Oxidation of aryl methyl sulfoxides by oxo(salen)manganese(V) complexes and the reactivity-selectivity principle

Article abstract of DOI:10.1021/jo9815756

The oxidation of various para-substituted phenyl methyl sulfoxides with several substituted oxo(salen)manganese(V) complexes follows an overall second-order kinetics that is first-order in sulfoxide and in oxo(salen)manganese(V) complex. Electron-releasing substituents in sulfoxides and electron-withdrawing substituents in oxo(salen)manganese(V) complexes enhance the rate of oxidation. The less nucleophilic sulfoxides are more sensitive to substituent effect (p = -2.44) compared to the corresponding sulfides (p = -1.86). These results are interpreted with a common mechanism involving the electrophilic attack of the oxidant on the sulfur center of the substrate. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reactions of various sulfoxides with a given oxo(salen)manganese(V) complex and vice versa. Mathematical treatment of the results leads to the conclusion that this redox system falls under strong reactivity-selectivity principle.

Full text of DOI:10.1021/jo9815756

2
232
J . Org. Chem. 1999, 64, 2232-2239
Oxid a tion of Ar yl Meth yl Su lfoxid es by Oxo(sa len )m a n ga n ese(V)
Com p lexes a n d th e Rea ctivity-Selectivity P r in cip le†
,
‡
‡
,§
Arunachalam Chellamani,* Periasamy Kulanthaipandi, and Seenivasan Rajagopal*
Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627 012, and
School of Chemistry, Madurai Kamaraj University, Madurai 625 021, India
Received August 4, 1998 (Revised Manuscript Received December 1, 1998)
The oxidation of various para-substituted phenyl methyl sulfoxides with several substituted oxo-
salen)manganese(V) complexes follows an overall second-order kinetics that is first-order in
(
sulfoxide and in oxo(salen)manganese(V) complex. Electron-releasing substituents in sulfoxides
and electron-withdrawing substituents in oxo(salen)manganese(V) complexes enhance the rate of
oxidation. The less nucleophilic sulfoxides are more sensitive to substituent effect (F ) -2.44)
compared to the corresponding sulfides (F ) -1.86). These results are interpreted with a common
mechanism involving the electrophilic attack of the oxidant on the sulfur center of the substrate.
Correlation analyses show the presence of an inverse relationship between reactivity and selectivity
in the reactions of various sulfoxides with a given oxo(salen)manganese(V) complex and vice versa.
Mathematical treatment of the results leads to the conclusion that this redox system falls under
strong reactivity-selectivity principle.
In tr od u ction
Organic sulfides and sulfoxides are interesting sub-
F values (selectivity) in conjunction with the low reactiv-
ity of sulfoxides compared to sulfides leads us to infer
that the selectivity decreases with the decreasing reactiv-
strates that have many similarities but some differences
in their reactions with oxidants. Whereas organic sulfides
behave always as strong nucleophiles, sulfoxides act as
biphilic substrates if strong and rather unselective
oxidants are used. It is well known that organic sulfox-
ides are less powerful nucleophiles compared to the
corresponding organic sulfides and thus less reactive
towards electrophilic reagents. With many oxidants, it
6-10
ity of sulfoxides compared to sulfides.
Recently, we have reported the mechanism of the
oxidation of organic sulfides by oxo(salen)manganese(V)
complexes and also the successful application of the
11
reactivity-selectivity principle (RSP) to this oxidation.
To understand whether the organic sulfoxides follow a
mechanism similar to that of sulfides towards the oxidant
oxo(salen)manganese(V) and the relevance of RSP in this
redox system, we have studied spectrophotometrically the
kinetics of oxidation of several aryl methyl sulfoxides
with the six oxo(salen)manganese(V) complexes 2a -f
s
has been established that the ratio of reactivity, k /kso,
is very large and the nucleophilic reactivity of thioethers
is more affected by structural effects than that of
sulfoxides.1 Comparison of reaction constant (F) values
-5
III
+
-
generated in situ from the corresponding[Mn (salen)] PF
6
points out that the values for thioether oxidation are
complexes and PhIO as represented in eq 1; the results
of these studies are analyzed in this article.
consistently larger than those of sulfoxides.6
-10
The low
†
Dedicated to Professor C. Srinivasan on his sixtieth year.
Manonmaniam Sundaranar University.
Madurai Kamaraj University.
‡
§
(1) (a) Di Furia, F.; Modena, G. Pure Appl. Chem. 1982, 54, 1853
and references cited therein. (b) Bethell, D.; Dunn, S. F. C.; Khodaei,
M. M.; Newall, A. R. J . Chem. Soc., Perkin Trans. 2 1989, 1829. (c)
Klumpp, G. W. Reactivity in Organic Chemistry; Wiley: New York,
1
982; pp 213-218.
(
2) (a) Andersen, K. K. In The Chemistry of Sulfones and Sulfoxides;
Patai, S., Rappoport, Z., Stirling, C. J . M., Eds.; J ohn Wiley and Sons:
New York, 1988; Chapter 3, pp 56-94. (b) Carreno, M. C. Chem. Rev.
1
995, 95, 1717.
(3) Conte, V.; Di Furia, F. In Catalytic Oxidations with Hydrogen
Peroxide as Oxidant; Strukul, G., Ed.; Kluwer Academic Publisher:
Dordrecht, 1992; pp 223-252.
(4) Bonchio, M.; Campestrini, S.; Conte, V.; Di Furia, F.; Moro, S.
Tetrahedron 1995, 51, 12363. (b) Ballistreri, F. P.; Tomaselli, G. A.;
Toscano, R. M.; Bonchio, M.; Conte, V.; Di Furia, F. Tetrahedron Lett.
1
994, 35, 8041.
5) Bonchio, M.; Conte, V.; De Conciliis, M. A.; Di Furia, F.;
Ballisteri, F. P.; Tomaselli, G. A.; Toscano, R. M. J . Org. Chem. 1995,
0, 4475 and references cited therein.
(
6
Although we have assumed the active Mn species to
be the reactive (salen)Mn(V) oxo intermediate ((salen)-
(6) Modena, G.; Maioli, I. Gazz. Chim. Ital. 1957, 87, 1306.
(7) Cerniani, A.; Modena, G. Gazz. Chim. Ital. 1959, 89, 843.
(8) (a) Ballistreri, F. P.; Tomaselli, G. A.; Toscano, R. M.; Conte, V.;
Di Furia, F. J . Am. Chem. Soc. 1991, 113, 6209. (b) Adam, W.; Haas,
W.; Lohray, B. B. J . Am. Chem. Soc. 1991, 113, 6202.
(10) Campestrini, S.; Conte, V.; Di Furia, F.; Modena, G.; Bortolini,
O. J . Org. Chem. 1988, 53, 5721.
(11) (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.
(9) (a) Bortolini, O.; Di Furia, F.; Modena, G.; Scardellato, C.;
Scrimin, P. J . Mol. Catal. 1981, 11, 107. (b) Bortolini, O.; Di Furia, F.;
Modena, G. J . Mol. Catal. 1982, 14, 53.
1
0.1021/jo9815756 CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/13/1999

Oxidation of Aryl Methyl Sulfoxides
J . Org. Chem., Vol. 64, No. 7, 1999 2233
lated solid was found to be impure compared to an in
situ generated solution of 2a , on the basis of their
reported spectroscopic characterization. Therefore, the
oxomanganese(V) complexes were generated in situ for
the studies reported here. It is pertinent to recall that
Kochi and co-workers have taken the formation of peaks
at 530 and 550 nm as spectral evidence of the oxoman-
2 2
ganese(V) complex when the parent salen and 5,5′-(NO ) -
salen were taken as the ligands.13
Kin etics of Su lfoxid e Oxid a tion w ith Oxo(sa len )-
m a n ga n ese(V). Reaction rates were measured spectro-
photometrically in acetonitrile at 25 °C by following the
decay of the oxomanganese(V) species at 680 nm. At a
constant initial concentration of sulfoxide, constant val-
ues of k
tration of 2a (Table 1); this, coupled with the observation
of linear log(A - A ) versus time plots (r > 0.995),
1
were obtained upon varying the initial concen-
t
∞
ensures that the order in 2a is one. Inspection of data in
Table 1 reveals that variation of the initial concentration
of sulfoxide at constant [2a ] resulted in variation of the
1 1
k values. A plot of k versus [sulfoxide] yielded a straight
line passing through the origin (Figure 2), indicating that
the reaction is overall second-order and first-order in each
reactant. Similar kinetics were observed for the oxidation
of substituted phenyl methyl sulfoxides with oxomanga-
nese(V) complexes 2a -f (Figure 2). Therefore, the rate law
can be depicted as in eq 3.
F igu r e 1. (a) Absorption spectrum of 1a (0.0018 M) in
acetonitrile taken in a 1 cm cuvette. (b) Absorption spectrum
of oxo(salen)manganese(V) complex 2a (0.0018 M).
V
Mn dO) by comparing the present spectral observations
with the previous reports,11 we could not isolate this
active species. It is pertinent to point out that to date no
-
d[2]/dt ) k [2][sulfoxide]
(3)
2
(
salen)Mn(V) oxo species have yielded to structural
The effect of substituents at the para position of the
phenyl ring of phenyl methyl sulfoxide was studied using
several para-substituted phenyl methyl sulfoxides. The
second-order rate constants for the reaction of these para-
substituted sulfoxides with 2a are given in Table 2.
Electron-releasing substituents in the phenyl ring ac-
celerate the rate, whereas electron-withdrawing substit-
characterization, although Groves et al. and others have
characterized oxomanganese(V) porphyrin complexes in
recent years.12 We have chosen the salen ligand because
it is similar to porphyrin and the electronic and steric
nature of the metal complex can be tuned by introducing
electron-withdrawing and electron-releasing substituents
and bulky groups in the ligand.
uents produce the opposite effect. The log k
2
values show
better correlation with σ (Figure 3; r ) 0.997; F ) -2.44
p
Resu lts a n d Discu ssion
+
-
+
(
(
0.60; s ) 0.079) than with σ /σ (r ) 0.984; F ) -1.54
0.89; s ) 0.184). The negative F value indicates an
Oxid a tive Con ver sion of Ma n ga n ese(III) to Oxo-
accumulation of positive charge at the sulfur center, and
the magnitude of the F value indicates the extent of
charge development on the sulfur atom in the transition
state of the rate-determining step.14
m a n ga n ese(V) Sp ecies. The stirring of clear brown
III
solutions of (salen)Mn complexes 1a -f in acetonitrile
with iodosobenzene consistently led to the formation of
oxomanganese(V) species 2a -f as shown in eq 1. The
formation of oxomanganese(V) species is invariably as-
sociated with the following two changes: (i) the light
The electronic effects on the reactivity of oxomanga-
nese(V) complexes can be studied by having the substit-
uents at the 5,5′-positions that are para to the pair of
ligating oxygen atoms.13 The effect of altering the elec-
tronic nature of the oxidant on the rate of oxidation of
methyl phenyl sulfoxide has been studied using oxo-
brown color darkens, and (ii) the characteristic peak of
III
(
salen)Mn at λmax ≈ 350 nm disappears and a new
absorption band at λmax ≈ 530 nm appears (Figure 1).
The dark brown solution, on standing, faded to the
original light brown within 2-3 h. When methyl phenyl
sulfoxide (MPSO) was added to the dark brown solution,
the fading occurred in less than 40-50 min and methyl
phenyl sulfone was isolated in 79% yield (eq 2). The
absorption spectrum of the final solution coincided with
(salen)manganese(V) complexes 2a -d . The second-order
rate constants measured from this study are given in
Tables 1 and 2. It is seen that the electron-withdrawing
substituents at the 5,5′-positions of the salen ligand
enhance the rate, whereas electron-releasing substituents
III
that of the original (salen)Mn complex.
(12) (a) Groves, J . T.; Lee, J .; Marla, S. S. J . Am. Chem. Soc. 1997,
1
19, 6269. (b) Feichtingr, D.; Plattner, D. A. Angew. Chem., Int. Ed.
(
salen)MnIII
Engl. 1997, 36, 1718. (c) Collins, T. J .; Gordon-Wylies, W. J . Am. Chem.
Soc. 1989, 111, 4511. (d) Collins, T. J .; Powell, R. D.; Slehodnick, C.;
Liffelman, E. S. J . Am. Chem. Soc. 1990, 112, 899. (e) MacDonnel, F.
M.; Fackler, N. L. P.; Stern, C.; O’Halloran, T. V. J . Am. Chem. Soc.
PhSOMe
8 PhSO Me + PhI
(2)
2
PhIO
We have attempted to get a stable oxo(salen)MnV com-
plex. We actually obtained a solid product from the dark
brown solution of 2a by pouring the solution into a pool
of ether cooled to -40 °C. The dark brown solid was
filtered at low temperature. Upon dissolution, the iso-
1
994, 116, 7431.
(13) (a) Srinivasan, K.; Michaud, P.; Kochi, J . K. J . Am. Chem. Soc.
986, 108, 2309. (b) Srinivasan, K.; Perrier, S.; Kochi, J . K. J . Mol.
1
Catal. 1986, 36, 297.
14) J ohnson, C. D. Hammett Equation; Cambridge University: New
York, 1980.
(

2
234 J . Org. Chem., Vol. 64, No. 7, 1999
Chellamani et al.
Ta ble 1. Ra te Con sta n ts for th e Oxid a tion of MP SO by Oxo(sa len )m a n ga n ese(V) Com p lexes 2a -f in Aceton itr ile
a t 25 °Ca
02[MPSO], M
10 [2], M
3
10 k1(obs), s
4
b
-1
10 k1(dec), s
4
c
-1
10 k1, s
4
d
-1
10 k2, M s-1
3
e
-1
1
2
a
2
2
2
2
2
2
1
3
4
7
0
0
0
0
0
0
0
0
0
0
0.83
1.24
1.65
1.80
2.02
3.04
1.8
1.8
1.8
1.8
1.8
11.4 ( 0.4
11.7 ( 0.6
11.0 ( 0.2
11.2 ( 0.3
11.3 ( 0.3
11.5 ( 0.5
7.96 ( 0.18
14.3 ( 0.4
17.6 ( 0.7
27.5 ( 1.1
36.6 ( 1.7
5.26 ( 0.14
5.23 ( 0.25
4.72 ( 0.05
4.84 ( 0.09
5.15 ( 0.17
5.02 ( 0.22
4.84 ( 0.09
4.84 ( 0.09
4.84 ( 0.09
4.84 ( 0.09
4.84 ( 0.09
6.14 ( 0.26
6.47 ( 0.35
6.28 ( 0.15
6.36 ( 0.21
6.15 ( 0.13
6.48 ( 0.28
3.12 ( 0.09
9.46 ( 0.31
12.8 ( 0.6
22.7 ( 1.0
31.8 ( 1.6
3.07 ( 0.13
3.24 ( 0.18
3.14 ( 0.08
3.18 ( 0.11
3.08 ( 0.07
3.24 ( 0.14
3.12 ( 0.09
3.15 ( 0.10
3.20 ( 0.15
3.24 ( 0.14
3.18 ( 0.16
1
00
2
2
2
2
b
c
d
e
8
1.8
1.8
1.8
1.8
5.06 ( 0.06
7.15 ( 0.17
10.5 ( 0.3
16.0 ( 0.4
3.91 ( 0.04
3.91 ( 0.04
3.91 ( 0.04
3.91 ( 0.04
1.15 ( 0.02
3.24 ( 0.13
6.59 ( 0.26
12.1 ( 0.4
1.44 ( 0.03
1.62 ( 0.07
1.65 ( 0.07
1.51 ( 0.05
2
4
8
0
0
0
4
0
0
0
1.8
1.8
1.8
1.8
7.14 ( 0.23
10.3 ( 0.3
15.8 ( 0.7
26.4 ( 1.0
5.09 ( 0.16
5.09 ( 0.16
5.09 ( 0.16
5.09 ( 0.16
2.05 ( 0.07
5.21 ( 0.14
10.7 ( 0.5
21.3 ( 0.8
5.13 ( 0.18
5.21 ( 0.14
5.35 ( 0.25
5.33 ( 0.20
1
2
4
4
0
0
0
1.8
1.8
1.8
1.8
13.0 ( 0.3
26.4 ( 1.0
45.1 ( 1.3
81.9 ( 4.1
5.14 ( 0.21
5.14 ( 0.21
5.14 ( 0.21
5.14 ( 0.21
7.84 ( 0.09
21.3 ( 0.8
40.0 ( 1.1
76.8 ( 3.9
19.6 ( 0.2
21.3 ( 0.8
20.0 ( 0.6
19.2 ( 1.0
1
2
4
8
0
0
0
1.8
1.8
1.8
1.8
6.38 ( 0.18
9.34 ( 0.34
14.0 ( 0.4
22.7 ( 0.9
4.52 ( 0.11
4.52 ( 0.11
4.52 ( 0.11
4.52 ( 0.11
1.86 ( 0.07
4.82 ( 0.23
9.48 ( 0.29
18.2 ( 0.79
2.33 ( 0.09
2.41 ( 0.12
2.37 ( 0.07
2.28 ( 0.10
2
4
8
2
f
8
0
0
0
1.8
1.8
1.8
1.8
5.61 ( 0.16
7.71 ( 0.23
11.0 ( 0.5
19.1 ( 0.8
4.13 ( 0.14
4.13 ( 0.14
4.13 ( 0.14
4.13 ( 0.14
1.48 ( 0.02
3.58 ( 0.09
6.87 ( 0.36
15.0 ( 0.7
1.85 ( 0.03
1.79 ( 0.05
1.72 ( 0.09
1.88 ( 0.08
2
4
8
a
As determined by a spectrophotometric technique following the disappearance of the oxo complex at 680 nm; the error quoted in k
values is the 95% confidence limit of the Student t test. b Estimated from pseudo-first-order plots over 20% reaction. Estimated from
first-order plots over 50-60% reaction in the absence of sulfoxide. Obtained as k1 ) k1(obs) - k1(dec). e Individual k2 values estimated as
k1/[sulfoxide].
c
d
retard it. Hammett correlation of log k
an excellent linear relationship with a slope of 0.52 (
.04 (Figure 4; r ) 0.999; s ) 0.013). The positive F value
indicates the buildup of negative charge on the metal
center in the transition state of the rate-determining
step.14
In principle, this sizable electronic effect may be
ascribed to any of several factors. The substituents may
effect changes in the Mn-oxo bond length in the reactive
oxo(salen)MnV intermediate, thereby altering the non-
bonding substrate-ligand interactions. However, such
effects on metal-oxo bond length are reported to be
small.15 Further, the observed results are contrary to
what one can expect from this hypothesis. Because
electron-withdrawing groups should shorten the Mn-oxo
bond length, it must lead to a decrease in reactivity
rather than an increase as observed experimentally
2
versus Σσ
p
shows
tenuating its reactivity and thus generating a relatively
milder oxidant. Similarly, electron-withdrawing substit-
uents are expected to destabilize the Mn(V) oxo inter-
mediate, making it a more reactive oxidant.16 This aspect
can be substantiated from the E° values of the Mn(II)/
Mn(III) redox couple. There is an excellent correlation
between E° values of the Mn(II)/Mn(III) redox couple and
0
Σσ of the 5,5′-substituents (Figure 5; r ) 0.999; slope )
p
0.24). Thus, the effect of introducing a 5,5′-substituent
in the salen ligand on the redox potential of the metal-
oxo complex is mainly responsible for the substituent
effect on the rate of the oxygenation reaction. The mea-
surement of the redox potential of the couple Mn(IV)/
Mn(V) is not attempted because Mn(IV) and Mn(V) are
not stable under the electrochemical reaction conditions.
The results of the correlation of kinetic and electrochemi-
cal data with Hammett σ constants point out an interest-
ing contrast between the oxidant and substrate toward
the substituent effect. The dramatic difference between
the effect of substitution on the substrate versus that on
(compare the reactivity of 2a and 2d with sulfoxides).
Thus, an alternative explanation must be attempted.
The introduction of the electron-donating substituents
in the salen ligand of the Mn(III) complex is expected to
stabilize the high-valent Mn(V) oxo intermediate, at-
(16) (a) Hughes, D. L.; Smith, G. B.; Liu, J .; Dezeny, G. C.;
Senanayake, C. H.; Larsen, R. D.; Verhoeven, T. R.; Reider, P. J . J .
Org. Chem. 1997, 62, 2222 and references cited therein. (b) Palucki,
M.; Finney, N. S.; Pospisil, P. J .; Guler, M. L.; Ishida, T.; J acobsen, E.
N. J . Am. Chem. Soc. 1998, 120, 948.
(15) Nugent, W. A.; Mayer, J . M. Metal-Ligand Multiple Bonds;
Wiley: New York, 1988; pp 148-152.

Oxidation of Aryl Methyl Sulfoxides
J . Org. Chem., Vol. 64, No. 7, 1999 2235
oxidant on the sulfur of the sulfoxide.1
9-26
However, a
single-electron transfer mechanism from the sulfoxide to
the metal ion in the rate-determining step has been
postulated in the Cr(VI),2
7,28
cytochrome P-450, and
29
tetrahexylammonium tetrakis(diperoxomolybdo)phos-
phate30 oxidations of substituted phenyl methyl sulfox-
ides. On the other hand, in the alkaline peroxobenzoic
3
1-33
acid and peroxide oxidation,
as well as in the fluo-
renone carbonyl oxide34 and permanganate ion oxida-
tion of sulfoxides, a nucleophilic attack of the oxidant on
the sulfoxide in the rate-determining step has been
suggested.
35
In the present study, the formation of the sulfone in
the absence of other oxygen sources (i.e., at inert atmo-
sphere and in non-aqueous solvent) undoubtedly estab-
lishes that the oxygen atom incorporated into the sulfone
is derived from the oxomanganese(V) ions. The results
obtained from the studies on influence of changes in the
electronic nature of the substrate and oxidant throw more
light on the mechanism of oxygen atom transfer. The
observed F value of -2.44 ( 0.60 is close to the F values
found for the oxidation of substituted phenyl methyl
1
9
sulfoxides by potassium bromate (F ) -2.05) and by
chloramine-T20 (F ) -2.02). These reactions were char-
acterized as proceeding through an electrophilic attack
by a halogen ion on the sulfur center of the sulfoxide in
2
2
the rate-determining step. In the dioxiranes, gem-
2
3
24
dialkylperoxonium ion, Cr(VI)-oxalic acid, peroxomono-
sulfate,25 and oxo(phosphine)ruthenium(IV) complexes
oxidation of substituted phenyl methyl sulfoxides, al-
though low F values of -0.76, -0.83, -0.93, -0.40, and
26
F igu r e 2. Plots of k
a) p-BrC SOMe with 2a , (b) MPSO with 2b, (c) MPSO with
e, (d) MPSO with 2a , (e) MPSO with 2c, (f) p-OMeC SOMe
with 2a , and (g) MPSO with 2d .
1
versus [sulfoxide] for the oxidation of
(
2
6 4
H
6 4
H
-
N
0.42, respectively, have been observed, an S 2 mecha-
nism has been postulated for all these reactions. In the
the salen ligand is that the oxidant has an extremely
electrophilic oxygen that has very little interaction with
the aromatic portion of the salen. This absence of
electronic communication could be due to weak M-O π
bonding or a weak π interaction between the salen oxygen
and the metal. The slope value (+0.24) obtained in the
present study, because the observed F value is high and
log k
2
values are better correlated with σ
p
, one may
anticipate a S
N
2 mechanism involving the rate-determin-
ing electrophilic attack of the oxidant on the sulfur of
the sulfoxide.
A clear picture of the mechanism of this reaction may
emerge by comparing the present results with those
p
E° versus Σσ plot (Figure 5) supports this postulation
regarding electronic interaction between ligand and
metal and suggests that regardless of oxidation state this
interaction is weak.17 It is interesting to compare the
present results with those observed for the oxidation of
(19) Srinivasan, C.; J egatheesan, P. P.; Arumugam, N. Indian J .
Chem. 1986, 25A, 678.
(20) Ganapathy, K.; J ayagandhi, P. Int. J . Chem. Kinet. 1983, 15,
1
1
129.
organic sulfides. The electronic effect is more pro-
nounced with sulfoxide oxidation and the F value is
uniformly higher with sulfoxides (Tables 2 and 3).
The effect on the reaction rate of introducing substit-
uents at the 7-position of the salen ligand of the oxoman-
ganese(V) complex was studied using 2a , 2e, and 2f for
the oxidation of methyl phenyl sulfoxide. The rate data
in Table 1 show that the presence of a methyl or phenyl
group at the 7-position slightly reduces the rate, as in
(
21) Srinivasan, C.; J egatheesan, P. P. Indian J . Chem. 1989, 28A,
50.
(22) Murray, R. W.; J eyaraman, R.; Krishna pillai, M. J . Org. Chem.
987, 52, 746.
23) Bloodworth, A. J .; Melvin, T.; Mitchell, J . C. J . Org. Chem. 1988,
2
1
5
(
3, 1078.
(24) Srinivasan, C.; J egatheesan, P. P.; Rajagopal, S.; Arumugam,
N. Can. J . Chem. 1987, 65, 2421.
25) Suthakaran, R.; Subramanian, P.; Srinivasan, C. Proc. Indian
(
Acad. Sci. Chem. Sci. 1986, 97, 555.
(26) Acquaye, J . H.; Muller, J . G.; Takeuchi, K. J . Inorg. Chem. 1993,
32, 160.
1
1
the case of sulfide oxidation. Thus, the steric effect
(
27) (a) Baliah, V.; Satyanarayana, P. V. V. Indian J . Chem. 1978,
A16, 966; (b) 1980, B19, 620.
28) Srinivasan, C.; Venkatasamy, R.; Rajagopal, S. Indian J . Chem.
981, 20A, 505.
29) Watanabe, Y.; Takashi, I.; Oae, S. Tetrahedron Lett. 1982, 533.
(30) Ballistreri, F. P.; Bazzo, A.; Tomaselli, G. A.; Toscano, R. M. J .
Org. Chem. 1992, 57, 7074.
31) (a) Curci, R.; Modena, G. Tetrahedron Lett. 1963, 1749; 1965,
V
observed with Mn complexes is little, which is contrary
(
to the substantial steric effect noted in the oxo(salen)-
1
V
18
Cr complexes oxidation of alkynes.
(
Mech a n ism of Oxygen Atom Tr a n sfer fr om Oxo-
m a n ga n ese(V) to Su lfoxid es. Three types of mecha-
nisms have been proposed so far for the oxidation of
sulfoxides. Most of the reactions proceed by a mechanism
involving a rate-determining electrophilic attack of the
(
8
63. (b) Tetrahedron 1966, 22, 1227.
(32) (a) Curci, R.; Giovine, A.; Modena, G. Tetrahedron 1966, 22,
235. (b) Curci, R.; Di Furia, F.; Modena, G. J . Chem. Soc., Perkin
1
Trans 2. 1977, 576.
33) Ogata, Y.; Suyama, S. Chem. Ind. 1971, 707.
(34) Sawaki, Y.; Kato, H.; Ogata, Y. J . Am. Chem. Soc. 1981, 103,
3832.
(
(
17) The authors thank one of the referees for pointing out this
interesting aspect.
18) Righter, B.; Srihari, S.; Hunter, S.; Masnovi, J . J . Am. Chem.
Soc. 1993, 115, 3918.
(
(35) Rajagopal, S.; Sivasubramaniam, G.; Suthakaran, R.; Srini-
vasan, C. Proc. Indian Acad. Sci. Chem. Sci. 1991, 103, 637.

2
236 J . Org. Chem., Vol. 64, No. 7, 1999
Chellamani et al.
Ta ble 2. Secon d -Or d er Ra te Con sta n ts a n d G Va lu es for th e Rea ction s of p a r a -Su bstitu ted P h en yl Meth yl Su lfoxid es
a ,b
(
p-XC6H4SOMe) w ith 2a -d in Aceton itr ile a t 25 °C
oxo(salen)manganese(V) complex, 10 k2, M s-1
3
-1
no.
X
2b
2a
2c
2d
Fd
(r)
1
2
3
4
5
6
OMe
Me
H
13.5 ( 0.7
17.6 ( 0.6
23.4 ( 1.1
41.7 ( 2.0
0.235 ( 0.040
0.377 ( 0.074
0.518 ( 0.036
0.559 ( 0.121
0.609 ( 0.152
0.710 ( 0.068
(0.999)
(0.998)
(0.999)
(0.997)
(0.994)
(0.999)
5.00 ( 0.20
1.62 ( 0.07
0.515 ( 0.020
0.360 ( 0.010
0.018 ( 0.002
7.25 ( 0.15
3.18 ( 0.11
1.21 ( 0.04
0.920 ( 0.050
0.040 ( 0.004
11.2 ( 0.2
30.2 ( 1.4
5.35 ( 0.25
2.24 ( 0.08
1.95 ( 0.05
0.078 ( 0.006
20.0 ( 0.6
Cl
7.95 ( 0.25
7.05 ( 0.30
0.550 ( 0.016
Br
NO2
c
e
F
-2.67 ( 0.76
(0.996)
-2.44 ( 0.60
(0.997)
-2.29 ( 0.70
(0.995)
-1.80 ( 0.68
(0.993)
(
r)
a
b
The error quoted in k2 is the 95% confidence limit of the Student t test. General conditons: [2] ) 0.0018 M; [sulfoxide] ) 0.20 M.
c
d
[
Sulfoxide] ) 0.50 M. The values were obtained by correlating log k2 with Σσp, for the reaction of various oxo(salen)manganese(V)
e
complexes with a given sulfoxide. The values were obtained by correlating log k2 with σp for the reaction of various sulfoxides with a
given oxo(salen)manganese(V) complex.
F igu r e 4. Hammett plot for the oxidation of MPSO by
substituted oxo(salen)maganese(V) complexes.
F igu r e 3. Hammett plot for the oxidation of aryl methyl
sulfoxides by 2a . The points are referred to by the same
numbers as in Table 2.
V
observed for the oxo(salen)Mn complexes oxidation of
organic sulfides1 (Table 3). It is surprising that the
reactivity of organic sulfides and sulfoxides towards these
Mn(V) complexes is comparable and the F values are
always higher with sulfoxides (Tables 2 and 3). As far
as we know, it seems to be a first example where less
nucleophilic sulfoxides are more sensitive to substituent
effect compared to sulfides. It is interesting to recall that
sulfoxides react faster than sulfides with the same
oxidant only if the sulfoxide behaves as an electrophile
rather than a nucleophile and the F value becomes
positive. Because the F value observed in the present
study is negative (F ) -2.44), the sulfoxides behave only
as strong nucleophiles. We take these interesting results
1
N
as evidence for the operation of an S 2 mechanism for
the oxidation of organic sulfoxides. These interesting
substituent effects observed with organic sulfides and
sulfoxides tempted us to postulate different mechanisms
for oxidation of these two substrates. To account for the
kinetic results observed with organic sulfides, a mecha-
nism involving an electron transfer from sulfide to Mn-
F igu r e 5. Plot of E° for the Mn(II)/Mn(III) redox couple versus
Σσ values of the substitutent in catalysts 1a -d .
p
However, all arguments presented above undoubtedly
support the S 2 mechanism for the oxidation of aryl
N
methyl sulfoxides by oxo(salen)manganese(V) complexes
(Scheme 2). The proposed mechanism envisages the
formation of intermediate I in the slow step, which then
(
V) in the rate-determining step has been postulated
(
Scheme 1) in our earlier report.11

Oxidation of Aryl Methyl Sulfoxides
J . Org. Chem., Vol. 64, No. 7, 1999 2237
Ta ble 3. Secon d -Or d er Ra te Con sta n ts a n d G Va lu es for th e Rea ction s of p a r a -Su bstitu ted Th ioa n isoles (p-XC6H4SMe)
w ith Su bstitu ted Oxo Com p lexes in Aceton itr ile a t 25 °Ca
oxo(salen)manganese(V) complex, 10 k2, M s-1
3
-1
X
2b
2a
2c
2d
F
(r)
OMe
Me
H
F
Cl
22.8 ( 1.2
26.4 ( 1.6
12.0 ( 0.8
6.70 ( 0.39
4.89 ( 0.12
2.67 ( 0.29
2.50 ( 0.11
1.28 ( 0.15
0.71 ( 0.08
0.24 ( 0.04
36.4 ( 2.5
21.7 ( 1.7
14.2 ( 1.0
11.6 ( 0.2
8.38 ( 0.38
7.36 ( 0.41
3.46 ( 0.03
2.34 ( 0.04
0.78 ( 0.08
52.0 ( 3.1
43.6 ( 1.8
40.8 ( 3.1
30.6 ( 1.5
19.0 ( 1.4
17.1 ( 0.2
9.15 ( 0.12
4.91 ( 0.25
1.65 ( 0.03
0.176 ( 0.042
0.332 ( 0.075
0.474 ( 0.078
0.477 ( 0.110
0.515 ( 0.192
0.520 ( 0.177
0.532 ( 0.141
0.533 ( 0.219
0.554 ( 0.228
(0.988)
(0.989)
(0.994)
(0.989)
(0.971)
(0.976)
(0.985)
(0.965)
(0.964)
9.25 ( 0.40
4.37 ( 0.18
3.27 ( 0.23
1.73 ( 0.12
1.48 ( 0.17
0.74 ( 0.11
0.40 ( 0.06
0.12 ( 0.03
Br
CO2H
COMe
NO2
F
-2.06 ( 0.19
(0.995)
-1.85 ( 0.16
(0.995)
-1.52 ( 0.18
(0.992)
-1.44 ( 0.32
(0.970)
(
r)
a
Data taken from ref 11b.
Sch em e 1
value. The reverse is the case with less nucleophilic
substrates, with sulfoxides giving a comparatively higher
F value.36
To demonstrate the electrophilic nature of the oxidant
and charge separation in the transition state, the influ-
ence of adding trichloroacetic acid and changing the
solvent composition on the rate of oxidation of organic
sulfides and sulfoxides has been studied; the data are
collected in Table 4. The increase in concentration of acid,
as well as the increase in the polarity of the medium,
favors the rate of oxidation of sulfides and sulfoxides. The
addition of acid leads to the protonation of the oxidant,
and the protonated species is more electrophilic, thereby
favoring the reaction. The increase in rate with the
increase in the polarity of the medium points out the
formation of a charge-separated transition state, which
Sch em e 2
is in favor of the S
N
2 mechanism in the oxo(salen)MnV
oxidation of organic sulfides and sulfoxides.
The oxomanganese(V) oxidation of para-substituted
phenyl methyl sulfoxides was carried out at four different
temperatures, and the thermodynamic parameters evalu-
ated using the Eyring equation are collected along with
2
k values in Table 5. Although the correlation between
q
q
∆
H and ∆S is not satisfactory, (r ) 0.953), a plot of log
k
2
at 20 °C versus log k at 40 °C is linear (r ) 0.999;
2
slope ) 0.875 ( 0.132; s ) 0.044), indicating that all the
sulfoxides are oxidized by a similar mechanism. The
enthalpy and entropy of activation for the self decay of
oxo(salen)manganese(V) complex are found to be 91.0 kJ
decomposes to give (salen)MnIII and sulfone as the
products. The rate acceleration by electron-withdrawing
groups on the 5,5′-positions of salen (2c and 2d ) and by
the electron-donating groups in aryl methyl sulfoxides
supports this formulation. The pathway proceeding
through the oxametallacyclic intermediate has been
excluded, because the reaction involves little steric effect
when bulky methyl and phenyl groups are introduced in
the 7,7′-positions (Table 1).1 When we look carefully at
the rate constant and reaction constant (F) data collected
in Tables 2 and 3, we can infer that they point out a
similar mechanism for the oxo(salen)manganese(V) oxi-
dation of organic sulfides and sulfoxides. Although less
nucleophilic sulfoxides are more sensitive to substituent
-1
-1
-1
mol and -6.0 J K mol , respectively.
Rea ctivity-Selectivity P r in cip le. The observation
III
of strong RSP in the Mn -salen catalyzed PhIO oxida-
tion of organic sulfides prompted us to extend the study
to organic sulfoxides. The second-order rate constants for
the reactions of various para-substituted phenyl methyl
sulfoxides with each of the oxo(salen)manganese(V)
complexes 2a -d are given in Table 2, and those for the
organic sulfides are summarized in Table 3.
2b
The last row of Table 2 contains F values for substitu-
ent variation in the phenyl methyl sulfoxide for each oxo
complex, and the last column shows reaction constants
for substituent variation in the oxo complex for each
phenyl methyl sulfoxide. The F values in Table 2 show
that there is a significant variation of reaction constants
effect, the k
times more than the k
2
values obtained for sulfides are always 2-3
values observed for the corre-
2
sponding sulfoxides. These data are in accordance with
the Hammond postulate, i.e., consistent with a common
S 2 mechanism wherein only the position of the transi-
N
tion state changes from one substrate class to the next.
The more electron-rich sulfides should have an earlier
transition state for interaction with an electrophilic metal
oxo complex and will thus exhibit less charge on S and a
weaker influence of the substituent, leading to a low F
(selectivity) when we vary the nature of the substituent
in either the oxo complex or the sulfoxide. Also, it is
(
36) The authors thank the referees for suggesting the application
of the Hammond postulate to interpret the data with a similar S
like mechanism for the oxidation of both substrates.
N
2-

2
238 J . Org. Chem., Vol. 64, No. 7, 1999
Chellamani et al.
Ta ble 4. Effect of Ad d in g Acid a n d Ch a n gin g th e Solven t Com p osition on th e Ra te of Oxid a tion of Meth yl P h en yl
a ,b
Su lfid e a n d Su lfoxid e by 2a a t 25 °C
4
c
-1
4
d
-1
1
0 k1, s
10 k1, s
1
02[acid], M
sulfide
.36 ( 0.31
sulfoxide
CH3CN-H2O (v/v)
sulfide
sulfoxide
6
3.12 ( 0.09
4.06 ( 0.14
6.45 ( 0.29
18.7 ( 0.8
39.1 ( 1.3
60/40
70/30
75/25
80/20
90/10
100/0
110 ( 6
38.1 ( 1.8
24.9 ( 1.3
21.3 ( 0.9
16.7 ( 0.5
11.6 ( 0.4
6.36 ( 0.21
1
2
5
.0
.0
.0
9.71 ( 0.43
21.3 ( 0.9
46.5 ( 2.1
148 ( 7
77.6 ( 3.9
63.1 ( 2.7
47.9 ( 2.2
30.9 ( 1.5
13.4 ( 0.4
1
4
0.0
0.0
457 ( 29
137 ( 8
a
General conditions: [2a ] ) 0.0018 M; solvent: CH3CN, unless otherwise noted. b In the evaluation of rate constants, the
self-decomposition of 2a at different [acid] and solvent composition is taken into account. [Substrate] ) 0.1 M. d [Substrate] ) 0.2 M.
c
Ta ble 5. Secon d -Or d er Ra te Con sta n ts a n d Activa tion P a r a m eter s for th e Oxid a tion of Su lfoxid es (p-XC6H4SOCH3) by
2
a in Aceton itr ile a t F ou r Differ en t Tem p er a tu r esa
03k2, M-1 s-1
1
q
-1
-∆S , J K mol-1
q
-1
X
293 K
298 K
303 K
313 K
∆H , kJ mol
OMeb
Me
H
9.26 ( 0.36
17.5 ( 0.4
24.9 ( 1.0
55.7 ( 1.8
62.5
60.9
71.9
79.8
72.0
89.8
69.9
81.3
50.8
33.5
62.9
28.1
4.83 ( 0.13
7.25 ( 0.15
3.18 ( 0.11
1.21 ( 0.04
0.920 ( 0.050
0.040 ( 0.004
12.0 ( 0.4
26.1 ( 0.7
2.24 ( 0.08
6.15 ( 0.25
1.87 ( 0.07
1.32 ( 0.06
0.074 ( 0.006
15.3 ( 0.6
Cl
0.640 ( 0.035
0.465 ( 0.020
0.020 ( 0.004
5.90 ( 0.20
3.60 ( 0.15
0.240 ( 0.014
Br
NO2
c
F
-2.53 ( 0.48
-2.44 ( 0.60
-2.39 ( 0.54
-2.21 ( 0.63
a
General conditions: [2a ] ) 0.0018 M; [sulfoxide] ) 0.2 M. b [Sulfoxide] ) 0.1 M. c [Sulfoxide] ) 0.5 M.
Ta ble 6. Resu lts of Cor r ela tion betw een log kF i a n d log kSi Accor d in g to eq 4 for Ar yl Meth yl Su lfoxid es
oxo(salen)manganese(V) complexes (F and S)
results
2a and 2b
0.997
2c and 2b
0.991
2c and 2a
0.998
2d and 2b
0.983
2d and 2a
0.993
2d and 2c
0.996
r
b
∆
0.912 ( 0.078
0.280
0.852 ( 0.182
0.518
0.937 ( 0.056
0.238
0.664 ( 0.198
1.05
0.733 ( 0.142
0.773
0.784 ( 0.110
0.535
apparent that as the reactivity of either sulfoxide or oxo
complex decreases, the F value increases, i.e., there is an
inverse relationship between reactivity and selectivity in
both cases. These results are very similar to those
1
1
observed with aryl methyl sulfides (Table 3).
Hence, we have attempted to analyze the data by
3
7
applying a method formulated by Exner to verify the
operation of RSP. Accordingly, the rate data reported in
Table 2 were subjected to mathematical treatment using
eqs 4 and 5.
log kFi ) a + b log kSi + εi
log k )/N
(4)
(5)
∆
) (∑i log kFi -
∑i
Si
where kFi and kSi are the second-order rate constants for
the reactions of fast and slow reagents (oxo complexes),
i
respectively, with each sulfoxide, ε is the error of the log
k
Fi versus log kSi correlation, and ∆ is the mean difference.
The values of b and ∆ were calculated for all of the six
possible combinations of one fast and one slow reagent
(among the four oxo complexes) with a series of similar
substrates (six sulfoxides). The results summarized in
Table 6 show a valid RSP in all cases as the value of b is
less than unity and ∆ is not too small.
In a system involving more than one reagent and the
same set of substrates, the existence of a “magic point”37
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
F igu r e 6. Log kFi versus log kSi plot for the reactions of aryl
methyl sulfoxides with (a) 2a and 2b, (b) 2c and 2b, (c) 2c
and 2a , (d) 2d and 2b, (e) 2d and 2a , and (f) 2d and 2c.
versa. In this aspect, the present system reveals interest-
ing results (Figure 6). The correlations involving nitro-
substituted oxo complex 2d (lines d-f) produce one magic
point, y
o
(1), and the correlations involving other oxo
(2).
(37) Exner, O. J . Chem. Soc., Perkin Trans 2 1993, 973.
complexes (lines a-c) produce another magic point, y
o

Oxidation of Aryl Methyl Sulfoxides
J . Org. Chem., Vol. 64, No. 7, 1999 2239
Both magic points are situated on the side of high
reactivity, as expected for a strong RSP as observed in
the solvolysis37,38 of various meta- and para-substituted
allyl benzensulfonates with various alcohols and oxo-
conditions ([sulfoxide] > [oxo complex]) in acetonitrile at 25 (
0
.1 °C by monitoring the decay of the oxo complex at 680
1
1a,13
nm.
The rate constants were computed from the linear least-
squares plots of log(A - A ) versus time, where A is the
absorbance at time t and A is the experimentally determined
t
∞
t
1
1
(salen)manganese(V) oxidation of organic sulfides. The
∞
fact that the present system obeys RSP may be consid-
ered as evidence that the same mechanism is operating
in all reactions.
infinity point. The first-order rate constants k1(dec) for the
autodecomposition of oxomanganese(V) were determined from
the first-order plots up to 50-60% of reaction. The plots for
the decay of the oxo complex in the presence of sulfoxide were
linear over 20% of reaction, and the pseudo-first-order rate
constants k1(obs) were determined from the disappearance of
Exp er im en ta l Section
the oxo complex up to this extent. The values of k
1
were
) k1(obs) - k1(dec). The second-order rate
constants were obtained by the relation k ) k /[sulfoxide].
Ma ter ia ls. All the aryl methyl sulfoxides were prepared
obtained1
1,41
as k
1
from the corresponding sulfides according to the literature
3
9
2
1
procedure. All the sulfoxides were purified by vacuum
distillation/recrystallization from suitable solvents. The physi-
cal constants of the sulfoxides were found to agree with
P r od u ct An a lysis. The reaction mixture from an actual
kinetic run was subjected to vacuum evaporation, and the
residue was then extracted with diethyl ether. The ethereal
literature values.39 The HPLC analyses of each sulfoxide
showed the presence of a single entity, and no impurity peak
extract was dried over anhydrous Na
evaporated. The product was dissolved in CH
2
SO
4
, and the solvent was
Cl , and GC
1
2
2
appears in the H NMR spectra. Acetonitrile (GR, E. Merck)
analyses of the samples showed the presence of sulfone and
iodobenzene. The yield of sulfone (75-94%) depended on the
sulfoxide and oxo complex employed. The lower yield of sulfone
may be attributed to the competitive self-decomposition of the
was first refluxed over P
2 5
O for 5 h and then distilled.
Iodosylbenzene was prepared from (diacetoxyiodo)benzene by
4
0
a standard method.
III
+
-
6
complexes were synthesized accord-
The [Mn (salen)] PF
ing to a known procedure.
studies of all of the complexes were found to be identical with
III
1
1a,13
oxomanganese(V) complex to the Mn complex, which is
The IR and UV-vis spectral
III
confirmed by the near quantitative recovery of Mn complex
1
3
11a
from the reaction mixture.
literature data. The oxo complexes 2a -f were generated
Stoich iom etr y. To determine the stoichiometry, the reac-
tion was carried out under the experimental conditions ([2] )
0.0018 M; [PhSOMe] ) 0.2 M). The resultant solution after
the completion of the reaction gave sulfone in ca. 79% yield
in situ by stirring magnetically 0.15 mmol of finely powdered
PhIO in 5 ml of an acetonitrile solution containing 0.015 mmol
of the corresponding MnIII(salen) complex for 5 min under
nitrogen, followed by filtration at ice temperature to remove
the undissolved iodosylbenzene. The conversion of MnIII to MnV
III
and Mn (salen) complex in ca. 95% yield. Accordingly, the
stoichiometry of the reaction can be represented by eq 6.
may be considered quantitative in the sense that prolonged
stirring causes no change in the initial absorption spectrum
of the MnV solution. Because the oxomanganese(V) complex
undergoes autodecomposition, the solutions were prepared
freshly for each kinetic run.
V
+
OdMn (salen) + PhSOMe f
III
+
Mn (salen) + PhSO2Me (6)
Ack n ow led gm en t. We thank CSIR, New Delhi, for
providing financial assistance in the form of a project.
P.K. thanks the U.G.C., New Delhi, and Manonmaniam
Sundaranar University, Tirunelveli, for an award of a
Fellowship under FIP. He also thanks the Principal and
Management of Sri K.G.S. Arts College, Srivaikuntam,
for granting permission to accept the fellowship.
Kin etic Mea su r em en ts. Reaction mixtures for kinetic runs
were prepared by quickly mixing the solutions of the oxo
complex and the sulfoxide in varying volumes so that in each
run the total volume was 5 ml. The kinetics was followed in a
Perkin-Elmer UV-vis spectrophotometer (Lambda 3B) fitted
with a thermostated cell compartment under pseudo-first-order
(38) Sendega, R. V.; Vizgert, R. V.; Mikhalevich, M. K. Reakts.
J O9815756
Sposobnost Org. Soedin. 1970, 7, 512.
(
(
39) Price, C. C.; Hydock, J . J . J . Am. Chem. Soc. 1952, 74, 1943.
40) Saltzman, H.; Sharefkin, J . G. Organic Synthesis; J ohn Wiley
(41) Curci, R.; Giannattasio, S.; Sciacovelli, O.; Troisi, L. Tetrahedron
1984, 40, 2763.
and Sons: New York, 1973; Vol. V, p 658.