Thion (RCSOH), selenon (RCSeOH), and telluron (RCTeOH) acids as predominant species

Article abstract of DOI:10.1021/ja953035l

Thiocarboxylic acids, such as selenocarboxylic acids, exist predominantly in the thioxo form (RCSOH, thion acid) in polar solvents such as tetrahydrofuran (THF) at temperatures below -50°C. Tellurocarboxylic acids (5) were observed for the first time by a

Full text of DOI:10.1021/ja953035l

1262
J. Am. Chem. Soc. 1996, 118, 1262-1267
Thion (RCSOH), Selenon (RCSeOH), and Telluron (RCTeOH)
Acids as Predominant Species
Shinzi Kato,* Yasuyuki Kawahara, Hideki Kageyama, Ryo Yamada,
Osamu Niyomura, Toshiaki Murai, and Takahiro Kanda
Contribution from the Department of Chemistry, Faculty of Engineering, Gifu UniVersity,
1-1 Yanagido, Gifu 501-11, Japan
ReceiVed September 5, 1995X
Abstract: Thiocarboxylic acids, such as selenocarboxylic acids, exist predominantly in the thioxo form (RCSOH,
thion acid) in polar solvents such as tetrahydrofuran (THF) at temperatures below -50 °C. Tellurocarboxylic acids
(5) were observed for the first time by acidolysis of the corresponding cesium tellurocarboxylates with hydrogen
chloride. The telluroic acids (6) exist predominantly in the telluroxo form (RCTeOH, telluron acid) in THF at
temperatures below -70 °C. Telluron acids were reddish to blue violet for the aliphatics (R ) alkyl) and dark green
for the aromatics (R ) aryl) and reacted with aryl isocyanates at -70 °C to give crystalline acyl carbamoyl tellurides
in good yields.
Introduction
Scheme 1
There are generally considered to be six kinds of mono-
chalcogenocarboxylic acids in which one of the two oxygen
atoms of the carboxylic acid is replaced by S, Se, or Te (Scheme
1). However, with exception of thiocarboxylic acid (1), which
is an important class of compounds biologically with a great
variety of uses in chemistry,1 there has been little known about
these compounds. Historically, the first synthesis of a thiocar-
boxylic acid was reported in 1854 by Kekule´, who prepared
thioacetic acid.2 Since then, thiocarboxylic acid has been
considered to exist in the thiol form 1 (thiol acid). In contrast,
the thioxo form 2 (hereafter called thion acid), the tautomer of
1, has remained elusive, even spectroscopically.3 We report
here that thion acid 2 is the predominat species in polar solvents
at low temperature. In relation to thio- (1) and selenocarboxylic
acids (3),4 we attempted to prepare tellurocarboxylic acids (5).
In this article, we report for the first time tellurocarboxylic acids
which exist predominantly as the telluroxo form 6 (hereafter
called telluron acid) in polar solvents at low temperature.
X Abstract published in AdVance ACS Abstracts, January 15, 1996.
(1) (a) Bauer, W.; Khu¨ren, K. In Methoden der Organischen Chemie,
5th ed.; Falbe, J., Ed.; Georg Thieme Publishers: Stuttgart, Germany, 1985;
pp 832-890. (b) Abeles, R. H.; Frey, P. A.; Jencks, W. P. Biochemistry;
Jones and Bartlett: Boston, MA, 1992. (c) Ishii, Y.; Nakayama, J. In
Organic Functional Group Transformations; Rees, C. W., Ed.; Pergamon
Press: New York, 1995; Vol. 5, Chapter 5.12.
Results and Discussion
By analogy with carboxylic acids, the structures of thiocar-
boxylic acids have been investigated in great detail using various
techniques, including cryoscopic,5 spectroscopic,5,6 chemical,7
and theoretical methods.6r,s,8 These investigations have strength-
ened the belief that the thiol form 1 best describes the properties
of these acids. The thion acid 2 (thioxo form) has probably
evaded clear experimental identification because of the small
energy difference between the two forms.8a Very recently, we
have isolated a series of selenocarboxylic acids and found that
their structures in a polar solvent are the selenoxo form (4)
(RCSeOH, selenon acid) at low temperature.4 This prompted
us to reexamine thiocarboxylic acids.
(2) Kekule´, A. Ann. Chem. Pharm. 1854, 90, 309.
(3) The possibility of the presence of a tautomer 2 of 1 was described
by Hantzsch and Scharf in 1913: Hantzsch, A.; Scharf, E. Ber. Dtsch. Chem.
Ges. 1913, 46, 3570. Reports that favor a detectable amount of the thion
acid from spectral data (infrared6b,c,g and UV/visible spectra6q) are not based
on unambiguously assigned bands.
(4) Kageyama, H.; Murai, T.; Kanda, T.; Kato, S. J. Am. Chem. Soc.
1994, 116, 2195.
(5) Murthy, A. S. N.; Rao, C. N. R.; Rao, B. D. N.; Venkateswarhu, P.
Trans. Faraday Soc. 1962, 58, 855.
(6) For infrared, see: (a) Kohlrausch, K. W. F.; Pongratz, A. Z. Phyz.
Chem. B 1934, 27, 176. (b) Sheppard, N. Trans. Faraday Soc. 1949, 45,
693. (c) Crouch, W. W. J. Am. Chem. Soc. 1952, 74, 2926. (d) Mecke,
R.; Spiesecke, H. Chem. Ber. 1956, 89, 1110. (e) Sjo¨berg, B. Acta Chem.
Scand. 1957, 11, 945. (f) Nyquest, R. A.; J. Potts, W. Spectrochim. Acta
1959, 15, 514. (g) Ginzburg, I. M.; Loginova, L. A. Opt. Spectrom. 1966,
20, 130. (h) Engler, V. R.; Gattow, G. Z. Anorg. Allg. Chem. 1972, 388,
78. (i) Beumer, A. E.; Harkema, S. Acta Crystallogr. 1973, B29, 682. (j)
Bicca de Alencastro, R.; Sandorfy, C. Can. J. Chem. 1973, 51, 1443. (k)
Crowder, G. A. Appl. Spectrosc. 1973, 27, 440. (l) Randhawa, H. S.; Rao,
C. N. R. J. Mol. Struct. 1974, 21, 123. (m) Crowder, G. A.; Robertson, E.;
Potter, K. Can. J. Spectrosc. 1975, 20, 49. (n) Crowder, G. J. Mol. Struct.
1976, 32, 207. (o) Randhawa, H. S.; Meese, C. O.; Walter, W. J. Mol.
Struct. 1977, 36, 25. (p) Randhawa, H. S.; Walter, W. J. Mol. Struct. 1977,
38, 89. For UV/vis, see: (q) Sheinker, Yu N.; Ioffe, S. T.; Kabachnik, M.
IzV. Akad. Nauk. SSSR., Otd. Khim. Nauk. 1960, 1571 (English Transl. p
1463). (r) Nagata, S.; Yamabe, T.; Fukui, K. J. Phys. Chem. 1975, 79,
2335. (s) Yamabe, T.; Akagi, K.; Nagata, S.; Kato, H.; Fukui, K. J. Phys.
Chem. 1976, 80, 611. For NMR, see refs 6a,b.
When 4-methoxybenzenecarbothioic acid (1d) (R ) 4-CH3-
OC6H4) was dissolved in a polar solvent such as THF, the color
of the solution changed from a light yellow to a deep yellow,
with an absorption maximum at 413 nm,9 whereas hexane or
(7) (a) Holmberg, B. Ark. Kemi, Mineral. Geol. 1938, 12B, No. 47. (b)
Holmberg, B.; Schjinberg, C. Ark. Kemi, Mineral. Geol. 1940, 14A, No. 2.
(8) (a) Naito, T.; Ohashi, O.; Yamaguchi, I. J. Mol. Spectrosc. 1977,
68, 32. (b) Fausto, R.; Batista de Carbalho, L. A. E.; Teixeira-Dias, J. J.
C. J. Mol. Struct. (THEOCHEM) 1987, 150, 38. For thioformic acid, see:
(c) George, P.; Bock, C. W.; Schmiedekamp, A. S. J. Mol. Struct.
(THEOCHEM) 1981, 76, 363. (d) Fausto, R.; Batista de Carbalho, L. A.
E.; Teixeira-Dias, J. J. C.; Ramos, V. J. Chem. Soc., Faraday Trans. 1989,
2, 1945. (e) Fausto, R.; Batista de Carbalho, L. A. E.; Teixeira-Dias, J. J.
C. J. Mol. Struct. 1990, 207, 67.
0002-7863/96/1518-1262$12.00/0 © 1996 American Chemical Society

Thion, Selenon, and Telluron Acids
J. Am. Chem. Soc., Vol. 118, No. 6, 1996 1263
Figure 1. Electron spectra in CH2Cl2 solution and in THF solution
for 4-methoxybenzenecarbothioic acid at 20 °C.
Figure 2. Comparison of IR spectra in THF solution and in the solid
state for 4-methoxybenzenecarbothioic acid at 20 °C.
CH2Cl2 solutions of 1d were light yellow and showed no
absorption in the region 350-500 nm (Figure 1). The IR and
NMR spectra of the deep yellow solution showed some critical
differences from those in the solid state (light yellow) or in
dichloromethane solution. As shown in Figure 2, neither of
the strong ν(CdO) (1670 cm-1) and ν(SsH) bands (2512 cm-1
)
in the solid state can be observed in THF solution. In THF,
the 1H and 13C NMR spectra do not show signals corresponding
to SsH and CdO moieties. Upon measuring these spectra at
a lower temperature of -70 °C, new broad signals near δ 14
and 212 appeared which were ascribed to the OH proton and
the CdS carbon,10 respectively. Upon further lowering of the
temperature to -90 °C, these broad signals became markedly
sharp at δ 14.52 and 212.3 (Figure 3A),11-13 indicating clearly
that tautomeric equilibrium exists between the thiol (I) and
thioxo forms (II) in a polar solvent (eq 1) (Table 1). A close
Figure 3. Variable-temperature NMR spectra in THF-d8 solution for
thiocarboxylic acids.
(thion acid) at low temperature in polar solVents such as THF.
To our knowledge, no explanation for the yellow chromophore
(10) The 13CdS values of the known compounds having the CdS moiety
are as follows. 4-CH3OC6H4CSOCH3: δ 211.3 (CDCl3), 212.8 (THF-d8).
PhCSOCH3: 213.4 (THF-d8): δ 212.2 (CDCl3) (Pedersen, B. S.; Sheiby,
S.; Klausen, K.; Lawesson, S. O. Bull. Soc. Chim. Belg. 1978, 42, 3556).
4-CH3OC6H4CSOSiMe3: δ 220.6 (CDCl3), 221.3 (THF-d8). No equilibrium
between RCOSSiMe3 and RCSOSiMe3 is observed (Kato, S.; Mitani, T.;
Mizuta, M. Bull. Chem. Soc. Jpn. 1972, 45, 244. Mikoliajczyk, M.;
Kielbasinski, O.; Schiebel, H. M. J. Chem. Soc., Perkin Trans. 1 1976,
564). An equilibrium between t-BuCOTeSiMe3 and t-BuCTeOSiMe3 has
been reported (Severengiz, T.; du Mont, W. W. J. Chem. Soc., Chem.
Commun. 1987, 820). Cf. the 13CdO values of ArCOSCH3. 4-CH3OC6H4-
COSCH3: δ 184.2 (CDCl3), 184.8 (THF-d8). PhCOSCH3: δ 185.0 (THF-
d8), 185.9 (CDCl3).
1
similarity of H and 13C NMR spectra has been observed in
other polar solvents such as acetone and methanol (Table 2)
and for other thiocarboxylic acids such as benzenecarbothioic
and 4-chloro- and 4-methylbenzenecarbothioic acids in THF
solution. In addition, for 2-methoxybenzenecarbothioic and
thioacetic acids, 1H and 13C NMR signals indicating the
existence of the two forms were observed at -90 °C, respec-
tively14 (Figure 3B,C). These results apparently suggest that
thiocarboxylic acids exist predominantly as the thioxo form II
(11) Proton ratio of OH/SH ) 2.99:0.01 (-90 °C) (on the basis of a
proton ratio of OH/CH3 ) 2.99:3.00).
(9) UV/vis λmax (log ꢀ) (THF): 289 (4.40), 413 (2.13). PhCSOCH3, UV/
vis λmax (log ꢀ): 418 (2.20) (hexane) (Ohno, A.; Koizumi, T.; Ohishi, Y.;
Tsuchihashi, G. Bull. Chem. Soc. Jpn. 1969, 42, 3556).
(12) Two groups have reported the existence of thioacetic6b,c and
trichlorothioacetic O-acids6g (thioxo form II) on the basis of the IR bands
at 1220 and 1160 cm-1, respectively.

1264 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
Kato et al.
Table 1. Thion and Thioxo Forms of RCOSH (R ) 4-CH3OC6H4)
Table 3. IR and NMR Spectral Data of
4-Methoxybenzenecarboselenoic Acid at 23 °C
a
in THF-d8
NMR (δ)
IR (cm-1
)
O
S
THF-d8
solvent
1H (SeH)
CdO
ν(CdO)
[solvent]
R
S
H
R
O H
1680 (s)a
1682 (s)
1697 (s)
1693 (w)
1698 (w)
[CHCl3]
[CH2Cl2]
[CCl4]
[THF]
[Et2O]
b
CDCl3
CD2Cl2
C6D6
THF-d8
CD3OD
acetone-d6
2.59
2.60
2.45
189.6
189.6
189.6
temp (°C)
%
%
23
0
-30
-50
-70
-90
50?
50?
212.4 (w)a
40
21
13
60
79
87
4.99
a s ) strong, w ) weak.
<1
>99
a On the basis of the proton ratio of OH and CH3 groups. b The SH
proton is not detected.
Table 4. Selenol and Selenoxo Forms of RCOSeH (R )
4-CH3OC6H4) in THF-d8
O
Se
THF-d8
Table 2. IR and NMR Spectral Data of
4-Methoxybenzenecarbothioic Acid at 23 °C
R
Se
H
R
O
H
NMR (δ)
IR (cm-1
)
b
temp (°C)
%
%
solvent
1H (SH)
13CdO
ν(CdO)
[solvent]
23
0
-30
-70
-90
50?
50?
CDCl3
CD2Cl2
C6D6
THF-d8
CD3OD
acetone-d6
4.48
4.49
4.20
188.5
188.7
187.8
1664 (s)a
1682 (s)
1679 (s)
1670 (w)
1698 (w)
[CHCl3]
[CH2Cl2]
[CCl4]
[THF]
[Et2O]
25
9
<1
75
91
>99
a On the basis of the proton ratio of OH and CH3 groups. b Se-H
proton is not detected.
a s ) strong, w ) weak.
ν(SesH) bands, which are observed in the solid state or in
nonpolar solvent such as dichloromethane, disappear or become
very small in a polar solvent such as THF, acetone, or methanol.
of a thiocarboxylic acid has been described in the literature.
Presumably, the absorption maximum at 413 nm observed for
1d can be attributed to the n-π* transition of the thiocarbonyl
group in the thion acid 2. Thionoacetic acid is more stable than
thiolacetic acid,15 while the reverse is true for the monosulfur
formic acid.6l,s However, thiocarboxylic acids exist only in the
thioxo form (2) at low temperature despite the fact that they
require double bonding between carbon and sulfur, which is
more energetically unfavorable than that between carbon and
oxygen.16 In a polar solvent, hydrogen-bonding interactions
with the solvent may enable the electrons on the thiocarboxyl
group to delocalize more easily. In this system, the equilibrium
in eq 1 is shifted to II. This can be explained by the notion
that the proton on a thiocarboxyl group localizes on the more
electronegative atom, i.e. oxygen rather than sulfur, which
enables the involvement of the CdS bond in II.
1
In the H, 13C, and 77Se NMR spectra, signals of SeH, OH,
CdO, and CdSe moieties are not detected (Table 3, see ref 4
(Figure 3)). Upon measuring these spectra at -70 °C, new
broad signals near δ 15, 220, and 750 ppm appear, which can
be ascribed to the OH proton, CdSe carbon, and CdSe
selenium, respectively. These broad signals become markedly
sharp, indicating clearly that tautomeric equilibrium exists
between the selenol (III) and the selenoxo (IV) forms in a polar
solvent (eq 2, Table 4).
As reported previously,4 selenocarboxylic acids also exist in
selenoxo form (4) rather than the selenol form (3) in polar
solvent at low temperature. Thus, both the strong ν(CdO) and
Tellurocarboxylic acids are of interest both for comparison
with thio- and selenocarboxylic acids and for understanding the
bonding between tellurium and hydrogen. However, the
synthesis of such heavy chalcogenocarboxylic acids has not been
possible because of their extreme instability and the difficulty
of preparing alkali metal tellurocarboxylates, which are the most
important starting compounds for the preparation of such labile
chalcogenocarboxylic acids. Very recently, we succeeded in
isolating rubidium and cesium tellurocarboxylates as crystals.17
This encouraged us to prepare tellurocarboxylic acids.
When a solution of an excess of dry hydrogen chloride in
ether was added to a solution of cesium 4-methoxybenzenecar-
botelluroate (7a) in THF at -90 °C, the dark red solution of
the salt immediately changed to dark green with the precipitation
of CsCl. The 1H and 13C NMR spectra did not show any signals
corresponding to the expected TeH and CO moieties, respec-
tively (Figure 4). Instead, sharp signals were observed at δ
16.02 and 222.9, which are ascribed to the OH proton and the
CdTe carbon, respectively.18 Upon allowing the temperature
to rise to -70 °C, the signals broadened substantially, indicating
(13) It has been reported that the thioxo form can be observed more
readily at a higher temperature than at a lower temperature. There are a
few ill-defined in OH proton chemical shifts for thioacetic acid.6p
(14) Acetic acid is well-known to form cyclic dimers, both in the gas
phase and in a dilute solution (Gordy, W. J. Chem. Phys. 1946, 21, 1217.).
Vibrational spectra of thioacetic,6d,j,m thiopropionic,6k and thiobenzoic
acids5,6g,j have been interpreted in terms of hydrogen-bonded dimers A and
B (self-association) in the solid state and in a dilute solution. However, no
evidence was obtained, indicating the presence of the cyclic dimer C of a
thion acid in a polar solvent such as THF. Further comment on the self-
association will be left for the discussion.
(15) The energy of the hydrogen bond of CH3COSH does not exceed
25.1-33.4 kJ/mol.6b
(16) The synthesis and characterization of carbon-chalcogen multiple
bond are of current research interest. For review, see: Okazaki, R. J. Synth.
Org. Chem. Jpn. 1988, 46, 1149. (b) Usov, V. A.; Timokhina, L. V.;
Voronkov, M. G. Sulfur Rep. 1992, 12, 95-158. (d) Guziec, F. S., Jr. In
The Chemistry of Organic Selenium and Tellurium Compounds; Patai, S.,
Ed.; John Wiley & Sons: New York, 1987; Vol. 2, pp 215-273. (e) Guziec,
F. S., Jr. In Organoselenium Chemistry; Liotta, D., Ed.; Wiley-Interscience:
New York, 1987; p 77.
(17) (a) Kawahara, Y.; Kato, S.; Kanda, T.; Murai, T. Chem. Lett. 1995,
87. (b) Kawahara, Y.; Kato, S.; Kanda, T.; Murai, T.; Ebihara, M. Bull.
Chem. Soc. Jpn. 1995, 68, 3507.

Thion, Selenon, and Telluron Acids
J. Am. Chem. Soc., Vol. 118, No. 6, 1996 1265
Scheme 2
at δ 535, which changed instantly to a dark green with further
addition of THF, indicating the formation of 9a with an
absorption maximum at 652 nm. However, the direct addition
of THF to the yellow solid at -78 °C led to a dark green
solution of 9a. Although tellurol acid 8 is too labile to allow
us to measure the TeH proton and carbonyl carbon chemical
shifts, these results indicate that, at low temperature, tellurol
acid 8a exists as a yellow solid or a yellow toluene solution. In
a polar solvent, the hydrogen-bonding interaction of 8a with
the solvent may enable the electrons on the tellurocarboxyl group
to delocalize more easily. As in thio- and selenocarboxylic
acids, the fact that the equilibrium in eq 3 is shifted to VI may
be explained by the realization that the proton on the telluro-
carboxyl group localizes on the more negative oxygen atom
than the tellurium, and this enables the involvement of the CdTe
bond in VI.
Figure 4. Variable-temperature NMR spectra in THF-d8 solution for
4-methoxybenezenecarbotelluroic O-acid (9a).
Figure 5. Electron spectra in THF solution for 4-methoxybenzenecar-
botelluroic O-acid (9a) at -90 °C.
The resulting tellurocarboxylic acids were extremely sensitive
toward oxygen and temperature. Tellurol acids 8 appear to be
considerably more labile than telluron acids 9. For example,
when exposed to air, 4-methoxybenzenecarbotelluroic O-acid
(9a) in THF solution immediately liberates black tellurium
(tellurium mirror), even at -90 °C.21 However, under an Ar
conditions, no appreciable change in the solution was observed
at -78 °C for at least 1 min. In contrast, the tellurol acid 8a
gradually decomposed to change from yellow brown to gray
even at -90 °C.
We previously found that dithiocarboxylic acids reacted with
aryl isocyanates to give thioacyl carbamoyl sulfides (RCS-S-
CONHR′, R, R′ ) alkyl, aryl) in good yields, and in which the
existence of hydrogen bonding between the NH hydrogen and
thiocarbonyl sulfur atoms was suggested by infrared study.22
Expecting the formation of tellurium analogues, we reacted
tellurocarboxylic O-acids with aryl isocyanates (eq 4). In fact,
the existence of some dynamic NMR exchange process (prob-
ably involving making and breaking of the hydrogen bond (eq
3). The 125Te NMR spectrum showed a characteristic signal at
δ 952 due to the CdTe group.19 Furthermore, the UV/visible
spectrum at -73 °C showed an absorption maximum at 652
nm which was ascribed to the n-π* transitions of the CdTe
group (Figure 5).20 These results apparently suggest that the
tellurocarboxylic acid exists predominantly in the telluroxo form
(VI) (telluron acid) (4-methoxybenzenecarbotelluroic O-acid
(9a)) in THF at low temperature. Under similar conditions,
acidolysis of cesium 4-methylbenzenecarbotelluroate (7b) and
2,2-dimethylpropanetelluroate (7c) with HCl led to dark green
(λmax 672 nm) and blue violet solutions (λmax 594 nm),
respectively, indicating the formation of the corresponding
tellurocarboxylic acids 9b and 9c.
Next, to confirm the existence of a tellurol acid 5, acidolysis
of the salt 7a with hydrogen chloride was carried out without
solvents. Thus, when an excess of pure liquid hydrogen chloride
was added to 7a at -195 °C and the temperature was raised to
-90 °C, the dark brown salt quickly changed to a yellow solid
near -110 °C. Addition of toluene to the yellow solid at -90
°C gave a yellow solution with a characteristic 125Te NMR signal
(18) The values of 13C NMR resonances corresponding to CdTe
moieties: 4-CH3C6H4CTeOSi(CH3)3 (CDCl3): δ 223.717a; 4-CH3OC6H4-
CTeOSi(CH3)3 (CDCl3): δ 221.617b; t-C4H9CTeOSiCH2(t-Bu) (CDCl3): δ
229.418a-c [(a) Barret, A. G. M.; Barton, D. H. R.; Reed, R. W. J. Chem.
Soc., Chem. Commun. 1979, 645-647. (b) Barret, A. G. M.; Reed, R. W.;
Barton, D. H. R. J. Chem. Soc., Perkin Trans. 1 1980, 2191-2195. t-C4H9-
CTeOSi(CH3)3 (CDCl3): (c) Severengis, T.; du Mont, W. W. J. Chem. Soc.,
Chem. Commun. 1987, 820-821].
4-methoxybenzenecarbotelluroic O-acid (9a) reacted with phenyl
and 4-methylphenyl isocyanates at -70 °C to give the corre-
sponding acyl carbamoyl tellurides 10a and 10b in good yields
(19) The values of 125Te NMR resonances corresponding to CdTe
moieties. 4-CH3-C6H4CTeOSiCH3)3 (CDCl3): δ 1438.17a 4-CH3OC6H4-
CTeOSi(CH3)3 (CDCl3): δ 1303.17b t-C4H9CTeOSi(CH3)3 (CDCl3): δ
1418.18c
(21) Removal of the black tellurium from the decomposition products
by filtration, followed by evaporation under reduced pressure, gave a yellow
oil in which the presence of bis(4-methoxybenzoyl) telluride was suggested
(20) The values of n-π* transitions of CdTe group. 4-CH3C6H4-
CTeOSi(CH3)3 (CHCl3): 700 nm.17a 2-CH3OC6H4CTeOSi(CH3)3 (CH3CN):
670 nm.17b 4-CH3OC6H4CTeOSi(CH3)3 (CHCl3): 682 nm.17b
1
by mass and H and 13C NMR spectral studies. Presumably the telluride
would be formed by detelluration of the corresponding diacyl ditelluride.

1266 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
Kato et al.
as pale yellow crystals (eq 4). In addition, the reaction with
4-methylbenzenecarbotelluroic O-acid (9b) under the same
conditions gave a good isolated yield of the corresponding
addition product 10c (eq 4). In these reactions, the use of
dichloromethane instead of THF as a solvent resulted in
analogous yields of 10. The NH proton chemical shifts of 10
were observed in downfield region below δ 10, suggesting that
10 exists as a strongly intramolecularly hydrogen-bonded species
11. Recently, analogous intramolecular six-membered-ring struc-
synthesis of a thiocarboxylic acid, have opened up the possibility
of a new chemistry for a class of compounds of hitherto
unknown chalcogenon acids (RCEOH, E ) S, Se, Te). In
addition, the presence of these acids may be important in
understanding not only the role of the chalcogeno elements,
especially S and Se, in biological systems but also the results
of studies concerning the addition of thiocarboxylic acids to
unsaturated bonds.
Experimental Section
General Procedure. Melting points were determined with a
Yanagimoto micro-melting point apparatus and are uncorrected. The
IR spectra were measured on a Perkin-Elmer FT-IR 1640 instrument.
1
The Electron spectra were taken by a JASCO Ubest-55. The H and
13C NMR spectra were recorded in 10% solutions on a JEOL A-400
(400 and 100 MHz, respectively) instrument with tetramethylsilane as
an internal standard. The 125Te NMR spectra were recorded on a JEOL
A-400 (85.3 MHz) with dimethyl telluride as an external standard.
Elemental analyses were performed by the Elemental Analysis Center
of Kyoto University.
tures arising from hydrogen-bonding between the NH hydrogen
and the carbonyl oxygen or thiocarbonyl sulfur atom have been
observed by X-ray structural analysis for 2,6-dimethoxybenzoyl
4-methylbenzenecarbamoyl selenides23 and 4-methoxythioben-
zoyl 4-methylbenzenecarbamoyl sulfides,24 respectively.
In summary, we have demonstrated the existence of thion
acid 2 as a predominat species and have reported the first
observation of a tellurocarboxylic acid. Tautomeric equilibrium
between the thiol and thioxo forms and between the tellurol
and telluroxo forms was demonstrated spectroscopically. These
results and those obtained for selenocarboxylic acids10 lead to
the general conclusion that, in monochalcogenocarboxylic acids
such as thio-, seleno-, and tellurocarboxylic acids, the chalco-
genoxo form IX is an actual and predominant species in apolar
solvents at low temperature despite the involvement of the
energetically unfavorable CdE bond (E ) S, Se, Te), whereas
chalcogenol acid VII (chalcogenol form) is a predominant
species in both the solid state and nonpolar solvents (Scheme
3). In these classes of compounds involving heteroatom-
containing allylic anion systems, electronegativity and hydrogen-
bonding interaction may play important roles in controlling the
degree of the localization of the electrons and in the formation
of double bonding between carbon and third-, forth-, and fifth-
row elements. The present findings, 140 years after the first
Materials. Thioacetic (1f) and benzenecarbothioic (1a) acids were
of commercial grade and distilled before use. 4-Methyl- (1b), 4-chloro-
(1c), 4-methoxy- (1d), and 2-methoxybenzenecarbothioic acids (1e)
were prepared by HCl acidolysis of the corresponding potassium salts
which were prepared according to the literature25a and purified by
vacuum distillation or by recrystallization from ether and/or hexane.
1
C6H5COSH (1a): liquid; IR (neat) 2582 (SH), 1672 (CdO) cm-1; H
NMR (CDCl3) δ 5.07 (s, 1H, SH), 7.3-8.1 (m, 5H, Ar); 13C NMR
(CDCl3) (23 °C) δ 127.9, 128.1, 128.7, 133.9 (Ar), 190.2 (CdO).
4-CH3C6H4COSH (1b): mp 42.5-43 °C (lit.25c 43-44 °C); IR (KBr)
2510 (SH), 1651 (CdO) cm-1; 1H NMR (CDCl3) δ 2.31 (s, 3H, CH3),
4.44 (bs, 1H,SH), 7.14 (d, 2H, Ar, 8.2 Hz), 7.69 (d, 2H, Ar, 8.2 Hz);
13C NMR (CDCl3) (23 °C) δ 21.7 (CH3), 127.9, 128.0, 129.3, 129.4,
134.1, 144.9 (Ar), 189.7 (CdO). 4-ClC6H4COSH (1c): IR (KBr) 2508
1
(SH), 1658 (CdO) cm-1; H NMR (CDCl3) δ 4.63 (s, 1H, SH), 7.34
(d, 2H, Ar, J ) 8.6 Hz), 7.84 (d, 2H, Ar, J ) 8.6 Hz); 13C NMR (CDCl3)
129.0, 129.1, 129.2, 129.3, 134.9, 140.7 (Ar), 188.9 (CdO). 4-CH3-
OC6H4COSH (1d): mp 81-83 °C (lit.25b 82-83 °C); IR (KBr) 2512
(SH), 1652 (CdO); (THF) 3572, 3507 (OH), 2682 (SH), 1670 (CdO)
1
cm-1; H NMR (CDCl3) δ 3.87 (s, 3H, CH3), 4.48 (s, 1H, SH), 6.40
(d, 2H, Ar, 8.4 Hz), 7.87, (d, 2H, Ar, 8.4 Hz); 13C NMR (CDCl3) (23
°C) δ 55.6 (CH3O), 113.9, 129.5, 130.2, 164.2 (Ar), 189.2 (CdO) (-50
°C); (CD2Cl2) δ 4.49 (SH), 188.7 (CdO); UV/vis λmax (ꢀ) (cyclohexane)
273 (4.40), 283 (4.36); UV/vis λmax (CH2Cl2) 279 nm. 2-CH3OC6H4-
COSH (1e): oil; IR (neat) 2552 (SH), 1641 (CdO); IR (THF) 3571,
3507 (OH), 2682 (SH), 1650 (CdO); (CD2Cl2) 3650 (OH), 2564 (SH),
Scheme 3
1
1645 (CdO) cm-1; H NMR (CD2Cl2) δ 3.96 (s, 3H, CH3), 4.92 (s,
1H, SH), 7.0-8.0 (m, 4H, Ar); IR (THF-d8) δ 3.91 (s, 3H, CH3O),
7.0-7.8 (m, 4H, Ar); 13C NMR (CD2Cl2) δ 25.3 (CH3O), 113.1, 121.1,
1
130.0, 135.1 (Ar), 159.4 (CdO). CH3COSH (1f): H NMR (CDCl3)
(23 °C) δ 2.39 (s, 3H, CH3), 4.77 (SH); 13C NMR (CDCl3) (23 °C) δ
32.6 (CH3), 194.0 (CdO). 4-methoxybenzenecarboselenoic O-acid was
prepared by the method described previously.4 Cesium 2,2-dimethyl-
propanetelluroates and 4-methyl-, and 4-methoxybenzenecarbotel-
luroates were prepared by the reaction of the corresponding O-tri-
methylsilyl tellurocarboxylates with cesium fluoride or by the reaction
of acyl chlorides with sodium telluride.17
1
Values of H and 13C NMR Corresponding to OH and/or SH
and CdS and/or CdO Moieties of Thiocarboxylic Acids. C6H5-
CSOH (1a): (THF-d8) (-90 °C) δ 14.28 (OH), 213.5 (CdS); (-70
°C) 14.68 (OH), 213.5 (CdS); (-50 °C) 14.52 (OH), 213.3 (CdS);
(CD3OD) (-80 °C) 15.60 (OH), 214.0 (CdS). 4-CH3C6H4-CSOH (1b):
(THF-d8) (-90 °C) δ 14.62 (OH), 210.8 (CdS); (-70 °C) 14.59 (OH),
210.8 (CdS); (-50 °C) 14.43 (OH), 210.9 (CdS) 4-ClC6H4CSOH (1c):
(THF-d8) (-90 °C) δ 14.90 (OH), 212.0 (CdS); (-70 °C) 14.80 (OH),
212.0 (CdS); (-50 °C) 212.2 (CdS). 4-CH3OC6H4CSOH (1d): (THF)
(-90 °C) δ 14.52 (OH), 212.3 CdS); (-70 °C) 14.37 (OH), 212. 2
(22) For the reaction with RCSSH, see: (a) Kato, S.; Mitani, T.; Mizuta,
M. Bull. Chem. Soc. Jpn. 1972, 45, 3653. We have postulated an analogous
intramolecular six-member-ring structure stabilized by hydrogen-bonding
between the thiocarbonyl sulfur and NH hydrogen atoms. For the reaction
with RCOSH, see: Motoki, S.; Saito, T.; Kagami, H. Bull. Chem. Soc. Jpn.
1974, 44, 775.
(23) Kageyama, H.; Asada, K.; Maejima, K.; Kanda, T.; Murai, T.; Kato,
S. The 67th Spring Annual Meeting of The Japan Chemical Society, Tokyo,
Abstr. II, March 1994; p 1320, (4HX-07).
(24) Ogawa, T.; Asada, K.; Kageyama, H.; Kanda, T.; Murai, T.; Kato,
S. The 69th Spring Annual Meeting of The Japan Chemical Society, Kyoto,
Abstr. II, March 1995; p 1304, (2HX-30).
(25) (a) Nobel, P. L., Jr.; Tarbel, D. S. Org. Synth. 1963, Collect. Vol.
4, 924. (b) Bloch, I.; Bergmann, M. Ber. Dtsch. Chem. Ges. 1920, 53,
961. (c) Weigert, F. Ber. Dtsch. Chem. Ges. 1903, 36, 1011.

Thion, Selenon, and Telluron Acids
J. Am. Chem. Soc., Vol. 118, No. 6, 1996 1267
(CdS); (acetone-d6) (-90 °C) δ 14.60 (OH), 193.4 (CdO), 211.5
(CdS); (C6D6) (23 °C) 4.26 (SH), 187.8 (CO). 2-CH3OC6H4CSOH
(1e): (THF-d8) (-90 °C) δ 5.86 (SH), 14.68 (OH), 188.0 (CdO), 217.8
(CdS); (-70 °C) 5.71 (SH), 188.0 (CdO); (-50 °C) 5.71 (SH), 188.0
(CdO); (-30 °C) 5.71 (SH), 188.0 (CdO). CH3CSOH (1f): (THF-
d8) (-90 °C) δ 6.44 (SH), 14.38 (OH), 195.5 (CdO), 221.2 (CdS);
(-70 °C) 6.25 (SH), 14.24 (OH), 195.2 (CdS); (-50 °C) 6.08 (SH),
195.0 (CdS); (-30 °C) 5.95 (b, SH); (0 °C) 5.81 (b, SH); (20 °C)
5.84 (b, SH).
Reaction of 4-Methoxybenezenecarbotelluroic O-Acid (9a) with
Phenyl Isocyanate. A solution of dry hydrogen chloride (2.66 mmol)
in ether (2.66 mL) was added dropwise to a suspension of cesium
4-methoxybenzenecarbotelluroate (7a) (0.181 g, 0.45 mmol) in tet-
rahydrofuran (7 mL) at -78 °C, and the mixture was stirred at -70
°C for 2 h. The color changed from a dark red to a dark green,
indicating the formation of 4-methoxybenzenecarbotelluroic O-acid (9a).
A solution of phenyl isocyanate (0.158 g, 1.33 mmol) in tetrahydrofuran
(4 mL) was added dropwise, and the mixture was stirred for 6 h. The
insoluble part (CsCl and black tellurium) was filtered off, and the
solvent was evaporated under reduced pressure to give 0.138 g (81%,
purity >98% by 1H NMR) of 4-methoxybenzoyl phenylcarbamoyl
telluride (10a) as microfine crystals. Recrystallization of the crystals
from a mixed solvent of dichloromethane and hexane (1:6) at -15 °C
afforded 0.065 g (38%) of chemically pure 10a as pale yellow needles:
mp 71 °C (dec); IR (KBr) 3324 (NH) 3037, 2985, 1701 (CdO), 1687
(CdO), 1594, 1571, 1545, 1303, 1266, 1165, 1026, 876, 860, 841,
752, 648, 613, 562, 502 cm-1; 1H NMR (CDCl3) δ 3.82 (s, 3H, CH3O),
6.80-7.7 (m, 9H, Ar), 10.13 (bs, 1H, NH); 13C NMR (CDCl3) δ 55.8
(CH3O), 114.6, 119.8, 124.8, 129.1, 130.1, 135.2, 137.2, 165.4 (Ar),
148.6 (NCO), 201.5 (CdO); 125Te NMR (CDCl3) δ 891.6.
1
Values of H and 13C NMR Corresponding to OH and/or SeH
and CdSe and/or CdO Moieties of 4-Methoxybenzenecarboselenoic
Acid (9a). 4-CH3OC6H4CSeOH: (THF-d8) (-90 °C) δ 15.29 (OH),
222.2 (CdSe); (-70 °C) 15.16 (OH), 222. 2 (CdSe); (-50 °C) 15.08
(OH), 221.8 (CdSe); (-30 °C) 14.7(br) (OH), 216.0 (CdSe).
Preparation of Tellurocarboxylic Acids. All manipulations were
carried out under an argon atmosphere.26
Preparation of 4-Methoxybenzenecarbotelluroic O-Acid (9a). To
a suspension of cesium 4-methoxybenzenecarbotelluroate (7a) (0.181
g, 0.45 mmol) in THF-d8 (5 mL) was added a solution of dry hydrogen
chloride (0.9 mmol) in ether (0.9 mL) at -78 °C. The mixture was
stirred at this temperature for 1 h. The color of the reaction mixture
instantly changed from dark red to dark green. Removal of CsCl by
filtration and subsequent evaporation of the excess of hydrogen chloride
and ether from the filtrate under reduced pressure afforded a dark green
4-Methoxybenzoyl 4-Methylbenzenecarbamoyl Telluride (10b).
In a way similar to the reaction with phenyl isocyanate, the reaction of
freshly prepared 4-methoxybenzenecarbotelluroic O-acid (9a) (0.45
mmol) [from a solution of dry hydrogen chloride (2.66 mmol) in ether
(2.66 mL) and cesium 4-methoxybenzenecarbotelluroate (7a) (0.181
g, 0.45 mmol) in tetrahydrofuran (10 mL)] with 4-methylphenyl
isocyanate (0.100 g, 1.25 mmol) in tetrahydrofuran (4 mL) for 6 h to
1
THF-d8 solution of 4-methoxybenzenecarbotelluroic O-Acid (9a): H
NMR (THF-d8) (-90 °C) δ 3.85 (s, 3H, CH3O), 6.9-8.3 (m, 4H, Ar),
16.2 (s, 1H, OH); 13C NMR (THF-d8) (-90 °C) δ 21.7 (CH3O), 127.9,
134.3, 141.4, 148.2 (Ar), 222.9 (CdTe); 125Te NMR (THF-d8) (-90
°C) δ 952; UV/vis (THF) (-90 °C) 284, 442.0, 652 nm.
1
give 0.256 g (89%, purity >99% by H NMR) of 4-methoxybenzoyl
4-methylphenylcarbamoyl telluride (10b) as a microfine solid and
recrystallization of the solid from a mixed solvent of dichloromethane
and hexane (1:6) at -20 °C afforded 0.061 g (21%) of chemically pure
10b as pale yellow needles: mp 90 °C (dec); IR (KBr) 3345, 3048,
2985, 1690 (CdO), 1593, 1571, 1507, 1432, 1310, 1268, 1220, 1164,
1023, 858, 835, 806, 780, 740, 648, 611, 580, 502, 490, 432 cm-1; 1H
NMR (CDCl3) δ 2.24 (s, 3H, CH3), 3.79 (s, 3H, CH3O), 6.90, 6.91,
7.09, 7.11, 7.43, 7.45, 7.69, 7.71 (8H, Ar), 10.05 (s, 1H, NH); 13C
NMR (CDCl3) δ 21. 0 (CH3), 55.8 (CH3O), 114.2, 114.6, 119.8, 129.6,
130.1, 131.1, 132.5 134.9, 135.3 (Ar), 148.5 (NCO), 165.4 (Ar), 201.5
(CdO); 125Te NMR (CDCl3) δ 888.0. Anal. Calcd for C15H15NO3Te:
C, 48.42; H, 3.81. Found: C, 48.13; H, 3.69.
Attempted Preparation of 4-Methoxybenezenecarbotelluroic Te-
Acid (4-CH3OC6H4COTeH) (8a). Dry hydrogen chloride gas (ca. 3
mL) was introduced onto cesium 4-methoxybenzenecarbotelluroate (7a)
(0.459 g, 1.16 mmol) in an NMR tube (Φ 5 × 150 mm) at -195 °C.
The excess of HCl was carefully evaporated upon raising the temper-
ature to -78 °C [the reaction occurred at ca. -110 °C (HCl; mp -114
°C), and the dark brown of the salts changed to a yellow solid of
4-methoxybenzenecarbotelluroic Te-Acid (8a) containing CsCl]. Cau-
tion: the NMR tube is disrupted explosiVely by a Violent eVolution of
HCl gas during the preparation of the 1H and 13C NMR samples. When
cooled to -90 °C, toluene-d8 (3 mL) was added to the yellow solid to
give a slightly yellow solution [125Te NMR (toluene-d8) (-90 °C) δ
535 (TeH)]. Upon addition of THF (1 mL) to the toluene-d8 solution,
there was an immediate change to a dark green solution [UV/vis
(THF)(-90 °C) 652 nm]. The visible spectrum was consistent with
that of the telluron acid 9a.
4-Methylbenzoyl Phenylcarbamoyl Telluride (10c). In a way
similar to the preparation of 10b, the reaction of freshly prepared
4-methylbenzenecarbotelluroic O-acid (9b) (1.0 mmol) [from a solution
of dry hydrogen chloride (2.66 mmol) in ether (2.66 mL) and sodium
4-methylbenzenecarbotelluroate (0.270 g, 1.0 mmol) in tetrahydrofuran
(10 mL)] with phenyl isocyanate (0.122 g, 1.03 mmol) in tetrahydro-
Preparation of 4-Methylbenezenecarbotelluroic O-Acid (9b). To
a suspension of cesium 4-methylbenzenecarbotelluroate (7b) (0.181 g,
0.45 mmol) in THF-d8 (5 mL) was added a solution of dry hydrogen
chloride (0.9 mmol) in ether (0.9 mL) at -78 °C. The mixture was
stirred at this temperature for 1 h. The color of the reaction mixture
instantly changed from a dark red to a dark green. Removal of CsCl
by filtration gave a THF solution of 4-methylbenzenecarbotelluroic
1
furan (5 mL) for 6 h gave 0.358 g (93%, purity >97% by H NMR))
of 4-methylbenzoyl phenylcarbamoyl telluride (10c) as a white-yellow
microfine solid. Recrystallization of the solid from a mixed solvent
of dichloromethane and hexane (1:6) at -20 °C afforded 0.091 g (24%)
of chemically pure 10c as pale yellow columns: mp 81 °C (dec); IR
(KBr) 3255, 3180, 3048, 2980, 1699 (CdO), 1654, 1597, 1545, 1495,
1444, 1306, 1293, 1236, 1207, 1169, 1142, 1075, 877, 856, 800, 754,
610, 578, 548, 505, 462 cm-1; 1H NMR (CDCl3) δ 2.35 (s, 3H, CH3),
7.0-7.7 (9H, Ar), 10.05 (s, 1H, NH); 13C NMR (CDCl3) δ 21. 9 (CH3),
119.8, 124.9, 127.6, 129.1, 130.1, 137.2, 139.9, 146.8 (Ar), 148.8
(NCO), 203.8 (CdO); 125Te NMR (CDCl3) δ 910.2.
1
O-acid (9b): H NMR (THF-d8) (-90 °C) δ 2.14 (s, 3H, CH3), 6.94,
6.95 (d, 2H, Ar), 7.71, 7.93 (d, 2H, Ar), 16.48 (s, 1H, OH); 13C NMR
(THF-d8) (-90 °C) δ 57.1 (CH3O), 115.4, 134.3, 143.1, 148.2 (Ar),
223.1 (CdTe) (?); 125Te NMR (THF-d8) (-90 °C) δ 1024 (?); UV/vis
(THF) (-90 °C) 673 nm.
2,2-Dimethylpropanetelluroic O-Acid (9c). To a suspension of
cesium 2,2-dimethylpropanetelluroate (7c) (0.181 g, 1.40 mmol) in
THF-d8 (5 mL) was added a solution of dry hydrogen chloride (2.8
mmol) in ether (2.8 mL) at -78 °C. The mixture was stirred at this
temperature for 1 h. The color of the reaction mixture instantly changed
from a yellow brown to blue-violet (a THF solution of 2,2-dimethyl-
propanetelluroic O-acid (9c)): UV/vis (THF) (-90 °C) 594 nm.
Acknowledgment. This work was supported partially by a
Grant in Aid for Scientific Research on Priority Area of Reactive
Organometallics No. 05236102 and by a Grant in-Aid-for
Scientific Research (B) from the Ministry of Education, Science,
Sports and Culture, Japanese Government. We greatly appreci-
ate additional support of our program by Miyoshi Oil & Fat
Co., Ltd., Research Fund.
(26) The term “under argon” means that the system was evacuated with
an oil pump and refilled with argon at least three times and that a positive
(ca. 3 Torr) of argon was maintained during the experiment.
JA953035L