(NHC)palladium complexes from hydroxy-functionalized imidazolium salts as catalyst for the microwave-accelerated fluorine-free Hiyama reaction

Article abstract of DOI:10.1002/ejoc.201101110

Palladium acetate reacts with 1-benzyl-3-(2-hydroxy-2-phenylethyl)-1H- imidazolium chloride to give the corresponding bis(carbene)palladium(II) complex 5 [(NHC)₂PdCl₂] by direct metalation. The X-ray diffraction structure of the complex is reported. In the presence of NaOH solution (50 % H₂O, w/w) in air and under microwave irradiation, the bis(NHC)pallalium(II) complex 5 is an active precatalyst for the Hiyama coupling of aryl halides with trialkoxy(aryl)silanes, providing biaryls and heterobiaryls. For some deactivated aryl bromides and for aryl chlorides, the corresponding cross-coupling reactions require tetrabutylammonium bromide (TBAB) as additive. The fluoride-free Hiyama reactions are performed with low (NHC)palladium loadings (0.1 mol-%). The active catalyst can also be prepared in situ from Pd(OAc)₂ and the corresponding imidazolium salt 4.

Full text of DOI:10.1002/ejoc.201101110

FULL PAPER
DOI: 10.1002/ejoc.201101110
(NHC)Palladium Complexes from Hydroxy-Functionalized Imidazolium Salts
as Catalyst for the Microwave-Accelerated Fluorine-Free Hiyama Reaction
Itziar Peñafiel,[a] Isidro M. Pastor,*[a] Miguel Yus,*[a] Miguel A. Esteruelas,*[b]
Montserrat Oliván,[b] and Enrique Oñate[b]
Dedicated to Professor Carmen Nájera on the occasion of her 60th birthday
Keywords: Carbene ligands / Cross-coupling / Nitrogen heterocycles / Palladium / Synthetic methods
Palladium acetate reacts with 1-benzyl-3-(2-hydroxy-2-
phenylethyl)-1H-imidazolium chloride to give the corre-
silanes, providing biaryls and heterobiaryls. For some deacti-
vated aryl bromides and for aryl chlorides, the corresponding
cross-coupling reactions require tetrabutylammonium bro-
mide (TBAB) as additive. The fluoride-free Hiyama reactions
are performed with low (NHC)palladium loadings (0.1 mol-
%). The active catalyst can also be prepared in situ from
Pd(OAc)2 and the corresponding imidazolium salt 4.
sponding bis(carbene)palladium(II) complex
5 [(NHC)2-
PdCl2] by direct metalation. The X-ray diffraction structure
of the complex is reported. In the presence of NaOH solution
(50% H2O, w/w) in air and under microwave irradiation, the
bis(NHC)pallalium(II) complex 5 is an active precatalyst for
the Hiyama coupling of aryl halides with trialkoxy(aryl)-
as a new type of ligands that can improve a range of cata-
lytic processes.[5]
Introduction
The pioneering work done independently by Wanzlick
Imidazolium derivatives with a range of functionalities
and Öfele showed the potential of N-heterocyclic carbenes have been employed in the preparation of different metal
(NHC) as ligands for transition metals.[1] NHCs are consid- complexes.[6–9] Hydroxy-functionalized imidazolium salts
ered as singlet carbenes having the carbene centre directly can be easily prepared by ring opening of epoxides.[10]
bonded to at least one nitrogen atom within the heterocy- Double deprotonation by employing 2 equiv. of base is usu-
cle.[2] In contrast to other carbenes, which usually have elec- ally employed to prepare the alkali-metal–carbene deriva-
trophilic character, NHCs behave as nucleophilic species tives, which are subsequently used in the preparation of
due to their strong σ-electron-donating properties. Thus, functionalized-NHC–transition-metal complexes.[11] Alter-
NHCs have a tendency to form stronger bonds with metal natively, direct metalation is possible by the use of basic
centres than most classical ligands, such as phosphanes.[3] metal precursors, which results in deprotonation of the
Indeed, the use of NHCs instead of phosphanes as ligands imidazolium salt and the formation of the corresponding
in many catalytic processes gives an advantage to the cata- NHC–metal complex.[12] Recently, we have reported the re-
lyst during the course of the reaction due to the ability of action of osmium hydride complex 1 with imidazolium salts
the NHC to transfer electron density to the metal centre.[4] that leads to the formation of the NHC–osmium complexes
In catalysis, phosphanes not only serve as simple ligands 2 and 3 by direct metalation of the imidazolium salts and
but can also be designed to support additional functional dehydrogenation of the hydroxy moiety (Scheme 1).[13]
groups that can modify the properties of the catalyst. It is
likely that functionalized NHC compounds can be prepared
[a] Departamento de Química Orgánica, Facultad de Ciencias and
Instituto de Síntesis Orgánica (ISO), Universidad de Alicante,
Apdo. 99, 03080 Alicante, Spain
Fax: +34-965-903-549
E-mail: ipastor@ua.es;
yus@ua.es
[b] Departamento de Química Inorgánica-Instituto de Síntesis
Química y Catálisis Homogénea (ISQCH), Universidad de
Zaragoza-CSIC,
Scheme 1. Formation of NHC–Os complexes by direct deproton-
ation.
50009 Zaragoza, Spain
E-mail: maester@unizar.es
7174
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Catalysts for the Microwave-Accelerated Fluorine-Free Hiyama Reaction
Metal-catalyzed cross-coupling reactions are particularly Sect.) as a consequence of the chirality of the CH carbon
useful transformations in modern organic synthesis, with atom of the CH2CH(OH)Ph substituent. Figure 1 shows
the palladium-based catalysts being of paramount impor- the X-ray structure of one of the diastereoisomers. It is a
tance.[14] Among the different organometallic reagents that trans square-planar isomer [C–Pd–C 179.5(2)°; Cl–Pd–Cl
can be employed in these transformations, the organosilicon 179.86(6)°], with Pd–C and Pd–Cl bond lengths of 2.019(5)
derivatives are very appealing because of their high stability, and 2.038(5) Å and 2.3021(15) and 2.3048(16) Å, respec-
low cost, non-toxicity, and ready accessibility. Despite these tively. One of the NHC ligands has (R) stereochemistry
advantages, the Hiyama coupling[15] has not received as [C(12)], and the second ligand has (S) stereochemistry
much attention as other cross-coupling processes,[16] such [C(30)].[23] The hydroxy hydrogen atoms point out towards
as the Negishi, Suzuki–Miyaura, and Stille reactions. An the chlorido ligands of adjacent molecules. The separations
essential characteristic of the Hiyama reaction is that the between these atoms of 2.34 [H(1A)···Cl(2A)] and 2.40
nucleophilicity of the organosilicon reagent needs to be en- [H(2A)···Cl(1A)] Å are smaller than the sum of the
hanced, usually by the addition of an anionic activator, with van der Waals radii of hydrogen and chlorine [rvdW(H) =
fluoride being the most commonly employed. The coupling 1.20 Å, rvdW(Cl) = 1.75 Å], which is in agreement with the
of aryl halides and organosilicon derivatives have been pro- presence of intermolecular hydrogen bonds.[24]
moted by the use of different bases (i.e., NaOH, KOH,
K2CO3, Cs2CO3, TMSOK, tBuOK) instead of the fluoride
ion.[17] More recently, Nájera and Alacid have described the
fluoride-free Hiyama reaction of trialkoxy(aryl)silanes and
trialkoxy(vinyl)silanes with aryl bromides and chlorides
using sodium hydroxide as silicon activator under conven-
tional heating and microwave irradiation[18] and employing
very low palladium loadings (0.001–1 mol-%).[19]
Palladium(II)–NHC complexes exhibit a remarkable sta-
bility, but have so far been examined only in a few palla-
dium-mediated reactions.[20] The combination of palladium
acetate with different imidazolium precursors, such as
Scheme 2. Preparation of [(NHC)2PdCl2] complex 5 by direct met-
alation.
IPr·HCl [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-
ylidene][21] has been shown to afford an efficient catalyst
for the Hiyama cross-coupling reaction of organosiloxanes
mediated by fluoride ion. A series of robust [(NHC)-
PdCl2(pyridine)] precatalysts derived from C4–C5-saturated
imidazolium salts have been shown to be active in the fluo-
ride-free Hiyama coupling of aryl halides with trimethoxy-
(phenyl and vinyl)silane in the presence of NaOH as base in
a mixed aqueous medium (dioxane/water).[22] Under these
reaction conditions, by using NHC ligands, high catalyst
loadings are needed (2–3 mol-% of Pd). We wish to report
on the preparation of a well-defined complex [(NHC)2-
PdCl2] by direct metalation of a hydroxy-functionalized
imidazolium salt and palladium acetate. The complex is
shown to catalyze the Hiyama cross-coupling reaction be-
tween arylsiloxanes and aryl halides under microwave irra-
diation and in the absence of fluoride ion.
Figure 1. Molecular diagram of complex 5. Selected bond lengths
[Å] and angles [°]: Pd(1)–C(1) 2.038(5), Pd(1)–C(19) 2.019(5),
Pd(1)–Cl(1A) 2.3021(15), Pd(1)–Cl(2A) 2.3048(16); C(1)–Pd(1)–
C(19) 179.5(2), Cl(1A)–Pd(1)–Cl(2A) 179.86(6), C(1)–Pd(1)–
Cl(1A) 92.41(14), C(1)–Pd(1)–Cl(2A) 87.72(14), C(19)–Pd(1)–
Cl(1A) 87.27(15), C(19)–Pd(1)–Cl(2A) 92.60(15).
Results and Discussion
Treatment of imidazolium chloride salt 4 with palladium
acetate in a 2:1 ratio under reflux in tetrahydrofuran led to
The bis(NHC)palladium complex 5 turned out to be an
the formation of the bis(carbene)palladium(II) complex 5 effective catalyst in the fluorine-free Hiyama cross-coupling
(Scheme 2). The basic precursor Pd(OAc)2 deprotonates the reaction. The initial experiments were carried out with 4-
imidazolium salt, generating acetic acid and the carbene li- bromoanisole (6a) and trimethoxy(phenyl)silane (7;
gands that coordinate to the palladium atom to afford the 1.5 equiv.) in the presence of complex 5 (0.1 mol-% of Pd)
final product. Complex 5 was isolated as an air-stable grey- and sodium hydroxide as base, giving the biaryl 8a (Table 1,
1
ish solid in 47% yield. The H and 13C{1H} NMR spectra Entries 1 and 2). First, the possibility of performing the re-
of the solid indicated that, in solution at room temperature, action under conventional heating (120 °C in a pressure
complex 5 exists as a mixture of diastereoisomers (see Exp. tube) or under microwave heating (120 °C with irradiation
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FULL PAPER
of 80 W) were studied,[25] with better yields in shorter times 5). Other sodium bases either gave lower yields than sodium
being obtained under the latter conditions (Table 1, En- hydroxide or induced no reaction at all (Table 2, Entries 6–
tries 1 and 2). The use of water as solvent, under conven- 8).
tional heating, gave the final product 8a with higher yield.
Similar results were obtained, under both heating condi-
Table 2. Base screening for the Hiyama cross-coupling reaction be-
tions, by performing the reaction in the presence of
tween 4-bromoanisole (6) and phenylsiloxane 7 under microwave
irradiation.[a]
Pd(OAc)2 and salt 4 in a 1:2 ratio (Table 1, compare En-
tries 3 and 4 with Entries 1 and 2, respectively). The use of
NHC as ligand had a positive effect on the reaction; when
the reaction was performed with Pd(OAc)2 in the absence
of salt 4, only 43% of the coupling product 8a was obtained
(Table 1, Entry 5). A decrease of the reaction time to 30 min
resulted in a reduction of the yield to 84% (Table 1, En-
try 6). The product 8a was obtained in lower yield with
both higher and lower amounts of the catalytic system
Pd(OAc)2/4 (Table 1, Entries 7 and 8, respectively). As ex-
pected, worse results were observed when a 1:1 ratio of pal-
ladium/imidazolium salt was employed (Table 1, Entry 9).
Entry
Base (equiv.)[b]
Yield [%][c]
1
2
3
4
5
6
7
8
LiOH (2.5)
NaOH (2.5)
NaOH (1.5)
KOH (2.5)
CsOH (2.5)
NaOtBu (2.5)
NaOAc (2.5)
Na2CO3 (2.5)
7
84 (10)[d]
60
72
13
51
74
–
Table 1. Hiyama cross-coupling reaction between 4-bromoanisole
(6a) and phenylsiloxane 7 by employing (NHC)palladium com-
plex.[a]
[a] Reaction performed with 4-bromoanisole (1 mmol) and phenyl-
siloxane (1.5 mmol), Pd(OAc)2 (0.001 mmol), imidazolium salt 4
(0.002 mmol), base (see column), microwave irradiation (80 W,
120 °C), 30 min. [b] The base was added as an aqueous solution of
50% (w/w). [c] Yields were calculated by GLC analysis employing
an internal standard (tridecane). [d] The yields when the NaOH
was added as a solid without any water are given.
Entry Catalyst (mol-%) Heating[b] Time [h]
Yield [%][c]
1
2
3
5 (0.1)
5 (0.1)
Δ
MW
Δ
18
1
18
41 (71)[d]
94
The optimal conditions were employed for the Hiyama
cross-coupling reactions of a range of aryl bromides 6 with
trimethoxy(phenyl)silane. Thus, a mixture of these reagents,
Pd(OAc)2 and NHC precursor 4 was irradiated in the mi-
crowave equipment (80 W, 120 °C, 1 h) in the presence of
NaOH (50%, w/w; aqueous solution), producing the corre-
sponding biaryl products 8 (Table 3). The coupling of both
deactivated aryl bromides [i.e., 4-bromoanisole (6a) and 4-
bromophenol (6b)] and activated 4-bromoacetophenone
(6c), with 7, produced the corresponding biaryl products
8a–c in good yield (Table 3, Entries 1–3). Similarly, the aryl-
ation of deactivated 1-bromo-2-methylbenzene proceeded
very successfully under the same reaction conditions with-
out the addition of any additive (Table 3, Entry 4).[18b] (Bi-
phenyl-4-yl)acetic acid[26] (8e) was prepared in 67% yield
by the reaction of 4-bromophenylacetic acid and siloxane 7
(Table 3, Entry 5). In contrast, the coupling of heteroaryl
bromides (i.e., 3-bromopyridine or 3-bromothiophene) with
phenylsiloxane gave lower yields (Table 3, Entries 6 and 8,
4 (0.2)
25 (69)[d]
Pd(OAc)2 (0.1)
4 (0.2)
Pd(OAc)2 (0.1)
Pd(OAc)2 (0.1)
4 (0.2)
Pd(OAc)2 (0.1)
4 (1)
Pd(OAc)2 (0.5)
4 (0.1)
4
MW
1
99 (92)[d]
5
6
MW
MW
1
0.5
43
84
7
8
9
MW
MW
MW
0.5
0.5
0.5
68
37
47
Pd(OAc)2 (0.05)
4 (0.1)
Pd(OAc)2 (0.1)
[a] Reaction performed with 4-bromoanisole (1 mmol) and phenyl-
siloxane (1.5 mmol) by using 2.5 equiv. of NaOH (50% aqueous
solution). [b] Conventional heating (Δ): pressure tube at 120 °C;
MW: microwave irradiation at 120 °C (80 W). [c] Yields were calcu-
lated by GLC analysis employing an internal standard (tridecane).
[d] The yields of the reaction with H2O (1 mL) as solvent are given
in parentheses.
The base (NaOH) was required to promote the Hiyama respectively). The use of 1 equiv. of tetrabutylammonium
reaction and favour the transmetalation process;[18c] as ex- bromide (TBAB), which is known to stabilize colloidal pal-
pected, the use of lower amounts of base reduced the yield ladium nanoparticles,[27] allowed the yield of 4-phenylpyr-
(Table 2, Entry 3). Thereafter, different bases were consid- idine (8f) (Table 3, Entry 7) to be improved. However, only
ered under the microwave conditions; however, no signifi- a slight increase in the yield of compound 8g was observed
cant improvement over the results obtained with sodium when TBAB was used (Table 3, Entry 9).
hydroxide were observed. It was also found that it was nec-
The cross-coupling reactions of a range of chloroarenes
essary to add the sodium hydroxide as a 50% aqueous solu- 9 with siloxane 7 to give the corresponding biaryls 8 were
tion (Table 2, Entry 2, footnote [d]). Other alkaline hydrox- performed with the catalytic system Pd(OAc)2/4 in the pres-
ides gave lower yields, although use of potassium salt was ence of TBAB and NaOH (50%, w/w; aqueous solution)
better than cesium or lithium salts (Table 2, Entries 1, 4 and under microwave irradiation (Table 3). In the absence of
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Catalysts for the Microwave-Accelerated Fluorine-Free Hiyama Reaction
Table 3. Hiyama coupling of aryl bromides and chlorides with tri- moarenes (Table 3, compare Entries 3 and 10, 1 and 12, and
methoxy(phenyl)silane by the catalytic system 4/Pd(OAc)2.[a]
7 and 13). Chlorobenzene derivatives bearing an electron-
withdrawing group [i.e., 4-chloroacetophenone and 1-
chloro-4-(trifluoromethyl)benzene] gave the final product
with good isolated yield (Table 3, Entries 10 and 11). Other
aryl chlorides, such as 4-chloroanisole, 2-chloropyridine, 3-
chloroaniline, and 4-chloroquinaldine, reacted to a lesser
extent, producing the final product with moderate to low
yields (Table 3, Entries 12–15).
Regarding other silicon-based reagents, aryl silatranes[28]
have been described as being good coupling reagents in the
fluoride-induced Hiyama reaction.[29] When phenylsilatrane
was treated with bromide 6a under the optimized condi-
tions to produce the corresponding product 8a, the yield
was lower than when siloxane 7 was used (Table 4, Entry 1).
Additionally, the commercially available trimethoxy(4-
methylphenyl)silane and triethoxy(4-methoxyphenyl)silane
were found to be suitable reagents for the Hiyama reaction
Table 4. Hiyama coupling of aryl bromides with siloxanes by the
catalytic system 4/Pd(OAc)2.[a]
[a] Reaction performed with trimethoxy(phenyl)silane (1.5 mmol)
and aryl halide (1 mmol) by using 2.5 equiv. of NaOH (50%, w/
w; aqueous solution), Pd(OAc)2 (0.001 mmol), imidazolium salt 4
(0.002 mmol), microwave irradiation (80 W, 120 °C), 1 h. [b] TBAB
(1 equiv. with respect to the aryl bromide) was used when indicated.
[c] Isolated yield of pure product after purification by flash
chromatography (silica gel; hexane/ethyl acetate mixtures); the yield
calculated by H1 NMR analysis using N,N-diphenylformamide as
[a] Reaction performed with silane derivative (1.5 mmol) and aryl
bromide (1 mmol) by using 2.5 equiv. of NaOH (50%, w/w; aque-
internal standard are given in parentheses. [d] Isolated yield of N-
acetyl-3-phenylaniline: the coupling product was acetylated prior
to purification.
ous solution), Pd(OAc)2 (0.001 mmol), imidazolium salt
4
(0.002 mmol), microwave irradiation (80 W, 120 °C), 1 h. [b] Iso-
lated yield of pure product after purification by flash chromatog-
raphy (silica gel; hexane/ethyl acetate mixtures); the yield calculated
by H1 NMR analysis using N,N-diphenylformamide as internal
standard is given in parentheses. [c] TBAB (1 equiv. with respect to
TBAB, the reaction with aryl chlorides did not give any
product in any case. As expected, the chloro derivatives gave
lower yields of compounds 8 than the corresponding bro- the aryl bromide) was used.
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I. M. Pastor, M. Yus, M. A. Esteruelas et al.
FULL PAPER
imidazolium salt 4 (1.0 mmol, 0.31 g) in THF (10 mL). The reac-
tion mixture was stirred at reflux temperature for 45 min. After
allowing the mixture to cool to room temperature, the resulting
mixture was concentrated to dryness. Diethyl ether was added to
the residue, and the mixture was stirred at room temperature for
2 h. The solid formed was filtered and recrystallized in acetone.
Yield: 0.18 g (47%); m.p. 200 °C (Me2CO; dec.). C36H36Cl2N4O2Pd
(734.01): calcd. C 59.01, H 4.95, N 7.65; found C 58.76, H 4.83, N
under the described conditions, although the coupling
products were isolated with only moderated yields (Table 4,
Entries 2–5).
Conclusions
The bis(NHC)PdII complex described here allows the
NaOH-promoted Hiyama reaction to be performed under
microwave irradiation. The fluoride-free catalytic system
needs low catalytic loadings compared to other (carbene)-
7.01. HRMS (ESI): calcd. for C36H36ClN4O2Pd [M
–
Cl]+
697.1566; found 697.1725; calcd. for C36H35N4O2Pd [M – 2 Cl –
H]+ 661.1803; found 661.1973. IR: ν = 3334 (br., O–H), 1061 [s,
˜
1
ν(C–O)]. H NMR (400 MHz, CD2Cl2): δ = 7.53–7.14 (40 H, Ph),
palladium complexes. The catalytic system can be prepared 6.92 (d, JH,H = 2 Hz, 1 H, CH im), 6.90 (d, JH,H = 2 Hz, 1 H, CH
im), 6.74 (d, JH,H = 2 Hz, 1 H, CH im), 6.73 (d, JH,H = 2 Hz, 1 H,
CH im), 6.66 (d, JH,H = 2 Hz, 1 H, CH im), 6.65 (d, JH,H = 2 Hz,
in situ by mixing the precursors [i.e., Pd(OAc)2 and the cor-
responding imidazolium salt 4] without the need to isolate
the resulting complex 5. Aryl bromides and chlorides give
coupling products with siloxanes with satisfactory results,
although chloride substrates require the inclusion of TBAB
as an additive, and the final products are obtained with
lower yields than when using the corresponding bromides.
1 H, CH im), 6.59 (d, JH,H = 2 Hz, 1 H, CH im), 6.53 (d, JH,H
=
2 Hz, 1 H, CH im), 5.86 (AB spin system, JA,B = 14.6 Hz, Δν =
47 Hz, 2 H, CH2Ph), 5.86 (AB spin system, JA,B = 14.8 Hz, Δν =
19 Hz, 2 H, CH2Ph), 5.67–5.65 (m, 4 H, CH2Ph), 5.59 (dd, JH,H
=
5.2, 6.8 Hz, 1 H, CHOH), 5.55 (dd, JH,H = 5.2, 7.6 Hz, 1 H,
CHOH), 5.42 (dd, JH,H = 3.6, 9.2 Hz, 1 H, CHOH), 5.40 (dd, JH,H
= 3.2, 8.8 Hz, 1 H, CHOH), 4.99–4.89 (m, 2 H, CH2), 4.75–6.62 (m,
6 H, CH2), 3.8 (v. br., OH) ppm. 13C NMR (100 MHz, CD2Cl2): δ
= 169.93, 169.89, 169.87, 169.85 (all s, Pd-C), 141.6, 141.5, 141.4,
136.6 (all s, Cipso-Ar), 129.6, 129.2, 129.1, 129.0, 128.9, 128.8, 128.7,
128.6, 128.4, 128.2, 128.1, 128.0, 126.5, 126.4, 126.3, 126.2 (all s,
CH-Ar), 123.0, 122.9, 122.8, 120.9, 120.8, 120.7 (all s, CH im),
73.9, 73.8 (both s, CHOH), 58.4, 58.3, 58.2 (all s, CH2Ph), 54.7,
54.4 [both s, CH2CH(OH)] ppm.
Experimental Section
General Remarks: All commercially available reagents (Acros, Ald-
rich, Fluka) were used without further purification. Melting points
were determined with a Reichert Thermovar hot plate apparatus
and are uncorrected. NMR spectra were recorded with a Bruker
Avance 300 or a Bruker Avance 400 (300 and 400 MHz for 1H
NMR, and 75 and 100 MHz for 13C NMR, respectively) by using
CDCl3 as solvent and TMS as internal standard, except where
otherwise stated; chemical shifts (δ) are given in ppm and coupling
constants (J) in Hz. Mass spectra (EI) were obtained at 70 eV with
an Agilent 5973 spectrometer, fragment ions are given as m/z values
with relative intensities (%) in parentheses. High-resolution electro-
spray mass spectra were acquired with a MicroTOF-Q hybrid qua-
drupole time-of-flight spectrometer (Bruker Daltonics, Bremen,
Germany). Elemental analyses were carried out with a Perkin–El-
mer 2400 CHNS/O analyzer. Infrared spectra were recorded with a
Perkin–Elmer Spectrum 100 spectrometer as neat solids. Analytical
TLC was performed on Merck aluminum sheets with silica gel 60
F254. Silica gel 60 (0.04–0.06 mm) was employed for flash
chromatography. Microwave reactions were performed with a CEM
Discover Synthesis Unit (CEM Corp., Matthews, NC) with a con-
tinuous focused microwave power delivery system in glass vessels
(10 mL) sealed with a septum under magnetic stirring.
General Procedure for the Hiyama Coupling of Aryl Halides with
Siloxanes: A 10 mL microwave vessel was charged with Pd(OAc)2
(0.001 mmol, 0.2 mg), imidazolium salt 4 (0.002 mmol, 0.6 mg),
aryl halide (1 mmol), arylsiloxane (1.5 mmol), and TBAB if indi-
cated (see Tables 3 and 4). An aqueous solution of NaOH (0.1 mL;
50%, w/w) was added dropwise, the vessel was sealed with a sep-
tum, and the mixture was heated in air at 120 °C by microwave
irradiation (80 W) for 60 min. After allowing the reaction mixture
to cool to room temperature, it was extracted with diethyl ether
(5ϫ 5 mL), and the combined organic layers were filtered through
a pad of Celite and anhydrous Na2SO4, and then concentrated. The
obtained crude product was purified either by recrystallization in
MeOH/H2O, or by flash chromatography on silica gel eluting with
diethyl ether/hexane mixtures.
4-Methoxybiphenyl (8a):[30] White solid; m.p. 91 °C (MeOH/H2O).
1H NMR (400 MHz, CDCl3): δ = 7.54 (m, 4 H), 7.42 (m, 2 H),
7.31 (m, 1 H), 6.98 (m, 2 H), 3.85 (s, 3 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 159.8, 141.5, 134.4, 129.4, 128.8, 127.4,
127.3, 114.8, 56.0 ppm. MS: m/z (%) = 184 (100) [M+], 169 (43),
141 (37), 115 (27).
Preparation of 3-Benzyl-1-(2-hydroxy-2-phenylethyl)imidazolium
Chloride (4):[13] Benzyl chloride (5.5 mmol, 0.63 mL) was added at
room temperature to a solution of 1-(2-hydroxy-2-phenylethyl)-
imidazole[10] (5.0 mmol, 0.94 g) in acetonitrile (15 mL), and the re-
sulting mixture was stirred at 80 °C for 16 h. The solution was con-
centrated to dryness, and acetone was added to the residue to af-
ford a white solid, which was washed with further portions of cold
acetone and dried in vacuo. Yield: 0.94 g (60%); m.p. 113–115 °C
4-Hydroxybiphenyl (8b):[31] White solid; m.p. 166–168 °C (MeOH/
1
H2O). H NMR (400 MHz, CDCl3): δ = 7.54 (m, 2 H), 7.48 (m, 2
H), 7.41 (m, 2 H), 7.30 (m, 1 H), 6.91 (m, 2 H), 1.25 (br. s, 1 H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 155.05, 140.7, 134.0, 128.7,
128.4, 115.6 ppm. MS: m/z (%) = 170 (100) [M+], 141 (15), 115
(15).
1
(Me2CO). H NMR (400 MHz, CDCl3): δ = 10.00 (s, 1 H), 7.45–
7.25 (m, 10 H), 7.22 (s, 1 H), 6.99 (s, 1 H), 6.34 (s, 1 H), 5.33 (s, 2
H), 5.26 (dd, J = 7.6, 3.2 Hz, 1 H), 4.73 (dd, J = 14.0, 3.2 Hz, 1
H), 4.39 (dd, J = 14.0, 7.6 Hz, 1 H) ppm. 13C NMR (100 MHz,
CDCl3): δ = 140.0, 137.4, 132.7, 129.4, 128.7, 128.5, 127.8, 126.0,
123.5, 120.6, 70.9, 56.9, 53.3 ppm.
4-Phenylacetophenone (8c):[32] White solid; m.p. 122–124 °C
(MeOH/H2O). 1H NMR (400 MHz, CDCl3): δ = 8.03 (m, 2 H),
7.68 (m, 2 H), 7.63 (m, 2 H), 7.50–7.37 (m, 3 H), 2.64 (s, 3 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 197.7 (CO), 145.7, 139.8, 135.8,
128.9, 128.8, 128.2, 127.3, 127.2, 26.6 ppm. MS: m/z (%) = 196 (51)
[M+], 180 (100), 153 (30).
Preparation of Complex 5: Pd(OAc)2 (0.53 mmol, 0.12 g) was added
at room temperature to a Schlenk flask containing a solution of
7178
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 7174–7181

Catalysts for the Microwave-Accelerated Fluorine-Free Hiyama Reaction
2-Methylbiphenyl (8d):[31] Yellow oil. 1H NMR (400 MHz, CDCl3): 130.2, 129.9, 126.9, 125.7, 113.4, 55.3, 20.5 ppm. MS: m/z (%) =
δ = 7.43–7.34 (m, 2 H), 7.33–7.30 (m, 3 H), 7.26–7.20 (m, 4 H), 198 (100) [M+], 183 (27), 167 (21), 165 (23), 155 (23), 153 (17).
2.27 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 141.9, 135.3,
4-(4-Methoxyphenyl)pyridine (8n):[38] Yellow solid; m.p. 53–55 °C
130.3, 129.7, 129.1, 128.0, 127.2, 126.7, 125.7, 20.4 ppm. MS: m/z
(MeOH/H2O). 1H NMR (400 MHz, CDCl3): δ = 8.61 (m, 2 H),
(%) = 168 (100) [M+], 167 (92), 153 (38).
7.60 (d, J = 11.7 Hz, 2 H), 7.46 (m, 2 H), 7.01 (d, J = 11.7 Hz, 2
H), 3.87 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 160.5,
147.7, 130.3, 150.1, 128.1, 121.0, 114.5, 55.4 ppm. MS: m/z (%) =
185 (100) [M+], 170 (25), 142 (30), 115 (18).
Biphenyl-4-ylacetic Acid (8e):[32] White solid; m.p. 163.164 °C
(MeOH/H2O). 1H NMR (400 MHz, CDCl3): δ = 7.56 (m, 9 H),
3.69 (s, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 177.0, 140.7,
140.3, 132.3, 129.7, 128.6, 127.6, 127.3, 127.2, 40.6 ppm. MS: m/z
(%) = 212 (77) [M+], 168 (20), 167 (100).
3-(4-Methoxyphenyl)thiophene (8o):[39] White solid; m.p. 128–
1
129 °C (MeOH/H2O). H NMR (300 MHz, CDCl3): δ = 7.52 (d, J
1
4-Phenylpyridine (8f):[32] Brown oil. H NMR (400 MHz, CDCl3):
= 8.9 Hz, 2 H), 7.34 (m, 3 H), 6.94 (d, J = 8.9 Hz, 2 H), 3.8 (s, 3
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 158.8, 142.0, 128.7,
127.5, 126.2, 126.0, 118.9, 114.2, 55.3 ppm. MS: m/z (%) = 190
(100) [M+], 175 (79), 147 (42).
δ = 8.67–8.65 (m, 2 H), 7.66–7.63 (m, 2 H), 7.52–7.44 (m, 5 H)
ppm. 13C NMR (100 MHz, CDCl3): δ = 139.3, 138.1, 150.7, 148.3,
129.1, 129.0, 127.0, 121.6 ppm. MS: m/z (%) = 155 (100) [M+], 128
(14), 127 (14).
X-ray Data: Suitable crystals for X-ray diffraction of 5 were ob-
tained by slow concentration of a solution in acetone. X-ray data
were collected with a Bruker Smart APEX CCD diffractometer
equipped with a normal focus, 2.4 kW sealed tube source (Mo radi-
ation, λ = 0.71073 Å) operating at 50 kV and 40 mA. Data were
collected over the complete sphere by a combination of four sets.
Each frame exposure time was 40 s covering 0.3° in ω. Data were
corrected for absorption by using a multiscan method applied with
the SADABS program.[40] The structure was solved by Patterson
(Pd atom) and conventional Fourier techniques and refined by full-
matrix least squares on F2 with SHELXL97.[41] Anisotropic param-
eters were used in the last cycles of refinement for all non-hydrogen
and non-disordered atoms. The hydrogen atoms were observed or
calculated and refined freely or by using a restricted riding model;
0.2 molecules of water were observed in the asymmetric unit as
crystallization solvent.
3-Phenylthiophene (8g):[31] White solid; m.p. 90–91 °C (MeOH/
H2O). H NMR (400 MHz, CDCl3): δ = 7.58 (d, J = 8.4 Hz, 2 H),
7.43–7.42 (m, 1 H), 7.40–7.34 (m, 4 H), 7.32–7.31 (m, 1 H) ppm.
13C NMR (100 MHz, CDCl3): δ = 142.3, 135.8, 128.7, 127.1, 126.4,
126.3, 126.2, 120.2 ppm. MS: m/z (%) = 160 (100) [M+], 128 (14),
127 (14).
1
4-(Trifluoromethyl)biphenyl (8h):[31] Colourless solid; m.p. 70–72 °C
(MeOH/H2O). 1H NMR (400 MHz, CDCl3): δ = 7.68 (s, 4 H), 7.59
(m, 2 H), 7.49–7.38 (m, 3 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 144.7, 139.7, 129.4 (2JC,F = 32 Hz), 128.9, 128.2, 127.4, 127.3,
125.7 (3JC,F = 4 Hz), 125.0 (1JC,F = 272 Hz) ppm. MS: m/z (%) =
202 (100) [M+], 202 (11), 152 (21).
2-Phenylpyridine (8i):[33] Colourless oil. 1H NMR (300 MHz,
CDCl3): δ = 8.67 (d, J = 7.6 Hz, 1 H), 8.00–7.97 (m, 2 H), 7.70–
7.68 (m, 2 H), 7.49–7.41 (m, 3 H), 7.20–7.16 (m, 1 H), 7.27–7.22
(m, 1 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 157.5, 149.7,
139.4, 136.7, 128.9, 128.7, 127.0, 126.8, 122.1, 120.6 ppm. MS: m/z
(%) = 156 (13) [M+ + 1], 155 (100) [M+], 154 (76), 127 (12), 77
(15).
Crystal Data for 5: C36H36Cl2N4O2Pd·0.2H2O; MW 737.59; colour-
less, irregular block (0.14ϫ0.10ϫ0.04 mm); monoclinic; space
group P21/c; a = 6.667(3) Å, b = 16.606(8) Å, c = 30.463(13) Å, β
= 94.749(15)°; V = 3361(3) Å3; Z = 4; Dcalcd. = 1.458 gcm–3; F(000)
= 1512; T = 100(2) K; μ = 0.750 mm–1. 27720 measured reflections
(2θ = 3–57°, ω scans 0.3°), 7868 unique (Rint = 0.1251); minimum/
maximum transmission factors 0.660/0.862. Final agreement fac-
tors were R1 = 0.0611 [4775 observed reflections, IϾ2σ(I)] and
wR2 = 0.1601; data/restraints/parameters 7868/12/404; GoF =
1.011. Largest peak/hole 1.020/–1.417 e/Å3. CCDC-836553 (5) con-
tains the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
N-(Biphenyl-3-yl)acetamide (8j):[34] Yellow solid; m.p. 142 °C
(MeOH/H2O). 1H NMR (400 MHz, CDCl3): δ = 7.70 (s, 1 H), 7.60
(d, J = 8.4 Hz, 2 H), 7.53 (d, J = 8.0 Hz, 1 H), 7.35–7.50 (m, 5 H),
2.23 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 167.2, 144.1,
143.2, 138.6, 128.9, 128.5, 125.7, 126.1, 123.6, 118.4, 118.2,
25.5 ppm. MS: m/z (%) = 211 (100) [M+], 196 (69), 168 (43), 153
(11).
2-Methyl-4-phenylquinoline (8k):[35] Yellow oil. 1H NMR
(400 MHz, CDCl3): δ = 8.09 (d, J = 8.5 Hz, 1 H), 7.85 (d, J =
8.4 Hz, 1 H), 7.69 (m, 1 H), 7.54–7.48 (m, 5 H), 7.44 (m, 1 H), 7.24
(s, 1 H), 2.78 (s, 3 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
142.3, 135.8, 158.5, 148.5, 138.1, 136.2, 129.5, 129.3, 129.0, 128.4,
128.3, 125.7, 125.6, 122.2, 25.3 ppm. MS: m/z (%) = 219 (100) [M+],
204 (16), 176 (11).
Acknowledgments
Financial support from the Ministerio de Ciencia e Innovación
(MICINN) of Spain (Project Nos. CTQ2007-65218, CTQ2008-
00810, Consolider Ingenio 2010 CSD2007-00006), the Generalitat
Valenciana (PROMETEO/2009/0349 and FEDER), the Diputa-
ción General de Aragón (E35), and the European Social Fund is
acknowledged.
4-(4-Methylphenyl)acetophenone (8l):[36] Colourless solid; m.p. 118–
119 °C (MeOH/H2O). 1H NMR (300 MHz, CDCl3): δ = 8.02 (app.
dt, J = 8.6, 1.9 Hz, 2 H), 7.67 (app. dt, J = 8.6, 1.9 Hz, 2 H), 7.54
(m, 2 H), 7.28 (m, 2 H), 2.63 (s, 3 H), 2.41 (s, 3 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 197.9, 145.7, 138.2, 136.9, 135.6, 129.7,
128.9, 127.1, 126.9, 26.6, 21.2 ppm. MS: m/z (%) = 210 (52) [M+],
195 (100), 165 (28), 152 (34).
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Eur. J. Org. Chem. 2011, 7174–7181
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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Received: July 29, 2011
Published Online: October 20, 2011
Eur. J. Org. Chem. 2011, 7174–7181
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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