A new, practical synthesis of organotellurium compounds from organic halides and silyl tellurides. Remarkable effects of polar solvents and leaving groups

Article abstract of DOI:10.1016/S0040-4039(01)00931-5

Silyl tellurides react with organic halides to give the corresponding organotellurium compounds and silyl halides in good to excellent yields. Substitution proceeds in polar solvents, such as acetonitrile, but not in nonpolar solvents under identical conditions. The leaving group also plays a significant role, with alkyl bromides being the most reactive, alkyl chlorides less so and alkyl iodides the least reactive. After removal of the volatile silyl halides and solvent under vacuum, the essentially pure organotellurium compounds, which can be used directly as precursors for carbocations, carbanions, and carbon-centered radicals, were obtained.

Full text of DOI:10.1016/S0040-4039(01)00931-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 5061–5064
A new, practical synthesis of organotellurium compounds from
organic halides and silyl tellurides. Remarkable effects of polar
solvents and leaving groups
Shigeru Yamago,* Kazunori Iida and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University,
Yoshida Hon-machi, Sakyo-ku, Kyoto 606-8501, Japan
Received 16 May 2001; accepted 1 June 2001
Abstract—Silyl tellurides react with organic halides to give the corresponding organotellurium compounds and silyl halides in
good to excellent yields. Substitution proceeds in polar solvents, such as acetonitrile, but not in nonpolar solvents under identical
conditions. The leaving group also plays a significant role, with alkyl bromides being the most reactive, alkyl chlorides less so and
alkyl iodides the least reactive. After removal of the volatile silyl halides and solvent under vacuum, the essentially pure
organotellurium compounds, which can be used directly as precursors for carbocations, carbanions, and carbon-centered radicals,
were obtained. © 2001 Elsevier Science Ltd. All rights reserved.
Divalent organotellurium compounds have attracted a
great deal of attention1 because of their synthetic util-
ity. Since they serve as excellent precursors of carb-
anions by transmetallation,2 of carbocations by oxida-
tion,3,4 and of carbon-centered radicals by reaction with
an efficient and practical synthetic route, which requires
no troublesome workup and/or purification.
We have recently reported that silyl tellurides show
unique reactivity toward carbonyl compounds and
quinones, especially in polar solvents.12 During our
investigation of silyl tellurides, we found that they react
with organic halides in polar solvents without cata-
lysts.13 We report here a new and practical synthesis of
organotellurium compounds by the thermal substitu-
tion reaction of trimethylsilyl phenyl telluride (1) and
organic halides (Eq. (1)).14 The reaction proceeds under
neutral conditions and affords the corresponding
organotellurium compounds in good to excellent yields.
Since the trimethylsilyl halide generated was easily
removed under reduced pressure, we obtained the
essentially pure organotellurium compounds, which
could be used directly for further synthetic
transformations.
radical mediators or by photolysis and thermolysis,5,6
a
number of methods for their preparation have been
developed. Most of these syntheses rely on nucleophilic
substitution of lithium or sodium tellurolates with
appropriate electrophiles, such as organic halides and
sulfonates.1a,7 Various new metal tellurates, e.g. stannyl
tellurides8 and samarium tellurides,9 have been recently
employed to increase efficiency and selectivity.
Homolytic substitution of ditellurides with carbon-cen-
tered radicals,10 and condensation of alcohols with
telluro-cyanates have also been developed.11 However,
the problem with the synthesis of organotellurium com-
pounds is often not the reaction but the isolation and
purification of the products. In many cases, certain
organotellurium compounds are unstable under air or
routine purification conditions, such as silica gel chro-
matography. As a result, organotellurium compounds
are often used in the following step without purifica-
tion. Therefore, it would be highly desirable to develop
(1)
We first examined the reaction of 1-decyl bromide (66.4
mg, 0.30 mmol) and 1 (83.4 mg, 0.30 mmol) in acetoni-
trile (1.0 mL) at 60°C for 12 h, and found that the
reaction proceeded smoothly to give 1-decyl phenyltel-
luride in quantitative yield, as judged by 1H NMR
(Table 1, entry 1). The reaction also produced
Keywords: tellurium; silicon; silyl tellurides; substitution; organo-
tellurium compounds.
* Corresponding authors. Tel.: +81-75-753-5661; fax: +81-
75-753-5911 (S.Y.); tel.: +81-75-753-5651; fax: +81-75-753-
5911 (J.Y.); e-mail: yamago@sbchem.kyoto-u.ac.jp; yoshida
@sbchem.kyoto-u.ac.jp
1
trimethylsilyl bromide, which was characterized by H
NMR in the experiments carried out in CD3CN. The
progress of the reaction was found to be strongly
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00931-5

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S. Yamago et al. / Tetrahedron Letters 42 (2001) 5061–5064
influenced by the polarity of the solvent. Thus, the
reaction took place efficiently in polar solvents, e.g.
acetonitrile, DMF, pyridine, and the ionic liquid (1-
The present reaction was applied to a variety of benzyl,
allyl, and alkyl halides; its scope and efficiency are
summarized in Table 1. Benzyl halides are especially
good substrates, and both bromides and chlorides
reacted to give the corresponding tellurides in quantita-
tive yields (entries 4–7). Substitution took place exclu-
sively at the sp3 carbonꢀchlorine bond over the sp2
carbonꢀchlorine bond (entries 5 and 6). The secondary
benzyl bromide also afforded the desired product in
good yield (entry 7), while allyl halides, equally good
substrates, furnished the desired products in excellent
yields (entries 8 and 9). It is worth noting that unstable
benzylic and allylic tellurides could be easily character-
butyl-3-methyl-1-imidazolium
tetrafluoroborate),15
while no reaction occurred in non-polar solvents, e.g.
THF and toluene, under similar conditions. Acetonitrile
was selected as the solvent for the following investigation
because of its volatility.
We next examined the effect of leaving groups and found
that, surprisingly, alkyl bromides were most reactive,
alkyl chlorides less so and alkyl iodides the least reactive.
Thus, even though the reaction of 1-decyl chloride had
not gone to completion after 24 h at 60°C, the desired
product formed in 87% yield (entry 2). Conversely, only
half the amount of 1-decyl iodide was converted to the
product under similar conditions (entry 3).16
1
ized by H and 13C NMR after removal of trimethyl-
silyl bromide and solvent under vacuum, followed by the
introduction of deuterated solvents. Primary alkyl bro-
mides are also quite reactive (entries 1, 10 and 11).
Table 1. Synthesis of organotellurium compounds from organic halides and silyl telluride 1a

S. Yamago et al. / Tetrahedron Letters 42 (2001) 5061–5064
5063
CNXy
(3.0 equiv)
TePh
Ph
Ph
(48%)
+
Ph
Ph
hν, 100 oC
NXy
1) 1/MeCN
2) evacuation
CD3CN
3
4 ( 40%)
Ph
TePh
Ph
Br
1) PhCHO
2) H3O+
n-BuLi
(1.1 equiv)
2
Ph
Ph
Ph
Li
THF, -78 oC
OH
6 (75%)
5
Figure 1. One-pot generation of benzyl radical and anion from benzyl bromide.
introduced to the crude 2, and to the resulting solution
was added BuLi in hexane (1.53 M solution, 0.22 mL,
0.33 mmol) at −78°C. The resulting mixture was stirred
for 15 min at this temperature, and benzaldehyde (33.5
mL, 0.33 mmol) was added. After being stirred for 30
min at −78°C, the reaction mixture was quenched by
addition of methanol (1.0 mL). After the usual workup,
purification by silica gel chromatography afforded 8 in
75% yield (44.5 mg, 0.23 mmol).
a-Amino tellurides, which are excellent precursors for
a-amino radicals,6f could be easily prepared from the
corresponding chlorides (entry 11). Secondary alkyl
bromides were found to be moderately reactive (entry
12), while tertiary alkyl bromides were unreactive (entry
13). Aryl halides, such as bromo- and iodobenzene, did
not react under similar conditions, whereas 2-pyridyl
bromide reacted to give the corresponding telluride in
moderate yield (entry 14). This is probably because the
strong directing effect of the 2-pyridinyl group
enhances the reactivity.17
Acknowledgements
A synthetic advantage of the current method is the ease
of workup. Because the side products formed are the
volatile trimethylsilyl halides, the essentially pure prod-
ucts could be easily obtained after removal of the silyl
halide and the solvent under vacuum. The crude
product could then be used for further synthetic trans-
formations. For example, benzyl phenyl telluride (2),
which was obtained in essentially pure form by the
reaction of benzyl bromide and 1 followed by evacua-
tion, was used directly as a precursor of the benzyl
radical and anion species (Fig. 1). Thus, the crude 2
was heated with 2,6-dimethylphenyl isonitrile (CNXy)
under irradiation to obtain the imidoylated product 4
and 1,2-diphenylethane by way of the benzyl radical 3.6f
Furthermore, treatment of the crude 2 with one equiv.
of n-BuLi selectively cleaved the benzylic CꢀTe bond to
generate the benzyl lithium 5,18 which added to ben-
zaldehyde to give 6 in good yield.
This work is partly supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education,
Culture and Sports.
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