o-directed lithiation of acylated hydroxythiophenes

Article abstract of DOI:10.3987/COM-13-S(S)91

O-Carbamates derived from 2-hydroxy- and 3-hydroxy-thiophenes generated o-directed carbanions, which were transformed to the corresponding methyl sulfides and methyl and trimethylsilyl substituted thiophenes.

Full text of DOI:10.3987/COM-13-S(S)91

HETEROCYCLES, Vol. 88, No. 2, 2014
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HETEROCYCLES, Vol. 88, No. 2, 2014, pp. 1565 - 1572. © 2014 The Japan Institute of Heterocyclic Chemistry
Received, 12th July, 2013, Accepted, 26th August, 2013, Published online, 9th September, 2013
DOI: 10.3987/COM-13-S(S)91
o-DIRECTED LITHIATION OF ACYLATED HYDROXYTHIOPHENES
Ineta Vendina, Anete Parkova, and Peteris Trapencieris*
Department of Organic Chemistry, Latvian Institute of Organic Synthesis,
Aizkraukles iela 21, Riga, LV-1006, Latvia; peteris@osi.lv
Abstract – O-Carbamates derived from 2-hydroxy- and 3-hydroxy-thiophenes
generated o-directed carbanions, which were transformed to the corresponding
methyl sulfides and methyl and trimethylsilyl substituted thiophenes.
Dedicated to Professor Viktoras Sniečkus, an eminent organic chemist and
th
initiator of Balticum Organicum Syntheticum (BOS), in occasion of his 77
birthday
Directed ortho-metalation (DoM) is one of the most powerful strategies for the synthesis of
1
2
polysubstituted aromatics. Victor Snieckus was one of the pioneers for employing DoM and its more
3
powerful analog – directed remote metalation (DreM) as the tools in the hands of organic chemists. For
4
the synthesis of furocoumarins, both methods are employed together.
In general, the DoM comprises the deprotonation of a site ortho- to a heteroatom-containing directing
metalation group (DMG) by a strong base, an alkyllithium reagent, leading to the ortho-lithiated species
1
5
6
A. Among the most synthetically useful DMGs are tertiary amides and O-carbamates.
DMG
Li
A
The main synthetic potential of DoM reaction is shown on substituted arene O-carbamates (benzenes and
naphthalenes). Only limited publications are shown for functionalisation of nitrogen heterocycles:
7
pyridines. Deprotonation of unsubstituted thiophenes is going preferentially in 2-position and is
8
reviewed, but functionalized and substituted thiophenes had a lack of information about the directions of
lithiation reaction.

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HETEROCYCLES, Vol. 88, No. 2, 2014
At the same time, some alternatives for functionalisation of thiophenes are known: new C-N bond
9
formation in Buchwald-Hartwig coupling by employing anilines with electron-donating and
1
0
electron-accepting substituents and β-selective C-H bond arylation of thiophenes with iodoarenes. To
the best of our knowledge, there are no systematic deprotonation studies of hydroxythiophenes.
Substituted hydroxythiophenes are good analogs of aromatic compounds as starting materials for new
anticancer drug synthesis. Our synthetic aims are connected with functionalized thiophenes as starting
materials, keeping the oxygen functionality in thiophene ring, for the synthesis of new heterocyclic
systems. Therefore we have used the corresponding O-carbamates 3 and 10 prepared from
2
-hydroxythiophene 2 (Scheme 1) and 3-hydroxythiophene 9 (Scheme 2) as starting materials for directed
ortho-lithiation reactions. Better yield (77%) of 2-O-carbamate 3 was obtained starting from isolated
-hydroxythiophene 2.
2
R
R
O
O
+
O+
5a,b
+
OH
O
O R
4a,b
O
O
S
S
S
S
S
S
S
NEt2
NEt2
1
2
3
7a,b
6
a; R=Me
b; R=SMe
Scheme 1
1
1
Acetylation of thiophenes 2 and 9 turns to be sluggish because tautomerism of hydroxythiophenes. At
the same time there are obvious differences in the tautomerisation process. Thus, ketonization of
11
2
-hydroxybenzothiophenes is 40 times faster than of 3-hydroxybenzothiophenes. Recently the first
systematic study of the properties (general stability, tautomerism and NMR spectroscopic parameters) of
1
2
3
2
3
-hydroxythiophene 9 was published. 2-Hydroxythiophene in CDCl
3
solution exists in keto form
-Keto completely, and therefore acylation reaction was performed in DMF. At the same time,
1
2
-hydroxythiophene in CDCl solution exists in 2-Keto and 2-Enol form equilibria 74:26.
3
OH
-Enol
O
S
2
S
2-Keto
Reaction conditions for lithiation of aryl O-carbamates are very important, because higher temperatures
1
3
are used for the Fries rearrangement of aryl O-carbamates. Therefore we have tried to use for thiophene

HETEROCYCLES, Vol. 88, No. 2, 2014
1567
derivatives standard lithiation conditions applied for o-directed metallation of aromatic compounds - with
o
s-BuLi at -78 C and checked also other lithiation procedures. At the beginning we have deprotonated
carbamate 3 with freshly prepared LDA. In all the experiments at -78 °C and at -4 °C, after quenching
with MeI as an electrophile only starting materials with some impurities were isolated. Further
deprotonation of carbamate 3 with 1.5 eq. of n-BuLi in the presence of 1.5 eq. of TMEDA at -8 °C and
-
30 °C gave a mixture of products, however no more starting material remained. At -78 °C reaction is
going more under control and only two products were obtained – deacylation product 6 in the keto- form
1
and 3-methylthiophene 7a in ratio 40:60 detected after H NMR. These results lead us to a conclusion,
that even at -78 °C deacylation happens and product 6 is out of expected DoM reaction.
Reaction with s-BuLi and TMEDA did not support DoM reaction and only 5-deprotonation as well as
deacylation of O-carbamate occurred. Thus, obtained compound 5b exists in a keto form with a
2
characteristic chemical shifts for 3-CH protons: 4.06 as a doublet with J = 3.2 Hz. No ortho-directed
lithiation products were observed in the case of methylthiolation agent dimethyl disulfide used as an
electrofile in the reaction.
Thus, all deprotonation agents (LDA, n-BuLi, s-BuLi) used in our experiments for substrate 3 partly
cleaved O-carbamate functionality, and only low yields of o-directed lithiation products were obtained.
DoM was observed only with n-BuLi as a base, but s-BuLi deprotonated only 5-position of thiophene
ring.
O
O
B(OH)2
OH
O
OH
O
O
NEt2
NEt2
+
+
S
R
S
S
S
S
S
S
8
9
10
11a,b
13a-c
12
a; R=Me
b; R=SMe
c; R=SiMe3
Scheme 2
1
4
Simple 3-hydroxythiophenes are often very sensitive and air-unstable and exist in two tautomeric forms.
1
2
The ratio of keto- and enol-forms depend on the solvent used in the experimental conditions, and simple
-hydroxythiophenes can react in 2-position. Therefore we have used thiophene O-carbamates 3 and 10 as
3
starting materials for two reasons – to investigate ortho-directing power of O-dimethylcarbamate
lithiation directing group and to avoid hydroxythiophene tautomerism. After analysis of results for
substrate 3, we turned to use n-BuLi + TMEDA for the 3-hydroxy analog 10 deprotonation studies. First
experiments showed that carbamate 10 did not deprotonate with n-BuLi at -78 °C, therefore we

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HETEROCYCLES, Vol. 88, No. 2, 2014
performed reactions at higher temperatures. Thus, reaction of carbamate 10 with 1.1 eq. of n-BuLi and
1.1.eq. of TMEDA at -30 °C and quenching of obtained carbanion with MeI, two products were obtained
–
expected 2-methylthiophene 13a (68%) and deacylated ketone 12 (17%). Quenching of obtained
o-lithiation directed carbanion with dimethyldisulfide gave 2 products – 2-methylthiothiophenes 11b
33%) and 13b (37%). Only TMS-Cl as an electrophile gave 2-TMS thiophene 13c as a sole product in
(
3
8% yield. Increase of the reaction temperature to 0 °C, did not improve the yields of target thiophenes
1
3a-c. Moreover, reaction mixtures became more complicated, because of the side reactions - Fries
rearrangement and O-carbamate deacylation.
Thus, O-carbamates derived from 2-hydroxy- and 3-hydroxythiophenes in contrary to the corresponding
1
benzene derivatives generated o-directed carbanions only with n-BuLi and TMEDA at -78 °C and -30 °C,
correspondingly. These generated carbanions were transformed to the corresponding methyl sulfides,
methyl and trimethylsilyl substituted thiophenes in moderate yields.
EXPERIMENTAL
1
The H NMR spectra were recorded on a Varian Mercury plus 400 (400 MHz) spectrometer, internal
1
3
standard was TMS (solvent CDCl
400 MHz) spectrometer, internal standard was CHCl
Alufolien plates of Kieselgel 60. Column chromatography was carried out on Kieselgel (Acros), 0.023 –
.070 mm, pore diameter ca 6 nm. Dichloromethane was distilled from CaH . Tetrahydrofuran (THF) and
diethyl ether (Et O) were distilled from Na/benzophenone. Thiophene and thiophen-3-ylboronic acid
were purchased from Acros. 2-Hydroxythiophene 1 was prepared after the patent procedure.
3
). The C NMR spectra were recorded on a Varian Mercury plus 400
(
3
(δ 77.0 ppm). TLC was carried out on DC
0
2
2
1
5
2
-Diethylcarbamoyloxythiophene (3)
To an ethereal solution (6 mL) of thiophene (0.96 mL, 11.9 mmol) was added n-BuLi (1.8M solution in
hexanes, 6.6 mL, 11.9 mmol) at room temperature and stirred for 40 min. Then reaction mixture was
o
cooled to -78 C and triethylborate (2.92 mL, 16.6 mmol) was added and stirred at -78 °C for 2 h. The
cold bath was removed and 30% aqueous H O (3.0 mL) was added dropwise at -50 °C and reaction
2
2
mixture was allowed reflux. After addition the solution was allowed to reflux for additional 30 min.
Reaction mixture was acidified with 6N HCl to pH=1. The resulted mixture was extracted with Et
3x10 mL) and combined ethereal solution was washed with 10% aqueous ferrous ammonium sulfate
solution (10 mL), washed with water (10 mL), and dried over Na SO . Et O was removed by heating to
2 °C (with no vaccum). Thiophen-2-ol as an orange ethereal solution was used immediately in the next
2
O
(
2
4
2
4
reaction. To the obtained thiophen-2-ol solution was added anhydrous DMF (10 mL) and DABCO (1.85
g, 16.5 mmol) and stirred for 30 min at room temperature. Diethylcarbamoyl chloride (2.10 mL, 16.5

HETEROCYCLES, Vol. 88, No. 2, 2014
1569
mmol) was added and dark solution was stirred for 17 h. The reaction was quenched with water (50 mL),
extracted with EtOAc (3x15 mL), combined EtOAc solution was dried over Na SO and concentrated in
2
4
vacuo. The crude residue was purified by flash column chromatography (eluent 1:9 EtOAc – petroleum
1
ether) to yield 2-diethylcarbamoyloxythiophene (1.12 g, 51%) as a yellowish liquid; H NMR (400 MHz,
CDCl )  1.16-1.28 (m, 6H), 3.34-3.46 (m, 4H), 6.62 (dd, J = 1.7, 3.7 Hz, 1H), 6.79 (dd, J = 3.7, 5.8 Hz,
3
1
3
1
1
H), 6.83 (dd, J = 1.7, 5.8 Hz, 1H); C NMR (CDCl
23.11, 152.66, 153.33.
3
)  13.19, 14.13, 41.92, 42.53, 112.43, 117.49,
5
-Methylthio-3H-thiophen-2-one (5b)
TMEDA (0.15 mL, 0.98 mmol) was dissolved in dry THF (2 mL) and cooled to -78 °C, s-BuLi (1.3 M
solution in cyclohexane, 0.75 mL, 0.98 mmol) was added and stirred for 1 h at -78 °C. Then
2
-diethylcarbamoyloxythiophene (3) (0.13 g, 0.65 mmol) solution in dry THF (2 mL) was added
dropwise and reaction mixture was stirred at -78 °C for 2 h. Me (87 µL, 0.98 mmol) was added at
78 °C and stirred for 40 min and the reaction mixture was allowed to warm to room temperature and then
stirred overnight. Reaction mixture was quenched with saturated aqueous NH Cl (5 mL), extracted with
EtOAc (3x10 mL), washed with brine, dried over Na SO and concentrated in vacuo. The crude residue
2 2
S
-
4
2
4
was purified by column chromatography (eluent 1:4 EtOAc – petroleum ether) to yield
1
5
2
-methylthio-3H-thiophen-2-one (29 mg, 31%) as a slightly yellow liquid; H NMR (400 MHz, CDCl
.41 (s, 3H), 4.06 (d, J = 3.2 Hz, 2H), 6.87 (d, J = 3.2 Hz, 1H).
3
) 
3
-Methyl-2-diethylcarbamoyloxythiophene (7a)
TMEDA (0.11 mL, 0.75 mmol) was dissolved in dry THF (2 mL) and cooled to -78 °C, n-BuLi (1.8 M
solution in hexanes, 0.42 mL, 0.75 mmol) was added and stirred for 40 min at -78 °C. Then
2
-diethylcarbamoyloxythiophene (3; 0.10 g, 0.50 mmol) solution in dry THF (1 mL) was added dropwise
and reaction mixture was stirred at -78 °C for 2 h. MeI (50 µL, 0.75 mmol) solution in the dry THF (0.3
mL) was added at -78 °C and stirred for 40 min and the reaction mixture was allowed to warm to room
temperature and stirred overnight. Reaction mixture was quenched with saturated aqueous NH
extracted with EtOAc (3x10 mL), dried over Na SO and concentrated in vacuo. The crude residue was
purified by column chromatography (eluent 1:9 EtOAc
4
Cl (5 mL),
2
4
–
petroleum ether) to yield
1
3
-methyl-2-diethylcarbamoyloxythiophene (12 mg, 11%) as a slightly yellow liquid; H NMR (400 MHz,
CDCl )  1.12 (t, J = 7.0 Hz, 3H), 1.26 (t, J = 7.0 Hz, 3H), 2.11 (s, 3H), 3.40 (q, J = 7.0 Hz, 2H), 3.44 (q,
3
1
3
J = 7.0 Hz, 2H), 6.66 (d, J = 5.8 Hz, 1H), 6.78 (d, J = 5.8 Hz, 1H); C NMR (CDCl
4.10, 41.99, 42.58, 116.32, 122.13, 126.22, 146.96, 152.75.
3
)  11.85, 13.27,
1

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HETEROCYCLES, Vol. 88, No. 2, 2014
3
-Diethylcarbamoyloxythiophene (10)
To an ethereal solution (10 mL) of thiophen-3-ylboronic acid (0.50 g, 3.91 mmol) was added 10%
aqueous H (3.0 mL) and the reaction mixture was refluxed for 2 h and the layers were separated. The
aqueous layer was extracted with Et O (2x5 mL) and combined ethereal solution was washed with 10%
aqueous ferrous ammonium sulfate solution (10 mL), washed with water (10 mL), and dried over Na SO
O was removed by heating to 42 °C (with no vaccum). Thiophen-3-ol (9) as an orange ethereal
solution was used immediately in the next reaction. To the obtained thiophen-3-ol Et O solution was
2 2
O
2
2
4
.
1
6
Et
2
2
added anhydrous DMF (5.0 mL) and DABCO (0.44 g, 3.91 mmol) and stirred for 1 h at room temperature.
Diethylcarbamoyl chloride (0.53 g, 3.91 mmol) was added and dark solution was stirred for 1 h. After
completion of the reaction, it was quenched with water (10 mL), extracted with EtOAc (3x5 mL),
combined EtOAc solution was dried over Na
purified by flash column chromatography (eluent 1:9 EtOAc – petroleum ether) to yield
-diethylcarbamoyloxythiophene (0.41 g, 53%) as a colorless liquid.
2 4
SO and concentrated in vacuo. The crude residue was
3
-1 1
IR (neat) 1721, 2975 cm ; H NMR (400 MHz, CDCl
3
)  1.15-1.30 (m, 6H), 3.33-3.47 (m, 4H), 6.93 (dd,
13
J = 1.4, 5.3 Hz, 1H), 7.02 (dd, J = 1.4, 3.4 Hz, 1H), 7.21 (dd, J = 3.4, 5.3 Hz, 1H); C NMR (CDCl ) 
3
1
3.30, 14.16, 41.89, 42.28, 109.97, 121.72, 123.78, 147.98, 153.48.
2
-Methyl-3-diethylcarbamoyloxythiophene (13a)
TMEDA (66 μL, 0.44 mmol) was dissolved in dry THF (1.5 mL) and cooled to -30 °C. n-BuLi (1.8 M
solution in hexanes, 0.25 mL, 0.44 mmol) was added and stirred for 10 min at -30 °C before
3
-ethylcarbamoyloxythiophene (10; 80 mg, 0.40 mmol) solution in dry THF (0.5 mL) was added
dropwise and reaction mixture was stirred at -30 °C for 2 h. MeI (28 µL, 0.44 mmol) solution in dry THF
0.2 mL) was added to the generated thiophene carbanion. After 1 h at -30 °C reaction mixture was
allowed to reach room temperature in 40 min, and quenched with saturated aqueous NH Cl (5 mL),
extracted with EtOAc (3x5 mL), dried over Na SO and concentrated in vacuo. The crude residue was
(
4
2
4
purified by column chromatography (eluent 1:9 EtOAc
–
petroleum ether) to yield
2
-methyl-3-diethylcarbamoyloxythiophene (40 mg, 47%) as light yellow liquid.
-1
IR (neat) 1718, 2975 cm ; H NMR (400 MHz, CDCl )  1.23 (m, 6H), 3.40 (m, 4H), 2.29 (s, 3H), 6.84 (d,
3
1
3
J = 5.4 Hz, 1H), 6.98 (d, J = 5.4 Hz, 1H); C NMR (CDCl )  11.09, 13.30, 14.14, 41.89, 42.31, 119.84,
3
1
22.34, 123.66, 144.13, 153.62.
2
-Methylthio-3-hydroxythiophene (11b) and 2-methylthio-3-diethylcarbamoyloxythiophene (13b)
TMEDA (0.7 mL, 0.44 mmol) was dissolved in dry THF (10 mL) and cooled to -30 °C. n-BuLi (1.8 M

HETEROCYCLES, Vol. 88, No. 2, 2014
1571
solution in hexanes, 2.5 mL, 4.42 mmol) was added and stirred for 10 min at -30 °C before
-ethylcarbamoyloxythiophene (10; 0.80 g, 4.01 mmol) solution in dry THF (1.0 mL) was added
dropwise and reaction mixture was stirred at -30 °C for 2 h. Me (0.4 mL, 4.42 mmol) was added to the
generated thiophene carbanion. After 1 h at -30 °C reaction mixture was allowed to reach room
temperature overnight, and quenched with saturated aqueous NH Cl (10 mL), extracted with EtOAc
and concentrated in vacuo. The crude residue was purified by column
) to yield 2-methylthio-3-diethylcarbamoyloxythiophene 13b (0.37 g,
3
2 2
S
4
(
3x10 mL), dried over Na
2
SO
4
chromatography (eluent CH
2
Cl
2
3
7%) as a slightly yellow liquid and 2-methylthio-3-hydroxythiophene 11b (0.20 g, 34%) as a light
liquid.
-1 1
1
1b: IR (neat) 1535, 2920, 3429 cm ; H NMR (400 MHz, CDCl
3
)  2.28 (s, 3H), 5.89 (s, 1H), 6.74 (d, J
1
3
=
5.8 Hz, 1H), 7.22 (d, J = 5.8 Hz, 1H); C NMR  22.09, 107.26, 118.05, 127.67, 156.56.
-1 1
1
3b: IR (neat) 1732, 2932 cm ; H NMR (400 MHz, CDCl
3
)  1.21 (t, J = 7.0 Hz, 3H), 1.30 (t, J = 7.0
Hz, 3H), 3.39 (q, J = 7.0 Hz, 2H), 3.47 (q, J = 7.0 Hz, 2H), 6.96 (d, J = 5.8 Hz, 1H), 7.24 (d, J = 5.8 Hz,
1
3
1
H); C NMR (CDCl )  13.32, 14.13, 21.16, 42.16, 42.43, 121.38, 122.90, 125.69, 149.56, 153.25.
3
2
-Trimethylsilyl-3-diethylcarbamoyloxythiophene (13c)
TMEDA (83 µL, 0.55 mmol) was dissolved in dry THF (1.5 mL) and cooled to -30 °C. n-BuLi (1.8 M
solution in hexanes, 0.31 mL, 0.55 mmol) was added and stirred for 10 min at -30 °C before
3
-ethylcarbamoyloxythiophene (10; 100 mg, 0.50 mmol) solution in dry THF (0.5 mL) was added
dropwise and reaction mixture was stirred at -30 °C for 2 h. MeSiCl (70 µL, 0.55 mmol) solution in dry
THF (0.2 mL) was added to the generated thiophene carbanion. After 1 h at -30 °C reactions mixture was
allowed to reach room temperature in 40 min, and was quenched with saturated aqueous NH
extracted with EtOAc (3x5 mL), dried over Na SO and concentrated in vacuo. The crude residue was
purified by column chromatography (eluent 1:9 EtOAc petroleum ether) to yield
-trimethylsilyl-3-diethylcarbamoyloxythiophene (51 mg, 38%) as a light liquid.
4
Cl (5 mL),
2
4
–
2
-1 1
IR (neat) 1723, 2973 cm ; H NMR (400 MHz, CDCl
3
)  0.30 (s, 3H), 1.19 (t, J = 7.1 Hz, 3H), 1.24 (t, J
=
7.0 Hz, 3H), 3.38 (q, J = 7.1 Hz, 2H), 3.44 (q, J = 7.0 Hz, 2H), 7.00 (d, J = 5.0 Hz, 1H), 7.42 (d, J = 5.0
1
3
Hz, 1H); C NMR (CDCl )  -0.53, 13.26, 14.18, 41.63, 42.04, 122.75, 123.67, 128.91, 153.73, 153.84.
3
ACKNOWLEDGEMENTS
This study was supported by The Latvian National Research Programme 2010–2013. “BIOMEDICINE”.
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