Factors controlling photochemical cleavage of the energetically unfavorable Ph-Se bond of alkyl phenyl selenides

Article abstract of DOI:10.1021/jo701447a

(Chemical Equation Presented) Primary photochemical paths of alkyl phenyl selenides (1) were investigated, and an origin of large deviations in the chemical yields of products obtained by carbon radical reactions induced by photolysis of phenyl selenides was clarified. KrF excimer laser photolyses of n-pentyl phenyl selenide (1a) yielded 1-pentene (2a), n-pentane (3a), n-decane (4a), dipentyl selenide (5a), benzene (6), dipentyl diselenide (7a), and diphenyl diselenide (7) as major photoproducts, with compounds 2a, 3a, 4a, 5a, and 7 formed by pentyl-Se bond cleavage, and 5a, 6, and 7a by Ph-Se bond cleavage. The selectivity of the photoproducts revealed the occurrence of an unexpected amount of Ph-Se bond cleavage (35% in n-hexane at 248 nm) during photolysis. Solvent viscosity, wavelength of light, and the structure of alkyl substituents were the major factors that controlled Ph-Se bond cleavage. The ratio of Ph-Se bond cleavage decreased with increasing solvent viscosity and laser wavelength. The effect of alkyl substituents on the ratio of bond cleavages, Ph-Se/total C-Se, was investigated for five alkyl phenyl selenides; the ratio decreased in the order pentyl > 2-methylallyl > allyl > 1-ethylpropyl > tert-butyl groups. The contribution of Ph-Se bond cleavage is most probably the origin of the large deviations in the yields of radical reactions induced by photolyses of 1, which can be minimized by selecting appropriate solvents and wavelength of light.

Full text of DOI:10.1021/jo701447a

Factors Controlling Photochemical Cleavage of the Energetically
Unfavorable Ph-Se Bond of Alkyl Phenyl Selenides
Akihiko Ouchi,*,†,‡ Suyou Liu,†,§ Zhong Li,†,§ S. Ajaya Kumar,†,§ Toshiaki Suzuki,†
Takeshi Hyugano,‡ and Haruo Kitahara¶
National Institute of AdVanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565,
Japan, the UniVersity of Tsukuba, Ibaraki 305-8571, Japan, and Faculty of Education, Hirosaki
UniVersity, Hirosaki 036-8560, Japan
ouchi.akihiko@aist.go.jp
ReceiVed July 4, 2007
Primary photochemical paths of alkyl phenyl selenides (1) were investigated, and an origin of large
deviations in the chemical yields of products obtained by carbon radical reactions induced by photolysis
of phenyl selenides was clarified. KrF excimer laser photolyses of n-pentyl phenyl selenide (1a) yielded
1-pentene (2a), n-pentane (3a), n-decane (4a), dipentyl selenide (5a), benzene (6), dipentyl diselenide
(7a), and diphenyl diselenide (7) as major photoproducts, with compounds 2a, 3a, 4a, 5a, and 7 formed
by pentyl-Se bond cleavage, and 5a, 6, and 7a by Ph-Se bond cleavage. The selectivity of the
photoproducts revealed the occurrence of an unexpected amount of Ph-Se bond cleavage (35% in n-hexane
at 248 nm) during photolysis. Solvent viscosity, wavelength of light, and the structure of alkyl substituents
were the major factors that controlled Ph-Se bond cleavage. The ratio of Ph-Se bond cleavage decreased
with increasing solvent viscosity and laser wavelength. The effect of alkyl substituents on the ratio of
bond cleavages, Ph-Se/total C-Se, was investigated for five alkyl phenyl selenides; the ratio decreased
in the order pentyl > 2-methylallyl > allyl > 1-ethylpropyl > tert-butyl groups. The contribution of
Ph-Se bond cleavage is most probably the origin of the large deviations in the yields of radical reactions
induced by photolyses of 1, which can be minimized by selecting appropriate solvents and wavelength
of light.
Introduction
selenides undergo preferential cleavage of alkyl-Se rather than
Ph-Se bonds by photolysis because C-Se bond dissociation
energy is greater for Ph-Se than for alkyl-Se bonds.2 On the
basis of this belief, a number of reactions using photochemical
PhSe-alkyl bond cleavage have been reported for various alkyl
phenyl selenides,3 but the photochemical selectivity of Ph-Se
and alkyl-Se bond cleavage has not been investigated system-
atically. Indeed, large deviations in the yields of expected
products are often observed in these reactions, for which the
reason has not been clarified so far.
The phenylselenyl group is a useful functional group that is
often used in organic synthesis. One of the applications of the
phenylselenyl group is C-C bond formation through radical
reactions, in which carbon radicals are generated by PhSe-C
bond cleavage.1 It is generally believed that alkyl phenyl
† AIST.
‡ University of Tsukuba.
¶ Hirosaki University.
§ Present affiliations: Central South University, P. R. China (S.L.), East China
University of Science and Technology, P. R. China (Z.L.), St. Mary’s College,
India (S.A.K.), and RIKEN Wako Institute, Japan (T.S.).
(1) (a) Martens, J.; Praekcke, K. J. Organomet. Chem. 1980, 198, 321-
351. (b) Goldschmidt, Z. In The Chemistry of Organic Selenium and
Tellurium Compounds; Patai, S., Ed.; John Wiley & Sons: Chichester, 1987;
Vol. 2, Chapter 5. (c) Schiesser, C. H.; Wild, L. M. Tetrahedron 1996, 52,
13265-13314. (d) Renaud, P. Top. Curr. Chem. 2000, 208, 81-112.
(2) Dissociation energies of C-Se bonds are 59.9 kcal‚mol-1 for Et2Se
and 71 kcal‚mol-1 for Ph2Se: Batt, L. In The Chemistry of Organic Selenium
and Tellurium Compounds; Patai, S., Rappoport, Z., Eds.; John Wiley &
Sons: Chichester, 1986: Vol. 1, p 159.
10.1021/jo701447a CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/16/2007
8700
J. Org. Chem. 2007, 72, 8700-8706

Photochemical CleaVage of UnfaVorable Ph-Se Bond
SCHEME 1. The Two Primary Photochemical Paths of 1
To understand the origin of the large deviations in the yields
and to improve the synthetic efficiency of the reactions, it is
necessary to understand the photochemical reactivity of the
selenides. Therefore, we conducted a detailed study of the
photolysis of simple alkyl phenyl selenides: n-pentyl (1a),
1-ethylpropyl (1b), tert-butyl (1c), allyl (1d), and 2-methylallyl
(1e) phenyl selenides (Scheme 1). To the best of our knowledge,
systematic studies on the photochemical reactivity of C-Se bond
cleavage of selenides have not been conducted. In the present
study, we found a significant amount of Se-Ph bond cleavage
(Path B in Scheme 1) in addition to the normal alkyl-SePh
bond cleavage (Path A in Scheme 1), the ratio of which was
dependent on the wavelength of light, solvent viscosity, and
the structure of alkyl groups. On the basis of these results, the
synthetic efficiency of the reactions of organoselenium com-
pounds can be increased by selecting suitable reaction conditions
revealed in the present study.
FIGURE 1. Absorption spectra of 1a-e. 1a (red); 1b (orange); 1c
(purple); 1d (green); 1e (blue). Concentration: 10-4 M in n-hexane,
optical path: 10 mm. The wavelengths of the ArF, KrF, and XeCl
excimer laser emissions are marked in the figure.
Results and Discussion
Absorption Spectra of 1a-e. Figure 1 shows the absorption
spectra of 1a-e. The figure shows the presence of four
absorption bands for each compound, which are attributed to
the PhSe moiety of the molecules. The photolyses were
conducted with ArF (193 nm), KrF (248 nm), and XeCl (308
nm) excimer lasers, with emissions corresponding to the
different absorption bands of 1. The absorption at 308 nm
corresponds to a secondary absorption band of a phenyl ring
(π-π*, 1Lb),4 that at 248 nm to π-π* (1Lb)4 absorption and to
an n-π* transition of the Se atom,5 and that at 193 nm to higher
π-π* transitions.
KrF Excimer Laser Photolysis of 1a. First, photolysis of
1a was conducted with a KrF excimer laser. Scheme 2 shows
the major photoproducts of 1a in n-hexane at room temperature.
The product distribution during KrF excimer laser photolysis
is shown in Figure 2. With the decrease in 1a, photoproducts
2a-5a, 6, 7, and 7a were formed with different selectivity.6 A
significant increase in the selectivity of 6 and a decrease in 7
were observed as photolysis progressed, in contrast to small
FIGURE 2. KrF excimer laser photolysis of 1a. Decrease in 1a
(magenta b) and selectivity6 of 1-pentene (2a, black 0), n-pentane (3a,
red O), n-decane (4a, green 3), dipentyl selenide (5a, turquoise ]),
benzene (6, blue 4), diphenyl diselenide (7, brown "), and dipentyl
diselenide (7a, orange <) as a function of the number of the laser
shots.14 Concentration: 10-3 M in n-hexane, optical path: 1 mm, laser
fluence: 100 mJ cm-2 pulse-1, room temperature.
changes in the selectivity of 2a-5a and 7a. The considerable
changes in 6 and 7 indicate the presence of a secondary
photolysis of 7 to form 6, which was confirmed by a KrF laser
photolysis of 7 in n-hexane.7
Figure 2 also shows a slight increase in the selectivity of 2a
and 3a, with a small decrease in the selectivity of 5a as
photolysis progressed. The small change in selectivity of 2a,
3a, and 5a can be explained by the generation of 2a and 3a via
secondary photolysis of 5a, which was confirmed by a KrF laser
photolysis of 5a in n-hexane.7
(3) (a) Byers, J. H.; Lane, G. C. J. Org. Chem. 1993, 58, 3355-3360.
(b) Renaud, P.; Vionnet, J.-P. J. Org. Chem. 1993, 58, 5895-5896. (c)
Curran, D. P.; Eichenberger, E.; Collis, M.; Roepel, M. G.; Thoma, G. J.
Am. Chem. Soc. 1994, 116, 4279-4288. (d) Ryu, I.; Muraoka, H.; Kambe,
N.; Komatsu, M.; Sonoda, N. J. Org. Chem. 1996, 61, 6396-6403. (e)
Back, T. G.; Gladstone, P. L.; Parrez, M. J. Org. Chem. 1996, 61, 3806-
3814. (f) Wackernagel, F.; Schwitter, U.; Giese, B. Tetrahedron Lett. 1997,
38, 2657-2660. (g) Ouchi, A.; Koga, Y. J. Org. Chem. 1997, 62, 7376-
7383. (h) Buers, J. H.; Shaughnessy, E. A.; Mackie, T. Heterocycles 1998,
48, 2071-2077. (i) Ouchi, A.; Sakuragi, M.; Kitahara, H.; Zandomeneghi,
M. J. Org. Chem. 2000, 65, 2350-2357. (j) Ouchi, A.; Li, Z.; Sakuragi,
M.; Majima, T. J. Am. Chem. Soc. 2003, 125, 1104-1108. (k) Murakami,
S.; Kim, S.; Ishii, H.; Fuchigami, T. Synlett 2004, 815-818. (l) Murakami,
S.; Ishii, H.; Fuchigami, T. J. Fluorine Chem. 2004, 125, 609-614. (m)
Hong, I. S.; Greenberg, M. M. Org. Lett. 2004, 6, 5011-5013. (n) Pandey,
G.; Gadre, S. R. Acc. Chem. Res. 2004, 37, 201-210.
Primary Products of KrF Excimer Laser Photolysis of 1a.
The formation of 2a, 3a, and 4a can be explained by the
reactions of pentyl radicals that were formed by the expected
primary photochemical cleavage of pentyl-Se bonds. However,
a considerable amount of benzene (6) was also surprisingly
formed, even after a single laser shot. This result indicates that
6 was also formed by a primary photochemical process via an
unexpected photochemical Ph-Se bond cleavage to generate
phenyl radicals and subsequent hydrogen abstractions because
the occurrence of secondary photolysis is unlikely within the
pulse duration of the KrF laser (23 ns, fwhm). Furthermore,
the possibility of the formation of phenyl radicals by thermal
decomposition of photochemically generated PhSe radicals can
be also excluded because PhSe radicals are reported to undergo
(4) Jaffe´, H. H.; Orchin, M. Theory and Application of UltraViolet
Spectroscopy; John Wiley & Sons: New York, 1962; Chapter 12, Section
12.2.
(5) Mason, W. R. J. Phys. Chem. 1996, 100, 8139-8143.
(6) Product selectivity is based on the consumption of 1.
(7) Original data are provided as Supporting Information.
J. Org. Chem, Vol. 72, No. 23, 2007 8701

Ouchi et al.
SCHEME 2. Major Photoproducts of 1a
predominant recombination to 7.8 The formation of 5a, which
is a coupling product of pentyl and pentylselenyl radicals, also
supports the presence of primary Ph-Se bond cleavage.
Another possible primary process for the formation of 6 is a
reaction between photochemically generated pentyl radicals and
1a to form 5a and phenyl radicals (eq 1) and successive
hydrogen abstraction by the phenyl radicals. However, this
reaction path can be excluded from the major process because
7a, which cannot be formed by the process shown in eq 1, was
observed in the photolysis. In addition, Figure 2 shows that the
selectivity of diphenyl diselenide (7) and dipentyl diselenide
(7a) is 29 and 17%, respectively, and if we assume that the
efficiency for the formation of diselenides is the same between
7 and 7a, the ratio 29/17 ) 1.7 is similar to the ratio of pentyl-
Se and Ph-Se bond cleavage, 58/31 ) 1.9 (see Table 1).
were conducted with 1a-d11, for which the results are shown in
Figure 4. The slope for the yield of 6 is unity, but that for 6-d1
is 1.6. The result for 6-d1 indicates that 40% of 6-d1 was formed
by one-photon processes and 60% by a two-photon process.
The deuterium atom of the one-photon processes is most
probably abstracted from pentyl-d11-selenyl radicals, both inside
and outside of the solvent cages, and that of the two-photon
process from pentyl-d11 radicals. Possible deuterium abstraction
from the starting material 1a-d11 can be neglected because such
radicals generated from 1a-d11 would yield products that are
different from the primary products, while the mass balance of
the primary products was found to be 97.1% (vide supra).
From the selectivity of 6 of 30.9%, it was calculated that
hydrogen abstraction by phenyl radicals proceeded at levels of
28.4% from the solvent, 1.0% from pentylselenyl radicals, and
1.5% from pentyl radicals. The two-photon process observed
in this experiment is contradictory to the laser intensity results
TABLE 1. Selectivity of C-Se Bond Cleavages of 1a
selectivity of C-Se bond cleavage (%)b
ratiof
(Ph-Se/total)
light sourcea
pentyl-Sec
Ph-Sed
totale
ArF laser
(193 nm)
KrF laser
(248 nm)
XeCl laser
(308 nm)
38.5
25.0
63.5
0.39
0.35
0.29
The total selectivity of the major photoproducts from pentyl
and phenyl radicals generated by primary photochemical
processes, that is, the sum of the selectivity for 2a, 3a, 4a, 5a,
and 6 after one laser shot, was 89.3%, which indicates that most
of the primary photochemical processes can be explained by
analyses of the major products. However, the total selectivity
increases to 97.1% when the selectivity of minor products is
included, that is, pentylbenzene, undecane, and 2- and 4-pen-
tylphenyl phenyl diselenides.7
Formation of Benzene (6) in KrF Excimer Laser Pho-
tolysis of 1a. Details on the formation of 6 were further
investigated by experiments on the effect of laser intensity.
Figure 3 shows the consumption of 1a and the yields of 2a-4a
and 6 as a function of KrF laser fluence in a double-logarithmic
plot. The experiments were conducted with a single laser pulse
to avoid secondary photochemical processes. It is evident that
the slope for consumption of 1a and for yields of 2a, 3a, and
6 is unity, whereas that for the yield of 4a is 2. These results
indicate that the consumption of 1a and the formation of 2a,
3a, and 6 proceeded by one-photon processes, while the
formation of 4a is by a two-photon process. The fact that 6
was formed by a one-photon process is clear evidence that 6 is
a primary photoproduct formed by initial Ph-Se bond cleavage
and successive hydrogen abstraction by the phenyl radicals thus
generated.
58.4
68.4
30.9
27.6
89.3
96.0
a Laser photolysis conditions: 10-3 M 1a in n-hexane, optical path: 1
mm, room temperature; laser intensity: 1.25 × 1017 photons cm-2 pulse-1
,
number of laser shots: 1 shot (ArF and KrF) and 100 shots (XeCl). Original
data are provided as Supporting Information. b Product selectivity is based
on the consumption of 1, and results are the average of at least two
independent runs. c Selectivity of (2a + 3a + 4a + 5a). d Selectivity of 6.
e Selectivity of (2a + 3a + 4a + 5a + 6). f Selectivity ratio of 6/(2a + 3a
+ 4a + 5a + 6).
The hydrogen source for the formation of 6 from phenyl
radicals was investigated by KrF laser photolysis of n-C5D11-
SePh (1a-d11) in n-hexane. GC-MS analysis of photochemically
generated 6 showed that 92% was C6H6 (6) and 8% was C6H5D
(6-d1). This result indicates that 92% of the hydrogen was
abstracted from the solvent. To clarify the source of the
deuterium in 6-d1, experiments on the effect of laser intensity
FIGURE 3. Fluence dependence on the KrF excimer laser photolysis
of 1a. Consumption of 1a (magenta b) and yields of 1-pentene (2a,
black 0), n-pentane (3a, red O), n-decane (4a, green 3), and benzene
(6, blue 4) as a function of the laser fluence.14 Concentration: 10-3
M
in n-hexane, number of laser shots: 1 shot, optical path: 1 mm, room
(8) Ito, O. J. Am. Chem. Soc. 1983, 105, 850-853.
8702 J. Org. Chem., Vol. 72, No. 23, 2007
temperature.

Photochemical CleaVage of UnfaVorable Ph-Se Bond
SCHEME 3. Plausible Reaction Mechanism for the Formation of Major Photoproducts of 1a (The Selectivity6 of the Primary
Products and Reaction Paths Are Also Indicated)
for the formation of 6 from 1a (cf. Figure 3), but this can be
rationalized on the basis of a small contribution of the two-
photon process in the formation of 6 compared to the one-photon
processes.
hydrogen abstractions from the solvent. This was confirmed by
the photolysis of 1a-d11 in n-hexane; GC-MS analysis of
photochemically generated 3a showed a major peak at m/e 83
corresponding to C5HD11, which provides evidence of hydrogen
abstraction from the solvent.
The selectivity of 2a for one laser shot was 22.6% and as
1.7% was formed by disproportionation and 1.5% by hydrogen
abstraction by phenyl radicals, 19.4% of 2a was formed by other
processes. Other possible paths for the formation of 2a are (i)
an intramolecular â-H elimination10 in 1a, (ii) hydrogen
abstractions from pentyl radicals by PhSe or pentylselenyl
radicals on the outside of the solvent cage, and (iii) hydrogen
abstraction of pentyl radicals by PhSe radicals within the solvent
cage.
To determine the path responsible for the formation of 2a
from the three possible paths, experiments on the effect of
solvent viscosity were conducted. A series of experiments was
conducted in n-heptane, n-dodecane, n-tetradecane, n-hexade-
cane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, and
cyclodecane.7 Figure 5 shows the selectivity of 2a, 3a, 4a, 5a,
and 6 and the consumption of 1a for a single KrF laser shot as
a function of solvent viscosity.11
Figure 5a clearly shows that the selectivity of 2a increased
with increasing solvent viscosity. If the formation of 2a
proceeded by path (i), an intramolecular â-H elimination, 2a
and PhSeH would be formed within the solvent cage, so the
solvent viscosity should not have much effect on the selectivity
of 2a; however, this was not the case, as shown in Figure 5a.
FIGURE 4. Laser fluence effect on the yield of benzene (6, O) and
benzene-d1 (6-d1, b) in KrF excimer laser photolysis of 1a-d11 as a
function of the laser fluence.14 Concentration: 10-3 M in n-hexane,
number of laser shots: 1 shot, optical path: 1 mm, room temperature.
Reaction Mechanism of KrF Excimer Laser Photolysis
of 1a. Scheme 3 shows a plausible reaction mechanism for the
formation of primary photoproducts of 1a, with selectivity6 of
the major products also presented. Although the mass balance
of photoproducts was good, there are still some byproducts that
were not identified. However, we think that there is only a very
small contribution of secondary photolysis in these unidentified
products because the occurrence of secondary photolyses is less
probable particularly at one laser shot (within ca. 20 ns).
The requirement for two photons for the formation of 4a is
consistent with the reaction mechanism shown in Scheme 3.
Considering that the selectivity of 4a for one laser shot was
11.5% and the disproportionation/coupling ratio of primary alkyl
radicals is 0.15,9 it was calculated that 1.7% of 2a and 3a was
formed by disproportionation. The selectivity of 3a for one laser
shot was 6.0%, so that 4.3% of 3a should be formed by
It has been reported that the fraction of geminate radical pairs
escaping from the solvent cage is a linear function of (1/η)n,
where η is the solvent viscosity and the value of n is between
0.512 and 1.0.13 This means that the selectivity of products
formed from free radical species outside the solvent cage will
(10) (a) Pickett, N. L.; Foster, D. F.; Maung, N.; Cole-Hamilton, D. J.
J. Mater. Chem. 1999, 9, 3005-3014. (b) Pola, J.; Ouchi, A. J. Organomet.
Chem. 2001, 629, 93-96.
(11) Riddick, J. A.; Toops, E. E., Jr. Technique of Organic Chemistry,
2nd ed.; Interscience Publishers: New York, 1955; Vol. VII, Chapter III.
(12) Koenig, T.; Deinzer, M. J. Am. Chem. Soc. 1968, 90, 7014-7019.
(13) Niki, E.; Kamiya, Y. J. Am. Chem. Soc. 1974, 96, 2129-2133.
(14) Results are the average of at least two independent runs.
(15) In the case of tetradecane, the selectivity of 5a was not determined
owing to the overlapping of the peaks for tetradecane and 5a in GLC
analyses.
(9) Fossey, J.; Lefort, D.; Sorba, J. Free Radicals in Organic Chemistry;
John Wiley & Sons: Chichester, 1995; p 124, Table 10.3.
J. Org. Chem, Vol. 72, No. 23, 2007 8703

Ouchi et al.
SCHEME 4. Major Photoproducts of 1b and 1c
decrease with increasing solvent viscosity. Indeed, the selectivity
of products formed solely on the outside of the solvent cage
(i.e., 4a and 5a) decreased with increasing solvent viscosity.
However, the trend for selectivity of 2a was opposite to the
case expected from path (ii).
The trend observed for selectivity of 2a is consistent with
the result expected from path (iii). The contribution of hydrogen
abstraction from pentyl radicals by PhSe radicals within the
solvent cage to form 2a will increase when the radical pair is
retained within the cage for a longer time. The decrease in
consumption of 1a with increasing solvent viscosity indicates
an increase in the recombination probability for pentyl and PhSe
radicals owing to inhibition of the escape of radical pairs from
the cage, which is also in accord with the trend expected from
path (iii). Therefore, it is concluded that 19.4% of the formation
of 2a can be attributed to hydrogen abstraction from pentyl
radicals by PhSe radicals within the solvent cage. Indeed, the
formation of PhSeH was confirmed by HPLC analyses.
Solvent Viscosity Effect on the Selectivity of Pentyl-Se
and Ph-Se Bond Cleavage. From the primary photochemical
paths of 1a shown in Scheme 3, we know that the ratio of
pentyl-Se and Ph-Se bond cleavage (Path A vs Path B) is
equal to the ratio of the selectivity (2a + 3a + 4a + 5a) versus
6.
Figure 6 shows the selectivity of pentyl-Se and Ph-Se bond
cleavage as a function of solvent viscosity. The figure clearly
shows the presence of a considerable degree of Ph-Se bond
cleavage in the photolysis of 1a, which was unexpected
previously. The selectivity of Ph-Se bond cleavage increased
with decreasing solvent viscosity and vice versa for the
selectivity of pentyl-Se bond cleavage.
Wavelength Effect on the Selectivity of C-Se Bond
Cleavage. The effect of wavelength on the selectivity of pentyl-
Se and Ph-Se bond cleavage in 1a was studied using ArF, KrF,
and XeCl lasers, with results summarized in Table 1.7 Owing
to a very low molar absorption coefficient at 308 nm (cf. Figure
1), XeCl laser photolysis was very slow so that the XeCl laser
required more laser shots to achieve sufficient conversion of
1a compared to that with the KrF and ArF lasers.
Table 1 indicates that the ratio of Ph-Se bond cleavage
increased with decreasing laser wavelength. The change in the
ratio probably reflects the participation of different excited states
involved in photolysis because the emission wavelength of each
laser corresponds to different absorption bands (cf. Figure 1).
FIGURE 5. Selectivity6 of (a) 1-pentene (2a), (b) n-pentane (3a), (c)
n-decane (4a), (d) dipentyl selenide (5a),11,15 (e) benzene (6), and (f)
consumption of pentyl phenyl selenide (1a) in the KrF excimer laser
photolysis of 1a as a function of the solvent viscosity.11,14 Concentra-
FIGURE 6. Selectivity6 of pentyl-Se (b, O: 2a + 3a + 4a + 5a)15
and Ph-Se (9, 0: 6) bond cleavage in KrF excimer laser photolysis
of 1a as a function of the solvent viscosity.14 Concentration: 10-3 M,
tion: 10-3 M, optical path: 1 mm, laser fluence: 100 mJ cm-2 pulse-1
,
optical path: 1 mm, KrF excimer laser fluence: 100 mJ cm-2 pulse-1
,
number of laser shots: 1 shot, room temperature. Open symbol:
number of laser shots: 1 shot. Open symbols: cycloalkanes; closed
cycloalkanes; closed symbol: n-alkanes.
symbols: n-alkanes.
8704 J. Org. Chem., Vol. 72, No. 23, 2007

Photochemical CleaVage of UnfaVorable Ph-Se Bond
SCHEME 5. Major Photoproducts of 1d and 1e
For allylic groups (1d, 1e), the ratio of Ph-Se bond cleavage
is between that for primary and secondary alkyl groups.
Disproportionation is not possible for allylic radicals, so that
coupling and hydrogen abstraction are the possible radical
reactions. However, owing to the stability of allylic radicals,
the major reactions of allyl and 2-methylallyl radicals are
coupling reactions, which is in agreement with the results for
KrF laser photolyses of 1d and 1e.7 The high selectivity of
alkylcyclohexanes (8d, 8e), which are also coupling products
of cyclohexyl radicals formed by hydrogen abstraction from
solvent molecules by phenyl radicals and allylic radicals, can
be also explained by the stability of allylic radicals.
The ratio of Ph-Se bond cleavage by photolysis with a low-
pressure mercury lamp, which has major emission (254 nm)
similar to that of the KrF laser, was 0.39 (1 min irradiation).
The ratio was slightly higher than for the KrF laser, which can
be explained by the formation of 6 by the previously mentioned
secondary photolysis of 7.
Effect of Alkyl Groups on the Selectivity of C-Se Bond
Cleavage. The effect of alkyl substituents on the selectivity of
Ph-Se and alkyl-Se bond cleavage was investigated using
n-pentyl (1a), 1-ethylpropyl (1b), tert-butyl (1c), allyl (1d), and
2-methylallyl (1e) phenyl selenides.
The major photoproducts of 1b and 1c are shown in Scheme
4.7 In analogy with the reaction mechanism of 1a shown in
Scheme 3, the selectivity of 1-ethylpropyl-Se bond cleavage
is given by the sum of the selectivity for 2b-cis, 2b-trans, 3a,
4b, and 5b and that of the Ph-Se bond by 6. Similarly, for 1c,
the selectivity of tert-butyl-Se bond cleavage is given by the
sum of the selectivity for 2c, 3c, 4c, and 5c and that of the
Ph-Se bond by 6.
Scheme 5 shows the major photolysis products of 1d and
1e.7 Interestingly, a significant amount of corresponding alkyl-
cyclohexanes (8d, 8e) was detected in these cases. The selectiv-
ity of alkyl-Se bond cleavage is presented as the sum of the
selectivity for 3, 4, 5, and 8 and that of the Ph-Se bond by 6
for both 1d and 1e.
Experimental Section
Synthesis of Selenides, Diselenides, and 2-Methylallylcyclo-
hexane (8e). NMR spectra were recorded with CDCl3 as solvent.
As internal standards, TMS was used for 1H NMR (δ: 0 ppm) and
CDCl3 for 13C NMR NMR (δ: 77.0 ppm) analyses.
Synthesis of Alkyl Phenyl Selenides 1a-e. Pentyl-d11 phenyl
selenide (1a-d11), 1-ethylpropyl phenyl selenide (1b), allyl phenyl
selenide (1d), and 2-methylallyl phenyl selenide (1e) were synthe-
sized similar to that of pentyl phenyl selenide (1a).16
Pentyl Phenyl Selenide (1a).17,18 Diphenyl diselenide (3.41 g,
10.9 mmol) was dissolved in 30 mL of EtOH and cooled with an
ice-water bath. To the solution were added 0.83 g (21.9 mmol) of
sodium borohydride in portions and then 3.00 g (19.9 mmol) of
1-bromopentane, both under a nitrogen atmosphere, and the solution
was stirred at room temperature for 3 h. The reaction mixture was
poured into water, extracted with ether, and the organic portion
was washed successively with aqueous Na2CO3 and brine. The ether
solution was dried over anhydrous Na2SO4, the solvent was removed
in vacuo, and the crude product was purified by a distillation using
a 15 cm Vigreux column: Yield 3.53 g (78%), bp 56.5-57.5 °C
1
(0.16 Torr) [lit.17 116 °C (5.0 Torr)], faintly yellow oil; H NMR
δ 0.88 (t, 3H, J ) 7.0 Hz), 1.35 (m, 4H), 1.71 (tt, 2H, J ) 7.5, 6.7
Hz), 2.91 (t, 2H, J ) 7.5 Hz), 7.15-7.33 (m, 3H), 7.42-7.53 (m,
2H); 13C NMR δ 13.9, 22.1, 27.8, 29.8, 32.0, 126.5, 128.9, 130.6,
132.3; IR (neat) 466, 474, 671, 691, 735, 779, 999, 1022, 1074,
1105, 1156, 1194, 1244, 1267, 1298, 1379, 1437, 1466, 1478, 1580,
1743, 1794, 1873, 1945, 2859, 2872, 2928, 2959, 3000, 3017, 3059,
3072 cm-1; MS, m/e (relative intensity) 27 (10), 29 (15), 39 (10),
41 (23), 43 (92), 51 (12), 71 (12), 55 (8), 71 (12), 77 (22), 78 (41),
91 (12), 154 (22), 155 (23), 156 (50), 157 (17), 158 (100), 159 (9),
160 (18), 224 (9, M+), 225 (8, M+), 226 (23, M+), 228 (46, M+),
230 (9, M+). Anal. Calcd for C11H16Se (%): C, 58.15; H, 7.10.
Found (%): C, 57.82; H, 7.01.
The selectivity of C-Se bond cleavage for 1a-e is sum-
marized in Table 2. As the alkyl substituents changed from
primary to secondary and tertiary groups, the ratio of Ph-Se
bond cleavage decreased, which is probably due to the decrease
of alkyl-Se bond energies in this order. At the same time, the
selectivity of the coupling products (4) decreased with increasing
olefins 2.7 The decrease in selectivity for 4 can be explained
by the increase in the ratio of the rate constants for dispropor-
tionation/recombination as the alkyl group changes from primary
(0.15) to secondary (1.2) and tertiary (4.5) groups.9
tert-Butyl Phenyl Selenide (1c).19,20 A solution of 2.47 g (26.1
mmol) of phenylselenyl chloride in 50 mL of dry ether cooled with
an ice-water bath was added to 7.6 mL of tert-butyllithium (1.7
M, 31.3 mmol) in pentane, and the mixture was stirred at room
temperature under a nitrogen atmosphere for 0.5 h. The reaction
mixture was poured into water, extracted with ether, and the organic
portion was washed successively with aqueous Na2CO3 and brine.
The ether solution was dried over anhydrous Na2SO4, and the
solvent was removed in vacuo to give 4.52 g of crude product.
The crude product was purified by a distillation using a 15 cm
Vigreux column to give 2.78 g (31%) of pure 1c as a faintly yellow
oil: bp 33-34 °C (0.23 Torr) [lit.20 57-58 °C (0.8 Torr)]; 1H NMR
TABLE 2. Selectivity of C-Se Bond Cleavages of 1a-e
selectivity of C-Se bond cleavage (%)a
ratio
(Ph-Se/total)
substrate
alkyl-Se
Ph-Se
total
1a
1b
1c
1d
1e
55.5b
31.8c
17.8c
2.9c
87.3d
0.36e
79.8b
97.6d
0.18e
92.4b
95.3d
0.03e
61.1f (68.3)g
48.8f (56.1)g
21.9c
22.7c
83.0h (90.2)i
71.5h (78.8)i
0.26j (0.24)k
0.32j (0.29)k
a Photolysis conditions: 10-3 M 1a-e in cyclohexane, optical path: 1
mm, KrF laser fluence: 1.25 × 1017 photons cm-2 pulse-1, room
temperature, number of irradiated KrF laser shots: 1 shots. Product
selectivity is based on the consumption of 1, and results are the average of
at least two independent runs. Original data are provided as Supporting
Information. b Selectivity of (2 + 3 + 4 + 5). c Selectivity of 6. d Selectivity
of (2 + 3 + 4 + 5 + 6). e Selectivity ratio of 6/(2 + 3 + 4 + 5 + 6).
f Selectivity of (3 + 4 + 5). g Selectivity of (3 + 4 + 5 + 8). h Selectivity
of (3 + 4 + 5 + 6). i Selectivity of (3 + 4 + 5 + 6 + 8). j Selectivity ratio
of 6/(3 + 4 + 5 + 6). k Selectivity ratio of 6/(3 + 4 + 5 + 6 + 8).
(16) Analytical and spectroscopic data are provided as Supporting
Information.
(17) Foster, D. G. Recl. TraV. Chim. Pays-Bas. 1935, 447-457.
(18) Duddeck, H.; Wagner, P.; Rys, B. Magn. Reson. Chem. 1993, 31,
736-742.
(19) Ranu, B. C.; Mandal, T.; Samanta, S. Org. Lett. 2003, 5, 1439-
1441.
(20) Gabdrakhmanov, F. G. Sb. Aspir. Rab. Kazan. Gos. UniV. Khim.
Geol. 1967, 85-90; Chem. Abstr. 1969, 70, 96328g.
J. Org. Chem, Vol. 72, No. 23, 2007 8705

Ouchi et al.
δ 1.41 (s, 9H), 7.23-7.43 (m, 3H), 7.65 (dd, 2H, J ) 7.8, 1.7 Hz);
13C NMR δ 32.1, 43.1, 128.3, 128.4, 128.6, 138.2; IR (neat) 476,
521, 673, 694, 741, 774, 914, 1001, 1022, 1065, 1075, 1154, 1219,
1302, 1364, 1391, 1437, 1452, 1460, 1476, 1580, 1758, 1808, 1881,
1952, 2859, 2890, 2917, 2936, 2957, 2973, 3058, 3073 cm-1; MS,
m/e (relative intensity) 29 (20), 39 (12), 41 (37), 50 (5), 51 (11),
55 (5), 56 (7), 57 (100), 77 (22), 78 (44), 84 (6), 154 (19), 155
(19), 156 (42), 157 (14), 158 (84), 159 (8), 160 (15), 212 (7, M+),
214 (15, M+). High-resolution MS calcd for C10H14Se: 212.0267,
214.0260. Found: 212.0259, 214.0267.
Synthesis of Dialkyl Selenides 5a,b,d,e. Diallyl selenide (5d)
was synthesized similar to that of dipentyl selenide (5a) and bis-
(2-methylallyl) selenide (5e) by a conventional procedure.16
Dipentyl Selenide (5a).21 1-Bromopentane (19.1 g, 126 mmol)
was added to a suspension of 7.5 g (60 mmol) of Na2Se in dry
THF (100 mL). After completion of the addition, the reaction
mixture was stirred at room temperature under a nitrogen atmo-
sphere. The mixture was then poured into water, extracted with
ether, and the organic portion was washed with brine and dried
over anhydrous MgSO4. The solvent was then removed in vacuo,
and crude product was purified by silica gel column chromatography
(hexane): Yield 4.5 g (34%), faintly yellow oil; 1H NMR (CDCl3)
δ 0.90 (t, 3H, J ) 7.2 Hz), 1.33 (m, 4H), 1.66 (tt, 2H, J ) 7.5, 7.5
Hz), 2.55 (t, 2H, J ) 7.5 Hz); 13C NMR (CDCl3) δ 14.0, 22.2,
23.9, 30.4, 32.2; IR (neat) 564, 646, 727, 820, 903, 922, 962, 1034,
1059, 1072, 1105, 1190, 1240, 1267, 1294, 1333, 1342, 1377, 1420,
1464, 2624, 2664, 2731, 2857, 2870, 2924, 2955 cm-1; MS, m/e
(relative intensity) 27 (10), 29 (15), 39 (6), 41 (34), 42 (10), 43
(100), 55 (12), 69 (29), 70 (31), 71 (81), 109 (7), 148 (8), 149
(12), 150 (15), 151 (16), 152 (26), 220 (7, M+), 222 (15, M+).
Anal. Calcd for C10H22Se (%): C, 54.29; H, 10.01. Found (%): C,
54.44; H, 10.20.
The following authentic samples were purchased and used as
bought: benzene (spectral grade), n-pentane (spectral grade),
1-pentene, n-decane, 1-hexene, pentylbenzene, undecane, cis-2-
pentene, trans-2-pentene, 3,4-diethylhexane, 2,2,3,3,-tetrameth-
ylbutane, 1,5-hexadiene, 2,5-dimethyl-1,5-hexadiene, allylcyclo-
hexane, diphenyl diselenide, di-tert-butyl selenide, and standard
gases 2-methylpropane (0.997% in N2) and 2-methylpropene (8.10%
in N2).
Excimer Laser Photolyses. Photolyses of 1 were conducted on
50 µL of 10-3 M solutions under a nitrogen atmosphere at room
temperature using a synthetic quartz cell of 10 mm width and 1
mm optical path. The solutions were degassed with three freeze-
pump-thaw cycles before photolysis. The lasers used were ArF,
KrF, and XeCl excimer lasers [pulsewidth (fwhm): 20 ns (typical),
23 and 20 ns (typical), respectively]. Experiments on the fluence
dependence were conducted similarly using a single laser pulse in
the same experimental setup.
Analysis of the Photolysis Products. The consumption of 1 and
the product yield were analyzed on a capillary GLC (Neutrabond-
1, 60 m, 0.25 mmID, GL Sciences Inc.) and a HPLC pump
(Supersphere 100, RP-8e, 250 mm, 4 mmID, Merck) fitted with a
UV detector by comparison with authentic samples. MS analyses
of the photoproducts of 1a-d11 were conducted using a GC-MS
spectrometer equipped with a capillary column (Neutrabond-1,
60 m, 0.25 mmID, GL Sciences Inc.).
Supporting Information Available: Analytical and spectro-
scopic data of 1a-d11, 1b, 1d, 1e, 5b, 5d, 5e, and 8e. Synthetic
procedure of 5b. Results on the low-pressure mercury lamp, and
ArF and XeCl excimer laser photolyses of 1a in n-hexane. Results
on the KrF excimer laser photolysis of 1a in n-hexane, n-heptane,
n-dodecane, n-tetradecane, n-hexadecane, cyclopentane, cyclohex-
ane, cycloheptane, cyclooctane, and cyclodecane. Results on the
KrF excimer laser photolyses of 5a and 7 in n-hexane. Results on
the KrF excimer laser photolyses of 1a-e in cyclohexane. NMR
spectra of 1a-e, 1a-d11, 5a,b,d,e, and 8e. This material is available
free of charge via the Internet at http://pubs.acs.org.
Photolysis. General Aspects. The following solvents for the
photolyses were purchased and used as bought or after purification
by standard procedures: n-hexane (spectral grade), n-heptane
(spectral grade), cyclopentane (spectral grade), and cyclohexane
(spectral grade), n-dodecane, n-tetradecane, n-hexadecane, cyclo-
heptane, cyclooctane, and cyclodecane.
(21) Gladysz, J. A.; Hornby, J. L.; Garbe, J. E. J. Org. Chem. 1978, 43,
1204-1208.
JO701447A
8706 J. Org. Chem., Vol. 72, No. 23, 2007