Copper-catalyzed cross-coupling of aryl- and heteroaryltriethoxysilanes with aryl and heteroaryl iodides and bromides

Article abstract of DOI:10.1055/s-0033-1339108

Copper(I)-catalyzed coupling of aryl- and heteroaryltriethoxysilanes with aryl and heteroaryl iodides is described. The transformation also proceeds with activated aryl bromides, but in this case it requires a stoichiometric amount of the catalyst for best product yields. The current reaction requires a P,N-based bidentate ligand for aryl-aryl coupling while it proceeds without external ligands for aryl-heteroaryl coupling to afford good product yields. The reaction protocol can also be applied to achieve biarylation of diiodoarenes in reasonable yields. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1339108

SPECIAL TOPIC
▌1933
sCpeociapl tpopeicr-Catalyzed Cross-Coupling of Aryl- and Heteroaryltriethoxysilanes
with Aryl and Heteroaryl Iodides and Bromides
Copper-Catalyzed Cross-Coupling
Santosh K. Gurung, Surendra Thapa, Bijay Shrestha, Ramesh Giri*
Department of Chemistry & Chemical Biology, The University of New Mexico, Albuquerque, NM 87131, USA
Fax +1(505)2772609; E-mail: rgiri@unm.edu
Received: 28.02.2014; Accepted after revision: 13.04.2014
ation of (NHC)Cu(Ar) species, was also proposed previ-
Abstract: Copper(I)-catalyzed coupling of aryl- and heteroaryl-
ously by the Hoveyda group14 in the conjugate addition of
triethoxysilanes with aryl and heteroaryl iodides is described. The
these organosilanes to cyclic enones. Existence of similar
organocopper intermediates can also be envisaged in re-
transformation also proceeds with activated aryl bromides, but in
this case it requires a stoichiometric amount of the catalyst for best
product yields. The current reaction requires a P,N-based bidentate lated transformations catalyzed by copper salts that utilize
organosilicon reagents15 or alkynes6,16 as starting materi-
als.
ligand for aryl–aryl coupling while it proceeds without external li-
gands for aryl–heteroaryl coupling to afford good product yields.
The reaction protocol can also be applied to achieve biarylation of
Despite these literature precedents, copper-catalyzed cou-
plings of arylsilicon reagents with aryl halides were not
known until 2013.12 At that time we reported the first
Cu(I)-catalyzed cross-coupling of aryl- and heteroaryl-
triethoxysilanes with aryl- and heteroaryl iodides.17 We
discovered that a combination of a bidentate P,N-ligand
PN-1 and copper(I) iodide generated an active catalyst
that allowed the coupling reaction to proceed efficiently,
providing the biaryl products in good to excellent yields
(Scheme 1).
diiodoarenes in reasonable yields.
Key words: aryltriethoxysilane, biarylation, catalytic cycle, cross-
coupling, P,N-ligand
Organosilicon reagents remain some of the most widely
utilized compounds in organic chemistry due to their ease
of synthesis, handling, high stabilities, and low toxicities.1
In particular, their use in Hiyama coupling has made these
organosilicon compounds highly attractive in organic
synthesis for the formation of carbon–carbon (C–C)
bonds.1,2 Of note, these organometallic reagents have
been extensively utilized in the synthesis of natural prod-
ucts and pharmaceutical molecules.2c,d,3
Ar Si(OEt)3
CuI (10 mol%)
aryl or heteroaryl
Ar
PN-1 (10 mol%)
NMe2
+
R
CsF (1.5 equiv)
DMF, 120 °C, 24 h
I
PR2
Recently, tremendous advances have been made in em-
ploying the inexpensive and earth-abundant first row late
transition metals (Fe, Co, Ni, and Cu) in cross-coupling
reactions as palladium substitutes.4 In this regard, the Fu
group reported nickel-catalyzed cross-couplings of trialk-
oxy(aryl)silanes with alkyl halides; this demonstrated that
the Hiyama coupling could be conducted with non-noble
metals with a high level of efficacy.5 Copper-based cata-
lysts have also emerged recently as viable alternatives to
palladium to form C–C bonds for the Sonogashira,6
Stille,7 Suzuki–Miyaura,6d,8 and Heck9 couplings.10 One
case of Hiyama coupling promoted by stoichiometric
amounts of copper salts11 was previously reported using a
limited number of special molecules.12 Recently, the Ball
group demonstrated that trialkoxy(aryl)- and trialkoxy(al-
lyl)silanes could undergo facile transmetalation with
(NHC)Cu(F) (NHC = N-heterocyclic carbene) to generate
(NHC)Cu(Ar) and (NHC)Cu(allyl) complexes, which
were shown to be reactive towards aldehydes, acyl ha-
lides, and allylic substrates.13 Transmetalation of aryltri-
fluoro- and trifluoro(vinyl)silanes with (NHC)Cu(Br) in
the presence of a fluoride source, which led to the gener-
R
up to 94%
R = Ph (PN-1)
R = t-Bu (PN-2)
aryl or heteroaryl
Scheme 1 Copper(I)-catalyzed coupling of aryltriethoxysilanes with
aryl iodides
Based on our mechanistic studies with an independently
synthesized [PhCu] intermediate17 and utilizing literature
reports,13,14 we also proposed a catalytic cycle (Scheme 2)
for this reaction. Unlike the palladium(0)-based catalyst in
which oxidative addition of aryl halides occurs first, the
catalytic cycle utilizing the isoelectronic d10 copper(I)
complex begins with initial transmetalation of the organo-
silicon reagents followed by reaction of the newly gener-
ated (PN-1)CuAr1 species with aryl halides Ar2X. The
sequential occurrence of the elementary steps in this new
catalytic cycle18 was further supported by our recent de-
tailed mechanistic studies of a related (PN-2)Cu(I)-cata-
lyzed Suzuki–Miyaura coupling.8i
In this article, we would like to report our further studies
on the copper(I)-catalyzed Hiyama coupling that demon-
strates a broader synthetic scope using new substrates. We
further show that when the reaction is conducted in the
presence of the more sterically hindered and electron-rich
ligand PN-2, the reaction also proceeds with activated
SYNTHESIS 2014, 46, 1933–1937
Advanced online publication: 28.05.2014
DOI: 10.1055/s-0033-1339108; Art ID: ss-2014-c0146-st
© Georg Thieme Verlag Stuttgart · New York
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1934
S. K. Gurung et al.
SPECIAL TOPIC
Ar2
Table 1 Coupling of Aryltriethoxysilanes with Aryl Iodides in the
Presence of PN-1a
Ar1
(PN)Cu
Ar1 Ar2
Entry Ar1Si(OEt)3
Ar2I
Product, yieldb
Ar2
I
I
Cl
Ar1
(PN)Cu
(PN)Cu
I
Si(OEt)3
Cl
FSi(OEt)3 or
[F2Si(OEt)3]–[Cs]+
CsF
1
(PN)Cu
F
Ar1Si(OEt)3 or,
[Ar1SiF(OEt)3]–[Cs]+
CsI
I
OMe
MeO
Scheme 2 Proposed catalytic cycle
1, 36%
Br
Si(OEt)3
aryl bromides. In addition, we have demonstrated that the
current reaction protocol can be extended to dihaloarenes
for multiple arylation with organosilicon reagents.
Br
2
The results of our newer studies on the substrate scope for
aryl-aryl couplings in the presence of 10 mol% each of
CuI and PN-1 are summarized in Table 1. The reaction
tolerates both electron-rich and electron-poor aryl-
triethoxysilane reagents such as triethoxy(4-methoxyphe-
nyl)silane, triethoxy(4-tolyl)silane, and (4-chloro-
phenyl)triethoxysilane (Table 1, entries 1–5). Similarly,
the reaction can also be conducted with aryl iodides bear-
ing halide substituents such as bromide and chloride (en-
tries 1 and 2), in addition to electron-rich and electron-
poor aryl iodides (entries 3–5).
I
2, 68%
Cl
Si(OEt)3
OMe
3
I
Cl
MeO
1, 52%
We also demonstrated further substrate scope of the cur-
rent transformation for aryl-heteroaryl couplings and the
results are summarized in Table 2. Reactions with these
substrates containing at least one heterocyclic moiety ei-
ther on the organosilicon reagent or the aryl iodide were
conducted with 10 mol% copper(I) iodide in the absence
of ligands. The reaction proceeds well for aryl–heteroaryl
couplings and also advantageously tolerates halide sub-
stituents such as chloride and bromide, affording cross-
coupled products in good to excellent yields.
F3C
CF3
Si(OEt)3
F3C
CF3
4
I
OMe
OMe
3, 30%
F3C
We further demonstrated that the current reaction protocol
can also be applied to achieve biarylation of diiodoarenes
using both aryl- and heteroaryltriethoxysilanes in reason-
able yields. As shown in Table 3, phenyl- and thienyl-
triethoxysilanes were coupled with 1,2- and 1,4-
diiodobenzenes to afford 1,2-diphenylated and 1,2- and
1,4-dithienylated benzenes in 35–51% yields.
Si(OEt)3
F3C
5
I
OMe
OMe
4, 44%
a Reactions conditions: standard conditions from Scheme 1 on a 1.0-
mmol scale for 48 h.
We also found that when PN-1 is replaced in the reaction
system with the more electron-rich and sterically hindered
ligand PN-2, the reaction could be conducted with activat-
ed aryl bromides despite affording the coupled products in
low yields. However, when 50 mol% to stoichiometric
amounts of each of copper(I) iodide and PN-2 were used,
excellent yields of the products were obtained. As pre-
sented in Table 4, we were able to cross-couple a variety
of aryltriethoxysilanes with electron-poor aryl bromides
under these reaction conditions (entries 1–4).
b Isolated yields. Mass balance is accounted for by the unreacted start-
ing materials.
In summary, we have demonstrated a broad substrate
scope of the copper(I)-catalyzed Hiyama-type couplings.
The couplings of aryl- and heteroaryltriethoxysilanes pro-
ceed well with aryl and heteroaryl iodides and activated
aryl bromides. In general, the reaction requires a P,N-
based ligand (e.g., PN-1 or PN-2) for aryl–aryl couplings
Synthesis 2014, 46, 1933–1937
© Georg Thieme Verlag Stuttgart · New York

SPECIAL TOPIC
Copper-Catalyzed Cross-Coupling
1935
Table 3 Diarylation of Dihaloarenes with Aryl- and Heteroaryl-
Table 2 Coupling of Aryl- and Heteroaryltriethoxysilanes with Aryl
and Heteroaryl Iodides in the Absence of PN-1a
triethoxysilanesa
Entry Ar1Si(OEt)3
Ar2I
Product, yieldb
Entry Ar1Si(OEt)3
Ar2I
Product, yieldb
Si(OEt)3
I
Si(OEt)3
I
1
1
I
Cl
N
Cl
N
5, 58%
10, 35%c
OMe
Si(OEt)3
S
I
Si(OEt)3
I
2
2
S
Cl
N
I
OMe
Cl
N
S
6, 53%
11, 40%
OMe
Si(OEt)3
I
Si(OEt)3
I
I
3
3
S
S
S
Cl
Cl
N
OMe
N
12, 51%
7, 54%
a Reaction conditions: standard conditions from Scheme 1 on a 1.0-
mmol scale for 48 h for aryl-aryl couplings. Standard conditions from
Scheme 1 on a 1.0-mmol scale for 48 h for aryl–heteroaryl couplings
in the absence of PN-2.
OMe
Si(OEt)3
I
b Isolated yields. Mass balance is accounted for by the unreacted start-
ing materials.
N
4
N
N
c CuI (20 mol%) and PN-2 (20 mol%) were used.
N
OMe
8, 35%
MHz, respectively) and internally referenced to the residual solvent
signals of CDCl3 for 1H NMR at δ = 7.26 and for 13C CDCl3 at δ =
77.16, and C6F6 for 19F NMR at δ = –164.9. NMR spectra for new
compounds were recorded at the NMR Facilities, Department of
Chemistry and Chemical Biology, University of New Mexico.
Compounds 1–4, 6–8, 13, and 14 were identified based on their 1H
and 13C NMR spectra by comparison with the literature data of the
corresponding compounds.8i
Si(OEt)3
5
I
S
S
Br
Br
9, 78%
a Reactions conditions: standard conditions from Scheme 1 on a 1.0-
mmol scale for 48 h in the absence of PN-1. Less than 3% homocou-
pling product was observed in most cases.
CuI-Catalyzed Couplings of Aryltriethoxysilanes with Aryl Io-
dides and Bromides: General Procedure A (in the Presence of
Ligand PN-1 or PN-2)
b Isolated yields. Mass balance is accounted for by the unreacted start-
ing materials.
In a glovebox, aryl iodide (1.0 mmol), CsF (228 mg, 1.5 mmol), CuI
(19.0 mg, 0.10 mmol), and PN-1 ligand (30 mg, 0.10 mmol) were
weighed into a 4-dram borosilicate scintillation vial and dissolved
in DMF (5 mL). Aryltriethoxysilane (1.0 mmol) was then added to
the mixture, and the vial was tightly capped with a poly-seal cone-
lined urea cap (Wheaton). The mixture was taken out of the glove-
box and placed in an oil bath pre-heated to 120 °C with vigorous
stirring. After 48 h, the mixture was cooled to r.t., diluted with EtOAc
(15 mL) and washed with H2O (3 × 5 mL). The aqueous fraction
was back-extracted with EtOAc (3 × 5 mL). The combined organic
fractions were dried (Na2SO4) and cotton-filtered, and the solvent
was removed on a rotary evaporator. The product was purified by
column chromatography (silica gel, hexanes, hexanes–Et2O, or hex-
anes–EtOAc).
and proceeds in the absence of any ancillary ligands for
aryl–heteroaryl couplings. The current reaction protocol
can be easily extended to the biarylation of diiodoarenes,
affording the cross-coupled products in reasonable yields.
All the reactions and handling of chemicals were done inside a N2-
filled glovebox unless stated otherwise. All the glassware were
properly dried in an oven before use. Bulk solvents and chemicals
1
were purchased from commercial sources. H, 13C, and 19F NMR
spectra were recorded on a Bruker instrument (300, 75, and 282
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2014, 46, 1933–1937

1936
S. K. Gurung et al.
SPECIAL TOPIC
7-Chloro-4-(4-tolyl)quinoline (5)
Table 4 Coupling of Aryltriethoxysilanes with Aryl Bromides in the
Presence of PN-2a
Purification by column chromatography (silica gel) gave 5 (147.1
mg, 58%) as a white solid.
Entry Ar1Si(OEt)3
Ar2Br
Product, yieldb
1H NMR (300 MHz, CDCl3): δ = 2.41 (s, 3 H), 7.35 (m, 5 H), 7.44
(dd, J = 9.0, 2.1 Hz, 1 H), 7.88 (d, J = 9.0 Hz, 1 H), 8.153 (d, J = 2.4
Hz, 1 H), 8.92 (d, J = 4.2 Hz, 1 H).
13C NMR (75 MHz, CDCl3): δ = 21.4, 121.6, 125.5, 127.6, 128.9,
Br
129.5, 134.8, 135.3, 138.8, 148.8, 149.4, 151.20.
GC-MS (EI): m/z = 253.7 [M]+.
Si(OEt)3
1
2-(2-Bromophenyl)thiophene (9)19
Following general procedure B using triethoxy(2-thienyl)silane and
1-bromo-2-iodobenzene for 48 h with purification by column chro-
matography (silica gel, hexane) gave 9 (186.5 mg, 78%) as a color-
less oil.
1H NMR (300 MHz, CDCl3): δ = 7.12 (dd, J = 5.1, 3.6 Hz, 1 H),
7.19 (dt, J = 7.8, 1.8 Hz, 1 H), 7.33 (m, 2 H), 7.40 (dd, J = 5.1, 1.2
Hz, 1 H), 7.50 (dd, J = 7.5, 1.5 Hz, 1 H), 7.69 (dd, J = 8.1, 1.2 Hz,
1 H).
10, 64% (15%)c
CF3
Si(OEt)3
Br
2
13C NMR (75 MHz, CDCl3): δ = 123.0, 126.2, 127.1, 127.5, 127.9,
129.2, 132.1, 133.8, 135.5, 141.9.
GC-MS (EI): m/z = 239.0 [M]+.
F3C
13, 78% (23%)
p-Terphenyl (10)20
CF3
Reaction of triethoxy(phenyl)silane with 1,4-diiodobenzene (Table
3) with purification by column chromatography (silica gel, hexane)
gave 10 (81 mg, 35%) as a white solid. Reaction of triethoxy(phe-
nyl)silane with 4-bromobiphenyl (0.5-mmol scale, Table 4) also
gave 10 (74 mg, 64%) as a white solid.
1H NMR (300 MHz, CDCl3): δ = 7.36 (m, 2 H), 7.47 (m, 4 H), 7.66
(m, 8 H).
Si(OEt)3
Br
3
F3C
OMe
MeO
13C NMR (75 MHz, CDCl3): δ = 127.2, 127.5, 127.7, 129.0, 140.3,
140.9.
14, 53% (15%)
GC-MS (EI): m/z = 230.0 [M]+.
CF3
Si(OEt)3
Br
1,4-Bis(2-thienyl)benzene (11)21
Following general procedure B using triethoxy(2-thienyl)silane and
1,4-diiodobenzene for 48 h with purification by column chromatog-
raphy (silica gel, hexane) gave 11 (97 mg, 40%) as a white solid.
1H NMR (300 MHz, CDCl3): δ = 7.10 (dd, J = 5.1, 1.5 Hz, 2 H),
7.30 (dd, J = 5.1, 0.9 Hz, 2 H), 7.34 (dd, J = 3.6, 1.2 Hz, 2 H), 7.62
(s, 4 H).
13C NMR (75 MHz, CDCl3): δ = 123.2, 125.0, 126.4, 128.3, 133.6,
144.1.
GC-MS (EI): m/z = 242.1 [M]+.
4
CF3
MeO
OMe
4, 88% (20%)
a Reaction conditions: standard conditions from Scheme 1 on a 0.5-
mmol scale using CuI (100 mol%) and PN-2 (100 mol%) for 24 h.
b Isolated yields. The values in parenthesis represent GC yields when
CuI (10 mol%) and PN-2 (10 mol%) were used. Mass balance is ac-
counted for by the unreacted starting materials.
c CuI (50 mol%) and PN-2 (50 mol%) were used.
1,2-Bis(2-thienyl)benzene (12)
Following general procedure B using triethoxy(2-thienyl)silane and
1,2-diiodobenzene for 48 h with purification by column chromatog-
raphy (silica gel, hexane) gave 12 (123 mg, 51%) as a colorless oil.
1H NMR (300 MHz, CDCl3): δ = 7.18 (dd, J = 3.6, 1.2 Hz, 2 H),
7.25 (dd, J = 5.1, 1.5 Hz, 2 H), 7.56 (dd, J = 5.1, 1.2 Hz, 2 H), 7.66
(m, 2 H), 7.80 (m, 2 H).
13C NMR (75 MHz, CDCl3): δ = 126.1, 127.1, 127.2, 128.0, 131.2,
134.0, 142.8.
GC-MS (EI): m/z = 242.1 [M]+.
CuI-Catalyzed Couplings of Aryltriethoxysilanes with Aryl Io-
dides and Bromides: General Procedure B (in the Absence of
Ligand)
In a glovebox, aryl iodide (1.0 mmol), CsF (228 mg, 1.5 mmol), and
CuI (19.0 mg, 0.1 mmol) were weighed into a 4-dram borosilicate
scintillation vial and dissolved in DMF (5 mL). Aryltriethoxysilane
(1.0 mmol) was then added to the mixture, and the vial was tightly
capped with a poly-seal cone-lined urea cap (Wheaton). The mix-
ture was taken out of the glovebox and placed in an oil bath pre-
heated to 120 °C with vigorous stirring. After 48 h, the mixture was
cooled to r.t., diluted with EtOAc (15 mL), and washed with H2O (2
× 5 mL). The aqueous fraction was back-extracted with EtOAc (3 ×
5 mL). The combined organic fractions were dried (Na2SO4) and
cotton-filtered, and the solvent was removed on a rotary evaporator.
The product was purified by column chromatography (silica gel,
hexanes, hexanes–Et2O, or hexanes–EtOAc).
Acknowledgment
We thank the University of New Mexico (UNM) for financial sup-
port, and upgrades to the NMR (NSF grants CHE08-40523 and
CHE09-46690) and Mass Spectrometry Facilities. The Bruker X-
ray diffractometer was purchased via an NSF CRIF:MU award to
UNM (CHE04-43580).
Synthesis 2014, 46, 1933–1937
© Georg Thieme Verlag Stuttgart · New York

SPECIAL TOPIC
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