Photodeoxygenation and α-cleavage reactions of selenoxides

Article abstract of DOI:10.1246/bcsj.76.201

Upon UV-irradiation, diphenyl and dibenzyl selenoxides underwent deoxygenation and α-cleavage competitively to give selenides, and diselenides and others, respectively. A mechanism of bimolecular photodeoxygenation is proposed, since the yields of the photoproducts were dependent on the initial concentration of the selenoxides.

Full text of DOI:10.1246/bcsj.76.201

c. Jpn.
Bull. Chem. Soc. Jpn., 76, 201–202 (2003)
201
e Chemical
ciety of Ja-
n
Short Articles
e Chemical
ciety of Ja-
n
α
Photodeoxygenation and -Cleavage
Reactions of Selenoxides
Y. Yamazaki et al.
Yuko Yamazaki, Takahiro Tsuchiya,
and Tadashi Hasegawa*
03
1
2
Department of Chemistry, Tokyo Gakugei University,
4-1-1, Nukuikitamachi, Koganei, Tokyo 184-8501
SJA8
09-2673
2728
(Received May 21, 2002)
Scheme 1.
elimination of the oxygen atom from 1. If phenol is formed
through the reaction of an oxygen atom with solvent benzene,2
phenol should not be formed when 1a is irradiated in solvents
other than benzene. When 1a was irradiated in acetonitrile,
nearly the same amount of phenol as that in benzene was
formed. Therefore, phenol was not produced by the oxygen-
ation of solvent benzene, and atomic oxygen was not formed in
the photodeoxygenation of 1.
02
02
Upon UV-irradiation, diphenyl and dibenzyl selenox-
ides underwent deoxygenation and α-cleavage competitively
to give selenides, and diselenides and others, respectively. A
mechanism of bimolecular photodeoxygenation is proposed,
since the yields of the photoproducts were dependent on the
initial concentration of the selenoxides.
.1
.2
3.1
The formation of 2 may be explained in terms of the recom-
bination of a radical pair formed through α-cleavage.3 Ac-
cording to this mechanism, cross-coupled selenides must be
formed when a mixture of two kinds of selenoxides is irradiat-
ed. However, only a small amount of cross-coupling selenides,
benzyl phenyl selenides, was detected when a mixture of 1a
and 1b was irradiated in benzene. This result indicates that α-
cleavage is not the main process for producing 2.
The initial concentration of the starting material shows little
influence on the product distribution in unimolecular photore-
actions of most organic compounds. However, the yields of
the photoproducts from 1 depended on the initial concentration
of 1. The ratio of the yield of the selenide 2a to that of the di-
selenide 3a, Y2a/Y3a, increased along with an increase in the
initial concentration of 1a (Table 1). The variation of Y2a/Y3a
indicates that the photodeoxygenation of the selenoxide pro-
ceeded not unimolecularly, but bimolecularly.
The photochemistry of organic sulfur compounds has been
well-studied. From a mechanistic point of view, the photode-
oxygenation of sulfoxides is particularly interesting,1 since
three fundamentally different mechanisms have been proposed
for the deoxygenation.2–4 The photochemical reactions of
selenides5 and diselenides6 have been studied compared with
that of the corresponding organic sulfur compounds. However,
only a few papers have been reported on the photochemistry of
selenoxides.7 We report here on the photochemical behavior of
selenoxides.
The irradiation of diphenyl selenoxide (1a, 2.1 × 10−2 mol
dm−3) in benzene under nitrogen with a high-pressure mercury
lamp gave diphenyl selenide (2a), diphenyl diselenide (3a), bi-
phenyl (4a), and phenol (5a) in 52, 48, 25, and 15% yields, re-
spectively, at 6% conversion of 1a. The irradiation of dibenzyl
selenoxide (1b, 2.5 × 10−2 mol dm−3) under similar condi-
tions gave dibenzyl selenide (2b), dibenzyl diselenide (3b),
and benzaldehyde (6b) in 64, 32, and 38% yields, respectively,
together with a trace of 1,2-diphenylethane (4b) and benzyl
alcohol (5b) at 17% conversion of 1b. Only a trace of 4a was
formed when 1a was irradiated in acetonitrile, while d5-
biphenyl (m/e 159) was detected when 1a was irradiated in d6-
benzene. These results indicate that α-cleavage of 1a occurs to
give phenyl radicals, a great majority of which then react with
benzene to yield 4a. The recombination of α-cleavage frag-
ments, the phenyl or benzyl radical and the seleninyl radical, in
a solvent cage would give the selenenic ester RSeOR, as in the
case of sulfenic ester formation in the photoreaction of sulfox-
ides,1,8 though the selenenic ester could not be detected in
TLC, GC, LC, and NMR analyses. The formation of com-
pounds 3, 5, and 6 can be explained in terms of a secondary re-
action of the selenenic ester.9 The formation of the diselenide
3 can also be explained in terms of the decomposition of inter-
mediate 7, as shown in Scheme 1.
Four possible structures may be considered for the bimolec-
ular intermediate produced from two molecules of 1. How-
ever, structures Ⅱ and Ⅳ (Fig. 1), which contain a Se–O–Se–O
bond, are presumably excluded, because no selenones were
detected in the photoreaction of the selenoxide 1. Structure Ⅰ
seems to be plausible for the intermediate, though the possibil-
Table 1.
Dependence of Product Yields on the Initial
Concentration of 1a
a)
Initial concentration
Consumption
of 1a/%
Y2a
Y3a
of 1a/10−2 mol dm−3
2.1
6.2
10.0
6
5
8
1.1 (52/48)
1.6
2.6
a) The ratio of the yield of the selenide 2a to that of the di-
selenide 3a. The yields were calculated assuming that one
mole of 2a is formed from one mole of 1a and one mole of
3a from two moles of 1a.
The formation of 2 may be explained in terms of direct

202 Bull. Chem. Soc. Jpn., 76, No. 1 (2003)
© 2003 The Chemical Society of Japan
were used.
References
1
W. S. Jenks, D. D. Gregory, Y. Guo, W. Lee, and T.
Tetzlaff, in “Molecular and Supramolecular Photochemistry, Vol.
1, Organic Photochemistry,” ed by V. Ramamurthy and K. S.
Schanze, Marcel Dekker, New York (1997), p. 1.
Fig. 1.
ity of structure Ⅲ cannot be excluded. The sulfur analogue of I
was proposed in a sensitized photoreaction of sulfoxide.4c
In conclusion, selenoxides 1 undergo photodeoxygenation
competing with α-cleavage, and the photodeoxygenation pro-
ceeds through a bimolecular intermediate having a Se–O–O–
Se bond.
2
a) D. D. Gregory, Z. Wan, and W. S. Jenks, J. Am. Chem.
Soc., 119, 94 (1997). b) E. Lucien and A. Greer, J. Org. Chem.,
66, 4576 (2001).
3
R. Ludersdorf, I. Khait, K. A. Muszkat, K. Praefcke, and P.
Margaretha, Phosphorus and Sulfur, 12, 37 (1981); I. Khait, R.
Ludersdorf, K. A. Muszkat, and K. Praefcke, J. Chem. Soc.,
Perkins Trans. 2, 1981, 1417.
Experimental
4
a) M. S. E. F. El Amoudi, P. Geneste, and J.-L. Olive, J.
Org. Chem., 46, 4258 (1981). b) G. M. Gurria and G. H. Posner, J.
Org. Chem., 38, 2419 (1973). c) J. R. Shelton and K. E. Davis, Int.
J. Sulfur Chem., 8, 217 (1973).
IR and UV spectra were recorded with JASCO IR A-3 and
JASCO UV-505 spectrometers, respectively. 1H and 13C NMR
spectra were measured with a JEOL FX90Q spectrometer using
tetramethylsilane as an internal standard. GC-MS spectra were
measured with a Varian Saturn 2000R. An Ushio 450 W high-
pressure mercury lamp was used as an irradiation source. Selen-
oxides 1 were prepared according to a literature procedure.10
General Procedure for Irradiation of 1. A solution of selen-
oxide 1 in benzene or acetonitrile was irradiated for 2 h at 20 °C
under nitrogen with a 450 W high-pressure mercury lamp. A po-
tassium chromate filter solution was used to isolate the 313 nm
line. The starting material 1 and products 2–4 and 6 were ana-
lyzed using a Gasukuro Kogyo 570B high-pressure liquid chro-
matograph with a Model 511 single-wave UV detector (254 nm).
An Inertsil ODS-2 column (4.6 × 150 mm) was used, and a mix-
ture of acetonitrile–water (v/v = 1/1) was used as the moving
phase at a flow rate of 6 cm3 min−1. Naphthalene was used as a
calibrant for the analyses. Product 5 was determined on a
Shimadzu GC-4A gas chromatograph equipped with a flame ion-
ization detector using a 2 m column containing Apiezon Grease L
on Neopak 1A. In the photolysis of a mixture of 1a (λmMaexOH 222
5
See, for example: A. Ouchi and Y. Koga, J. Org. Chem., 62,
7376 (1997); I. Ryu, H. Muraoka, N. Kambe, M. Komatsu, and N.
Sonoda, J. Org. Chem., 61, 6396 (1996); J. H. Byers and G. C.
Lane, J. Org. Chem., 58, 3355 (1993).
6
See, for example: A. Ogawa, M. Doi, K. Tsuchii and T.
Hirao, Tetrahedron Lett., 42, 2317 (2001); A. Ogawa, R.
Obayashi, M. Doi, N. Sonoda, and T. Hirao, J. Org. Chem., 63,
4277 (1998).
7
a) T. Tezuka, H. Suzuki, and H. Miyazaki, Tetrahedron
Lett., 49, 4885 (1978). b) K. Kobayashi, S. Shinhara, M.
Moriyama, T. Fujii, E. Horn, A. Yabe, and N. Furukawa, Tetrahe-
dron Lett., 40, 5211 (1999).
8
N. Furukawa, Bull. Chem. Soc. Jpn., 70, 2571 (1997); Y.
Guo, A. P. Darmanyan, and W. S. Jenks, Tetrahedron Lett., 38,
8619 (1997); I. W. J. Still, in “The Chemistry of Sulphones and
Sulphoxides,” ed by S. Patai, Z. Rappoport and C. J. M. Stirling,
John Wiley & Sons Ltd. (1988) Chapt. 18, p. 873.
9
I. D. Entwistle, R. A. W. Johnstone, and J. H. Varley, J.
benzene
MeOH
benzene
Chem. Soc. Chem. Commun., 1976, 61.
10 M. R. Detty, J. Org. Chem., 45, 274 (1980).
(ε 11850), ε
= 24) and 1b (λ
232 (ε 19260), ε
=
313
max
313
10), 6.6 × 10−3 mol dm−3 of 1a and 1.6 × 10−2 mol dm−3 of 1b