olefin-palladium complex is usually weaker than that in the
olefin-rhodium complex.6 It is probably due to these reasons
that palladium blacking out is generally observed in pal-
ladium-catalyzed reactions, which considerably erodes the
catalytic efficiency and might constitute the obstruction to
the success of diene-palladium-catalyzed asymmetric reac-
tions.
In recent years, our group introduced a family of diene
ligands bearing a [3.3.0]bicyclic backbone and successfully
applied them in rhodium-catalyzed reactions.3a,h,n,7 As an
extension, we launched a project to explore the application
of these diene ligands in palladium-catalyzed asymmetric
reactions. First, we attempted to prepare the chiral diene-palla-
dium catalysts. After several experiments, to our delight,
ligand 1a was found to coordinate with palladium by a ligand
exchange from bis(benzonitrile)palladium(II) chloride to
furnish the corresponding complex 2a, which is stable for
months in air at room temperature (Scheme 1).8 Under the
The Suzuki-Miyaura coupling reaction, which is one of
the most efficient methods to form axially chiral biaryls,9
was selected to evaluate the catalytic ability of these
complexes. Thus, the coupling reaction between aryl bromide
3a and aryl boronic acid 4 was first examined.8 When the
reaction was carried out in DME-H2O at 60 °C with 5 mol
% of 2a as the catalyst and KOH as the base, the desired
coupling product 5a was obtained in 22% yield and 14% ee
with observation of palladium black formation. After various
solvents and bases were screened, it turned out that toluene
and Cs2CO310 were the optimal combination which delivered
the coupling product in 52% yield and 60% ee (entry 1, Table
1). More catalysts were then evaluated (entries 2-8, Table
Table 1. Screening Ligands and Optimization Experimentsa
Scheme 1.
Preparation of the Pd(II)-Diene Complexa
entry Pd(II)-diene (mol %) ligand (mol %) yieldb (%) eec (%)
1
2
3
4
5
6
7
8
2a (5)
2b (5)
2c (5)
2d (5)
2e (5)
2f (5)
2g (5)
2h (5)
2a (5)
2d (5)
2d (5)
2d (5)
2d (3)
2d (5)
-
-
-
-
-
-
-
52
55
62
65
47
40
42
64
45
67
70
70
62
78
60
30
52
75
55
12
0
a Reaction was carried out with diene ligand (0.5 mmol, 1 equiv) and
PdCl2(PhCN)2 (0.5 mmol, 1 equiv) in benzene (10 mL) at rt overnight.
-
51
0
9
ent-1a (5)
1d (5)
1d (15)
1d (25)
1d (9)
1d (15)
10
11
12
13
14d
79
81
82
80
90
same conditions, the complexes 2b-h were also successfully
obtained from 1b-h, respectively.
(3) For selected recent examples, see: (a) Wang, Z. Q.; Feng, C. G.;
Zhang, S. S.; Xu, M. H.; Lin, G. Q. Angew. Chem., Int. Ed. 2010, 49, 5780.
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Chem., Int. Ed. 2010, 49, 3969. (d) Luo, Y.; Carnell, A. Angew. Chem.,
Int. Ed. 2010, 49, 2750. (e) Nishimura, T.; Yasuhara, Y.; Sawano, T.;
Hayashi, T. J. Am. Chem. Soc. 2010, 132, 7872. (f) Shintani, R.; Soh, Y. T.;
Hayashi, T. Org. Lett. 2010, 12, 4106. (g) Brown, M. K.; Corey, E. J. Org.
Lett. 2010, 12, 172. (h) Shao, C.; Yu, H. J.; Wu, N. Y.; Feng, C. G.; Lin,
G. Q. Org. Lett. 2010, 12, 3820. (i) Cao, Z.; Du, H. Org. Lett. 2010, 12,
2602. (j) Shintani, R.; Tsutsumi, Y.; Nagaosa, M.; Nishimura, T.; Hayashi,
T. J. Am. Chem. Soc. 2009, 131, 13588. (k) Fournier, P.; Fiammengo, R.;
Ja¨schke, A. Angew. Chem., Int. Ed. 2009, 48, 4426. (l) Hu, X.; Zhuang,
M.; Cao, Z.; Du, H. Org. Lett. 2009, 11, 4744. (m) Mahoney, S. J.; Dumas,
A. M.; Fillion, E. Org. Lett. 2009, 11, 5346. (n) Feng, C. G.; Wang, Z. Q.;
Shao, C.; Xu, M. H.; Lin, G. Q. Org. Lett. 2008, 10, 4101. (o) Okamoto,
K.; Hayashi, T.; Rawal, V. H. Org. Lett. 2008, 10, 4387. (p) Gendrineau,
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a The reaction was carried out with 3 (0.4 mmol, 1 equiv), 4 (0.6 mmol,
1.5 equiv), and Cs2CO3 (1 mmol, 2.5 equiv) in dry toluene under nitrogen
at 80 °C for 0.5 h unless otherwise noted. b Isolated yield. c Determined by
HPLC on a Chiralcel OJ-H Column. d The reaction was carried out at 25
°C for 48 h.
1). It was found that all the tested catalysts with chiral diene
ligands bearing either electron-rich or electron-deficient
substituents could catalyze the reaction with similar ef-
ficiency, while 2d gave the best yield as well as ee value
(entry 4 in Table 1).
Interestingly, when 5 mol % of catalyst 2a along with 5
mol % of the corresponding antipode ligand, (R,R)-1a, was
(4) Viciu, M. S.; Nolan, S. P. In Topics in organometallic chemistry:
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14, p 241.
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(5) An example atempt using diene as chiral ligand: Grundl, M. A.;
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