Charge-Transfer Quenching of Excited Ketones by Selenides

Article abstract of DOI:10.1246/bcsj.76.2413

Diphenyl selenide (k_q = 2.5 × 10⁹ mol ^-1 dm³ s-1) and selenoxide (2.5 × 108) quenched the Type II reaction of butyrophenone through a charge-transfer process more efficiently than the corresponding sulfide (1.5 × 108) and sulfoxide. The irradiation of 2-(benzylseleno)ethyl benzoylacetate gave bis[2-(benzoylacetoxy)ethyl] diselenide and 1,2-diphenylethane.

Full text of DOI:10.1246/bcsj.76.2413

Short Articles
Bull. Chem. Soc. Jpn., 76, 2413–2414 (2003) 2413
ꢀ
Because the encounter distance (a) is 0.7 nm and the relative
ꢁ
2
9
6:48e N
1
2
Charge-Transfer Quenching
of Excited Ketones by Selenides
C ¼
ꢂ
ð2Þ
4
p"0a
"
37:5
5
6
dielectric constant (") is 2.292 for benzene, the C value for
ꢂ1
benzene is 75.98 kJ mol . The reduction potential (EredðAÞ)
Yuko Yamazaki, Takayuki Nakamura,
ꢀ
of butyrophenone was assumed to be the same as that of valer-
7
and Tadashi Hasegawa
ophenone (ꢂ2:07 V). Because the Typebphotoreaction of bu-
ꢀ
the excitation energy E in Eq. 1 has been proposed to be the
8
tyrophenone proceeds from its n,ꢁ triplet excited state and
ꢀ
triplet energy of ketone in the excited CT complex formation
Department of Chemistry, Tokyo Gakugei University,
4
-1-1 Nukuikitamachi, Koganei, Tokyo 184-8501
9
between a ketone and an electron donor, the triplet energy of
ꢂ1
8
Received May 7, 2003; E-mail: tadashi@u-gakugei.ac.jp
butyrophenone in benzene (308.4 kJ mol ) was used for the
excitation energy. The oxidation potential (EoxðDÞ) of diphenyl
selenide and sulfide were determined to be 1.14 and 1.51 V, re-
spectively, from their cyclic voltammograms. The free-energy
changes (ꢀG) in the CT quenching of the Typebreaction of
9
ꢂ1
3
ꢂ1
Diphenyl selenide (kq ¼ 2:5 ꢁ 10 mol dm s ) and
8
selenoxide (2:5 ꢁ 10 ) quenched the Typebreaction of bu-
tyrophenone through a charge-transfer process more efficient-
8
butyrophenone by diphenyl selenide and sulfide in benzene
ꢂ1
ly than the corresponding sulfide (1:5 ꢁ 10 ) and sulfoxide.
were estimated to be ꢂ74:64 and ꢂ38:95 kJ mol
,
respectively. Dibutyl sulfide quenches triplet butyrophenone
The irradiation of 2-(benzylseleno)ethyl benzoylacetate gave
bis[2-(benzoylacetoxy)ethyl] diselenide and 1,2-diphenyl-
ethane.
2
via a CT process. Because the oxidation potential of the sul-
1
fide is ca. 1.8 V, ꢀG for the CT quenching by the sulfide is ca.
ꢂ1
ꢂ10:9 kJ mol . These results support the idea of CT quench-
ing of the Type b reaction by diphenyl selenide and sulfide
(Table 1).
Quenching of excited molecules is an important process in
organic photochemistry. Sulfides quench excited ketones via
The kqꢀ and kq values of diphenyl selenoxide were also de-
1,2
ꢂ1
3
8
ꢂ1
3
ꢂ1
,
a charge-transfer (CT) process.
Little attention has been
termined to be 33 mol dm and 2:5 ꢁ 10 mol dm s
focused on the quenching ability of simple selenides and
selenoxides. We report here on the CT quenching of photo-
reactions of ketones by selenides.
When diphenyl selenide was used as a quencher, a Stern–
Volmer plot for the Typebreaction of butyrophenone in ben-
zene showed a linear relationship with a slope of kqꢀ (326
respectively. Because the oxidation potential of the selenoxide
was 2.05 V, ꢀG in the quenching of the Typebreaction was
ꢂ1
estimated to be 13.18 kJ mol . The triplet energy of the selen-
ꢂ1
oxide was determined to be 351.9 kJ mol from its phosphor-
escence spectrum measured at 77 K in EPA. This value is 39.4
ꢂ1
ꢂ1
kJ mol higher than the triplet energy of butyrophenone (312.5
ꢂ1
3
10
kJ mol in EPA at 77 K). Each 5.9 kJ mol of endothermic-
ꢂ1
mol dm ). The quenching rate constant (kq) was calculated
9
ꢂ1
3
ꢂ1
ꢄ
to be 2:5 ꢁ 10 mol dm s , assuming that the lifetime (ꢀ)
ity in an energy-transfer step occurring at near 25 C slows
ꢀ
ꢂ7
3
of the n,ꢁ triplet state of butyrophenone is 1:3 ꢁ 10 s.
The kqꢀ and kq values for diphenyl sulfide were also determined
down the rate by a factor of 10, and essentially no quenching
of donor triplets occurs when the triplet energy of a donor is
less than that of an acceptor by more than several times of
ꢂ1
3
8
ꢂ1
3
ꢂ1
to be 20 mol dm and 1:5 ꢁ 10 mol dm s , respectively.
Because the ionization potential of selenium is smaller than that
of sulfur, quenching by the selenide would involve the CT
process.
ꢂ1 11
4.2 kJ mol
phenyl selenoxide involves a CT process.
.
These results indicate that the quenching by di-
Intramolecular CT quenching would occur in a molecule
having both ketone and selenide chromophores, and proton
transfer may follow the CT interaction. We recently reported
that 2-(alkylthio)ethyl benzoylacetates underwent photocycli-
The feasibility of the CT interaction can be estimated by us-
4
ing the Rehm–Weller equation:
ꢀ
ꢀ
G ¼ ꢂE þ 96:48 fEoxðDÞ ꢂ EredðAÞg ꢂ C
ð1Þ
The term C, for taking into account the electrostatic energy in a
12
ꢃ
solvent, can be calculated from the following equation:
zation via remote proton transfer. However, the irradiation
of 2-(benzylseleno)ethyl benzoylacetate (1) in benzene did
not give cyclization products (Scheme 1). The oxidation poten-
5
Table 1. Kinetic Parameters for the TypebReaction of Butyrophenone, Oxidation Potential of Quenchers, and
Free Energy Change for the CT Quenching of the TypebReaction
aÞ
ꢂ1
3
9
ꢂ1
3
ꢂ1
ꢂ1
ꢀG/kJ mol
Quenchers
kqꢀ /mol dm
kq=10 mol dm sec
Eox/V
bÞ
2
,5-Dimethyl-2,4-hexadiene
Diphenyl selenide
Diphenyl sulfide
Diphenyl selenoxide
Diphenyl sulfoxide
Dibutyl sulfide
660
5.1
2.5
0.15
0.25
0.0015
0.35
326
20
33
1.14
1.51
2.05
2.10
ꢂ74:64
ꢂ38:95
13.18
17.99
ꢂ10:9
cÞ
0.2
cÞ
dÞ
45
ca. 1.8
a) In benzene. b) Ref. 3 (ꢀ ¼ 1:3 ꢁ 10ꢂ7 s). c) Ref. 2. d) Ref. 1.
Published on the web December 15, 2003; DOI 10.1246/bcsj.76.2413

2
414 Bull. Chem. Soc. Jpn., 76, No. 12 (2003)
ꢀ 2003 The Chemical Society of Japan
Scheme 1.
3
tial of 1 was determined to be 1.52 V from its cyclic
voltammogram. This value is nearly equal to that of diphenyl
sulfide, and indicates that the intramolecular CT interaction in 1
is energetically allowed to occur. Back electron transfer from
the ketyl anion radical part to the selenide cation radical part in
the CT intermediate produced from 1 should occur much faster
than intramolecular proton transfer.
In conclusion, selenides act as more effective CT quenchers,
but less efficient reducing agents than the corresponding sul-
fides.
solution of 1 (0.53 g, 1.5 mmol) in 50 cm of benzene was placed
in a Pyrex vessel and irradiated for 15 h under nitrogen with a 450-
W high-pressure mercury lamp. After removal of the solvent, the
residue was chromatographed on a silica-gel column. Elution with
hexane–acetone (v/v = 3/1) gave 0.10 g of the unreacted starting
material 1, 0.30 g (93%) of bis[2-(benzoylacetoxy)ethyl] disele-
nide (2) and 0.06 g (58%) of 1,2-diphenylethane (3). The structure
of 3 was determined by a direct comparison with a commercially
ꢄ
available material. 2: mp 53.5–54.5 C; IR (KBr) 1620, 1680,
ꢂ1
1
and 1740 cm ; H NMR (CDCl3) ꢂ 3.09 (3.2H, t, J ¼ 7:1 Hz, 2
ꢁ
SeCH2), 3.14 (0.8H, t, J ¼ 6:2 Hz, 2 ꢁ SeCH2), 4.01 (3.2H,
s, 2 ꢁ CH2), 4.43 (3.2H, t, J ¼ 7:1 Hz, 2 ꢁ OCH2), 4.44 (0.8H,
t, J ¼ 6:2 Hz, 2 ꢁ OCH2), 4.50 (0.4H, s, C=CH), 7.3–7.5 (6H,
m, aromatic), 7.9–8.0 (4H, m, aromatic), and 12.42 (0.4H, s,
OH). Found: C, 48.68; H, 4.11%. Calcd for C22H22O6Se2: C,
Experimental
The IR spectra were recorded with a JASCO IR Report-100
1
spectrometer. The H NMR spectra were measured with a JEOL
FX-90Q spectrometer using tetramethylsilane as an internal
standard. An Ushio 450-W high-pressure mercury lamp was used
as an irradiation source. Phosphorescence spectra were recorded
with a HITACHI F4500 spectrometer.
4
8.90; H, 4.10%.
Cyclic Voltammetry. A three-electrode arrangement with the
saturated calomel electrode (SCE) as a reference, a platinum coil as
a counter electrode, and a platinum disc as a working electrode was
used. Dry acetonitrile containing 1 mol dm of Et4NClO4 was
used as the supporting electrolyte solution. Experiments were per-
Quenching Rate Determination. Benzene solutions of buty-
ꢂ3
ꢂ3
rophenone (ca. 0.05 or 0.1 mol dm ) containing a known concen-
ꢂ3
tration of hexadecane (ca. 0.005 mol dm ) as a calibrant and ap-
ꢄ
formed at 25 C in solutions deaerated by passing argon. The peak
ꢂ1
propriate concentrations of a quencher were placed in 150 ꢁ 15
mm Pyrex culture tubes. The tubes were degassed by three
freeze-pump-thaw cycles and then sealed. Irradiation was per-
formed on a ‘merry-go-round’ apparatus with an Ushio 450-W
high-pressure mercury lamp. A potassium chromate filter solution
was used to isolate the 313 nm line. Analyses of the irradiated
samples were performed on a Shimadzu GC-8A gas chromato-
graph equipped with a flame ionization detector, which was con-
nected to a Shimadzu C-R6A Chromatopac integrator, using a 2
m column containing 15% propylene glycol succinate on Uniport
B.
potential values are those obtained at a sweep rate of 100 mV s
.
References
1
(1973).
2
3057 (1987).
3
7495 (1972).
J. B. Guttenplan and S. G. Cohen, J. Org. Chem., 38, 2001
P. J. Wagner and M. J. Lindstom, J. Am. Chem. Soc., 109,
P. J. Wagner and A. E. Kemppainen, J. Am. Chem. Soc., 94,
4
5
D. Rehm and A. Weller, Isr. J. Chem., 8, 259 (1970).
J. Mattary, J. Runsink, J. Gersdorf, T. Rumbach, and C. Ly,
Preparation of 2-(Benzylseleno)ethyl Benzoylacetate (1).
Ethyl 2-benzoylacetate (0.97 g, 5 mmol), 2-benzylselenoethanol
Helv. Chim. Acta, 69, 442 (1986).
J. A. Dean, ‘‘Lange’s Handbook of Chemistry,’’ 15th ed,
McGraw Hill, New York (1999), 5.105.
7
(1981).
8
(
were dissolved in 20 dm of benzene. The mixture was placed
1.14 g, 5 mmol), and a small amount of p-toluenesulfonic acid
3
6
3
in a 50-dm round-bottomed flask and refluxed for 67 h. After re-
moval of the solvent under reduced pressure, the residue was chro-
matographed on a silica-gel column. Elusion with hexane–acetone
P. J. Wagner and E. J. Siebert, J. Am. Chem. Soc., 103, 7329
P. J. Wagner, M. J. Thomas, and E. Harris, J. Am. Chem.
(
(
(
v/v = 8/1) gave 1.60 g (84%) of 1 as a pink viscous oil. 1: IR
Soc., 98, 7675 (1976).
9 J. B. Guttenplan and S. G. Cohen, J. Am. Chem. Soc., 94,
4040 (1972).
10 J. N. Pitts, Jr., D. R. Burley, J. C. Mani, and A. D.
Broadbent, J. Am. Chem. Soc., 90, 5902 (1968).
11 N. J. Turro, ‘‘Modern Molecular Photochemistry,’’ Benja-
min/Cumming, Menlo Park (1978), Chap. 9.
12 Y. Yamazaki, T. Miyagawa, and T. Hasegawa, J. Chem.
Soc., Perkin Trans. 1, 1997, 2979.
neat) 1625, 1685, and 1740 cm ; 1H NMR (CDCl3) ꢂ 2.65
ꢂ1
1.4H, t, J ¼ 6:6 Hz, SeCH2), 2.73 (0.6H, t, J ¼ 6:2 Hz, SeCH2),
3
.79 (1.4H, s, CH2), 3.98 (2H, s, CH2Ph), 4.20 (0.6H, t, J ¼ 6:2
Hz, OCH2), 4.28 (1.4H, t, J ¼ 6:6 Hz, OCH2), 5.65 (0.3H, s,
C=CH), 7.3–7.8 (8H, m, aromatic), 7.9–8.1 (2H, m, aromatic),
and 12.46 (0.3H, s, OH). Found: C, 59.66; H, 5.00%. Calcd for
C18H18O3Se: C, 59.84; H, 5.02%.
Irradiation of 2-(Benzylseleno)ethyl Benzoylacetate (1).
A