Aryl(triethyl)silanes for Biaryl and Teraryl Synthesis by Copper(II)-Catalyzed Cross-Coupling Reaction

Article abstract of DOI:10.1002/anie.201608667

Aryl(triethyl)silanes are found to undergo cross-coupling with iodoarenes in the presence of catalytic amounts of CuBr₂and Ph-Davephos, as well as cesium fluoride as a stoichiometric base. Because the silicon reagents are readily accessible through catalytic C?H silylation of aromatic substrates, the net transformation allows coupling of aromatic hydrocarbons with iodoarenes via triethylsilylation.

Full text of DOI:10.1002/anie.201608667

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International Edition: DOI: 10.1002/anie.201608667
German Edition: DOI: 10.1002/ange.201608667
Cross-Coupling Reactions
Aryl(triethyl)silanes for Biaryl and Teraryl Synthesis by Copper(II)-
Catalyzed Cross-Coupling Reaction
Takeshi Komiyama, Yasunori Minami,* and Tamejiro Hiyama*
Abstract: Aryl(triethyl)silanes are found to undergo cross-
coupling with iodoarenes in the presence of catalytic amounts
of CuBr2 and Ph-Davephos, as well as cesium fluoride as
a stoichiometric base. Because the silicon reagents are readily
air- and moisture-sensitive.[5] On the other hand, tetraorgano-
silanes are characterized by high stability and solubility, but
most of them are inert to the reaction. To promote cross-
coupling of stable silanes, a functional group such as allyl, (2-
hydroxymethyl)phenyl, or 2-furyl is introduced on the silicon
center.[6–8] This modification, however, requires multistep
preparation and sometimes results in serious instability. Thus,
the cross-coupling using trialkylsilyl-substituted arenes is an
ideal and challenging goal. Reported herein is an advanced
solution to employ aryl(triehyl)silanes for the cross-coupling
reactions.
Because 2,1,3-benzothiadiazole plays significant roles in
organic electronics as a p-electron acceptor,[9] we first
examined the preparation of a disilylated benzothiadiazole
by straightforward silylation, and found that a double silyla-
tion of 5,6-difluoro-2,1,3-benzothiadiazole (1a) with triethyl-
silane (2a) in excess gave 4,7-bis(triethylsilyl)benzothiadia-
zole 3a in 94% yield with an Ir catalyst, tetramethylphenan-
throline ligand, and norbornene as a hydrogen accetptor in
diisopropyl ether [Eq. (1)].[10] Tributylsilyl and trihexylsilyl
groups could be also introduced into 1a (Supporting Infor-
mation). In contrast, triethoxysilane failed to give any
silylated product (Table S1). In light of the fact that 3a is
not accessible by the lithiation/silylation of 1a, the direct
silylation approach is straightforward and extremely efficient.
À
accessible through catalytic C H silylation of aromatic
substrates, the net transformation allows coupling of aromatic
hydrocarbons with iodoarenes via triethylsilylation.
T
he cross-coupling reaction is a straightforward carbon–
carbon bond-forming transformation between p-conjugate
molecules and allows construction of a wide variety of p-
conjugated molecular systems. Substrates and reagents for
such transformations, however, often suffer from low solubil-
ities due to their planar and rigid skeletons. Thus, alkyl chains
are introduced in either or both the reactants, and soluble but
toxic organotin reagents are applied rather than organoboron
reagents.[1] In this sense, organosilicon reagents have advan-
tages in view of stability, low toxicity, handling, and good
solubility. In particular, aryl(triethyl)silanes are easily acces-
sible not only through the reaction of organolithium or
-magnesium reagents with chlorosilanes, but also through
[2]
À
catalytic C H silylation with hydrosilanes. However, cross-
coupling of such silanes has been limited (Scheme 1).[2j,3,4]
Scheme 1. Ideal silicon-based cross-coupling strategy using aryl-
(triethyl)silanes.
With doubly silylated reagent 3a in hand, we next
examined the coupling with haloarenes under various palla-
dium catalytic conditions, but all of our attempts failed
(Table 1, Entry 1). Thus, we screened other metal catalysts
and discovered that 3a reacted with 2.1 equivalents of p-
iodoanisole (4a) in the presence of copper(II) bromide
(purity 99.999%, 10 mol%), Ph-Davephos [PPh2{(2-NMe2-
C6H4)C6H4}, 10 mol%], and CsF (2.5 equiv) in DMI at 1508C
for 24 h to produce doubly coupled product 5aa in 94% yield
(Entry 2).[11] Copper(II) chloride also gave 5aa, albeit in
a lower yield and a mono-coupled product, p-anisyl-5,6-
difluoro-7-triethylsilyl-2,1,3-benzothiadiazole, in 34% yield
(Entry 3). Copper salts such as Cu(OAc)2, CuF2, CuI, and
CuBr did not show any catalytic activity (Entries 4–7), in
sharp contrast to the reported Cu(I)-catalyzed cross-coupling
using silicon reagents.[12,13] Reaction with CuBr2 in the dark
gave 5aa in a yield similar to Entry 2 (Entry 8), showing that
Organosilicon reagents reactive enough for cross-coupling
usually need heteroatoms such as oxygen or halogens on the
silicon to enhance their reactivity toward a nucleophilic
activator, but such coupling-active silicon reagents are in turn
[*] T. Komiyama
Department of Applied Chemistry
Chuo University
Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan)
Dr. Y. Minami, Prof. Dr. T. Hiyama
Research and Development Initiative
Chuo University
Kasuga, Bunkyo-ku, Tokyo 112-8551 (Japan)
E-mail: yminami@kc.chuo-u.ac.jp
thiyama@kc.chuo-u.ac.jp
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201608667.
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Table 1: Reaction of 4,7-disilyl-5,6-difluorobenzothiadiazole with 4a.[a]
Table 2: Cross-coupling of 3a with 4.[a]
Entry
Ar in 4
5, Yield [%][b]
1
4b
4c
4d
4e
4 f
4g
C6H4-3-OMe
C6H4-2-OMe
C6H4-4-Cl
C6H4-4-Br
C6H4-4-F
5ab, 90
5ac, 72
5ad, 89
5ae, 91
5af, 85
5ag, 67
2[c]
3
Entry
3, Si
Metal Source
Yield
[%][b]
4
5
6
1
2
3
4
3a, SiEt3
3a, SiEt3
3a, SiEt3
3a, SiEt3
Pd(dba)2
<1
94[c]
20[d]
11
CuBr2 (99.999%)
CuCl2 (99.999%)
Cu(OAc)2 (98%)
CuF2 (99.5%)
C6H4-4-CO2Et
7
4h
4i
5ah, 77
5ai, 92
5
3a, SiEt3
3
8[d]
6
7
8
9
10
11
12
3a, SiEt3
3a, SiEt3
CuI (99.999%)
CuBr (99.999%)
CuBr2 (99.999%)
CuBr2 (99.999%)
CuBr2 (99.999%)
CuBr2 (99.999%)
CuBr2 (99.999%)
<1
5
89
9[d]
4j
5aj, 92
5ak, 57
3a, SiEt3 (in the dark)
3a’, Si(nBu)3
3a’’, Si(nHex)3
3a’’’, SiMe2Ph
3a’’’’, SiMe2[3,5-(CF3)2C6H3]
85[c,e]
84[f]
63
10[d]
4k
<1
[a] Unless otherwise noted, a mixture of 3a (1 equiv), 4a (2.1 equiv),
CuBr2 (10 mol%), Ph-Davephos (10 mol%), CsF (2.5 equiv), and DMI
(3m) was heated at 1508C for 24 h. [b] NMR yields. [c] Isolated yield.
[d] 4-p-Anisyl-5,6-difluoro-7-triethylsilyl-2,1,3-benzothiadiazole was pro-
duced in 34% yield. [e] 48 h. [f] 4 d.
11[d]
4l
5al, 65
[a] Unless otherwise noted, a mixture of 3a (0.3 mmol), 4 (2.1 equiv),
CuBr2 (0.1 equiv), Ph-Davephos (0.1 equiv), and CsF (2.5 equiv) in DMI
(3m) was heated at 1508C for 24 h. [b] Isolated yields. [c] 48 h. [d] KF
(2.5 equiv), 18-Crown-6 (2.5 equiv), and CPME (cyclopentyl methyl ether,
3m) were used instead of CsF and DMI.
light is not promoting the present cross-coupling. Other
ligands and solvents under the CuBr2 catalytic conditions
were unsuccessful (Supporting Information).[14] We next
examined the effect of silyl groups. Tributylsilyl and trihexyl-
silyl groups could be utilized in this reaction, albeit in longer
reaction times (Entries 9 and 10). On the other hand, the
reaction using 3a’’’ having a phenyldimethylsilyl group
afforded 5aa in 63% yield with generation of 4-methoxybi-
phenyl (4%) (Entry 11). The tendency that an electron-
deficient aryl group on silicon reacts with iodoarene in
preference to an electron-rich one is contrary to that of the
normal cross-coupling.[5e,15] Use of 3a’’’’ containing a 3,5-
bis(trifluoromethyl)phenyldimethylsilyl group gave an insolu-
ble mixture that did not include 5aa (Entry 12).
The scope of iodoarenes was surveyed briefly under the
double cross-coupling conditions for 3a, and the results are
summarized in Table 2. m-Iodoanisole (4b) coupled doubly to
give 5ab in an excellent yield (Entry 1). As o-iodoanisole (4c)
underwent the double cross-coupling without any problem to
form 5ac (Entry 2), moderate steric hindrance appears
acceptable. Chlorine, bromine, and fluorine were tolerated
in the reaction to provide 5ad, 5ae, and 5af smoothly
(Entries 3–5). This observation allows preparation of various
functionalized monomers for polyarylene synthesis.[16] Ethyl
4-iodobenzoate (4g) as an electron-deficient aryl electrophile
could give product 5ag (Entry 6). Introduction of 4-diphenyl-
aminophenyl groups to the difluorobenzothiadiazole core
occurred successfully (Entry 7). Whereas the reaction of 3a
with 4-morpholinoiodobenzene (4i) gave 5ai in a moderate
yield (< 50%) under the optimized conditions, further
optimization of the conditions gave 5ai in 92% yield using
KF (2.5 equiv), 18-crown-6 (2.5 equiv), and CPME (Entry 8).
Under the KF/18-crown-6/CPME conditions, 2-iodo-9,9-
dimethylfluorene (4j), N-(4-iodophenyl)carbazole (4k), and
3-iodo-N-phenylcarbazole (4l) were converted into 5aj, 5ak,
and 5al (Entries 9–11).
We next turned our attention to the scope of the aryl
group in Ar-SiEt3 (Table 3).[17] We focused on a thiophene
motif as an electron-rich aryl group, and examined the double
cross-coupling reaction of 2,5-bis(triethylsilyl)thiophene (3b)
and its 3,4-ethylenedioxy analogue 3c with iodoarene 4.[18] To
our delight, they gave the corresponding products bearing
alternately an electron-rich thiophene ring and electron-
deficient aryl group like p-cyanophenyl (4m) in good to high
yields (Entries 1–4). The fact that only a small amount of
mono-coupled products accompanied the reaction indicates
that the mono-coupled arylsilanes are more reactive than the
starting disilylarenes. 2,5-Bis(triethylsilyl)furan (3d) could
also be used (Entry 5). Additionally, 1,4-bis(triethylsilyl)-
2,3,5,6-tetrafluorobenzene (3e) reacted with 4a or 4h to give
5ea or 5eh, respectively (Entries 6 and 7).
Single cross-coupling was examined next. The five-mem-
bered heterocycle, 2-triethylsilylbenzo[b]thiophene (6a),
cross-coupled with 4a, 4m, or 6-iodoquinoline (4n), smoothly
to give the corresponding coupled product 7aa, 7am, or 7an
(Entries 8–10). 2-Triethylsilylindole (6b) successfully coupled
with 4a to produce 7ba (Entry 11). Use of 3,5-
2
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Table 3: Cross-coupling of various aryl(triethyl)silanes with 4.[a]
bis(trifluoromethyl)phenyl(triethyl)silane (6c) gave biaryl
7ca in a moderate yield (Entry 12). The fact that the reaction
of 4-trietylsilylbenzonitrile (6d) with 4a gave 7da, albeit in
a low yield (Entry 13), indicates that the electron-accept-
ability of silylated arenes is the key for effective cross-
coupling.[19] The cross-coupling reaction of 2-triethylsilylpyr-
idine (6e) and 3-triethylsilylpyridine (6 f) smoothly gave 7ea
and 7 fa (Entries 14 and 15). It is advantageous that 2-
triethylsilylpyridine is more stable and easier to handle than
notoriously unstable 2-pyridylboronic acid and its esters.[20]
The double coupling was applied to the reaction of 6a
with 2,7-diiodo-9,9-dimethylfluorene (8) to give rise to 9 in
99% yield [Eq. (2)]. To demonstrate the scalability of the
cross-coupling, a gram-scale reaction of 6a with 4a was
carried out, and the desired product 7aa was readily isolated
in a quantitative yield [Eq. (3)]. Moreover, we investigated
the sila-Sonogashira coupling[21] using 1-trimethylsilyl-2-phe-
nylacetylene (10) and obtained unsymmetrical diarylacety-
lene 11 in 93% yield [Eq. (4)].
Entry
Arylsilane
4
Product, Yield [%][b]
1[c]
3b
4a
5ba, 70
2
3
4
3c
3c
3c
4a
4g
4m
5ca, 83 (X=OMe)
5cg, 91 (X=CO2Et)
5cm, 97 (X=CN)
5
3d
4a
5da, 58
6
7
3e
3e
4a
4h
5ea, 74 (X=OMe)
5eh, 57 (X=NPh2)
8
9
6a
6a
4a
4m
7aa, 90 (X=OMe)
7am, 92 (X=CN)
10
11
6a
6b
4n
4a
7an, 98%
7ba, 87
12
13
6c
6d
4a
4a
7ca, 56
7da, 28
To gain insight into the mechanism of the cross-coupling
reaction, we monitored the reaction of 6a with 4a by 1H and
19F NMR, and detected the formation of a quantitative
amount of F-SiEt3 (12) with no trace of ArSi(Et)2OH being
observed [Eq. (5)].[7c] Because CuII can be reduced to CuI
through a single electron transfer (SET) accompanied by
generation of a radical species,[22] we attempted radical clock
experiments using 2-allyloxyiodobenzene (4o). If the corre-
sponding aryl radical is generated from the iodoarene,
intramolecular cyclization of the aryl radical is expected to
occur (kcyclization = 7.9 ꢀ 109 sÀ1).[23] We observed that, in lieu of
a cyclized product 13 or 14, normal coupled product 7co was
produced in 18% yield [Eq. (6)]. Moreover, 3-homoallyl-2-
silyl-benzothiophene (6g) reacted with 4a to give only normal
coupled product 7ga (66%) without formation of a cyclized
product 15 or 16 [Eq. (7)].[24] These facts indicate that either
free aryl radical intermediates are hardly generated from both
iodoarenes and arylsilanes, or the coupling involving a radical
intermediate[25] proceeds very quickly and smoothly.[26]
14
6e
4a
7ea, 72
15
6 f
4a
7 fa, 71
[a] Condition A (Entries 1 to 7): 3 (0.3 mmol), 4 (2.1 equiv), CuBr2
(0.1 equiv), Ph-Davephos (0.1 equiv), and CsF (2.5 equiv) in DMI (3m)
at 1508C for 24 h. Condition B (Entries 8 to 15): 6 (0.3 mmol), 4
(1.0 equiv), CuBr2 (0.1 equiv), Ph-Davephos (0.1 equiv), and CsF
(1,3 equiv) in DMI (3m) at 1508C for 24 h. [b] Isolated yields. [c] In DMI
(2m) at 1558C.
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À
silicon reagents are easily accessed by catalytic C H silyla-
tion, and thus rapid synthesis of oligoarenes via cross-
coupling is now available. Thus, our current efforts are
being focused on extending the scope of this reaction using
various aryltrialkylsilanes and understanding the reaction
mechanisms in detail.
Acknowledgements
This work was partially supported financially by Sumitomo
Chemicals, by a Grants in-Aid for Young Scientists (B)
(25870747 to Y.M.) from JSPS, and by ACT-C from JST.
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À
Keywords: arenes · C H silylation · copper · cross-coupling ·
silanes
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[21] Y. Nishihara, S. Noyori, T. Okamoto, M. Suetsugu, M. Iwasaki,
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[12] For a copper-mediated cross-coupling reaction, see: a) H. Ito, H.
Sensui, K. Arimoto, K. Miura, A. Hosomi, Chem. Lett. 1997, 639.
For copper-catalyzed cross-coupling reactions, see: b) S. K.
Gurung, S. Thapa, A. S. Vangala, R. Giri, Org. Lett. 2013, 15,
5378; c) A. Tsubouchi, D. Muramatsu, T. Takeda, Angew. Chem.
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Commun. 2014, 50, 8018; f) R. Giri, S. Thapa, Synlett 2015, 709.
[13] The fact that copper(I) salts were not effective for the reaction
indicates that the mechanism of the present cross-coupling is
different from the reported Cu(I)-catalyzed cross-coupling of
aryl(trialkoxy)silanes (Ref. [12]).
[14] It should be noted that the use of PPh3 as the most general
phosphine ligand instead of Ph-Davephos also produced 5aa in
82% yield under the same conditions of Table 1, Entry 2.
[15] Y. Hatanaka, K.-i. Goda, Y. Okahara, T. Hiyama, Tetrahedron
1994, 50, 8301.
[16] For selected reviews on cross-coupling polymerization, see: a) J.
Sakamoto, M. Rehahn, G. Wegner, A. D. Schlꢃter, Macromol.
Rapid Commun. 2009, 30, 653; b) M. J. Robb, S.-Y. Ku, C. J.
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A. J. McNeil, Macromolecules 2013, 46, 8395; e) T. Yokozawa, Y.
Nanashima, Y. Ohta, ACS Macro Lett. 2012, 1, 862.
[22] For examples, see: a) G. Sorin, R. Martinez Mallorquin, Y.
Contie, A. Baralle, M. Malacria, J.-P. Goddard, L. Fensterbank,
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8903; b) T. W. Liwosz, S. R. Chemler, Org. Lett. 2013, 15, 3034;
c) M. L. Deb, S. S. Dey, I. Bento, M. Teresa, T. Barros, C. D.
Maycock, Angew. Chem. Int. Ed. 2013, 52, 9791; Angew. Chem.
2013, 125, 9973.
[23] A. N. Abeywickrema, A. L. J. Beckwith, J. Chem. Soc. Chem.
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[24] Radical cyclization of N-allyl(2-halo-3-thienyl)carbamates was
reported. D. Brugier, F. Outurquin, C. Paulmier, J. Chem. Soc.
Perkin Trans. 1 2001, 37.
[25] For examples of reactions with silicates involving radical
intermediates, see: a) J. Yoshida, K. Tamao, T. Kakui, A.
Kurita, M. Murata, K. Yamada, K. Kumada, Organometallics
1982, 1, 369; b) V. Corcꢁ, L. Chamoreau, E. Derat, J.-P. Goddard,
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[26] The reaction of 6a with 4a was performed in the presence of
a catalytic amount of TEMPO, galvinoxyl, or BHT (bis-3,5-tert-
butyl-4-hydroxytoluene) as a radical scavenger and 7aa was
afforded in a lower yield, indicating that radical species may be
involved in this cross-coupling. Alternatively, such reagents may
simply deactivate the copper catalyst. For a proposed reaction
mechanism, see the Supporting Information. A full understand-
ing of the reaction mechanism will be the topic of future
investigation.
[17] The silicon reagents other than 6d and 6e were prepared by
dehydrogenative silylation. See the Supporting Information.
[18] Thiophene units having aryl groups at C2 position are effective
components for various functional oligomers and polymers. See
selected papers: a) J. Roncali, P. Blanchard, P. Frꢄre, J. Mater.
Chem. 2005, 15, 1589; b) A. Mishra, M. K. R. Fischer, P. Bꢅuerle,
Angew. Chem. Int. Ed. 2009, 48, 2474; Angew. Chem. 2009, 121,
2510; c) K. Takimiya, I. Osaka, M. Nakano, Chem. Mater. 2014,
26, 587; d) C. Wetzel, E. Brier, A. Vogt, A. Mishra, E. Mena-
Osteritz, P. Bꢅuerle, Angew. Chem. Int. Ed. 2015, 54, 12334;
Received: September 5, 2016
Revised: October 14, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Cross-Coupling Reactions
T. Komiyama, Y. Minami,*
T. Hiyama*
&&&& — &&&&
Aryl(triethyl)silanes for Biaryl and Teraryl
Synthesis by Copper(II)-Catalyzed Cross-
Coupling Reaction
Simple aryl(triethyl)silanes are demon-
strated to undergo cross-coupling with
iodoarenes to form biaryls and teraryls.
The reaction is characterized by a diverse
substrate scope, including 4,7-bis(trie-
thylsilyl)-5,6-difluorobenzothiadiazole
and thiophene derivatives, and the use of
CuBr2 as a crucial catalyst. The employed
aryl(triethyl)silanes are accessible by Ir-
À
catalyzed C H silylation.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!