Pomerantz et al.
temperature over 1 h. The reaction mixture was acidified with
1 M HCl and 100 mL of ether was added. The aqueous layer
was extracted with ether (3 × 50 mL) and the combined
organic layer was washed with water, dried (MgSO4), and
concentrated to give the crude product, which was recrystal-
lized from ethanol-water (1:4) to give 2-bromothiophene-3-
carboxylic acid as an off white crystalline solid (2.11 g, 68%).
Mp: 176-8 °C (lit.36,37 mp 178-9 °C). 1H NMR: δ 9.7-10.5
(1H, v broad), 7.25 (1H, d, J ) 5.5 Hz), 7.44 (1H, d, J ) 5.9
Hz). 13C NMR: δ 122.1, 126.2, 129.9, 130.3, 167.2. IR: 702,
735, 831, 891, 999, 1189, 1295, 1432, 1450, 1526, 1683 cm-1
.
Anal. Calcd for C5H3O2SBr: C, 29.00; H, 1.46. Found: C, 29.05,
H, 1.68.
Gen er a l P r oced u r e for Dia zom eth a n e Ester ifica tion
of Th iop h en e Ca r boxylic Acid s. The thiophene carboxylic
acid (3 mmol) was dissolved in 20 mL of ether and cooled to 0
°C, and then 50 mL of a cold diazomethane (5 mmol) solution
in ether38 was added and allowed to warm to room temperature
over 3 h. The solvent was removed under reduced pressure,
leaving the methyl ester.
Meth yl 2-br om oth iop h en e-3-ca r boxyla te (9)22 was pre-
pared from 2-bromothiophene-3-carboxylic acid to yield a pale
1
yellow oil in 100% yield. H NMR: δ 3.87 (3H, s), 7.21 (1H, d,
F IGURE 5. Some resonance forms for compounds 3-5.
J ) 5.8 Hz), 7.34 (1H, d, J ) 5.8 Hz).
Meth yl 2-br om oth iop h en e-4-ca r boxyla te (11)22 was pre-
pared from 2-bromothiophene-4-carboxylic acid39,40 in 100%
TABLE 5. UV Sp ectr a of Com p ou n d s 3-5 (THF
solu tion s)
1
yield. Mp: 43-4 °C (lit.22 mp 43 °C). H NMR: δ 3.85(3H, s),
compd
λmax (nm) (ꢀ)
7.46 (1H, d, J ) 1.5 Hz), 7.98 (1H, d, J ) 1.5 Hz).
Meth yl th iop h en e-3-ca r boxyla te22 was prepared from
thiophene-3-carboxylic acid as a colorless oil in 100% yield.
1H NMR: δ 3.86(3H, s), 7.30 (1H, dd, J ) 5.1, 3.2 Hz), 7.52
(1H, dd, J ) 5.1, 1.2 Hz), 8.10 (1H, dd, J ) 3.2, 1.2 Hz).
Gen er a l P r oced u r e for th e Syn th esis of Dim eth yl 2,2′
-Bith iop h en e-4,4′-d ica r boxyla te (3) a n d Dim eth yl 2,2′
-Bith iop h en e-3,3′-d ica r boxyla te (5). The methyl 2-bro-
mothiophenecarboxylate (2 mmol) was dissolved in 0.5 mL of
dry DMF, copper powder (508 mg, 8.0 mg-atom) was added,
and the mixture was heated at 145 °C under an argon
atmosphere for 2 days. The reaction mixture was cooled,
dissolved in 10 mL of CHCl3, filtered, and dried (MgSO4). The
residue was chromatographed on silica gel, eluting with ethyl
acetate:hexane (1:4) to give the bithiophene as a white powder.
Dim et h yl 2,2′-b it h iop h en e-4,4′-d ica r boxyla t e (3) was
prepared from 11 in 51% yield. Mp: 175-6 °C. 1H NMR: δ
3.88 (6H, s), 7.58 (2H, d, J ) 1.4 Hz), 8.00 (2H, d, J ) 1.4 Hz).
13C NMR: δ 52.1, 124.7, 132.0, 134.2, 137.0, 162.8. IR (KBr):
3
4
5
233 (25 000)
248 (22 000)
254 (15 000)
304 (7 600)
313 (13 000)
307 (7 000)
and the structure of 4 is in agreement with our previous
suggestion, based on calculations, that there is an
electrostatic attraction between the 3′-carbonyl oxygen
(partial negative charge) and the sulfur of the distal ring
(partial positive charge), resulting in a stabilization of
the planar structure. In dimethyl 2,2′-bithiophene-3,3′-
dicarboxylate (5) the thiophene rings are severely twisted
(S-C-C-S dihedral angle 74.8°) and the stable confor-
mation is with the sulfur atoms syn. This is the first time
a simple bithiophene (or oligothiophene) has been dem-
onstrated to completely adopt the syn conformation.
740, 843, 867, 987, 1091, 1243, 1346, 1443, 1524, 1726 cm-1
UV: λmax 304 (ꢀ 7600), 233 nm (ꢀ 25 000). Anal. Calcd for
12H10O4S2: C, 51.04; H, 3.57; S 22.71. Found: C, 51.11; H,
.
Exp er im en ta l Section
C
Gen er a l. Melting points are uncorrected. NMR spectra
were recorded using CDCl3 solutions at 500.16 MHz with TMS
(δ ) 0.00) as the internal reference for 1H and at 125.78 MHz
with CDCl3 (δ ) 77.00) as the reference for 13C. FT-infrared
spectra were recorded using KBr pellets or as a thin film
between NaCl plates, and UV-vis spectra were taken using
THF solutions. Elemental analyses were done by either
Quantitative Technologies Inc., White House, NJ or using a
Perkin-Elmer 2400 CHN analyzer at the University of Texas
at Arlington.
2-Br om oth iop h en e-3-ca r boxylic Acid . To a solution of
diisopropylamine (3.03 g, 30 mmol) in 40 mL of dry tetrahy-
drofuran at -78 °C was added n-butyllithium (19.6 mL of a
1.43 M solution in hexane, 30 mmol) under an argon atmo-
sphere. After stirring for 25 min, a solution of thiophene-3-
carboxylic acid (1.92 g, 15 mmol) in 10 mL of dry tetrahydro-
furan was added slowly during a 10 min period, the mixture
was allowed to stand at -78 °C for an additional 20 min and
it was then treated with a solution of CBr4 (4.98 g,15 mmol)
in 10 mL of dry tetrahydrofuran. The cooling bath was
removed and the solution was allowed to warm to room
3.40; S, 23.01. The sample for single-crystal X-ray crystal-
lography was prepared by recrystallization from ethyl acetate:
hexane (1:9).
Dim et h yl 2,2′-b it h iop h en e-3,3′-d ica r boxyla t e (5) was
prepared from 9 in 55% yield. Mp: 148-9 °C. 1H NMR: δ 3.69
(6H, s), 7.34 (2H, d, J ) 5.4 Hz), 7.52 (2H, d, J ) 5.4 Hz). 13C
NMR: δ 51.8, 125.9, 129.6, 131.3, 140.0, 163.2. IR (KBr): 744,
1002, 1156, 1247, 1286, 1433, 1722 cm-1. UV: λmax 307 (ꢀ 7000),
254 nm (ꢀ 15 000). Anal. Calcd for C12H10O4S2: C, 51.04; H,
3.57; S 22.71. Found: C, 51.32; H, 3.78; S, 22.61. The sample
for single-crystal X-ray crystallography was prepared by
recrystallization from ethyl acetate:hexane (1:9).
Dim eth yl 2,2′-Bith iop h en e-3,4′-d ica r boxyla te (4). Meth-
yl 2-(tri-n-butylstannyl)thiophene-3-carboxylate (10) was first
(36) Campaigne, E.; LeSuer, W. M. J . Am. Chem. Soc. 1949, 71, 333.
(37) Gronowitz, S.; Pettersson, K. J . Heterocycl. Chem. 1976, 13,
1099.
(38) Arndt, F. In Organic Syntheses; Wiley: New York, 1943; Collect.
Vol. II, p 165.
(39) Campaigne, E.; Bourgeois, R. C. J . Am. Chem. Soc. 1954, 76,
2445.
(35) J affe´, H. H.; Orchin, M. Theory and Applications of Ultraviolet
Spectroscopy; Wiley: New York, 1962; Chapter 17.
(40) Cheng, Y. Ph.D. Dissertation, The University of Texas at
Arlington, Arlington, TX, 1998.
6936 J . Org. Chem., Vol. 67, No. 20, 2002