Optically active dinuclear palladium complexes containing a Pd-Pd bond: Preparation and enantioinduction ability in asymmetric ring-opening reactions

Article abstract of DOI:10.1021/ol900533h

Optically active dinuclear palladium complexes containing a Pd-Pd bond were prepared by using (R,R)-bis(tert-butylmethylphosphino)methane ((R,R)-t-Bu-MiniPHOS). The dinuclear palladium complexes coupled with silver triflate exhibited good to excellent ena

Full text of DOI:10.1021/ol900533h

ORGANIC
LETTERS
2009
Vol. 11, No. 11
2245-2248
Optically Active Dinuclear Palladium
Complexes Containing a Pd-Pd Bond:
Preparation and Enantioinduction Ability
in Asymmetric Ring-Opening Reactions
Tomokazu Ogura, Kazuhiro Yoshida, Akira Yanagisawa, and Tsuneo Imamoto*
Department of Chemistry, Graduate School of Science, Chiba UniVersity, Yayoi-cho,
Inage-ku, Chiba 263-8522, Japan
imamoto@faculty.chiba-u.jp
Received March 14, 2009
ABSTRACT
Optically active dinuclear palladium complexes containing a Pd-Pd bond were prepared by using (R,R)-bis(tert-butylmethylphosphino)methane
((R,R)-t-Bu-MiniPHOS). The dinuclear palladium complexes coupled with silver triflate exhibited good to excellent enantioselectivities up to
99% in palladium-catalyzed alkylative ring-opening reactions of azabenzonorbornadienes.
Among many transition-metal-catalyzed asymmetric reac-
tions, chiral palladium complexes are known to be powerful
and frequently employed catalysts for the synthesis of various
kinds of optically active compounds.1 Almost all of the chiral
palladium complexes reported hitherto consist of a mono-
nuclear metal center, and as far as we know, there is no report
of chiral dinuclear complexes containing a Pd-Pd bond.2,3
On the other hand, chiral dinuclear rhodium or ruthenium
complexes have been extensively investigated in the field
of asymmetric catalysis, and some of the explored reactions
have been employed for the large-scale production of useful
optically active compounds.4,5 Such preceding successful
results motivated us to study the preparation and application
(3) Many interesting reactions of dinuclear palladium complexes includ-
ing catalytic reactions have been reported. (a) Benner, L. S.; Olmstead,
M. M.; Hope, H.; Balch, A. L. J. Organomet. Chem. 1978, 153, C31-C35.
(b) Balch, A. L.; Hunt, C. T.; Lee, C.-L.; Olmstead, M. M.; Farr, J. P.
J. Am. Chem. Soc. 1981, 103, 3764–3772. (c) Kullberg, M. L.; Kublak,
C. P. Organometallics 1984, 3, 632–634. (d) Yamamoto, H.; Shinoda, S.;
Saito, Y. J. Mol. Catal. 1985, 30, 259–266. (e) Kullberg, M. L.; Kubiak,
C. P. Inorg. Chem. 1986, 25, 26–30. (f) Young, S. J.; Kellenberger, B.;
Reibenspies, J. H.; Himmel, S. E.; Manning, M.; Anderson, O. P.; Stille,
J. K. J. Am. Chem. Soc. 1988, 110, 5744–5753. (g) Davies, J. A.; Dutremez,
S.; Pinkerton, A. A.; Vilmer, M. Organometallics 1991, 10, 2956–2958.
(h) Davies, J. A.; Kirschbaum, K.; Kluwe, C. Organometallics 1994, 13,
3664–3670. (i) Besenyei, G.; Pa´rka´nyi, L.; Foch, I.; Sima´ndi, L. I.; Ka´lma´n,
A. Chem. Commun. 1997, 1143–1144. (j) Ishii, H.; Ueda, M.; Takeuchi,
K.; Asai, M. J. Mol. Catal. A: Chem. 1999, 144, 477–480. (k) Stambili,
J. P.; Kuwano, R.; Hartwig, J. F. Angew. Chem., Int. Ed. 2002, 41, 4746–
4748. (l) Richmond, M. K.; Scott, S. L.; Yap, G. P. A.; Alper, H.
Organometallics 2002, 21, 3395–3400. (m) Besenyei, G.; Pa´rka´nyi, L.;
Szalontai, G.; Holly, S.; Pa´pai, I.; Keresztury, G.; Nagy, A. J. Chem. Soc.,
Dalton Trans. 2004, 2041–2050. (n) Hama, T.; Hartwig, J. F. Org. Lett.
2008, 10, 1545–1548.
(1) For excellent reviews dealing with Pd-catalyzed asymmetric reac-
tions, see: (a) Tsuji, J. Palladium Reagents and Catalysts; Innovations in
Organic Synthesis; Wiley, Chichester, 1995. (b) Trost, B. M.; Van Vranken,
D. L. Chem. ReV. 1996, 96, 395–422. (c) Sodeoka, M.; Hamashima, Y.
Pure Appl. Chem. 2008, 80, 763–776. (d) Mohr, J. T.; Stoltz, B. M. Chem.
Asian J. 2007, 2, 1476–1491. (e) Tietze, L. F.; Ila, H.; Bell, H. P. Chem.
ReV. 2004, 104, 3453–3516. (f) Handbook of Organopalladium Chemistry
for Organic Synthesis; Negishi, E., Ed.; Wiley: Hoboken, 2002. (g)
Ogasawara, M.; Hayashi, T. In Catalytic Asymmetric Synthesis, 2nd ed.;
Ojima, I., Ed.; Wiley: New York, 2000; pp 651-674.
(2) For representative papers dealing with dinuclear palladium complexes
containing a Pd-Pd bond, see: (a) Balch, A. L.; Benner, L. S. Inorg. Chem.
1982, 21, 47. (b) Kullberg, M. L.; Lemke, F. R.; Powell, D. R.; Kublak,
C. P. Inorg. Chem. 1985, 24, 3589–3593. (c) Krafft, T. E.; Hejna, C. I.;
Smith, J. S. Inorg. Chem. 1990, 29, 2682–2688
.
10.1021/ol900533h CCC: $40.75
Published on Web 05/04/2009
 2009 American Chemical Society

of chiral dinuclear palladium complexes. Herein we describe
the first example of optically active dinuclear palladium
complexes with a chiral diphosphine ligand and their
potential utility as chiral catalysts in asymmetric carbon-
carbon bond-forming reactions.
The molecular structure of bromide complex 1b was
determined by single-crystal X-ray analysis. The ORTEP
drawing shown in Figure 1 clearly indicates that the complex
Our idea was based on the preparation of achiral dinuclear
palladium(I) complexes with methylene-bridged diphosphine
ligands, which was reported by Balch and Benner.2a We
envisioned that the use of an optically active diphosphine
instead of an achiral one would provide the desired palladium
complex. This idea was realized by the use of (R,R)-bis(tert-
butylmethylphosphino)methane (abbreviated as (R,R)-t-Bu-
MiniPHOS), which was previously synthesized in our
laboratory and was proven to be an efficient ligand in several
representative asymmetric catalyses.6 The preparation of the
target dinuclear palladium complexes is shown in Scheme
1. Initially obtained chloride complex 1a was converted into
Figure 1. Crystal structure of complex 1b; hydrogen atoms are
omitted for clarity.
consists of two fused five-membered palladacycles containing
a Pd-Pd bond. In this complex, each palladium metal forms
a square planar structure, and the two five-membered rings
have almost the same conformational geometries, being
largely distorted each other.8 A notable fact is that the bulky
tert-butyl groups occupy quasi-axial positions rather than
quasi-equatorial spaces, avoiding the steric repulsion between
the tert-butyl group and the bromine atom. The complex
possesses two stereochemically equivalent palladium atoms,
and we expect that this imposed asymmetric environment
may lead to high enantioselectivity in catalytic asymmetric
reactions.
Scheme 1. Preparation of Dinuclear Palladium Complexes 1a-c
The enantioinduction ability of these complexes was tested
in the Pd-catalyzed asymmetric ring-opening reaction of
oxabenzonorbornadiene (2).9 Preliminary experimental re-
sults are summarized in Table 1. The reactions using 1a as
catalyst (3 mol %) without additives at 20 °C in dichlo-
romethane, 1,2-dichloroethane, or toluene proceeded slug-
gishly, and after 24 h, the corresponding ring-opening product
3 was obtained in the indicated yields with low to moderate
enantiomeric excesses (ee) (entries 1-3). The same reaction
in toluene at higher temperatures afforded the product in good
yields with improved ee (entries 4-6). Use of 1,2-dichlo-
roethane or THF under refluxing conditions resulted in low
yields and decreased enantioselectivities (entries 7 and 8),
compared with the reaction using toluene as solvent.
Complexes 1b and 1c were also used to examine the effects
the corresponding bromide complex 1b and iodide complex
1c by reacting with NaBr and NaI, respectively.7
(4) For asymmetric reactions catalyzed by rhodium dinuclear complexes,
see: (a) Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic Methods
for Organic Synthesis with Diazo Compounds; VCH: Weinheim, 1998. (b)
Doyle, M. P.; Forbes, D. C. Chem. ReV. 1998, 98, 911–935. (c) Takahashi,
T.; Tsutsui, H.; Tamura, M.; Kitagaki, S.; Nakajima, M.; Hashimoto, S.
Chem. Commun. 2001, 1604–1605. (d) Davies, H. M. L.; Beckwith, R. E.
Chem. ReV. 2003, 103, 2861–2904. (e) Minami, K.; Saito, H.; Tsutsui, H.;
Nambu, H.; Anada, M.; Hashimoto, S. AdV. Synth. Catal. 2005, 347, 1483–
1487. (f) Denton, J. R.; Davies, H. M. L. Org. Lett. 2009, 11, 787–790,
and references cited therein.
(8) The dihedral angle between the two Pd-square planes is ap-
proximately 40°. CCDC-718690 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax:
(+44)1223-336-033; or deposit@ccdc.cam.ac.uk).
(9) For representative papers dealing with asymmetric ring-opening
reactions of oxabenzonorbornadienes and azabenzonorbornadienes, see: (a)
Lautens, M.; Renaud, J.-L.; Hiebert, S. J. Am. Chem. Soc. 2000, 122, 1804–
1805. (b) Lautens, M.; Fagnou, K.; Hiebert, S. Acc. Chem. Res. 2003, 36,
48–58. (c) Lautens, M.; Hiebert, S. J. Am. Chem. Soc. 2004, 126, 1437–
1447. (d) Li, M.; Yan, X.-X.; Hong, W.; Zhu, X.-Z.; Cao, B.-X.; Sun, J.;
Hou, X.-L. Org. Lett. 2004, 6, 2833–2835. (e) Cabrera, S.; Arraya´s, R. G.;
Carretero, J. C. Angew. Chem., Int. Ed. 2004, 43, 3944–3947. (f) Cabrera,
S.; Arraya´s, R. G.; Alonso, I.; Carretero, J. J. Am. Chem. Soc. 2005, 127,
17938–17947. (g) Cho, Y.-H.; Zunic, V.; Senboku, H.; Olsen, M.; Lautens,
M. J. Am. Chem. Soc. 2006, 128, 6837–6846.
(5) For diruthenium-catalyzed asymmetric reactions, see: (a) Nishiba-
yashi, Y.; Onodera, G.; Inada, Y.; Hidai, M.; Uemura, S. Organometallics
2003, 22, 873–876. (b) Inada, Y.; Nishibayashi, Y.; Uemura, S. Angew.
Chem., Int. Ed. 2005, 44, 7715–7717. (c) Matsuzawa, H.; Miyake, Y.;
Nishibayashi, Y. Angew. Chem., Int. Ed. 2007, 46, 6488–6491.
(6) (a) Yamanoi, Y.; Imamoto, T. J. Org. Chem. 1999, 64, 2988. (b)
Gridnev, I. D.; Yamanoi, Y.; Higashi, N.; Tsuruta, H.; Yasutake, M.;
Imamoto, T. AdV. Synth. Catal. 2001, 343, 118–136.
(7) The chloride complex 1a was isolated as an orange powder that was
proved to be a 57:43 mixture of two conformational isomers, and the
bromide complex 1b isolated was a 90:10 mixture of two conformers. The
iodide complex 1c was isolated as a single conformer.
2246
Org. Lett., Vol. 11, No. 11, 2009

Table 1. Asymmetric Ring-Opening Reaction of
Oxabenzonorbornadiene (2) Catalyzed by 1a-c
Table 2. Effects of Silver Triflate in Ring-Opening Reaction of
4a
entry catalyst
solvent
CH2Cl2
T (°C) time (h) yielda (%) ee (%)
entry
complex
additive
time (h)
yieldb (%)
ee (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
20
Cl(CH2)2Cl 20
24
24
24
4
71
19
30
28
45
53
60
55
46
24
60
63
60
51
59
59
1
2
3c
4
5
6
7
1a
1a
1a
1b
1b
1c
1c
3
0.5
3
0.5
0.5
3
72
91
75
77
87
70
88
50
73
67
66
72
72
72
AgOTf
AgOTf
toluene
toluene
toluene
toluene
Cl(CH2)2Cl reflux
THF
toluene
toluene
toluene
toluene
toluene
toluene
20
86
50
74
80
0.5
71
AgOTf
AgOTf
reflux
0.5
0.5
4
0.5
0.5
0.5
0.5
0.5
0.5
66
56
13
68
0.5
reflux
80
80
80
80
a Absolute configuration was determined by comparison of the chiral
HPLC retention time with the data described in the literature.9e b Isolated
yield. c The reaction was carried out by the use of 1a (3 mol %) and AgOTf
(3.3 mol %).
28, 74b
92
1ac
1ad
1ae
1af
63
73
70
80
80
a Isolated yield. b After 4 h. c AgOTf (6.6 mol %) was added as an
additive. d AgF (6.6 mol %) was added. e AgBF4 (6.6 mol %) was added.
f AgPF6 (6.6 mol %) was added.
nadienes. The summarized results are shown in Table
3. All reactions of substrates of N-tosyl series (4a-e) with
Table 3. Asymmetric Ring-Opening Reactions of
Azabenzonorbornadienes
of bromide and iodide counterions, and the obtained results
indicate that both chemical yields and enantioselectivities
were not markedly affected by these halides (entries 9 and
10).10 On the other hand, use of silver salts, especially silver
triflate, as additive increased the chemical yields (entry 11).11
Based on the results mentioned above, the catalyst system
(complexes 1a-c/AgOTf) was applied to the ring-opening
reaction of azabenzonorbornadiene derivative 4a with di-
methylzinc. The results are summarized in Table 2 together
with those obtained by the use of 1a-c alone. Remarkable
rate acceleration was observed when AgOTf/1a-c was used
in a 2:1 ratio (entries 2, 5, and 7). This is probably due to
the formation of a dicationic palladium complex that is more
reactive than the parent complexes.12
substrate
time
(h) product (%) (%)
yielda ee
entry
4
R1
R2
R2Zn
Me2Zn 0.5
Me2Zn
Me2Zn 0.5
Me2Zn
Me2Zn 0.5
1
2
3
4
5
6
7
8
9
4a
H
Ts
Ts
Ts
Ts
5a
5b
5c
5d
5e
5f
88
79
94
91
88
83
74
66
75
72
70
72
88
99
95
74
99
99
26
98
80
91
4b Me
4c MeO
4d BnO
2
1
The scope of reaction with this catalytic system was
examined by using variously substituted azabenzonorbor-
4e MOMO Ts
4f MeO
4g MeO
4h MeO
4i MeO
PhSO2 Me2Zn 0.5
b
Ar1SO2 Me2Zn 0.5
5g
5h
5i
5j
5k
c
Ar2SO2 Me2Zn 4.5
Ms
Ts
Ts
(10) Fagnou, K.; Lautens, M. Angew. Chem., Int. Ed. 2002, 41, 26–47.
(11) Carretero and co-workers generated cationic Pd complexes by
treatment of palladium chloride complexes with NaB(ArF)4 or AgPF6
and demonstrated their high catalytic activities in alkylative ring opening
of oxabenzonorbornadienes and azabenzonorbornadienes. See refs 9d
and 9e.
Me2Zn 0.5
Et2Zn 24
Et2Zn 24
10 4c MeO
11 4d BnO
a Isolatedyield.b Ar1SO2 ) 4-O2NC6H4SO2.c Ar2SO2 ) 2,4,6-(i-Pr)3C6H2SO2.
(12) Addition of 2 equiv of AgOTf to each solution of 1a-c in toluene-
d8 afforded the same NMR signals (1H NMR (400 MHz) δ 1.12 (m, 36H),
1.28 (br s, 12H), 1.70 (quin, J ) 42.5 Hz, 4H); 31P NMR (161 MHz) δ
2.68). On the other hand, addition of 1 equiv of AgOTf to a solution of 1c
in toluene-d8 gave different signals (1H NMR (400 MHz) δ 1.16 (m, 36H),
1.60 (br s, 12H), 2.15 (quin, J ) 39.5 Hz, 4H); 31P NMR (161 MHz) δ
-3.62 (s)). A monocationic species was probably generated in the latter
case.
dimethylzinc proceeded smoothly to give the expected
products (5a-e) in high yields (entries 1-5). The enanti-
oselectivities of these reactions are largely dependent on the
substituent (R1) on the benzene ring. Thus, excellent enan-
tiomeric excesses of up to 99% were observed for MeO and
BnO derivatives (entries 3 and 4), and the reactions of other
derivatives (4a,b,e) resulted in 72-88% selectivities (entries
1, 2, and 5). The very high enantioselectivity of the reaction
(13) In order to confirm the existence of a dimeric palladium species
throughout the course of the reaction, we carried out the reaction in toluene-
d8 using a stoichiometric amount of 1c/AgOTf (1:2). After the mixture was
1
kept at 80 °C for 0.5 h, NaI was added. The H and 31P NMR spectra of
the mixture showed almost the same signals as those of dimeric complex
1c.
Org. Lett., Vol. 11, No. 11, 2009
2247

of the methoxy derivative was not affected by changing the
sulfonyl substituent on the nitrogen atom (entries 6-9),
except the substrate had a bulkier substituent (entry 8). These
enantioselectivities compare well with the highest value
reported hitherto for this transformation.9 On the other hand,
reactions of these substrates with diethylzinc required longer
reaction times (24 h) to obtain acceptable chemical yields;
the enantiomeric excesses of the products were found to be
80% and 91%, respectively (entries 10 and 11).
as dimeric species in the reactions of 4b and 4i with
dimethylzinc, but considerable differences in ee’s of the
products (71% vs 88%; 75% vs 98%) were observed. These
results indicate that the actual catalytic species generated
from the dimeric palladium complex 1c is not the same as
that generated from the monomeric palladium complex 6.13
In summary, we prepared optically active dipalladium
complexes containing a Pd-Pd bond using a methylene-
bridged P-chiral diphosphine ligand ((R,R)-t-Bu-MiniPHOS).
The complexes that coupled with silver triflate exhibited high
catalytic activity in the asymmetric ring-opening reactions
of azabenzonorbornadienes with dimethylzinc. These findings
demonstrate the potential utility of the dinuclear palladium
complexes as chiral catalysts in other catalytic asymmetric
reactions.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No.
19028009, “Chemistry of Concerto Catalysis”) from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy, Japan.
Supporting Information Available: Experimental pro-
cedures and characterization data of new compounds and
details of X-ray crystallographic analysis for 1b. This
material is available free of charge via the Internet at
http://pubs.acs.org.
It is also of interest to compare these dimeric palladium-
catalyzed reactions with those catalyzed by a mononuclear
palladium complex 6, which was prepared by the reaction
of PdCl2(cod) with (R,R)-t-Bu-MiniPHOS. The complex 6
coupled with silver triflate exhibited similar catalytic activity
OL900533H
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Org. Lett., Vol. 11, No. 11, 2009