A. Chellamani et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 104–110
109
Table 6
Results of correlations between log kFi and log kSi according to Eq. (8).
Results
Complexes (F & S)
a and 1b
1
1c and 1b
1d and 1b
1c and 1a
1d and 1a
1d and 1c
r
0.998
0.997
0.997
0.999
0.999
0.999
b
ꢃ
0.99 ± 0.04
1.00 ± 0.03
1.00 ± 0.04
1.01 ± 0.01
1.01 ± 0.01
1.00 ± 0.00
0.093
0.291
0.668
0.197
0.574
0.377
four complexes) with a series of similar substrates (six sulfoxides)
plex and will thus exhibit less positive charge on S atom and a
weaker influence of the substituents, leading to a low ꢂ value. The
less sensitivity of sulfoxides in the first and second systems can be
attributed to the less nucleophilic character of sulfoxides compared
to the corresponding sulfides in accordance with the observation
in the earlier studies [51] that in the electrophilic oxidation by
peroxides the reactivity of sulfoxides is considerably less affected
by substituent effects than that of sulfides. The less sensitivity of
sulfoxides in the fifth system is in contrast with the Hammond
postulate for a common SN2 mechanism and has been attributed
(
Fig. 6). The results summarized in Table 6 show an indifferent RSP
(
RSP(?)) in all the cases as the value of b is nearly one. As expected
for a RSP(?), the lines in Fig. 6 are parallel to one another and pro-
duce no magic point. It may be mentioned here that a strong RSP has
been observed in our earlier studies of oxidation of aryl methyl sul-
fides and sulfoxides with (salen)MnIII/PhIO and (salen)Mn /NaOCl
catalytic systems [22,24,26,28].
III
3.5. Comparison of reactivity of various catalytic systems
V
towards organic sulfides and sulfoxides
to the poor electrophilic nature of oxo(salen)Cr –sulfoxide adduct
V
compared to the oxo(salen)Cr complex.
Comparison of the studies of oxidations of aryl methyl sul-
III
fides and the corresponding sulfoxides by (salen)Mn /H O
2
2
4. Conclusion
III
III
[
29,present study], (salen)Fe /PhIO [9,48], (salen)Mn /PhIO
III
III
[
21,24], (salen)Mn /NaOCl [26,28] and (salen)Cr /PhIO [25,49]
Aryl methyl sulfoxides are efficiently oxidized to the corre-
catalytic systems discloses some interesting results. In all the five
systems, both the substrates act as nucleophiles and the sulfides
III
sponding sulfones by the (salen)Mn /H O catalytic system. Based
2
2
on kinetic results and substituent effect studies with respect to
both sulfoxide and complex a Michaelis–Menten type mecha-
III
are more reactive. The same reactive species, (salen)Mn OOH
V
in the first system and oxo(salen)M in the next three systems,
III
nism involving (salen)Mn OOH complex as reactive species has
oxidizes both sulfides and sulfoxides with a similar mechanism,
Michaelis–Menten type in the first two systems and SN2 type in
the third and the fourth systems. In the fifth system, the mecha-
nism is similar (SN2 type), but the reactive species are different,
been proposed. The results in the present study have been com-
pared with those obtained in our earlier studies of oxidation
III
of organic sulfides and sulfoxides with the (salen)Mn /PhIO,
III
III
(
salen)Mn /NaOCl, (salen)Fe /PhIO and (salen)CrIII/PhIO catalytic
V
V
oxo(salen)Cr for sulfide oxidation and oxo(salen)Cr –sulfoxide
adduct for sulfoxide oxidation. The reaction constant (ꢂ) values
observed for substituent variation in the para-position of phenyl
ring of sulfides and sulfoxides show that sulfides are more sensi-
tive in the first, second and fifth systems and sulfoxides are more
sensitive in the third and fourth systems. The more sensitivity of
sulfoxides in the third and fourth systems is in consistent with the
Hammond postulate for a common SN2 mechanism for both the
substrates. That is, as the sulfides are electron-rich compared to the
corresponding sulfoxides, they have an earlier transition state for
interaction with the electrophilic oxo(salen)manganese(V) com-
systems. Both Hammett correlations and Exner’s method of math-
ematical verification point out an indifferent RSP in the present
study.
Acknowledgement
A.C. thanks Manonmaniam Sundaranar University for the finan-
cial assistance from UGC-UAG.
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