Highly efficient generation of ammonium eneselenolates, their reactions and electronic properties

Article abstract of DOI:10.1246/cl.2000.368

The ammonium eneselenolates generated from selenothioic acid S-esters and ammonium fluoride were reacted with carbon electrophiles to furnish ketene selenothioacetals with high stereoselectivities. The spectroscopic properties of the ammonium eneselenolat

Full text of DOI:10.1246/cl.2000.368

368
Chemistry Letters 2000
Highly Efficient Generation of Ammonium Eneselenolates,
Their Reactions and Electronic Properties
Toshiaki Murai, Shuuya Hayakawa, and Shinzi Kato*
Department of Chemistry, Faculty of Engineering, Gifu University, Yanagido, Gifu 501-1193
(Received January 12, 2000; CL-000038)
The ammonium eneselenolates generated from selenothioic
acid S-esters and ammonium fluoride were reacted with carbon
electrophiles to furnish ketene selenothioacetals with high stereo-
selectivities. The spectroscopic properties of the ammonium ene-
selenolates have suggested that the electrons on the selenium
atom efficiently delocalize on the carbon-carbon double bond.
Conjugate anions such as allylic anions and enolates are
one of the most important chemical species in organic chem-
istry.1 The introduction of a heavier atom to conjugate anions
is of great interest. For example, sulfur isologues of enolates
have been extensively studied.2 In contrast, only a limited
examples of metal enolates involving a selenium atom, i.e. ene-
selenolates have been known to be formed from vinyl metallic
species and a selenium atom.3 Very recently, we have dis-
closed that selenium counterpart of lithium enolates, i.e., lithi-
um eneselenolates were generated by the treatment of selenoth-
ioic acid S-esters with LDA4 similarly to the ordinary esters and
thioesters. We report herein highly efficient generation, reac-
tion and electronic properties of ammonium eneselenolates.
The purple selenothioacetic acid S-butyl ester 1a turned
light brown almost instantly by adding a THF solution of tetra-
butylammonium fluoride (eq 1).5 The aqueous workup of the
reaction mixture gave a complex mixture, but a similar treat-
ment of the ester 1a with ammonium fluoride in the presence of
a variety of carbon electrophiles gave the products 3, which
cannot be easily obtained by other methods.6 The results are
summarized in Table 1.7
The use of methyl iodide and ethyl bromide afforded ketene
selenothioacetals 3a and 3b within 1 min in 85 and 81% yields,
respectively (entries 1 and 2).8 In the reaction of epibromohy-
drin the substitution reaction selectively took place at the
bromine-substituted carbon atom to give the product 3c (entry
3). The use of allylic halides gave the γ,δ-unsaturated selenoth-
ioic acid S-esters 1c, 1f, and 1g in good yields (entries 4–6). In
these reactions the carbon atoms having substituents such as
methyl, ethoxycarbonyl, and phenyl groups were selectively
introduced to the α-carbon atom to the selenocarbonyl group.
The deprotonation with ammonium fluoride was also appli-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
369
cable to α-monosubstituted and disubstituted selenothioic acid
esters 1b-1e. The subsequent treatment of 2b–2e with alkyl
halides gave ketene selenothioacetals 3d – 3h (entries 7–12).
Noteworthy is the ratio of E and Z isomers of the ketene
selenothioacetal 3d. The generation of ammonium eneseleno-
late 2b was complete within one minute, but the quick addition
of methyl iodide gave both isomers of 3d in a nealy equal ratio
(entry 7). The stereoselective formation of 3d was attained by
mixing the ester 1b with ammonium fluoride for 30 min, fol-
lowed by the addition of MeI (entry 8). Benzoylation of 2c gave
the product 3f exclusively as a Z-isomer (entry 10). Even α-di-
subsituted esters 1d and 1e were converted to ketene selenoth-
ioacetals 3g and 3h with high stereoselectivity (entry 12).
The ammonium salts 2 were stable enough to monitor
NMR spectra. The results are shown in Table 2.
pling constants between the carbon atom and the selenium atom
of the ammonium salts 2b (1J = 187.5) and 2c (1J = 175.7).
They are close to the normal value of the carbon-selenium dou-
ble bond (ca. 200 Hz),11 and this is in good agreement with a
partial double bond character of the carbon-selenium bond in 2.
Thus, this properties may allow for the easy isomerization of
two isomers of 2.
In summary, we have found the first example of ammoni-
um eneselenolates from selenothioic acid S-esters. Alkylations
of ammonium eneselenolates provided highly stereoselective
synthetic methods for ketene selenothioacetals. Electronic
properties of eneselenolates have also been disclosed. Further
studies on the application of the present system are in progress.
This work was supported in part by a Grant-in-Aid for
Scientific Research on Priority Areas (No. 11120222) and by
Grant-in-Aid for Scientific Research (No. 9355032) from the
Ministry of Education, Science, Sports and Culture, Japan.
References and Notes
1
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2
3
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Typical experimental procedure for the synthesis of ketene
selenothioacetals: To a solution of THF (5 mL) and selenothioic
acid S-butyl ester 1 (1 mmol) were added a THF solution of tetra-
butylammonium fluoride (1.5 mmol) and alkyl halides (1 mmol)
successively at 0 °C. The mixture was then stirred for 30 min at
0 °C, poured into water and extracted with Et2O three times. The
combined organic layers were dried over MgSO4 and concentrat-
ed. The residue was chromatographed through silica gel column
with hexane–Et2O as eluent to give the corresponding ketene
selenothioacetals 3.
NMR spectra of ammonium eneselenolates 2b and 2c have
clearly indicated that they were formed as the single stereoiso-
mer of 2b and 2c despite the fact that the stereoselectivity of
alkylation of 2b depended on the reaction time as shown in
Table 1. The stereochemistry of 2b and 2c was assigned as Z-
geometries on the basis of their phase sensitive NOESY spec-
tra. Thus the generation of the ammonium eneselenolates may
take place with no stereoselectivity, but the isomerization of E-
2 to Z-2 may smoothly proceed through the rotation of the car-
bon-carbon double bond in 2 (Scheme 1).9
6
A. Krief, in "Comprehensive Organometallic Chemistry," ed by
E. W. Abel, F. G. A. Stone, and G. Wilkinson, Pergamon, Oxford
(1995), Vol. 11, p. 515.
7
8
All new compounds gave satisfactory spectral and microanalytical
data.
The high reactivity of the ammonium eneselenolate 2a toward
alkyl halides is in sharp contrast to the reactivity of lithium ene-
selenolates generated from 1a and LDA. The methylation of
lithium eneselenolates gave 3a only in low yields.
Very recently, the isomerization between Z and E-isomers of
lithium enethiolates was observed for the first time: A. L. Schwan
and M. D. Refvik, Synlett, 1998, 96.
9
The selenium atom of ketene selenothioacetals Z-3d and Z-
3e was observed at 202.2 and 231.4 ppm, respectively, in 77Se
NMR spectra. It should be noted that those of ammonium salts
2b and 2c was shifted lower field by about 50 ppm. The seleni-
um atom having negative charge is generally observed in the
field higher than 0 ppm.10 In contrast, the selenium of ammoni-
um salts 2b and 2c is strongly deshielded even if it has an
anionic character. This may be explained by noting that the
electrons on the selenium atom efficiently delocalize on the car-
bon-carbon double bond. Further evidence for the delocaliza-
tion of the electrons in ammonium eneselenolates 2 is the cou-
10 The NMR sample was prepared as follows. The reaction mixture
of esters 1 (0.5 mmol) and a THF solution of tetrabutylammoni-
um fluoride (0.75 mmol) was concentrated in vacuo at room tem-
perature over 1 h. Then, to the remaining viscous liquid was
added THF-d8 (0.8 mL).
11 H. Duddeck, Prog. NMR Spectrosc., 27, 1 (1995); T. M.
Klapötke and M. Broschag, "Compilation of Reported 77Se NMR
Chemical Shifts," Wiley, New York, (1996).
12 E. R. Cullen, F. S. Guziec, Jr., C. J. Murphy, T. C. Wong, and K.
K. Andersen, J. Am. Chem. Soc., 103, 7055 (1981); J. Peng, M.
E. Barr, D. A. Ashburn, J. D. Odom, R. B. Dunlap, and L. A.
Silks III, J. Org. Chem., 59, 4977 (1994).