Sulfoxidation of Thioketones and Sulfines
J . Org. Chem., Vol. 64, No. 17, 1999 6375
Sch em e 1
previously unknown compounds (p-FC
CF CSO. The synthesis and isolation of those
compounds has been reported separately.
6 4 2
H ) CSO and (m-
3
6 4 2
C H )
2
9
Kin etics of Th iok eton e Oxid a tion . Control experi-
ments for reactions between thioketones and hydrogen
3
peroxide, without MTO, proceeded more than 10 times
more slowly than those measured with the catalyst
present. Thus the uncatalyzed reaction can be neglected
entirely. It was also shown that oxidation by atmospheric
oxygen or photoactivation in laboratory light were not
important. No effort was made to exclude either.
those in the predecessor compound as the number of RedO
bonds is reduced at each stage.
Kin etic Da ta . Both initial rate and full time course
analyses were employed to determine the kinetics. To deter-
mine initial rates, the absorbance-time data from spectro-
photometric kinetics or the integrated intensities-time data
The typical ranges of concentrations used for kinetics
experiments on thioketones in these studies were 0.01-
0.1 mM thioketone, 0.1-1 mM hydrogen peroxide, and
1
from H NMR kinetics were converted to concentration-time
values. The initial rate for each experiment was obtained by
1
-100 µM MTO. The analysis of the kinetic data requires
fitting each curve to a polynomial function: [Y]
t
) [Y]
was taken as the
. Under some circumstances the reaction
0
- m
1
t
an examination of the general scheme by which MTO
activates hydrogen peroxide. The general mechanism has
2
3
-
m
2
t - m
3 1
t ... The value of parameter m
initial reaction rate, v
i
3
0-32
now been established.
The reactions, shown in
was first-order with respect to the thioketone concentration.
In such cases the absorbance-time data were fitted to the
equation
Scheme 1,30 feature (a) conversion of MTO to two suc-
cessive peroxorhenium complexes, (b) attack of substrate
Y at an oxygen of the peroxo group, and (c) transfer of
that oxygen to Y. This general picture requires a few
refinements in special cases, as recently reviewed, but
it otherwise suffices quite well.
Abs ) Abs + (Abs - Abs ) exp(-k t)
(2)
t
∞
0
∞
ψ
3
3
Tr a p p in g of SO w ith Dien es. 4,4′-Difluorothiobenzophe-
none S-oxide (13 mM) and MTO (12 mM) were mixed in
acetonitrile containing 0.1 M HOTf to stabilize the MTO.17
Hydrogen peroxide (250 mM) was then added to the solution,
The kinetic data for the thioketones could be analyzed
by accounting for only the left-hand loop in Scheme 1.
That is so because these experiments were carried at low
followed immediately by 2,3-dimethyl-1,3-butadiene (100 mM).
The reaction was monitored by 1H NMR. After 1.5 h, 2,5-
[H O ], where any contribution from B can be ignored.
2
2
dihydro-3,4-dimethylthiophene-1-oxide (5.0 mM) was formed.
This compound is the known product of the reaction between
Under these circumstances the rate of the reaction takes
the form
1
9
sulfur monoxide and the diene. The detected spectrum
1
matched that of the known compound: H (CD
2
3
CN) δ 3.85 (d,
k k [H O ][Y][Re]
k k [H O ][Y][Re]
1 3 2 2 T
1
3
2
2
T
H), 3.61 (d, 2H), 1.77 (s, 6H). The remainder of the diene (46
v )
=
(3)
mM) had been oxidized to diol products. MTO-catalyzed
oxidations of dienes with hydrogen peroxide have been well-
k-1 + k [Y] + k [H O ]
k-1 + k [Y]
3
3
1
2
2
2
0
characterized.
in which [Re]
Under the conditions used for the thioketones, the k
] denominator term was entirely negligible. The
T
stands for the total rhenium concentration.
The same procedure using 4,4′-dimethoxythiobenzophenone
and thiobenzophenone S-oxides with the same trapping re-
agent, 2,3-dimethyl-1,3-butadiene, was used. In these cases
the same SO trapped product was obtained.
1
-
2 2
[H O
expression for the reaction rate in eq 3 states that the
variation of the initial rate with the concentration of the
thioketone will take the form of a rectangular hyperbola,
with the rate attaining a thioketone-independent plateau
Resu lts
Id en tifica tion a n d Isola tion of th e Su lfin es. The
initially formed sulfines were identified by their UV-
at high concentrations. The same is true for values of k
as a function of [R CS]. These experiments were carried
out with (p-MeOC CdS and with thiocamphor. Data
for both compounds are displayed in Figure 1. From both
ψ
2
1
vis, H NMR, and mass spectra in comparison with those
6
4 2
H )
7
,8,11,14,15,21-28
of the known compounds.
These spectra and
those of the starting thiones and final product ketones
are given in the Supporting Information. The only
exceptions to that are two of the sulfines, which have
evidently not been previously reported. These are the
-1 -1
experiments, the value k
1
) 15.5 ( 0.6 L mol
s
was
-1
1
obtained. Given K
1
) 91 L mol from H NMR experi-
ments cited previously, we obtain the value k-1 ) 0.17
-1
s
. Such determinations were needed to substantiate the
1
model and establish values of k and k-1 in this medium,
(
18) Espenson, J . H.; Abu-Omar, M. M. Adv. Chem. Ser. 1997, 253,
9-134.
19) Tardif, S. L.; Rys, A.; Abrams, C. B.; Abu-Yousef, I. A.; Leste-
Laserre, P. B.; Schults, E. K. V.; Harpp, D. N. Tetrahedron 1997, 53,
2225.
but the kinetic data at the plateau provided no new data
and no information about the reaction of the thioketone
itself.
In most of the determinations, therefore, the thioketone
concentration was kept deliberately low, such that eq 3
9
(
1
(
(
20) Tan, H.; Espenson, J . H. Inorg. Chem. 1998, 37, 467-472.
21) Maccagnani, G.; Innocenti, A.; Zani, P.; Battaglia, A. J . Chem.
Soc., Perkin Trans. 2 1987, 1113.
22) Dahn, H.; Pechy, P.; Toan, V. V.; Bonini, B. F.; Lunazzi, L. J .
Chem. Soc., Perkin Trans. 2 1993, 10, 1881.
3
could be simplified even further by dropping the k term
in the denominator. The initial rate could then be
accurately represented by
(
(
23) Zwanenburg, B. Tetrahedron 1971, 27, 1731.
24) Tangerman, A.; Zwanenburg, B. J . Chem. Soc., Perkin Trans.
(
2
2
1
1973, 458.
25) Tangerman, A.; Zwanenburg, B. J . Chem. Soc., Perkin Trans.
1974, 1141.
26) Veenstra, G. E.; Zwanenburg, B. Recl. Trav. Chim. Pays-Bas
976, 95, 37.
27) Huisgen, R.; Mloston, G.; Polborn, K.; Palacios-Gambra, F.
Liebigs Ann. Org. Bioorg. Chem. 1997, 1, 187.
28) Kuipers, J . A. M.; Lammerink, B. H. M.; Still, K. W. J .;
Zwanenburg, B. Synthesis 1981, 4, 295.
v ) K k [R CdS] [H O ] [Re]
T
(4)
(
i
1
3
2
0
2
2 0
(
The test of this kinetic equation is presented in Figure
(
(29) Huang, R.; Espenson, J . H., submitted for publication.
(30) Abu-Omar, M. M.; Appleman, E. H.; Espenson, J . H. Inorg.
Chem. 1996, 35, 7751-7757.
(