Synthesis of α-phenylseleno-α,β-unsaturated esters by Wittig-type reactions. Studies on the Diels-Alder reaction

Article abstract of DOI:10.1016/S0022-328X(00)00685-9

Ethyl α-phenylseleno-α,β-unsaturated esters were prepared by the reaction of tri-ethyl phosphonoacetate anion and ethoxycarbonyl(phenylselenenyl)methylidene(triphenyl)phosphorane with aliphatic and aromatic aldehydes. The α-phenylseleno unsaturated esters

Full text of DOI:10.1016/S0022-328X(00)00685-9

Journal of Organometallic Chemistry 623 (2001) 131–136
www.elsevier.nl/locate/jorganchem
Synthesis of a-phenylseleno-a,b-unsaturated esters by Wittig-type
reactions. Studies on the Diels–Alder reaction
Claudio C. Silveira *, Marta R.S. Nunes, Elson Wendling, Antonio L. Braga
Departamento de Quimica, Uni6ersidade Federal de Santa Maria, Caixa Postal 5001, 97105-970 Santa Maria, RS, Brazil
Received 28 July 2000; accepted 13 September 2000
Abstract
Ethyl a-phenylseleno-a,b-unsaturated esters were prepared by the reaction of tri-ethyl phosphonoacetate anion and ethoxycar-
bonyl(phenylselenenyl)methylidene(triphenyl)phosphorane with aliphatic and aromatic aldehydes. The a-phenylseleno unsaturated
esters were obtained as mixtures of E/Z-isomers in medium to good yields and in moderate yields, respectively. a-Phenylselenenyl
acrylate was used as dienophile in a Diels–Alder reaction. On reaction with isoprene only the para isomer was obtained while
reaction with cyclopentadiene gave a 1:1 mixture of exo/endo isomers in good yield. © 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Wittig reaction; Diels–Alder reactions; Selenium and compounds; Carboxylic esters; Microwave heating
1. Introduction
hydes, under acidic conditions [8]. These methods suffer
from the high cost and the toxicity of arsenium com-
pounds or from the low yields and selectivity of the
second method. All other methods lack generality [6].
Recently we described the preparation of such com-
pounds by the reaction of aldehydes with tellurium
ylids of a-bromo-a-phenylseleno acetate [6].
Organoselenium compounds play an important role
in modern organic synthesis [1]. Of the different classes
of organoselenium compounds, vinylic selenides consti-
tute a very useful group. They have been reviewed
recently [2].
Considerable efforts have been directed to extend the
Wittig and Wittig–Horner reactions to the preparation
of a-metal and non-metal substituted olefins [3], such as
vinylic chalcogenides [2], in view of their applications in
organic synthesis. The appropriate a-substituted phos-
phonium salt or phosphonate are the obvious starting
materials for these reactions [2,4].
a-Phenylseleno-a,b-unsaturated esters, a potentially
very useful class of vinylic selenides [5], are scarcely
described in the literature, with few general methods
reported for their preparation [5,6]. They have been
prepared by a Wittig-type reaction of methoxycar-
bonyl(phenylselenenyl)methylidene(triphenyl)arsorane
with aldehydes [7]. The reaction showed a medium to
high Z-stereoselectivity. They have also been prepared
from ethyl(phenylseleno)acetate and aromatic alde-
In view of the fact that vinylic chalcogenides have
great potential in organic synthesis [1,2] and in view of
the advantages of the Wittig reaction, we developed
methods for the synthesis of a-phenylseleno-a,b-unsatu-
rated esters based on the reaction of sodium(phenylse-
leno)methanetriethylphosphono acetate and carbo-
ethoxy(phenylseleno)methylideno(triphenyl)phosphor-
ane with aldehydes.
2. Results and discussion
2.1. Reaction of sodium(phenylseleno)methane
triethylphosphonoacetate with aldehydes
In the first part of the present study, a new method
for the synthesis of a-phenylseleno-a,b-unsaturated es-
ters was developed, based on a Wittig–Horner-type
reaction using as starting material triethylphosphonoac-
* Corresponding author. Tel./fax: +011-55-55 220 8754.
E-mail address: silveira@quimica.ufsm.br (C.C. Silveira).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00685-9

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C.C. Sil6eira et al. / Journal of Organometallic Chemistry 623 (2001) 131–136
etate (1). Thus, the treatment of a tetrahydrofuran
solution of 1 at 0°C with a base, followed by phenylse-
lenenyl bromide, generates the intermediate selenophos-
phonate 2, which reacts in situ with another base
equivalent, followed by an aldehyde, to give the desired
olefinic product 3, according to Scheme 1.
There have been reports in the literature [7,9] that the
anion of 2 would be totally inert to the reaction with
carbonyl compounds, in a Wittig–Horner-type reac-
tion. This low reactivity of the anion of 2 would be
anticipated as a result of three groups which stabilize a
carbanion at the a-position. However, in order to
achieve good yields in Wittig–Horner reactions, the
presence of a further electron-withdrawing a-sub-
stituent on the phosphonate anions is required [10].
Thus, we decided to perform the reaction as shown in
Scheme 1. Indeed desired products were obtained with
both aromatic and aliphatic aldehydes, in yields rang-
ing from 49 to 89% (Table 1).
In order to find the best experimental conditions, a
detailed study was performed using benzaldehyde as the
standard aldehyde. The selenophosphonate 2 was gen-
erated in situ for convenience in view of an observed
low stability, although Paulmier described its prepara-
tion some years ago [11]. Initially, looking for the best
base, lithium derived bases, like LDA, Li-hexamethyld-
isilylamide (Li-HMDS) and lithiumdicyclohexylamide
were employed, but the yields were very low or the
products were not formed. Better results were obtained
by the use of sodium derivatives, like Na-HMDS and
NaH. The best results were observed with NaH as base
(73% yield) which was used for the present study with
all other aldehydes. Among the solvents examined,
THF turned out to be the best choice.
The use of higher amounts of PhSeBr or the use of
PhSeI (generated from PhSeSePh and I2), did not re-
duce its formation. This byproduct could be easily
removed by Kugelrohr distillation prior to the purifica-
tion of the product by column chromatography. We
observed that protection from light was useful, particu-
larly when longer reaction times were necessary (ben-
zaldehyde,
p-chlorobenzaldehyde,
furfural
and
tolualdehyde). It is noteworthy that the reaction
worked well even for formaldehyde, to give acrylate 3f,
the simplest member of this class of compounds. It was
used successfully as substrate for an investigation of the
reactivity as a dienophile in a Diels–Alder reaction of
such selenides (vide infra).
The reactions were also performed with several dif-
ferent ketones, but in all cases we were not able to
detect the formation of the desired product, which it is
not a surprising result in view of the expected low
reactivity and the bulkiness of the tri-substituted carb-
anion 2a-Na.
Concerning the stereochemistry of these olefinations,
the formation of a mixture of E and Z isomers was
observed, with a small preference for the E isomer. The
ratios of the geometrical E and Z isomers and other
experimental details are given in Table 1. The geometri-
cal isomers were assigned by the chemical shifts of the
1
vinylic hydrogens in the H-NMR spectra. Thus, the
signals of the vinylic hydrogen are shifted ca. 1 ppm
downfield for the Z-isomers as compared to E-isomers.
This absorption position has been well established in
several previous studies [6]. For example, the Z isomer
on the reaction with benzadehyde showed the vinylic
hydrogen at l=8.15 and the E isomer resonates at
l=7.04. The Z/E ratio was determined by GC analysis
from both the crude reaction mixture and the purified
compounds, and confirmed by comparing 1H-NMR
with some previously described compounds [6].
In all cases, for both aliphatic and aromatic alde-
hydes, the formation of a byproduct, identified as the
a,b-unsaturated ester free of selenium, was isolated.
Scheme 1.
Table 1
Synthesis of a-phenylseleno-a,b-unsaturated esters according to Scheme 1
b
Entry
R in 3
Conditions
Time (h)
Yield (%) a
E/Z
3a
3b
3c
3d
3e
3f
2-furyl
Reflux THF
Reflux THF
Reflux THF
Reflux THF
Reflux THF
Reflux THF
r.t.
6
3
12
12
6
0.45
1
3
87
73
65
73
49
76
89
86
65:35
40:60
60:40
67:33
60:40
4-NO2C6H4
4-ClC6H4
C6H5
4-CH3C6H4
H
3g
3h
n-CH3CH2CH2
n-CH3(CH2)4
65:35
70:30
Reflux THF
a Isolated yield after column chromatography.
b Determined by GC from the crude reaction mixture.

C.C. Sil6eira et al. / Journal of Organometallic Chemistry 623 (2001) 131–136
133
Scheme 2.
2.2. Reaction of ethoxycarbonyl(phenylseleno)-
methylidene(triphenyl)phosphoranes with aldehydes
under microwa6e irradiation
which is quite acceptable, considering the very low
reactivity of the starting phosphorane 5. Noteworthy is
the beneficial effect of microwave irradiation for the
unreactive species, making possible reactions that by
other methods might be very difficult. Unfortunately,
the corresponding vinylic selenides were obtained as
mixtures of isomers, but with reversed stereochemistry,
compared to the products obtained by reactions via
selenophosphonates (vide infra). In a similar way, in
the present case the formation of the same byproducts
was observed, the a,b-unsaturated ester, and variable
amounts of diphenyl diselenide.
Some years ago, Petragnani and Moura Campos [12]
described the preparation of the selenophosphorane 5,
a white crystalline solid, very stable to storage at room
temperature, which was shown to be totally inert in
Wittig-type reactions with carbonyl compounds [12].
The preparation of the phosphorane 5 has been formu-
lated to proceed via a transylidation reaction between
carbethoxymethylideno(triphenyl)phosphorane (4) and
arylselenenyl bromide (Scheme 2) [12]. We found that
the selenophosphorane 5 could also be prepared conve-
niently by the reaction of equimolar amounts of car-
bethoxymethylideno(triphenyl)phosphorane (4) and
phenylselenenyl bromide, treating the resulting salt im-
mediatly with aqueous sodium hydroxide solution, to
give the desired phosphorane in 60% yield after recrys-
talization (Scheme 2).
In recent years, microwave irradiation has been
found to be a very useful tool for some transformations
in organic synthesis [13]. Thus, we decided to study the
Wittig-type reaction of phosphorane 5 with aldehydes
under microwave irradiation. Several a-phenylseleno-
a,b-unsaturated esters could be prepared successfully by
this route (Scheme 3, Table 2).
2.3. Study on the reacti6ity of 3f in a Diels–Alder
reaction
The Diels–Alder reaction is one of the most useful
and powerful tools in preparative organic chemistry
[14]. In this respect we decided to perform studies on
the reaction of a-phenylseleno-a,b-unsaturated esters as
dienophiles in Diels–Alder reactions with commercially
available dienes. For our study, we decided to use the
compound 3f, the simplest reagent of this class of com-
pounds. Although 3f is expected to be a good
Initially, with the standard conditions, with THF as
solvent under reflux, products were not detected after 2
days, which is in accordance with previous observation
[12]. For the microwave irradiation experiments, a do-
mestic apparatus was used and the reactions were con-
ducted in an open flask. The reaction was studied in
detail having benzaldehyde as the standard aldehyde,
following the reaction by TLC and GC. The first
experiments were performed in the absence of catalyst
and in toluene as the solvent, with very low yields of up
to 15%. By removal of the solvent and the use of a
catalytic amount of benzoic acid, better results were
obtained. In the case of solid aldehydes, it was neces-
sary to use a small amount of solvent (toluene), enough
to give a homogeneous solution. The yields were also
dependent on the power of the microwave apparatus
used. The best results were obtained at 560 W of
irradiation, with a reaction time ranging from 3 to 10
min.
Scheme 3.
Table 2
a-Phenylseleno-a,b-unsaturated esters prepared by microwave (560
W) irradiation
b
Entry R in 3
Time (min) Yield (%) a E/Z ratio
3a
3b
3c
3d
3e
3f
2-furyl
9
10
6
6
3
10
6
6
41
41
25
40
10
48
44
50
22:78
30:70
23:77
25:75
43:57
4-NO2C6H4
4-ClC6H4
C6H5
4-CH3C6H4
H
3g
3h
n-CH3CH2CH2
n-CH3(CH2)4
12:88
17:83
a Isolated yield after column chromatography.
b Determined by GC from the crude reaction mixture.
The results, presented in Table 2, showed that mod-
erate yields of the desired products could be obtained,

134
C.C. Sil6eira et al. / Journal of Organometallic Chemistry 623 (2001) 131–136
acquired on a Bruker IFS 28 spectrometer and elemen-
tal analyses were performed on a Vario EL elemental
analysis system. The microwave reactions were per-
formed on a domestic apparatus, Panasonic NN 6556.
Merck silica gel 60 (230–400 mesh) was used for flash
chromatography. THF was distilled over sodium–ben-
zophenone immediately before use. Triethyl phospho-
noacetate [15] and carbethoxymethylene(triphenyl)-
phosphorane [16] were prepared by literature methods.
Scheme 4.
3.2. General procedure for the synthesis of
h-phenylseleno-h,i-unsaturated esters (3a–3h) from the
phosphonate 1
dienophile, to the best of our knowledge, there are no
reports on its Diels–Alder reactivity.
The first experiment with 3f was performed with
isoprene at high temperatures, which gave negative
results. With cyclopentadiene it was possible to detect
the formation of the desired product, albeit in low
yield. The use of solvents like toluene and acetone does
not improve the results. However, the use of ZnBr2 as
catalyst resulted in the formation of the desired adduct
by reaction with cyclopentadiene in 89% isolated yield
(Scheme 4). Under the same conditions, the reaction
also gave a good result with isoprene as the diene.
The reactions were performed using an excess of the
diene (up to 10 equivalents) and 1.2 equivalents of the
Lewis acid in dichloromethane as the solvent. The
Diels–Alder reaction of 3f with isoprene proceeded
regioselectively to give 6 in 77% isolated yield after 48
h at room temperature. The formation of the other
regioisomer was not detected by NMR. The formation
of the para isomer was also confirmed by a NOESY
NMR study.
The reaction of 3f with cyclopentadiene, under the
conditions applied, showed no stereoselectivity, and the
adduct 7 was obtained with an endo/exo ratio of 1:1.
The compounds 6 and 7 could serve as versatile
synthetic intermediates for selenium-containing poly-
cyclic systems. A significant feature of the reaction is
that the dienophile can act as a propiolic ester synthon,
through the removal of phenylseleninic acid from the
corresponding selenoxide.
To a round bottom flask equipped with a reflux
condenser containing a suspension of NaH (0.072 g, 3
mmol) in THF (10 ml), at 0°C under nitrogen, was
added triethylphosphonoacetate (1) (0.231 g, 1 mmol),
followed by a THF (5 ml) solution of phenylselenenyl
bromide (0.35 g, 1.5 mmol). Immediate reaction was
observed, with disappearance of the deeper color of
PhSeBr, leaving the solution yellow. The aldehyde (1.5
mmol) was added after a few minutes, the ice-bath was
removed and the reaction was heated under reflux for
the time indicated in Table 1. The reaction mixture was
cooled, treated with water (5 ml) and a saturated
aqueous solution of NH4Cl (50 ml) and extracted with
ethyl acetate (2×25 ml). The organic phase was dried
over MgSO4 and the solvent removed in vacuo. The
residue was purified by Kughelrohr distillation (to re-
move a byproduct, see text) followed by column chro-
matography in silica gel, eluting with hexane–ethyl
acetate (99:1), to yield 3a–3h [17].
3.3. Preparation of carbethoxymethylidene-
(phenylseleno)triphenylphosphorane (5)
[Ph3PꢀC(SePh)CO2Et]
In a 100 ml flask containing a solution of car-
bethoxymethylene (triphenyl)phosphorane [15] (3.48 g,
10 mmol) in dichloromethane (10 ml), was added,
dropwise, a solution of phenylselenenyl bromide (2.3 g,
10 mmol) in dichloromethane (10 ml). The mixture was
stirred at room temperature for 5 min and the solvent
was removed under vacuum. The residue was dissolved
in water (50 ml), a few drops of an aqueous solution of
phenolphthaleine were added, followed by a 0.1 N
aqueous solution of NaOH until a pink color remained.
3. Experimental
3.1. General
1H- and 13C-NMR spectra of CDCl3 solutions were
recorded on a 200 MHz, or on a 400 MHz Bruker
DPX-series NMR spectrometer. Chemical shifts are
expressed as parts per million (ppm) downfield from
tetramethylsilane as an internal standard. Mass spectra
(EI) were obtained at 70 eV with a Hewlett Packard
EM/CG HP-5988A spectrometer, infrared spectra were
The
aqueous
solution
was
extracted
with
dichloromethane, dried over MgSO4, the solvent re-
moved in vacuo and the solid residue recrystallized
from methanol to yield 5 (3.0 g, 60%); m.p. 178–180°C
(literature value 182°C [12]).

C.C. Sil6eira et al. / Journal of Organometallic Chemistry 623 (2001) 131–136
135
3.4. General procedure for the synthesis of
252 (60), 172 (100).
h-phenylseleno-h,i-unsaturated esters (3a–3h) from the
phosphoranes 5
Ethyl(E+Z)-2-phenylseleno-2-nonenoate (3h): oil.
IR (film): 1710 cm−1 (CꢀO). lH (200 MHz, CDCl3) 0.87
(t, 3H, J=7.0 Hz); 1.11 and 1.15 (2t, 3H, J=7.0 Hz);
1.25–1.68 (m, 8H); 2.45 (q, 2H, J=7.0 Hz); 4.06 and
4.10 (2q, 2H, J=7.0); 6.28 (t, 1H, E isomer); 7.23–7.57
(m, 6H, includes Z isomer).
A 15 ml round bottom containing the selenophospho-
rane 5 (0.5 g, 1 mmol), benzoic acid (ꢀ10 mg) the
aldehyde (5 mmol) and toluene (1 ml; for the solid
aldehydes) was irradiated in a domestic microwave oven
(560 W) for the time indicated in Table 2. The reaction
mixture was cooled to room temperature, water was
added and the reaction was extracted with ethyl acetate
(2×25 ml). The combined organic phases were dried
over MgSO4 and the solvent was removed in vacuo. The
product was purified as above.
Ethyl(E+Z)-2-phenylseleno-3-(2-furyl)-2-propeno-
ate (3a): oil. IR (film) 1716 cm−1 (CꢀO). lH (200 MHz,
CDCl3) 1.03 and 1.14 (2t, 3H, J=7.2 Hz); 4.04 and 4.14
(2q, 2H, J=7.2 Hz); 6.36 (dd, 1H, J=3.6 and 2 Hz);
6.46 (dd, 1H, J=3.6 and 2 Hz); 6.69 (dd, 1H, J=3.4
and 0.4 Hz); 6.77 (s, E isomer); 8.03 (s, Z isomer)
(E+Z; 1H); 7.14–7.6 (m, 6H).
Ethyl(E+Z)-2-phenylseleno-3-(4-nitrophenyl)-2-pro-
penoate (3b): yellow–orange oil. IR (film): 1699 cm−1
(CꢀO). lH (200 MHz, CDCl3) 1.08 (t, 3H, J=7.2 Hz);
4.05 (q, 2H, J=7.2 Hz); 6.85 (s, E isomer); 8.05 (s, Z
isomer) (E+Z; 1H); 7.19–7.73 (m, 7H); 8.11–8.20 (m,
2H).
Ethyl(E+Z)-2-phenylseleno-3-(4-chlorophenyl)-2-
propenoate (3c): oil. IR (film): 1715 cm−1 (CꢀO). lH
(200 MHz, CDCl3)1.04 and 1.05 (2t, 3H, J=7.0 Hz);
4.02 and 4.06 (2q, 2H, J=7.0 Hz); 6.94 (s, 1H, E vinylic
H); 8.06 (s, 1H, Z vinylic H); 7.15–7.69 (m, 9H). Anal.
Found: C, 55.8; H, 4.1. C17H15ClO2Se requires: C, 55.8;
H, 4.0%.
3.5. Typical procedure for the Diels–Alder reaction
To a suspension of dry ZnBr2 (1.2 mmol, 0.27 g) in
dichloromethane (2 ml) at 0°C was added a solution of
3f (1 mmol, 0.25 g) in dichloromethane (2 ml). The
mixture was stirred for 10 min and the diene (10 mmol)
was added. The reaction was stirred for 48 h at room
temperature (for isoprene) or for 3 h at 0°C (for
cyclopentadiene). After completion,
a
saturated
aqueous solution of NaHCO3 (50 ml) was added, ex-
tracted with dichloromethane (2×25 ml); the organic
phase was dried over MgSO4 and the solvent removed
in vacuo. The residue was purified by column chro-
matography on silica gel, eluting with hexane–ethyl
acetate (99:1).
Ethyl-4-methyl-1-phenylselanyl-3-cyclohexene-1-car-
boxylate (6): oil. IR (film): 1720 cm−1 (CꢀO). lH (400
MHz, CDCl3) 1.12 (t, 3H, J=7.0 Hz); 1.62 (s, 3H);
1.97–2.18 (m, 4H); 2.43 (AB quart, 2d, J=18 Hz); 4.02
(q, 2H, J=7.0 Hz); 5.29 (s, 1H); 7.23–7.60 (m, 5H).
13C-NMR (100 MHz, CDCl3) lC 13.74; 23.08; 28.37;
30.16; 33.45; 48.16; 60.49; 118.57; 125.64; 127.48; 128.41;
128.90; 131.28; 133.37; 137.14; 173.01. m/z 323
(M+); 167 (40); 93 (100); 77 (40). Anal. Found: C, 59.55;
H, 6.33. C16H20O2Se requires: C, 59.25; H, 6.22%.
Ethyl-2-phenylselanylbicyclo[2.2.1]hept-5-ene-2-car-
boxylate (7): oil. IR (film): 1727 cm−1 (CꢀO). 1H-NMR
lH (400 MHz, CDCl3) exo+endo: 1.07 and 1.08 (2t,
3H, J=7.0 Hz); 3.79–3.90 (m, 2H); 7.20–7.55 (m, 5H);
exo isomer 1.22 (d, 1H, J=9.2); 1.60–1.64 (m, 1H);
1.93 (dd, 1H, J=12.8 and 3.6 Hz); 2.15 (dd, 1H,
J=12.8 and 2.4 Hz); 2.92 (br s, 1H); 3.44 (s, 1H); 6.15
(m, 1H); 6.22 (m, 1H); endo isomer 1.36 (dd, 1H,
J=12.8 and 2.8 Hz); 1.60–1.65 (m, 1H); 2.15 (d, 1H,
J=8.4 Hz); 2.61 (dd, 1H, J=12.8 and 3.6 Hz); 2.92 (br
s, 1H); 3.07 (s, 1H); 5.84 (m, 1H); 6.09 (m, 1H).
13C-NMR (100 MHz, CDCl3) lC 13.75; 13.82; 37.80;
37.98; 41.93; 42.76; 47.50; 48.02; 49.38; 49.50; 55.17;
56.15; 60.40; 60.85; 128.26; 128.37; 128.60; 128.86;
133.82; 134.65 138.26; 138.88; 173.46; 174.03. Anal.
Found: C, 60.2; H, 5.60. C16H18O2Se requires: C, 59.8;
H, 5.60%.
Ethyl(E+Z)-2-phenylseleno-3-phenyl-2-propenoate
(3d): oil. IR (film): 1714 cm−1 (CꢀO). lH (200 MHz,
CDCl3) 1.00 and 1.03 (2t, 3H, J=7.0 Hz); 4.02 and 4.05
(2q, 2H, J=7.0 Hz); 7.16–7.62 (m, 10H); 7.04 (s, 1H,
E vinylic H); 8.15 (s, 1H, Z vinylic H). Anal. Found: C,
61.8; H, 4.8. C17H16O2Se requires: C, 61.6; H, 4.8%.
Ethyl(E+Z)-2-phenylseleno-3-(4-methylphenyl)-2-
propenoate (3e): oil. IR (film): 1714 cm−1 (CꢀO). lH
(200 MHz, CDCl3) 1.03 (t, 3H, J=7.0 Hz); 2.34 and
2.39 (2s, 3H); 4.05 (q, 2H, J=7.0 Hz); 7.06–7.60 (m,
10H, includes E vinylic H); 8.15 (s, 1H, Z isomer;
literature 8.19 [5]).
Ethyl-2-phenylseleno-acrylate (3f): oil. IR (film): 1725
cm−1 (CꢀO). lH (200 MHz, CDCl3) 1.30 (t, 3H, J=7.0
Hz); 4.26 (q, 2H, J=7.0 Hz); 5.34 (s, 1H); 6.65 (s, 1H);
7.3–7.6 (m, 5H).
Ethyl(E+Z)-2-phenylseleno-2-hexenoate (3g): oil.
IR (film): 1710 cm−1 (CꢀO). lH (200 MHz, CDCl3) 0.89
(t, 3H, J=7.0 Hz); 0.98 (t, 3H, J=7.0 Hz); 1.02–1.6 (m,
2H); 2.42 and 2.46 (2q, 2H, J=7.3 Hz); 4.09 and
4.12 (q, 2H J=7.0 Hz); 6.29 (t, J=7.7, 1H, E isomer);
7.17–7.53 (m, 6H, includes Z isomer). m/z 298 (M, 97%),
Acknowledgements
The authors thank the following agencies for finan-
cial support: FAPERGS, CNPq and PADCT. Thanks

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C.C. Sil6eira et al. / Journal of Organometallic Chemistry 623 (2001) 131–136
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