Highly efficient and reusable polyaniline-supported palladium catalysts for open-air oxidative heck reactions under base-and ligand-free conditions

Article abstract of DOI:10.1002/adsc.200800329

Oxidative Heck reactions of arylboronic acids and alkenes are carried out in the presence of polyaniline-supported palladium catalysts using air as a co-oxidant to afford the Heck products with excellent yields and selectivities under base-and ligand-free conditions. The catalyst was recovered by simple filtration and used for several cycles with consistent activity.

Full text of DOI:10.1002/adsc.200800329

COMMUNICATIONS
DOI: 10.1002/adsc.200800329
Highly Efficient and Reusable Polyaniline-Supported Palladium
Catalysts for Open-Air Oxidative Heck Reactions under Base-
and Ligand-Free Conditions
Pravin R. Likhar,a,* Moumita Roy,a Sarabindu Roy,a M. S. Subhas,a
M. Lakshmi Kantam,a and B. Sreedhara
a
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderbad – 500607, India
Fax : (+91)-40-2716-0921; phone: (+91)-40-2719-3510; e-mail: plikhar@iict.res.in
Received: May 30, 2008; Revised: July 27, 2008; Published online: August 19, 2008
Abstract: Oxidative Heck reactions of arylboronic
acids and alkenes are carried out in the presence of
polyaniline-supported palladium catalysts using air
as a co-oxidant to afford the Heck products with
excellent yields and selectivities under base- and
ligand-free conditions. The catalyst was recovered
by simple filtration and used for several cycles with
consistent activity.
the presence of air using palladium catalysts and ni-
trogenous ligands.[5b]
The reported catalytic systems for the OH reaction
so far attempted are in homogeneous condition and
are associated with one or more problems of the ho-
mogeneous catalysis such as non-reusability of the
palladium catalysts, contamination of the products
with the metal, tedious work-up procedure, etc.
Hence, from economical as well as environmental
points of view, the development of a heterogeneous
catalytic system for the OH reaction is of great inter-
est. Although several heterogeneous palladium cata-
lysts are reported for the conventional Heck coupling
of aryl halides,[6–7] there is no citation for the hetero-
geneous palladium-catalyzed OH reactions.
Keywords: oxidative Heck reaction; palladium;
polyaniline; reusable catalyst
The palladium-catalyzed oxidative Heck (OH) cou-
In continuation of our work on the development of
pling of arylboronic acids and alkenes has attracted heterogeneous palladium catalysts for carbon-carbon
considerable attention of organic chemists in recent bond forming reactions,[7] herein we report a highly
times.[1–5] This is mainly because of the large diversity efficient and reusable polyaniline-supported palladi-
of readily accessible arylboronic acids, their moisture- um catalyst for OH coupling of arylboronic acids with
and air-stability, low toxicity and easy removal of the alkenes in the absence of external oxidants and under
boron-derived by-products coupled with the high base- and ligand-free conditions.
chemo- and regioselectivity of the reaction. The oxi-
Polyaniline (PANI) is one of the most widely stud-
dative Heck reaction was first discovered by Heck ied conducting polymers for electronic and optical ap-
using a stoichiometric amount of palladium salts[1] plications[8] due to its environmental stability and in-
and subsequently its catalytic version was developed teresting redox properties. Recently, PANI has been
by Uemura,[2] Mori[3] and Jung et al.[4] using palladium receiving considerable attention for catalytic reactions
salts in presence of a co-oxidant. Usually a metal salt as a catalyst and support owing to its highly conduct-
[3,4a]
such as Cu
A
or molecular oxygen[4] are used ing and redox properties.[9] Furthermore, its easy
as co-oxidants to reoxidize Pd(0) in the OH reactions preparative protocol from non-expensive starting ma-
which in turn generates a stoichiometric amount of terial (aniline), controllable doping levels through an
metal waste or cause problems associated with the acid doping/base dedoping process, inert nature, and
use of handling pure oxygen gas, reducing the utility non-solubility in most organic solvents and water
of the procedure for large-scale applications. More- make it an idle support in heterogeneous catalysis. By
over, most of the methods developed so far require exploiting these properties, polyaniline-supported pal-
the presences of bases and external ligands for the ladium (PA-Pd1) catalysts have been reported earlier
smooth reaction. Recently, Larhed reported palladi- by our group for the effective Suzuki coupling of aryl
um-catalyzed OH reactions in the presence of an or- halides and arylboronic acids.[10] We assumed that
ganic base and nitrogenous ligands under air. Later, poly
ACHTREaUNG nilines are good nitrogeneous ligands, which
the same group described a base-free OH reaction in may facilitate the efficient reoxidation of Pd(0) by
1968
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1968 – 1974

Highly Efficient and Reusable Polyaniline-Supported Palladium Catalysts
COMMUNICATIONS
Scheme 1. The OH reaction of 1a and 2a in the presence of PA-Pd1.
molecular oxygen during an oxidative Heck reaction loading, etc, (Table 1). From Table 1, it is evident that
and make the catalysts stable and also recyclable. addition of base leads to the formation of biphenyl 4
Polyaniline and polyaniline-supported palladium (homocoupled product from 1a) which eventually di-
catalysts were synthesized and characterized accord- minished the overall yield of the OH product
ing to our earlier report.[9f,g,10] Polyaniline has quinoid (Table 1, entry 1). In the absence of base, we obtained
(Q) and benzenoid (B) units along with amine and the Heck product in 94% yield with 99% selectivity
imine functional moieties, which can hold the metal over the 1,4-addition product without the formation
complexes. The characteristic vibrational absorbances of 4. Among the several solvents screened, CH3CN
of PANI are located at 1584, 1488, 1376, 1308, 1156 was the solvent of choice in terms of the yield
and 824 cmÀ1 and are attributed to the stretching vi- (Table 1, entry 2). A decrease in the palladium load-
brations of quinoid (nC N +nC C), benzenoid (nC
)
ing from 5 mol% to 2.5 mol% afforded the product in
=
=
=
C
À
units of the polymer, deformations of the C N bond, 60% in 12 h with a decreased selectivity of the Heck
À
stretching vibrations of the C N bond, in plane defor- product (Table 1, entry 6). The use of molecular
mations of CH bonds present in the aromatic rings of oxygen and Cu
(OAc)2 as oxidants under dry condi-
the undoped polymer and the out of plane deforma- tions afforded 94% and 32% isolated yields, respec-
tions of CH bonds in 1,4-substituted aromatic ring, re- tively (Table 1, entries 7 and 8). At room temperature,
spectively. Initially, we studied the reaction between no product formation was observed (Table 1, entry 9).
phenylboronic acid (1a) and butyl acrylate (2a) in the
We have also performed a comparative study be-
presence of PA-Pd1 as the model OH reaction tween the catalytic activity of the PA-Pd1 with other
(Scheme 1) and subsequent work was focused on the heterogeneous palladium catalysts which are highly
optimization of this reaction using various reaction active in the conventional Heck coupling of aryl hal-
parameters such as temperature, solvent, catalyst ides.[7] We have carried out the OH reaction of 1a and
Table 1. Optimization of reaction conditions for the oxidative Heck coupling of 1a and 2a using PA-Pd1.[a]
Entry X [mol%] Temperature [8C] Solvent
Base
Time [h] Yield [%][b] Selectivity (3a:3a’)[c] Yield [%][b]
4
1
2
3
4
5
6
5
5
5
5
5
2.5
5
5
5
80
80
80
110
100
80
80
80
CH3CN K3PO4
4
4
4
4
4
12
4
4
24
55
94
12
5
10
60
94
32
n.r.
77:23
99:1
99:1
99:1
99:1
82:18
99:1
99:1
–
15
–
–
–
–
–
–
—
–
CH3CN
EtOH
–
–
–
–
–
–
–
–
Toluene
H2O
CH3CN
CH3CN
CH3CN
CH3CN
7[d]
8[e]
9
30
[a]
Reaction conditions: 1a (1 mmol), 2a (3 mmol), PA-Pd1
under air.
ACHTRE(UNG x mol%), solvent (3 mL), stirred at the appropriate temperature
[b]
[c]
[d]
[e]
Isolated yield of 3a and 3a’.
1
Determined by H NMR and GC.
Using molecular oxygen as a reoxidant under dry reaction conditions.
Using Cu(OAc)2 as a reoxidant under dry reaction conditions.
ACHTREUNG
Adv. Synth. Catal. 2008, 350, 1968 – 1974
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1969

Pravin R. Likhar et al.
COMMUNICATIONS
2a under the optimized conditions in the presence of entries 2 and 3), layered double hydroxide-supported
these heterogeneous catalysts and the results are sum- nanopalladium [LDH-Pd(0)] as well as PdCl42À(LDH-
marized in Table 2. It was observed that commercially PdCl42À) (Table 2, entries 4 and 5), and highly basic
magnesium-lanthanum mixed oxide-supported nano-
palladium [Mg-LaO-Pd(0)] (Table 2, entry 6) are not
Table 2. Efficiency of different catalysts in the oxidative
effective in the OH reaction. 3-Aminopropyl-modi-
fied silica-supported palladium catalyst [SiO2-NH2-
Pd(II)] was also turned out to be ineffective (Table 2,
entry 7). Polyaniline-supported palladium chloride
(PA-Pd1) and palladium acetate (PA-Pd2) (5 mol%
of Pd), afforded comparable yields of 3a obtained
with the same selectivity (Table 2, entries 8 and 9).
The turnover numbers of both the catalysts are found
to be same.
Heck coupling of 1a and 2a.[a]
Entry
Catalyst
Yield [%][b]
(3a+3a’)
Selectivity
ACHTREUNG
(3a:3a’)[c]
A
1
10%-Pd/C
0
–
2[d]
3[d]
4[e]
5[e]
6[f]
7[g]
8
NAP-Mg-PdCl4
NAP-Mg-Pd(0)
LDH-PdCl4
LDH-Pd(0)
Mg-LaO-Pd(0)
SiO2-NH2-Pd(II)
PA-Pd1
5
0
5
0
70:30
–
78:22
–
0
–
Based on literature reports,[4b–d] we propose that the
PA-Pd1-catalyzed OH reaction proceeds through the
pathway as shown in Scheme 2. Here polyaniline is
acting as a support as well as a macro ligand (as it has
several amine and imine moieties) which facilitates
the smooth oxidation of Pd-H to Pd(II) in the pres-
ence of air. A similar kind of activation of oxygen by
polyaniline was earlier reported in the palladium-cat-
alyzed alcohol oxidation reaction.[11]
To widen the scope of the catalyst PA-Pd1, we have
carried out the OH reactions of a diverse range of ar-
ylboronic acids with butyl acrylate under the opti-
mized conditions (Table 3). From Table 3, it can be
seen that the electronic effects of the substituents
present on the arylboronic acid do not have a pro-
nounced effect on the overall yield of the products
(Table 3, entries 1–8).
10
94
93
41
99:01
99:01
99:01
99:01
9
PA-Pd2
PA-Pd2
10[h]
[a]
Reaction conditions: 1a (1 mmol), 2a (3 mmol), catalyst
(5 mol%), CH3CN (3 mL), stirred at 808C under air for
4 h.
[b]
[c]
[d]
[e]
[f]
Isolated yield of 3a and 3a’.
1
Determined by H NMR and GC.
Ref.[7a]
Ref.[7c]
Ref.[7d]
[g]
3-Aminopropyl-modified silica gel (9% functionalization,
Aldrich) supported PdCl2. loading of Pd (0.41 mmolg)À1.
Reaction performed at room temperature (308C).
[h]
available, Pd/C (Table 2, entry 1), nanocrystalline
The selectivity of the Heck products varies from
magnesium oxide-supported nanopalladium [NAP- one substrate to other (i.e., 92–100%) but we were
Mg-Pd(0)] and PdCl42À(NAP-Mg-PdCl42À) (Table 2, not able to observe any direct relationship with the
Scheme 2. Plausible mechanism of the PA-Pd1-catalyzed OH reaction.
1970
asc.wiley-vch.de
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1968 – 1974

Highly Efficient and Reusable Polyaniline-Supported Palladium Catalysts
COMMUNICATIONS
Table 3. OH reactions of different arylboronic acids with butyl acrylate in presence of PA-Pd1.[a]
[a]
Reaction conditions: arylboronic acid (1 mmol), 2a (3 mmol), PA-Pd1 (5 mol%), CH3CN
(3 mL), 808C, air.
[b]
Isolated yield.
[c]
1
Determined by H NMR and GC.
electronic nature of the arylboronic acids. With 4-(hy- 1,4-addition product (Table 4, entry 5). In the case of
droxymethyl)phenylboronic acid, no product forma- the electron-rich olefins, p-methoxystyrene and tert-
tion was observed even after 24 h (Table 3, entry 9). pentyl vinyl ether, only the p-methoxystilbene prod-
1-Naphthylboronic acid afforded the product in 92% uct was obtained (52%) in 24 h while no product was
yield with excellent selectivity for the Heck product obtained in the latter case (Table 4, entries 6 and 7).
(Table 3, entry 10).
2-Cyclohexenone which is extensively explored for
In an attempt to expand the scope further, we have the 1,4-addition reaction, afforded the Heck product
employed several alkenes in the OH reaction with exclusively in 55% yield (Table 4, entry 8).
phenylboronic acids and the results are shown in
Next we studied the reusability of PA-Pd1 in the
Table 4. All the acrylates reacted smoothly to afford OH reaction of 1a with 2a (Table 5). After comple-
the Heck product selectively in high yields (Table 4, tion of the reaction, the catalyst was filtered and
entries 1–4). Acrylonitrile gave the Heck product in washed with ethyl acetate, followed by methanol and
60% yield with moderate trans selectivity (Table 4, acetone and air dried. The recovered catalyst was em-
entry 3). When we employed styrene in the reaction, ployed in the next run and almost consistent activity
trans-stilbene was obtained exclusively without any was observed for up to five cycles. The activity of the
Adv. Synth. Catal. 2008, 350, 1968 – 1974
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1971

Pravin R. Likhar et al.
Table 4. The OH reaction of different alkenes with phenylboronic acid in the presence of PA-Pd1.[a]
COMMUNICATIONS
[a]
Reaction conditions: 1a (1 mmol), alkene (3 mmol), PA-Pd1 (5 mol%), CH3CN (3 mL), 808C,
air.
[b]
[c]
[d]
[e]
Isolated yield.
Selectivity of Heck product determined by 1 H NMR and GC.
trans:cis=60:40.
Only b isomer obtained.
catalyst was slightly diminished in the 6th cycle. The (after 6th cycle) were found to be 0.510 mmolgÀ1 and
palladium contents of the fresh and used catalyst 0.509 mmolgÀ1, respectively.
When we performed an XPS study of the catalyst
recovered from the 1st cycle and the 5th cycle, we ob-
served that some of the palladium was reduced and
after the 5th cycle 20% of the total palladium was re-
Table 5. Reusability study of PA-Pd1 in the OH reaction of
1a and 2a.[a]
duced to the zero state [see Figure 1. Pd 3d5/2 in
Pd(II): 337 eV and in Pd(0): 335 eV]. This reduction
Cycle
1
2
3
4
5
6
of the palladium (II) may account for the slightly di-
minished activity of the catalyst. Next, we checked
the nature of the catalysis. For this study, the OH re-
action of 1a with 2a was terminated after 15 min
(yield 13%) and the catalyst filtered off under hot
conditions. The filtrate was stirred under the reaction
Yield[b] [%]
94
94
92
89
88
72 (92)[c]
[a]
Reaction conditions: 1a (1 mmol), 2a (3 mmol), PA-Pd1
(5 mol%), CH3CN (3 mL), 808C, air, 4 h.
Isolated yield.
[b]
[c]
Reaction time 10 h.
1972
asc.wiley-vch.de
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1968 – 1974

Highly Efficient and Reusable Polyaniline-Supported Palladium Catalysts
COMMUNICATIONS
PA-Pd2:[10] Polyaniline (300 mg) was charged into a
round-bottomed flask containing an acetonitrile solution
(25 mL) of palladium acetate (0.6 mmol) and stirred under
nitrogen atmosphere for 48 h. The resultant catalyst was fil-
tered off and washed with acetonitrile followed by acetone.
The residue was dried in air for 24 h to afford the black cat-
alyst PA-Pd2. The palladium content in PA-Pd2 catalyst was
analyzed by the ICP-AES technique (Pd 0.43 mmolgÀ1).
Preparation of LDH-Pd(0) and LDH-PdCl42À: LDH-
2
Pd(0) and LDH-PdCl4 catalysts were prepared according to
procedure reported in the literature.[7a]
2À
Preparation of NAP-Mg-PdCl4
and NAP-Mg-Pd(0):
2À
NAP-Mg-PdCl4 and NAP-Mg-Pd(0) catalysts were pre-
pared according to procedure reported in the literature.[7c]
Preparation of Mg-LaO-Pd(0): Mg-LaO-Pd(0) catalyst
was prepared according to procedure reported in the litera-
ture.[7d]
Preparation of SiO2-NH2-Pd(II): 3-Aminopropyl-modified
silica gel (9% functionalization, Aldrich; 1 g) was suspended
in 25 mL of CH3CN and PdCl2 (0.5 mmol) and stirred under
N2 for 12 h to afford the SiO2-NH2-Pd(II). The catalyst was
filtered, washed with a copious amount of CH3CN followed
by acetone and dried in air. The loading of Pd was estimated
as 0.41 mmolgÀ1.
Figure 1. XPS narrow scan for Pd (a) in fresh PA-Pd1 and
(b) in used (after 5th cycle) PA-Pd1.
General Procedure for the OH Reaction
conditions for another 4 h, no increase in yield was
observed. When the filtrate was tested for palladium
by the ICP-AES technique, no palladium was detect-
In an oven-dried 10-mL round-bottomed flask, arylboronic
acid (1 mmol), alkene (3 mmol), PA-Pd1 (5 mol%) and
CH3CN (3 mL) were charged and stirred at 808C under air.
ed. This study clearly proves the heterogeneous After completion of the reaction, as monitored by TLC, the
catalyst was filtered. The filtrate was diluted with ethyl ace-
tate and washed with 10% NaOH solution, followed by sa-
turated NaCl solution. The combined organic layer was con-
centrated under reduced pressure to get the crude product
which was purified by Kügelrohr distillation using a Büchi
Glass Oven B-585 Kügelrohr apparatus. All the products
nature of the catalyst.
The work-up of the reaction was further simplified
by filtering the catalyst and isolation of the pure prod-
uct by a distillation method as there was no other side
product. Thus after removal of the catalyst from the
reaction mixture, the solvent and excess alkenes were
removed under reduced pressure by Kügelrohr distil-
lation to afford the pure product (>95% purity,
checked by NMR and GC).
1
were characterized by H NMR and MS and compared with
the literature values.[3–5]
Reusability Study of the Catalyst
In conclusion, we have shown for the first time a
heterogeneous palladium catalyst for the open air OH
reactions under base- and ligand-free conditions. The
scope of the catalyst is well documented with various
arylboronic acids and alkenes. Heterogeneity and re-
usability of the catalyst are also extensively studied.
After completion of the reaction, the catalyst was filtered
and washed with ethyl acetate, followed by methanol and
acetone and air dried. The recovered catalyst was employed
in the next run.
Acknowledgements
Experimental Section
M. R. thanks UGC, New Delhi, S. R. and M. S. S. thank
CSIR, New Delhi for providing research fellowships.
Preparation of Polyaniline-Supported Palladium
Catalyst
PA-Pd1:[10] Polyaniline (300 mg) was charged into a round-
bottomed flask containing an acetonitrile solution (25 mL) References
of palladium chloride (0.6 mmol) and stirred under a nitro-
gen atmosphere for 48 h. The resultant catalyst was filtered
off and washed with acetonitrile followed by acetone. The
residue was dried in air for 24 h to afford the black catalyst
PA-Pd1. The palladium content in PA-Pd1 catalyst was ana-
lyzed by the ICP-AES technique (Pd 0.51 mmolgÀ1).
[1] H. A. Dieck, R. F. Heck, J. Org. Chem. 1975, 40, 1083.
[2] C. S. Cho, S. Uemura, J. Organomet. Chem. 1994, 465,
85.
[3] X. Du, M. Suguro, K. Hirabayashi, A. Mori, Org. Lett.
2001, 3, 3313.
Adv. Synth. Catal. 2008, 350, 1968 – 1974
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1973

Pravin R. Likhar et al.
COMMUNICATIONS
[4] a) J. P. Parrish, Y. C. Jung, S. I. Shin, K. W. Jung, J. Org.
Chem. 2002, 67, 7127; b) M. M. S. Andappan, P. Nils-
son, M. Larhed, Molecular Diversity 2003, 7, 97;
c) Y. C. Jung, R. K. Mishra, C. H. Yoon, K. W. Jung,
Org. Lett. 2003, 5, 2231; d) M. M. S. Andappan, P. Nils-
son, M. Larhed, Chem. Commun. 2004, 218; e) C. H.
Yoon, K. Yoo, S. W. Yi, R. K. Mishra, K. W. Jung, Org.
Lett. 2004, 6, 4037; f) K. S. Yoo, C. H. Yoon, K. W.
Jung, J. Am. Chem. Soc. 2006, 128, 16384; g) C.
Adamo, C. Amatore, I. Ciofini, A. Jutand, H. Lakmini,
J. Am.Chem. Soc. 2006, 28, 6829; h) P. A. Enquist, P.
Nilsson, P. Sjoberg, M. Larhed, J. Org. Chem. 2006, 71,
8779.
[5] a) P-A. Enquist, J. Lindh, P. Nilsson, M. Larhed, Green
Chem. 2006, 8, 338; b) J. Lindh, P-A. Enquist, A. Pilot-
ti, P. Nilsson, M. Larhed, J. Org. Chem. 2007, 72, 7957.
[6] a) V. Farina, Adv. Synth. Catal. 2004, 346, 1553; b) K.
Kohler, S. S. Prockl, W. Kleist, Current Org. Chem.
2006, 10, 1585; c) L. Yin, J. Liebscher, Chem. Rev. 2007,
107, 133.
Synth. Catal. 2007, 349, 1141; f) H. Li, L. Wang, P. Li,
Synthesis 2007, 1635; g) M. J. Climent, A. Corma, S.
Iborra, M. Mifsud, Adv. Synth. Catal. 2007, 349, 1949;
h) P. Zheng, W. Zheng, J. Catal. 2007, 250, 324.
[8] a) Handbookof Organic Conductive Molecules and
Polymers, (Ed.: H. S. Nalwa), John Wiley & Sons, Chi-
chester, 1997, Vols. 1–4; b) T. A. Skoheim, R. L. Else-
nbaumer, J. R. Reynolds, Handbookof Conducting
Polymers, 2nd edn., Marcel Dekker, New York, 1998.
[9] a) S. Velusamy, M. Ahamed, T. Punniyamurthy, Org.
Lett. 2004, 6, 4821; b) G. Kowalski, J. Pielichowski, Syn-
lett 2002, 2107; c) E. N. Prabhakaran, J. Iqbal, J. Org.
Chem. 1999, 64, 3339; d) B. C. Das, J. Iqbal, Tetrahe-
dron Lett. 1997, 38, 1235; e) K. Pielichowski, J. Pieli-
chowski, J. Iqbal, P. Gurtat, Appl. Catal. A: Gen. 1997,
161, L25; f) B. M. Choudary, M. Roy, S. Roy, M. L.
Kantam, B. Sreedhar, K. V. Kumar, Adv. Synth. Catal.
2006, 348, 1734; g) M. L. Kantam, M. Roy, S. Roy,
M. S. Subhas, B. Sreedhar, B. M. Choudary, R. L. De, J.
Mol. Catal. A: Chem. 2007, 265, 244.
[10] M. L. Kantam, M. Roy, S. Roy, B. Sreedhar, S. S. Mad-
havendra, B. M. Choudary, R. L. De, Tetrahedron 2007,
63, 8002.
[11] a) T. Hirao, M. Higuchi, B. Hatano, I. lkeda, Tetrahe-
dron Lett. 1995, 36, 5925; b) T. Hirao, M. Higuchi, l.
lkeda, Y. Ohshiro, J. Chem. Soc. Chem. Commun. 1993,
194.
[7] a) B. M. Choudary, S. Madhi, N. S. Chowdari, M. L.
Kantam, B. Sreedhar, J. Am. Chem. Soc. 2002, 124,
14127; b) S. S. Prockl, W. Kleist, M. A. Gruber, K.
Koehler, Angew. Chem. Int. Ed. 2004, 43, 1881; c) M. L.
Kantam, S. Roy, M. Roy, B. Sreedhar, B. M. Choudary,
Adv. Synth. Catal. 2005, 347, 2002; d) A. Cwik, Z. Hell,
F. Figueras, Adv. Synth. Catal. 2006, 348, 523; e) M. L.
Kantam, K. B. S. Kumar, P. Srinivas, B. Sreedhar, Adv.
1974
asc.wiley-vch.de
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 1968 – 1974