Diphenylphosphinite ionic liquid (IL-OPPh2): A solvent and ligand for palladium-catalyzed silylation and dehalogenation reaction of aryl halides with triethylsilane

Article abstract of DOI:10.1016/j.jorganchem.2010.01.001

The use of an imidazolium-based phosphinite ionic liquid (IL-OPPh₂) as both solvent and ligand for Pd offers an efficient catalytic system for silylation of aryl iodides, bromides and also chlorides by triethylsilane in the presence of Cs₂CO₃. In the absence of base, this system is also performed for catalytic dehalogenation of aryl halides. The ionic liquid containing its corresponding Pd(0) complex can be easily recovered and reused in several runs without losing its efficiency.

Full text of DOI:10.1016/j.jorganchem.2010.01.001

Journal of Organometallic Chemistry 695 (2010) 887–890
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Diphenylphosphinite ionic liquid (IL-OPPh2): A solvent and ligand
for palladium-catalyzed silylation and dehalogenation reaction
of aryl halides with triethylsilane
a,
b
Nasser Iranpoor a, , Habib Firouzabadi , Roya Azadi
*
*
a Department of Chemistry, Shiraz University, Shiraz 71454, Iran
b Department of Chemistry, College of Sciences, Shahid Chamran University, Ahvaz 61357-43169, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 19 May 2009
Received in revised form 12 December 2009
Accepted 5 January 2010
Available online 11 January 2010
The use of an imidazolium-based phosphinite ionic liquid (IL-OPPh2) as both solvent and ligand for Pd
offers an efficient catalytic system for silylation of aryl iodides, bromides and also chlorides by triethyl-
silane in the presence of Cs2CO3. In the absence of base, this system is also performed for catalytic dehalo-
genation of aryl halides. The ionic liquid containing its corresponding Pd(0) complex can be easily
recovered and reused in several runs without losing its efficiency.
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Ionic liquid
Palladium
Aryl halide
Silylation
Dehalogenation
Triethylsilane
1. Introduction
In the continuation of our studies on the use of diph-
enylphosphinite ionic liquid (IL-OPPh2) [23] both as potential com-
Arylsilanes are considered as one of the important intermedi-
ates in modern organic chemistry, and therefore, numerous syn-
thetic methods have been proposed for their preparation [1].
Traditional synthetic routes to arylsilanes consist of the reaction
of silicon electrophiles with aryl Grignard or aryllithium com-
pounds [2–4]. Drawback of this methodology is the limited num-
ber of substrates available. Another method is the catalytic
conversion of C–H bonds to C–Si bonds in the presence of transi-
tion-metal complexes [5,6]. Different research groups separately
reported on the transition-metal-catalyzed coupling reaction of
aryl halides with trialkoxysilanes [7–11]. Although, trialkylsilanes
(e.g., Et3SiH) have not been found suitable silylating agent in some
catalytic systems [12], recently, their use to attain the silylation of
aryl halides under Pd and Pt catalysis have been reported [13,14].
Similar transformation has been achieved using Pd2(dba)3/P(o-tol)3
and Me6Si2 as a silylating agent [15,16]. Conversion of aryl halides
to aryltrialkylsilane by trialkylsilane has also been reported under
rhodium catalyzes [17–19]. As part of the recent developments, io-
nic liquids as green solvents [20,21] have also been considered as
potential media for this purpose [22].
plexing agent and also media for Pd(0)-catalyzed reactions [24,25],
we now report on its use for the efficient silylation of ArX (X = I, Br,
Cl) with triethylsilane in the presence of Cs2CO3 and also dehalo-
genation of aryl halides in the absence of base at 80–120 °C
(Scheme 1).
Among the studied bases for silylation of bromobenzene with
PdCl2 in this ionic liquid, Cs2CO3 was found to be more suitable
(Table 1, entry 2). The silylation of aryl halides in the presence of
trialkylsilane is competitive with dehalogenation to form the re-
duced arene. In this reaction, it was observed that in the absence
of base, the reduced arene is formed as the major product (Table
1, entry 1). As shown in Table 1, entry 2, the reaction of bromoben-
zene with Et3SiH in the presence of Cs2CO3 gave only 5% benzene
as the side product. Therefore, we selected Cs2CO3 as the most suit-
able base under our optimized reaction conditions for the silylation
reaction.
Under our optimized reaction conditions, 0.5 mmole of IL-
OPPh2, 0.05 mmole of PdCl2, 1.5 mmole of Cs2CO3, and 1.5 mmole
of Et3SiH, the silylated products were obtained in good yields for
a wide array of aryl iodides and bromides at 80 °C and for chloro-
benzene at 120 °C. Triethylsilylbenzene and 4-triethylsilyltoluene
were obtained in high yields from the reaction of iodobenzene,
bromobenzene and 4-bromotoluene, respectively (Table 2, entries
1, 3, 4). The presence of an electron-donating substituent like
* Corresponding authors. Tel.: +98 711 2287600; fax: +98 711 2280926.
E-mail addresses: iranpoor@chem.susc.ac.ir (N. Iranpoor), firouzabadi@chem.
susc.ac.ir (H. Firouzabadi).
0022-328X/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.01.001

888
N. Iranpoor et al. / Journal of Organometallic Chemistry 695 (2010) 887–890
methoxy on the iodobenzene increased the reaction time from 1 to
SiEt3
2 h (Table 2, entries 1, 2). The longer reaction time of electron-defi-
cient bromides could be due to the lower solubility of these com-
pounds in this ionic liquid (Table 2, entries 5–7). By using 3-
bromopyridine and triethylsilane as substrates satisfactory result
was obtained and its silylated product was isolated in 90% yield
(Table 2, entry 9). Chlorobenzene was also successfully converted
into triethylsilylbenzene in the presence of this catalytic system
at 120 °C (Table 2, entry 10). Also, the results of dehalogenation
of aryl halides in the absence of base are tabulated in Table 2.
The desired products were simply isolated by diethyl ether
extraction. Then the remaining mixture which contains the ionic li-
quid and its corresponding Pd(0) complex was washed with water
followed by diethyl ether and dried under vacuum. The presence of
Pd(0) in the complex before and after use was studied by X-ray
Powder Diffraction. The powder X-ray diffraction (XRD) patterns
for the catalyst showed the expected pattern of Pd(0) (Fig. 1).
The same results were also obtained for the XRD patterns of the
catalysts after its use for at least six cycles in the coupling reaction
of bromobenzene with triethylsilane in the presence of Cs2CO3 to
give triethyl(phenyl)silane (Table 3). The obtained diffraction rings
can be ascribed to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) crystallo-
graphic planes of the Pd(0) particles which reveals the excellent
stability and recovery of the catalysts. In addition, the absence of
the peak at 420 nm due to Pd(II) supports the complete conversion
of Pd(II) to Pd(0).
PdCl2/ IL-OPPh2/Cs2CO3
80-120 oC, 1-7 h, 80-90%
X
R
R
Et3SiH
+
PdCl2/ IL-OPPh2
80-120 oC, 0.5-5 h, 84-91%
R
R=Me, OMe, CN,
COMe, NO2, Cl
X= Cl, Br, I
OPPh2
H3C
N
N
PF6
IL-OPPh2
Scheme 1.
Table 1
Effect of different bases on silylation of bromobenzene.a
Entry
Base
Time (h)
Silylation (dehalogenation)
Conversionb (%)
1
2
3
4
5
None
3
2
0 (100)
95 (5)
Cs2CO3
Na2CO3
NaOAc
Et3N
3
1
1.5
80 (20)
85 (15)
90 (10)
Although, the exact mechanism of these reactions is not clear,
we proposed that the silylation reactions could occur through the
generally accepted PdCl2/IL-OPPh2/base association with genera-
tion of the active species Pd(0) followed by the oxidative addition
of triethylsilane and aryl halide to give Pd(IV) species (I). In the
presence of base, the rapid base-promoted elimination of HX fol-
a
Reaction conditions: 0.5 mmol of IL-OPPh2, 0.05 mmole of PdCl2, 1.0 mmole of
bromobenzene, 1.5 mmole of Et3SiH and 1.5 mmole of base.
b
GC yield of benzene as dehalogenated product is shown in the parentheses.
Table 2
PdCl2/IL-OPPh2-catalyzed silylation and dehalogenation of aryl halides with Et3SiH.
Entry
ArX
Silylation reactiona
Dehalogenation reactionb
Time (h)
1
Isolated yieldc (%)
85 (7)
Time (h)
0.5
Conversiond (isolated) (%)
1
2
3
4
5
6
100
I
2
2
2
4
5
80 (5)
85 (5)
83 (5)
80 (8)
82 (7)
1.5
1
100 (91)
100
MeO
I
Br
1.5
4.5
4
100
Br
95 (84)
97 (90)
NC
O
Br
Br
Br
7
8
9
6
5
5
80 (8)
85 (5)
90 (7)
4
89 (80)
93 (87)
90 (85)
O2N
Cl
4
Br
4.5
Br
N
10
7
89 (4)e
5
88e
Cl
a
b
c
The reaction was performed in the presence of Cs2CO3 as the base.
The reaction was performed without adding Cs2CO3.
GC yield of the dehalogenated product is shown in the parentheses.
GC yield using n-octane as an internal standard and the isolated yield is shown in the parentheses.
The reaction was performed at 120 °C.
d
e

N. Iranpoor et al. / Journal of Organometallic Chemistry 695 (2010) 887–890
889
Fig. 1. Powder XRD patterns resulted from the mixture of PdCl2 and IL-OPPh2.
Table 3
PdCl2, IL-OPPh2
Silylation of bromobenzene using recycled ionic liquid and its Pd complex.
base
Cycle
Conversion (%)
Yielda (%)
Et3SiH
1
2
3
4
5
6
7
100
100
100
100
100
100
93
85
83
80
80
82
82
74
ArSiEt3
(IL-OPPh2)2Pd(0)
SiEt3X
II
SiEt3
H
II
SiEt3
X
SiEt3
Ar
II
(IL-OPPh2)2Pd
a
(IL-OPPh )
(IL-OPPh2)2Pd
2 2 Pd
Isolated yields.
base
HX
Path b
ArH
rich and also neutral aryl halides using triethylsilane as silylating
reagents in the presence of Cs2CO3. This catalytic system can also
be applied for dehalogenation reaction of aryl halides in the ab-
sence of base. The reusability of this phosphinite ionic liquid to-
gether with its IL/Pd(0) complex, simplicity of the isolation of the
product by extraction in diethyl ether, and the possibility of apply-
ing the system to aryl iodides, bromides and chlorides can also be
considered as strong practical advantages of this method.
Ar-X
Path a
Ar
SiEt3
IV
base
(IL-OPPh )
2 2Pd
H
X
(I)
Scheme 2.
lowed by the reductive elimination can offer the silylated product
(Ar-SiEt3) together with the Pd(0) complex to continue the cycle
(Path a). In the absence of base, the reductive elimination of (I) of-
fers ArH and Et3SiX (Path b) (Scheme 2). In order to see the effect of
produced HX on the silylation process, we performed a reaction be-
tween triethyl(phenyl)silane and equimolar of HBr in diph-
enylphosphinite ionic liquid at 80 °C for 5 h. Monitoring of the
reaction did not show the formation of any product and the unre-
acted triethyl(phenyl)silane was isolated in 81% yield. This result
shows that in the silylation process, the produced HX has no effect
on the silylated product.
3. Experimental
The chemicals were obtained from Fluka or Merck chemical
companies and used without further purification. The progress of
the reactions was followed with TLC using silica gel SILG/UV 254
plates or GLC on a Shimadzu model GC-10A instrument. IR spectra
were run on a Shimadzu FTIR-8300 spectrophotometer. The 1H
NMR and 13C NMR spectra were recorded on a Bruker Avance
DPX-250, FT-NMR spectrometer. X-ray diffractions were obtained
using XRD, D8, Avance, Bruker, axs.
3.1. Typical procedure for the silylation of bromobenzene with
triethylsilane in the presence of PdCl2/IL-OPPh2
2. Conclusion
In summary, the present paper shows that this phosphinite io-
nic liquid (IL-OPPh2) as both the reaction media and ligand for
Pd(0) proved to be a recyclable and efficient catalytic system for di-
rect trialkylsilyl transfer to varieties of electron-deficient, electron-
PdCl2 (0.05 mmole, 8.83 mg) and CsCO3 (1.5 mmole, 0.29 g)
were added to a flask equipped with a condenser containing IL-
OPPh2 (0.5 mmole, 0.23 g). The flask was placed in an 80 °C oil bath
and stirred for 15 min. Then Et3SiH (1.5 mmole, 0.19 mL) and bro-

890
N. Iranpoor et al. / Journal of Organometallic Chemistry 695 (2010) 887–890
mobenzene (1.0 mmole, 0.105 mL) were added to the mixture,
respectively. GC and TLC of the reaction mixture showed the com-
pletion of the reaction after 2 h. After completion of the reaction,
the mixture was cooled to room temperature and tri-
ethyl(phenyl)silane was extracted with diethyl ether (3 Â 5 mL).
Evaporation of the solvent followed by chromatography on a short
column of silica gel using n-hexane/ethyl acetate (5/1) as eluent
gave triethyl(phenyl)silane in 85% yield. 1H NMR (250 MHz, CDCl3)
7.55–7.50 (m, 3H), 6.88–6.85 (m, 2H), 0.98 (t, 9H, J = 7.5 Hz), 0.80
(q, 6H, J = 7.5 Hz) ppm; 13C NMR (60 MHz, CDCl3) 142.3, 133.2,
128.1, 126.5, 7.5, 3.4 ppm.
followed by chromatography on a short column of silica gel using
n-hexane/ethyl acetate (5/1) as eluent gave triethyl(phenyl)silane
in 83% yield. The remaining mixture which contains the ionic li-
quid and its corresponding Pd complex was washed with water
(2 mL) followed by diethyl ether (2 mL) and dried in vacuum. The
obtained mixture was reused as mentioned above for the next run.
Acknowledgments
We are thankful for a grant for this work from Organization of
Management and Planning of Iran and also the Shiraz University
Research council.
3.2. Typical procedure for the dehalogenation of 4-bromoacetophenone
with triethylsilane in the presence of PdCl2/IL-OPPh2
References
To a flask equipped with a condenser containing IL-OPPh2
(0.5 mmole, 0.23 g), PdCl2 (0.05 mmole, 8.83 mg) was added. The
flask was placed in an 80 °C oil bath for aryl iodides and bromides
and at 120 °C for aryl chlorides and stirred for 15 min. Then, Et3SiH
(1.5 mmole, 0.19 mL) and 4-bromoacetophenone (1.0 mmole,
0.19 g) were added to the mixture, respectively. GC and TLC anal-
ysis of the reaction mixture showed the completion of the reaction
after 4 h. After completion of reaction, the mixture was cooled to
room temperature and acetophenone was extracted with diethyl
ether (3 Â 5 mL). Evaporation of the solvent gave the crude product
which was purified by chromatography on a short column of silica
gel using n-hexane/ethyl acetate (5/1) as eluent gave acetophe-
none in 90% yield. 1H NMR (250 MHz, CDCl3), 7.86 (d, 2H), 7.37–
7.45 (m, 3H), 2.76 (s, 3H) ppm; 13C NMR (60 MHz, CDCl3) 197.5,
136.8, 133.2, 128.7, 128.8, 27.8 ppm.
[1] K.A. Horn, Chem. Rev. 95 (1995) 1317.
[2] S. Lulinski, J. Serwatowski, J. Org. Chem. 68 (2003) 9384–9388.
[3] K. Nishide, T. Miyamoto, K. Kumar, S.I. Ohsugi, M. Node, Tetrahedron Lett. 43
(2002) 8569–8573.
[4] A.S. Manoso, C. Ahn, A. Soheili, C.J. Handy, R. Correira, W.M. Seganish, P.
DeShong, J. Org. Chem. 69 (2004) 8305–8314.
[5] N. Tsukada, J.F. Hartwig, J. Am. Chem. Soc. 127 (2005) 5022.
[6] S.B. Bennet, C. Eaborn, R.A. Jackson, R. Pearce, J. Organomet. Chem. 28 (1971)
59.
[7] M. Murata, K. Suzuki, S. Watanabe, Y. Masuda, J. Org. Chem. 62 (1997) 8569.
[8] A.S. Manoso, P. DeShong, J. Org. Chem. 66 (2001) 7449.
[9] S.E. Denmark, J.M. Kallemeyn, Org. Lett. 5 (2003) 3483.
[10] K. Komuro, K. Ishizaki, H. Suzuki, Touagouseikenkyu-nenpo 6 (2003) 24.
[11] C.J. Handy, A.S. Manoso, W.T. McElroy, W.M. Seganish, P. DeShong,
Tetrahedron 61 (2005) 12201.
[12] R. Boukherroub, C. Chatgilialoglu, G. Manuel, Organometallics 15 (1996) 1508.
[13] Y. Yamanoi, J. Org. Chem. 70 (2005) 9607–9609.
[14] A. Hamze, O. Provot, M. Alami, J.-D. Brion, Org. Lett. 7 (2006) 931.
[15] M. Murata, H. Ohara, R. Oiwa, S. Watanabe, Y. Masuda, Synthesis (2006) 1771.
[16] E. McNeill, T.E. Barder, S.L. Buchwald, Org. Lett. 9 (2007) 3785.
[17] Y. Yamanoi, H. Nishihara, Tetrahedron Lett. (2006) 7157.
[18] Y. Yamanoi, T. Taira, J. Sato, I. Nakamula, H. Nishihara, Org. Lett. 9 (2007) 4543.
[19] Y. Yamanoi, H. Nishihara, J. Org. Chem. 73 (2008) 6671.
[20] (a) T. Tu, W. Assenmacher, H. Peterlik, G. Schnakenburg, K.H. Dötz, Angew.
Chem., Int. Ed. 47 (2008) 7127;
3.3. Typical procedure for the silylation of bromobenzene with
triethylsilane in the presence of recycled ionic liquid and its Pd
complex
(b) E. Elamparuthi, T. Linker, Angew. Chem., Int. Ed. 48 (2009) 1853.
[21] (a) V. Strehmel, H. Rexhausen, P. Strauch, Tetrahedron Lett. 51 (2010) 747;
(b) M. Anouti, J. Jones, A. Boisset, J. Jacquemin, M. Caillon-Caravanier, D.
Lemordant, J. Colloid Interf. Sci. 340 (2009) 104;
(c) H.B. Han, J. Nie, K. Liu, W.K. Li, W.F. Feng, M. Armand, H. Matsumoto, Z.B.
Zhou, Electrochim. Acta 55 (2010) 1221.
[22] M. Iizuka, Y. Kondo, Eur. J. Org. Chem. (2008) 1161.
[23] N. Iranpoor, H. Firouzabadi, R. Azadi, Tetrahedron Lett. 47 (2006) 5531–5534.
[24] N. Iranpoor, H. Firouzabadi, R. Azadi, Eur. J. Org. Chem. (2007) 2197–2201.
[25] N. Iranpoor, H. Firouzabadi, R. Azadi, J. Organomet. Chem. 693 (2008) 2469–
2472.
To a flask equipped with a condenser containing recycled ionic
liquid and its Pd complex; CsCO3 (1.5 mmole, 0.29 g), Et3SiH
(1.5 mmole, 0.19 mL) and bromobenzene (1.0 mmole, 0.105 mL)
were added, respectively. The flask was placed in an 80 °C oil bath.
GC and TLC of the reaction mixture showed the completion of the
reaction after 2 h. After completion of the reaction, the mixture
was cooled to room temperature and triethyl(phenyl)silane was
extracted with diethyl ether (3 Â 5 mL). Evaporation of the solvent