Favorable effect of ortho substituents on selenoxide syn elimination of n-alkyl aryl selenoxides

Article abstract of DOI:10.1016/S0040-4039(00)00879-0

n-Alkyl aryl selenides with an ortho substituent were found to be superior to the corresponding para isomers as starting materials for the preparation of terminal olefins when hydrogen peroxide was used as the oxidant, because the para isomers afforded unwanted selenones and a primary alcohol as by-products. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00879-0

Tetrahedron Letters 41 (2000) 5557±5560
Favorable e€ect of ortho substituents on selenoxide syn
elimination of n-alkyl aryl selenoxides
Shinsei Sayama and Tetsuo Onami*
Department of Chemistry, Fukushima Medical University, Fukushima 960-1295, Japan
Received 23 March 2000; revised 22 May 2000; accepted 26 May 2000
Abstract
n-Alkyl aryl selenides with an ortho substituent were found to be superior to the corresponding para
isomers as starting materials for the preparation of terminal ole®ns when hydrogen peroxide was used as
the oxidant, because the para isomers a€orded unwanted selenones and a primary alcohol as by-products.
# 2000 Elsevier Science Ltd. All rights reserved.
Sharpless1 and other groups2,3 have reported that an electron-withdrawing substituent on the
benzene ring accelerated the selenoxide syn elimination of alkyl aryl selenoxides, and have
recommended the use of n-alkyl o-nitrophenyl selenides as starting materials for the preparation
of terminal ole®ns. However, the reason why the o-nitro derivatives are superior to the corre-
sponding p-nitro ones has not been clearly rationalized. In order to clarify this reason, the selen-
oxide elimination reaction of n-alkyl ortho-substituted phenyl selenides has been compared with
that of the corresponding para-substituted isomers using hydrogen peroxide as the oxidant. In
this communication, we would like to report on the favorable e€ect of ortho substituents on the
selenoxide elimination reaction.
The ole®n-forming reaction by the oxidation of alkyl aryl selenides with hydrogen peroxide is a
two-step reaction. First, the selenides are oxidized to selenoxides, and then a syn elimination
reaction of the selenoxides takes place to yield ole®ns. In order to compare the overall reaction of
n-alkyl ortho-substituted phenyl selenides with that of the para-substituted compounds, the pro-
1
gress of the reactions was followed by gas±liquid chromatography (GLC) and H NMR. The
reactions were conducted in tetrahydrofuran (THF) as usual, using aryl 1-dodecyl selenides 1a±1g
as the substrates and 30% hydrogen peroxide as the oxidant. In nuclear magnetic resonance
(NMR) measurements, however, the reactions were performed in acetone-d6 instead of THF-d8,
because non-deuterated THF in the solvent has resonance signals at 3.6±3.9 ppm, which overlap
with the signals of the reaction products.
* Corresponding author.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00879-0

5558
1
First, the progress of the reactions was monitored by 270 MHz H NMR. Every reaction rate
in the ®rst step (selenide to selenoxide) was not proportional to the product of the concentrations
of selenides 1a±1g and hydrogen peroxide. This is presumably because ArSe(O)OOH, which
formed gradually during the oxidation reaction, catalyzed the reaction in the ®rst step.2,4 6
Therefore, the half life (t1/2) of each selenide has been used to compare the reaction rates of the
®rst step. The results are given in Table 1. Selenides 1a, 1b, 1c and 1d have an electron-with-
drawing substituent on the benzene ring, and were oxidized to selenoxides more slowly than
selenides 1e, 1f or 1g. This could be explained by the fact that an electron-withdrawing sub-
stituent on the benzene ring diminishes the electron density of the lone pair of electrons on the
selenium atom.7,8
Table 1
Oxidation of selenide with H2O2 to selenoxide and its syn elimination to ole®n in acetone-d6
a
According to the literature,1 a selenide such as 1a, which has a nitro group at the ortho position
of the benzene ring, is expected to be the optimal compound for the preparation of terminal ole®n
3. However, this selenide was also the most resistant to oxidation to selenoxide in the ®rst step,
and its half life was approximately twice as long as that of p-nitro-substituted isomer 1b. This
slow reaction can be compensated by the subsequent fast step. The syn elimination reaction of
o-nitro substituted selenoxide 2a took place about 1.9 times as fast as that of the p-nitro-substituted
compound (see below). The rate of ole®n formation will be ®rst-order in selenoxides, because syn
elimination of selenoxides is a unimolecular reaction.9 Actually, ®rst-order kinetics were observed
for this second step.10 As can be seen in Table 1, the elimination rate of the selenoxides is faster
(2b>2d>2f) if the para substituent on the benzene ring is more electron-withdrawing. Such a
relationship can be seen in the corresponding ortho isomers (2a>2c>2e) and has been pointed
out by Sharpless et al.1
Comparing the elimination rate of each of the ortho-substituted selenoxides with that of the
corresponding para-substituted isomers, the former was faster than the latter (see k1 in Table 1):

5559
in the case of the nitro substituent, ortho-substituted selenoxide 2a decomposed to the ole®n
about 1.9 times as fast as para-substituted selenoxide 2b. Similar results were observed both in
methoxycarbonyl-substituted (2c versus 2d: 6.5 times) and methyl-substituted selenoxides (2e
versus 2f: 12 times). The selenium atoms in the o-nitro-substituted and o-methoxycarbonyl-
substituted selenoxides could be trigonal bipyramidal rather than pyramidal due to the coordination
of the substituents with the selenium atoms in the ortho position.11 These selenoxides would
consequently decompose faster than the corresponding para isomers, since the selenoxide oxygen
atom is closer to the leaving hydrogen atom in the trigonal bipyramid than in the pyramid
structure. This may be the reason why 2a and 2c decomposed faster than 2b and 2d, respectively.
o-Methyl-substituted selenoxide 2e, in which the selenium atom is pyramidal, is sterically more
hindered than the para isomer and hence would decompose to the products more rapidly in order
to relieve the steric strain.
As can be seen in Table 2, the yields of ole®n 3 are very high (98%, 97% after 18.5 h) for the
oxidation of o-nitro- and o-methoxycarbonyl-substituted selenides (1a, 1c), whereas in their para
isomers (1b, 1d) the yields are not as high (79%, 68% after 18.5 h) and a few percent of an
unexpected alcohol (1-dodecanol) was detected as a by-product. The same alcohol was also
obtained from p-methyl-substituted selenide 1f as well as from unsubstituted selenide 1g.12 On the
other hand, only trace amounts (less than 0.5%) of the alcohol were found in the reaction mixture
of ortho-substituted selenides (1a, 1c, 1e). Eventually, dodecyl 4-tolyl selenone13 was isolated from
the reaction mixture of selenide 1f, and decomposed gradually to the alcohol. It is known that
treatment of n-alkyl aryl selenones with water a€ords n-alkyl alcohols.14 The structure of the
selenone was characterized by NMR and high resolution mass spectroscopy (MS) analyses. The
77Se NMR chemical shift of the selenone (996 ppm relative to Me2Se) was di€erent from that of the
related benzeneseleninic acid (ca. 1173 ppm)15 and close to the chemical shift of selenones (ca. 970
ppm).16,17 Meanwhile, in the case of aryl selenoxides with an ortho substituent, almost no such
side reaction occurred. This is presumably because the ortho substituent would obstruct the
approach of hydrogen peroxide or arylperseleninic acid to the central selenium atom.
Table 2
Oxidation of aryl dodecyl selenide to 1-dodecene with H2O2 in THFa

5560
In conclusion, n-alkyl aryl selenides with an electron-withdrawing ortho substituent on the
benzene ring are more suitable compounds than the corresponding para isomers for the preparation
of terminal ole®ns, because the selenoxides of the former decompose to the ole®ns faster than
those of the latter, and hardly undergo further oxidation to selenones leading to the production
of unwanted primary alcohols. Although the o-nitro group in n-alkyl aryl selenides is one of the
best groups for the preparation of terminal ole®ns,18 the o-methoxycarbonyl group could also be
suitable, since selenide 1c was converted into terminal ole®n 3 in a high yield of 97% after 18.5 h
(see Table 2).
Acknowledgements
This work was supported by a Grant-in-Aid for Scienti®c Research (C) from the Japan Society
for the Promotion of Science (No. 11640541).
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(mol dm^3) of the selenoxide after complete disappearance of the selenide, and a-x is the concentration of the
remaining selenoxide after time t. However, in the cases of entries 2, 4, 6 and 7, calculations were made by multiplying
the obtained rate constant by the ratio of the formation rate of the ole®n to the consumption rate of the selenoxide,
because some selenoxides were oxidized to selenones in the presence of a large excess of H2O2 (see text).
11. (a) Baiwir, M.; Llabres, G.; Dideberg, O.; Dupont, L.; Piette, J. L. Acta Cryst. 1975, B31, 2188; (b) Hargittai, I.;
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12. In the case of selenides with a secondary alkyl group, for example, 1-methylheptyl phenyl selenide, the oxidation
reaction a€orded a small amount (1.3%) of 2-octanol as a by-product: Sayama, S.; Onami, T. Unpublished results.
13. 1H NMR (270 MHz, CDCl3) ꢀ=0.87 (3H, t, J=6.6 Hz), 1.92 (2H, quintet, J=7.8 Hz), 2.47 (3H, s), 3.44 (2H, m,
CH2-Se), 7.43 (2H, d, J=8.3 Hz), 7.85 (2H, d, J=8.3 Hz); 13C NMR (CDCl3) ꢀ=14.0 (CH3), 21.6 (CH3), 59.9
(CH2-Se), 127.0, 130.7, 138.2, 145.3; 77Se NMR (CDCl3) 996 ppm relative to Me2Se; anal. by high resolution MS:
found m/z: 372.1566. Calcd for C19H32O2Se: M, 372.1567.
14. Krief, A.; Dumont, W.; Denis, J.-N. J. Chem. Soc., Chem. Commun. 1985, 571.
15. Luthra, N. P.; Odom, J. D. The Chemistry of Organic Selenium and Tellurium Compounds; Patai, S.; Rappoport,
Z., Eds.; John Wiley & Sons: New York, 1986; Chapter 6.
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18. n-Alkyl 2-pyridyl selenides are also good starting materials for the preparation of terminal ole®ns: Toshimitsu, A.;
Owada, H.; Uemura, S.; Okano, M. Tetrahedron Lett. 1980, 21, 5037.