Regiospecific silylation of 2,5-dibromothiophene: A reinvestigation

Article abstract of DOI:10.1016/S0040-4039(01)00068-5

Lithiation of 2,5-dibromothiophene by LDA with ensuing silylation proceeds regiospecifically in accordance with the halogen dance mechanism to yield 3,5-dibromo-2-trimethylsilylthiophene or 3,4-dibromo-2,5-bis(trimethylsilyl)thiophene depending on the ratio of reagents. The outcome of the reaction was confirmed by further chemical transformations of the latter compound to 2,5-bis(trimethylsilyl)thiophene and 2,5-bis(trimethylsilyl)thiophene-1,1-dioxide. The structures of the compounds obtained were determined by ¹H, ¹³C and ²⁹Si NMR, and X-ray analysis in the case of a sulfone.

Full text of DOI:10.1016/S0040-4039(01)00068-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 2039–2041
Regiospecific silylation of 2,5-dibromothiophene: a reinvestigation
Edmunds Lukevics,* Pavel Arsenyan, Sergey Belyakov, Juris Popelis and Olga Pudova
Latvian Institute of Organic Synthesis, Aizkraukles 21, Riga LV-1006, Latvia
Received 29 November 2000; revised 18 December 2000; accepted 10 January 2001
Abstract—Lithiation of 2,5-dibromothiophene by LDA with ensuing silylation proceeds regiospecifically in accordance with the
halogen dance mechanism to yield 3,5-dibromo-2-trimethylsilylthiophene or 3,4-dibromo-2,5-bis(trimethylsilyl)thiophene depend-
ing on the ratio of reagents. The outcome of the reaction was confirmed by further chemical transformations of the latter
compound to 2,5-bis(trimethylsilyl)thiophene and 2,5-bis(trimethylsilyl)thiophene-1,1-dioxide. The structures of the compounds
1
obtained were determined by H, 13C and 29Si NMR, and X-ray analysis in the case of a sulfone. © 2001 Elsevier Science Ltd.
All rights reserved.
The interaction of aromatic and heteroaromatic (furan,
thiophene, pyrazole, imidazole, isothiazole, pyridine)
bromides with bases such as lithium, sodium or potas-
sium amides under appropriate conditions can lead to
the formation of rearranged products. These reactions
were considered in the literature as halogen migration,
halogen scrambling, halogen dance or base-catalyzed
halogen dance reactions.1 Some examples of regioselec-
tive halogen dance rearrangements for different thio-
phene derivatives have been published.2 The structure
of lithiothiophene, formed in the reaction of 2,5-dibro-
mothiophene with lithium diisopropylamide (LDA) in
THF or ether at −70°C, was found to be 3,5-dibromo-
2-lithiothiophene as a result of base-catalyzed halogen
dance mechanism. The following treatment of this lithio
derivative with methyl iodide, ethyl iodide, allyl bro-
mide, N,N-dimethylformamide, chloroformates, vari-
ous oxiranes, cyclohexanone or dimethyl disulfide was
used to prepare the corresponding 2-substituted 3,5-
dibromothiophenes.2a However, according to the litera-
ture data,3 quenching of the lithio derivative obtained
from 2,5-dibromothiophene with trimethylchlorosilane
gave 2,5-dibromo-3-trimethylsilylthiophene 1. Later
Fro¨hlich2a showed that 3,5-dibromo-2-trimethylsilylth-
iophene 2 prepared from 2,3-dibromothiophene (LDA,
−78°C, THF, Me3SiCl) has the same spectroscopic
characteristics as the compound wrongly ascribed as
thiophene 1,3 indirectly confirming the halogen dance
mechanism in the case of 2,5-dibromothiophene
silylation.
We attempted to prepare thiophene 1 by this method
1
(LDA, −78°C, THF, Me3SiCl). The results of H and
13C NMR spectroscopy did not allow us to determine
the structure of the reaction product definitely; how-
ever, 29Si NMR spectroscopic data for this compound
(l=−4.68 ppm) clearly confirmed the formation of the
isomeric 3,5-dibromo-2-trimethylsilylthiophene 24 bear-
ing a silyl group at position 2 of the heterocycle, since
the signals (l 29Si) for the trimethylsilyl group in posi-
tion 3 of the thiophene ring are shifted to a higher field
(−7 to −8 ppm). The lithiation of compound 2 by LDA
in THF at −78°C also proceeds with a halogen dance
rearrangement to yield symmetrical 3,4-dibromo-2,5-
bis(trimethylsilyl)thiophene 3,5 due to the fact that the
29Si chemical shift of this derivative is −4.23 ppm.
Compound 3 was prepared using direct metallation of
2,5-dibromothiophene with two equivalents of LDA
(45% yield), as well as by lithiation of tetra-
bromothiophene with two equivalents of BuLi (40%
yield) and subsequent silylation (Scheme 1).
Additional confirmation of the position of the silyl
group in thiophene 3 was obtained by its debromina-
tion (BuLi, −78°C, THF; H2O) to 2,5-bis(trimethylsi-
lyl)thiophene 4.6 Oxidation of silyl derivative 4 by
m-chloroperbenzoic acid in methylene chloride at room
temperature gave the corresponding sulfone 5.6
The structure of thiophene-1,1-dioxide 5 was deter-
mined by X-ray crystallography7 (Fig. 1). In the crystal
the molecules of sulfone 5 lie in special positions. Both
crystallographically independent molecules in the crys-
tals lie in mutually perpendicular mirror planes m.
Molecules A pack perpendicularly to the y crystallo-
graphic axis while molecules B are perpendicular to the
Keywords: bromine dance reaction; lithiation; thiophene; X-ray crys-
tal structure.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00068-5

2040
E. Luke6ics et al. / Tetrahedron Letters 42 (2001) 2039–2041
Scheme 1. (a) (i) LDA, THF, −78°C; (ii) Me3SiCl. (b) (i) 2LDA, THF, −78°C; (ii) 2Me3SiCl. (c) (i) 2BuLi, THF, −78°C; (ii)
2Me3SiCl. (d) (i) 2BuLi, THF, −78°C; (ii) H2O. (e) m-CPBA, CH2Cl2, 20°C.
differs from 2-tert-butyl-5-trimethylsilylthiophene-1,1-
dioxide.
References
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Figure 1. Molecular structure of 2,5-bis(trimethylsilyl)thio-
,
phene-1,1-dioxide 5. Selected bond distances (A) and angles
(°): SꢀC(2), 1.775(5), 1.7648(5); C(2)ꢀC(3), 1.2691(8), 1.299(7);
C(3)ꢀC(4), 1.4622(12); SiꢀC(2), 1.8862(6), 1.7648(5); C(2)ꢀSꢀ
C(2%), 95.84(3).
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2995; (b) Fro¨hlich, J.; Hametner, C.; Kalt, W. Monatsh.
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4. 3,5-Dibromo-2-trimethylsilylthiophene (2): 70% yield; MS,
m/e 312 (M ). 1H NMR (200 MHz, CDCl3/TMS) l
+
’
(ppm): 0.36 (s, Me3Si, 9H), 7.01 (s, H4, 1H); 13C NMR
(50.31 MHz, CDCl3/TMS) l (ppm): −0.9, 116.5, 119.5,
134.4, 137.6; 29Si NMR (39.74 MHz, CDCl3/TMS) l
(ppm): −4.68 (Me3Si). Anal. calcd for C7H10Br2SSi: C,
26.77; H, 3.21; S, 10.21. Found: C, 26.65; H, 3.24; S,
10.30.
Figure 2. Molecular packing of sulfone 5.
5. 3,4-Dibromo-2,5-bis(trimethylsilyl)thiophene (3): 45%
yield (from 2,5-dibromothiophene); 62% yield (from 2);
z axis (Fig. 2). The five-membered heterocycle is planar.
The oxygen atoms of the SO2 unit lie roughly the same
distance above and below the five-membered ring. The
main geometric parameters of the studied sulfone 5 are
in good agreement with those previously found for
2,5-disubstituted thiophene-1,1-dioxides structures.6a,c,8
Silylsulfone 5 is similar to 2-trimethylsilyl-5-trimethyl-
germyl- and 2,5-bis(trimethylgermyl)thiophene-1,1-
dioxides in terms of molecular packing; however, it
+
1
’
MS, m/e 386 (M ). H NMR (200 MHz, CDCl3/TMS) l
(ppm): 0.38 (s, Me3Si); 13C NMR (50.31 MHz, CDCl3/
TMS) l (ppm): −1.2, 122.2, 140.6; 29Si NMR (39.74 MHz,
CDCl3/TMS) l (ppm): −4.23 (Me3Si). Anal. calcd for
C10H18Br2SSi2: C, 31.09; H, 4.69; S, 8.30. Found: C, 31.15;
H, 4.74; S, 8.33.
6. (a) Furukawa, N.; Hoshiai, H.; Shibutani, T.; Higaki, M.;
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diffractometer, Mo-radiation (u=0.71073 A); absorption
coefficient 0.346 mm−1; 2qmax=50.0°; index ranges 0<h<
18, 0<k<18 and 0<l<7; number of independent reflections
1443; full-matrix least-squares on F2; goodness-of-fit
0.901; extinction coefficient 0.018(2); final R-factor 0.0515.
Atomic coordinates and components of temperature factor
tensors are deposited at the Cambridge Crystallographic
Data Centre (the CCDC deposition number is 153353).
8. Vorontsova, L. G. Zh. Strukt. Khim. 1966, 7, 240.
7. X-ray data: empirical formula C10H20O2SSi2; molecular
weight 260.50; crystal system, orthorhombic; unit cell
,
dimensions a=15.363(3), b=15.577(3) and c=6.491(1) A;
3
,
V=1553.4(5) A ; Z=4; space group Pmmn; Dcalcd=1.114
g/cm3; F(000)=560; data collection on a ‘Syntex P21’
.
.