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N. Sofikiti et al. / Tetrahedron Letters 45 (2004) 1335–1337
1O2
Ph
O
O
S
O
+ Ph Se
O
O
Ar Se
O
O O
O O
O
O
S
Me
Me
S
Ar
Ar
Me
Me
Ar
Ar
O
Se
Ph
+
Ar
Me
Me
Ar
S
Se
S
Se
O
Se
O
Se
Me
Me
O
Me
Me
Ar
Ar
Ph
Ar
IV
O
Ar Se
O
O
S
Mechanism 1, Ar = p-ClPh
k
p-Me / kH = 1.0
k
k
p-MeO / kH = 3.1
p-Cl / kp-MeO = 1.4
k
p-Cl / kH = 3.8
p-Cl / kp-Me = 4.1
Ar
+
Me
Me
k
Me
Me
Scheme 3. Competition of aryl selenoxides in their co-photooxygen-
ation with (CH3)2S.
O
O O
O
Se
S
Ar
Ar
Me
Me
O
S
Ar
Ar
O
Se
O
O
Se
S
O
Me
Me
Ar
Ar
IV
O
O
S
unsuccessful. Upon mixing diphenyl selenoxide with
(PhO)3P, oxygen atom transfer occurs from the selen-
oxide to phosphite to form a selenide and phosphate.11
Ar
+
Ar Se
O
Mechanism 2, Ar = p-MeOPh
Me
Me
In a mechanistic study of the selenoxide trapping reac-
tion, we performed kinetic competition for oxidation
between diphenyl selenoxide and several aryl selenox-
ides12;13 with the sulfide being the limiting reagent. The
results are presented in Scheme 3. While bis(p-meth-
oxyphenyl) selenoxide (electron-donating substituent) is
3.1 times more reactive compared to diphenyl selenox-
ide, the bis(p-chlorophenyl) selenoxide (electron-with-
drawing substituent) is also 3.8 times more reactive than
Ph2SeO. These results were verified by performing
kinetic competition experiments between p-Cl- and
p-MeO-phenyl selenoxides; a relative rate of kCl/
kMeO ¼ 1.4, was determined.
O
Ar Se
O
S
O
O
S
Me
O
Me
Ar
Ar
Me
Me
O
Se
S
O
Ar
+
Se
Me
Me
O
IV
O
Ar
Ar
δ−
O
O
O
S
O
S
Ar
Ar
Me
δ+
δ+
Se
[3+2] pericyclic
cycloaddition
Ar
Ar
Me
Me
Se
Me
O
O
δ−
TS for Ar = p-ClPh
TS for Ar = p-MeOPh
Scheme 4. Proposed mechanisms for the trapping reaction of per-
sulfoxide by aryl selenoxides.
The surprising feature of these results is that both,
electron donors (MeO) and electron acceptors (Cl)
accelerate the trapping reaction. This can be explained
by invoking the two possible mechanisms (1 and 2)
shown in Scheme 4. Due to the electron withdrawing
substituent (Cl), the Se atom in bis(p-chlorophenyl)
selenoxide is more electrophilic than that of diphenyl
selenoxide. Therefore, nucleophilic attack from the
negatively charged oxygen of dimethyl persulfoxide to
the Se atom can be reasonably assumed to occur,
forming the five-membered ring intermediate IV
(mechanism 1), which in turn, collapses to the products.
On the other hand, in bis(p-methoxyphenyl) selenoxide,
the Se atom is significantly less electrophilic compared
to diphenyl selenoxide, while the oxygen atom of the
Se@O moiety is highly nucleophilic (see resonance
structures below). Thus, it is reasonable to assume that
ing a concerted [3+2] pericyclic cycloaddition to form
intermediate IV, as shown in Scheme 4. The degree of
bond formation in the transition state for the formation
of IV depends on the nature of the aryl substituents. In
the trapping reaction of bis(p-methoxyphenyl) selenox-
ide, the new S–O bond has been substantially formed,
while the new Se–O bond is very loose and partially
formed. On the other hand, in the reaction of bis(p-
chlorophenyl) selenoxide, the Se–O bond has been
formed extensively, while the newly formed S–O bond is
very loose. We cannot differentiate between the two
mechanistic scenarios based on the current results.
In the next step, we examined the efficiency of aryl sel-
enides in trapping the intermediate(s) in the co-pho-
tooxygenation with dimethyl sulfide. Their sulfur
analogues (aryl sulfides) have been reported5 to be
rather poor trapping reagents. Aryl selenides are unreac-
1
tive with O2 in dichloromethane, even after prolonged
O
O
photooxygenation times (45 min), in accordance with
the recent observations by Krief and Lonez.14 Yet, in the
co-photooxygenation of 2 equiv of dimethyl sulfide with
1 equiv of diphenyl selenide, a mixture of diphenyl sel-
enone (major) and diphenyl selenoxide (minor) were
formed (ꢀ50–60% conversion of Ph2Se), indicative that
selenides also trap the intermediate(s). By using >4 equiv
of dimethyl sulfide relative to diphenyl selenide, almost
quantitative conversion to diphenyl selenone was
observed. This was also found for the case of bis(p-
chlorophenyl) and bis(p-methoxyphenyl) selenides,
which required approximately 4–5 equiv of dimethyl
sulfide to achieve complete oxidation to selenones. It is
Se
Se
MeO
OMe
MeO
OMe
nucleophilic attack from the oxygen atom of selenoxide
to the electrophilic sulfur atom of persulfoxide is taking
place (mechanism 2, in Scheme 4). Again, intermediate
IV is formed, which collapses to the final products. The
relative participation of each mechanism depends on the
nature of the aryl substituents, with donors favoring
mechanism 2 and acceptors favoring mechanism 1. An
alternative rationalization can be envisioned, consider-
Sofikiti, Nikoletta
