Highly chemoselective synthesis of functionalized diselenides from alkyl halides using benzyltriethylammonium tetrathiomolybdate

Article abstract of DOI:10.1039/a701512g

A variety of functionalized selenocyanates generated in situ from the corresponding alkyl halides undergo a facile reductive coupling on treatment with benzyltriethylammonium tetrathiomolybdate 1 under very mild conditions to give the corresponding disele

Full text of DOI:10.1039/a701512g

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Highly chemoselective synthesis of functionalized diselenides from alkyl halides
using benzyltriethylammonium tetrathiomolybdate
Kandikere R. Prabhu and Srinivasan Chandrasekaran*
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India
A variety of functionalized selenocyanates generated in situ
from the corresponding alkyl halides undergo a facile
reductive coupling on treatment with benzyltriethylammon-
ium tetrathiomolybdate 1 under very mild conditions to give
the corresponding diselenides in very good yields.
sulfur ligand to MoVI to produce MoIV species 15.10 Further
reaction of 15 with 1 results in the formation of alkyl diselenides
22
and Mo
2
S
8
.§
Table 1 Formation of diselenides
Organoselenium compounds have emerged as important rea-
gents and intermediates in organic synthesis and they also have
biological importance as proteins containing selenium are
Entry
Starting material
t/h
a
Product
Yield (%)b,c
1
Br
Se
2
essential components of certain bacterial and mammalian
enzyme systems.2 Hence a convenient synthesis of these
X
X
1
2
3
4
2a X = H
0.5
2
3a
3b
3c
3d
89
90
86
95
selenium compounds in general and functionalised organic
diselenides in particular continues to be a challenging task.
From the large number of methods available for the synthesis
of diselenides, the most important routes involve either the
2b X = NO2
2c X = CN
2d X = CO2Et
1
6
3
reaction of metal diselenides on alkyl halides or the preparation
4
5
6
HO
Me
O
Br
24
HO
( )6
Se
Se
96
85
of selenols which can be subjected to further oxidation.
(
)6
2
2
5
Recently Salama and Bernad reported that organic selenocya-
4
5
3
nates, on treatment with LiEt BH followed by air oxidation,
l
0.5
0.5
Me
give the corresponding diselenides in good yields. Most of the
methods available are generally useful only for the synthesis of
simple diselenides lacking reactive functional groups.
( )7
( )7
6
7
O
In our earlier work we have demonstrated that benzyl-
7
8
90
88
Br
Se
triethylammonium tetrathiomolybdate [PhCH
2
NEt
3
]
2
MoS
4
1 is
EtO
EtO
2
6
a useful sulfur transfer reagent in organic synthesis as well as
a reagent that can mediate reductive dimerization of alkyl
azides7 and organic thiocyanates.8 In exploring further the
utility of induced internal redox reactions of tetrathiomolybdate
8
9
Br
Br
Se
Se
1
in organic synthesis it appeared quite attractive to study the
0.5
reductive dimerization of organic selenocyanates with 1. Here
we present the results of such an investigation.
Treatment of a number of alkyl halides with KSeCN (1.1
equiv.) in MeCN (25 °C, 0.5 h) generated the corresponding
alkyl selenocyanates in situ which, on further reaction with
tetrathiomolybdate 1 (1.2 equiv., 25 °C, 0.5–24 h), led to the
formation of the alkyl diselenides in high yields (85–96%).†
The results are summarized in Table 1. As can be seen, except
in the case of hydroxy bromide 4 (entry 5), all the substrates
reacted with great facility in a short period of time (0.5–6.0 h).
Particularly noteworthy are the successful reactions of the
substrates 2b,c,d, 8 and 12, which contain easily reducible
groups such as nitro, cyano, ester and keto functionalities. The
1
0
11
O
O
Ph
Ph
9
4
87
Br
Se
2
1
2
13
a
Time refers to conversion of selenocyanate to diselenide. b Refers to
c
isolated yield.
data.
All the products exhibited expected analytical and spectral
5
9
3 4
methodology involving use of LiEt BH or NaBH for the
S
S
synthesis of diselenides cannot be applied successfully in many
of these cases. This kind of high chemoselectivity obtainable in
a room temperature reaction is very unusual. The facile
synthesis of a cyclic 8-membered diselenide 11 from dibromide
S–
S–
S
VI
S
S
VI
+ 2CN –
Mo
S
+ 2 RSeCN
Mo
RSe
RSe
1
0 (entry 8) in high yield also demonstrates the high potential of
1
4
the methodology. Unlike the earlier reported reactions of alkyl
selenocyanates, which proceed via alkyl selenol interme-
diates,5 we believe that the present methodology does not
,9
S
involve alkyl selenols as intermediates.‡ A tentative mechanism
2–
MoS42–
RSe
RSe
S
based on the reactivity of alkyl thiocyanates8 with tetra-
IV
Mo2S8
+ RSeSeR
Mo
thiomolybdate 1 is presented in the Scheme 1. It is likely that, in
S
22
the reaction of organic selenocyanates, the attack of MoS
4
on
S
the selenium of selenocyanate leads to mononuclear molybde-
num species of type 14. The second stage of the reaction can be
represented as an induced internal electron-transfer from a
1
5
Scheme 1
Chem. Commun., 1997
1021

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In conclusion, we have demonstrated that a variety of
functionalized alkyl diselenides can be synthesised in high yield
from alkyl halides via alkyl selenocyanates under mild
conditions. Further work is in progress to extend this method-
ology to unsymmetrical diselenides and seleno sulfides.
We thank the Department of Science and Technology, New
Delhi, for financial support of this investigation.
References
1
D. Liotta and R. Monahan III, Science, 1986, 231, 356 and references
cited therein.
2
T. C. Stadman, Science, 1974, 183, 915; D. Illman, Chem. Eng., 1994,
53; K. Schwarz and Fredga, Bioinorg. Chem., 1974, 3, 153.
3 J. Y. Wang, W. Cui and Y. Hu, J. Chem. Soc., Perkin. Trans. 1, 1994,
2341 and references cited therein.
4
A. Krief, A. F. De Mahien, W. Dumont and M. Trabelsi, Synthesis,
988, 131 and references cited therein.
1
Footnotes
5
6
P. Salama and C. Bernad, Tetrahedron Lett., 1995, 36, 5711.
A. R. Ramesha and S. Chandrsekaran, Synth. Commun., 1992, 22, 3277;
A. R. Ramesha and S. Chandrasekaran, J. Org. Chem., 1994, 59,
*
†
E-mail: scn@orgchem.iisc.ernet.in
No trace of selenol could be detected in the reaction mixture, and
1
354.
A. R. Ramesha, S. Bhat and S. Chandrasekaran, J. Org. Chem., 1995,
0, 7682.
K. R. Prabhu, A. R. Ramesha and S. Chandrasekaran, J. Org. Chem.,
995, 60, 7142.
tetrathiomolybdate 1 does not convert selenols to diselenides.
Typical procedure. To a solution of benzyl bromide 2a (1 mmol) in MeCN
5 ml) was added KSeCN (1.1 mmol). The mixture was stirred at room
7
8
9
‡
(
6
temperature (25 °C) for 0.5 h (until the starting material had disappeared, as
shown by TLC), and benzyltriethylammonium tetrathiomolybdate 1 (1.2
mmol) was added. The mixture stirred for 0.5 h, the solvent was evaporated
1
Cordora-reys, E. VandenHoven, A. Mohammad and B. M. Pinto, Can.
J. Chem., 1995, 73, 113.
in vacuo and the black residue thus formed was extracted with CH
Et O (1:9, 20 ml 3 5) and filtered through a Celite pad. The filtrate was
concentrated and the crude product was purified by column chromatography
silica gel, light petroleum (60–80 °C)–ethyl acetate, 9:1) to yield dibenzyl
2 2
Cl –
1
0 E. L. Coyle, M. H. Harmer, G. N. George, M. Daage and E. I. Steifel,
2
Inorg. Chem., 1990, 29, 14.
(
3
diselenide 3a as a solid, mp 90–91 °C (lit., 90–91 °C) in 87% yield.
The IR spectrum (KBr) of the molybdenum byproduct shows a strong
§
Received in Cambridge, UK, 4th March 1997; Com.
7/01512G
21
absorption at 515 cm assignable to a persulfido molybdenum species.
1022
Chem. Commun., 1997