Highly regioselective hydroselenation of inactivated terminal alkynes using diselenide-Ph2P(O)H mixed systems under visible-light irradiation

Article abstract of DOI:10.1016/j.tetlet.2013.07.127

Upon visible-light irradiation, we could achieve a highly regioselective hydroselenation of inactivated terminal alkynes to give 1-(organylseleno)-1- alkenes by using diselenide-Ph₂P(O)H mixed systems. The photoinduced hydroselenation presented in this study is advantageous as it does not involve the handling of oxygen-sensitive and foul-smelling selenols.

Full text of DOI:10.1016/j.tetlet.2013.07.127

Tetrahedron Letters 54 (2013) 5453–5456
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Tetrahedron Letters
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Highly regioselective hydroselenation of inactivated terminal
alkynes using diselenide–Ph2P(O)H mixed systems under
visible-light irradiation
⇑
Yohsuke Kobiki, Shin-ichi Kawaguchi, Akiya Ogawa
Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Upon visible-light irradiation, we could achieve a highly regioselective hydroselenation of inactivated
terminal alkynes to give 1-(organylseleno)-1-alkenes by using diselenide–Ph2P(O)H mixed systems.
The photoinduced hydroselenation presented in this study is advantageous as it does not involve the
handling of oxygen-sensitive and foul-smelling selenols.
Received 21 May 2013
Revised 17 July 2013
Accepted 25 July 2013
Available online 2 August 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Photoinduced hydroselenation
Diselenide
Diphenylphosphine oxide
Vinyl selenide
Terminal alkynes
In recent years, the chemistry of organoselenium compounds
has attracted a lot of interest as they can be used in a variety of
applications such as biological active agents, functionalized mole-
cules for electronic materials, and synthetic intermediates.1 Among
organoselenium compounds, vinyl selenides are useful intermedi-
ates for the synthesis of various vinyl and carbonylcompounds.2
One of the most straightforward methods for the synthesis of vinyl
selenides is the hydroselenation of alkynes with selenols.3 The
hydroselenation of alkynes has been studied under a variety of
reaction conditions such as Michael-type addition,4 radical addi-
tion,4c,d,5 and transition-metal-catalyzed addition6 of selenols to al-
kynes. Their regio- and/or stereoselectivities can be controlled by
the choice of reaction conditions. However, these methods some-
times suffer from drawbacks such as the unpleasant smell of sele-
nols and the rapid oxidation of selenols to their corresponding
diselenides by atmosphericoxygen.7 Therefore, developing a meth-
od for the hydroselenation of alkynes without using selenols as the
selenium source is strongly desired. Thus, we focused on the use of
diorganyl diselenides instead of selenols, because the diselenides
are stable under an oxidizing atmosphere and do not have a foul
smell. The hydroselenation of ‘activated alkynes’ (electron-defi-
cient alkynes having electron-withdrawing groups such as car-
bonyl and aryl groups) using diselenides has already been
achieved by mixing diselenides and alkynes with appropriate
reductants through ionic reaction pathway.8 In contrast, effective
hydroselenation of inactivated alkynes (relatively electron-rich al-
kynes) using diselenides and reductant has not been achieved hith-
erto.9 Herein, we report a highlyregioselective hydroselenation of
inactivated alkynes 1 under visible-light irradiation by using a
diselenide–Ph2P(O)H mixed system. The above reaction affords vi-
nyl selenides 4 (Eq. (1)).
h
ν
(>300 nm)
(R'Se)2
+
R
R
SeR'
ð1Þ
Ph2P(O)H (3a)
1
2
4
The photoinduced hydroselenation of alkynes with selenols via
the formation of radicals is a useful and convenient method for the
synthesis of vinyl selenides. If hydroselenation is performed using
diselenides instead of selenols, it is essential to first reduce the
diselenides to selenols. Moreover, in order to carry out hydrosele-
nation as described above, high concentrations of starting material
and neutral reaction conditions are needed. However, existing
methods for the reduction of diselenides to selenols generally re-
quire acidic or basic conditions and/or large quantities of solvent.10
Therefore these existing methods are not suitable for hydroselena-
tion with diselenides via the formation of radicals. Thus, we
investigated the hydroselenation of alkynes with diselenides
by combining it with the reduction of diselenides using
hydrogensources.
⇑
First, we examined the reaction of 1-octyne (1a) (1.0 mmol)
with diphenyl diselenide (2a) (0.20 mmol) in the presence of
Corresponding author. Tel./fax: +81 72 254 9290.
E-mail address: ogawa@chem.osakafu-u.ac.jp (A. Ogawa).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.127

5454
Y. Kobiki et al. / Tetrahedron Letters 54 (2013) 5453–5456
several hydrogen sources upon photoirradiation with a tungsten
lamp (500 W) through a sealed Pyrex tube (h >300 nm) (Table 1).
phenylseleno)-1-octene (4ab) in good yields with preferential
cis-stereoselectivity (entry 2). In contrast, hydroselenation using
bis(p-trifluoromethylphenyl) diselendes (2c) proceeded only
slightly under the same conditions (entry 3). After the reaction,
diselenide (2c) wascompletely consumed leaving behind large
amounts of p-trifluoromethylbenzeneselenol. It is known that the
photoinduced hydroselenation of alkynes with selenols is acceler-
ated in the co-presence of small amounts of diselenide.5 Therefore,
we examined the hydroselenation of 2c under milder reducing
conditions (i.e., 1.2 equiv of Ph2P(O)H and CHCl3 (0.1 mL)), so as
to not consume the diselenide entirely. Carrying out the reaction
in the presence of CHCl3 as the solvent significantly improved
the yield of 1-(p-trifluoromethylphenylseleno)-1-octene (4ac).
When other diaryl diselenides (2d–g) were employed for the
hydroselenation, 1-(arylseleno)-1-octenes (4ad–ag) were obtain-
edin good yields under mild conditions (entries 5–8). This hydro-
selenation reaction could also be applied to other aliphatic
alkynes, such as 6-chloro-1-hexyne (1b) and 5-methyl-1-hexyne
(1c) (entries 9–12). Diselenides bearing a 1- or 2-naphthyl group
(2h–i) could also be used for this hydroselenation, although longer
reaction times were required (entries 11 and 12). Aromatic alkynes
can also be employed in this hydroselenation reaction (entry 13).
The use of di-n-butyl diselenide (2j) successfully gave the corre-
sponding adduct having an n-butylseleno group. This result was
noteworthy because butaneselenol is difficult to handle and has
a particularly bad smell (entry 14).7 The hydroselenation of an
internal alkyne (4-octyne, 1e) or an alkene (1-decene,9) was not ef-
fec tive (entries 15 and 16).
The pathway for the present hydroselenation of alkynes using
diselenides and diphenylphosphine oxide is discussed below.
Although diselenides react smoothly with diphenylphosphine
oxide to give the corresponding selenols and selenophosphine
oxide, a small amount of diselenides remained.6f Previously, our
group reported that the photoinduced hydroselenation of alkynes
with benzeneselenol was catalyzed effectively by the addition of
small amounts of diphenyl diselenide.5 Based on our previous
study, diselenides can catalyze the addition reaction of selenols
to alkynes in the present hydroselenation reaction as well. On Ta-
ble 2, entry 3, the hydroselenation did not proceed efficiently. In
this case, 77Se NMR measurements of the resulting mixture
(formed after the reaction) indicated that the diselenide had been
m
When 0.24 mmol of diphenylphosphine oxide (3a) was used as the
hydrogen source, 0.15 mmol of 1-(phenylseleno)-1-octene (4aa)
(77%) was obtained and 0.18 mmol of selenophosphine oxide
(PhSe–P(O)Ph2) was detected by 31P and 77Se NMR11 (entry 1). Un-
der theseconditions, the bisselenated compound (5aa) was also ob-
tained as a byproduct, because 2a could also add to alkynes under
photoirradiation.12 Use of diphenylphosphine (3b) instead of 3a
also afforded 4aa in good yields (entry 2). However, the complex
mixture also contained hydrophosphination13 and phosphinosele-
nation14 products of 1a. On the other hand, diethyl phosphite (3c)
did not promote the desired hydroselenation (entry 3). Tris(tri-
methylsilyl)silane (6a) caused the hydroselenation of 1a, along
with the formation of the corresponding selenosilane (PhSe–
Si(SiMe3)3), whichwas detected using 29Si and 77Se NMR (entry
4).15 However, 6a also caused undesired bisselenation resulting
in the formation of 5aa in measurable yield. In contrast, the reac-
tion with another hydrosilane, such as triethylsilane (6b), gave
only 5aa (entry 5). Tri-n-butyltin hydride (7), a typical hydrogen
source, afforded the hydroselenation product in moderate yield
with good selectivity (entry 6). When 7 was used as hydrogen
source, a byproduct bearing a selenium-heteroatom bond, sele-
nostannane (PhSe–SnnBu3) was detected by 77Se and 117Sn, 119Sn
NMR.16,17 1,4-Cyclohexadiene (8) was found to be an unsuitable
hydrogen source for hydroselenation, as it afforded 4aa in poor
yields and the bisselenated product 5aa in large amounts (entry
7). These results clearly indicated that 3a was the most appropriate
hydrogen source for this hydroselenation reaction. Next, the effects
of the (PhSe)2/hydrogen source molar ratios, concentration of the
starting materials, and solvent on the reaction were investigated.
The hydroselenation product 4aa was obtained selectively in excel-
lent yield when 2.0 equiv of 3a were used (entry 8). Although 4aa
was obtained in high yields when CHCl3 was used as the solvent,
the selectivity was reduced (entry 9). When hydroselenation was
conducted in low concentrations of the solvent, theyields of the de-
sired product decreased (entries 10 and 11).
Next, we investigated the substrate scope of this hydroselena-
tion reaction (Table 2).18 Hydroselenation of an aliphatic alkyne,
1-octyne (1a), using bis(p-methylphenyl) diselenides (2b) and
2.0 equiv of 3a proceeded regioselectively to afford 1-(p-methyl-
Table 1
Hydroselenation of 1-octyne (1a) with diphenyl diselenide (2a) and several hydrogen sources
PhSe
hν (>300 nm)
nHex
1a
nHex
nHex
SePh
(PhSe)2
hydrogen source
+
+
+
SePh
solvent, 7 h, 40 °C
2a
4aa
5aa
1.0 mmol
0.20 mmol
Entry
Hydrogen source
(PhSe)2:hydrogen sourcea
Solvent
Yield of 4aab,c
Yield of 5aab (%)
1
2
3
4
5
6
7
8
9
Ph2P(O)H (3a)
Ph2PH (3b)
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:2
None
None
None
None
None
None
None
None
77% [E/Z = 8/92]
74%
Trace
32% [E/Z = 13/87]
0%
39% [E/Z = 15/85]
13% [E/Z = 37/63]
93% (81%) [E/Z = 21/79]
93% (77%) [E/Z = 26/74]
54% [E/Z = 25/75]
6
12
89
31
94
Trace
78
0
4
0
0
(EtO)2P(O)H (3c)
(Me3Si)3SiH (6a)
Et3SiH (6b)
nBu3SnH (7)
1,4-Cyclohexadiene (8)
Ph2P(O)H (3a)
Ph2P(O)H (3a)
Ph2P(O)H (3a)
Ph2P(O)H (3a)
1:1.2
1:2
1:2
CHCl3 (0.1 mL)
CHCl3 (0.5 mL)
C6H6 (0.5 mL)
10
11
35% [E/Z = 22/78]
a
b
c
Molar ratios.
Determined by 1H NMR. Isolated yield is shown in parentheses.
Yields are calculated based on molecular weight of 2a.

Y. Kobiki et al. / Tetrahedron Letters 54 (2013) 5453–5456
5455
Table 2
Hydroselenation of several alkynes (1) with several diselenide (2) and diphenylphosphine oxide (3)
hν (>300 nm)
CHCl3 (0.1 mL), 7 h, 40 o
(R2Se)2
Ph2P(O)H
SeR2
R1
R1
+
+
C
3a
1
2
4
0.20 mmol
1.2 equiv.
Entry
1
R2
1:2a
4
Yield of 4 [E/Z]b
nHex
nHex
Se
5:1
1a
X
X
1c
2c
3c
4
5
6
X@H (2a)
=p-CH3 (2b)
=p-CF3 (2c)
X@H (4aa)
=p-CH3 (4ab)
=p-CF3 (4ac)
81% [E/Z = 21/79]
83% [E/Z = 18/82]
(10%)
90% [E/Z = 32/68]
63% [E/Z = 14/86]
85% [E/Z = 27/73]
65% [E/Z = 16/84]
81% [E/Z = 18/82]
=p-F (2d)
=p-F (4ad)
=m-CH3 (2e)
=m-CF3 (2f)
=m-F (2g)
=m-CH3 (4ae)
=m-CF3 (4af)
=m-F (4ag)
7
8
Cl
Cl
Se
5:1
1b
X
X
9
10
X@H (2a)
=p-CH3 (2b)
X@H (4ba)
=p-CH3 (4bb)
77% [E/Z = 24/76]
68% [E/Z = 7/93]
Se
4ch
11d
12d
5:1
5:1
63% [E/Z = 25/75]
1c
2h
Se
4ci
85% [E/Z = 12/88]
2i
F3C
H3C
Se
13
14c,e
15
1.2:1
5:1
92% [E/Z = 39/61]
90% [E/Z = 29/71]
CF3
1d
H3C
4df
2f
2j
nHex
nHex
Se
4aj
nPr
1a
nPr
nPr
nPr
Se
5:1
5:1
(24%) [E/Z = 12/88]
1e
2a
2a
4ea
nOct
nOct
Se
16c,f
(14%)
9
10
a
b
c
d
e
f
Molar ratios.
Isolated yields (NMR yields are shown in the parentheses). Yields are calculated based on molecular weight of 2.
Ph2P(O)H 3a (0.4 mmol, 2.0 equiv) was used in the absence of solvent (condition of entry 8 in Table 2).
Irradiated for 15 h.
Stirred for 1 h beforeirradiation.
Irradiated for 24 h.
converted completely to selenol and selenophosphine oxide and no
diselenide was detected. On the other hand, when the hydrosele-
nation proceeded well, small amounts of diselenides were de-
tected. Therefore, it appears that in order for the hydroselenation
reaction to proceed well, small amounts of diselenides were
needed during the reaction. Since the rate of diselenide reduction
with diphenylphosphine oxide depends on the substituent in the
diselenide, tuning the reaction conditions is important for this
reaction. On the basis of these considerations, a plausible reaction
pathway for the hydroselenation is shown in Scheme 1. First,
visible-light irradiation triggers homolytic cleavage of the
Se–Se single bond in the unreacted diselenide,19 generating the
corresponding seleno radicals. Next, attack of the seleno radical
on the terminal carbon of alkynes leads to vinyl radical
intermediates. The vinyl radicals are captured by selenols20 to
produce hydroselenation products21 along with regeneration of
the seleno radicals. Moreover, Ph2P(O)SeR, which is generated as
a by-product, does not add to alkynes. This is in contrast to the
addition reaction of unoxidized Ph2PSePh and alkynes.14 This is
also an important reason that makes the present hydroselenation
system so simple.
In conclusion, we have developed a highly regioselective hydro-
selenation of inactivated terminal alkynes by using novel disele-
nide–Ph2P(O)H mixed systems. It has been found that regulation
of the reducing conditions can control the desired hydroselenation.
The method for the hydroselenation of alkynes discussed in this
study is user-friendly because it does not involve the use of
selenols.

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Y. Kobiki et al. / Tetrahedron Letters 54 (2013) 5453–5456
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RSe
H
R'
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7. Paulmier, C. In Selenium Reagent and Intermediates in Organic Synthesis;
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(RSe)2
Ph2P(O)H
+
8. (a) Okamura, H.; Miura, M.; Kosugi, K.; Takei, H. Tetrahedron Lett. 1980, 21, 87;
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RSe P(O)Ph2
Scheme 1. A plausible reaction pathway for hydroselenation using the diselenide–
Ph2P(O)H mixed systems.
9. Recently, the one-pot hydroselenation of alkynes using diselenides and NaBH4
in an ionic liquid was reported. See, Lenardão, E. J.; Dutra, L. G.; Saraiva, M. T.;
Jacob, R. G.; Perin, G. TetrahedronLett. 2007, 48, 8011.
Acknowledgments
10. (a) Bogert, M. T.; Stull, A. J. Am. Chem. Soc. 2011, 1927, 49; (b) Baker, J. W.;
Barrett, G. F. C.; Tweed, W. T. J. Chem. Soc. 1952, 2831; (c) Günther, W. H. H. J.
Org.Chem. 1966, 31, 1202; (d) Ogawa, A.; Nishiyama, Y.; Kambe, N.; Murai, S.;
Sonoda, N. TetrahedronLett. 1987, 28, 3271; (e) Santi, C.; Santoro, S.; Testraferri,
L.; Tiecco, M. Synlett 2008, 1471.
This work is supported by Grant-in-Aid for Scientific Research
(C, 23550057), from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan.
11. Spectral data of PhSe–P(O)Ph2:31P NMR (CDCl3)
d
40.4 (with
2 satellites
JP–Se = 381 Hz); 77Se NMR (CDCl3) d 381 ppm (d, JSe–P = 381 Hz).
12. (a) Back, T. G.; Krishna, M. V. J. Org. Chem. 1998, 53, 2533; (b) Ogawa, A.;
Yokoyama, K.; Yokoyama, H.; Masawaki, T.; Kamble, N.; Sonoda, N. J. Org. Chem.
1991, 56, 5721; (c) Nomoto, A.; Higuchi, Y.; Kobiki, Y.; Ogawa, A. Mini-Rev. Med.
Chem. 2013, 13, 814.
Supplementary data
Supplementary data (general comments, spectral and analytical
data, and copies of 1H NMR,13C NMR, and 77Se NMR spectra of vinyl
selenides) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2013.07.127.
13. Mitchell, T. N.; Heesche, K. J. Organomet. Chem. 1991, 409, 163.
14. Kawaguchi, S.-i; Shirai, T.; Ohe, T.; Nomoto, A.; Sonoda, M.; Ogawa, A. J. Org.
Chem. 2009, 74, 1751.
15. Spectral data of PhSe–Si(SiMe3)3: 29Si NMR (CDCl3) d ꢀ59.1, ꢀ114.7; 77Se NMR
(CDCl3) d ꢀ46.8 ppm.
16. Spectral data of PhSe–SnnBu3: 77Se NMR (CDCl3)
d
ꢀ27 (with
4 satellites
References and notes
JSe–117Sn = 957 Hz, JSe–119Sn = 999 Hz); 117Sn NMR (CDCl3) d 66.6 ppm (with 2
satellites J117Sn–Se = 957 Hz); 119Sn NMR (CDCl3) d 65.6 ppm (with 2 satellites
J119Sn–Se = 999 Hz).
1. (a) Selected representative reviews: Organic Selenium Compounds: Their
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17. It was reported that selenostannane was synthesized from diphenyl diselenide
and tri-n-butyltin hydride. See, Crich, D.; Hwang, J.-T.; Gastaldi, S.; Recupero,
F.; Wink, D. J. J. Org.Chem. 1999, 64, 2877.
18. General Procedure for Photoinduced Hydroselenation of Alkynes or Alkenes with
Diselenides and Diphenylphosphine Oxide: Diphenylphosphine oxide (48.5 mg,
0.24 mmol), diselenide (0.20 mmol), alkyne or alkene (1.0 mmol), and
chloroform (0.10 mL) were placed in
a sealed Pyrex glass tube under
John Wiley
& Sons: New York, 2012; Vol. 3,; (b) Tingoli, M; Tiecco, M.;
nitrogen. The mixture was irradiated with a tungsten lamp (500 W) for 7 h.
After the reaction, purification of the crude product was performed by
preparative TLC or silica gel column chromatography using hexane as eluent.
19. When the reaction was conducted in dark condition, only trace amount of 4
was obtained.
20. In the reaction system, adequate amount of selenol and diphenylphosphine
oxide are present. Selenols are known as effective hydrogen source for the
carbon-centered radicals (the rate constant k = 2.1 ꢁ 109 Mꢀ1 sꢀ1, Newcomb,
M. Choi, S.-Y. Horner, J. H. J. Org, Chem. 1999, 64, 1225). Although the capturing
ability of carbon-centered radicals of diphenylphosphine oxide has not been
investigated directly, this ability of other P–H bond compound
(dicyclohexylphosphine) has been known (k = 2.5 ꢁ 103 Mꢀ1 sꢀ1, Franz, J. A.
Suleman, N. K.Alnajjar, M. S. J. Org. Chem. 1986, 51, 19). Based on these rate
constants, we assume that selenols are almost the only hydrogen source to
vinyl radicals.
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R.; Cai, M. Synth. Commun. 2008, 38, 4170.
3. Coupling reaction of diselenides with vinyl compounds was reported as an
alternative method to synthesize the vinyl selenides. See, (a) Wang, L.; Wang,
M.; Huang, F. Synlett 2005, 2007; (b) Fukuzawa, S.-i.; Tanihara, D.; Kikuchi, S.
Synlett 2006, 2145; (c) Chang, D.; Bao, W. Synlett 2006, 1786; (d) Ranu, B. C.;
Chattopadhyay, K.; Banerjee, S. J. Org.Chem. 2006, 71, 423; (e) Taniguchi, N. J.
Org. Chem. 2007, 72, 1241; (f) Wang, Z.; Mo, H.; Bao, W. Synlett 2007, 91; (g)
Braga, A. L.; Barcellos, T.; Paixão, M. W.; Deobald, A. M.; Godoi, M.; Stefani, H.
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21. Once-formed (Z)-4 cannot isomerize to (E)-4 under photoirradiation
(k >300 nm).