β-nitro xanthates as olefin precursors

Article abstract of DOI:10.1021/ol035027c

(Matrix presented) Potassium O-ethyl xanthate readily adds to α,β-unsaturated nitro compounds to give stable β-nitro xanthates, which undergo tin-free elimination to form olefins in good yield and good E selectivity upon simple heating with lauroyl peroxi

Full text of DOI:10.1021/ol035027c

ORGANIC
LETTERS
2003
Vol. 5, No. 16
2907-2909
â-Nitro Xanthates as Olefin Precursors
Gilles Ouvry, Be´atrice Quiclet-Sire, and Samir Z. Zard*
Laboratoire de Synthe`se Organique associe´ au CNRS, Ecole Polytechnique,
91128 Palaiseau, France
zard@poly.polytechnique.fr
Received June 8, 2003
ABSTRACT
Potassium O-ethyl xanthate readily adds to r,â-unsaturated nitro compounds to give stable â-nitro xanthates, which undergo tin-free elimination
to form olefins in good yield and good E selectivity upon simple heating with lauroyl peroxide in refluxing 1,2-dichloroethane.
Radical elimination of nitrogen dioxide to form olefins was
developed by Kornblum et al.1 and by Ono et al.2 in early
investigations on the radical reactivity of nitro groups and
gem-dinitro compounds. This concept was used later by
Barton et al. in their tin-free olefin synthesis.3 The process
relies on the reaction of methyl radicals, generated from
stoichiometric amounts of N-acetoxy-2-thiopyridone (Barton
ester), with the relatively unstable â-nitro trithiocarbonates.
Recently, Yao has uncovered an efficient styrene synthesis
starting with â-nitrostyrenes and exploiting once again the
â-elimination of nitrogen dioxide.4
is usually an efficient reaction and has already been applied
to the synthesis of â-nitro dithiocarbamates.6 We were
pleased to see that the addition of 4 equiv of potassium
O-ethyl xanthate to a solution of a vinylic nitro compound
in acetic acid readily furnished the desired compound
(Scheme 1).
Scheme 1. Synthesis of â-Nitro Xanthate 2
As part of our program aimed at developing useful tin-
free radical procedures, we explored the reactivity of â-nitro
xanthates.5 Indeed, these compounds should also undergo
elimination in the presence of a source of reactive radicals
to give the corresponding olefins.
We applied this reaction to various nitro-olefins.7 The
reaction is compatible with mono- or disubstituted vinylic
nitro compounds. The yield is generally good and the
substitution pattern can be varied substantially (Table 1). The
â-nitro xanthates were always isolated as a mixture of
diastereoisomers and were easily handled and stored, in
contrast to the analogous trithiocarbonates.3
We have also found that these compounds could be readily
obtained directly from the acetylated Henry adducts.
We anticipated that the xanthate salt, even in the presence
of acetic acid, would be sufficiently basic to deprotonate R
To ascertain the feasibility of this approach to olefins, we
needed a simple way to access such â-nitro xanthates. The
Michael addition of nucleophiles to vinylic nitro compounds
(1) Kornblum, N.; Boyd, S. D.; Pinnick, H. W.; Smith, R. G. J. Am.
Chem. Soc. 1971, 93, 4316.
(2) Ono, N.; Miyake, H.; Tamura, R.; Kaji, A. Tetrahedron Lett. 1981,
18, 1705. Ono, N.; Miyake, H.; Kamimura, A. Tetrahedron 1985, 41, 4013.
(3) Barton, D. H. R.; Jaszberenyi, J. Cs.; Tachdjian, C. Tetrahedron Lett.
1991, 32, 2703. For a similar application see: Kobertz, W. R.; Bertozzi, C.
R.; Bednarski, N. D. J. Org. Chem. 1996, 61, 1894.
(4) Yao, C.-H.; Chu, C.-M.; Liu, J.-T. J. Org. Chem. 1998, 63, 719.
Liu, J.-T.; Yao, C.-F. Tetrahedron Lett. 2001, 42, 6147. Liu, J.-T.; Jang,
Y.-J.; Shih, Y.-K.; Hu, S.-R.; Chu, C.-M.; Yao, C.-F. J. Org. Chem. 2001,
66, 6021.
(5) For reviews see: Zard, S. Z. Angew. Chem., Int. Ed. Engl. 1997, 36,
672. Zard, S. Z. In Radicals in Organic Synthesis; Renaud, P., Sibi, M. P.,
Eds.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 1, pp 90-108.
(6) Guo, B.; Ge, Z.; Cheng, T.; Li, R. Synth. Commun. 2001, 31, 3021.
(7) Nitro olefins 1 were either commercillay available or synthesized
according to the classical Knœvenagel reaction: Jones, G. Org. React. 1967,
15, 204-599.
10.1021/ol035027c CCC: $25.00 © 2003 American Chemical Society
Published on Web 07/08/2003

examples (Scheme 3). In both cases, the acetylated Henry
adducts were not purified but were directly treated with
potassium O-ethyl xanthate in acetonitrile and acetic acid.
In the case of compound 2g, we used the modified Henry
conditions described by Go´mez-Sa´nchez.8
Table 1. Synthesis â-Nitro Xanthate 2
Scheme 3. Synthesis â-Nitro Xanthate 2a
a Reagents and conditions: (a) isobutyraldehyde, Et3N, TBAF,
TBDMSCl, THF, 0 °C; (b) Ac2O, DMAP, CH2Cl2, rt; (c) KSC(S)-
OEt, MeCN, AcOH; (d) MeNO2, Et3N.
Not unexpectedly, â-nitro xanthates are sensitive to base
and treatment with triethylamine regenerates the correspond-
ing nitro-olefin. The reversibility of the addition of the
xanthate salt can, however, be used to advantage by trapping
the resulting Michael adduct with an electrophile.
Indeed, the resulting anion could be readily captured with
formaldehyde, as illustrated by the transformations in Scheme
4. Unfortunately, we have not yet been successul in extending
Scheme 4. Functionalization of â-Nitro Xanthate 2
to the nitro group, and thus generate the vinylic nitro
compound as an intermediate (Scheme 2). The xanthate salt
Scheme 2. Synthesis of â-Nitro Xanthate 2 from Acetylated
Henry Adducts
this multicomponent process to other less reactive electro-
philes.
With a versatile approach to â-nitro xanthates in hand,
we examined their behavior under the action of a radical
would then add to the olefin and form the expected
compound. We successfully applied this variation to two
(8) Ferna´ndez, R.; Gasch, C.; Go´mez-Sa´nchez, A.; V´ılchez, J. E.
Tetrahedron Lett.1991, 32, 3225.
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Org. Lett., Vol. 5, No. 16, 2003

source, namely stoichiometric amounts of lauroyl peroxide
(DLP). We found that this cheap and safe peroxide, with a
half-life of 1-2 h at 80 °C, was quite convenient and
effective in promoting radical xanthate transfer reactions.5
The transformation we expected to accomplish is summarized
in Scheme 5,
Table 2. Radical Elimination of â-Nitro Xanthates 2
Scheme 5. General Scheme
The process is not a chain mechanism because nitrogen
dioxide is not a suitable chain carrier, and 3 equiv of initiator
were needed for total consumption of the starting material.
Presumably, the nitrogen dioxide produced in the process
scavenges a portion of the undecyl radicals generated in the
thermal decomposition of lauroyl peroxide. The results are
compiled in Table 2. The reactions were generally quite clean
and the yield good. Radical elimination of the nitrogen
dioxide is not stereospecific and both diastereoisomers of
the â-nitro xanthates give the thermodynamically most stable
olefin with E geometry. In other words, bond rotation is faster
than â-elimination of the nitrogen dioxide.
The reaction is efficient for the synthesis of mono-, di-,
or trisubstituted olefins. As for most radical reactions, it is
compatible with many functional groups (free alcohol, ethers,
esters, cyano). More interestingly, the reaction is compatible
with the presence of other nitro groups (compound 3h) and
with cyclopropane derivatives (compound 3f). In the latter
case, either the elimination of nitrogen dioxide is faster than
ring opening of the cyclopropane or the opening of the
cyclopropane is reversible under the reaction conditions. We
favor the former hypothesis in view of the high rate of
xanthate transfer. We would have expected this process to
compete successfully with the reverse 3-exo ring closure.
No ring-opened xanthate was observed. In any case, all these
examples highlight once again the remarkable chemoselec-
tivity of the xanthate-mediated radical chemistry. The
synthesis of dienes (compound 3d) is especially noteworthy
in this respect.
1
a E/Z ratio determined by H NMR.
compounds9 with the generality and mildness of the xanthate
radical fragmentation.
Acknowledgment. This paper is dedicated with respect
and affection to the memory of Professor Sylvestre Julia.
We also thank Ce´cile Blache`re for participating in this work
as part of an undergraduate research project.
Supporting Information Available: Detailed experi-
mental procedures and spectra data. This material is available
free of charge via the Internet at http://pubs.acs.org.
In summary, these preliminary results demonstrate the
potential of â-nitro xanthates as olefin precursors. These
compounds are easy to prepare and the structures that one
can construct can be quite complex. The olefination method
thus efficiently combines the vast ionic chemistry of nitro
OL035027C
(9) For general reviews on this chemistry see: (a) Barrett, A. G. M.
Chem. Soc. ReV. 1991, 20, 95. (b) Ono, N. The Nitro Group in Organic
Synthesis; Wiley-VCH: Weinheim, Germany, 2001.
Org. Lett., Vol. 5, No. 16, 2003
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