Rhodium-catalyzed annulative coupling of 3-phenylthiophenes with alkynes involving double C-H bond cleavages

Article abstract of DOI:10.1002/chem.201303847

Double C-H bond activation took place efficiently upon treatment of 3-phenylthiophenes with alkynes in the presence of a rhodium catalyst and a copper salt oxidant to form the corresponding naphthothiophene derivatives. Dehydrogenative coupling with alkenes was also found to occur on the phenyl moiety rather than the thiophene ring. These reactions provide straightforward synthetic methods for p-conjugated molecules involving a thiophene unit from readily available, simple building blocks.

Full text of DOI:10.1002/chem.201303847

DOI: 10.1002/chem.201303847
Communication
&
Catalysis
Rhodium-Catalyzed Annulative Coupling of 3-Phenylthiophenes
with Alkynes Involving Double C-H Bond Cleavages
Tomonori Iitsuka,[a] Koji Hirano,[a] Tetsuya Satoh,*[a, b] and Masahiro Miura*[a]
Abstract: Double CÀH bond activation took place effi-
ciently upon treatment of 3-phenylthiophenes with al-
kynes in the presence of a rhodium catalyst and a copper
salt oxidant to form the corresponding naphthothiophene
derivatives. Dehydrogenative coupling with alkenes was
also found to occur on the phenyl moiety rather than the
thiophene ring. These reactions provide straightforward
synthetic methods for p-conjugated molecules involving
a thiophene unit from readily available, simple building
blocks.
Transition-metal-catalyzed direct CÀH functionalization has
Scheme 1. Transition-metal-catalyzed dehydrogenative annulation.
been recognized as a powerful tool in organic synthesis.[1] In
particular, the dehydrogenative annulation of aromatic sub-
strates with internal alkynes through CÀH bond cleavage,
pounds have attracted considerable attention because of their
alkyne insertion, and cyclization provides an atom- and step-
economical route to polycyclic arenes and heteroarenes from
simple building blocks.[2] In the initial CÀH activation step, het-
eroatom-containing groups, such as hydroxy-, carboxy-, and
amido-functions, are usually employed as directing groups for
regioselective CÀH bond cleavage and functionalization at
their ortho position (Scheme 1a). Recently, not only such s-co-
ordinating directing groups but also p-electron units including
alkenyl, alkynyl, aryl, heteroaryl, and cyano groups have been
found to trigger ortho CÀH bond functionalization
(Scheme 1b).[2,3] Actually, the annulative couplings of some
azoles with alkynes involving CÀH bond cleavage at the proxi-
mal positions of the nitrogen-containing rings have been re-
ported.[2,4] However, relevant couplings utilizing other hetero-
cycles including the most challenging annulation of phenyl-
thiophenes still remain unprecedented. Such reactions appear
to provide simple pathways from readily available substrates
to useful p-conjugated molecules bearing a thiophene unit. It
is well-known that thiophene-containing fused polycyclic com-
applicability to electronic devices, such as organic field effect
transistors (OFETs), organic light-emitting diodes (OLEDs), and
organic photovoltaics (OPVs),[5] as well as to therapeutics.[6]
During our research on the rhodium-[7,8] and ruthenium-cata-
lyzed[9,10] dehydrogenative coupling, we found that 3-phenyl-
thiophenes undergo dehydrogenative annulation upon treat-
ment with alkynes in the presence of a rhodium catalyst and
a copper salt oxidant to furnish thiophene-containing tricyclic
frameworks. The results obtained with this novel reaction are
described herein.
In an initial attempt, 3-phenylthiophene (1a) (0.2 mmol) was
treated with diphenylacetylene (2a) (0.1 mmol) in the presence
of [Cp*RhCl2]2 (0.005 mmol) and Cu(OAc)2·H2O (0.2 mmol) as
a catalyst and an oxidant, respectively, in toluene at 1108C for
7 h under N2. As the dehydrogenative annulation product, 4,5-
diphenylnaphtho[2,1-b]thiophene (3aa) was obtained in 53%
yield (Table 1, entry 1). Other silver and copper salts, such as
AgOAc, Cu(2-ethylhexanoate)2, Cu(OCOCF3)2, and CuCO3, were
less effective as oxidants (entries 2–5). Elevating the bath tem-
perature to 1258C improved the yield of 3aa to 70% (entry 6),
although further elevation was not effective (entry 7). Interest-
ingly, the addition of a catalytic amount of Cs2CO3 (0.03 mmol)
was found to significantly enhance the reaction efficiency to
afford 3aa in 90% yield (entry 8). Other bases including CsOAc
and KOMe were comparably effective as Cs2CO3 (entries 9–14).
In diglyme, DMF, or PhCF3, the product yield decreased (en-
tries 15–17). The scale-up did not affect the product yield
(entry 18). Decreasing the amount of the Rh catalyst or 1a
slightly reduced the yield of 3aa (entries 19 and 20, respective-
ly).
[a] T. Iitsuka, Dr. K. Hirano, Prof. Dr. T. Satoh, Prof. Dr. M. Miura
Department of Applied Chemistry
Faculty of Engineering, Osaka University
Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-7362
E-mail: satoh@chem.eng.osaka-u.ac.jp
miura@chem.eng.osaka-u.ac.jp
[b] Prof. Dr. T. Satoh
JST, ACT-C
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303847.
Chem. Eur. J. 2014, 20, 385 – 389
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Communication
Table 1. Reaction of 3-phenylthiophene (1a) with diphenylacetylene
(2a).[a]
Table 2. Reaction of 3-phenylthiophenes 1 with alkynes 2.[a]
Entry
1
2
t
[h]
Product(s), Yield [%][b]
Entry Oxidant
Additive Solvent
T [oC] Yield of 3aa [%][b]
1
Cu(OAc)2·H2O
–
–
–
–
–
–
–
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
110
110
110
110
110
125
135
125
125
125
125
125
125
125
53
33
16
0
2
AgOAc
[c]
3
Cu(2-EH)2
4
5
Cu(OCOCF3)2
CuCO3
0
6
7
Cu(OAc)2·H2O
Cu(OAc)2·H2O
70
53
90 (84)
79
86
72
74
90
82
59
12
79
90
84
86
8
9
Cu(OAc)2·H2O Cs2CO3
Cu(OAc)2·H2O K2CO3
Cu(OAc)2·H2O CsOAc
Cu(OAc)2·H2O KOAc
Cu(OAc)2·H2O CsOPiv
Cu(OAc)2·H2O KOMe
Cu(OAc)2·H2O DABCO
Cu(OAc)2·H2O Cs2CO3
Cu(OAc)2·H2O Cs2CO3
Cu(OAc)2·H2O Cs2CO3
Cu(OAc)2·H2O Cs2CO3
Cu(OAc)2·H2O Cs2CO3
Cu(OAc)2·H2O Cs2CO3
1
2
3
4
1a
2b: R=Me
2c: R=tBu
2d: R=OMe
2e: R=CF3
10 3ab: R=Me, 86
24 3ac: R=tBu, 65
7
7
10
11
12
13
14
15
16
17
18[d]
19[e]
20[f]
3ad: R=OMe, 76
3ae: R=CF3, 76
diglyme 125
DMF
PhCF3
toluene
toluene
toluene
125
125
125
125
125
5[c]
6[c]
1a
2 f: R=nPr
2g: R=nC7H15
7
7
3af: R=nPr, 79
3ag: R=nC7H15, 85
[a] Reaction conditions: 1a (2 equiv), 2a (0.1 mmol, 1 equiv), [Cp*RhCl2]2
(5 mol%), oxidant (2 equiv), additive (30 mol%), solvent (1 mL) under N2
for 7 h. [b] GC yield based on the amount of 2a used. Value in parenthe-
ses indicates yield after purification. [c] 2-EH=2-ethylhexanoate. [d] With
1a (2 equiv), 2a (0.2 mmol, 1 equiv), [Cp*RhCl2]2 (5 mol%), Cu(OAc)2·H2O
(2 equiv), Cs2CO3 (30 mol%) in toluene (2 mL) for 10 h. [e] With 1a
(2 equiv), 2a (0.2 mmol, 1 equiv), [Cp*RhCl2]2 (5 mol%), Cu(OAc)2·H2O
(2 equiv), Cs2CO3 (30 mol%) in toluene (1 mL) for 10 h. [f] With 1a
(1.2 equiv), 2a (0.2 mmol, 1 equiv), [Cp*RhCl2]2 (5 mol%), Cu(OAc)2·H2O
(2 equiv), Cs2CO3 (30 mol%) in toluene (1 mL) for 10 h.
7[c]
1a
2h
2a
7
3ah, 50
3ah’, 12
8
9
10
1b: R=Me
1c: R=OMe
1d: R=Cl
10 3ba: R=Me, 85
24 3ca: R=OMe, 66
7
Under the optimized reaction conditions (Table 1, entry 18),
1a underwent the annulation with various p-substituted di-
phenylacetylenes 2b–e to produce the corresponding 4,5-di-
arylnaphthothiophenes 3ab–e in 65–86% yields (Table 2, en-
tries 1–4). Dialkylacetylenes 2 f and 2g also reacted with 1a
smoothly to produce 4,5-dialkylnaphthothiophenes 3af and
3ag in good yields (entries 5 and 6). The reaction with unsym-
metrical 1-phenyl-1-propyne (2h) gave 5-methyl-4-phenyl-
naphthothiophene 3ah predominantly, along with a minor
amount of 4-methyl-5-phenyl isomer 3ah’ (entry 7). The annu-
lation reactions of 3-(p-substituted phenyl)thiophenes 1b–d
with 2a took place to produce 7-substituted 4,5-diphenylnaph-
thothiophenes 3ba, 3ca, and 3da, respectively (entries 8–10).
Electron-deficient 1d tends to react more rapidly than elec-
tron-rich 1b and 1c. It should be noted that m-fluoro substi-
tuted 1e reacted at the sterically more hindered position to
form 3ea exclusively (entry 11).[11] The annulation appears to
occur at the electron-deficient position preferentially. 3-Phenyl-
benzothiophene (1 f) also underwent the annulation efficiently
3da: R=Cl, 98
11
1e
2a
2a
7
3ea, 87
12[d] 1 f
24 3 fa, 70
[a] Reaction conditions: 1 (2 equiv), 2a (0.2 mmol, 1 equiv), [Cp*RhCl2]2
(5 mol%), Cu(OAc)2·H2O (2 equiv), Cs2CO3 (30 mol%) in toluene (2 mL) at
1258C under N2 for 7 h. [b] Isolated yield. [c] With 1a (2 equiv), 2 (0.1 mmol,
1 equiv), [Cp*RhCl2]2 (5 mol%), Cu(OAc)2·H2O (2 equiv), Cs2CO3 (30 mol%) in
toluene (1 mL). [d] At 1208C.
Under similar conditions, the coupling of 1a using alkenes
as coupling partners in place of alkynes was next examined.
Treatment of 1a (0.2 mmol) with styrene (4a) (0.1 mmol) in the
to furnish
a
tetracyclic benzo[b]naphtho[1,2-d]thiophene
framework (entry 12). This reaction proceeded more smoothly
presence
of
[Cp*RhCl2]2
(0.005 mmol),
Cu(OAc)2·H2O
at 1208C.
(0.2 mmol), and Cs2CO3 (0.03 mmol) in toluene at 1258C for 7 h
Chem. Eur. J. 2014, 20, 385 – 389
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Scheme 2. Reaction of 1a with alkenes 4.
gave (E)-3-(2-styrylphenyl)thiophene (5a) in 66% yield as
a single major product (Scheme 2). Interestingly, no other alke-
nylated product was detected. Similarly, the reaction with
butyl acrylate (4b) also afforded 2’-alkenylated product 5b se-
lectively. These facts indicate that the CÀH bond at the 2’-posi-
tion of 1a, rather than that on the thiophene moiety, is selec-
tively cleaved under the present conditions. Therefore, in the
reaction of 1 with alkynes under similar conditions, the CÀH
bond at the 2’-position seems to be cleaved at the initial step
to form an intermediate A (step a in Scheme 3).
Figure 1. Time-course of the yield of 3aa in the reaction of 1a with 2a with
(squares) or without (diamonds) addition of AcOH. Reaction conditions: 1a
(2 equiv), 2a (0.2 mmol, 1 equiv), [Cp*RhCl2]2 (5 mol%), Cu(OAc)2·H2O
(2 equiv), Cs2CO3 (30 mol%), AcOH (4 equiv), in toluene (2 mL) under N2 at
1258C.
Scheme 4. Reaction of [D5]-1a with 2 f.
(step a in Scheme 3) is likely irreversible. Next, an intermolecu-
lar competition reaction of [D0]-1a/[D5]-1a with 2 f was exam-
ined. As shown in Scheme 5, the annulation products [D0]-3af
Scheme 3. Plausible mechanism for the reactions of 1a with 2 and 4.
Then, A may undergo alkyne insertion into the resulting ArÀ and [D4]-3af were obtained in a ratio of 1.9:1 (kH/kD), suggest-
Rh bond and a second CÀH bond cleavage on the thiophene
ring to form a seven-membered rhodacycle B (step b in
Scheme 3). Through the subsequent reductive elimination and
reoxidation steps, product 3 and an active RhIII species may be
formed. In the reactions with alkenes 4, the common inter-
mediate A may undergo alkene insertion (step c) and b-hydro-
gen elimination to release product 5 and HX.
ing that the rate-determining step involves the CÀH(D) bond
cleavage at the 2’-position of 1a.[12] Two parallel reactions
using [D0]-1a and [D5]-1a were also performed separately and
showed a similar KIE value of 1.7 (see the Supporting Informa-
tion). The observed KIE as well as substituent effect (Table 2,
entries 8–11) may suggest that the CÀH bond cleavage step
As shown in Table 1, the addition of appropriate bases, such
as Cs2CO3 and KOMe, significantly promoted the reaction of 1a
with 2a (entries 8–14 versus 6). One of possible roles of the
base seems to be the capture of acid HX formed during the re-
action. Therefore, the effect of the acid was investigated brief-
ly. The addition of AcOH (4 equiv) was found to considerably
retard the reaction of 1a with 2a, as shown in Figure 1.
To provide additional mechanistic information, deuterated
substrate, 3-([D5]phenyl)thiophene ([D5]-1a), was treated with
2 f under standard conditions (Scheme 4). After 2 h, no signifi-
cant D–H exchange at the ortho positions of recovered [D5]-1a
or at the 9-position of produced [D4]-3af was observed. This
result indicates that the initial CÀH(D) bond cleavage step
Scheme 5. Competition reaction of [D0]-1a and [D5]-1a with 2 f.
Chem. Eur. J. 2014, 20, 385 – 389
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Communication
possibly involves an acetate-assisted deprotonation[11a,b,13]
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In summary, we have demonstrated that variously substitut-
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the rhodium-catalyzed dehydrogenative annulation of 3-phe-
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Experimental Section
General procedure for the reactions of 3-phenylthiophenes
with alkynes
To a 20 mL two-necked flask with a reflux condenser, a balloon,
and a rubber cup were added phenylthiophene 1 (0.4 mmol),
alkyne 2 (0.2 mmol), [Cp*RhCl2]2 (0.01 mmol, 6 mg), Cu(OAc)2·H2O
(0.4 mmol, 80 mg), Cs2CO3 (0.06 mmol, 20 mg), 1-methylnaphtha-
lene (ca. 30 mg) as internal standard, and toluene (2 mL). Then, the
resulting mixture was stirred under nitrogen at 1258C (bath tem-
perature) for 7 h. After cooling, the reaction mixture was extracted
with ethyl acetate (100 mL) and ethylenediamine (2 mL). The or-
ganic layer was washed by water (100 mL, three times), and dried
over Na2SO4. After evaporation of the solvents under vacuum,
product 3 was isolated by column chromatography on silica gel
using hexane as an eluent. Characterization data of products are
summarized in the Supporting Information.
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Acknowledgements
This work was partly supported by Grants-in-Aid from MEXT,
JSPS, and JST, Japan.
Keywords: annulation · CÀC coupling · CÀH activation ·
homogeneous catalysis · rhodium
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Received: October 2, 2013
Published online on November 29, 2013
Chem. Eur. J. 2014, 20, 385 – 389
www.chemeurj.org
389
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim