PETROV et al.
1728
tion of potassium 2-(2-hydroxyaryl)ethynethiolate IIa–
IIc. Further an intramolecular proton transfer furnished
thioketene derivatives IIIa–IIIc. As a result of the
intramolecular cyclization involving the hydroxy group and
the thioketene fragment after the subsequent acidifying
of anions IVa–IVc we obtained benzo[b]furan-2-thiols
Va–Vc.
analogous to the fragmentation of ions of 2-alkylsulfanyl-
benzo[b]furans [8], and it confirmed the structure of
benzo[b]furan-2-thiols Va–Vc.
Benzo[b]furan-2-thiol (Va). To 0.3 g (1.68 mmol)
of azole Ia in 10 ml of freshly distilled DMF was added
in one portion 0.46 g (3.37 mmol) of freshly calcined
potassium carbonate. The mixture was boiled for 1 h at
stirring (TLC monitoring by the disappearance of the initial
compound and by the end of nitrogen liberation). The
reaction mixture was cooled, the light-brown dispersion
obtained was poured into 80 ml of cold water, and concn,
HCl was added till acid reaction. The separated red-yellow
flaky precipitate was filtered off, washed with cold water,
dissolved in 50 ml of water containing 2 equiv of KOH,
and the solution was stirred with activated carbon at 30°C
for several min, and then it was filtered. To the solution
obtained concn. HCl was again added till acid reaction.
The separated yellow flaky precipitate was thoroughly
washed with cold water and dried in a vacuum. Yield
0.17 g (68%). Light-yellow crystals, mp 89–91°C (mp
95–96°C [4]), Rf 0.17 (ethyl acetate–hexane, 1 : 2).
1H NMR spectrum (CDCl3), ppm: 3.70 s (SH), 4.23 s
(CH2), 6.79 s (H3, Ht), 7.18, 7.46 (Harom). 13C NMR
spectrum (CDCl3), ppm: 48.06 (CH2), 110.11, 110.40,
110.83, 119.80, 122.56, 123.96, 124.00, 124.53, 125.18,
128.25, 128.41, 142.79(C8, thiol); 155.79(C2, thiol),
158.21(C8, thione) 216.31(C=S). Mass spectrum, m/z (Irel,
%): 150 (100) [M]+, 121 (50) [M – O – CH]+, 90 (21),
77 (19), 63 (18), 51 (23), 39 (32). Found, %: C 63.67,
63.92; H 4.23, 4.34. C8H6OS. Calculated, %: C 63.97;
H 4.03. M 150.20.
The structure of compounds Va–Vc was proved by
1H, 13C NMR, and mass spectra and by comparison of
constants for the compounds already described in the
literature.
1H NMR spectra of thiols Va–Vc registered in CDCl3
at room temperature showed that under these conditions
compounds Va–Vc exist as mixtures of tautomers. The
proton at the sulfur atom in the enethiol tautomer appeared
as a singlet in the region 3.69–4.02 ppm and had the same
intensity as the signal of the H3 proton of the benzofuran
ring that was observed as a singlet in the region 6.70–
6.85 ppm.At the same time two protons of the CH2 group
of the benzofuran ring corresponding to the thione form
gave rise to a singlet in the region 4.19–4.40 ppm. The
proton signals of the benzene rings of benzo[b]furan-2-
thiols Va–Vc appeared as a complex multiplet in the
region 7.0–8.0 ppm. The signals from the methyl group
protons of benzo[b]furan-2-thiols Va–Vc give rise to
two singlets in the region 2.2–2.5 ppm. Thus with a certain
degree of inaccuracy it is possible from the intensity of
these signals to estimate the ratio of the enethiol and thione
tautomers of benzofurans Va–Vc.
The ratio of the enethiol and thione forms for
benzo[b]furan-2-thiol (Va) was 1.04:1; for 6-methyl-
benzo[b]furan-2-thiol (Vb), 0.68:1; for 5-methyl-
benzo[b]furan-2-thiol (Vc), 0.61:1. In [2] for benzo-
[b]furan-2-thiol (Va) in benzene was found the ratio of
enethiol and thione forms equal 1:1.
In 13C NMR spectra of benzo[b]furan-2-thiols Va and
Vc the 13C signals of carbon atoms corresponding to
enethiol and thione tautomers are well resolved. The
methylene group signal of the thione tautomer δC(CH2)
appeared in the region 48.6–48.13 ppm, and the signal of
the thiocarbonyl group δC(C=S), at 215.8–216.73 ppm.
The characteristic 13C signals of the enethiol form were
observed in the δC(C3) 110.7–111.3 and δC(C2) 150.5–
155.79 ppm.
6-Methylbenzo[b]furan-2-thiol (Vb) was obtained
similarly. Yield after reprecipitation from water 32%.
Light-yellow crystals, mp 60–64°C, Rf 0.3 (ethyl acetate–
1
hexane, 1:2). H NMR spectrum (CDCl3), ppm: 2.38 s
(CH3, thione), 2.44 C (CH3, thiol), 3.66 s (SH), 4.18 s
(CH2), 6.74 s (H3, Ht), 6.98, 7.33 m (Harom). Mass
spectrum, m/z (Irel, %): 164 (54) [M]+, 163 (7) [M – H]+,
135 (29) [M – O – CH]+, 131 (13) [M – H – S]+, 91 (45),
77 (49), 63 (32), 51 (65), 39 (100). Found, %: C 65.93,
66.12; H 4.76, 4.99. C9H8OS. Calculated, %: C 65.82;
H 4.91. M 164.22.
5-Methylbenzo[b]furan-2-thiol (Vc) was obtained
similarly. Yield after reprecipitation from water 61%, after
recrystallization from methanol 55%. Light-yellow needle
crystals, mp 83–85°C, Rf 0.63 (ethyl acetate–hexane,
In the mass spectra of benzo[b]furan-2-thiols Va–Vc
peaks of the molecular ions were observed whose isotope
composition was consistent with the calculated one.
Further fragmentation of the molecular ions was
1
1:2). H NMR spectrum (CDCl3), ppm: 2.33 s (CH3,
thione), 2.40 s (CH3, thiol), 3.68 s (SH), 4.16 s (CH2),
6.69 s (H3, Ht), 7.06, 7.24 (Harom). 13C NMR spectrum
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 45 No. 11 2009