Studies on tellurium-containing heterocycles. Part 10. 2-benzotelluropyrylium salts: First preparation and reactions with Grignard reagents

Article abstract of DOI:10.1039/a804036b

The first simple preparation of 2-benzotelluropyrylium salts 4 from o-ethynylbenzyl bromides 1 in two steps, and the characteristic reaction of these salts 4 with benzylmagnesium bromide and other Grignard reagents are described.

Full text of DOI:10.1039/a804036b

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Studies on tellurium-containing heterocycles. Part 10.1
2-Benzotelluropyrylium salts: first preparation and reactions
with Grignard reagents
Haruki Sashida* and Kazuo Ohyanagi
Faculty of Pharmaceutical Sciences, Hokuriku University, Kanagawa-machi, Kanazawa
920-1181, Japan
The first simple preparation of 2-benzotelluropyrylium
salts 4 from o-ethynylbenzyl bromides 1 in two steps, and
the characteristic reaction of these salts 4 with benzyl-
magnesium bromide and other Grignard reagents are
described.
isotellurochromenes 3 were produced by the 6-endo-dig ring
closure of 2 at the sp carbon atom of the acetylenic moiety.
The 5-exo-dig reaction products were not obtained. The
isochromenes 3 were treated with triphenylcarbenium tetra-
Ϫ
fluoroboranuide (Ph3Cϩ BF4 ) in MeNO2 at room temperature
to give the desired 2-benzotelluropyrylium tetrafluoro-
boranuides 4 (4a: 94% yield, 4b: 89% yield) as stable pale green
prisms. The salts 4 are the first synthetic examples of 2-
benzotelluropyrylium salts. As expected, 4 have high reactivities
and are easily decomposed on contact with water or alcohol.
Thus, in order to examine the chemical behavior of the novel
telluropyrylium salts 4 towards nucleophiles, the reaction of 4
with some Grignard reagents was carried out. Contrary to our
expectations, treatment of the salts 4 with Grignard reagents,
such as methyl-, ethyl- or phenyl-magnesium bromide (iodide)
in THF at 0 ЊC resulted in a self-coupling reaction at the C-1
position to give 1,1Ј-bis(isotellurochromenyl) 5 as the sole char-
acterized product in moderate yield (Scheme 2). However, when
benzylmagnesium bromide was used as the Grignard reagent in
Et2O at 0 ЊC, 1-benzylisotellurochromenes 6 (6a: 48% yield, 6b:
21% yield), normal coupling products, were obtained. In this
case, no dimers 5 were obtained. It is already known that
the reaction of telluraxanthylium salts8 with reducing agents,
such as zinc powder or with nucleophiles, such as ammonium
chloride, affords the telluraxanthyl dimers, produced via a free
radical mechanism. This result clearly supports the present
formation of 5 proceeding via a similar radical mechanism.
In contrast, a similar reaction of their selenium analogs, the
2-benzoselenopyrylium salts, with any Grignard reagent gave
the normal corresponding 1-substituted isoselenochromenes in
good yields.
The chemistry (structure and physical properties, syntheses
and reactions) of not only the pyrylium salts2 but also the
thiopyrylium salts3 has been widely investigated and is now
well established. Recently, extensive synthetic studies on their
selenium4 and tellurium5 analogs have been undertaken,
however the title compounds, the 2-benzotelluropyrylium salts,
have not been prepared until now. We have previously focused
on the synthesis of various tellurium- or selenium-containing
heterocycles6 by an intramolecular cyclization of the tellurol
or selenol moieties to an ethynyl group. In our continuing
studies, we herein describe the successful synthesis of the 2-
benzotelluropyrylium salts and their reactions with Grignard
reagents.
The synthesis of the 2-benzotelluropyrylium salts 4 is shown
in Scheme 1. In order to obtain isotellurochromenes 3, the
C
C
R
C
C
R
i
Br
TeH
1
2
a: R = But
b: R = H
ii
In addition, treatment of 1-benzyl-3-tert-butylisotelluro-
Ϫ
chromene 6a with Ph3Cϩ BF4 gave 1-benzyltelluropyrylium
1
tetrafluoroboranuide 7a as the sole product. The H NMR
R
R
spectrum of the crude isolated telluropyrylium salt 7a in
CD3CN showed two tert-butyl signals at δ 1.59 and 1.29 in a
ratio of 2:3, indicating the presence of the salt 7a and the
benzylidene compound 8a. Treatment of the salt 7a with base
or by alumina column chromatography using methylene
chloride as the eluent quantitatively gave 8a. This finding
suggests that the telluropyrylium salt 7a exists in the solvent as
an equilibrium mixture of 7a and 8a. The stereochemistry of
the olefin moiety in 8a was determined by a nuclear Overhauser
enhancement (NOE) measurement. A NOE was observed
between the benzylidene 1Ј-H and the aromatic 8-H in the 400
iii
+
Te
Te
–
BF4
3
4
Scheme 1 Reagents and conditions: i, NaHTe (1.2 equiv.), DMF, 0 ЊC,
1 h; ii, EtOH, 90 ЊC, 1–3 h; iii, Ph3Cϩ BF4 (1 equiv.), MeNO2, room
Ϫ
temp., 30 min
precursors for the preparation of the telluropyrylium salts
4, the conversion from o-ethynylbenzyl bromides 1 to the
telluroles 2 was examined. Treatment of the benzyl bromides 1
with sodium hydrogen telluride7 in DMF at 0 ЊC, followed by
addition of EtOH, and then heating at 90 ЊC, resulted in a
direct ring closure to afford isotellurochromenes 3 (3a: 84%
yield, 3b: 64% yield) via the benzyltellurol intermediates 2.† The
1
MHz H NMR spectrum of 8a. Thus, the olefin moiety was
determined to have (Z)-stereochemistry. In the case of a similar
treatment of the 3-unsubstituted isotellurochromenes 6b with
Ϫ
Ph3Cϩ BF4 , the formation of salt 7b and the benzylidene
1
compound 8b was confirmed by the H NMR spectrum in
CD3NO2. However, both 7b and 8b were too unstable to isolate.
The proposed mechanism for the formation of 8 is shown in
Scheme 2.
† The formation of compounds 2 was characterized by the isolation of
bis(o-ethynylbenzyl) ditellurides, which were obtained by potassium
ferricyanide oxidation of 2 before heating in ethanol.
J. Chem. Soc., Perkin Trans. 1, 1998
2123

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Ϫ
νmax(KBr)/cmϪ1 1054 (BF4 ); δH(CD3CN, 400 MHz) 1.68 (9H, s,
But), Ph-H [8.00 (1H, ddd, J 8.8, 7.2, 1.5), 8.34 (1H, br d, J 8.4),
8.47 (1H, ddd, J 8.4, 7.2, 1.5), 8.49 (1H, br d, J 8.8)], 9.14 (1H, s,
4-H), 13.22 (1H, s, 1-H); δC(CD3CN, 100 MHz) 33.34 (q),
43.80 (s), 131.42 (d), 132.07 (d), 135.40 (d), 137.84 (d),
139.05 (d), 139.85 (s), 143.69 (s), 182.85 (s), 188.83 (s)
(Anal. Calc. for C13H15BF4Te: C, 40.49; H, 3.93. Found: C,
40.35; H, 3.75%).
R
R
R
+
Te
Te
i
Te
BF4
–
4
5
ii
1-Benzylisotellurochromene 6a
A solution of PhCH2MgBr (4 mmol) in Et2O (4 ml) was added
to a suspended mixture of telluropyrylium salt 4a (3 mmol) in
Et2O (10 ml) and the mixture was stirred at room temperature
for 30 min. The reaction mixture was quenched by the addition
of aqueous NH4Cl, and extracted with Et2O. The ethereal
fractions were washed with brine, dried (MgSO4) and evapor-
ated. The residue was chromatographed on silica gel using
hexane–CH2Cl2 (20:1) as eluent to give 6a, yellow oil;
δH(CDCl3, 90 MHz) 1.31 (9H, s, But), 3.21 (2H, d, J 7.1,
CH2Ph), 3.96 (1H, br t, J 7.1, 1-H), 6.78 (1H, s, 4-H), 6.86–7.26
(9H, m, Ar-H); HRMS m/z Calc. for C20H22Te: 392.0785.
Found: C, 392.0781.
R
R
iii
+
Te
Te
CH2Ph
–
BF4
CH2Ph
6
7
5
4
3
R
+
R
6
7
Te
Ph
Te
2
Selected data for the telluropyrylium salt 7a. Yield 71%, pale
1
8
green prisms, mp 105–108 ЊC (decomp.); νmax(KBr)/cmϪ1 1052
H
1'
H
Ph
Ϫ
(BF4 ); δH(CD3CN, 90 MHz) 1.55 (9H, s, But), 4.82 (2H, s,
H
–
BF4
CH2Ph), 7.22–7.59 (9H, m, Ph-H), 8.92 (1H, s, 4-H) (Anal.
Calc. for C20H21BF4Te: C, 50.49; H, 4.45. Found: C, 50.64; H,
4.47%).
Selected data for the benzylidene compound 8a. Yellow oil;
δH(CDCl3, 400 MHz) 1.24 (9H, s, But), 6.86 (1H, s, 4-H), 7.18–
7.45 (8H, m, Ph-H), 7.48 (1H, s, 1Ј-H), 7.53 (1H, br d, J 7.8,
8-H); HRMS m/z Calc. for C20H20Te: 390.0628. Found:
390.0627.
8
9
a: R = But
b: R = H
Scheme 2 Reagents and conditions: i, EtMgBr (MeMgI, PhMgBr),
THF, 0 ЊC, 30 min; ii, PhCH2MgBr, Et2O, 0 ЊC, 30 min; iii, Ph3Cϩ BF4
Ϫ
(1 equiv.), MeNO2, room temp., 2 h
References
For part 9, see H. Sashida, Heterocycles, 1998, 48, 631.
Experimental
1
2
Melting points were measured on a Yanagimoto micro melting
point hot stage apparatus and are uncorrected. IR spectra were
determined with a Hitachi 270-30 spectrometer. Mass spectra
(MS) and HRMS were recorded on a JEOL JMS-DX300
instrument. NMR spectra were determined with a JEOL
EX-90A (90 MHz) or JEOL JNM-GSX 400 (400 MHz)
spectrometer in CDCl3 or CD3CN using tetramethylsilane as
internal standard and J values are given in Hz.
(a) A. T. Balaban, A. Dinculescu, G. N. Dorofeenko, G. W. Fischer,
A. V. Koblik, V. V. Mezhritskii and W. Schroth, Advances in
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G. Doddi and G. Ercolani, Advances in Heterocyclic Chemistry:
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Bull. Soc. Roy. Sci. Liege, 1973, 42, 146.
(a) I. D. Sadekov and V. Minkin, Advances in Heterocyclic
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vol. 63, ed. A. R. Katritzky, Academic Press, Inc., London, 1995,
pp. 1–60; (b) I. D. Sadekov, A. A. Ladatko, E. I. Sadekova, G. N.
Dorofeenko and V. I. Minkin, Khim. Geterotsikl. Soedin., 1981, 343
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3
4
Isotellurochromene 3a
A solution of o-ethynylbenzyl bromide 1 (2.51 g, 10 mmol) in
DMF (10 ml) was slowly added to a solution of sodium
hydrogen telluride (12 mmol), which was freshly prepared from
tellurium dust (1.53 g) and sodium borohydride (0.54 g) in
DMF (40 ml) at 0 ЊC under an argon atmosphere. The reaction
mixture was stirred under the conditions for 1 h. Ethanol (40
ml) was added to the reaction mixture, and then the whole
mixture was heated at 90 ЊC with stirring for 1–3 h. After
addition of water, the aqueous mixture was extracted with ben-
zene. The organic extracts were washed with water and brine,
dried (MgSO4) and evaporated. The resulting residue was
chromatographed on silica gel using n-hexane as eluent to give
pure 3a, yellow prisms, mp 63 ЊC (from n-hexane); δH(CDCl3,
90 MHz) 1.29 (9H, s, But), 3.79 (2H, s, 1-H2), 6.64 (1H, s, 4-H),
7.10–7.26 (4H, m, Ar-H) (HRMS m/z Calc. for C13H16Te:
302.0315. Found: 302.0313).
5
6
7
8
J. Liu, M. Qui and X. Zhou, Synth. Commun., 1990, 20, 2759.
A. A. Bumber, E. P. Ivakhnenko, A. A. Ladatko, A. A.
Arutyunyants, I. D. Sadekov and O. Yu. Okhlobistin, Khim.
Geterotsikl. Soedin., 1988, 1196 (Chem. Abstr., 1989, 111, 38 699h).
2-Benzotelluropyrylium salt 4a
Ph3Cϩ BF4Ϫ (1.88 g, 5.5 mmol) was added to a stirred solution
of the isochromene 3a (1.51 g, 5 mmol) in dry MeNO2 (10 ml)
and the mixture was stirred at room temperature for 30 min. To
the reaction mixture was added dry Et2O to precipitate the tel-
luropyrylium salt 4a, pale green prisms, mp 101 ЊC (decomp.);
Paper 8/04036B
Received 29th May 1998
Accepted 29th May 1998
2124
J. Chem. Soc., Perkin Trans. 1, 1998