Asymmetric azidoselenenylation of alkenes: A key step for the synthesis of enantiomerically enriched nitrogen-containing compounds

Article abstract of DOI:10.1002/anie.200351229

Chiral, nonracemic azidoselenides such as 2 are useful intermediates for the synthesis of enantiomerically enriched nitrogen-containing compounds (e.g. 3). The asymmetric electrophilic azidoselenenylation of a variety of alkenes with the sulfur-containing

Full text of DOI:10.1002/anie.200351229

Angewandte
Chemie
azide,[3] or iodine azide.[4] Considerably improved results
were obtained by using organoselenium reagents. The first
example of the azidoselenenylation of alkenes was reported
by Hassner and Amarasekara.[5] The reaction was effected
with PhSeCl and sodium azide in DMSO and proceeded
through the formation of a cyclic seleniranium ion inter-
mediate, which then underwent ring opening by nucleophilic
attackof the azide anion. The addition products therefore
resulted from a stereospecific trans addition. However, the
reaction was not regiospecific. Similarly, the reaction of
exocyclic alkenes with N-(phenylseleno)phthalimide and
azidotrimethylsilane gave rise to a mixture of regioisomers.[6]
More recently we reported that the stereospecific azidosele-
nenylation of alkenes can be carried out more conveniently
with phenylselenenyl triflate and sodium azide in acetoni-
trile.[7] We have also reported the use of an azido radical to
promote the azidoselenenylation of olefins. The reaction is, of
course, not stereospecific in this case, and the anti-Markow-
nikoff addition products are formed.[8]
We report the first example of a remarkable asymmetric
electrophilic azidoselenenylation of olefins that occurs with a
very high level of facial selectivity. This process is made
possible by the use of chiral, nonracemic selenium reagents.
During the last 10 years several research groups have
developed simple and efficient procedures for the preparation
of chiral, nonracemic diselenides.[9] These compounds have
been employed in various asymmetric reactions, mainly as
precursors of electrophilic reagents,[9] but also as catalysts[10]
or as a source of chiral selenium anions.[11] A common
characteristic of all chiral diselenides studied is the close
proximity of a heteroatom (oxygen or nitrogen) that can
interact with selenium. We recently described the synthesis of
the sulfur-containing diselenides di-2-[(1S)-1-(methylthio)-
ethyl]phenyl diselenide (1)[12] and di-2-methoxy-6-[(1S)-1-
(methylthio)ethyl]phenyl diselenide (2).[13] Electrophilic
reagents derived from these diselenides were used to effect
asymmetric hydroxyselenenylation,[12,13] methoxyselenenyla-
tion,[12,13] and cyclofunctionalization reactions,[14] which pro-
ceeded with very high facial selectivity under very mild
experimental conditions.
Asymmetric Azidoselenenylation
Asymmetric Azidoselenenylation of Alkenes:
A Key Step for the Synthesis of Enantiomerically
Enriched Nitrogen-Containing Compounds**
Marcello Tiecco,* Lorenzo Testaferri, Claudio Santi,
Cristina Tomassini, Francesca Marini, Luana Bagnoli,
and Andrea Temperini
Organic azides are versatile starting materials for the syn-
thesis of a variety of nitrogen-containing compounds. The
azido group can react with both nucleophilic and electrophilic
reagents and can be used in 1,3-dipolar cycloaddition
reactions.[1] One of the most convenient ways to produce
organic azides is the electrophilic addition to alkenes of an
appropriate reagent, such as hydrazoic acid,[2] mercuric
Preliminary experiments on asymmetric azidoselenenyla-
tion were carried out on styrene with the chiral diselenides 1–
5. Upon treatment with bromine and silver triflate, 1–5 were
converted into the corresponding electrophilic selenenyl
triflate reagents 6–10. These reacted with styrene (11) in the
presence of 1 equivalent of sodium azide to afford a mixture
of the corresponding diastereomeric addition products 12–16
(Scheme 1).
[*] Prof. M. Tiecco, L. Testaferri, C. Santi, C. Tomassini, F. Marini,
L. Bagnoli, A. Temperini
The observed diastereomeric ratios and chemical yields
are summarized in Table 1. The excellent selectivity observed
Dipartimento di Chimica e Tecnologia del Farmaco
Sezione di Chimica Organica, Università di Perugia
06123-Perugia (Italy)
Table 1: Asymmetric azidoselenenylation of styrene.
Fax: (+39)075-585-5116
E-mail: tiecco@unipg.it
Diselenide
t [h]
Yield [%]
d.r.
[**] Financial support from MURST, National Project “Stereoselezione
in Sintesi Organica. Metodologie ed Applicazioni”, the University of
Perugia, Progetti di Ateneo, and CNR, Rome is gratefully acknowl-
edged.
1
2
3
4
5
22
21
20
21
30
90
90
70
10
28
91:9
97:3
52:48
87:13
75:25
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 3131 – 3133
DOI: 10.1002/anie.200351229
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Communications
(Table 2, entries 1, 2, 3, and 6) with Markownikoff orienta-
tion. The diastereomeric ratios were determined from the
1H NMR spectra of the crude reaction mixtures and con-
firmed after purification by column chromatography. Excel-
lent levels of diastereoselectivity were obtained in all cases.
The major isomers of the azidoselenides 13 and 18–20 are
depicted in Table 2. In the cases of 13, 19, and 20 these were
determined after conversion into the known oxazolines[15] 23,
26, and 27 (Scheme 2). For this purpose the azides were
reduced to the corresponding amines, which were then
treated in situ with CH3COCl at 08C. The acetamido
selenides 22, 24, and 25 thus obtained underwent a stereo-
specific SN2 intramolecular deselenenylation upon treatment
Scheme 1. Asymmetric azidoselenenylation of styrene.
with the diselenides 1 and 2 seems to indicate that the
interaction of the selenium atom with the sulfur atom is
stronger than its interaction with the other heteroa-
toms (oxygen or nitrogen) used in previous investiga-
tions.
On the basis of these results all
further reactions were carried out with
the diselenide 2 as precursor to the
electrophilic arylselenenyl triflate 7.
Experimental conditions, chemical
yields, and diastereomeric ratios for
the reactions of 7 with a variety of alkenes are reported
in Table 2.
Scheme 2. Conversion of azidoselenides into optically active oxazolines.
The azidoselenenylation products were obtained in every
case as an inseparable mixture of the two possible diaste-
reomers. The results reported in Table 2 indicate that this
azidoselenenylation reaction is a stereospecific trans addition
(Table 2, entries 2, 4, and 5) that occurs regioselectively
with SO2Cl2 to afford the oxazolines 23, 26, and 27,
respectively.[15] The absolute configurations of compounds
17 and 18 were assigned by analogy.
To highlight the importance of these compounds as
synthetic intermediates, some of the azidoselenides were
then transformed into other enan-
tiomerically enriched nitrogen-con-
taining compounds. The benzoyl
Table 2: Asymmetric azidoselenenylation of alkenes with the diselenide 2.[a]
Entry
Alkene
Azidoselenide
t [h]
Yield [%]
90
d.r.
derivative 28 was prepared from
20 as indicated in Scheme 3. The
corresponding selenoxide, obtained
by treatment of 28 with meta-chlor-
operbenzoic acid (mCPBA),[16]
underwent spontaneous deselene-
nylation to afford the optically
active aziridine 29 by an intramo-
lecular nucleophilic substitution,
and the a,b-unsaturated amide 30
by an elimination process.
1
styrene (11)
13
17
18
19
21
97:3
2
3
4
b-methylstyrene
a-methylstyrene
(E)-4-octene
20
70
60
98:2
99:1
18
2495
95:5
Enantiomerically
enriched
azides can be also conveniently
5
6
(E)-5-decene
20
21
20
20
95
70
95:5 employed in 1,3-dipolar cycloaddi-
tions to allow the synthesis of tri-
azoles.[17] Thus, as indicated in
1-methyl-1-cyclohexene
95:5
Scheme 4, the azide 13 was treated
with dimethyl acetylenedicarboxy-
[a] In a typical experiment, bromine (0.5 mmol) and silver triflate (1.1 mmol) were added to a solution of
the diselenide 2 (0.5 mmol) in MeCN (2.5 mL) at 08C. After 15 min the mixture was cooled to À308C
and sodium azide (1.0 mmol) was added. The reaction mixture was stirred for 30 min, then the alkene
(1.0 mmol) was added, and the mixture was allowed to warm to room temperature gradually. Upon
completion of the reaction (monitored by TLC and GC–MS), the mixture was filtered through anhydrous
K2CO3, and the filtrate was concentrated under vacuum.
late to give the triazole 31.[17] Dese-
lenenylation of 31 with triphenyltin
hydride and AIBN (azobisisobuty-
ronitrile) then afforded the triazole
32. The enantiomeric excess of 32
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Chemie
[10] a) M. Tiecco, L. Testaferri, C. Santi, C. Tomassini, F. Marini, L.
Bagnoli, A. Temperini, Tetrahedron: Asymmetry 2000, 11, 4645 –
4650; b) C. Santi, T. Wirth, Tetrahedron: Asymmetry 1999, 10,
1019 – 1023.
[11] T. Wirth, Tetrahedron 1999, 55, 1 – 28.
[12] M. Tiecco, L. Testaferri, L. Bagnoli, F. Marini, A. Temperini, C.
Tomassini, C. Santi, Tetrahedron Lett. 2000, 41, 3241 – 3245.
[13] M. Tiecco, L. Testaferri, C. Santi, C. Tomassini, F. Marini, L.
Bagnoli, A. Temperini, Chem. Eur. J. 2002, 8, 1118 – 1124.
[14] a) M. Tiecco, L. Testaferri, F. Marini, S. Sternativo, A. Temper-
ini, L. Bagnoli, C. Santi, Tetrahedron: Asymmetry 2001, 12,
1493 – 1502; b) M. Tiecco, L. Testaferri, L. Bagnoli, V. Purga-
torio, A. Temperini, F. Marini, C. Santi, Tetrahedron: Asymmetry
2001, 12, 3297 – 3304.
Scheme 3. Preparation of optically active aziridines.
[15] M. Tiecco, L. Testaferri, C. Santi, C. Tomassini, F.
Marini, L. Bagnoli, A. Temperini, Eur. J. Org.
Chem. 2000, 3451 – 3457.
[16] V. R. Ward, M. A. Cooper, A. D. Ward, J. Chem.
Soc. Perkin Trans. 1 2001, 944 – 945.
[17] G. Broggini, G. Molteni, G. Zucchi, Synthesis 1995,
647 – 648.
Scheme 4. Conversion of azidoselenides into optically active triazoles.
was identical to the diastereomeric excess of the starting
azide.
In conclusion, we have reported the first example of the
highly enantioselective addition of a nitrogen nucleophile to a
carbon–carbon double bond, which was made possible by the
use of chiral, nonracemic electrophilic selenium reagents. An
important aspect of this new reaction lies in the synthetic
applications of the resulting azidoselenides. These products
can be conveniently used in the synthesis of a variety of
nitrogen-containing derivatives of very high optical purity.
Received: February 19, 2003 [Z51229]
Keywords: alkenes · asymmetric synthesis · azides ·
.
heterocycles · selenium
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Angew. Chem. Int. Ed. 2003, 42, 3131 – 3133
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