3566 J . Org. Chem., Vol. 62, No. 11, 1997
Bohra et al.
Ta ble 8. Cor r ela tion of Ra te of Oxid a tion of Alk yl
P h en yl Su lfid es w ith th e P a velich -Ta ft Equ a tion a
(S × 100)
Ps )
(9)
(L + D + S)
temp, K
F*
δ
R2
sd
The values of PD and PS are also recorded in Table 7.
The value of PD for the oxidation of ortho- and para-
substituted aryl methyl sulfides is ca. 52%, whereas its
value for the meta-substituted sulfides is ca. 43%. The
value of Ps shows that the steric effect is considerable in
this reaction.
283
293
303
313
-2.47 ( 0.06
-2.53 ( 0.12
-2.30 ( 0.02
-2.18 ( 0.05
0.76 ( 0.01
0.76 ( 0.02
0.69 ( 0.01
0.63 ( 0.01
0.9998
0.9991
0.9999
0.9997
0.01
0.01
0.01
0.01
a
Number of data points ) 5.
Sch em e 1
In earlier studies of oxidation of sulfides involving a
direct oxygen transfer via an electrophilic attack on the
sulfide-sulfur, the reaction constants were negative but
of relatively small magnitude, e.g. by hydrogen peroxide
(-1.13),19 periodate (-1.40),20 permanganate (-1.52),21
and peroxydisulfate (-0.56).22 Large negative reaction
constants were exhibited by oxidations involving forma-
tion of halogeno-sulfonium cations, e.g. by chloramine-T
(-4.25),23 bromine (-3.2),24 and N-bromoacetamide
(-3.75).25 In the oxidation by N-chloroacetamide (NCA),26
the values of field (FI) and resonance (F+R) at 298 K are
-1.3 and -1.7, respectively.
(ii) Alk yl P h en yl Su lfid es. The rates of oxidation
of alkyl phenyl sulfides did not yield any significant
correlation separately with Taft’s σ* or Es values. The
rates were therefore analyzed in terms of Pavelich-
Taft’s27 dual substituent-parameter (DSP) eq 10.
log k2 ) F*σ* + δEs + log l0
(10)
The correlations are excellent (Table 8). Though the
number of compounds is small (five) for any analysis by
the DSP equation, the results can be used qualitatively.
The negative polar reaction constant confirms that the
electron-donating power of the alkyl group enhances the
reaction rate. The steric effect plays a minor inhibitory
role.
Sch em e 2
Mech a n ism
In view of the absence of any effect of radical scavenger,
acrylonitrile, on the reaction rate, it is unlikely that a
one-electron reaction, giving rise to free radicals, is
operative in this oxidation. The observed Michaelis-
Menten kinetics with respect to sulfides led us to suggest
the formation of a 1:1 complex of BBCP and sulfides in
a rapid preequilibrium. With the present data it is not
possible to definitely state the nature of the intermediate
complex. Theoretical calculations28 have shown that
there is a substantial amount of charge transfer from the
metal to the oxygens in permanganate, and the manga-
nese is essentially dipositive as in reaction 1. The most
logical mode of interaction between sulfides and BBCP
would, therefore, be nucleophilic attack at the metal.
Donation of an unshared pair of electrons to an empty
d-orbital on the metal would result in the formation of a
coordinate covalent bond as in reaction 2. The initially
formed intermediate is likely to undergo a further rapid
reaction in which the incipient oxide and sulfonium ions
bond to form a highly structured intermediate (3) that
would rearrange to give a sulfoxide and manganese(V)
(Scheme 1). The observed acid-catalysis may well be due
to the successive protonation of the intermediate 2 prior
to the further reactions (Scheme 2).
Exp er im en ta l Section
Ma ter ia ls. The sulfides were either commercial products
or prepared by known methods,29-35 and were purified by
distillation under reduced pressure or crystallization. Their
purity was checked by comparing their boiling or melting
points with the literature values. BBCP was prepared by the
(19) Modena, G.; Maioli, L. Gazz. Chim. Ital. 1957, 87, 1306.
(20) Ruff, F.; Kucsman, A. J . Chem. Soc., Perkin Trans. 2 1985, 683.
(21) Banerji, K. K. Tetrahedron 1988, 44, 2969.
(22) Srinivasan, C.; Kuthalingam, P.; Arumugam, N. Can. J . Chem.
1978, 56, 3043.
(23) Ruff, F.; Kucsman, A. J . Chem. Soc., Perkin Trans. 2 1975, 509.
(24) Miotti, U.; Modena, G.; Sadea, L. J . Chem. Soc., B 1975, 802.
(25) Perumal, S.; Alagumalai, S.; Selvaraj, S.; Arumugam, N.
Tetrahedron 1986, 42, 4867.
(26) Agarwal, A.; Bhatt, P.; Banerji, K. K. J . Phys. Org. Chem. 1990,
3, 174.
(27) Pavelich, W. H.; Taft, R. W. J . Am. Chem. Soc. 1956, 79, 4935.
(28) Ziegler, T.; Rauk, A.; Baerends, E. J . Chem. Phys. 1976, 16,
209.
(29) Ruff, F.; Kucsman, A. J . Chem. Soc., Perkin Trans. 2 1985, 683.
(30) Srinivasan, C.; Kuthalingam, P.; Chelamani, A.; Rajagopal, S.;
Arumugam, N. Proc. Indian Acad. Sci., Chem. Sci. 1984, 95, 157.
(31) Zuncke, T.; Swartz, H. Chem. Ber. 1913, 46, 775. Ibid. 1915,
48, 1242.
(32) Hofmann, A. W. Chem. Ber. 1887, 20, 2260.
(33) Gilman, H.; Gainer, G. C. J . Am. Chem. Soc. 1949, 71, 1747.
(34) Saggiomo, A. J .; Craig, P. N.; Gordon, M. J . J . Org. Chem. 1958,
23, 1906.
(35) Nodiff, E. A.; Lipschutz, S.; Craig, P. N.; Gordon, M. J . J . Org.
Chem. 1960, 25, 60.