Highly active Pd(II) catalysts with trans-bidentate pyridine ligands for the Heck reaction

Article abstract of DOI:10.1021/ol026161k

(Matrix presented) The air-, water-, and heat-stable palladium(II) complexes 2a and 2b are prepared by the reaction of palladium(II) salts with the new transbidentate nitrogen ligands, 1,2-bis(2-pyridylethynyl)benzenes. The structure of complex 2a has bee

Full text of DOI:10.1021/ol026161k

ORGANIC
LETTERS
2002
Vol. 4, No. 15
2545-2547
Highly Active Pd(II) Catalysts with
trans-Bidentate Pyridine Ligands for the
Heck Reaction
Tomikazu Kawano,* Tatsuji Shinomaru, and Ikuo Ueda*
The Institute of Scientific and Industrial Research, Osaka UniVersity,
Mihogaoka, Ibaraki, Osaka 567-0047, Japan
kawano@sanken.osaka-u.ac.jp
Received May 9, 2002
ABSTRACT
The air-, water-, and heat-stable palladium(II) complexes 2a and 2b are prepared by the reaction of palladium(II) salts with the new trans-
bidentate nitrogen ligands, 1,2-bis(2-pyridylethynyl)benzenes. The structure of complex 2a has been confirmed by X-ray structure analysis.
The complexes efficiently catalyze the Heck olefination of aryl iodides and provide a good yield under phosphine-free conditions. The reaction
is very sensitive to the nature of the chelating ligand.
The Heck reaction is one of the most widely used palladium-
catalyzed carbon-carbon bond-forming reactions in organic
synthesis.1 The reaction is normally carried out in the
presence of phosphine ligands and a base under an inert
atmosphere. Therefore, the development of the catalyst under
phosphine-free conditions would be an important achieve-
ment. To date, a number of reports on phosphine-free catalyst
systems for the Heck reaction have been made.2 However,
far less attention has been devoted to phosphine-free
nitrogen-based catalysts,3 though nitrogen-based catalysts are
also known to show high activity for the palladium-catalyzed
Wacker process.4
with palladium(II) salts is expected to yield a trans-chelated
palladium(II) complex exclusively as a result of the rigidity
of the ligand. In this paper, we report the synthesis of
palladium(II) complexes from 1,2-bis(2-pyridylethynyl)-
benzene ligands and their catalytic activity for the Heck
olefination of aryl halides. The investigation of the reactivity
(2) For example, see: (a) Peris, E.; Loch, J. A.; Mata, J.; Crabtree, R.
H. Chem. Commun. 2001, 201. (b) Herrmann, W. A.; Elison, M.; Fischer,
J.; Ko¨cher, C.; Artus, G. R. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 2371.
(c) Tulloch, A. A. D.; Danopoulos, A. A.; Tooze, R. P.; Cafferkey, S. M.;
Kleinhenz, S.; Hursthouse, M. B. Chem. Commun. 2000, 1247. (d) Ohff,
M.; Ohff, A.; Milstein, D. Chem. Commun. 1999, 357. (e) Gruber, A. S.;
Zim, D.; Ebeling, G.; Monteiro, A. L.; Dupont, J. Org. Lett. 2000, 2, 1287.
(f) Bergbreiter, D. E.; Osburn, P. L.; Liu, Y.-S. J. Am. Chem. Soc. 1999,
121, 9531. (g) Reetz, M. T.; Westermann, E. Angew. Chem., Int. Ed. 2000,
39, 165. (h) Reetz, M. T.; Westermann, E.; Lohmer, R.; Lohmer, G.
Tetrahedron Lett. 1998, 39, 8449. (i) Jeffery, T. In AdVances in Metal-
Organic Chemistry; Liebeskind, L. S., Ed.; JAI: London, 1996; Vol. 5, pp
153-260.
We recently reported that the reaction of copper(I) salts
with the nitrogen ligands, 1,2-bis(2-pyridylethynyl)benzenes,
gave rise to the formation of trans-chelated copper(I)
complexes in good yields.5 The coordination of the ligands
(3) Cabri, W.; Candiani, I.; Bedeschi, A.; Santi, R. Synlett 1992, 871.
(4) (a) ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A. Science
2000, 287, 1636. (b) Bianchi, D.; Bortolo, R.; D’Aloisio, R.; Ricci, M.
Angew. Chem., Int. Ed. 1999, 38, 706. (c) Nishimura, T.; Onoue, T.; Ohe,
K.; Uemura, S. J. Org. Chem. 1999, 64, 6750.
(5) Kawano, T.; Kuwana, J.; Shinomaru, T.; Du, C.-X.; Ueda, I. Chem.
Lett. 2001, 1230.
(1) For example, see: (a) Heck, R. F. Palladium Reagents in Organic
Synthesis; Academic Press: London, 1990. (b) Heck, R. F. In Comprehen-
siVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press:
Oxford, New York, 1991; Vol. 4, pp 833-863. (c) Beletskaya, I. P.;
Cheprakov, A. V. Chem. ReV. 2000, 100, 3009. (d) de Meijere, A.; Meyer,
F. E. Angew. Chem., Int. Ed. Engl. 1995, 33, 2379.
10.1021/ol026161k CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/25/2002

of the resulting palladium(II) complexes for the Heck reaction
would provide a new catalyst system for the Heck reaction
under phosphine-free conditions. Furthermore, the differences
in reactivity between trans-chelated complexes in this study
and a well-known cis-chelated palladium(II) complex, dichloro-
(2,2′-bipyridine)palladium(II),6 would be clarified.
The results of the catalytic Heck olefination of aryl iodides
and bromides using palladium(II) complexes 2a, 2b, and 3
are summarized in Table 1.
Table 1. Heck Couplings of Aryl Halides with tert-Butyl
Acrylate Using Bidentate Nitrogen Catalysts 2a, 2b, and 3a
catalyst,
mol %
time yield
entry
ArX
solvent (h)
(%)b
TONc
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
2a , 1.0
2a , 0.1
2a , 0.01 PhI
2a , 0.001 PhI
2a , 0.001 PhI
2b, 0.01 PhI
3, 1.0
2a , 1.0
2a , 0.01 4-Me-C6H4I
3, 1.0
PhI
PhI
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
17
17
17
17
48
17
17
17
17
17
17
17
17
72
96
98
95
72
80
86
82
83
91
88
90
73
85
1
96
981
9501
71900
79630
8594
82
83
9074
88
PhI
4-Me-C6H4I
4-Me-C6H4I
1-C10H7Id
Figure 1. Bidentate nitrogen ligands and their palladium(II)
complexes.
2a , 1.0
2a , 0.01 1-C10H7Id
3, 1.0
90
7345
85
1-C10H7Id
PhBr
PhBr
PhBr
4-Me-C6H4Br Et3N
4-Me-C6H4Br n-Bu3N 72
4-Me-C6H4Br Et3N
1-C10H7Bre
1-C10H7Bre
1-C10H7Bre
4-NO2-C6H4Br n-Bu3N
4-NO2-C6H4Br n-Bu3N
2a , 1.0
2a , 1.0
3, 1.0
2a , 1.0
2a , 1.0
3, 1.0
2a , 1.0
2a , 1.0
3, 1.0
2a , 1.0
3, 1.0
1
4
0.6
2.1
3.5
0.9
2.5
The ligands 1a and 1b were easily prepared in good yields
by the cross-coupling reaction of 1,2-bis(ethynyl)benzene7
with the corresponding 2-bromopyridine derivatives in amine
solvent in the presence of tetrakis(triphenylphosphine)-
palladium(0) at 100 °C. The new palladium(II) complex 2a
was obtained as yellow-orange crystals in quantitative yield
by the reaction of PdCl2(CH3CN)2 with 1a in dichloro-
methane at room temperature. The palladium(II) complex
2b8 was obtained as orange crystals in 82% yield by
treatment of PdCl2(CH3CN)2 with 1b in dichloromethane at
50 °C, whereas at room temperature 2b was isolated in only
56% yield. Obviously, the difference in reactivity between
these two metal complexations can be attributed to the
electron-donating ability of the substituent of the pyridine
ring. Palladium(II) complex 3, in which 2,2′-bipyridine (bpy)
coordinates palladium(II) ion in a cis-chelating mode, was
prepared as yellow crystals in 88% yield by the reaction of
PdCl2(cod)9 with bpy in dichloromethane at room temper-
ature. Complexes 2a, 2b, and 3 have been fully characterized
by spectroscopy (1H NMR, FAB mass) and by elemental
analysis. Moreover, the structure of 2a was confirmed by
X-ray crystallography. The crystal structure of complex 2a
is similar to that of complex 2b.10 Complexes 2a and 2b
were found to be thermally stable, and no degradation was
observed at temperatures up to at least 140 °C as a result of
the stabilizing chelate effect. In addition, these complexes
were not sensitive to oxygen or moisture; no change in their
efficiencies was observed even if the Heck coupling reactions
were carried out under aerobic conditions.
n-Bu3N 72
Et3N
4
72
72
0.6
2.1
3.5
0.9
2.5
55
0
72
72
Et3N
n-Bu3N 72
n-Bu3N 72
55
0
100
18
7
7
100
18
a Reaction conditions: aryl halide (20 mmol), tert-butyl acrylate (25
mmol), amine solvent (25 mmol), 100 °C. b Isolated yield. c TON: turnover
number (mol product/mol catalyst). d 1-C10H7I: 1-iodonaphthalene. e 1-
C10H7Br: 1-bromonaphthalene.
A black solid, presumably palladium black, precipitated
from the reaction mixture when 1.00 mol % of complexes
2a, 2b, and 3 was used. The use of every complex for the
Heck olefination of aryl iodides gives rise to the formation
of trans-acrylic acid esters exclusively in good yield (entry
1-13). When 0.001 mol % of 2a was used, a turnover
number (TON) of 79630 was achieved (entry 5). No marked
difference in the reactivity between 2a and 2b was observed
(entries 3 and 6). We have found that complexes 2a and 2b
are more effective catalysts than classical palladium(II) salts
because palladium(II) chloride alone is an active catalyst for
the Heck reactions when aryl iodides are used. As seen in
entry 5 of Table 1, for example, when 0.001 mol % of 2a
was used, a TON of 79630 was achieved; however,
PdCl2(PhCN)2 had a TON of only 4790 under similar
experimental conditions. On the other hand, complexes 2a
and 2b were not effective for the Heck olefination of less
reactive aryl bromides (entry 14-22). Among them, more
reactive 1-bromo-4-nitrobenzene only led to the formation
of the desired product in a quantitative yield (entry 23). The
reactivity for aryl iodides and bromides indicates that the
oxidative addition process is involved in the rate-determining
(6) McCormick, B. J.; Jaynes, E. N., Jr.; Kaplan, R. I. Inorg. Synth. 1971,
13, 216.
(7) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis
1980, 627.
(8) (a) Hu, Y.-Z.; Chamchoumis, C.; Grebowicz, J. S.; Thummel, R. P.
Inorg. Chem. 2002, 41, 2296. (b) Bosch, E.; Barnes, C. L. Inorg. Chem.
2001, 40, 3097.
(9) Drew, D.; Doyle, J. R. Inorg. Synth. 1990, 28, 348.
(10) See Supporting Information for full details.
2546
Org. Lett., Vol. 4, No. 15, 2002

step for the Heck reaction in this study. It is remarkable that,
for the Heck olefination of aryl bromides, complex 2a has a
better catalytic activity than complex 3 (especially entries
21 and 22).
Scheme 1 shows a plausible mechanism in our system with
trans-bidentate nitrogen complex 2. Oxidative addition of
This intermediate might be the trans-chelated complex A.
Oxidative addition to A affords the complex B, which is also
coordinated by the ligand in a trans-configuration. Migratory
insertion of olefin associated with the chelate opening in B
will allow production of the complex C. This B-to-C process
might be essential for determining the trend of reactivity for
complexes 2 and 3, because complex 3 with a cis-chelation
mode is a stronger chelated complex than complex 2 with a
trans-chelation mode. In addition, it is worth noting that the
rigidity of the ligand structure makes trans-to-cis isomer-
ization of complex B impossible. â-H elimination followed
by reductive elimination and chelate closure completes the
catalytic cycle.
Scheme 1. Plausible Mechanism
In conclusion, we have demonstrated that new palla-
dium(II) complexes derived from bidentate nitrogen ligands,
1,2-bis(2-pyridylethynyl)benzenes, are effective catalysts for
the Heck reactions of aryl iodides under phosphine-free
conditions. The new catalyst system is air-, water-, and heat-
stable. Further studies aimed at the improvement of the
catalytic activity of our nitrogen catalysts are in progress.
Acknowledgment. We thank the Material Analysis
Center of our institute for assisting us with elemental
analyses.
Supporting Information Available: Characterization
data and crystal data (CIF) for complex 2a. This material is
available free of charge via the Internet at http://pubs.acs.org.
aryl halide probably involves a palladium(0) complex, which
is most likely obtained by the preactivation of complex 2.
OL026161K
Org. Lett., Vol. 4, No. 15, 2002
2547