Imidazolium-based phosphinite ionic liquid (IL-OPPh2) as Pd ligand and solvent for selective dehalogenation or homocoupling of aryl halides

Article abstract of DOI:10.1016/j.jorganchem.2008.04.037

The use of an imidazolium-based phosphinite ionic liquid (IL-OPPh₂) as ligand for Pd offers an efficient reagent system for the selective dehalogenation or homocoupling of aryl halides in the presence of NaOPrⁱ or Et₃N, respectively. This ionic liquid plays a dual role as both the reaction media and also as the potential complexing agent with Pd via its phosphinite carrying group. The ionic liquid containing its corresponding Pd complex can be easily recovered and reused in several runs.

Full text of DOI:10.1016/j.jorganchem.2008.04.037

Journal of Organometallic Chemistry 693 (2008) 2469–2472
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Imidazolium-based phosphinite ionic liquid (IL-OPPh ) as Pd ligand and solvent
2
for selective dehalogenation or homocoupling of aryl halides
*
*
Nasser Iranpoor , Habib Firouzabadi , Roya Azadi
Department of Chemistry, Shiraz University, Shiraz 71454, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The use of an imidazolium-based phosphinite ionic liquid (IL-OPPh ) as ligand for Pd offers an efficient
2
Received 20 January 2008
Received in revised form 15 April 2008
Accepted 28 April 2008
reagent system for the selective dehalogenation or homocoupling of aryl halides in the presence of NaO-
i
Pr or Et
3
N, respectively. This ionic liquid plays a dual role as both the reaction media and also as the
potential complexing agent with Pd via its phosphinite carrying group. The ionic liquid containing its cor-
responding Pd complex can be easily recovered and reused in several runs.
Ó 2008 Elsevier B.V. All rights reserved.
Available online 6 May 2008
Keywords:
Phosphinite
Ionic liquid
Palladium
Aryl halide
Dehalogenation
Homocoupling
1
. Introduction
Much study has been devoted to develop new methods for
2. Results and discussion
We have recently introduced an imidazolium-based phosphi-
nite ionic liquid (Scheme 1, IL-OPPh ) for the Heck reaction[18].
This ionic liquid plays a dual role both as a potential complexing
agent via its phosphinite carrying group with Pd and also as the
media. Here, we report a new application of this ionic liquid as sol-
dehalogenation of aromatic halides from the synthetic [1] and
environmental [2] points of view. Usually the systems which have
been developed for the reduction of halogenated arenes employ
inorganic hydrides as hydrogen source and a variety of transi-
tion-metals in DMF, dioxane, toluene and or methanol as solvent
2
vent and ligand with catalytic amounts of PdCl
2
for the highly
[
3–12]. The use of ambient hydrogen pressure (balloon) as a source
selective and efficient dehalogenation or homocoupling of aryl ha-
lides in the presence of NaOPr or Et N, respectively, at 80 °C
3
i
of hydrogen with Pd/C-Et N for the dehalogenation of aryl chlo-
3
rides suffers from lack of chemoselectivity. With this system, the
reduction of the nitro group has also been observed [13].
(Scheme 1). This dual property provides the possibility of recycling
of the ionic liquid along with its corresponding Pd complex as cat-
alyst and eliminates the waste of the ligand and also the Pd
compound.
The desired products were simply isolated by diethyl ether
extraction. The use of this ionic liquid also offers avoiding the
use of high boiling solvents, which their separation from the reac-
tion mixtures is usually a tedious process.
Cross-coupling reactions have also been widely used by chem-
ists for the synthesis of biaryls via formation of carbon–carbon
bonds [14]. Early researches have focused mainly on finding new
coupling partners, however, attention during the last decade has
turned towards the development of milder reaction conditions
using new catalysts and reaction systems. In order to develop cat-
alysts that can operate at very low metal loading, the complex of
Pd with phosphines [15] and with N-heterocyclic carbenes (NHCs)
Among the studied bases for dehalogenation of bromobenzene
i
with PdCl
2
in this ionic liquid, NaOPr was found to be more suit-
[
16] in organic solvents has been the subject of intensive studies.
able for dehalogenation, probably due to the presence of b-hydro-
As part of the recent developments, ionic liquids as green solvents,
have also been considered as potential media for this purpose [17].
gen which can be transferred to reduce the aryl halide (Table 1,
Entry 1). Et
3
N and NaOAc were found to be more efficient for
N and
homocoupling of bromobenzene. The more reactivity of Et
3
NaOAc compared with sodium and cesium carbonates could be
*
Corresponding authors. Tel.: +98 711 2287600; fax: +98 711 2280926.
E-mail addresses: iranpoor@chem.susc.ac.ir (N. Iranpoor), firouzabadi@chem.
due to their more solubility in the reaction media (Table 1, Entries
i
susc.ac.ir (H. Firouzabadi).
4 and 5). We therefore selected NaOPr and Et
3
N as the suitable
0
022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.04.037

2
470
N. Iranpoor et al. / Journal of Organometallic Chemistry 693 (2008) 2469–2472
i
o
bromotoluene after 0.5 and 5 h, respectively (Table 2, Entries 1 and
NaOPr , 80 C
X
2). For electron-deficient bromides, elongations of the reaction
times along with the decrement of the yields of the products were
observed (Table 2, Entries 3–5). This is most probably due to the
less solubility of 4-bromobenzonitrile, 4-bromoacetophenone and
1-bromo-4-nitrobenzene in this ionic liquid. This method showed
to be useful for dehalogenation of compounds carrying sensitive
functional groups such as nitro, ketone and nitrile, so they re-
mained intact in the reaction mixture and were not reduced. Also,
PdCl (5 mole%)
2
R
o
Et N, 80 C
3
IL-OPPh2
R
R
R
R= Me, OMe, CN,
COMe, NO , Cl,
CO H, OH
OPPh2
N
N
2
PF6
2
2 2
we applied this PdCl /IL-OPPh complex for the dehalogenation of
X= Cl, Br, I
IL-OPPh2
heteroaryl halides. 3-Bromopyridine and 3-bromothiophene were
dehalogenated satisfactorily and the desired products were iso-
lated in 68% and 93% yields, respectively (Table 2, Entries 7 and
Scheme 1.
8
). The use of this system for dehalogenation of aryl chlorides
was also successful (Table 2, Entries 9–11). As the results shown
in Table 2, Entries 10 and 11 demonstrate that the reaction is also
applicable upon substrates carrying carboxylic and phenolic func-
tionalities. The presented data in Table 2, Entries 5 and 6 also show
the selective debromination reaction of 4-bromonitrobenzene and
4-chloro-bromobenzene without reduction of the reducible func-
tional groups in the molecule. Aryl iodides under present condi-
tions produce the coupling products rather than dehalogenation.
This observation is in agreement with the reported methods in
Table 1
Effect of different bases on dehalogenation and homocoupling of bromobenzene
a
Entry
Base
Time (h)
Yield (%) b
100c
i
1
2
3
4
5
NaOPr
1
8
8
5
1
d
Na
Cs
NaOAc
Et
2
CO
3
50
80
d
2
CO
3
100d
100d
3
N
a
Reaction conditions: 0.5 mmole of IL-OPPh
2
, 0.05 mmole of PdCl
2
, 1.0 mmole of
the literature for aryl iodides [3–5].
bromobenzene and 2.0 mmole of base.
i
By changing the base from NaOPr to Et
3
N, homocoupling was
b
GC yield using n-octane as internal standard.
Dehalogenation product.
Homocoupling product.
c
performed successfully for different substituted aryl halides. For
this purpose, bromobenzene (Table 3, Entry 3) was treated as a
d
2 2
model compound with 0.5 mmole IL-OPPh , 0.05 mmole of PdCl ,
and 2.0 mmole of Et N. The homocoupling product was obtained
3
bases for dehalogenation and homocoupling of aryl halides,
respectively.
in high yield. The optimized conditions were then applied to other
aryl halides to obtain homocoupling products. The results of this
investigation are shown in Table 3.
2
Under our optimized reaction conditions, 0.5 mmole IL-OPPh ,
i
0
2
.05 mmole of PdCl , and 2.0 mmole of NaOPr , the dehalogenated
products were obtained in excellent yields for a wide array of aryl
bromides and chlorides at 80 °C (Table 2). For example: a complete
conversion was obtained for the reaction of bromobenzene and 4-
Table 3
Homocoupling of aryl halides with PdCl
2
2 3
/IL-OPPh system using Et N as base
Entry
1
Ar-X
Product
Time (h)
0.25
Yield (%)a
95
Table 2
I
i
Dehalogenation of aryl halides with PdCl
2
/IL-OPPh
2
system using NaOPr as base
Entry
1
Ar-X
Product
Time (h)
0.5
Yield (%)a
2
3
4
5
6
7
8
9
MeO
I
MeO
OMe
1
0.25
15
15
15
15
12
15
90
90
75
62
69
58
80
60
89
Br
Br
2
3
4
5
6
Br
Br
Br
Br
Br
5
88
85
80
72
85
Br
NC
O
NC
O
13
15
15
13
NC
O
Br
NC
O
CN
O
Br
O N
O N
2
2
O N
Br
O N
NO2
2
2
Cl
N
S
Cl
Br
Cl
N
S
Br
Cl
N
S
Cl
7
8
9
24
4
75
93
N
S
B
Br
N
S
Br
10
5
94
Cl
24
36
24
90
80
83
1
0
1
HO C
Cl
Cl
HO C
11
Cl
18
20
78
80
2
2
1
HO
HO
12
HO C
C
HO C
CO H
2
2
2
a
a
In the case of volatile liquid products, the yields were determined by GC
analysis using n-octane as internal standard.
All products are known compounds and were identified by comparison of their
physical or spectral data with those of known samples [20–27].

N. Iranpoor et al. / Journal of Organometallic Chemistry 693 (2008) 2469–2472
2471
The recycling of the IL and its Pd complex can be achieved eas-
ily. After removal of the product by ether, the remaining mixture of
ionic liquid and its corresponding Pd complex was washed with
water to remove the produced triethylammonium halide in the
homocoupling reaction, and or sodium halide and acetone in the
dehalogenation reaction. Then the remained IL containing its Pd
complex can be either dried in a vacuum oven, or extracted with
n-octane as internal standard. In other cases, the product was sim-
ply extracted with diethyl ether (3 ꢀ 5 ml). Evaporation of the sol-
vent followed by chromatography on a short column of silica gel
gave the desired product. All the products are known compounds
and were identified by comparison of their physical and spectral
data with the literature.
Physical and spectral data of the products:
2 3
dichloromethane followed by drying (Na SO ) and evaporation of
the solvent. The recycled IL and its Pd complex were reused for
six runs in the reaction of bromobenzene for the dehalogenation
or coupling reaction without losing its efficiency.
4
.3. Biphenyl
White solid, m.p. 70–71 °C (lit. 69–70 °C) (Ref. [20]), 1H NMR
The use of mole ratio method for the complex formation be-
13
(
3 3
250 MHz, CDCl ): 7.63–7.31 (m, 10H), C NMR (62.5 MHz, CDCl ):
tween this IL and PdCl
ment with the ML structure for this complex [18b]. The active
catalyst can be formed by coordination of two molecules of phos-
phinite ionic liquid (IL-OPPh ) through P atom to Pd(II) center fol-
2
in the presence of base is in good agree-
1
45.5, 130.3, 127.4, 127.2.
2
0
4.4. 4,4 -Dimethylbiphenyl
2
lowed by the reduction to Pd(0) by another molecule of IL-OPPh
This reduction is generally known for Pd(II) compounds [19].
2
.
White solid, m.p. 119–120.5 °C (lit. 118–120 °C) (Ref. [21]), 1H
NMR (250 MHz, CDCl ): 7.52 (d, J = 8.1 Hz, 4H), 7.23 (d, J = 8.0 Hz,
4
3
1
3
H), 2.52 (s, 6H), C NMR (62.5 MHz, CDCl
3
): 140.2, 138.6, 130.5,
1
24.4, 21.0.
3
. Conclusion
0
4
.5. 4,4 -Dimethoxybiphenyl
In summary, we have shown the applicability of a imidazolium-
based phosphinite ionic liquid (IL-OPPh ) as both the reaction
2
White solid, m.p. = 177–178 °C (lit. 178–179 °C) (Ref. [22]), 1H
NMR (250 MHz, CDCl ): 7.47 (d, J = 8.6, 4H), 7.12 (d, J = 8.6, 4H),
3.75 (s, 6H), C NMR (62.5 MHz, CDCl ): 162.5, 135.4, 125.5,
media and the ligand of Pd(II) for the efficient dehalogenation
and homocoupling of a large variety of aryl halides. The use of this
3
1
3
i
3
complex in the presence of NaOPr was shown to be selective for
1
18.7, 57.8.
3
dehalogenation and with Et N was selective for homocoupling of
aryl halides. The reusability of this phosphinite ionic liquid con-
taining its Pd complex can also be considered as a strong practical
advantageous of this method.
0
4
.6. 4,4 -Dichlorobiphenyl
White solid, m.p. 142–143 °C (lit. 142–145 °C) (Ref. [21]), 1H
NMR (250 MHz, CDCl ): 7.55 (d, J = 8.23 Hz, 2H), 7.41 (d,
J = 8.23 Hz, 2H), C NMR (62.5 MHz, CDCl ): 138.5, 135.5, 129.0,
3
1
3
4
. Experimental
3
1
28.2.
The chemical 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
0
4.7. 4,4 -Dicyanobiphenyl
1
plates or GLC on a Shimadzu model GC-10A instrument. IR spectra
Colorless solid, m.p. 235–238 °C (lit. 236–240 °C) (Ref. [23]), H
1
were run on a Shimadzu FTIR-8300 spectrophotometer. The
H
NMR (250 MHz, CDCl ): 7.51 (d, J = 8.5 Hz, 4H), 7.75 (d, J = 8.5 Hz,
4H), C NMR (62.5 MHz, CDCl ): 109.2, 120.2, 126.5, 133.4, 148.6.
3
3
1
3
13
NMR and C NMR spectra were recorded on a Bruker Avance
DPX-250, FT-NMR spectrometer. Melting points were determined
on a Buchi 510 in open capillary tubes and are uncorrected.
0
4.8. 4,4 -Diacetylbiphenyl
White solid, m.p. 188–189 °C (lit. 189–190 °C) (Ref. [24]), 1H
NMR (250 MHz, CDCl ): 7.98 (d, J = 8.6 Hz, 4H), 7.70 (d, J = 8.6 Hz,
4
.1. General procedure for the homocoupling of aryl halides
3
1
3
To a flask containing phosphinite ionic liquid (0.5 mmole,
3
4H), 2.77 (s, 6H), C NMR (62.5 MHz, CDCl ): 28.9, 125.5, 127.8,
0
2 3
.23 g), PdCl (0.05 mmole, 8.8 mg) and Et N (2 mmole, 0.20 g)
138.2, 146.3, 195.8.
were added at 80 °C and stirred for 15 min under nitrogen atmo-
sphere. Then aryl halide (1.0 mmole) was added to the mixture.
GC and TLC of the reaction mixture showed the completion of
the reaction after 0.25–20 h. After completion of reaction, the mix-
ture was cooled to room temperature and the coupled product was
extracted with 3 ꢀ 5 ml of diethyl ether. (In the case of Table 3, En-
tries 12 and 13 dichloromethane was used.) Evaporation of the sol-
vent followed by chromatography on a short column of silica gel
gave the coupled product in 58–95% yield.
0
4.9. 4,4 -Dinitrobiphenyl
Yellow solid, m.p. 238–239 °C (lit. 240 °C) (Ref. [25]), 1H NMR
(250 MHz, CDCl ): 7.45 (d, J = 8.5 Hz, 4H), 7.85 (d, J = 8.5 Hz, 4H),
C NMR (62.5 MHz, CDCl ): 124.5, 126.3, 148.5, 150.5.
3
3
1
3
0
4.10. 3,3 -Bipyridyl
1
Orange oil, m.p. 231–232 °C (lit. 232 °C) (Ref. [26]), H NMR
(250 MHz, CDCl ): 7.45 (m, H5, 5 ), 7.91 (d, H4, 4 ), 8.6 (d, H6, 6 ),
3
.8 (s, H2, 2 ), C NMR (62.5 MHz, CDCl ): 124.3, 133.5, 136.8,
3
149.5, 151.4.
0
0
0
4.2. General procedure for the dehalogenation of aryl halides
0
13
8
i
PdCl
were added to
0.5 mmole, 0.23 g) under nitrogen atmosphere. The flask was
placed in an 80 °C oil bath and stirred for 15 min. Aryl halide
1.0 mmole) was then added to the mixture and stirred for 0.5–
6 h. The reaction was monitored by gas chromatography. In the
case of low boiling point products, GC yield was determined using
2
(0.05 mmole, 8.8 mg) and NaOPr (2.0 mmole, 0.13 g)
a
flask containing phosphinite ionic liquid
0
(
4.11. 3,3 -Bithiophen
White solid, m.p. 130–132 °C (lit. 131–133 °C) (Ref. [27]), 1H
NMR (250 MHz, CDCl
): 7.20 (d, 2 H), 7.30 (s, 1H), 13C NMR
(62.5 MHz, CDCl ): 93.3, 115.7, 118.2, 118.9.
(
3
3
3

2
472
N. Iranpoor et al. / Journal of Organometallic Chemistry 693 (2008) 2469–2472
[12] W. Bujis, Catal. Today 27 (1996) 159–166.
0
4
.12. Biphenyl-4,4 -dicarboxylic acid
[
13] Y. Monguchi, A. Kume, K. Hattori, T. Maegawa, H. Sajiki, Tetrhedron 62 (2006)
926–7933.
7
1
Solid, m.p. 300 °C (lit. >300 °C) [27], H NMR (250 MHz, CDCl
3
):
.75 (d, J = 8.5 Hz, 4H), 7.83 (d, J = 8.5 Hz, 4H), 12.5 (2H), 1 C NMR
): 127.5, 128.1, 135.6, 149.7, 170.4.
[
14] (a) E.W. Abel, F.G.A. Stone, G. Wilkinson (Eds.), Comprehensive
Organometallic Chemistry II, Pergamon Press, New York, 1995;
3
7
(
2
(
b) J. Tsuji, Transition Metal Reagents and Catalysts, Wiley, Bath, England,
000. pp. 56–76;
c) A. de Meijere, F. Diederich (Eds.), Metal-Catalyzed Cross-Coupling
Reactions, second ed., Wiley-VCH, Weinheim, Germany, 2004;
d) M. Beller, C. Bolm, Transition Metal for Organic Chemistry, vol. 1, Wiley-
VCH, Weinheim, Germany, 1998. pp. 158–193;
e) J. Hassan, Ch. Hathroubi, Ch. Gozzi, M. Lemaire, Tetrahedron 57 (2001)
7845–7855;
(
62.5 MHz, CDCl
3
Acknowledgements
(
We are thankful for a Grant for this work from Organization of
Management and Planning of Iran and also the Shiraz University
Research council.
(
(
(
(
f) W.M. Seganish, M.E. Mowery, Sh. Riggleman, Ph. DeShong, Tetrahedron 61
2005) 2117–2121;
g) X. Xu, D. Cheng, W. Pei, J. Org. Chem. 71 (2006) 6637–6639.
References
[15] (a) R. Bedford, C.S.J. Cazin, M.B. Hursthose, M.E. Light, V.J.M. Scordia, Dalton
Trans. (2004) 3864–3868;
[
1] (a) A.R. Pinder, Synthesis (1980) 425–452;
b) M. Hudlicy, in: B.M. Trost, I. Fleming (Eds.), Comprehensive Organic
Synthesis, vol. 8, Pergamon, Oxford, 1991, pp. 895–922;
c) F. Alonso, I.P. Beletskaya, M. Yus, Chem. Rev. 102 (2002) 4009–4091.
2] M.L. Hitchman, R.A. Spackman, N.C. Ross, C. Agra, Chem. Soc. Rev. 24 (1995)
23–430.
3] (a) P. Caubere, Angew. Chem., Int. Ed. Engl. 95 (1983) 597–611;
b) R. Nakao, H. Rhee, Y. Uozumi, Org. Lett. 7 (2005) 163–165.
4] (a) G. Guillaumet, L. Mordenti, P. Caubere, J. Organomet. Chem. 92 (1975) 43–
7;
(b) R. Bedford, C.S.J. Cazin, D. Holder, Coord. Chem. Rev. 248 (2004) 2283–
2321;
(c) R.B. Bedford, Chem. Commun. 21 (2003) 1787–1796;
(d) U. Nettekoven, F. Naud, A. Schnyder, H.-U. Blaser, Synlett (2004) 2549–
2552.
(
(
[
[
[
4
[16] O. Navarro, N. Marion, Y. Oonishi, R.A. Kelly III, S.P. Nolan, J. Org. Chem. 71
(2006) 685–692.
[17] (a) V. Calò, A. Nacci, A. Monopoli, Eur. J. Org. Chem. 71 (2006) 3791–3802;
(b) C.C. Cassol, A.P. Umpierre, G. Machado, S.I. Wolke, J. Dupont, J. Am. Chem.
Soc. 127 (2005) 3298–3299;
(
4
(
(
b) H. Li, S. Liao, Y. Xu, Chem. Lett. (1996) 1059–1060;
c) R. Boukherroub, C. Chatgilialoglu, G. Manuel, Organometallics 15 (1996)
(c) V. Calò, A. Nacci, A. Monopoli, F. Montigelli, J. Org. Chem. 70 (2005) 6040–
6044;
(d) G.A. Grasa, M.S. Viciu, J. Huang, S.P. Nolan, J. Org. Chem. 66 (2001) 7729–
7737;
1
508–1510;
d) F. Massicot, R. Schneider, Y. Fort, S. Illy- Cherrey, O. Tillement, Tetrahedron
6 (2000) 4765–4768.
5] (a) C.L. Cavallaro, Y. Liu, J. Schwartz, P. Smith, New J. Chem. 20 (1996) 253–
57;
(
5
(e) A. Corma, H. Garcı’a, A. Leyva, Tetrahedron 60 (2004) 8553–8560.
[18] (a) N. Iranpoor, H. Firouzabadi, R. Azadi, Tetrahedron Lett. 47 (2006) 5531–
5534;
(b) N. Iranpoor, H. Firouzabadi, R. Azadi, Eur. J. Org. Chem. 13 (2007) 2197–
2201.
[19] (a) L.A. Paquette, P.E. Wiedeman, P.C. Bulman-Page, Tetrahedron Lett. 26
(1985) 1611;
(b) B. Mucha, H.M.R. Hoffmann, Tetrahedron Lett. 30 (1989) 4489.
[20] G.W. Kabalka, L. Wang, R.M. Pagni, C.M. Hair, V. Namboodiri, Synthesis (2003)
217–222.
[21] D.J. Koza, E. Carita, Synthesis (2002) 2183–2186.
[22] S.E. Denmark, M.H. Ober, Org. Lett. 5 (2003) 1357–1360.
[23] M.A. Ogliaruso, L.A. Shadoff, E.I. Becker, J. Org. Chem. 28 (1963) 2725.
[24] A.M. Echavarren, J.K. Stille, J. Am. Chem. Soc. 109 (1987) 5478–5486.
[25] S. Padmanabhau, K.V. Gavaskar, D.J. Triggle, Synth. Commun. 26 (1996) 3109–
3113.
[
2
(
(
b) D. Villemin, B. Nechab, J. Chem. Res. Synop. 9 (2000) 432–434;
c) X. LiZhen, J.R. Han, R.H. Kang, M. Ouyang, Chin. Chem. Lett. 12 (2001) 287–
2
90;
(
(
(
d) W. Qiu, D.J. Burton, J. Org. Chem. 58 (1993) 419–423;
e) C.A. Margues, M. Selva, P. Tundo, J. Org. Chem. 60 (1995) 2430–2435;
f) S. Rajagopal, A.F. Spatola, J. Org. Chem. 60 (1995) 1347–1355.
[
[
6] C. Qian, D. Zhu, Y. Gu, J. Organomet. Chem. 401 (1991) 23–29.
7] (a) Y. Zhang, S. Liao, Y. Xu, Tetrahedron Lett. 35 (1994) 4599–4602;
(
(
b) M.S. Viciu, G.A. Grasa, S.P. Nolan, Organometallics 20 (2001) 3607–3612;
c) O. Navarro, H. Kaur, P. Mahjoor, S.P. Nolan, J. Org. Chem. 69 (2004) 3173–
3
180.
8] M.E. Cucullu, S.P. Nolan, T.R. Belderrain, R.H. Grubbs, Organometallics 18
1999) 1299–1304.
9] T. Okamoto, S. Oka, Bull. Chem. Soc. Jpn. 54 (1981) 1265–1266.
[
[
(
[26] (a) M. Tiecco, L. Testaferri, M. Tingoli, D. Chanelli, M. Montanucci, Synthesis
(1984) 736;
(b) G. Lunn, J. Org. Chem. 57 (1992) 6317.
[27] Y. Xie, G.K. Tan, Y.K. Yan, J.J. Vittal, S. Ch. Ng, T.S.A. Hor, J. Chem. Soc., Dalton
Trans. (1999) 773–779.
[
[
10] (a) M. Stiles, J. Org. Chem. 59 (1994) 5381–5385;
b) F. Alonso, G. Radivoy, M. Yus, Tetrahedron 55 (1999) 4441–4444.
11] S. Tabaei, H. Mohammad, C.U. Pittman, K.T. Mead, J. Org. Chem. 57 (1992)
669–6671.
(
6