Enantioselective reaction of imines and benzyl nitriles using palladium pincer complexes with C2-symmetric chiral bis(imidazoline)s

Article abstract of DOI:10.1002/adsc.201100626

Enantioselective reactions of a wide variety of benzyl nitriles with N-tosylimines catalyzed by novel chiral 1,3-bis(imidazolin-2-yl)benzene- palladium(II) [Phebim-Pd(II)] complexes have afforded the respective products in high yield with good enantiosele

Full text of DOI:10.1002/adsc.201100626

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DOI: 10.1002/adsc.201100626
Enantioselective Reaction of Imines and Benzyl Nitriles
Using Palladium Pincer Complexes with C2-Symmetric Chiral
Bis(imidazoline)s
Kengo Hyodo,a Shuichi Nakamura,a,* Kotaro Tsuji,a Takahiro Ogawa,a
Yasuhiro Funahashi,a and Norio Shibataa,*
a
Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa-ku,
Nagoya 466-8555, Japan
Fax : (+81)-52-735-5245; e-mail: snakamur@nitech.ac.jp or nozshiba@nitech.ac.jp
Received: August 6, 2011; Published online: December 8, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100626.
Abstract: Enantioselective reactions of a wide vari-
ety of benzyl nitriles with N-tosylimines catalyzed
by novel chiral 1,3-bis(imidazolin-2-yl)benzene-pal-
ladium(II) [Phebim-Pd(II)] complexes have afford-
ed the respective products in high yield with good
enantioselectivity. A reaction mechanism is pro-
posed based on X-ray crystal structures of palladi-
um complexes.
of a-cyano carbanions under mild reaction conditions.
To the best of our knowledge, there are only two re-
ports on catalytic enantioselective reactions of alkyl
nitriles with imines using this strategy.[5,6,7] Recently,
Shibasaki and co-workers have reported the enantio-
selective reaction of allyl nitriles with ketimines using
a chiral bisphosphine-Cu(I) catalyst to give b-amino
nitriles with high enantioselectivity.[5a] The enantiose-
lective reaction of benzyl nitriles with imines using
chiral bisphosphine-Pd(II) catalysts has been reported
by Szabꢀ and co-workers to give products with mod-
erate enantioselectivity (up to 71% ee).[5b] Despite the
impressive progress achieved in enantioselective reac-
tion of alkyl nitriles with imines using chiral catalysts,
the development of novel catalyst systems with ac-
Keywords: asymmetric catalysis; imidazolines;
Lewis acids; pincer complexes
The development of mild, catalytic and enantioselec- ceptable catalytic activity and stereoselectivity still re-
À
tive versions of C C bond-forming processes is a mains a major challenge. Recently, we developed
topic of paramount importance in modern organic chiral bis(imidazoline)-metal catalysts as highly tunea-
chemistry. In this context, the enantioselective reac- ble chiral catalysts.[8,9] Herein, our ongoing interest
tion of nucleophiles with imines using transition metal was extended to an enantioselective reaction of alkyl
catalysts continues to attract a great deal of interest, nitriles with imines using bis(imidazoline)-palladium
as it provides efficient access to chiral amines.[1] In pincer complexes. Recently, Gong and Song reported
particular, the reaction of alkyl nitriles as nucleophiles the synthesis of palladium and platinum complexes
with imines has attracted much attention,[2] because with chiral bis(imidazoline) ligands, which activated
the reaction affords chiral b-amino nitriles, which are the Friedel–Crafts reaction between b-nitrostyrene
important intermediates for biologically active com- and indole to give a product with 55% ee.[10] During
pounds.[3] However, the generation of a-cyano carban- the preparation of this manuscript, Gong and Song re-
ions is not a trivial task due to the low acidity of a- ported the enantioselective addition of diarylphos-
protons for alkyl nitriles (pKa 21–33 in DMSO).[4] The phines to enones using pincer catalysts for bis(imida-
deprotonation of the a-proton of nitriles requires zoline).[11] Although a pioneering study on pincer-type
strong bases, such as LDA and phosphazenes, which catalysts with bis(imidazoline) ligands has been made,
may trigger undesired side reactions, such as elimina- expanding the scope of catalytic enantioselective reac-
tion or epimerization of the product. Therefore, en- tions using pincer-type catalysts would be highly desir-
hancement of the acidity of alkyl nitriles by coordina- able.[12] We herein report the highly enantioselective
tion of transition metal catalysts to cyanide groups reaction of benzyl nitriles with imines using palladium
has become an attractive approach for the generation
Adv. Synth. Catal. 2011, 353, 3385 – 3390
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only 4a did not afford product 3aa, the addition of
5 mol% of AgOAc could enhance the reactivity of
benzyl nitrile to give product 3aa in good yield with a
moderate enantiomer ratio (entries 1 and 2).[13] The
reaction using the palladium complex of BINAP 5 or
SEGPHOS 6 as chiral ligands did not afford good re-
sults (entries 3 and 4). To improve stereoselectivity,
we optimized the structure of the bis(imidazoline)s.
The reaction using p-toluenesulfonylated catalyst 4b
afforded product 3aa with low enantioselectivity
Figure 1. Enantioselective reaction of alkyl nitriles with
imines using chiral palladium complexes.
pincer complexes with 1,3-bis(imidazolin-2-yl)benzene (entry 5). When catalyst 4c having 2,4,6-trimethylben-
as ligand (Phebim) (Figure 1). zoyl groups was used as a catalyst, to our delight,
The enantioselective reaction of benzyl nitrile 2a good diastereoselectivity and enantioselectivity were
(1.5 equiv.) with N-(p-toluenesulfonyl)imine 1a observed (entry 6). Unfortunately, more bulky cata-
(1.0 equiv.) was carried out by using 5 mol% of palla- lyst 4d did not afford product 3aa (entry 7). The addi-
dium catalysts (Table 1). Although the reaction using tion of molecular sieves 5ꢂ or a base such as Na2CO3
or K2CO3 improved the reactivity and yield of the re-
action (entries 8 and 9). However, a strong base such
as Cs2CO3 triggered the elimination reaction of
Table 1. Enantioselective reaction of benzyl nitrile 2a with
imine 1a using palladium catalysts 4–6.
sulfonACHTUNGERTNaNUNG mide from product 3aa giving 1,2-diphenyl-
AHCTUNGTREGaNNUN crylonitrile, therefore the yield of 3aa decreased
(entry 10). Reducing the amount of benzyl nitrile en-
hanced diastereoselectivity without the loss of yield
(entry 11). When the reaction was carried out at a
lower temperature, the enantioselectivity was slightly
improved over that at 08C (entry 12). The best enan-
tioselectivity was obtained when the reaction was car-
ried out at À208C in THF using a combination of 4c
Table 2. Enatioselective of benzyl nitrile 2a with various
imines 1a–l using 4c.
Run
Cat. Base
Time [h] Yield [%] dr[a]
er[b]
1[c]
2
3
4
5
4a
4a
5
–
–
–
–
–
–
–
60
60
60
60
60
60
60
–
–
–
79
15
18
74
51
–
85
99
86
99
97
58:42 78:22
75:25 55:45
72:28 54:46
62:38 66:34
90:10 93:7
6
4b
4c
4d
4c
4c
4c
Entry
1
3
Time [h] Yield [%] dr
er
6
7
–
–
1
1a
3aa
96
97
87
79
94
86
87
93
62
87
89
85
85
93:7
95:5
8[d]
9[d]
10[d]
Na2CO3 60
K2CO3 48
Cs2CO3 16
K2CO3
K2CO3
88:12 92:8
84:16 92:8
73:27 89:11
91:9
93:7
2[a]
3[a]
4
1b 3ba 144
1c 3ca 120
1d 3da 96
1e
1f
1g
1h 3ha 72
1i
1j
89:11 95:5
82:18 94:6
89:11 92:8
80:20 92:8
11[d,e] 4c
12[d,e,f] 4c
48
96
93:7
95:5
5
6
7
8
3ea
3fa
3ga
72
96
120
77:23
78:22
85:15 88:12
85:15 91:9
83:17 89:11
91:9
88:12 93:7
83:17 88:12
[a]
1
Diastereomeric ratio was determined by H NMR.
[b]
Enantiomeric ratio for a major diastereomer was deter-
mined by HPLC analysis using a chiral column.
Without AgOAc.
9
3ia
3ja
72
96
10
11
12
94:6
[c]
[d]
[e]
[f]
1k 3ka 144
1l 3la 168
MS 5 ꢂ was added.
Benzyl nitrile (1.25 equiv.) was used.
The reaction was carried out at À208C.
[a]
The reaction was carried out at À108C.
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Enantioselective Reaction of Imines and Benzyl Nitriles Using Palladium Pincer Complexes
with AgOAc and K2CO3 (entry 12). The absolute con-
figuration of the major diastereomer of 3aa was deter-
mined to be (1S,2R)-isomer by X-ray crystallography
analysis (see the Supporting Information).
With these optimized conditions, the reaction of
benzyl nitrile 2a with a series of imines 1a–l using 4c
was examined (Table 2). The reaction of various
imines 1a–l gave products 3aa–la in high yield with
high enantioselectivity (entries 1–12), although the re-
actions of 1b and 1c, which have an electron-donating
substitutent, decreased the reaction rate (entries 2
and 3), and the reaction with ortho-substituted imine
1f decreased the enantioselectivity (entry 6).
The reaction was also extended to different benzyl
nitriles 2a–k, most of which gave high enantioselectiv-
ity (Table 3). With both electron-donating and elec-
tron-withdrawing substituents, benzyl nitriles 2a–i
Scheme 1. Transformations of b-amino nitriles.
We next examined the transformation of the cyano
gave products 3aa–ai with high enantioselectivity (en- group in 3aa to an amino or amide group (Scheme 1).
tries 1–9). The chemical yield was excellent in most The reduction of the cyano group in 3aa using
cases. 2-Naphthylmethyl nitrile 2j and heteroatom- NiCl2·6H2O, NaBH4, and (Boc)2O afforded N-Boc
containing arylmethyl nitrile 2k also resulted in a high amine 7 without a loss of enantiopurity.[14] Hydrolysis
enantioselectivity. A maximum enantiomer ratio of of 3aa using PdACHTNUTRGNEG(UN OAc)2, PPh3, and CH3CH=NOH in
96:4 was obtained for p-chloro- and p-bromobenzyl refluxing aqueous EtOH gave b-amino amide 8 in
nitriles (entries 5 and 8). To the best of our knowl- good yield.[15]
edge, an enantiomeric ratio of 96:4 for product 3 is to
The proposed catalytic cycle for the generation of
date the best result for the asymmetric reaction of a the a-cyano carbanion and the reaction with imines is
benzyl nitrile with imines.
shown in Figure 2. The addition of AgOAc causes the
exchange reaction of bromide in 4c to acetate (com-
plex A). Complex A acts as an efficient Lewis acid
catalyst as well as a base. Coordination of the cyano
group in benzyl nitrile to the palladium atom of com-
plex A affords cationic complex B, which can be
easily deprotonated by acetate or K2CO3 because the
acidity of the a-proton is enhanced.[16,17] The next step
is the nucleophilic reaction of palladium ketenimide
(complex C) with N-sulfonylimines to give complex
D, which subsequently undergoes protonation and de-
complexation to give product 3a and regenerate com-
plex A.
Table 3. Enantioselctive reactions of various benzyl nitriles
2a–k with imine 1a using 4c.
In order to consider the reaction mechanism, the
structures of the palladium pincer complex 4c was
confirmed by X-ray crystal analysis (Figure 3). The
bisACTHUNRTGNEUNG(imidazoline) ligand is coordinated to the Pd(II)
center via two nitrogens in imidazoline and one
carbon in the aryl group in a tridentate manner. The
palladium pincer complex 4c possesses an approxi-
mate C2-symmetry. The complex 4c features a nearly
square planar Pd(II) center, and the sum of the
angles about the Pd atom is 3608. It was found that
the five rings comprising two imidazolines, two palla-
dacycles and benzene were approximately coplanar.
From the above considerations and the absolute
configuration of products, Figure 4 shows a proposed
transition state for the enantioselective reaction of
palladium ketenimide with imine using 4c. The a-
carbanion of benzyl nitriles is coordinated to palladi-
um avoiding the steric repulsion between the bulky
Entry
2
3
Time [h] Yield [%] dr
er
1
2a
3aa
96
97
90
87
95
99
89
89
99
99
92
83
93:7
95:5
2[a]
3[a]
4
2b 3ab 216
2c 3ac 240
2d 3ad 120
2e
2f
2g
2h 3ah 72
2i
2j
86:14 90:10
87:13
93:7
95:5
95:5
5[b]
6[b]
7[b]
8[b]
9[b]
10
11
3ae
3af
3ag
72
120
72
89:11 96:4
78:22 83:17
76:24 92:8
92:8
91:9
96:4
95:5
3ai
3aj
66
72
90:10 95:5
85:15 93:7
2k 3ak 72
[a]
The reaction was carried out at 08C.
The reaction was carried out at À308C.
[b]
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Kengo Hyodo et al.
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Figure 2. Proposed reaction cycle of the reaction of benzyl nitrile with an imine using 4c.
Figure 3. X-ray crystal structure of 4c·2H2O.
2,4,6-trimethylphenyl groups in 4c and the phenyl aryl group in N-tosylimine and the aryl group in palla-
group in benzyl nitrile. The Si-face of the carbanionic dium ketenimide, the Re-face of the imine reacts with
center of palladium ketenimide is shielded by bulky palladium ketenimide to give the (1S,2R)-isomer.
2,4,6-trimethylphenyl groups, while imine approaches
In conclusion, we have developed an enantioselec-
the Re-face of the carbanionic center of the palladium tive reaction of benzyl nitriles with N-sulfonylimines
ketenimide. Avoiding steric repulsion between the using novel chiral pincer complexes between 1,3-bis-
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Enantioselective Reaction of Imines and Benzyl Nitriles Using Palladium Pincer Complexes
Figure 4. Proposed state for the reaction of benzyl nitrile with an imine using 4c.
References
ACHTUNGTRENNUNG(imidazolin-2-yl)benzene having a bulky substituent
and Pd(II) to give products with good diastereo- and
enantioselectivity. A range of imines and benzyl ni-
triles can be tolerated in the process. This process
offers a simple and efficient route for the synthesis of
functionalized b-amino nitriles and their derivatives.
Further experiments are in progress to study the
scope of this process and the potential application of
the bis(imidazoline)-palladium pincer catalyst to
other reactions.
[1] a) S. Kobayashi, H. Ishitani, Chem. Rev. 1999, 99, 1069–
1094; b) G. K. Friestad, A. K. Mathies, Tetrahedron
2007, 63, 2541–2569; c) D. Ferraris, Tetrahedron 2007,
63, 9581–9597; d) S. Kobayashi, Y. Mori, J. S. Fossey,
M. M. Salter, Chem. Rev. 2011, 111, 2626–2704, and ref-
erences cited therein.
[2] a) N. Kumagai, S. Matsunaga, M. Shibasaki, J. Am.
Chem. Soc. 2004, 126, 13632–13633; b) N. Kumagai, S.
Matsunaga, M. Shibasaki, Chem. Commun. 2005, 3600–
3602; c) Y. Kawano, H. Fujisawa, T. Mukaiyama,
Chem. Lett. 2005, 34, 1134–1135; d) L. Viteva, T. Go-
spodova, Y. Stefanovsky, S. Simova, Tetrahedron 2005,
61, 5855–5865; e) T. Mukaiyama, M. Michida, Chem.
Lett. 2007, 36, 1244–1245; f) N. Kumagai, S. Matsunaga,
M. Shibasaki, Tetrahedron 2007, 63, 8598–8608; g) J.
Aydin, K. J. Szabꢀ, Org. Lett. 2008, 10, 2881–2884;
h) L. Yin, M. Kanai, M. Shibasaki, J. Am. Chem. Soc.
2009, 131, 9610–9611; i) P. B. Gonzꢃlez, R. Lopez, C.
Palomo, J. Org. Chem. 2010, 75, 3920–3922.
Experimental Section
Typical Procedure for Benzyl Nitrile with N-
Tosylimines Catalyzed by Chiral Phebim-Pd(II)
Complexes
ACHTUNGTRENNUNG(1S,2R)-2-(4-Methylbenzenesulfonyl)amino-1,2-diphenylpro-
pionitrile (3aa): Benzyl nitrile (2a) (34.2 mL, 0.296 mmol)
was added to a mixture of AgOAc (2.0 mg, 0.0119 mmol), 4c
(12.8 mg, 0.0119 mmol), K2CO3 (32.8 mg, 0.237 mmol) and
molecular sieves 5ꢂ (92 mg) in THF (0.75 mL) at À208C.
After stirring for 30 min, N-(4-methylbenzenesulfonyl)imine
1a (61.5 mg, 0.237 mmol) was added. After disappearance of
1a in the reaction mixture on TLC, saturated aqueous
NH4Cl (1.0 mL) was added. The mixture was filtered
through celite to remove molecular sieves 5ꢂ, and washed
with CH2Cl2 (10 mL). The mixture was extracted with
CH2Cl2. The combined organic layer was washed with satu-
rated aqueous NaCl, and dried over Na2SO4. Filtration and
removal of solvent under reduced pressure gave a residue,
which was purified by column chromatography (benzene/
CH3CN=95:5) giving (1S,2R)-3aa (86.4 mg, 97%) as a white
solid and 4c (10.0 mg, 78%) was recovered.
[3] a) M. Liu, M. P. Sibi, Tetrahedron 2002, 58, 7991–8035;
b) S. Abele, D. Seebach, Eur. J. Org. Chem. 2000, 1–15.
[4] a) F. G. Bordwell, Acc. Chem. Res. 1988, 21, 456–463;
b) J. P. Richard, G. Williamns, J. Gao, J. Am. Chem.
Soc. 1999, 121, 715–726.
[5] a) R. Yazaki, T. Nitabaru, N. Kumagai, M. Shibasaki, J.
Am. Chem. Soc. 2008, 130, 14477–14479; b) J. Aydin,
C. S. Conrad, K. J. Szabꢀ, Org. Lett. 2008, 10, 5175–
5178.
[6] For enantioselective reactions using nitrile activation
with chiral palladium species, see: a) M. A. Stark, G.
Jones, C. J. Richards, Organometallics 2000, 19, 1282–
1291; b) C. Mazet, L. H. Gade, Chem. Eur. J. 2003, 9,
1759–1767; c) K. Takenaka, M. Minakawa, Y. Uozumi,
J. Am. Soc. Chem. 2005, 127, 12273–12281; d) S. Jautze,
R. Peters, Angew. Chem. 2008, 120, 9424–9429; Angew.
Chem. Int. Ed. 2008, 47, 9284–9288.
[7] For enantioselective reaction of a-cyano carbanions
with aldehydes, see: a) K. Soai, Y. Hirose, S. Sakata,
Tetrahedron: Asymmetry 1992, 3, 677–680; b) P. R. Car-
lier, W. W.-F. Lam, N. C. Wan, I. D. Williams, Angew.
Chem. 1998, 110, 2374–2376; Angew. Chem. Int. Ed.
1998, 37, 2252–2254; c) Y. Suto, N. Kumagai, S. Matsu-
Acknowledgements
This work was supported by the Teijin Pharma Award in
Synthetic Organic Chemistry, Japan and a Sasagawa Scientif-
ic Research Grant from the Japan Science Society.
Adv. Synth. Catal. 2011, 353, 3385 – 3390
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
3389

Kengo Hyodo et al.
COMMUNICATIONS
naga, M. Kanai, M. Shibasaki, Org. Lett. 2003, 5, 3147–
3150; d) Y. Suto, R. Tsuji, M. Kanai, M. Shibasaki,
Org. Lett. 2005, 7, 3757–3760; e) R. Yazaki, N. Kuma-
gai, M. Shibasaki, J. Am. Chem. Soc. 2009, 131, 3195–
3196. For the reaction with ketones, see: f) R. Yazaki,
N. Kumagai, M. Shibasaki, J. Am. Chem. Soc. 2010,
132, 5522–5531. For the reaction with a,b-unsaturated
thioamides, see: g) Y. Yanagida, R. Yazaki, N. Kuma-
gai, M. Shibasaki, Angew. Chem. 2011, 123, 8056–8060;
Angew. Chem. Int. Ed. 2011, 50, 7910–7914. See also:
h) N. Kumagai, M. Shibasaki, Angew. Chem. 2011, 123,
4856–4868; Angew. Chem. Int. Ed. 2011, 50, 4760–4772.
[8] a) S. Nakamura, K. Hyodo, Y. Nakamura, N. Shibata,
T. Toru, Adv. Synth. Catal. 2008, 350, 1443–1448; b) S.
Nakamura, M. Ohara, Y. Nakamura, N. Shibata, T.
Toru, Chem. Eur. J. 2010, 16, 2360–2362; c) M. Ohara,
S. Nakamura, N. Shibata, Adv. Synth. Catal. 2011, in
press. For related recent studies from our group, see:
d) S. Nakamura, H. Nakashima, H. Sugimoto, H. Sano,
M. Hattori, N. Shibata, T. Toru, Chem. Eur. J. 2008, 14,
2145–2152; e) S. Nakamura, M. Hayashi, Y. Hiramatsu,
N. Shibata, Y. Funahashi, T. Toru, J. Am. Chem. Soc.
2009, 131, 18240–18241; f) S. Nakamura, H. Nakashima,
A. Yamamura, N. Shibata, T. Toru, Adv. Synth. Catal.
2008, 350, 1209–1212.
[9] For bis(imidazoline) ligands, see the review, a) H. Liu,
D.-M. Du, Adv. Synth. Catal. 2009, 351, 489–519, and
references cited therein. See also: b) T. Arai, T. Mizu-
kami, A. Yanagisawa, Org. Lett. 2007, 9, 1145–1147;
c) S. Jautze, P. Seiler, R. Peters, Chem. Eur. J. 2008, 14,
1430–1444; d) H. Huang, R. Peters, Angew. Chem.
2009, 121, 612–615; Angew. Chem. Int. Ed. 2009, 48,
604–606; e) S. Jautze, S. Diethelm, W. Frey, R. Peters,
Organometallics 2009, 28, 2001–2004; f) L. Cheng, J.
Dong, J. You, G. Gao, J. Lan, Chem. Eur. J. 2010, 16,
6761–6765; g) M. Weber, S. Jautze, W. Frey, R. Peters,
J. Am. Chem. Soc. 2010, 132, 12222–12225; h) H. Lin,
D.-M. Du, Adv. Synth. Catal. 2010, 352, 1113–1118;
i) D. C. Braddock, T. Cailleau, G. Cansell, S. A. Hermit-
age, R. H. Pouwer, J. M. Redmond, A. J. P. White, Tet-
rahedron: Asymmetry 2010, 21, 2911–2919; j) X. Du, H.
Liu, D.-M. Du, Eur. J. Org. Chem. 2011, 786–793; k) S.
Kim, K. Choi, Eur. J. Org. Chem. 2011, 4747–4750. For
tris(imidazoline)s, see: l) K. Murai, S. Fukushima, S.
Hayashi, Y. Takahara, H. Fujioka, Org. Lett. 2010, 12,
964–966; m) K. Murai, T. Matsushita, A. Nakamura, S.
Fukushima, M. Shimura, H. Fujioka, Angew. Chem.
2010, 122, 9360–9363; Angew. Chem. Int. Ed. 2010, 49,
9174–9177; n) K. Murai, S. Fukushima, A. Nakamura,
M. Shimura, H. Fujioka, Tetrahedron 2011, 67, 4862–
4868.
[10] a) X.-Q. Hao, J.-F. Gong, C.-X. Du, L.-Y. Wu, Y.-J. Wu,
M.-P. Song, Tetrahedron Lett. 2006, 47, 5033–5036;
b) L.-Y. Wu, X.-Q. Hao, Y.-X. Xu, M.-Q. Jia, Y.-N.
Wang, J.-F. Gong, M.-P. Song, Organometallics 2009,
28, 3360–3380; c) J.-L. Niu, X.-Q. Hao, J.-F. Gong, M.-
P. Song, Dalton Trans. 2011, 40, 5135–5150.
[11] M.-J. Yang, Y.-J. Liu, J.-F. Gong, M.-P. Song, Organo-
metallics 2011, 30, 3793–3803.
[12] For reviews on pincer catalysts, see: a) M. Albrecht, G.
van Koten, Angew. Chem. 2001, 113, 3866–3898;
Angew. Chem. Int. Ed. 2001, 40, 3750–3781; b) J. T. Sin-
gleton, Tetrahedron 2003, 59, 1837–1857; c) M. E. van
der Boom, D. Milstein, Chem. Rev. 2003, 103, 1759–
1792; d) E. Peris, R. H. Crabtree, Coord. Chem. Rev.
2004, 248, 2239–2246; e) N. Selander, K. J. Szabꢀ,
Chem. Rev. 2011, 111, 2048–2076. For recent reviews of
pincer type transition metal catalysts including bis-
AHCTUNGTREG(NNNU oxazolyl)phenyl ligands (Phebox), see: f) H. Nishiya-
ma, Chem. Soc. Rev. 2007, 36, 1133–1141; g) H. Nish-
iyama, J. Ito, Chem. Rec. 2007, 7, 159–166; h) H. Nish-
iyama, J. Ito, Chem. Commun. 2010, 203–212.
[13] We also examined other silver salts as the additive,
such as AgOTf, AgSbF6, AgBF4, AgClO4; however, the
reaction afforded product 3a either in low yields or
with lower enantioselectivities.
[14] a) S. Caddick, A. K. K. Haynes, D. B. Judd, M. R. V.
Williams, Tetrahedron Lett. 2000, 41, 3513–3516; b) S.
Caddick, D. B. Judd, A. K. K. Lewis, M. T. Reich,
M. R. V. Williams, Tetrahedron 2003, 59, 5417–5423.
[15] a) E. S. Kim, H. S. Kim, J. N. Kim, Tetrahedron Lett.
2009, 50, 2973–2975; b) E. S. Kim, H. S. Lee, J. N. Kim,
Tetrahedron Lett. 2009, 50, 6286–6289; c) E. S. Kim,
Y. M. Kim, J. N. Kim, Bull. Korean Chem. Soc. 2010,
31, 700–703.
[16] Q.-A. Chen, D.-S. Wang, Y.-G. Zhou, Chem. Commun.
2010, 46, 4043–4051.
[17] The reaction without AgOAc did not afford a product
because of the low basicity of bromide.
3390
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 3385 – 3390