Probing for Electronic and Steric Effects in the Peracid Oxidation of Thianthrene 5-Oxide

Article abstract of DOI:10.1021/jo960710g

A systematic examination of the electronic and steric effects in the peracid oxidation of thianthrene 5-oxide (SSO) was undertaken and has revealed valuable information on the oxidative reactivity of the SSO mechanistic probe toward peroxidic oxidants. Thus, no significant steric effect was found on the bisulfoxide (SOSO)/sulfone (SSO₂) product ratio for aliphatic peracids with varying size of alkyl groups [R = Me, Et, i-Pr, t-Bu, (n-Bu)(Et)CHCH₂]. However, a significant electronic effect was observed for CH₃CO₃H and CF₃CO₃H, for which the X_so values were 0.16 and 0.01, i.e., predominant oxidation of the sulfide functionality to the SOSO product by these oxidants. In addition, substituted perbenzoic acids displayed this trend. In contrast, the nucleophilic peracid anion led exclusively (X_so = 1.00) to oxidation of the sulfoxide site to the SSO₂ product. The oxygen transfer appears to be orbital-controlled since the more electrophilic oxidant is clearly the more reactive as well as more chemoselective, i.e., preferred attack at the more nucleophilic sulfide site in SSO.

Full text of DOI:10.1021/jo960710g

J . Org. Chem. 1997, 62, 115-119
115
P r obin g for Electr on ic a n d Ster ic Effects in th e P er a cid Oxid a tion
of Th ia n th r en e 5-Oxid e
Waldemar Adam* and Dieter Golsch
Institute of Organic Chemistry, University of W u¨ rzburg, Am Hubland, D-97074 W u¨ rzburg, Germany
Received April 18, 1996X
A systematic examination of the electronic and steric effects in the peracid oxidation of thianthrene
5
-oxide (SSO) was undertaken and has revealed valuable information on the oxidative reactivity
of the SSO mechanistic probe toward peroxidic oxidants. Thus, no significant steric effect was
found on the bisulfoxide (SOSO)/sulfone (SSO ) product ratio for aliphatic peracids with varying
size of alkyl groups [R ) Me, Et, i-Pr, t-Bu, (n-Bu)(Et)CHCH ]. However, a significant electronic
effect was observed for CH CO H and CF CO H, for which the XSO values were 0.16 and 0.01, i.e.,
2
2
3
3
3
3
predominant oxidation of the sulfide functionality to the SOSO product by these oxidants. In
addition, substituted perbenzoic acids displayed this trend. In contrast, the nucleophilic peracid
anion led exclusively (XSO ) 1.00) to oxidation of the sulfoxide site to the SSO product. The oxygen
2
transfer appears to be orbital-controlled since the more electrophilic oxidant is clearly the more
reactive as well as more chemoselective, i.e., preferred attack at the more nucleophilic sulfide site
in SSO.
In tr od u ction
acceptor (substrate), since clearly the sulfide terminal is
more nucleophilic than the sulfoxide site in SSO and,
thus, more prone toward attack by an electrophilic
oxidant. This would imply that the SSO probe differenti-
ates between the electrophilic character of the peroxidic
oxidant without ascribing nucleophilic reactivity to it.
During the last decade, the oxidation of thianthrene
-oxide (SSO) has been used as a mechanistic probe to
determine the electronic character of an oxidant (Scheme
5
1
-3
1
).
Thus, electrophilic oxidants preferentially attack
thianthrene 5-oxide (SSO) at its sulfide sulfur to afford
the thianthrene 5,10-dioxides (trans-SOSO and cis-
SOSO), while nucleophilic ones oxidize SSO predomi-
To explore whether this mechanistic rationale is gen-
eral for oxidants and not specific for metal peroxo
complexes, we decided to investigate nonmetal-type per-
oxidic oxidants, in particular organic peroxy acids. Their
choice rests on the fact that peracids are established
electrophilic oxidants and their mechanism of oxygen
nantly to the thianthrene 5,5-dioxide (SSO
sirable second oxidation (overoxidation) of all three
dioxides leads to trioxide SOSO (Scheme 1). The XSO
2
). An unde-
2
parameter was introduced as mole fraction of SSO
2
5
product,1 for which XSO ) 1.00 represents exclusive
a
transfer is well understood and not obscured by whether
substrate-oxidant complexation operates. For the varia-
tion of the electrophilic character of the peracid oxidant,
the aliphatic cases CH CO H versus CF CO H and a
3 3 3 3
series of para-substituted perbenzoic acids were selected,
for which the XSO parameter should reflect the electronic
nature of the oxidant. The steric properties were to be
probed for a set of aliphatic peracids, namely methyl,
ethyl, isopropyl, tert-butyl, and the neopentyl-type (n-Bu)-
oxidation of the sulfoxide site and XSO ) 0.00 exclusive
oxidation at the sulfide terminal. Prototype examples
1
b
are the peroxy anion (XSO ) 1.00) and protonated
hydrogen peroxide (XSO ) 0.05),1b which span essentially
the full range of this selectivity parameter. Clearly, the
peroxy anion must function as nucleophilic oxidant by
attack at the electrophilic sulfoxide functionality, while
protonated hydrogen peroxide serves as an electrophilic
oxidant through oxygen transfer to the nucleophilic
2
(Et)CHCH as alkyl groups, by assessing the cis-trans
ratio of the bisulfoxide product SOSO. The results of this
study and its mechanistic implications are reported
herewith.
sulfide site.
In a recent publication4 an alternative mechanistic
rationale was forwarded for metal peroxo complexes in
that “...all the oxidation reactions are electrophilic pro-
cesses...”, which argues that oxidation of the sulfoxide
site in SSO entails electrophilic attack at the lone pair
rather than nucleophilic addition to the sulfoxide func-
tionality. As a consequence, attack at the sulfoxide site
does not necessarily reflect the nucleophilicity of an
oxygen donor (peroxide) but rather that of the oxygen
Resu lts
The oxidation conditions, product compositions, and
1
b
XSO values with the new analytical protocol for the
various peracids are given in Table 1, together with
previously reported data for the two dioxiranes methyl-
(
trifluoromethyl)dioxirane (TFD) and dimethyldioxirane
X
Abstract published in Advance ACS Abstracts, December 15, 1996.
(DMD) (Table 1, entries 15 and 16). The data in Table 1
clearly confirm that the trifluoroperacetic acid (Table 1,
entry 1) is, as expected, the most electrophilic peracid
(1) (a) Adam, W.; Haas, W.; Sieker, G. J . Am. Chem. Soc. 1984, 106,
5
1
020-5022. (b) Adam, W.; Golsch, D. Chem. Ber. 1994, 127, 1111-
113.
(2) (a) Adam, W.; D u¨ rr, H.; Haas, W.; Lohray, B. B. Angew. Chem.
1
986, 98, 85-87; Angew. Chem., Int. Ed. Engl. 1986, 25, 101-103. (b)
Adam, W.; Golsch, D.; G o¨ rth, F. C. Chem. Eur. J . 1996, 2, 255-258.
(5) (a) Plesnicar, B. In The Chemistry of Peroxides; Patai, S., Ed.;
J ohn Wiley & Sons: New York, 1983; Part 2, p 543-545. (b) Drabowicz,
J .; Kielbasinski, P.; Mikolajczyk M. In The Chemistry of Sulphones
and Sulphoxides; Patai, S., Rappoport, Z., Stirling, C., Eds.; J ohn Wiley
& Sons: New York, 1988; pp 241-242. (c) Sawaki Y. In Organic
Peroxides; Ando, W., Ed.; J ohn Wiley & Sons: New York, 1992; pp
426-447.
(3) (a) Adam, W.; Lohray, B. B. Angew. Chem. 1986, 98, 185-186;
Angew. Chem., Int. Ed. Engl. 1986, 25, 188-189. (b) Adam, W.; Haas,
W.; Lohray, B. B. J . Am. Chem. Soc. 1991, 113, 6202-6208.
(4) Bonchio, M.; Conte, V.; De Conciliis, M. A.; Di Furia, F.;
Ballistreri, F. P.; Tomaselli, G. A.; Toscano, R. M. J . Org. Chem. 1995,
0, 4475-4480.
6
S0022-3263(96)00710-4 CCC: $14.00 © 1997 American Chemical Society

1
16 J . Org. Chem., Vol. 62, No. 1, 1997
Adam and Golsch
Sch em e 1
Ta ble 1. Oxid a tion a of Th ia n th r en e 5-Oxid e by P er oxid ic Oxid a n ts
b
products (%)
SOSO
entry
oxidant
F3CCO3H
F3CCO2H/UHP
MeCO3H
equiv of [O]
timed (min)
convn (%)
SSO2
cis-
trans-
SOSO2
XSOc
1
2
3
4
5
6
7
8
9
0.30
0.40
0.40
0.50
10
0.20
0.30
0.50
0.35
0.35
0.35
0.35
0.35
0.35
0.20
0.50
0.50
0.5
1 d
360
5 d
1 d
120
80
40
10
1
30
42
41
8.6
9.2
22
28
47
36
36
38
31
37
30
21
52
30
0.6
0.8
15
9.1
9.4
14
14
12
10
7.6
16
16
18
16
0.8
6.8
100
75
67
17
38
22
20
19
20
23
34
17
18
16
17
23
32
67
47
69
65
66
67
65
56
65
65
65
66
96
90
0
1.6
0
1.1
5.9
0
1.2
0.7
1.5
2.3
2.1
2.6
1.4
1.7
1.3
0.2
0.1
0
0.02
0.01
0.16
0.14
0.09
0.15
0.14
0.13
0.12
0.10
0.18
0.18
0.19
0.18
0.01
0.07
1.00
e
e
(CH3CO)2O/UHP
CCl3CN/H2O2f
p-MeOC4H6CO3H
C6H5CO3H
m-ClC6H4CO3H
p-NO2C6H4CO3H
3,5-(NO2)2C6H4CO3H
EtCO3H
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
360
360
360
360
i-PrCO3H
t-BuCO3H
(n-Bu)(Et)CHCH2CO3H
g
TFD
3.1
3.1
0
h
DMD
i
m-CPBA/NaOH
60
a
b
CH2Cl2/25 °C unless noted otherwise. HPLC analysis (RP-18, 64:34:2 MeOH/H2O/MeCN), error ( 0.5% of the stated values, normalized
c
d
on 100%. XSO is defined in Scheme 1; error ( 0.005 units. The reaction times were determined by the peroxide test (KI/starch paper).
e
Urea hydrogen peroxide adduct (UHP). f 1:20 CH2Cl2/MeOH was used as solvent. -78 °C. h -50 °C. H2O/CH2Cl2 as biphasic solvent
g
i
system; n-Bu4NOH as phase transfer catalyst.
examined herein. The fact that the CF
3
CO
2
H/UHP
2 2
9.2% conversion of SSO in 24 h with 10 equiv of H O ,
oxidant (Table 1, entry 2) possesses the same XSO value
within the experimental error) suggests also that in this
case CF CO H is the active oxygen transfer agent. When
compared to peracetic acid (entry 3), it is evident that
CF CO H is not only the more electrophilic and, thus,
more reactive, it is also the more selective in that it
almost exclusively oxidizes the sulfide site. Again, the
same XSO values (0.15 ( 0.01) for peracetic acid (Table
presumably because the rate of peroxyiminocarboxylic
acid formation is slow under these conditions.
For the substituted perbenzoic acids (Table 1, entries
(
3
3
6
-10) a similar reactivity behavior as for the aliphatic
3
3
derivatives is displayed, but the differentiation is much
less pronounced, i.e., only 0.05 XSO units, which is
definitely outside the experimental error ((0.005 units).
Thus, the p-methoxy derivative is the less electrophilic,
less reactive, and less selective species, while the 3,5-
dinitroperbenzoic acid (Table 1, entry 10) is more elec-
trophilic, more reactive, and also more selective in this
SO
1
1
, entry 3) and the acetic anhydride/UHP system (Table
, entry 4) establishes that also in the latter CH CO
3
3
H
is the active oxidant.
Also, the trichloroacetonitrile-hydrogen peroxide com-
bination (Table 1, entry 5) constitutes an electrophilic
oxidant, for which presumably the peroxyiminocarboxylic
set. In fact, in the Hammett plot of the X values a good
2
correlation (F ) -0.0987; r ) 0.991) was obtained [log
[XSO(X)/XSO(H)] versus σ(X); X are the substituents of the
perbenzoic acids (Table 1, entries 6-10)]. However, the
anion of m-CPBA (Table 1, entry 17), generated in the
H O/CH Cl biphasic medium with NaOH and tetra-
2 2
acid intervenes, produced by addition of the H O to the
nitrile functionality. Usually alkaline hydrogen peroxide
6
is employed for such oxidations, as the intermediary
2
2
2
peroxyiminocarboxylic acid serves as a mild epoxidizing
butylammonium hydroxide as phase transfer catalyst
agent. It should, however, be kept in mind that the CCl
3
-
gave exclusively SSO , the product of the nucleophilic
2
CN/H system is a rather ineffective oxidant, i.e., ca.
2
O
2
oxidation of SSO.
The steric demand in the peracid oxidation of SSO was
probed for the aliphatic peracids with the Et, i-Pr, t-Bu,
(
6) Hennig, H.-W.; Prescher, G.; Siegmeier, R. In Methoden der
Organischen Chemie (Houben-Weyl); Kropf, H., Ed.; Georg Thieme
Verlag: Stuttgart, 1988; Vol. E13, Organische Peroxo-Verbindungen,
pp 898 and 1276.
and (n-Bu)(Et)CHCH
in comparison to peracetic acid (Table 1, entry 3).
2
groups (Table 1, entries 11-14)

Peracid Oxidation of Thianthrene 5-Oxide
J . Org. Chem., Vol. 62, No. 1, 1997 117
Clearly, the steric effect is negligible since all five
derivatives possess not only nearly constant XSO values
of ca. 0.18, but also the cis-trans ratios of the bisulfoxide
SOSO are constant at 17:65 (Table 1, entries 3, 11, and
1
4).
Since the oxidation of SSO with peracids is conducted
under acidic conditions, it was relevant to check whether
there might be any isomerization of the trans- and the
cis-SOSO under the influence of acids [PM3 calculations
show that the cis isomer has a lower enthalpy of forma-
tion (19.9 kcal/mol) than the trans isomer (23.7 kcal/
mol)].2b Moreover, even redistribution of the oxygen
atoms between the various sulfur oxide products is
possible, which would falsify the product ratios. For
F igu r e 1. Electrophilic attack of peracids on the lone pairs
of the sulfide and sulfoxide functionalities.
example, it is known7 that sulfoxides are acylated by
trifluoroacetic anhydride at their nucleophilic oxygen
While the peri hydrogen atoms do encumber the attack
at the cis sulfide lone pair so that preferably trans-SOSO
is produced (Figure 1), the cis,trans-SOSO ratio remains
constant irrespective of the alkyl group attached to the
peracid carbon atom. Therefore, the similar XSO values
of nearly 0.2 (Table 1, entries 3 and 11-14) display
electronic effects in the ease of oxidation at the sulfide
versus sulfoxide sites for these electrophilic oxidants.
Since the electrophilic character of these aliphatic per-
atom and the electrophilically activated sulfoxide may
transfer its oxygen atom to organic substrates.7 Never-
theless, control experiments established that all the
reaction mixtures given in Table 1 are invariant in
product composition for a few days at room temperature,
thus making any isomerization after the oxidation rather
unlikely. Furthermore, neither dilute protic acids (acetic
4
acid, trifluoroacetic acid, HClO ) nor acetic anhydride did
isomerize the trans-SOSO to its cis isomer under the
applied conditions. However, when trifluoroacetic an-
hydride was added to the trans-SOSO solution, immedi-
ate isomerization was found and after several hours a
constant trans/ cis ratio of 36:64 was reached. We
propose that the isomerization occurs through a sulfurane
as was found in the reaction of chiral sulfoxides with
3 3
acids is expected to be nearly constant (except CF CO H,
Table 1, entry 1), the observed chemoselectivity must
arise from the distinct oxidative reactivity of the sulfide
and sulfoxide sites in the SSO probe, namely their
differentiated nucleophilicity.
4
As suggested for the metal peroxo complexes, also for
8
the electrophilic peracid oxidants the oxygen transfer
occurs by nucleophilic attack of the sulfide or the sulfox-
ide lone pair on the O-O bond of the peroxide. Since
sulfides are inherently more nucleophilic than sulfoxides,
for such typical electrophilic oxidants as peracids, sulfide
oxidation in SSO prevails and low XSO values (<0.2) are
observed. Unusual is the fact, however, that for the SSO
probe the relative oxidative reactivities of the sulfide
versus sulfoxide sites are so similar, i.e., only ca. 4-fold
(XSO ca. 0.2) compared to over 30-fold for comparable
acetic anhydride.
Of mechanistic interest is the deep violet color that is
immediately generated when trifluoroacetic anhydride is
added to a methylene chloride solution of SSO. The
absorption maximum at λ ) 550 nm and the character-
istic EPR spectrum (g ) 2.0081) establish that the
•
+
9-11
thianthrene radical cation (SS ) had been formed.
This radical cation is generally produced when thian-
9
threne is dissolved in fuming sulfuric acid, treated with
1
0
oxidants like nitrosonium tetrafluoroborate, or submit-
1
2
ted to electrolytic oxidation.11
sulfide/sulfoxide pairs. Since the two sulfur function-
alities are in conjugation in SSO, delocalization of a
sulfide lone pair onto the sulfoxide group (see structures
below) leads to a dipolar mesomeric structure, in which
Discu ssion
The present systematic examination of electronic and
steric effects in the peracid oxidation of thianthrene
5
-oxide (SSO) has revealed valuable information on the
oxidative reactivity of the SSO mechanistic probe for
nonmetal, peroxidic oxidants. Thus, the fact that the
aliphatic peracids (except the strongly electrophilic
F
3
CCO
trans-SOSO ratios of ca. 17:65 (Table 1, entries 3 and
1-14) convincingly demonstrates that steric effects do
not operate in this oxygen transfer. In view of the
established S 2-type mechanism for the oxygen transfer
3
H) all give XSO values within 0.17 ( 0.02 and cis,-
the sulfide site (sulfonium character) is now less nucleo-
philic and the sulfoxide one (sulfurane character) more
so. This moderates the relative reactivity of SSO toward
electrophilic oxidants, which constitutes an advantage for
this intramolecular mechanistic probe by responding
more sensitively to the electronic character in the oxygen
transfer process. Since the XSO values are accurately
determined by quantitative chromatographic analysis of
1
N
1
2
by peracids to sulfides, clearly the alkyl group is too
far removed to exercise steric effects (Figure 1).
(
7) Mancuso, A. J .; Swern, D. Synthesis 1981, 165-185.
(8) (a) Oae, S.; Kawai, T.; Furukawa, N.; Iwasaki, F. J . Chem. Soc.,
Perkin Trans. 2 1987, 405-411. (b) Oae, S.; Uchida, Y. Acc. Chem.
Res. 1991, 24, 202-208.
(
806. (b) Shine, H. J ., Dais, C. F., Small, R. J . J . Org. Chem. 1964, 29,
1-25.
(
2
the products SOSO and SSO , the oxidative reactivity
9) (a) Shine, H. J ., Piette, L. J . Am. Chem. Soc. 1962, 84, 4798-
and chemoselectivity of oxygen transfer agents can be
conveniently and reliably assessed by the SSO probe.
Superimposed on this nucleophilic differentiation of the
sulfide and sulfoxide oxygen atom acceptor are electronic
effects on the electrophilicity of the oxygen donor. This
is most clearly expressed in the XSO values (Table 1,
4
2
10) Boduszek, B.; Shine, H. J . J . Org. Chem. 1988, 53, 5142-5143.
11) Svanholm, U.; Parker, V. D. J . Chem. Soc., Perkin Trans. 2
(
1
973, 1594-1597.
12) Ballistreri, F. P.; Tomaselli, G. A.; Toscano, R. M.; Bonchio, M.;
Conte, V.; Di Furia, F. Tetrahedron Lett. 1994, 35, 8041-8044.
(

1
18 J . Org. Chem., Vol. 62, No. 1, 1997
Adam and Golsch
entries 1 and 3) for CF
3
CO
3
H (0.02) versus CH
3
CO
3
H
philic oxidants (HOO , RCO
3
-, carbonyl oxides, etc.) the
-
(
0.16), which confirm that the former is the more elec-
X
SO values are definitely larger than 0.5, and as XSO tends
to unity, the nucleophilicity of the oxidant increases. For
electrophilic oxidants, e.g., RCO H, dioxiranes, etc., the
SO values are definitely less than 0.5, and as XSO tends
trophilic oxidant. An appreciable differentiation of the
electrophilicities is also displayed for the dioxiranes TFD
3
(0.01, Table 1, entry 15) and DMD (0.07, Table 1, entry
X
1
6). As expected, the fluorinated dioxirane is definitely
to zero, the electrophilicity of the oxidants increases.
However, for such electrophilic oxidants both the sulfide
and sulfoxide functionalities are oxidized, respectively,
the more electrophilic oxidant. The electronic effects of
the perbenzoic acids (Table 1, entries 6-10), which fall
within the XSO range 0.10 for the dinitro derivative (Table
1
6
to SOSO and SSO
the peroxide bond. For these electrophilic oxidants the
SO values differentiate their relative electrophilic char-
2
by attack of the sulfur lone pair on
, entry 10) and 0.15 for the methoxy case (Table 1, entry
), are small, but the regular trend confirms the former
X
as the more and the latter as the less electrophilic one
in their attack on the lone pairs of the sulfide and
sulfoxide functionalities. Be this as it may, the m-CPBA
acter through the inherent nucleophilicity difference of
the sulfide and sulfoxide sites in SSO. Apparently, this
electrophilic oxygen transfer to the heteroatom is orbital-
controlled so that the more electrophilic oxidant is the
more reactive but also more chemoselective, i.e., prefer-
ential oxidation of SSO to SOSO, as confirmed for
1
b
anion (Table 1, entry 17), like the hydroperoxy anion
2
b
and carbonyl oxides, definitely is a nucleophilic oxidant.
For all of these almost exclusively SSO is oxidized to
SSO
2
by nucleophilic addition to the sulfoxide functional-
3 3 3 3
peracids (CF CO H versus CH CO H) or dioxiranes (TFD
ity rather than reaction at the lone pair.
versus DMD). Thus, the SSO probe (XSO value) is a
convenient and informative mechanistic tool to assess the
electronic nature of an oxygen transfer agent.
Of mechanistic significance is the general fact that for
all SSO oxidations reported herein the more electrophilic
oxidant is not only the more reactive, as would be
expected, but also the more chemoselective. This is
Exp er im en ta l Section
particularly evident for the pairs CF
, entry 1) versus CH CO H (0.16, Table 1, entry 3), TFD
0.01, Table 1, entry 15) versus DMD (0.07, Table 1, entry
6), and the 3,5-dinitro- (0.10, Table 1, entry 10) versus
3 3
CO H (0.02, Table
Gen er a l Asp ects. The HPLC analyses were carried out
as described elsewhere1b on an HPLC analytical system
equipped with a C-18 reversed-phase column by using the
1
3
3
(
1
3 2 3
ternary solvent mixture of CH OH/H O/CH CN (64:34:2) as
p-methoxy-substituted (0.15, Table 1, entry 6) perbenzoic
acids. For the latter pair, oxidation by the 3,5-dinitro
derivative is essentially instantaneous (ca. 1 min) and
the p-methoxy one requires ca. 2 h, yet the former is the
more selective! This clearly violates the reactivity-
selectivity principle (RSP), but numerous exceptions have
eluent. Detection was performed at λ ) 254 nm, and the
identification of the peaks was verified by their UV spectra
(250-450 nm) on a KONTRON detector 430. As internal
standard trans-1-phenyl-1-penten-3-one was added in chloro-
form after the reaction in the dark to avoid photoisomerization.
The EPR spectrum was recorded on a Bruker EPR 420
spectrometer.
been documented, especially for orbital-controlled reac-
All oxidations were run in distilled solvents (CH
, MeOH over Mg). Acetic acid and trifluoroacetic acid
were used without further purification.
Methylene chloride solutions of the peroxy acids RCO H (R
2 2
Cl over
tions.1
3a,b
Presumably, oxygen-transfer reactions to SSO
2 5
P O
with electrophilic oxidants are also orbital-controlled, and
their reactivity and selectivity are dictated by favorable
matching of HOMO (oxygen acceptor, SSO) and LUMO
3
) Et, i-Pr, t-Bu, (n-Bu)(Et)CHCH
2
-, Ph, p-MeOC
6 4
H -) were
prepared from their corresponding acid chlorides by treatment
(oxygen donor, peroxide) energies. As expected for such
1
5
with H
2
O
2
/NaOH according to reported procedures.
-, 3,5-(NO
were prepared from their corresponding
in MeSO H. The peroxide content of the
The
cases, the more electrophilic oxidant is the more reactive
and also more selective one, as nicely exhibited by the
solutions of the peracids with R ) p-NO
2
C
6
H
4
2 2
) -
C
6
H
3
- in CH
2
Cl
2
2
X
SO data in Table 1. Indeed, PM3 calculations on CH
CO H versus CF CO H, and DMD versus TFD, cor-
roborate this reactivity-selectivity trend.
tively, the nucleophilic oxidations with oxygen anions
3
-
16
acids with H
2
O
3
3
3
3
peracid solutions was determined by iodometry, and the
solutions were stored over at -20 °C.
Gen er a l P r oced u r es for th e Oxid a tion of Th ia n th r en e
5-Oxid e (SSO) w ith P er a cid s. To a solution of thianthrene
1
4
Alterna-
-
-
(
3
HOO , RCO , carbonyl oxides, etc.) are charge-con-
1
3c
5-oxide (23.2 mg, 0.100 mmol) in CH Cl2 (5 mL) was added
trolled.
2
the peracid stock solution (0.2-0.5 equiv). After the peroxide
was consumed (as tested by KI/starch paper), the residue was
analyzed by HPLC as described above without any workup.
The results are summarized in Table 1.
In conclusion, the sulfoxide entity in SSO can be
oxidized either by electrophilic oxidants (RCO H) through
2-type attack of the sulfoxide lone pair to the peroxide
3
S
N
-
bond or with nucleophilic oxidants (RCO
philic addition to the sulfoxide functionality to afford
SSO , both in competition with the electrophilic oxidation
at the sulfide site to produce SOSO. Thus, for nucleo-
3
) by nucleo-
Rea ction of Th ia n th r en e 5-Oxid e w ith UHP /F
3
C-
CO H. A solution of thianthrene 5-oxide (21.0 mg, 0.0904
2
2
mmol), F
0.0489 mmol) in CH
3
C-CO
2
H (103 mg, 0.904 mmol), and UHP (4.60 mg,
Cl (10 mL) was stirred at room temper-
2
2
ature. After 10 d the peroxide test was negative and the
sample was submitted to HPLC analysis as described above.
The results are summarized in Table 1.
(13) (a) Giese, B. Angew. Chem. 1977, 89, 162-173; Angew. Chem.,
Int. Ed. Engl. 1977, 16, 125-136. (b) Fukui, K. Fortschr. Chem.
Forsch. 1970, 15, 1-85. (c) Fleming, I. In Grenzorbitale und
Reaktionen organischer Verbindungen; VCH Verlagsgesellschaft:
Weinheim, 1988; pp 41-100.
Rea ction of Th ia n th r en e 5-Oxid e w ith UHP /(H
CO) O. A solution of thianthrene 5-oxide (25.2 mg, 0.109
mmol), acetic anhydride (5.00 mL), and UHP (6.54 mg, 0.0695
mmol) in CH Cl (11 mL) was stirred at room temperature (ca.
0 °C). After 8 d, CH Cl (20 mL) was added, and the solution
was washed with 10% aqueous NaHCO solution (10 mL) and
subsequently with water (2 × 20 mL). The organic layer was
washed with water (2 × 20 mL) and dried over MgSO . The
3
C-
2
(14) The LUMO of CH
3 3
CO H (0.29 eV) is significantly higher than
2
2
that of CF CO H (-1.06 eV), as expected for the electron-accepting
3
3
2
2
2
properties of the fluoro atoms of the latter, while in SSO the higher-
energy HOMO (-8.87 eV) is located on the sulfide and the HOMO-1
3
(
-9.30 eV) on the sulfoxide functionality. Thus, the lowest energy gap
pertains to the HOMO(sulfide)-LUMO(CF
implies higher reactivity as well as chemoselectivity for this
3 3
CO H) interaction, which
4
a
combination. The PM3 parameters were taken from: Stewart, J . J . P.
J . Comput. Chem. 1989, 10, 209-221; 1991, 12, 320-341. Software
package VAMP 5.0: Rauhut, G.; Chandrasekhar, J .; Alex, A.; Steinke,
T.; Clark, T. University of Erlangen-N u¨ rnberg, Germany, 1993.
(15) Ogata, Y.; Sawaki, Y. Tetrahedron 1967, 23, 3327-3332.
(16) Schneider, H.-J .; M u¨ ller, W. J . Org. Chem. 1985, 50, 4609-
4615.

Peracid Oxidation of Thianthrene 5-Oxide
J . Org. Chem., Vol. 62, No. 1, 1997 119
solvent was evaporated (30 °C/12 Torr), and the residue was
mmol) or HClO
4
(0.303 mL, 15 M, 4.55 mmol). The reaction
taken up in 5 mL of CH
HPLC analysis as described above. The results are sum-
marized in Table 1.
2
Cl
2
/ethanol (1:1) and submitted to
mixtures were directly injected into the HPLC, and their
trans/ cis ratio was determined. No isomerization of trans-
SOSO was found even after 2 days of treatment.
Rea ction of Th ia n th r en e 5-Oxid e w ith H
Thianthrene 5-oxide (23.2 mg, 0.0999 mmol) was dissolved in
CH Cl (0.5 mL) and methanol (10 mL). Trichloroacetonitrile
0.700 mL, 6.92 mmol) and a methanolic H solution (0.620
mL, 1.61 M, 0.999 mmol) were added at room temperature
and stirred. After 1 d, the peroxide was quenched with 0.5
mL of dimethyl sulfide. The solvent was evaporated (30 °C/
2 2 3
O /Cl C-CN.
Isom er iza tion of tr a n s-SOSO by Tr iflu or oa cetic An -
h yd r id e. To a solution of trans-SOSO (0.844 mg, 3.40 µmol)
2
2
in 5.50 mL of CH
2 3 2
Cl at 25 °C was added (F CCO) O (22.3 mg,
2
(
2 2
O
0
.108 mmol). After 26 h the reaction mixture was submitted
to the HPLC analysis and the trans/ cis ratio determined to
be 36:64, which was invariant after that time. Acetic anhy-
dride (541 mg, 5.30 mmol) did not cause any isomerization
under these conditions.
1
2 2
2 Torr), and the residue was taken up in 4 mL of CH Cl /
ethanol (1:1) and submitted to HPLC analysis as described
above. The results are summarized in Table 1.
Rea ction of Th ia n th r en e 5-Oxid e w ith m -Ch lor op er -
ben zoic Acid (m -CP BA)/Na OH. A methylene chloride solu-
tion of m-CPBA (0.302 mL, 0.153 M, 0.0463 mmol) was added
to a well-stirred solution of NaOH (95.0 mg, 2.38 mmol) and
n-Bu NOH (0.200 mL, 0.386 M, 0.0771 mmol) as phase-
4
transfer catalyst in water (1.00 mL) at ambient temperature.
To this well-stirred two-phase mixture was added a solution
Rea ction of SSO w ith Tr iflu or oa cetic An h yd r id e. To
a solution of SSO (0.790 mg, 3.40 µmol) in 5.50 mL of CH
at 25 °C was added a solution of trifluoroacetic anhydride (22.3
mg, 0.108 mmol) in CH Cl Immediately a violet color
2 2
Cl
2
2
.
appeared (λmax ) 550 nm) and an EPR spectrum showed the
9-11
characteristic signal at g ) 2.0081
radical cation.
for the thianthrene
Ack n ow led gm en t. We are grateful for generous
financial support from the Deutsche Forschungsgemein-
schaft (Schwerpunktprogramm “Peroxidchemie: Mech-
anistische und pr a¨ parative Aspekte des Sauerstofftrans-
fers”) and the Fonds der Chemischen Industrie. We
thank F. Kita for assistance with the PM3 calculations
and Dr. T. Linker for helpful discussions.
of thianthrene 5-oxide (20.0 mg, 0.0861 mmol) in CH
mL). After 1 h the peroxide test was negative, 3.00 mL of CH
Cl was added, and the organic layer was washed with water
2 2
Cl (3.00
2
-
2
(
2 × 10 mL) and separated. Small aliquots of the organic layer
were directly submitted to HPLC analysis as described above.
The results are summarized in Table 1.
Attem p ted Isom er iza tion of tr a n s-SOSO by Acid s. To
separate solutions of trans-SOSO (0.844 mg, 3.40 µmol) in 5.50
mL of CH
2
Cl
2
at 25 °C was added F
3
CCO
2
H (1.65 mL, 21.5
J O960710G