J . Org. Chem., Vol. 65, No. 11, 2000
Sevvel et al.
Ta ble 3. Secon d -Or d er Ra te Con sta n t, k2, Va lu es a t Th r ee Differ en t Tem p er a tu r e, a n d Va lu es of En th a lp y(∆Hq) a n d
-
En tr op y(∆Sq) of Activa tion for [OdCr V(sa len )]+P F 6 Oxid a tion of p-XC6H4SMe in CH3CNa
k2 × 104 M-1 s-1
308 K
X
288 K
298 K
(∆Hq), kJ mol-1
-(∆Sq), J K-1 mol-1
p-OMe
p-Me
H
p-Cl
p-Br
59.7 ( 2.0
26.1 ( 1.6
11.7 ( 0.5
7.10 ( 0.3
6.80 ( 0.4
1.02 ( 0.17
1.42 ( 0.1
112 ( 4.0
41.6 ( 2.5
20.1 ( 1.0
15.3 ( 1.0
11.6 ( 1.0
2.13 ( 0.1
3.20 ( 0.1
230 ( 7.60
90.1 ( 4.4
45.2 ( 2.0
32.2 ( 1.8
23.1 ( 1.7
5.00 ( 0.3
7.90 ( 0.4
48.1 ( 2.5
44.4 ( 4.2
48.6 ( 3.3
54.8 ( 4.2
44.4 ( 5.4
57.8 ( 6.7
62.4 ( 3.8
120.6 ( 9.2
141.5 ( 15.1
132.7 ( 12.1
114.7 ( 14.2
152.4 ( 19.3
121.8 ( 21.2
102.1 ( 9.2
p-COCH3
p-CO2H
a
-
General conditions: [OdCrV(salen)]+PF6 ) 0.001 M; [sulfide] ) 0.02 M.
molybdenum peroxypolyoxoanions,33 sulfamyloxaridines,34
phenyliodosodiacetate,35 pyridinium halochromates,26,36
and permanganate25 follow SN2 mechanism. However, a
dual mechanism has been proposed in some oxidation
reactions.19
PF6 with ArSMe are also in favor of the electrophilic
attack of oxidant on the substrate.38
-
In the present study, the nucleophilic attack of sulfide
on the Cr center may be excluded, as it is difficult to
explain the formation of sulfoxide from such a formula-
tion. The coordination of the substrate to the metal is
not significant, which is evident from the following
experimental observations: (i) when the substrate is
added to [OdCrV(salen)]+ there is little change in the
absorption spectrum of the latter. On the other hand,
addition of trichloroacetic acid, H2O, and ligands such
as imidazole and pyridine N-oxide to [OdCrV(salen)]+
produces a substantial shift in the λmax. These spectral
and kinetic observations point out that even if the
substrate binds to the oxidant, the binding is weak and
equilibrium constant for such binding is small. (ii) The
reaction is clearly first order in the substrate, and no
saturation kinetics with respect to substrate is observed.
If the reaction proceeds through electron transfer in the
rate-controlling step, CrIV would have been formed as the
intermediate. During the course of the reaction, we have
not observed any spectral evidence for CrIV. If CrIV is
formed during the reaction, addition of MnII will influence
the rate.37 When the reaction between [OdCrV(salen)]+
and PhSMe was carried out at different concentration of
MnII, no substantial change in the k1 value was observed.
This observation also rules out the formation of CrIV in
the rate-controlling step. The reactivity of similar electron
donors is in accordance with their oxidation potentials.
Oxidation potentials of diethyl sulfide and methyl phenyl
sulfide are 1.65 and 1.53 V, respectively. The higher
reactivity of diakyl sulfides compared to aryl methyl
sulfides also (Table 2) rules out the operation electron-
transfer mechanism. Baciocchi et al.31 postulated electron-
transfer mechanism for CeIV oxidation of organic sulfides
as they observed lower reactivity for dialkyl sulfides. The
kinetic and spectral observations led us to conclude that
the reaction proceeds through the electrophilic attack of
oxygen at the sulfur center of the organic sulfide (eq 5).
Furthermore, the values of thermodynamic parameters,
∆Hq and ∆Sq (Table 3) for the reaction of [OdCrV(salen)]+-
As the reaction is being carried out in pure acetonitrile
and the rate of reaction is not affected by the presence
of oxygen(similar results are observed under inert atmo-
sphere), the origin of oxygen for the product, ArSOMe,
is the oxygen in the OdCrV complex.
Th e Rea ctivity-Selectivity P r in cip le. Recently, we
have observed the operation of the reactivity-selectivity
principle (RSP) in the oxidation of organic sulfides and
sulfoxides by oxo(salen)manganese(V) complexes.13 As the
four oxo(salen)chromium(V) complexes chosen for the
present study show appreciable change in the reactivity
and the F values (selectivity), it is natural to check the
validity of RSP in this redox reaction also. The second-
order rate constants for the reaction of several para-
substituted phenyl methyl sulfides with each of the
oxo(salen)chromium(V) complexes 1a -d are collected in
Table 2.
Similar to earlier approaches, we have analyzed the
kinetic data using a method advocated by Exner39 to
verify the operation of RSP in the present study. Equa-
tions 6 and 7 were used to analyze the data given in Table
2
log kF ) a + b log kS + ꢀi
(6)
(7)
i
i
∆ ) ( log k
-
log k )/N
∑
∑
i
Fi
Si
i
In eqs 6 and 7, kFi and kSi are the second-order rate
constants for the reactions of fast and slow reagents (oxo
complexes), respectively, with each sulfide, ꢀi is the error
of the log kFi versus log kSi correlation, and ∆ is the mean
difference. The values of b and ∆ were calculated for all
of the five possible combinations of one fast and one slow
reagent (among the four oxo complexes) with a series of
similar substrates. The results summarized in Table 4
show a valid RSP in all cases as the value of b is less
than unity and ∆ is not too small.
(33) Arocoria, A.; Ballistreri, F. P.; Spina, E.; Tomaselli G. A.;
Toscano, R. M. Gazz. Chim. Ital. 1990, 120, 309.
(34) Davis, F. A.; McCanley, J . P.; Chattopachayay, S.; Herakai, M.;
Towson, J . C.; Watson W. H.; Taranaiepour, R. J . Am. Chem. Soc. 1987,
109, 3370.
(35) Srinivasan, C.; Chellamani A.; Kuthalingam, P. J . Org. Chem.
1982, 47, 428.
(36) Bohra, A.; Sharma P. K.; Banerji, K. K. J . Org. Chem. 1997,
62, 3562.
(37) Perez-Benito, J . F.; Arias, C. Can. J . Chem. 1992, 71, 649.
(38) Campestrini, S.; Conte, V.; Di Furia, F.; Modena, G. J . Org.
Chem. 1988, 53, 5721.
(39) Exner, O. J . Chem. Soc., Perkin Trans. 2 1993, 973.

Selective Oxidation of Organic Sulfides
J . Org. Chem., Vol. 65, No. 11, 2000 3339
Ta ble 4. Resu lts of Cor r ela tion betw een log kF i a n d log
kSi Accor d in g to Eq 6 for Ar yl Meth yl Su lfid es
Ta ble 5. Selective Oxid a tion of p-X-C6H4-S-CH3 a n d R2S
in CH3CN a t 298 K by Ia -C
oxo(salen)chromium(V) complexes (F and S)
1a
1b
1c
results 1a and 1b 1d and b 1a and 1c 1a and 1d 1b and 1c
reaction % of reaction % of reaction % of
time
(min)
sulf-
oxide
time
(min)
sulf-
oxide
time
(min)
sulf-
oxide
r
b
∆
0.973
0.465
1.11
0.973
0.625
1.55
0.973
0.843
0.691
0.981
0.730
0.285
0.993
0.548
0.682
X
Ha
150
240
60
52
96
95
93
60
61
40
30
60
150
25
99
99
99
90
55
57
42
27
180
95
Hb
OMe
Me
Cl
Br
COCH3
CO2H
35
35
180
180
180
180
99
87
53
50
42
30
75
25
240
240
240
240
150
150
150
150
R2S
Et
Pr
45
45
45
45
45
45
98
83
68
60
60
35
20
20
20
20
20
20
99
87
62
65
67
49
35
35
35
35
35
35
99
86
63
65
69
47
Pri
Bu
Bus
But
a
General conditions: [OdCrV(salen)]+ ClO4 0.002 M and [p-X-
b
C6H4-S-Me]/[R2S] 0.002 M. Longer period and other conditions
are same as a.
a polar mechanism favoring more selectivity. It is inter-
esting to compare the present results with those observed
with MnV-salen complexes toward organic sulfides. With
the parent oxo(salen)metal complex, the F value is -1.86
in MnV and -2.68 with CrV. This comparison seems to
point out that with CrV-salen oxidation of organic
sulfides the transition state is more productlike, leading
to more selective formation of sulfoxide. This leads to the
anticipation of superior enantioselectivity if chiral chro-
mium(V)-salen is chosen as the oxidant because a
relatively late transition state suggests a strong interac-
tion of the incoming sulfide with the chiral ligand around
the metal. To check the success of selectivity achieved
with oxo(salen)chromium(V) complexes, we have studied
the synthetic utility of this redox reaction and the details
are presented in Table 5. The data given in Table 5
highlight the importance of CrV-salen complex for the
selective oxidation of sulfide to sulfoxide. The percentage
conversion is highly sensitive to the nature of the
substituent in the phenyl ring of PhSMe and the struc-
ture of Cr(V) complex. The detailed study on the synthetic
aspects of this reaction will be reported separately.
Tem p er a tu r e Dep en d en ce. The kinetics of OdCrV
oxidation of aryl methyl sulfides have been followed in
three different temperatures and the values of enthalpy-
(∆Hq) and entropy(∆Sq) of activation evaluated from the
Eyring equation are collected in Table 3. The ∆Hq (44-
62 kJ mol-1) and ∆Sq (-152 to -102 J K-1mol-1) values
are in favor of two electron transfer rather than single
electron transfer in the rate-controlling step of the
reaction.41 The Hammett reaction constant, F, values at
288, 298, and 308 K are -2.10, -1.96, and -1.91,
respectively, pointing to a decrease in selectivity with an
increase in temperature, i.e., with an increase in the
reactivity. Thus, the redox reaction obeys the RSP.39
Com p a r ison w ith Ca r boxyla to-Bou n d Ch r om i-
u m (V) Ion Oxid a tion . Carboxylato-bound oxochromi-
um(V) ion oxidizes the organic sulfides21 to the corre-
sponding sulfoxides but less efficiently, and the reaction
is of total second-order, first-order in each reactant. On
F igu r e 4. Plot of log kFi versus with log kSi for the reactions
of aryl methyl sulfides with: [, 1a + 1b; 9, 1d + 1b; 2, 1a
and 1c; ×, 1a and 1d ; b, 1b and 1c.
In a reaction series involving more than one reagent
and the same set of substrates, the existence of a magic
point in the log kFi versus log kSi plots is an indication
for a strong RSP. The magic point represents some
limiting value of reactivity in which, for a particular
substrate, the reaction rate is independent of the reagent
and vice versa. In this aspect, the present system reveals
interesting results and the correlations produce one
magic point (Figure 4). The magic point is situated on
the side of high reactivity, as expected for a strong RSP.
It is interesting to point out that two magic points were
obtained in the oxo(salen)manganese(V) oxidation of
organic sulfides and sulfoxides. The observation that the
present reaction series obeys RSP may be taken as
evidence for the operation of similar mechanism in all
reactions.
Selective Oxid a tion of Or ga n ic Su lfid e to Su lfox-
id e. The selective oxidation of organic sulfide to sulfoxide
is of importance particularly in the preparation of chiral
sulfoxides, which are used as the starting materials in
the synthesis of a variety of chiral organic molecules.40
Though optically active MnV-salen complexes have at-
tracted the attention of chemists to use them in the
epoxidation of olefins and oxidation of alkyl aryl sulfides,
oxidations using other metallosalen complexes as cata-
lysts have not been fully examined. MnV-salen com-
plexes prefer radical pathway thus less selective. On the
other hand, CrV-salen complexes oxidation proceeds by
(40) Carreno, M. C. Chem. Rev. 1995, 95, 1717.
(41) Marcus, R. A. J . Phys. Chem. 1997, A: 101, 4072.

3340 J . Org. Chem., Vol. 65, No. 11, 2000
Sevvel et al.
iodosylbenzene. Ether is slowly added to the dark filtrate in
order to precipitate crystals of oxochromium(V) salts. The aryl
methyl sulfides were obtained from known synthetic methods,
and dialkyl sulfides (Aldrich) were used without further
purification. All other reagents were of AnalaR grade or used
after purification. The kinetic study of the reaction was
performed after confirming the purity of reactants and solvents
used in this system.
Sch em e 2
Kin etic Mea su r em en ts. A J ASCO UV-vis spectropho-
tometer (model 7800) was employed to record the absorption
spectra of CrIII and OdCrV complexes used in the present study
and to follow the kinetics of oxygen atom transfer reactions of
OdCrV complexes with organic sulfides. The kinetic studies
were carried out in CH3CN under pseudo-first-order conditions
with a sulfide, OdCrV complex ratio of at least 10:1. The
progress of the reaction was monitored by following the decay
of absorbance of OdCrV complex at definite time intervals at
the following wavelengths (λabmax) [OdCrV(salen)]+PF6-, 571
nm, [OdCrV(salen)]+ClO4-, 560 nm, [OdCrV-(salprn)]+ClO4
,
-
the basis of the UV-vis absorption and EPR spectral
studies and substituent and solvent effects, a mechanism
involving an electron transfer from sulfide to CrV in the
rate-determining step followed by fast attack of CrIV/CrV
on the sulfide cation radical to form products has been
postulated (Scheme 2).
570 nm, [OdCrV(salophen)]+ClO4-, 600 nm, [OdCrV(7,7-Me2-
salen)]+ClO4-, 610 nm. The reaction was followed up to 4 half-
life periods.
The pseudo-first-order rate constant (k1) for each kinetic run
was evaluated from the slope of linear plot of log o.d. vs time
by the method of least squares. The linearity of each fit is
confirmed in terms of the values of correlation coefficient (r)
and standard deviation(s). The second-order rate constant (k2)
is evaluated from the relation k2 ) k1/[sulfide]. The precision
of k values in all the kinetic runs is given in terms of 95%
confidence limit (CL) of the student’s t test. The thermody-
namic parameters, the enthalpy(∆Hq) and entropy (∆Sq) of
activation, were evaluated by the least-squares method from
the linear plot of log k2/T vs 1/T. The precision of ∆Hq and ∆Sq
values was calculated using the method of Petersen et al.45
Stoich iom etr y a n d P r od u ct An a lysis. The stoichiometry
of the reaction between OdCrV(salen) complex and sulfides
was studied by taking different ratios of concentration of
oxidant and substrate and was found to be 1:1 and represented
by eq 8
Thus, the inference is that the nature of the ligand
alters the mechanism of the reaction. It is interesting to
note the difference in the reactivity of two different Cr-
(V) complexes toward the same substrate, organic sulfide.
The carboxylato-bound complex carries a negative charge,
whereas the salen complex carries a positive charge
though both complexes have essentially the similar
square-pyramidal configuration about the CrV center.
Note that the relevant chromium-oxygen bond distance
in both complexes is similar (1.55 Å).42 Though we expect
similar rate constants for the oxidation of same substrate
with these two structurally similar oxochromium(V)
complexes, substantially higher rate constant is observed
with CrV-salen complex compared to carboxylato-bound
chromium(V). To account for this large difference in
reactivity different mechanisms are proposed, the elec-
trophilic attack of oxygen at the sulfur center for the
former and electron transfer from the sulfide to Cr(V) in
the latter. The higher reactivity of cationic oxo(salen) CrV
complex compared to that of anionic carboxylato bound
CrV complex seems to support the suggested electrophilic
oxygen transfer mechanism for the titled reaction. Simi-
lar behavior has been observed in the MoVI oxidation of
thioethers.43
OdCrV(salen) + RSR′ f CrIII(salen) + RSOR′
(8)
In a typical experiment, 0.2 mm of substrate (ArSMe) was
added to a 0.2 mm solution of oxochromium(V) complex in 5
mL of solvent (CH3CN). The solution was stirred at 298 K for
1-3 h depending upon the nature of sulfide and complex (vide
infra). After the removal of the solvent under reduced pressure,
the organic product was extracted with ether and dried and
the solvent removed. Then the resulting residue was analyzed
by GC and IR. The IR spectrum of the product was found to
be identical with that of sulfoxide (ArSOMe) having SdO
stretching frequency in the characteristic region 1070-1030
cm-1. The GC analysis of the product also indicated the
formation of sulfoxide as the only product under the present
experimental conditions. The absorption spectrum of the
Exp er im en ta l Section
inorganic product of the reaction was similar to that of CrIII
salen complex confirming the formation of sulfoxide and CrIII
salen complex as the products.
-
-
The ligands salen, salprn, salophen, and 7,7′-Me2salen and
CrIII complexes of these ligands were synthesized using
established procedures.6,8,16 The purity of ligands and CrIII
complexes was checked by spectral methods. Iodosylbenzene
was prepared by alkaline hydrolysis of iodosobenzene diacetate
according to the reported method.44 The oxo(salen)chromium-
(V) complexes 1a -d were obtained by the general procedure
described below: A slight excess of iodosylbenzene is added
to CrIII-salen complex dissolved in ∼25 mL of CH3CN. The
reaction mixture turned from orange to dark green. This slurry
is stirred for 20 min and then filtered to remove the unreacted
Ack n ow led gm en t. The authors thank Drs. T. Ra-
masami and Kanthimathi, CLRI, Chennai, for the
generous gift of [CrIII(salpren)]+. S.R. thanks CSIR, New
Delhi, and R.S., the UGC, New Delhi, for providing
financial assistance. R.S. also thanks Dr. K. Subrah-
maniam, Principal and The Management, Vivekananda
College, Tiruvedakam West, for their encouragement
and permission granted to carry out the work.
J O9916380
(42) Farrel, R. P.; Lay, P. A. Comments Inorg. Chem. 1992, 13, 133.
(43) Ballistreri, F. P.; Bazzo, A.; Tomaselli, G. A.; Toscano, R. M. J .
Org. Chem. 1992, 57, 7074.
(44) Saltzman, H.; Sharefkin, J . G. Organic Syntheses; Wiley: New
York, 1973; Collect. Vol. V, p 658.
(45) Petersen, R. C.; Markgraf, J . H.; Ross, S. D. J . Am. Chem. Soc.
1961, 83, 3819. Wilberg, K. B. Physical organic chemistry; Wiley: New
York, 1964; p 376.