Deprotonative metalation of chlorothiophene with grignard reagents and catalytic cis -2,6-dimethylpiperidine under mild conditions

Article abstract of DOI:10.1055/s-0033-1338385

Deprotonative metalation of chlorothiophene takes place with a catalytic amount of cis-2,6-dimethylpiperidine (DMP) and an alkyl Grignard reagent at room temperature for three hours to give the corresponding thienyl Grignard reagent. Polymerization leading to head-to-tail-type poly(3-substituted thiophene) with the thus metalated chlorothiophene proceeds in the presence of a nickel catalyst bearing an N-heterocyclic carbene (NHC) ligand. Palladium-catalyzed cross-coupling reaction with aryl bromides also gives arylated thiophenes in good to excellent yields while the C-Cl bond remains intact. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1338385

LETTER
▌1133
lDetteerprotonative Metalation of Chlorothiophene with Grignard Reagents and
Catalytic cis-2,6-Dimethylpiperidine under Mild Conditions
Proton Abstraction of Chlorothiophene
Shunsuke Tamba, Kenji Ide, Keisuke Shono, Atsushi Sugie, Atsunori Mori*
Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai, Nada, Kobe 657-8501, Japan
Fax +81(78)8036181; E-mail: amori@kobe-u.ac.jp
Received: 23.02.2013; Accepted after revision: 21.03.2013
for chlorothiophenes. We describe in this communication
Abstract: Deprotonative metalation of chlorothiophene takes place
that efficient metalation of 2-chloro-3-substituted thio-
with a catalytic amount of cis-2,6-dimethylpiperidine (DMP) and an
phene with a catalytic amount of cis-2,6-dimethylpiperi-
dine and Grignard reagent takes place at room
alkyl Grignard reagent at room temperature for three hours to give
the corresponding thienyl Grignard reagent. Polymerization leading
to head-to-tail-type poly(3-substituted thiophene) with the thus temperature and that the metalated species can be subject-
metalated chlorothiophene proceeds in the presence of a nickel cat-
alyst bearing an N-heterocyclic carbene (NHC) ligand. Palladium-
catalyzed cross-coupling reaction with aryl bromides also gives ar-
ed to cross-coupling polymerization and arylation reac-
tions in the presence of a transition-metal catalyst.
We first studied metalation of 2-chloro-3-hexylthiophene
(1) in the presence of ethylmagnesium chloride and a cat-
alytic amount of several amines. Table 1 summarizes the
results of metalation, which was confirmed by quenching
the generated thiophene-magnesium species with iodine,
leading to 2-chloro-3-hexyl-5-iodothiophene (2). Al-
though the use of dicyclohexylamine (Cy2NH) as a cata-
lyst showed superior catalytic activity to TMP at 60 °C for
one hour, performing the reaction at room temperature for
the same time resulted in only 44% yield of 2. When 1,8-
ylated thiophenes in good to excellent yields while the C–Cl bond
remains intact.
Key words: chlorothiophene, palladium, nickel, cross-coupling,
Grignard reagent
Thiophene is an important chemical structure in many
biologically important compounds1 as well as advanced
organic materials.2 Transition-metal-catalyzed cross-cou-
pling reaction of organometallic compounds and aryl ha-
lides is a powerful method for forming the arylated
thiophene motif.3 Thus, it is important to develop an effi-
cient metalation method for use in cross-coupling reac-
tions as well as for the generation of simple
organometallic nucleophiles. Knochel and co-workers de-
veloped an efficient magnesium amide, the Knochel–
Hauser base, chloromagnesium 2,2,6,6-tetramethylpiperi-
dide lithium chloride salt (TMPMgCl·LiCl), and demon-
Table 1 Deprotonative Metalation of Chlorothiophene 1a
Hex
Hex
RMgX (1 equiv)
amine (10 mol%)
I2
H
Cl
I
Cl
S
1
S
2
THF
Entry Amine
RMgX
Temp
(°C)
Time
(h)
Conv.
(%)b
strated
deprotonative
metalation
of
several
heteroaromatic compounds and subsequent C–C bond
formation with various electrophiles under mild condi-
tions.4 We have also reported recently that metalation of
several heteroaromatic compounds occurs with the com-
bined use of a catalytic amount of tetramethylpiperidine
(TMP) and a Grignard reagent, and subsequent transition-
metal-catalyzed reactions with the thus obtained thienyl
magnesium species leads to the cross-coupled products
and several π-conjugated polymers.5 Although metalation
using the combination of a catalytic amount of TMP and
Grignard reagent is effective for several thiophene deriv-
atives, the metalation required 60 °C and longer reaction
periods (12–24 h).
1
2
Cy2NH
Cy2NH
Cy2NH
DBUc
EtMgCl
EtMgCl
EtMgCl
EtMgCl
EtMgCl
60
r.t.
r.t.
60
60
60
60
r.t.
r.t.
r.t.
1
90
44
77
0
1
3
3
4
1
5
Ph2NH
1
11
69
93
60
42
94
66
6
Cy(Me)NH EtMgCl
1
7
DMPd
DMP
DMP
DMP
DMP
EtMgCl
EtMgCl
i-PrMgBr
EtMgCl
0.5
1
8
9
1
Because chlorothiophene is an interesting building block
for facile and efficient access to several biologically ac-
tive compounds that incorporate a chlorothiophene moi-
ety,6 we were interested in developing a modified protocol
10
11
3
EtMgCl + LiCl r.t.
1
a Reaction conditions: 2-chloro-3-hexylthiophene (0.5 mmol), Gri-
gnard reagent (0.5 mmol), amine (0.05 mmol), THF (0.5 mL).
b The conversion was estimated by 1H NMR analysis after quenching
the reaction mixture with iodine.
SYNLETT 2013, 24, 1133–1136
Advanced online publication: 12.04.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
c DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene.
DOI: 10.1055/s-0033-1338385; Art ID: ST-2013-U0171-L
© Georg Thieme Verlag Stuttgart · New York
d DMP: cis-2,6-dimethylpiperidine.

1134
S. Tamba et al.
LETTER
diazabicyclo[5.4.0]undec-7-ene (DBU) was used in the Catalytic generation of 2-chloro-3-hexyl-5-chloromagne-
reaction instead of Cy2NH, no iodinated product was ob- siothiophene was performed with 10 mol% DMP and Et-
tained. The reaction with aromatic secondary amine di- MgCl at room temperature for three hours.
phenylamine (Ph2NH) hardly took place at 60 °C for one Polymerization of the resulting solution was examined
hour. Compared with Cy2NH, less hindered cyclohexyl- with 1.0 mol% NiCl2(PPh3)IPr at room temperature for 24
methylamine was also ineffective. On the other hand, hours, which was the standard polymerization conditions
metalation with cis-2,6-dimethylpiperidine (DMP) employed previously, to afford poly(3-hexylthiophene)
showed superior catalytic activity to afford the iodinated (P3HT; 3) in 59% isolated yield with Mn of 13200 (Mw/Mn
product in 93% conversion, and the reaction at room tem- = 1.39) as shown in Scheme 1.7 Considering that the de-
perature also improved the yield of 2 to 60% after stirring protonation of 1 with catalytic Et2NH required a higher re-
for one hour. Thus, we surveyed the effect of Grignard re- action temperature and a longer period, the use of DMP
agent at room temperature. The reaction with more steri- allowed the total metalation-polymerization procedures to
cally hindered i-PrMgBr afforded 2 in inferior yield be conducted at room temperature within a shorter depro-
(42%) to that with EtMgCl. The addition of LiCl to EtMg- tonation period.
Cl appeared to slightly improve the yield of 2 to 66%.
When the reaction was carried out with 10 mol% Cy2NH
and EtMgCl at room temperature, the conversion of 1 was
found to be 77% after stirring for three hours (entry 3). By
We then examined the coupling reaction of 1 with 4-bro-
motoluene (4a) leading to 2-chloro-3-hexyl-5-(4-methyl-
phenyl)thiophene (5a); Table 2 summarizes the results.
contrast, the use of DMP instead of Cy2NH induced the
metalation much faster (94% conversion after 3 h) sug-
gesting that subsequent transition-metal-catalyzed reac-
tions after the metalation with DMP can be carried out
under such conditions.
Table 3 Pd-Catalyzed Coupling Reaction of 1 with Aryl Bromidesa
Hex
Hex
PdCl2dppf
Aryl–Br
cat. DMP
EtMgCl
H
Cl
Aryl
Cl
S
S
THF, r.t., 3 h
r.t., time
1
5
Hex
Hex
Aryl-Br
Product
Time
(h)
Yield
(%)b
NiCl2(PPh3)IPr
(1.0 mol%)
EtMgCl (1 equiv)
DMP (10 mol%)
H
Cl
S
S
r.t., 24 h
n
THF, r.t., 3 h
Hex
Cl
1
3
59%
F3C
Br
10
51
Mn = 13200, Mw/Mn = 1.39
F3C
S
4b
Scheme 1 Nickel-catalyzed polycondensation of chlorothiophene
5b
Hex
Cl
F
Br
F
18
14
14
22
68
76
83
86
Table 2 Optimization of Catalyst for the Coupling of 1 with Bro-
moarene 4aa
S
4c
5c
Br
Me
Hex
Cl
Br
4a
Hex
Cl
Hex
Cl
cat. DMP
EtMgCl
catalyst
S
(2.0 mol%)
H
Me
4d
4e
S
S
5d
5e
THF, r.t., 3 h
r.t., time
1
5a
Hex
Cl
Br
Entry
Catalyst
Time (h)
Yield (%)b
S
1
2
3
4
5
6c
NiCl2(PPh3)2
PdCl2(PPh3)2
PdCl2dppf
19
trace
0
17
Hex
Cl
MeO
Br
15
52
25
49
69
MeO
S
PdCl2(dt-bpf)
20
4f
5f
Pd-PEPPSI-SIPr
PdCl2dppf
23
Hex
Cl
Me
N
Me
N
15.5
Br
20
25
Me
a Reaction conditions (unless noted otherwise): (i) 1 (0.5 mmol), DMP
(10 mol%), EtMgCl (1.0 equiv), THF; (ii) catalyst (2.0 mol%), 4-bro-
motoluene (4a; 1.5 equiv).
S
Me
4g
5g
b Isolated yield.
a Reaction conditions: (i) 1 (0.5 mmol), DMP (10 mol%), EtMgCl
(1.5 equiv), THF; (ii) PdCl2dppf (2.0 mol%), aryl bromide (1.8
equiv).
c The reaction was carried out with EtMgCl (1.5 equiv) and 4a (1.8
equiv).
Synlett 2013, 24, 1133–1136
© Georg Thieme Verlag Stuttgart · New York

LETTER
Proton Abstraction of Chlorothiophene
1135
When the coupling reaction of 1 was carried out with catalyzed coupling reaction through C–Cl bond cleavage.
10 mol% DMP and an equimolar amount of EtMgCl for Chlorothiophene 1 was first subjected to the reaction with
deprotonation and with 2.0 mol% NiCl2(PPh3)2 at room iodocarbazole 6 in THF with DMP (10 mol%) and
temperature for 24 hours, only a trace amount of coupling EtMgCl (1.0 equiv) at room temperature for 22 hours to
product 5a was obtained. The coupling reaction was then undergo the coupling reaction (Scheme 2). Following ad-
performed with several palladium catalysts. When the dition of 1.2 equiv of 3-hexyl-5-chloromagnesiothiope-
coupling reaction was examined in the presence of hene, which was generated by selective metalation of 3-
PdCl2(PPh3)2 as catalyst, no coupling product 5a was ob- hexylthiophene and TMPMgCl·LiCl, and 2.0 mol%
tained because of the insufficient activity of the palladium NiCl2(PPh3)IPr afforded 2,5-diarylated thiophene 7 in
catalyst bearing PPh3 ligands. Drastic improvement was 44% overall yield. The obtained head-to-tail-type bithio-
observed when the reaction was carried out in the pres- phene, bearing a carbazole moiety, is a partial structure of
ence of 2.0 mol% PdCl2dppf {dppf: 1,1′-bis(diphe- MK-2 dye, which has been employed as dye-sensitized
nylphosphino)ferrocene} to afford the corresponding solar cells showing excellent power conversion efficien-
coupling product in 52% yield. Similar reaction with cy.11 Considering that the synthesis of such structures re-
PdCl2(dt-bpf) {dt-bpf: 1,1′-bis[di(tert-butylphosphi- quired three or four steps in our previous report,5d,12 the
no)]ferrocene} gave 5a in only 25% yield. When the cat- present method improved the synthetic efficiency of lin-
alyst was switched to Pd-PEPPSI-SIPr,8 which showed ear head-to-tail-type oligothiophene-bearing carbazole
excellent catalytic activity in the coupling reaction of 3- moieties.13
hexylthiophene,9 5a was obtained in 49% yield. The meta-
lation with EtMgCl (1.5 equiv) and the coupling reaction
In conclusion, the combination of a catalytic amount of
cis-2,6-dimethylpiperidine and Grignard reagent is an ef-
with 4-bromotoluene (1.8 equiv) in the presence of
ficient metalating system for chlorothiophene at the C–H
PdCl2dppf (2.0 mol%) improved the yield to 69%.
bond. Polycondensation of metalated thiophene took
The reaction conditions detailed in Table 2 were applied place in the presence of a nickel catalyst to afford the cor-
to the reaction of 1 and several aryl bromides, as summa- responding P3HT in good yield. Head-to-tail-type bithio-
rized in Table 3. The reaction with aryl bromides bearing phene, bearing a carbazole moiety, was also synthesized
an electron-withdrawing substituent proceeded within a by one-pot iterative Pd-catalyzed C–H arylation reaction
relatively short reaction period in good to excellent yield. and Ni-catalyzed coupling reaction through C–Cl bond
The use of bromobenzene (4d) and 2-bromonaphthalene cleavage.
(4e) as aryl bromide afforded the corresponding products
5d and 5e in 76 and 83% yields, respectively. On the other
hand, the reaction with an aryl bromide bearing a more
Acknowledgment
This work was partially supported by Kakenhi (B), by the Japan So-
ciety for the Promotion of Science (JSPS) and the Nagase Science
and Technology Foundation. S.T. appreciates JSPS for a Research
Fellowship for Young Scientist. Thiophene monomers were kindly
donated by Soken Chemical & Engineering Co., Ltd.
electron-donating group was less effective. The reaction
with 4-bromoanisole (4f) required relatively long reaction
period and the reaction with 4-bromo-N,N-dimethylani-
line (4g) gave only 25% yield.10
Thus, selective arylation at the 5-position of chlorothio-
phene leads to facile synthesis of regioregular head-to-
tail-type oligothiophene by one-pot iterative Pd-catalyzed
C–H coupling reaction at the 5-position and sequential Ni-
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
I
N
Hex
Hex
6 (1.2 equiv)
PdCl2dppf (2.0 mol%)
DMP (10 mol%)
EtMgCl (1 equiv)
Cl
H
Cl
S
S
THF, r.t., 3 h
r.t., 22 h
1
N
Hex
MgCl
S
Hex
(1.2 equiv)
S
NiCl2(PPh3)IPr (2.0 mol%)
S
r.t., 24 h
Hex
N
7, 44%
Scheme 2 One-pot synthesis of partial structure of MK dye
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1133–1136

1136
S. Tamba et al.
LETTER
(8) PEPPSI™: Pyridine-Enhanced Precatalyst Preparation
References and Notes
Stabilization and Initiation. SIPr: 1,3-bis(2,6-
diisopropylphenyl)imidazolidin-2-ylidene. See:
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(10) General procedure for arylation of chlorothiophene: To
a solution of 1.02 M EtMgCl (0.75 mL, 0.75 mmol) in THF
were added 2-chloro-3-hexylthiophene (1; 0.5 mmol, 101
mg) and cis-2,6-dimethylpiperidine (0.05 mmol 6.7 μL)
dropwise at room temperature and the resulting mixture was
stirred at room temperature for 3 h. THF (2 mL), 4-
bromotoluene (4a; 0.9 mmol, 0.11 mL), and PdCl2(dppf)
(8.2 mg, 0.01 mmol) were successively added and stirring
was continued at room temperature for 15.5 h. The reaction
mixture was poured into water and the organic materials
were extracted with diethyl ether. The organic layer was
washed with water twice and dried over anhydrous sodium
sulfate. The solvent was removed under reduced pressure to
leave a crude oil, which was purified by column
(5) (a) Tanaka, S.; Tanaka, D.; Sugie, A.; Mori, A. Tetrahedron
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chromatography on silica gel using hexane as an eluent to
afford 2-chloro-3-hexyl-5-(4′-methylphenyl)thiophene (5a;
101 mg). 1H NMR (300 MHz, CDCl3): δ = 0.89 (t, J =
6.9 Hz, 3 H), 1.25–1.43 (m, 6 H), 1.50–1.68 (m, 2 H), 2.35
(s, 3 H), 2.56 (t, J = 7.7 Hz, 2 H), 6.96 (s, 1 H), 7.16 (d, J =
8.1 Hz, 2 H), 7.39 (d, J = 8.2 Hz, 2 H); 13C NMR (75 MHz,
CDCl3): δ = 14.07, 21.13, 22.59, 28.15, 28.93, 29.59, 31.63,
123.02, 123.07, 125.24, 129.56, 131.08, 137.51, 140.21,
140.62; IR (neat): 809, 1041, 1453, 1513, 2856, 2925, 2954;
HRMS (DART-ESI+): m/z [M + H]+ calcd for C17H2235ClS:
293.1131; found: 293.1131.
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(7) General procedure for polymerization of
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chlorothiophene 1: To a solution of 1.02 M EtMgCl (0.50
mL, 0.5 mmol) in THF were added 2-chloro-3-
hexylthiophene (1; 0.5 mmol, 101 mg) and cis-2,6-
dimethylpiperidine (0.05 mmol 6.7 μL) dropwise, and the
resulting mixture was stirred at room temperature for 3 h.
THF (4.5 mL) and NiCl2(PPh3)IPr (3.9 mg, 0.005 mmol)
were successively added and stirring was continued at 25 °C
for 24 h. Hydrochloric acid (1.0 M, 20 mL) and methanol (50
mL) were added to form a precipitate. The mixture was
filtered and the residue was washed with methanol
repeatedly to leave a dark-purple solid, which was dried
under reduced pressure to afford poly(3-hexylthiophene) (3;
50 mg, 59% yield; Mn = 13200, Mw/Mn= 1.39).
(13) Dohi, Kita, and co-workers recently reported an efficient
three-step synthesis of this structure, see: Dohi, T.;
Yamaoka, N.; Nakamura, S.; Sumida, K.; Morimoto, K.;
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Synlett 2013, 24, 1133–1136
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