Selenothioic acid S-Esters: Synthesis, characterization, and trend for stability

Article abstract of DOI:10.1021/ja970462l

Selenothioic acid S-alkyl esters were synthesized from the reaction of terminal acetylenes with n-butyllithium, selenium, and alkanethiols in moderate to high yields. The use of substituted benzenethiols or (triphenylsilyl)-acetylene allowed for the isola

Full text of DOI:10.1021/ja970462l

8592
J. Am. Chem. Soc. 1997, 119, 8592-8597
Selenothioic Acid S-Esters: Synthesis, Characterization, and
Trend for Stability
Toshiaki Murai,* Kaori Kakami, Akihiro Hayashi, Toshihiro Komuro,
Hiroya Takada, Makiko Fujii, Takahiro Kanda, and Shinzi Kato*
Contribution from the Department of Chemistry, Faculty of Engineering, Gifu UniVersity,
Yanagido, Gifu 501-11, Japan
ReceiVed February 11, 1997X
Abstract: Selenothioic acid S-alkyl esters were synthesized from the reaction of terminal acetylenes with n-butyllithium,
selenium, and alkanethiols in moderate to high yields. The use of substituted benzenethiols or (triphenylsilyl)-
acetylene allowed for the isolation of S-aryl esters. The synthesis of R-aryl selenothioic acid S-alkyl esters was
attained by the acid-catalyzed reaction of selenoacetic acid Se-alkynyl esters with thiols in good yields. 77Se NMR
studies showed that the chemical shifts in a series of the esters were downfield of that in selenoester by about 500
ppm and were upfield of that in selenoketone by about 600 ppm. In the visible spectra the absorptions of the esters
were observed at about 340 and 568 nm. X-ray molecular structure analyses of R-silyl esters showed that the bond
distances in the selenocarbonyl group were 1.792 and 1.785 Å, respectively. The formation of 1,3-diselenetane was
confirmed from the decomposed products of S-phenyl ester. The trend for the stability of selenothioic acid S-esters
is discussed on the basis of these synthetic results.
Organoselenium compounds1 have played important roles in
In contrast, the chemistry of selenothioic acid S-esters, i.e.
selenocarbonyl compounds substituted with an organosulfur
group (RC(Se)SR′), has remained elusive.10 Attempts to
synthesize selenothioic acid S-esters were reported as early as
1962, and the esters have been noted to be unstable.11 Jensen
briefly described methods for synthesizing these esters using
selenoiminonium salts and hydrogen selenide,12 but no details
were made available. Since then, the only examples of such
esters have been two cyclic compounds with a selenothiocar-
boxyl group.13 Transition metal complexes of selenothioic acid
S-esters, which are generally more stable than uncomplexed
selenocarbonyl compounds, were not prepared until recently.14
Nonetheless, an organosulfur group is expected to have a
stabilizing effect, since dithioic acid esters (RC(S)SR′) are much
more stable than thioaldehydes and thioketones and have been
biological systems1d and organic syntheses1e as well as materials-
related chemistry.1f They are also used as spectroscopic probes
(for example, chiral derivatizing agents).1g This is mainly due
to the properties of the compounds, since their stability and
reactivity are variable even by subtle change of the structures.
To fully utilize these properties, it is of great importance to
design and synthesize new selenium-containing compounds
having appropriate stabilities and reactivities. Selenocarbonyl
compounds can be good candidates for this purpose. However,
recent studies on selenoaldehydes and selenoketones2 have
disclosed that these are too labile to be isolated unless they are
stabilized by resonance effects of heteroaromatic rings3a or are
protected by sterically bulky groups.3b,4 As a result, unprotected
selenoaldehydes5 and selenoketones6 have been obtained only
as their dimers or Diels-Alder adducts with dienes. Seleno-
carbonyl compounds substituted with nitrogen- and oxygen-
containing functional groups, i.e. selenoamides (RC(Se)NR′2)
and selenoesters (RC(Se)OR′),7 have been known to be much
more stable than selenoaldehydes and selenoketones. Even
enolizable derivatives8,9 have been isolated, but the purification
is fairly cumbersome because they are highly polar and ther-
mally labile.
(4) (a) Back, T. G.; Barton, D. H. R.; Britten-Kelly, M. R.; Guziec, F.
S., Jr J. Chem. Soc., Chem. Commun. 1975, 539. (b) Back, T. G.; Barton,
D. H. R.; Britten-Kelly, M. R.; Guziec, F. S., Jr. J. Chem. Soc., Perkin
Trans. 1 1976, 2079. (c) Okazaki, R.; Ishii, A.; Inamoto, N. J. Chem. Soc.,
Chem. Commun. 1983, 1429. (d) Guziec, F. S. Jr.; Moustakis, C. A. J.
Org. Chem. 1984, 49, 189. (e) Guziec, F. S., Jr.; SanFilippo, L. J.; Murphy,
C. J.; Moustakis, C. A.; Cullen, E. R. Tetrahedron 1985, 41, 4843. (f) Ishii,
A.; Okazaki, R.; Inamoto, N. Bull. Chem. Soc. Jpn. 1988, 61, 861. (g)
Okuma, K.; Kojima, K.; Kaneko, I.; Ohta, H. Chem. Lett. 1991, 1053. (h)
Brooks, P. R.; Counter, J. A.; Bishop, R.; Tiekink, E. R. T. Acta Crystallogr.
1991, C47, 1939. (i) Shimada, K.; Jin, N.; Fujimura, M.; Nagano, Y.; Kudoh,
E.; Takikawa, Y. Chem. Lett. 1992, 1843. (j) Ishii, A.; Ding, M.-X.;
Nakayama, J.; Hoshino, M. Chem. Lett. 1992, 2289. (k) Ding, M.-X.; Ishii,
A.; Nakayama, J.; Hoshino, M. Bull. Chem. Soc. Jpn. 1993, 66, 1714. (l)
Okuma, K.; Kojima, K.; Kaneko, I.; Tsujimoto, Y.; Ohta, H.; Yokomori,
Y. J. Chem. Soc., Perkin Trans. 1 1994, 2151.
(5) (a) Nakayama, J.; Akimoto, K.; Niijima, J.; Hoshino, M. Tetrahedron
Lett. 1987, 28, 4423. (b) Meinke, P. T.; Krafft, G. A. Tetrahedron Lett.
1987, 28, 5121. (c) Haas, A.; Spehr, M.; Chimia 1988, 42, 265. (d) Kirby,
G. W.; Trethewey, A. N. J. Chem. Soc., Perkin Trans. 1 1988, 1913. (e)
Meinke, P. T.; Krafft, G. A. J. Am. Chem. Soc. 1988, 110, 8671. (f) Erker,
G.; Hock, R.; Nolte, R. J. Am. Chem. Soc. 1988, 110, 624. (g) Segi, M.;
Nakajima, T.; Suga, S.; Murai, S.; Ryu, I.; Ogawa, A.; Sonoda, N. J. Am.
Chem. Soc. 1988, 110, 1976. (h) Takikawa, Y.; Uwano, A.; Watanabe, H.;
Asanuma, M.; Shimada, K. Tetrahedron Lett. 1989, 30, 6047. (i) Okuma,
K.; Komiya, Y.; Kaneko, I.; Tachibana, Y.; Iwata, E.; Ohta, H. Bull. Chem.
Soc. Jpn. 1990, 63, 1653. (j) Segi, M.; Kato, M.; Nakajima, T. Tetrahedron
Lett. 1991, 32, 7427. (k) Vallee´, Y.; Worell, M. J. Chem. Soc., Chem.
Commun. 1992, 1680. (l) Li, G. M.; Segi, M.; Nakajima, T. Tetrahedron
Lett. 1992, 33, 3515. (m) Duchenet, V.; Valle´e, Y. Tetrahedron Lett. 1993,
34, 4925. (n) Blau, H.; Grobe, J.; Van, D. L.; Mack, H.-G.; Oberhammer,
H. Chem. Ber. 1994, 127, 647.
X Abstract published in AdVance ACS Abstracts, August 15, 1997.
(1) For recent examples, see: (a) Paulmier, C. Selenium Reagents and
Intermediates in Organic Synthesis; Baldwin, J. E., Ed.; Pergamon Press:
Oxford, U.K., 1986. (b) The Chemistry of Organic Selenium and Tellurium
Compounds; Patai, S., Ed.; John Wiley & Sons: New York, 1986 and 1987;
Vol. 1 and 2. (c) Organoselenium Chemistry; Liotta, D., Ed.; Wiley-
Interscience: New York, 1987. (d) Reilly, C. Selenium in Food and Health;
Blackie Academic & Professional: London, 1996. (e) Krief, A. In
ComprehensiVe Organometallic Chemistry; Abel, W. W., Stone, F. G. A.,
Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol 11, p 515. (f)
O’Brien, P.; Nomura, R. J. Mater. Chem. 1995, 5, 1761. (g) Wu, R.; Silks,
L. A.; Odom, J. D.; Dunlap, R. B. Spectroscopy 1996, 11, 37.
(2) For recent reviews, see: (a) Pelloux-Le´on, N.; Valle´e, Y. Main Group
Chem. News 1994, 2, 22. (b) Okuma, K. J. Synth. Org. Chem. Jpn. 1995,
53, 218. (c) Segi, M.; Nakajima, T. J. Synth. Org. Chem. Jpn. 1995, 53,
678. (d) Guziec, F. S., Jr.; Guziec, L. J. In ComprehensiVe Organic
Functional Group Transformations; Katritzky, A. R., Meth-Cohn, O., Rees,
C. W., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 3, p 381.
(3) (a) Reid, D. H.; Webster, R. G.; Mckenzie, S. J. Chem. Soc., Perkin
Trans. 1 1979, 2334. (b) Okazaki, R.; Kumon, N.; Inamoto, N. J. Am. Chem.
Soc. 1989, 111, 5949.
S0002-7863(97)00462-9 CCC: $14.00 © 1997 American Chemical Society

Selenothioic Acid S-Esters
J. Am. Chem. Soc., Vol. 119, No. 37, 1997 8593
Results and Discussion
Table 1. Synthesis of Selenothioic Acid S-Esters from Terminal
Acetylenesa
The efficient synthesis of selenothioic acid S-esters 1a was
achieved by the following one-pot procedure (eq 1).17a The
results of the synthesis of the esters using terminal acetylenes
2 and a variety of thiols 3 are summarized in Table 1. To an
Et2O solution of (trimethylsilyl)acetylene (2a) were added 1
equiv of n-butyllithium and selenium powder. Next, 2 equiv
of 1-butanethiol (3b) was added to the reaction mixture at 0
°C, which was then stirred at 0-20 °C for 1.5 h. The reaction
(6) (a) Okuma, K.; Sakata, J.; Tachibana, Y.; Honda, T.; Ohta, T.
Tetrahedron Lett. 1987, 28, 6649. (b) Meinke, P. T.; Krafft, G. A. J. Am.
Chem. Soc. 1988, 110, 8679. (c) Nakayama, J.; Akimoto, K.; Hoshino, M.
J. Phys. Org. Chem. 1988, 1, 53. (d) Segi, M.; Koyama, T.; Nakajima, T.;
Suga, S.; Murai, S.; Sonoda, N. Tetrahedron Lett. 1989, 30, 2095. (e) Erker,
G.; Hock, R.; Kru¨ger, C.; Werner, S.; Kla¨rner, F.-G.; Artschwager-Perl, U.
Angew. Chem., Int. Ed. Engl. 1990, 29, 1067. (f) Abelman, M. W.
Tetrahedron Lett. 1991, 32, 7389. (g) Brooks, P. R.; Bishop, R.; Craig, D.
C.; Scudder, M. L. J. Org. Chem. 1993, 58, 5900. (h) Hock, R.; Hillenbrand,
S.; Erker, G.; Kru¨ger, C.; Werner, S. Chem. Ber. 1993, 126, 1895. (i) Back,
T. G.; Dyck, B. P.; Parvez, M. J. Chem. Soc., Chem. Commun. 1994, 515.
(j) Wilker, S.; Erker, G. J. Am. Chem. Soc. 1995, 117, 10922.
(7) For reviews, see: (a) Kato, S.; Murai, T.; Ishida, M. Org. Prep.
Proced. Int. 1986, 18, 369. (b) Ogawa, A.; Sonoda, N. In ComprehensiVe
Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford,
U.K., 1991; Vol. 6, p 461. (c) Ogawa, A.; Sonoda, N. ReV. Heteroatom
Chem. 1994, 10, 43. (d) Ishii A.; Nakayama, J. In ComprehensiVe Organic
Functional Group Transformations; Katritzky, A. R., Meth-Cohn, O., Rees,
C. W., Eds.; Pergamon: Oxford, U.K., 1995; Vol. 5, p 505. (e) Dell, C. P.
In ComprehensiVe Organic Functional Group Transformations; Katritzky,
A. R., Meth-Cohn, O., Rees, C. W., Eds.; Pergamon: Oxford, U.K., 1995;
Vol. 5, p 565.
(8) For synthesis and isolation of enolizable selenoamides, see: (a)
Collard-Charon, C.; Renson, M. Bull. Soc. Chim. Belg. 1963, 72, 304. (b)
Cohen, V. I. J. Org. Chem. 1977, 42, 2645. (c) Malek-Yazdi, F.; Yalpani,
M. Synthesis 1977, 328. (d) Sukhai, R. S; Jong, R.; Brandsma, L. Synthesis
1977, 888. (e) Harrit, N.; Rosenkilde, S.; Larsen, B. D.; Holm, A. J. Chem.
Soc., Perkin Trans. 1 1985, 907. (f) Ishihara, H.; Yoshimi, M.; Hara, N.;
Ando, H.; Kato, S. Bull. Chem. Soc. Jpn. 1990, 63, 835. (g) Takikawa, Y.;
Watanabe, H.; Sasaki, R.; Shimada, K. Bull. Chem. Soc. Jpn. 1994, 67,
876. (h) Otten, P. A.; Gen, A. Recl. TraV. Chim. Pays-Bas 1994, 113, 499.
(i) Murai, T.; Ezaka, T.; Niwa, N.; Kanda, T.; Kato, S. Synlett 1996, 865.
(9) For synthesis and isolation of enolizable selenoesters, see: (a) Malek-
Yazdi, F.; Yalpani, M. J. Org. Chem. 1976, 41, 729. (b) Reference 8b. (c)
Barton, D. H. R.; Hansen, P.-E.; Picker, K. J. Chem. Soc., Perkin 1 1977,
1723. (d) Reference 8f. (e) Segi, M.; Takahashi, T.; Ichinose, H.; Li, G.
M.; Nakajima, T. Tetrahedron Lett. 1992, 33, 7865. (f) Wright, S. W.
Tetrahedron Lett. 1994, 35, 1331.
(10) Murai, T.; Kato, S. In ComprehensiVe Organic Functional Group
Transformations; Katritzky, A. R., Meth-Cohn, O., Rees, C. W., Eds.;
Pergamon: Oxford, U.K., 1995; Vol. 5, p 562.
(11) (a) Collard-Charon, C.; Renson, M. Bull. Soc. Chim. Belg. 1962,
71, 563. (b) Schuijl, P. J. W.; Brandsma, L.; Arens, J. F. Recl. TraV. Chim.
Pays-Bas 1966, 85, 889.
(12) Jensen, K. A. In Organic Selenium Compounds: Their Chemistry
and Biology; Klayman, D. L., Gu¨nther, W. H. H., Eds.; Wiley-Inter-
science: New York, 1973; p 263.
a The reaction was carried out as follows unless otherwise noted:
Acetylene (1 mmol) was reacted with n-BuLi (1 mmol) in Et2O (5
mL) at 0 °C for 15 min. Se (1 mmol) was added to the solution, and
stirred. Then, to the reaction mixture was added thiol (2 mmol) and
stirred at 0-20 °C for 0.5-3 h. b Isolated yield. c The reaction mixture
of acetylene, BuLi, and Se was slowly dropped to thiol. d The reaction
was carried out on 50 mmol scale. e THF was used as a solvent.
studied in great depth.15 Recently, we reported the first synthesis
and isolation of aliphatic selenothioic acid S-alkyl esters.16 They
were less polar and were distillable blue-violet liquids. Exten-
sive studies have further established convenient methods for
the synthesis of these esters17 and have demonstrated their
unique properties in synthetic reactions.16,18
Structures 1a-c represent generalized forms of the esters.
Either an alkyl (R) or aryl (Ar) group is attached to the
selenocarbonyl group and sulfur atom of the esters. We initially
(13) (a) Wallmark, I.; Krackov, M. H.; Chu. S.-H.; Maunter, H. G. J.
Am. Chem. Soc. 1970, 92, 4447. (b) Michael, J. P.; Reid, D. H.; Rose, B.
G.; Speirs, R. A. J. Chem. Soc., Chem. Commun. 1988, 1494.
(14) (a) Raubenheimer, H. G.; Kruger, G. J.; Linford, L.; Marais, C. F.;
Otte, R.; Hattingh, J. T. Z.; Lombard, A. J. Chem. Soc., Dalton Trans.
1989, 1565. (b) Fischer, H.; Treier, K.; Troll, C. Chem. Ber. 1995, 128,
1149. (c) Fischer, H.; Treier, K.; Troll, C.; Stamph, R. J. Chem. Soc., Chem.
Commun. 1995, 2461.
(15) For reviews of dithioic acid esters, see: (a) Scheithauer, S.; Mayer,
R. In Topics in Sulfur Chemistry; Senning, A., Ed.; George Thieme
Publishers: Stuttgart, Germany, 1979; Vol. 4. (b) Ramadas, S. R.;
Srinivasan, P. S.; Ramachandran, J.; Sastry, V. V. S. K. Synthesis 1983,
605. (c) Kato, S.; Ishida, M. Sulfur Rep. 1988, 8, 155. (d) Kato, S.; Murai,
T. In Supplement B: The Chemistry of Acid DeriVatiVes; Patai, S., Ed.;
John Wiley & Sons: New York, 1992; Vol. 2, p 803. (e) Metzner, P.;
Thuillier, A. In Sulfur Reagents in Organic Synthesis; Academic Press: New
York, 1994; pp 36-43, 96-112.
found that aliphatic esters 1a were stable enough to be handled
in air. This result implied that 1b,c could also be isolated in
air. However, attempts to isolate S-aryl esters 1b have failed,17a
and 1c was more labile than 1a.17b These results have suggested
that the esters could be distinguished from other selenocarbonyl
compounds on the basis of their structure and stability. We
present here the results of detailed studies on the synthesis,
characterization, and structures of selenothioic acid S-esters 1.
(16) Kato, S.; Komuro, T.; Kanda, T.; Ishihara, H.; Murai, T. J. Am.
Chem. Soc. 1993, 115, 3000.

8594 J. Am. Chem. Soc., Vol. 119, No. 37, 1997
Murai et al.
Table 2. Synthesis of Selenothioic Acid S-Esters from 8a
ester
mixture turned dark pink via orange during the reaction.19
Purification of the mixture by column chromatography on silica
gel using hexane as the eluent gave a 98% yield of selenothio-
acetic acid S-butyl ester (4b) as a deep pink oil (entry 2). The
ester 4b could be handled even at room temperature, in marked
contrast to a previous report that 4b was unstable.10 Upon the
exposure of 4b to air for more than 8 h, red selenium began to
deposit. Similarly, the reaction with thiols 3a,c-e gave seleno-
thioacetic acid S-alkyl esters 4a,c-e in good to high yields. In
these cases, the trimethylsilyl group of 2a was completely
removed during chromatographic purification (entries 1-5 and
7-9). Thus, the system of 2a, n-butyllithium, and selenium
can be regarded as an agent for selenoacetylation of thiols. The
use of (triphenylsilyl)acetylene 2b gave R-silyl esters 5 in
moderate to good yields (entries 10-14). The esters 4a-c and
5a,b were obtained as deep pink liquids or solids and could be
stored below -10 °C for more than 1 year, whereas S-tert-
butyl ester 4d and S-benzyl ester 4e gradually decomposed after
purification. The reaction of 2a with benzenethiol 3f gave rise
to a blue solution, which was indicative of the formation of 4f,
but this instantly turned yellow in aqueous workup (entry 6).
To enhance the stability of S-aryl esters, substituted benzene-
thiols were used. Bulky substituents were also introduced to
the R-carbon atom of the selenocarbonyl group of the esters.
After several disappointing results, the reaction with thiols 3g-i
or the use of silylacetylene 2b gave S-aryl esters 4g-i, and 5c-e
(entries 7-9 and 12-14). Nevertheless, deep violet blue S-aryl
esters 4g-i and 5c-e turned yellow much more quickly than
S-alkyl esters 4a-c and 5a,b, even at low temperature.
compd
R
R′
yieldb (%)
7a
7b
7c
7d
7e
C6H5
C6H5
C6H5
4-ClC6H4
1-naphthyl
C2H5
48
70
55
46
40
n-C3H7
n-C4H9
i-C3H7
n-C3H7
a The reaction was carried out as follows: Se-alkynyl selenoester (1
mmol) was stirred with CF3COOH (0.3 mmol) and thiol (6 mmol) in
THF (2 mL) at 66 °C for 48 h. b Isolated yield.
a solvent did not give the corresponding esters, unlike the
reaction of silylacetylenes 2a,b, but the reaction of 2c and 2d
in THF proceeded smoothly to give 6a and 6b in yields of 34%
and 58%, respectively (entries 15 and 16). In contrast, the
reaction of 2e gave the corresponding R-phenyl ester 7a at most
in 10% yield (entry 17). The synthesis of R-aryl esters 7 was
attained by the acid-catalyzed reaction of Se-alkynyl esters 8
with thiols 3 in better yields (eq 3, Table 2). Although this
reaction required a high reaction temperature and longer reaction
time, it demonstrated the thermal stability of 7 under acidic
conditions.
Selected spectroscopic data for the esters are summarized in
Table 3. The data for selenoester 98f and selenoketone 1020
are also listed. In the 13C NMR spectra of esters 4b,d, 5a,c,d,
and 7c, the CdSe signal was observed in the range of 238.0 (
2.4 ppm, which is close to that of 9.8f In contrast, the CdSe
signal in the 77Se NMR spectra of 4b, 5a,c,d, and 7c (1487 (
64 ppm) was shifted to lower fields by about 500 ppm compared
to that of 9. In 4k, two signals, which are characteristic of
selenocarboxyl (SedC-O) and selenothiocarboxyl (SedC-S)
groups, were observed at 952.0 and 1557.4 ppm, respectively.
This difference may be explained by noting the degree to which
the lone-pair electrons on the oxygen or sulfur atom delocalize
on the selenocarbonyl group (eq 4).21 The contribution of
Both 2-hydroxyethanethiol (3m) and 1,2-ethanedithiol (3n)
were also reacted. Although the reaction with 3n gave a
complex mixture, the reaction with 3m produced a 44% yield
of selenothioacetic acid S-2-hydroxyethyl ester (4j) together with
a 41% yield of ester 4k (eq 2). A selenoacetyl group was
introduced not only to a mercapto group but also to a hydroxyl
group. The use of 2 equiv of 3m decreased the yield of 4k,
whereas the reaction with 0.5 equiv of 3m predominantly gave
4k. These results are in sharp contrast to the reaction using
alcohols instead of thiols, in which no selenoacetylation of
hydroxyl group took place.
resonance structures 12 and 13 is more important in 11 than in
1 but is still present in 1. In fact, the selenium atom in
selenoketone 10, which has no resonance structures of type 12
and type 13, appears at lower field than those of these esters.
In a series of S-alkyl esters, the chemical shift of tert-butyl ester
4d was further downfield than that in n-butyl ester 4b in the
77Se NMR spectra.
In the visible spectra of esters 4b,d, 5a,c,d, and 7c, the
absorptions were observed at about 340 and 574 ( 20 nm.
Absorption at longer wavelengths which may be ascribed to
n-π* transitions of the selenocarbonyl group of the esters were
substantially red-shifted by ca. 100 nm compared with that for
9. In the ester 4k, typical bathochromic shifts of the absorptions
in the UV-visible spectra were observed between SedC-O
and SedC-S (Figure 1), analogous to the findings with thioic
This synthetic method was applied to the reaction of terminal
acetylenes 2c-e (Table 1, entries 15-17). The use of Et2O as
(17) (a) Murai, T.; Hayashi, A.; Kanda, T.; Kato, S. Chem. Lett. 1993,
1469. (b) Murai, T.; Ogino, Y.; Mizutani, T.; Kanda, T.; Kato, S. J. Org.
Chem. 1995, 60, 2942. (c) Murai, T.; Kakami, K.; Itoh, N.; Kanda, T.;
Kato, S. Tetrahedron 1996, 52, 2839.
(18) (a) Murai, T.; Takada, H.; Kanda, T.; Kato, S. Tetrahedron Lett.
1994, 35, 8817. (b) Murai, T.; Takada, H.; Kanda, T.; Kato, S. Chem. Lett.
1995, 1057. (c) Murai, T.; Maeda, M.; Matsuoka, F.; Kanda, T.; Kato, S.
J. Chem. Soc., Chem. Commun. 1996, 1461. (d) Murai, T.; Fujii, M.; Kanda,
T.; Kato, S. Chem. Lett. 1996, 877.
(19) The reaction in eq 1 may begin with the generation of lithium
alkyneselenolate 22 from silylacetylene 2a, n-butyllithium, and selenium.
Then, the protonation of 22 with 3 may give rise to selenoketene
intermediate 23,8d,f,16 followed by the nucleophilic attack of lithium thiolates
derived from 3 on 23 to give the products.
(20) Cullen, E. R.; Guziec, F. S., Jr.; Murphy, C. J.; Wong, T. C.;
Andersen, K. K. J. Am. Chem. Soc. 1981, 103, 7055.
(21) (a) Cullen, E. R.; Guziec, F. S., Jr.; Murphy, C. J.; Wong, T. C.;
Andersen, K. K. J. Chem. Soc., Perkin Trans. 2 1982, 473. (b) Duddeck,
H. Prog. NMR Spectrosc. 1995, 27, 1.

Selenothioic Acid S-Esters
J. Am. Chem. Soc., Vol. 119, No. 37, 1997 8595
Table 3. Selected Spectroscopic Data of Selenothioic Acid
S-Esters
Figure 1. UV-visible spectrum of 4k in cyclohexane.
5d is thermally more labile than 5a, but no differences were
observed between 5a and 5d except that the SedC-S bond in
5a (1.724(6) Å) is shorter than that in 5d (1.741(4) Å). Finally,
the bond distances in CdSe should be noted. The average
values of the bond distances in the selenocarbonyl group of
selenoformaldehyde and selenoacetaldehyde have been estimated
to be 1.739-1.76 Å by microwave spectroscopy24 and molecular
orbital calculations.25 X-ray structure analyses have revealed
that the bond distance of CdSe in selenoketone 14 is 1.774(6)
Å,4h and that in 15 is 1.790(4) Å,4l whereas those in seleno-
a In CDCl3. b In cyclohexane. c Not observed.
acid O-esters and dithioic acid esters.15a The absorptions due
to SedC-O in 4k were at 273 and 467 nm, and those due to
SedC-S in 4k were at 341 and 571 nm.
The molecular structures of 5a,d were determined by X-ray
crystallography. For 5a, two independent molecules were
present in one asymmetric unit and their average data are shown.
ORTEP drawings of 5a and 5d are shown in Figures 2 and 3,
respectively. Selected bond distances and angles are listed in
Tables 4 and 5.
This is the first example of the X-ray molecular structure
analysis of selenothioic acid S-esters. We have observed several
characteristic features. First, the selenium atom and the alkyl
or aryl group adopt a cis conformation with respect to the C-S
bond, similar to ordinary esters,22 even if steric and possibly
electronic repulsion appear to exist between the selenium atom
and the substituents on the sulfur atom. Second, the torsion
angles of Si-C-CdSe of 5a and 5d are 76.0(6) and 68.9(4)°,
respectively. Thus, the esters exist in a bisecting conformation
in the solid state, whereas R-silyl acetic acid S-butyl ester has
been reported to exist in an eclipsed conformation.23 Third,
amides have been reported to be about 1.83 Å.26 The bond
distances of the CdSe in esters 5a and 5d are 1.792(7) and
1.785(4) Å, respectively, and are rather close to those of
selenoketones.
Finally, the decomposition products of S-aryl esters 5c-e
were examined. These esters gradually turned from deep violet
blue to yellow within 1 day even when they were stored below
-10 °C under Ar atmosphere, but no deposition of red selenium
was observed. Although the NMR spectra of the yellow mixture
derived from 5c-e were less informative, X-ray molecular
structure analysis27 of a single crystal from the decomposed
(24) (a) Hutchinson, M.; Kroto, H. W. J. Mol. Spectroscopy, 1978, 70,
347. (b) Brown, R. D.; Godfrey, P. D.; McNaughton, D. Chem. Phys. Lett.
1985, 118, 29.
(25) (a) Bock, H.; Aygen, S.; Rosmus, P.; Solouki, B.; Weissflog, E.
Chem. Ber. 1984, 117, 187. (b) Collins, S.; Back, T. G.; Rauk, A. J. Am.
Chem. Soc. 1985, 107, 6589.
(26) (a) Fischer, H.; Tiriliomis, A.; Gerbing, U.; Huber, B.; Mu¨ller, G.
J. Chem. Soc., Chem. Commun. 1987, 559. (b) Fischer, H.; Gerbing, U.;
Tiriliomis, A.; Mu¨ller, G.; Huber, B.; Riede, J.; Hofmann, J.; Burger, P.
Chem. Ber. 1988, 121, 2095. (c) Nakayama, J.; Mizumura, A.; Akiyama,
I.; Nishio, T.; Iida, I. Chem. Lett. 1994, 77. (d) Murai, T.; Mizutani, T.;
Kanda, T.; Kato, S. Heteroatom Chem. 1995, 6, 241.
(22) Eliel, E. In Stereochemistry of Organic Compounds; Wiley: New
York, 1995; p 618.
(23) Fronczek, F. R.; Larson, G. L.; Suarez, J. Acta Crystallogr. 1990,
C46, 2480.

8596 J. Am. Chem. Soc., Vol. 119, No. 37, 1997
Murai et al.
Table 5. Selected Bond Distances and Angles of 5d
Bond Distance (Å)
atom
atom
distance
Se(1)
S(1)
S(1)
Si(1)
C(1)
C(1)
C(1)
C(21)
C(2)
C(2)
1.785(4)
1.741(4)
1.771(4)
1.910(4)
1.476(4)
Bond Angle (deg)
atom
atom
atom
angle
Se(1)
Se(1)
S(1)
C(1)
Si(1)
C(1)
C(1)
C(1)
S(1)
C(2)
S(1)
123.5(2)
124.5(3)
112.0(3)
105.1(2)
114.8(3)
C(2)
C(2)
C(21)
C(1)
Torsion Angle (deg)
atom
atom
atom
atom
angle
Figure 2. ORTEP drawing of 5a. Thermal ellipsoids are drawn at the
50% probability level.
Se(1)
Se(1)
C(1)
C(1)
C(1)
C(1)
S(1)
S(1)
S(1)
C(2)
C(21)
C(21)
C(21)
Si(1)
C(22)
C(26)
-1.1(3)
68.9(4)
-95.8(4)
88.7(4)
Scheme 1
18, was reported to be reversible with 18 when it was dissolved
in C6D6 or CDCl3 (eq 6).6d In contrast, 16 was highly stable in
Figure 3. ORTEP drawing of 5d. Thermal ellipsoids are drawn at the
50% probability level.
Table 4. Selected Bond Distances and Angles of 5a
Bond Distance (Å)
atom
atom
distance
Se(1)
S(1)
S(1)
Si(1)
C(1)
C(1)
C(1)
C(21)
C(2)
C(2)
1.792(7)
1.724(6)
1.833(7)
1.924(7)
1.473(9)
Bond Angle (deg)
atom
atom
atom
angle
Se(1)
Se(1)
S(1)
C(1)
Si(1)
C(1)
C(1)
C(1)
S(1)
C(2)
S(1)
125.1(4)
122.6(5)
114.3(5)
106.3(3)
114.3(5)
a solvent, and no reversible process was observed. These results
suggest that S-aryl esters 1b are thermally labile and undergo
dimerization to 1,3-diselenethanes more easily than they undergo
aerial oxidation leading to thioesters similar to 17 (eq 5). This
is in sharp contrast to the case of aromatic selenothioic acid
S-alkyl esters 1c, which are easily oxidized to thioesters.17b
On the basis of the present and previous results,17b the general
trend for the stability of selenothioic acid S-esters 1 is proposed
in Scheme 1.
The aliphatic acid S-alkyl esters 1a are the most stable among
the four types of esters 1a-d. The substitution of an alkyl group
attached either to the selenocarbonyl group or to the sulfur atom
with an aryl group reduces the stability of esters 1b,c. The
aliphatic acid S-aryl esters 1b are prone to undergo dimerization,
whereas the aromatic acid S-alkyl esters 1c easily undergo aerial
C(2)
C(2)
C(21)
C(1)
Torsion Angle (deg)
atom
atom
atom
atom
angle
Se(1)
Se(1)
C(1)
C(1)
C(1)
C(1)
S(1)
S(1)
S(1)
C(2)
C(21)
C(21)
C(21)
Si(1)
C(22)
C(23)
-5.3(5)
76.0(6)
-152.3(5)
85.4(6)
products of 5c revealed that trans-1,3-diselenetane 16 was
formed, probably via the dimerization of 4f, which was
generated by the protodesilylation of 5c (eq 5). A similar
dimerized compound 19, which was formed from selenoketone

Selenothioic Acid S-Esters
J. Am. Chem. Soc., Vol. 119, No. 37, 1997 8597
hexane as the eluent to give 9.59 g (98%) of 4b as deep pink oils: IR
Scheme 2
1
(neat) 2957, 2861, 1461, 1137, 1097, 827 cm-1; H NMR (CDCl3) δ
0.95 (t, J ) 7.4 Hz, 3H, CH3), 1.47 (sex, J ) 7.4 Hz, 2H CH2), 1.72
(qui, J ) 7.4 Hz, 2H, CH2), 2.57 (s, 3H, CH3), 3.23 (t, J ) 7.4 Hz, 2H,
SCH2); 13C NMR (CDCl3) δ 13.7 (CH3), 22.2 (CH2), 28.9 (CH2), 41.5
(SCH2), 44.9 (CH3), 237.8 (CdSe); CIMS (m/z) 197.0 (M+ + 1). Anal.
Calcd for C6H12SSe: C, 36.92; H, 6.20. Found: C, 36.74; H, 6.12.
2-(Triphenylsilyl)ethaneselenothioic Acid S-1-Methylethyl Ester
(5a): IR (neat) 3044, 1426, 1111, 1086, 892, 699, 504, 487 cm-1; 1H
NMR (CDCl3) δ 1.10 (d, J ) 6.9 Hz, 6H, CH3), 3.64 (m, J ) 6.9 Hz,
1H, SCH), 3.83 (s, 2H, CH2), 7.25-7.41 (m, 9H, m,p-CH), 7.51-7.72
(m, 6H, o-CH); 13C NMR (CDCl3) δ 21.1 (CH3), 45.7 (SCH), 51.0
(CH2), 127.7, 129.9 (CH), 132.7 (ipso-C), 136.2 (CH), 235.6 (CdSe);
EIMS (m/z) 440 (M+). Anal. Calcd for C23H24SSeSi: C, 62.85; H, 5.50.
Found: C, 62.77; H, 5.46.
oxidation. Attempts to isolate aromatic acid S-aryl esters 1d
have not yet been successful. It is also noteworthy that
replacement of the n-butyl group attached to the sulfur atom in
1a with a tert-butyl group reduces the stability of the esters.
Accordingly, the introduction of a bulky group to esters 1 does
not necessarily enhance their stability. This tendency is in sharp
contrast to that in a series of better-known selenoesters7b,c and
dithioic acid esters,15 in which all types of derivatives with alkyl
and aryl groups have been reported to be stable. It should also
be noted that isolation of enolizable selenoaldehydes and
selenoketones has not yet been reported, whereas aromatic
derivatives can be isolated. More interestingly, the stability of
a series of diselenoic acid esters (RC(Se)SeR′) is also different
(Scheme 2).26d,28 Aliphatic diselenoic acid esters 20 are less
stable than aromatic acid esters 21.
2-(Triphenylsilyl)ethaneselenothioic Acid S-2,6-Dimethylphenyl
Ester (5d): IR (neat) 3051, 1428, 1110, 877, 733, 505, 490, 479 cm-1
;
1H NMR (CDCl3) δ 2.00 (s, 6H, CH3), 3.97 (br, 2H, CH2), 7.04 (d, J
) 7.3 Hz, 2H, m-CH(SAr)), 7.19 (t, J ) 7.6 Hz, 1H, p-CH(SPh)),
7.32-7.42 (m, 9H, m,p-CH), 7.66 (d, J ) 6.8 Hz, 6H, o-CH); 13C NMR
(CDCl3) δ 20.8 (CH3), 50.7 (CH2), 127.9, 128.4, 130.0, 130.5 (CH),
133.1 (ipso-C), 136.3 (CH), 136.4, 142.6 (ipso-C), 237.6 (CdSe); EIMS
m/z 502 (M+); Anal. Calcd for C28H26SSeSi: C, 67.04; H, 5.22.
Found: C, 66.68; H, 5.16.
X-ray Measurements. All measurement were carried out on a
Rigaku AFC7R diffractometer with graphite monochromated Mo KR
radiation (λ ) 0.710 69 Å). All of the structures were solved and
refined using the teXsan crystallographic software package on an IRIS
Indigo computer. X-ray quality crystals of 5a were obtained by
vaporization of the saturated hexane solution and of 5d were obtained
by vaporization of the diffused solution of hexane into dichloromethane
solution of the samples. The crystals were cut from the grown needles.
Each crystal mounted on a glass fiber was coated with an epoxy resin.
The cell dimensions were determined by a least-squares refinement of
the diffractometer angles for 25 automatically centered reflections. Three
standard reflections were measured every 150 reflections, and no decay
was detected. An empirical absorption correction (DIFABS30 ) was
applied. The structures were solved by direct method SHELX8631 and
expanded using DIRDIF92.32 Scattering factors for neutral atoms were
from Cromer and Waber33 and anomalous dispersion was used. The
weighting scheme employed was w ) [σ2(Fo) + p2(Fo)2/4]-1. A full-
matrix least-squares refinement was executed with non-hydrogen atoms
being anisotropic. The final least-squares cycle included fixed hydrogen
atoms at calculated positions of which each isotropic thermal parameter
was set to 1.2 times of that of the connecting atom. Crystal data,
measurement description, data collection, and refinement parameters
are available as a Supporting Information.
Conclusion
The synthesis, characterization, and general trend for the sta-
bility of selenothioic acid S-esters 1 have been demonstrated.
The reaction of terminal acetylenes 2a, 2b, 2c, and 2d with se-
lenium and thiols gave the corresponding esters 4-6 in good
to high yields. Regarding the synthesis of R-aryl esters, the
acid-catalyzed reaction of Se-alkynyl selenoesters 8 with thiols
3 was effective. Although there were no critical differences in
the spectroscopic data and structures of S-alkyl esters 1a and
S-aryl esters 1b, 1b was more labile than 1a. From the de-
composed products of 5c, the formation of 1,3-trans-diselenetane
was confirmed. Among four possible derivatives 1a-d, S-alkyl
esters 1a and particularly selenothioacetic acid S-alkyl esters
4a-c were found to be the most stable under air despite the
fact that they were not protected either sterically or electroni-
cally.
Experimental Section
General Informations. The materials are commercially available
except for (triphenylsilyl)acetylene (2b)29 and selenoacetic acid Se-
alkynyl esters 8.8f The 77Se NMR (76 MHz) spectra were obtained
from a JEOL R-400 spectrometer, and 77Se chemical shifts were
expressed in parts per million deshielded with respect to neat Me2Se.
All spectra were acquired in the proton-decoupled mode; generally
0.05∼0.3 mmol solutions in CDCl3 (0.4 mL) were used.
Acknowledgment. We are grateful to Dr. M. Ebihara and
Prof. Dr. T. Kawamura of Gifu University for determination of
the X-ray structures of 5a,d and 16. This work was supported
partially by a Grant-in-Aid for Scientific Research from the
Ministry of Education, Science, Sports and Culture, Japan.
Supporting Information Available: Experimental proce-
dure for eq 3, characterization data for 4a,c-e,g-k, 5b,c,e, 6,
and 7, and complete tables of crystallographic data, final atomic
coordinates and equivalent isotropic thermal parameters, bond
distances, bond angles, and torsion angles (23 pages). See any
current masthead page for ordering and Internet access
instructions.
General Procedure for the Synthesis of Selenothioic Acid S-Esters
from Terminal Acetylenes: Synthesis of Ethaneselenothioic Acid
S-Butyl Ester (4b). To a solution of Et2O (60 mL) and (trimethylsilyl)-
acetylene (2a) (7.1 mL, 50.0 mmol) was added n-butyllithium (1.6 M
in hexane, 31.3 mL, 50.0 mmol) at 0 °C. The solution was stirred at
this temperature for 15 min, and selenium powder (3.95 g, 50.0 mmol)
was added at 0 °C. The reaction mixture was warmed to 20 °C and
stirred for 15 min. To this was added 1-butanethiol (11 mL, 100.0
mmol) at 0 °C. The reaction mixture gradually turned deep pink during
the stirring at 0 °C for 30 min and at 20 °C for 1 h. Then, the resulting
solution was poured onto water and extracted with Et2O. The organic
layer was dried over anhydrous magnesium sulfate and concentrated.
The residue was purified by column chromatography on silica gel using
JA970462L
(30) Walker, N.; Stuart, D. Acta Crystallogr. 1983, A39, 158.
(31) Sheldrick, G. M. In Crystallographic Computing 3; Sheldrick, G.
M., Kruger, C., Goddard, R., Eds.; Oxford University Press: Oxford, U.K.,
1985; p 175.
(32) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF
program system, Technical Report of the Crystallography Laboratory;
University of Nijmegen: The Netherlands, 1992.
(33) Cromer, D. T.; Waber, J. T. In International Tables for X-ray
Crystallography; The Kynoch Press: Birmingham, England, 1974; Vol. 4,
Table 2.2A.
(27) The detail of the X-ray molecular structure analysis of 16 is available
as Supporting Information.
(28) Murai, T.; Mizutani, T.; Kanda, T.; Kato, S. J. Am. Chem. Soc.
1993, 115, 5823.
(29) Colvin, E. W. In Silicon Reagents in Organic Synthesis; Academic
Press: New York, 1985; p 45.