Enantioselective Desymmetrization of Bicyclic Hydrazines using a C2

Article pubs.acs.org/Organometallics

Enantioselective Desymmetrization of Bicyclic Hydrazines using a C2‑Symmetric N‑Heterocyclic Carbene (NHC) Palladium Complex as Catalyst Peng Gu,† Qin Xu,*,† and Min Shi*,†,‡ †

Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Mei-Long Road, Shanghai 200237, People’s Republic of China ‡ State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, People’s Republic of China S Supporting Information *

ABSTRACT: The first example of palladium-catalyzed enantioselective desymmetrization of 2,3-bicyclic hydrazines with arylboronic acids through a ring-opening process is described by using a chiral C2-symmetric N-heterocyclic carbene (NHC) palladium complex as the catalyst. The reaction can be performed under convenient conditions to give trans-1,2-disubstituted 3-cyclopentenes 3 with high regioselectivity in good to excellent yields (up to 95%) and moderate to good enantioselectivities (up to 88% ee).

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INTRODUCTION Transition-metal-catalyzed annulation processes have recently attracted much interest as efficient strategies for the construction of functional molecular frameworks in organic synthesis.1 Among the diverse transition metals, palladium, rhodium, nickel, and copper were found to be effective catalysts in the hetero- and carboannulation of unsaturated cycloalkanes, dienes, and alkynes, giving a wide variety of arene-containing heterocycles and carbocycles in good yields under mild conditions.2 Aza bicyclic alkenes, which could be prepared by simple Diels−Alder cycloadditions between cyclopentadiene and diazo compounds,3 exhibit great synthetic potential. The existence of a strained carbon−carbon double bond and N−N bond in symmetrical bicyclic hydrazines allows them to be easily activated by the metal catalysts. The regular modes of transition-metal-activated bicyclic hydrazines are shown in Scheme 1. The reactivity of aza bicyclic alkenes has been studied with various nucleophiles under transition-metal-catalyzed conditions, leading to the formation of highly stereoselective disubstituted cyclopentenes.4 Palladium complexes, which were found to be efficient transition-metal catalysts, have been applied in the reaction of aza bicyclic alkenes since 2001. For example, Kaufmann and co-workers reported the palladium-catalyzed stereoselective ring opening of bicyclic hydrazine with aryl iodides to afford the corresponding cyclopentenes and hydroarylated adducts in good yields.5 Then, Radhakrishnan and co-workers disclosed a series of the desymmetrization of bicyclic hydrazine with mono- or bis© 2013 American Chemical Society

centered nucleophiles in the presence of palladium complexes, affording substituted cyclopentenes and polycyclic compounds in good yields.6 Enantioselective desymmetrization of aza bicyclic alkenes with organoboronic acids has been considered an efficient way of generating chiral building blocks which have biological activities, such as a central nervous system (CNS) stimulant agent (Scheme 1).7 In this respect, Pineschi and co-workers reported the first example of chiral Rh-catalyzed asymmetric ring opening of bicyclic hydrazine with arylboronic acids, giving the adducts in moderate enantioselectivities (12−89% ee).8 Then, Lautens and co-workers reported the same reactions catalyzed by a series of chiral Rh phosphine complexes to give the desired products in excellent ee values (up to 99% ee).9 Different kinds of chiral products 3 and 4 could be obtained by choosing different chiral phosphine ligands. However, to the best of our knowledge, the palladium-catalyzed asymmetric ring opening of bicyclic hydrazine with arylboronic acids still remains unexplored. Only one example of palladium-catalyzed enantioselective desymmetrization of aza bicyclic alkenes with other nucleophiles such as phenols has been reported to give the ring-opened products with moderate enantioselectivities (up to 58% ee).10 Herein, we report an efficient chiral NHC− Pd-catalyzed enantioselective desymmetrization of aza bicyclic alkenes with arylboronic acids, affording the desired adducts in Received: November 1, 2013 Published: December 4, 2013 7575

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Scheme 1. Activation Modes of Bicyclic Hydrazines by Transition Metals

1

good yields along with moderate to good enantioselectivities (up to 88% ee).

H NMR spectroscopic data of the crude product mixture. Then several other bases were screened, and the results of these experiments are summarized in Table 1 (entries 1−6). It was found that when LiOH·H2O was used as the base, the major product 3aa was isolated in the highest product ratio of 15/1 along with the highest ee value of 78% (Table 1, entry 2). We assumed that the reactivity and selectivity of diaza bicycle 1a were enhanced due to the activation of dicarbonyl groups by lithium ion when LiOH·H2O was used as the base.12 Other organic solvents, such as 1,4-dioxane and toluene, have also been used for this reaction (Table 1, entries 7 and 8). In 1,4dioxane, only the ring-opened product 3aa was formed along with a slightly decreased enantioselectivity of 68% ee. Tetrahydrofuran (THF) was the best solvent for this reaction. Examination of the temperature revealed that carrying out the reaction at 25 °C gave the ring-opened product 3aa as the sole product in 95% yield along with an 88% ee value (Table 1, entry 10), in which the same adduct was obtained with 68% ee in the previous literature.9a,b The absolute configuration of the product 3aa was assigned as 1R,2S, which was confirmed by comparison of the specific rotation and the HPLC trace with those in the previous literature.9a,b Having established the optimal catalytic conditions, a variety of diaza bicycles 1 having diverse substituents on the ester groups were evaluated for reactions with phenylboronic acid

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RESULTS AND DISCUSSION Initially, we utilized the diaza bicycle 1a (1.0 equiv) and phenylboronic acid (2.0 equiv) as substrates using K2CO3 (1.0 equiv) as a base with the NHC−Pd(OAc)2 complex C1 (5 mol %) (Figure 1) as the catalyst to examine the reaction

Figure 1. Chiral NHC−Pd(OAc)2 complex C1.

outcome.11 The ring-opened product 3aa was obtained in 85% yield with 64% ee value at 50 °C in a THF and water (10/ 1) solvent mixture. The arylation adduct 4aa was also isolated, and the ratio of products 3aa and 4aa was confirmed as 8/1 by

Table 1. Optimization of the Reaction Conditions for Enantioselective Desymmetrization of Diaza Bicycles with Phenylboronic Acids

entrya

base

T (°C)

t (h)

solvent

3aa/4aab

yield of 3aa (%)c

ee of 3aa (%)d

1 2 3 4 5 6 7 8 9 10

K2CO3 LiOH·H2O KOH KF Et3N NaOAc LiOH·H2O LiOH·H2O LiOH·H2O LiOH·H2O

50 50 50 50 50 50 50 50 80 room temp

36 36 36 72 72 100 36 72 36 36

THF THF THF THF THF THF dioxane toluene THF THF

8/1 15/1 6/1 7/1 6/1 4/1 100/0

85 83 82 46 64 42 88 n.r. 65 95

64 78 68 64 44 56 68

9/1 100/0

78 88

a Reaction conditions: NHC−Pd catalyst (5 mol %), alkene (0.1 mmol), phenylboronic acid (0.2 mmol), and base (0.1 mmol); reaction carried out in solvent/water (10/1). bDetermined by 1H NMR spectra. cYield of isolated product 3aa. dEnantiomeric excess of product 3aa was determined by chiral HPLC.

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with diaza bicycle 1a under the optimal conditions. We found that the corresponding ring-opened products were obtained in good yields and moderate to good enantioselectivities (ee values up to 86%), depending on the electronic effect and steric bulkiness of the employed arylboronic acids. As for orthosubstituted arylboronic acids, the electron-deficient species generally led to high enantioselectivities (Table 2, entries 4 and 5), but electron-rich and more sterically hindered substrates showed poor reactivities and the reactions should be carried out at 50 °C, affording the ring-opened products 3ad and 3ae in 80% and 76% yields along with lower enantioselectivities, respectively (Table 2, entries 6 and 7). Under the same conditions, 2-methoxyphenylboronic acid gave the desired ringopened product 3af in a trace amount (Table 2, entry 8). In comparison, Lautens’s Rh phosphine complexes showed higher catalytic activities when these 2-substituted arylboronic acids were used as substrates, affording the corresponding ringopened products with excellent enantioselectivities. In the cases of para-substituted arylboronic acids, the electron-rich species gave the desired adduct in higher enantioselectivity: 86% ee (Table 2, entries 9 and 10). The proposed mechanism for the reaction is illustrated in Scheme 2 on the basis of the related literature and our previous works.11a,13 Initially, the NHC−Pd(OAc)2 complex C1 is transformed into the corresponding Pd hydroxo complex A in the presence of base and water. Transmetalation with arylboronic acid gives rise to the palladium aryl complex B, which can coordinate to alkene 1. The subsequent carbopalladation is presumed to be irreversible, leading to the formation of complex C with the activation of dicarbonyl groups by lithium ion. The complex C can directly produce the arylation product 4 through a protodemetalation process or can give the ring-opened product 3 via a β-nitrogen elimination and subsequent protodemetalation process. In conclusion, we have successfully developed an efficient catalytic system for the enantioselective desymmetrization of

under the optimal conditions. The isopropyl carboxylate protected product 3ba was produced in 89% yield and 82% ee (Table 2, entry 1). The less sterically hindered Cbz group Table 2. Enantioselective Ring Opening of Diaza Bicycles with Substituted Arylboronic Acids

entrya

R

Ar

yield (%)b

ee (%)c

1 2 3 4 5 6d 7d 8d 9 10

i-Pr (1b) Bn (1c) Et (1d) t-Bu (1a) t-Bu (1a) t-Bu (1a) t-Bu (1a) t-Bu (1a) t-Bu (1a) t-Bu (1a)

C6H5 (2a) C6H5 (2a) C6H5 (2a) 2-FC6H4 (2b) 2-CIC6H4/2c 1-naphthalene (2d) 2-MeC6H4 (2e) 2-MeOC6H4 (2f) 4-MeC6H4 (2g) 4-FC6H4 (2h)

89 (3ba) 92 (3ca) 95 (3da) 62 (3ab) 75 (3ac) 80 (3ad) 76 (3ae) trace (3af) 72 (3ag) 68 (3ah)

82 82 73 64 86 36 39 86 45

a

Reaction conditions: NHC−Pd catalyst (5 mol %), alkene (0.1 mmol), arylboronic acid (0.2 mmol) and LiOH-H20 (0.1 mmol); reaction carried out in THF/water (10/1) at room temperature for 72 h. bYield of isolated product 3. cEnantiomeric excess of product 3 was determined by chiral HPLC. dThe reaction was carried out at 50 °C.

substituted product 3ca and ethyl carboxylate protected product 3da were both obtained in excellent yields and good enantioselectivities as 82% and 73% ee, respectively (Table 2, entries 2 and 3). The slight decrease in enantioselectivity suggests that the steric hindrance of the N-protecting groups has a direct impact on the chiral induction. Encouraged by the above results, we next utilized various arylboronic acids to react Scheme 2. Plausible Reaction Mechanism

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(CHIRALPACK OD-H, hexane/iPrOH 90/10, 0.5 mL/min, 215 nm): tmajor = 16.9 min, tminor = 14.7 min. [α]D20 = −236.1 (c = 1.02, CH2Cl2). 1H NMR (400 MHz, CDCl3, TMS): δ 7.29−7.21 (m, 5H), 6.57 (br, 1H), 5.86 (br, 1H), 5.71 (br, 1H), 4.76 (br, 1H), 4.25−4.19 (m, 2H), 4.01 (br, 3H), 2.72−2.47 (br m, 2H), 1.29−1.25 (m, 3H), 1.14−0.96 (m, 3H). Di-tert-butyl (1R,2S)-N,N′-(2-(2′-Fluorophenyl)cyclopent-3-enyl)hydrazinedicarboxylate (3ab). 3ab is a known compound;9a 62% yield (24.3 mg). The ee value was 64%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 90/10, 0.8 mL/min, 225 nm): tmajor = 24.3 min, tminor = 15.7 min. [α]D20 = −49.7 (c = 1.23, CH2Cl2). 1 H NMR (400 MHz, CDCl3, TMS): δ 7.24−7.16 (m, 2H), 7.10 (t, J = 7.6 Hz, 1H), 6.99 (t, J = 9.2 Hz, 1H), 6.31 and 6.10 (coalescing br, 1H), 5.91−5.90 (br m, 1H), 5.66 (br, 1H), 4.75 (br, 1H), 4.27 (br, 1H), 2.27−2.60 (br m, 2H), 1.49−1.19 (br m, 18H). Di-tert-butyl (1R,2S)-N,N′-(2-(2′-Chlorophenyl)cyclopent-3-enyl)hydrazinedicarboxylate (3ac). 3ac is a known compound; 9b 75% yield (30.7 mg). The ee value was 86%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 90/10, 0.8 mL/min, 225 nm): tmajor = 16.6 min, tminor = 12.5 min. [α]D20 = −72.4 (c = 2.24, CH2Cl2). 1 H NMR (400 MHz, CDCl3, TMS): δ 7.31 (d, J = 7.2 Hz, 1H), 7.27− 7.22 (m, 2H), 7.16 (m, 1H), 6.34 and 6.16 (coalescing br, 1H), 5.93− 5.92 (br m, 1H), 5.60 (br, 1H), 4.81−4.69 (br m, 1H), 4.43 (br, 1H), 2.73−2.62 (m, 2H), 1.49−1.12 (m, 18H). Di-tert-butyl (1R,2S)-N,N′-(2-(1′-Naphthyl)cyclopent-3-enyl)hydrazinedicarboxylate (3ad). 3ad is a known compound;9a 80% yield (34.0 mg). The ee value was 36%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 80/20, 0.5 mL/min, 215 nm): tmajor = 19.9 min, tminor = 14.9 min. [α]D20 = −26.0 (c = 2.11, CH2Cl2). 1 H NMR (400 MHz, CDCl3, TMS): δ 8.20 (br, 1H), 7.85 (d, J = 7.6 Hz, 1H), 7.73 (d, J = 7.6 Hz, 1H), 7.50−7.34 (m, 4H), 6.40 (br, 1H), 5.96 (br, 1H), 5.83 (br, 1H), 4.89−4.72 (br m, 2H), 2.81−2.60 (m, 2H), 1.49−1.26 (br m, 18H). Di-tert-butyl (1R,2S)-N,N′-(2-(2′-Methylphenyl)cyclopent-3-enyl)hydrazinedicarboxylate (3ae). 3ae is a known compound;9a 76% yield (29.5 mg). The ee value was 39%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 98/2, 0.8 mL/min, 220 nm): tmajor = 56.4 min, tminor = 72.3 min. [α]D20 = −40.1 (c = 2.59, CH2Cl2). 1 H NMR (400 MHz, CDCl3, TMS): δ 7.19−7.05 (m, 4H), 6.21 and 6.06 (coalescing br, 1H), 5.90−5.85 (m, 1H), 5.64 (br, 1H), 4.75 (br, 1H), 4.18 (br, 1H), 2.80−2.52 (m, 2H), 2.34 (s, 3H), 1.58−1.10 (br m, 18H). Di-tert-butyl (1R,2S)-N,N′-(2-(4′-Methylphenyl)cyclopent-3-enyl)hydrazinedicarboxylate (3ag). 3ag is a colorless oil; 72% yield (28.0 mg). The ee value was 86%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 90/10, 0.5 mL/min, 214 nm): tmajor = 28.4 min, tminor = 25.5 min. [α]D20 = −76.5 (c = 1.70, CH2Cl2). 1H NMR (400 MHz, CDCl3, TMS): δ 7.20−7.05 (m, 4H), 6.26 (br, 1H), 5.89−5.80 (br m, 1H), 5.68 (br, 1H), 4.67 (br, 1H), 3.90 (br, 1H), 2.71−2.45 (m, 2H), 2.31 (s, 3H), 1.52−1.20 (br m, 18H). 13C NMR (100 MHz, CDCl3): δ 155.8, 154.8, 140.5, 135.9, 132.9, 129.7, 129.1, 127.4, 81.1 (2), 67.8 (br, rotamers), 53.4, 35.2, 28.1 (3), 28.0 (3), 21.0. MS (ESI): m/z 411.2 (M+ + Na, 100). HRMS (ESI): calcd for C22H32N2O4Na, 411.2260; found, 411.2254. Di-tert-butyl (1R,2S)-N,N′-(2-(4′-Fluorophenyl)cyclopent-3-enyl)hydrazinedicarboxylate (3ah). 3ah is a colorless oil; 68% yield (26.7 mg). The ee value was 45%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 95/5, 0.8 mL/min, 225 nm): tmajor = 29.9 min, tminor = 34.6 min. [α]D20 = −63.1 (c = 2.31, CH2Cl2). 1H NMR (400 MHz, CDCl3, TMS): δ 7.23 (br, 2H), 6.98 (t, J = 8.8 Hz, 2H), 6.26 (br, 1H), 5.90−5.82 (m, 1H), 5.67 (br, 1H), 4.66 (br, 1H), 4.17−3.38 (m, 1H), 2.73−2.43 (m, 2H), 1.58−1.07 (br m, 18H). 19F NMR (376 MHz, CDCl3, CFCl3): δ −116.72 and −117.17 (coalescing br). 13C NMR (100 MHz, CDCl3): δ 161.7 (d, J = 242 Hz), 155.9, 154.8, 139.4, 132.5, 130.1, 129.0 (d, J = 6.6 Hz), 115.1 (d, J = 21.2 Hz), 81.3 (2), 68.2 and 66.0 (br, rotamers), 53.0, 35.0, 28.1 (3), 27.9 (3). MS (ESI): m/z 415.2 (M+ + Na, 100). HRMS (ESI): calcd for C21H29N2O4FNa, 415.2004; found, 415.2009.

diaza bicycles with arylboronic acids catalyzed by the chiral C2symmetric NHC−Pd(OAc)2 complex C1 under mild conditions. The potential biologically relevant (1R,2S)-1-amine-2arylcyclopentenes 3 can be obtained in good to excellent yields (up to 95%) and moderate to good enantioselectivities (up to 88% ee). Using LiOH·H2O as the base can significantly improve the catalytic efficiency. We assume that the activation of two carbonyl groups in diaza bicycles by the lithium ion plays an important role in the reaction.

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EXPERIMENTAL SECTION

General Remarks. Optical rotations were determined in a solution of CH2Cl2 at 20 °C; [α]D values are given in units of 10−1 deg cm2 g−1. NMR spectra were recorded with a 400 MHz spectrometer in CDCl3. Chemical shifts are reported in ppm downfield from internal TMS. Mass spectra and HRMS were with a mass spectrometer. All reactions were monitored by TLC. Flash column chromatography was carried out using 300−400 mesh silica gel at increased pressure. All reactions were performed under argon using standard Schlenk techniques. The optical purities of products were determined by HPLC analysis with chiral columns (Chiralpak AD-H and OD-H columns, 4.6 × 250 mm). The NHC−Pd complex C1 was prepared according to the previously reported procedure.11a The aza bicycles 1a−d were prepared by literature procedures.9a,b The corresponding racemic products were prepared according to the reported procedure.6b Note: NMR analysis displayed very broad peaks for all ring-opened products 3 due to rotamers of the bis-carbamate hydrazine moiety. The resolution for both 1H and 13C NMR spectroscopic data did not improve significantly when the temperature was varied. Typical Procedure of the NHC−Palladium-Catalyzed Enantioselective Desymmetrization of Diaza Bicycles with Arylboronic Acids. NHC−Pd(OAc)2 complex C1 (5 μmol, 0.05 equiv), diaza bicycle 1 (0.1 mmol, 1.0 equiv), arylboronic acid (0.2 mmol, 2.0 equiv), and LiOH·H2O (0.1 mmol, 1.0 equiv) were placed in a tube under argon, and the mixed solvent of tetrahydrofuran and water (10/ 1) was added in the tube through a syringe. The mixture was stirred at room temperature for 72 h. After the reaction was complete, the solvent was removed under reduced pressure. The residue was purified by flash column chromatography on a silica gel column (5−10% EtOAc/petroleum ether as an eluent) to give the desired ring-opened products 3. Di-tert-butyl (1R,2S)-N,N′-(2-Phenylcyclopent-3-enyl)hydrazinedicarboxylate (3aa). 3aa is a known compound;9a 95% yield (35.2 mg). The ee value was 88%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 98/2, 0.8 mL/min, 225 nm): tmajor = 79.9 min, tminor = 89.2 min. [α]D20 = −172.1 (c = 1.59, CH2Cl2). 1H NMR (400 MHz, CDCl3, TMS): δ 7.37−7.19 (m, 5H), 6.26 (br, 1H), 5.87−5.85 (m, 1H), 5.70 (br m, 1H), 4.70 (br, 1H), 3.94 (br, 1H), 2.74−2.58 (br m, 2H), 1.49−1.11 (br m, 18H). Diisopropyl (1R,2S)-N,N′-(2-Phenylcyclopent-3-enyl)hydrazinedicarboxylate (3ba). 3ba is a known compound;6b 89% yield (30.7 mg). The ee value was 82%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 90/10, 0.5 mL/min, 215 nm): tmajor = 23.2 min, tminor = 18.0 min. [α]D20 = −147.5 (c = 1.40, CH2Cl2). 1H NMR (400 MHz, CDCl3, TMS): δ 7.31−7.19 (m, 5H), 6.39 (br, 1H), 5.87−5.85 (m, 1H), 5.71−5.70 (m, 1H), 5.03−4.95 (m, 1H), 4.77 (br, 2H), 3.99 (br, 1H), 2.71−2.59 (br m, 2H), 1.30−1.13 (br m, 12H). Dibenzyl (1R,2S)-N,N′-(2-Phenylcyclopent-3-enyl)hydrazinedicarboxylate (3ca). 3ca is a known compound;9b 92% yield (40.5 mg). The ee value was 82%, as determined by chiral HPLC (CHIRALPACK AD-H, hexane/iPrOH 90/10, 0.5 mL/min, 215 nm): tmajor = 52.8 min, tminor = 39.5 min. [α]D20 = −111.7 (c = 1.59, CH2Cl2). 1H NMR (400 MHz, CDCl3, TMS): δ 7.25−6.68 (m, 15H), 6.69 (br, 1H), 5.75 (br, 1H), 5.61 (br, 1H), 5.10 (br, 2H), 4.91 (br, 2H), 4.72 (br, 1H), 3.93 (br, 1H), 2.58−2.43 (br m, 2H). Diethyl (1R,2S)-N,N′-(2-Phenylcyclopent-3-enyl)hydrazinedicarboxylate (3da). 3da is a known compound;9a 95% yield (30.1 mg). The ee value was 73%, as determined by chiral HPLC 7578

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Catal. 2008, 350, 2893−2902. (c) Bexrud, J.; Lautens, M. Org. Lett. 2010, 12, 3160−3163. (10) Luna, A. P.; Cesario, M.; Bonin, M.; Micouin, L. Org. Lett. 2003, 5, 4771−4774. (11) (a) Zhang, T.; Shi, M. Chem. Eur. J. 2008, 14, 3759−3764. (b) Wang, W. F.; Zhang, T.; Shi, M. Organometallics 2010, 29, 928− 933. (c) Xu, Q.; Zhang, R.; Zhang, T.; Shi, M. J. Org. Chem. 2010, 75, 3935−3937. (d) Zhang, R.; Xu, Q.; Zhang, X.; Zhang, T.; Shi, M. Tetrahedron: Asymmetry 2010, 21, 1928−1935. (12) Liu, Z.; Gu, P.; Shi, M.; McDowell, P.; Li, G. G. Org. Lett. 2011, 9, 2314−2317. (13) (a) Lautens, M.; Hiebert, S.; Renaud, J. L. J. Am. Chem. Soc. 2001, 123, 6834−6839. (b) Lautens, M.; Dockendorff, C.; Fagnou, K.; Malicki, A. Org. Lett. 2002, 4, 1311−1314. (c) Lautens, M.; Dockendorff, C. Org. Lett. 2003, 5, 3695−3698.

ASSOCIATED CONTENT

S Supporting Information *

Figures giving NMR spectra and chiral HPLC traces of the compounds shown in Tables 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*M.S.: e-mail, [email protected]; fax, 86-21-64166128. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Shanghai Municipal Committee of Science and Technology (11JC1402600), the National Basic Research Program of China ((973)-2010CB833302), the Fundamental Research Funds for the Central Universities, and the National Natural Science Foundation of China for financial support (21072206, 20472096, 21372241, 20672127, 21102166, 21121062, 21302203, 21361140350, and 20732008).

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REFERENCES

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dx.doi.org/10.1021/om4010687 | Organometallics 2013, 32, 7575−7579