Pd-Catalyzed Highly Enantioselective Synthesis of Planar Chiral

Sep 17, 2015 - A Family of Chiral Ferrocenyl Diols: Modular Synthesis, Solid-State Characterization, and Application in Asymmetric Organocatalysis. Ch...
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Pd-Catalyzed Highly Enantioselective Synthesis of Planar Chiral Ferrocenylpyridine Derivatives De-Wei Gao, Chao Zheng, Qing Gu,* and Shu-Li You* State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, People’s Republic of China Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, People’s Republic of China S Supporting Information *

ABSTRACT: A highly efficient synthesis of planar chiral ferrocenylpyridine derivatives via Pd-catalyzed intramolecular C− H arylation was developed, and quantitative yields and excellent enantioselectivity were obtained for a wide range of substrates. Notably, the catalyst loading could be lowered to 0.2 mol %, which represents the highest catalytic efficiency found for asymmetric C− H bond activation (TON up to 495). These compounds could be easily transformed to pyridine N-oxides, displaying promising catalytic reactivity in the asymmetric opening of meso-epoxide. Moreover, computational investigations were conducted to clarify the origin of the excellent enantioselectivity. The compatibility of large-scale synthesis and low catalyst loading should enhance the practicality of the synthetic application of the current method.

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INTRODUCTION Planar chiral ferrocene derivatives have attracted the wide attention of synthetic chemists due to their broad application in asymmetric catalysis, biochemistry, and material science.1 For example, chiral ferrocene derivatives containing a pyridine core are effective nucleophilic catalysts or Lewis base catalysts, showing powerful ability in an array of asymmetric reactions (Figure 1).2 Therefore, a convenient method for introducing planar chirality to the ferrocene backbone is highly desirable.3

tion.7 Since 2013, several groups, including ours, have realized Pd-catalyzed asymmetric arylation, oxidative Heck, acylation, and annulation reactions to deliver planar chiral ferrocenes via direct C−H bond functionalization.8 However, these reactions still suffer from relatively low catalytic efficiency and narrow substrate scope.9 To address these issues, here we report a highly efficient enantioselective synthesis of planar chiral ferrocenylpyridine derivatives via Pd-catalyzed intramolecular C−H bond arylation with a broad substrate scope.

Figure 1. Selected planar chiral ferrocene-based ligands and catalysts.

RESULTS AND DISCUSSION Optimization of the Reaction Conditions. The reaction conditions were optimized using ferrocene-derived pyridine bromide 2a as a model substrate. Initially, we investigated the intramolecular C−H bond arylation reaction by screening different bis-phosphine ligands. These results are summarized in Table 1. Excellent yield and enantioselectivity of 3a were obtained using commercially available (Ra)-BINAP (entry 1; 99% yield and 98% ee). Other bis-phosphine ligands L2−L4 could also promote the reaction in good yields and enantioselectivities (entries 2−4). (Ra)-BINAP was chosen as the optimal ligand due to its ready availability. When K2CO3 was added instead of Cs2CO3, the desired product 3a was obtained in 99% yield and 98% ee (entry 5). However, Na2CO3 was not suitable for this transformation (entry 6). The absence of a base led to very low conversion (entry 7). It is notable that a C−H bond arylation reaction also worked well by lowering the loading of the catalyst (entry 8). Through further

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Apart from chiral resolution and the diastereoselective directed ortho metalation (DoM) approach,4 catalytic asymmetric synthesis has provided a facile and powerful alternative to access planar chiral ferrocenes.3a The groups of Uemura, Schmalz, Kündig, and Richards synthesized planar chiral ferrocenes via Pd-catalyzed desymmetrization reactions, which rely on discriminating two enantiotopic C−X bonds.5 Recently, Ogasawara and co-workers elegantly applied asymmetric ringclosing metathesis to synthesize planar chiral ferrocene scaffolds.6 Over the past several years, there has been significant progress toward Pd-catalyzed asymmetric C−H bond activa© 2015 American Chemical Society

Received: August 24, 2015 Published: September 17, 2015 4618

DOI: 10.1021/acs.organomet.5b00730 Organometallics 2015, 34, 4618−4625

Article

Organometallics

bromo-substituted ferrocene on the other Cp ring is tolerated, as illustrated by 2m, delivering the corresponding product 3m in 98% yield and 97% ee, which allows a late-stage functionalization via the transformation of the C−Br bond. The effect of steric hindrance of the Cp ring was evaluated. High reactivity and enantioselectivity were observed for substrates possessing a pentamethyl or pentaethyl Cp ring (3p−r). Of particular note, substrates with an electrondonating group on the pyridine ring are highly reactive under the standard reaction conditions, providing rapid access to planar chiral ferrocenes with potential as nucleophilic catalysts (3t,u). A planar chiral ferrocene such as 3v having a bulky pentaphenyl Cp ring could also be synthesized in 99% yield and 99% ee in the presence of 5 mol % of Pd(OAc)2 and 10 mol % of (Ra)-BINAP. Notably, the mild reaction conditions above are found to be compatible with the enantioselective C−H direct arylation reaction from ferrocene-derived pyridine chlorides (2a,b,h,p,r). The absolute configuration of the products was determined to be Rp by an X-ray crystallographic analysis of a single crystal of enantiopure 3s (see the Supporting Information for details). Origin of Enantioselectivity. In order to probe the origin of the excellent enantioselectivity in the current asymmetric C− H functionalization reactions, DFT calculations10 were conducted on the basis of the generally accepted concerted metalation−deprotonation (CMD) mechanism.11 In order to simplify the model of computation, we chose pyridine bromide 2a as the model substrate and KHCO3 as the base (entry 9, Table 1),12 because in this case the bicarbonate ion will be the sole basic species in the reaction mixture, which works as the proton abstractor after the ligand exchange step. The C−H bond cleavage transition states TS-CMD-R and TS-CMD-S leading to the planar chiral ferrocene products in R and S configurations, respectively, were located (Figure 2). The Gibbs free energy of TS-CMD-S is higher than that of TS-CMD-R by 8.8 kcal/mol, which reproduces well the exceedingly high enantioselectivity observed experimentally. In each of these two transition states, the Pd(II) center is in the square-pyramidal coordination geometry. The two phosphorus atoms (P1 and P2) of the chiral ligand, one carbon atom (C2) of the pyridine ring, and the breaking C−H bond (C1···H1) on the Cp ring adopt the four coordination sites in the equatorial plane of the Pd(II) center. The bicarbonate anion approaches the Pd(II) center from the apical direction, and one oxygen atom (O2) abstracts the proton on the ferrocene. Notably, the coordination plane of the Pd(II) center in TS-CMD-S is highly distorted, which is demonstrated by a large dihedral angle D[P2−P1−C2−(C1··· H1)] (28.6°). However, the situation of TS-CMD-R is quite different; the four coordinating moieties are quite coplanar in that the dihedral angle D[P2−P1−C2−(C1···H1)] is only 4.3°. Apparently, the severe deviation from the ideal coordination mode to the Pd(II) center in TS-CMD-S is the crucial reason for its instability. This deviation might stem from the steric congestion between the ferrocene moiety and one phenyl group of the (Ra)-BINAP ligand in the quadrant II. In contrast, this disadvantageous interaction is avoided in TS-CMD-R because the ferrocene moiety in this structure is located in an open quadrant. Therefore, it is the biased interaction between the chiral ligand and the ferrocene moiety of the substrate that causes different coordination environments around the Pd(II) center in the two diastereomeric transition states, from which the enantioselectivity finally originates.

Table 1. Optimization of Reaction Conditions for Intramolecular C−H Bond Arylationa

entry

ligand

base

time (h)

yield (%)b

ee (%)c

1 2 3 4 5 6 7d 8e 9e,f 10e,g 11h 12f,g

L1 L2 L3 L4 L1 L1 L1 L1 L1 L1 L1 L1

Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 K2CO3 Na2CO3

7 10 10 10 10 18 18 22 35 7 40 40

99 99 81 99 99 99% ee). Analytical data for (Rp)-3h are as follows. [α]D20 = +953.1° (c = 0.01, acetone, > 99% ee). 1H NMR (400 MHz, CDCl3): δ 8.70−8.65 (m, 2H), 7.12 (s, 1H), 5.01−4.91 (m, 3H), 4.07 (s, 5H). 13C NMR (100 MHz, CDCl3): δ 193.7, 153.7, 152.0, 142.7, 135.7, 116.4, 86.7, 78.7, 77.0, 73.5, 68.0, 67.5. IR (film): 1625, 1448, 1390, 1371, 1296, 1077, 1033, 852, 817, 765 cm-1. HRMS (ESI): calcd for C 16 H 12 56 FeNO [M + H] + 290.0263, found 290.0266. The enantiomeric excess was determined by Daicel Chiralpak AD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(major) = 14.369 min, t(minor) = 17.378 min. Compound (Rp)-3i. Dark red liquid (94.2 mg, 99% yield, 98% ee). Analytical data for (Rp)-3i are as follows. [α]D20 = +45.6° (c = 0.01, acetone, 98% ee). 1H NMR (400 MHz, CDCl3): δ 8.24−8.22 (m, 1H), 7.55−7.51 (m, 1H), 6.89−6.85 (m, 1H), 4.98 (d, J = 6.0 Hz, 1H), 4.90 (d, J = 6.4 Hz, 1H), 4.84 (t, J = 2.0 Hz, 1H), 3.94−3.92 (m, 4H), 1.86−1.80 (m, 2H), 0.89−0.86 (m, 3H). 13C NMR (100 MHz, CDCl3): δ 192.2, 164.5, 152.2, 134.8, 129.4, 120.7, 95.9, 88.3, 79.6, 77.3, 72.5, 72.3, 72.2., 72.1, 68.1, 67.7, 20.3, 14.5. IR (film): 2965, 2929, 1682, 1592, 1568, 816, 782, 741 cm-1. HRMS (ESI): calcd for C 18 H 16 56 FeNO [M + H] + 318.0576, found 318.0581. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 10.312 min, t(major) = 17.568 min.

CONCLUSION In summary, a highly efficient Pd(0)-catalyzed C−H bond arylation reaction using commercially available (Ra)-BINAP to synthesize planar chiral ferrocenylpyridine derivatives has been developed, and the origin of the excellent enantioselectivity was also clarified by DFT calculations. The compatibility of the reaction conditions with large-scale synthesis and low catalyst loading should enhance the potential for practical applications. Moreover, the chiral products could be easily transformed to pyridine N-oxides, which show promising catalytic reactivity in the asymmetric opening of meso-epoxide. Application of the planar chiral ferrocenylpyridines to the synthesis of novel catalysts and ligands is currently ongoing in our laboratory.

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EXPERIMENTAL SECTION

General Procedure for the Enantioselective Synthesis of Planar Chiral Ferrocene. In a 10 mL Schlenk-type sealed tube, substrate 2 (0.3 mmol), Pd(OAc)2 (1.7 mg, 0.0075 mmol, 2.5 mol %), (Ra)-BINAP (9.3 mg, 0.015 mmol, 5.0 mol %), K2CO3 (62.2 mg, 0.45 mmol, 1.5 equiv), and pivalic acid (9.2 mg, 0.09 mmol, 30 mol %) were dissolved in p-xylene (1.5 mL) under argon. The tube was sealed with a Teflon-lined cap, and the reaction mixture was stirred at 80 °C. After the reaction was complete (monitored by TLC), the reaction mixture was cooled to room temperature and concentrated under reduced pressure. The residue was purified by silica gel column chromatography (ethyl acetate/petroleum ether 1/10 to 2/1, v/v) to afford the desired product 3. Compound (Rp)-3a. Dark red solid (85.9 mg, 99% yield, 98% ee). Analytical data for (Rp)-3a are as follows. Mp: 73.1−75.6 °C. [α]D20 = +52.1° (c = 0.01, acetone, 98% ee). 1H NMR (400 MHz, CDCl3) δ: 8.31 (d, J = 4.4 Hz, 1H), 7.61 (dd, J = 7.6, 1.6 Hz, 1H), 6.95 (dd, J = 7.2, 5.2 Hz, 1H), 5.11 (d, J = 2.4 Hz, 1H), 5.03 (d, J = 2.4 Hz, 1H), 4.93−4.91 (m, 1H), 4.12 (s, 5H). 13C NMR (100 MHz, CDCl3): δ 192.4, 164.8, 152.4, 134.6, 129.6, 120.9, 88.2, 79.2, 76.8, 73.2, 67.6, 67.5. IR (film): 3090, 1687, 1592, 1565, 1383, 814, 784, 738 cm−1. HRMS (ESI): calcd for C16H1256FeNO [M + H]+ 290.0263, found 290.0263. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/ min, λ 254 nm, t(minor) = 12.712 min, t(major) = 16.251 min. Compound (Rp)-3b. Dark red solid (98.7 mg, 97% yield, 99% ee). Analytical data for (Rp)-3b are as follows. Mp: 174.0−176.4 °C. [α]D20 = −1071.5° (c = 0.01, acetone, 99% ee). 1H NMR (400 MHz, CDCl3): δ 8.00 (s, 1H), 7.86 (d, J = 8.4 Hz, 1H), 7.66 (d, J = 7.6 Hz, 1H), 7.57 (t, J = 7.2 Hz, 1H), 7.34 (t, J = 7.2 Hz, 1H), 5.22 (s, 1H), 5.04 (s, 1H), 4.89 (s, 1H), 4.04 (s, 5H). 13C NMR (100 MHz, CDCl3): δ 191.6, 162.4, 149.8, 133.7, 131.3, 130.2, 129.5, 128.8, 126.5, 126.4, 88.7, 82.3, 77.7, 73.0, 67.6, 67.2. IR (film): 2920, 1691, 1611, 1572, 1258, 1068, 1053, 827, 797, 755 cm−1. HRMS (ESI): calcd for C 20 H 14 56 FeNO [M + H] + 340.0419, found 340.0420. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(major) = 14.814 min, t(minor) = 16.702 min. Compound (Rp)-3c. Dark red liquid (105.3 mg, 98% yield, 98% ee). Analytical data for (Rp)-3c are as follows. [α]D20 = +620.6° (c = 0.01, acetone, 98% ee). 1H NMR (400 MHz, CDCl3): δ 7.86 (d, J = 6.4 Hz, 1H), 6.21 (d, J = 6.4 Hz, 1H), 4.99 (s, 1H), 4.86 (s, 1H), 4.71 (s, 1H), 4.13 (s, 5H), 3.52 (s, 4H), 1.93 (s, 4H). 13C NMR (100 MHz, CDCl3): δ 192.5, 167.2, 150.5, 149.5, 116.5, 108.3, 87.2, 79.5, 74.5, 72.7, 66.3, 66.2, 51.5, 25.6. IR (film): 2971, 2872, 1672, 1584, 1529, 1400, 1212, 1053, 805, 749 cm−1. HRMS (ESI): calcd for C 20 H 19 56 FeN 2O [M + H] + 359.0841, found 359.0841. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 37.892 min, t(major) = 44.813 min. Compound (Rp)-3d. Dark red solid (110.6 mg, 99% yield, 99% ee). Analytical data for (Rp)-3d are as follows. Mp: 161.2−163.5 °C. [α]D20 = +1878.3° (c = 0.01, acetone, 99% ee). 1H NMR (400 MHz, CDCl3): δ 7.97 (d, J = 6.0 Hz, 1H), 6.38 (d, J = 6.0 Hz, 1H), 5.01 (s, 1H), 4.89 4622

DOI: 10.1021/acs.organomet.5b00730 Organometallics 2015, 34, 4618−4625

Article

Organometallics

Hz, 2H), 1.23 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3): δ 191.6, 166.1, 162.9, 152.7, 140.3, 134.5, 130.1, 120.9, 118.7, 89.9, 83.6, 80.1, 77.5, 74.3, 73.8, 72.8, 72.6, 68.1, 67.8, 60.3, 14.4. IR (film): 2987, 2901, 1694, 1385, 1193, 1159, 1067 cm−1. HRMS (ESI): calcd for C 21 H18 56 FeNO 3 [M + H] + 388.0631, found 388.0633. The enantiomeric excess was determined by Daicel Chiralcel OJ-H (0.46 cm × 25 cm), hexanes/IPA 95/5, 1.0 mL/min, λ 254 nm, t(major) = 34.064 min, t(minor) = 40.517 min. Compound (Rp)-3p. Dark red solid (105.6 mg, 98% yield, 98% ee). Analytical data for (Rp)-3p are as follows. Mp: 198.9−200.5 °C. [α]D20 = −2556.5° (c = 0.01, acetone, 98% ee). 1H NMR (400 MHz, CDCl3): δ 8.25 (dd, J = 4.8, 2.0 Hz, 1H), 7.54 (dd, J = 7.2, 1.2 Hz, 1H), 6.84 (dd, J = 7.2, 5.2 Hz, 1H), 4.57 (d, J = 2.4 Hz, 1H), 4.53 (t, J = 2.4 Hz, 1H), 4.46 (d, J = 2.4 Hz, 1H), 1.44 (s, 15H). 13C NMR (100 MHz, CDCl3): δ 190.7, 163.5, 151.6, 135.2, 129.1, 120.1, 88.2, 84.7, 82.4, 80.9, 71.3, 70.3, 9.5. IR (film): 2987, 2921, 1680, 1571, 1392, 1244, 1078, 1037, 828, 781, 738 cm−1. HRMS (ESI): calcd for C 21 H 22 56 FeNO [M + H] + 360.1045, found 360.1048. The enantiomeric excess was determined by Daicel Chiralpak AD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 5.452 min, t(major) = 5.844 min. Compound (Rp)-3q. Dark red solid (127.5 mg, 99% yield, 99% ee). Analytical data for (Rp)-3q are as follows. Mp: 145.2−147.6 °C. [α]D20 = −543.0° (c = 0.01, acetone, 99% ee). 1H NMR (400 MHz, CDCl3): δ 8.28 (dd, J = 5.6, 2.0 Hz, 1H), 7.57 (dd, J = 7.2, 1.6 Hz, 1H), 6.86 (dd, J = 7.6, 5.6 Hz, 1H), 4.58 (d, J = 2.0 Hz, 1H), 4.53 (t, J = 2.4 Hz, 1H), 4.48 (d, J = 2.0 Hz, 1H), 1.92 (q, J = 7.6 Hz, 10H), 0.99 (t, J = 7.6 Hz, 15H). 13C NMR (100 MHz, CDCl3): δ 191.3, 163.7, 151.7, 135.3, 129.0, 120.3, 90.5, 88.4, 81.7, 80.9, 70.9, 70.0, 18.3, 16.2. IR (film): 2967, 2919, 2872, 2849, 1679, 1591, 1564, 1424, 1384, 1204, 1057, 829, 782, 737 cm−1. HRMS (ESI): calcd for C26H3256FeNO [M + H]+ 430.1828, found 430.1823. The enantiomeric excess was determined by Daicel Chiralpak AD-H (0.46 cm × 25 cm), hexanes/ IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 5.348 min, t(major) = 6.352 min. Compound (Rp)-3r. Dark red solid (121.6 mg, 99% yield, > 99% ee). Analytical data for (Rp)-3r are as follows. Mp: 260.4−262.5 °C. [α]D20 = −1213.9° (c = 0.01, acetone, > 99% ee). 1H NMR (400 MHz, CDCl3): δ 7.99 (s, 1H), 7.90 (d, J = 8.4 Hz, 1H), 7.67 (d, J = 7.6 Hz, 1H), 7.58 (t, J = 8.0 Hz, 1H), 7.34 (t, J = 7.6, 1H), 4.77 (d, J = 2.4 Hz, 1H), 4.58 (t, J = 2.0 Hz, 1H), 4.54 (d, J = 2.0 Hz, 1H), 1.42 (s, 15H). 13 C NMR (100 MHz, CDCl3): δ 189.9, 161.4, 149.8, 134.8, 130.9, 130.0, 128.7, 128.7, 126.5, 125.9, 88.8, 84.4, 84.0, 83.5, 71.2, 70.3, 9.5. IR (film): 2913, 1682, 1619, 1489, 1424, 1374, 819, 798 cm−1. HRMS (ESI): calcd for C25H25Br56FeNO [M + H]+ 490.0463, found 490.0461. The enantiomeric excess was determined by Daicel Chiralpak AS-H (0.46 cm × 25 cm), hexanes/IPA 40/1, 0.41 mL/ min, λ 254 nm, t(major) = 25.917 min, t(minor) = 33.140 min. Compound (Rp)-3s. Dark red solid (112.0 mg, 99% yield, 98% ee). Analytical data for (Rp)-3s are as follows. Mp: 273.2−275.0 °C. [α]D20 = −248.1° (c = 0.005, acetone, 98% ee). 1H NMR (400 MHz, CDCl3): δ 8.24−8.21 (m, 1H), 6.59−6.55 (m, 1H), 4.60 (s, 1H), 4.56 (s, 1H), 4.51 (s, 1H), 1.47 (s, 15H). 13C NMR (100 MHz, CDCl3): δ 187.2, 166.5 (d, J = 4.1 Hz), 162.8 (d, J = 273.3 Hz), 154.3 (d, J = 7.1 Hz), 121.0 (d, J = 8.9 Hz), 109.6 (d, J = 17.1 Hz), 87.3 (d, J = 4.4 Hz), 85.1, 82.8, 80.7, 71.7, 70.8, 9.5. 19F NMR (376 MHz, CDCl3): δ −108.2. IR (film): 2969, 2914, 1677, 1604, 1563, 1427, 1377, 1255, 1044, 1003, 829, 798 cm−1. HRMS (ESI): calcd for C21H21F56FeNO [M + H]+ 378.0951, found 378.0948. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/ IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 7.658 min, t(major) = 11.027 min. Compound (Rp)-3t. Dark red solid (123.4 mg, 96% yield, 99% ee). Analytical data for (Rp)-3t are as follows. Mp: 202.6−204.3 °C. [α]D20 = +734.3° (c = 0.01, acetone, 99% ee). 1H NMR (400 MHz, CDCl3): δ 7.84 (d, J = 6.4 Hz, 1H), 6.15 (d, J = 6.4 Hz, 1H), 4.45 (s, 1H), 4.32 (s, 1H), 4.29 (s, 1H), 3.55−3.47 (m, 4H), 1.91−1.88 (m, 4H), 1.54 (s, 15H). 13C NMR (100 MHz, CDCl3): δ 191.3, 165.8, 150.2, 149.4, 116.9, 107.9, 87.4, 83.7, 80.6, 80.0, 69.6, 69.3, 51.4, 25.6, 9.7. IR (film): 2969, 2902, 1656, 1583, 1531, 1477, 1456, 1382, 1053, 802 cm−1.

Compound (Rp)-3j. Dark red liquid (98.9 mg, 99% yield, 98% ee). Analytical data for (Rp)-3j are as follows. [α]D20 = +18.0° (c = 0.01, acetone, 98% ee). 1H NMR (400 MHz, CDCl3): δ 8.31 (d, J = 4.0 Hz, 1H), 7.60 (d, J = 7.2 Hz, 1H), 6.95 (dd, J = 7.2, 5.2 Hz, 1H), 5.05 (d, J = 2.0 Hz, 1H), 4.98 (d, J = 2.0 Hz, 1H), 4.90 (d, J = 2.0 Hz, 1H), 4.15 (s, 2H), 4.10 (s, 1H), 4.08 (s, 1H), 3.75 (AB, JAB = 11.6 Hz, 1H), 3.65 (BA, JBA = 11.2 Hz, 1H), 3.15 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 192.1, 164.3, 152.4, 134.7, 129.6, 121.0, 88.6, 87.8, 79.6, 77.2, 73.7, 73.4, 73.0, 73.0, 68.7, 67.9, 67.7, 58.1. IR (film): 2923, 1687, 1592, 1568, 1387, 1079, 819, 783, 741 cm−1. HRMS (ESI): calcd for C 18 H 16 56 FeNO2 [M + H] + 334.0525, found 334.0525. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 18.146 min, t(major) = 24.486 min. Compound (Rp)-3k. Dark red liquid (79.5 mg, 83% yield, 98% ee). Analytical data for (Rp)-3k are as follows. [α]D20 = −23.9° (c = 0.01, acetone, 98% ee). 1H NMR (400 MHz, CDCl3): δ 8.31 (d, J = 4.8 Hz, 1H), 7.64 (d, J = 7.2 Hz, 1H), 6.98 (t, J = 5.6 Hz, 1H), 5.16 (s, 1H), 5.06 (s, 1H), 4.97 (s, 1H), 4.19−4.09 (m, 4H), 4.02 (AB, JAB = 13.2 Hz, 1H), 3.82 (BA, JBA = 13.2 Hz, 1H), 3.01(br, 1H). 13C NMR (100 MHz, CDCl3): δ 192.5, 164.6, 152.3, 134.6, 130.0, 121.1, 94.1, 88.1, 79.5, 77.3, 72.5, 72.5, 72.4, 71.4, 68.4, 67.9, 58.6. IR (film): 3308, 1676, 1594, 1572, 1422, 1389, 1008, 820, 782, 740 cm−1. HRMS (ESI): calcd for C17H1456FeNO2 [M + H]+ 320.0368, found 320.0366. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 24.264 min, t(major) = 28.124 min. Compound (Rp)-3l. Dark red liquid (86.5 mg, 83% yield, 98% ee). Analytical data for (Rp)-3l are as follows. [α]D20 = −102.4° (c = 0.01, acetone, 98% ee). 1H NMR (400 MHz, CDCl3): δ 8.33 (d, J = 4.8 Hz, 1H), 7.64 (d, J = 7.2 Hz, 1H), 6.99 (d, J = 7.2, 5.2 Hz, 1H), 5.19 (d, J = 2.0 Hz, 1H), 5.09 (d, J = 2.4 Hz, 1H), 5.00 (t, J = 2.4 Hz, 1H), 4.14−4.08 (m, 4H), 2.46 (s, 1H), 1.38 (s, 3H), 1.31 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 192.7, 164.8, 152.4, 134.8, 130.0, 121.2, 105.0, 88.2, 79.3, 77.2, 73.2, 73.0, 70.4, 69.7, 68.7, 68.3, 68.0, 31.2, 30.9. IR (film): 3318, 1691, 1569, 1389, 838, 776, 680 cm−1. HRMS (ESI): calcd for C19H1856FeNO2 [M + H]+ 348.0681, found 348.0682. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 11.987 min, t(major) = 13.962 min. Compound (Rp)-3m. Dark red solid (108.2 mg, 98% yield, 97% ee). Analytical data for (Rp)-3m are as follows. Mp: 68.9−70.8 °C. [α]D20 = −24.4° (c = 0.01, acetone, 97% ee). 1H NMR (400 MHz, CDCl3): δ 8.40 (br, 1H), 7.67 (d, J = 7.2 Hz, 2H), 7.02−7.01 (m, 1H), 5.11 (s, 1H), 5.03 (s, 1H), 4.93 (s, 1H), 4.36 (br, 2H), 4.06 (br, 2H). 13C NMR (100 MHz, CDCl3): δ 191.7, 163.3, 152.7, 134.8, 130.1, 121.3, 80.7, 80.5, 78.8, 74.9, 74.5, 73.2, 71.5, 71.2, 69.0, 68.7. IR (film): 3077, 2921, 1688, 1591, 1415, 1386, 1078, 819, 782, 740 cm−1. HRMS (ESI): calcd for C16H11Br56FeNO [M + H]+ 367.9368, found 367.9371. The enantiomeric excess was determined by Daicel Chiralpak AD-H (0.46 cm × 25 cm), hexanes/IPA 40/1, 0.41 mL/ min, λ 254 nm, t(minor) = 58.507 min, t(major) = 64.300 min. Compound (Rp)-3n. Dark red liquid (88.5 mg, 93% yield, 99% ee). Analytical data for (Rp)-3n are as follows. [α]D20 = −68.1° (c = 0.01, acetone, 99% ee). 1H NMR (400 MHz, CDCl3): δ 9.57 (s, 1H), 8.39 (br, 1H), 7.69 (br, 1H), 7.03 (br, 1H), 5.16 (s, 1H), 5.02 (s, 1H), 4.91 (s, 1H), 4.75 (s, 2H), 4.46 (s, 1H), 4.34 (s, 1H). 13C NMR (100 MHz, CDCl3): δ 192.2, 191.5, 163.2, 153.1, 134.3, 130.3, 121.7, 89.9, 82.3, 79.8, 77.3, 76.9, 76.6, 73.2, 67.9, 67.8. IR (film): 2971, 2921, 1680, 1571, 1244, 1078, 828, 781, 738 cm−1. HRMS (ESI): calcd for C 17 H 12 56 FeNO2 [M + H] + 318.0212, found 318.0213. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/min, λ 254 nm, t(minor) = 41.790 min, t(major) = 46.884 min. Compound (Rp)-3o. Dark red solid (106.9 mg, 92% yield, 99% ee). Analytical data for (Rp)-3o are as follows. Mp: 56.9−58.1 °C. [α]D20 = +288.3° (c = 0.01, acetone, 99% ee). 1H NMR (400 MHz, CDCl3): δ 8.27 (s, 1H), 7.54 (d, J = 7.6 Hz, 1H), 6.88 (t, J = 6.0 Hz, 1H), 6.78 (d, J = 16.0 Hz, 1H), 5.63 (d, J = 16.0 Hz, 1H), 5.02 (s, 1H), 4.90 (s, 1H), 4.81 (s, 1H), 4.37 (s, 2H), 4.27 (s, 1H), 4.20 (s, 1H), 4.09 (q, J = 7.2 4623

DOI: 10.1021/acs.organomet.5b00730 Organometallics 2015, 34, 4618−4625

Article

Organometallics HRMS (ESI): calcd for C25H2956FeN2O [M + H]+ 429.1624, found 429.1625. The enantiomeric excess was determined by Daicel Chiralpak AD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/ min, λ 254 nm, t(major) = 8.573 min, t(minor) = 9.918 min. Compound (Rp)-3u. Dark red solid (117.1 mg, 97% yield, 99% ee). Analytical data for (Rp)-3u are as follows. Mp: 235.1−237.8 °C. [α]D20 = +653.8° (c = 0.01, acetone, 99% ee). 1H NMR (400 MHz, CDCl3): δ 7.91 (d, J = 6.0 Hz, 1H), 6.27 (d, J = 6.0 Hz, 1H), 4.47 (d, J = 2.4 Hz, 1H), 4.36 (t, J = 2.4 Hz, 1H), 4.32 (d, J = 2.4 Hz, 1H), 3.03 (s, 6H), 1.52 (s, 15H). 13C NMR (100 MHz, CDCl3): δ 191.1, 166.0, 153.6, 151.0, 118.7, 108.7, 87.4, 84.0, 80.5, 80.4, 69.9, 69.5, 42.9, 9.7. IR (film): 1666, 1581, 1529, 1378, 1034, 1008, 826, 809, 793 cm−1. HRMS (ESI): calcd for C23H2756FeN2O [M + H]+ 403.1467, found 403.1467. The enantiomeric excess was determined by Daicel Chiralcel OD-H (0.46 cm × 25 cm), hexanes/IPA 90/10, 1.0 mL/ min, λ 254 nm, t(minor) = 10.896 min, t(major) = 21.195 min. Compound (Rp)-3v. Dark red solid (172.8 mg, 86% yield, 98% ee). Analytical data for (Rp)-3v are as follows. Mp: 286.4−288.3 °C. [α]D20 = −105.1° (c = 0.01, acetone, 98% ee). 1H NMR (400 MHz, CDCl3): δ 8.21 (dd, J = 5.2, 1.6 Hz, 1H), 7.54 (dd, J = 7.6, 1.6 Hz, 1H), 7.14− 7.10 (m, 5H), 7.03−6.99 (m, 10H), 6.94 (dd, J = 7.6, 5.2 Hz, 1H), 6.87−6.85 (m, 10H), 5.06 (t, J = 2.4 Hz, 1H), 4.97 (d, J = 2.0 Hz, 1H), 4.85 (d, J = 2.4 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 191.9, 161.9, 152.8, 136.0, 133.4, 132.0, 129.7, 127.3, 126.7, 121.3, 91.9, 90.4, 84.8, 81.8, 73.1, 73.0. IR (film): 2988, 1689, 1610, 1571, 1393, 1256, 1053, 827, 797, 755 cm−1. HRMS (ESI): calcd for C46H3256FeNO [M + H]+ 670.1828, found 670.1818. The enantiomeric excess was determined by Daicel Chiralpak AD-H (0.46 cm × 25 cm), hexanes/ IPA 40/1, 0.41 mL/min, λ 254 nm, t(minor) = 23.491 min, t(major) = 25.278 min. Synthesis of Planar Chiral Pyridine N-Oxides and Their Application in Asymmetric Opening of meso-Epoxide. To a solution of 3 (0.3 mmol, 1.0 equiv) in DCM (2 mL) at −10 °C was added dimethyldioxirane (4.0 mL). The reaction mixture was stirred for 5 h until the reaction was complete. Then the solvent was removed under reduced pressure at −10 °C, and the residue was purified by silica gel column chromatography (ethyl acetate/methanol 1/15, v/v) to give 4 as a solid. In a dry flask containing cis-stilbene oxide 5 (25.0 mg, 0.127 mmol, 1.0 equiv), catalyst 4 (0.0127 mmol, 0.1 equiv), and DCM (1.0 mL), ethyldiisopropylamine (18.1 mg, 0.14 mmol, 1.1 equiv) was subsequently added by syringe. The reaction mixture was cooled to −78 °C. Tetrachlorosilane (23.9 mg, 0.14 mmol, 1.1 equiv) was added dropwise. After the reaction was complete, it was quenched by the addition of saturated KF/KH2PO4 (1/1, v/v). The resulting mixture was extracted with dichloromethane, and the organic layer was washed with brine, dried with Na2SO4, filtered, and concentrated. The residue was purified by silica gel column chromatography (ethyl acetate/ petroleum 1/10, v/v) to give 6 as an oil. Compound (Rp)-4a. Dark red solid (85.0 mg, 93% yield). Analytical data for (Rp)-4a are as follows. Mp: >250 °C. [α]D20 = −2006.4° (c = 0.0025, acetone). 1H NMR (400 MHz, CDCl3): δ 8.05 (br, 1H), 7.31 (br, 1H), 7.04 (br, 1H), 5.50 (br, 1H), 5.19−5.11 (m, 2H), 4.30 (s, 5H). 13C NMR (100 MHz, CDCl3): δ 189.8, 151.2, 142.8, 138.2, 123.0, 120.1, 82.1, 77.8, 73.5, 71.5, 68.5. IR (film): 3086, 1688, 1437, 1412, 1262, 1240, 1215, 1016, 827, 805, 747 cm−1. HRMS (ESI): calcd for C16H1256FeNO2 [M + H]+ 306.0212, found 306.0212. Compound (Rp)-4p. Dark red solid (100.0 mg, 89% yield). Analytical data for (Rp)-4p are as follows. Mp: >250 °C. [α]D20 = −5561.9° (c = 0.0025, acetone). 1H NMR (300 MHz, CDCl3): δ 7.99 (d, J = 6.9 Hz, 1H), 7.26 (d, J = 7.2 Hz, 1H), 6.92 (t, J = 6.9 Hz, 1H), 5.04 (d, J = 2.1 Hz, 1H), 4.72 (t, J = 2.4 Hz, 1H), 4.59 (d, J = 1.8 Hz, 1H), 1.55 (s, 15H). 13C NMR (75 MHz, CDCl3): δ 188.0, 150.9, 142.6, 138.9, 122.2, 119.8, 86.1, 84.0, 82.7, 79.9, 75.7, 71.5, 9.9. IR (film): 2972, 2902, 1679, 1440, 1408, 1382, 1258, 1067, 1035, 754 cm−1. HRMS (ESI): calcd for C21H2256FeNO2 [M + H]+ 376.0994, found 376.0992. Compound (Rp)-4q. Dark red solid (130.5 mg, 97% yield). Analytical data for (Rp)-4q are as follows. Mp: >250 °C. [α]D20 = −1967.4 (c = 0.005, acetone). 1H NMR (400 MHz, CDCl3): δ 7.97

(d, J = 6.8 Hz, 1H), 7.22 (d, J = 7.2 Hz, 1H), 6.88 (t, J = 7.2 Hz, 1H), 4.95 (d, J = 2.0 Hz, 1H), 4.64 (s, 1H), 4.54 (d, J = 2.4 Hz, 1H), 1.97 (q, J = 7.2 Hz, 10H), 1.00 (t, J = 7.6 Hz, 15H). 13C NMR (100 MHz, CDCl3): δ 188.2, 150.8, 142.4, 138.7, 122.0, 119.8, 91.6, 83.1, 82.6, 79.6, 75.1, 71.0, 18.3, 16.2. IR (film): 3095, 2965, 2928, 2872, 1687, 1443, 1262, 1012, 832, 807, 762, 671, 665 cm−1. HRMS (ESI): calcd for C26H3256FeNO2 [M + H]+ 446.1777, found 446.1776. Compound (Rp)-4v. Dark red solid (195.0 mg, 86% yield). Analytical data for (Rp)-4v are as follows. Mp: >250 °C. [α]D20 = −1052.2° (c = 0.005, acetone). 1H NMR (400 MHz, CDCl3): δ 7.85 (d, J = 6.8 Hz, 1H), 7.17−7.11 (m, 6H), 7.04−7.00 (m, 10H), 6.96 (d, J = 6.8 Hz, 1H), 6.93−6.90 (m, 10H), 5.40 (d, J = 2.0 Hz, 1H), 5.14 (t, J = 2.0 Hz, 1H), 4.90 (d, J = 2.4 Hz, 1H). 13C NMR (100 MHz, CDCl3): δ 189.3, 148.8, 143.4, 139.5, 133.1, 132.0, 127.3, 126.9, 123.3, 119.5, 91.0, 86.8, 85.6, 80.4, 75.6, 74.2. IR (film): 2988, 2901, 1695, 1442, 1410, 1268, 1244, 1075, 1026, 740, 699 cm−1. HRMS (ESI): calcd for C46H3256FeNO2 [M + H]+ 686.1777, found 686.1773. (1R,2R)-2-Chloro-1,2-diphenylethanol (6). Analytical data for 6 are as follows. [α]D20 = −13.5° (c = 0.97, ethanol, 66% ee). 1H NMR (400 MHz, CDCl3): δ 7.22−7.09 (m, 10H), 5.00−4.93 (m, 2H), 3.12 (d, J = 2.4 Hz, 1H). The enantiomeric excess was determined by Daicel Chiralpak AD-H (0.46 cm × 25 cm), hexanes/IPA 95/5, 1.0 mL/min, λ 254 nm, t(major) = 16.875 min, t(minor) = 18.096 min. The absolute configuration of product 6 was assigned as R,R by comparing the optical rotation with that reported in the literature.2a

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ASSOCIATED CONTENT

S Supporting Information *

. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00730. Experimental procedures and analysis data for all new compounds and details of the crystallographic structure analysis and computations (PDF) Crystallographic data for (Rp)-3s (CIF) Cartesian coordinates of the calculated structures (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for Q.G.: [email protected]. *E-mail for S.-L.Y.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Basic Research Program of China (973 Program 2015CB856600), the National Natural Science Foundation of China (21332009, 21421091, 21572250), and the Science and Technology Commission of Shanghai Municipality (13JC1406900) for generous financial support.

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REFERENCES

(1) For books see: (a) Ferrocenes; Hayashi, T., Togni, A., Eds.; VCH: Weinheim, Germany, 1995. (b) Metallocenes; Togni, A., Haltermann, R. L., Eds.; VCH: Weinheim, Germany, 1998. (c) Štěpnička, P., Ed. Ferrocenes; Wiley: Chichester, U.K., 2008. (d) Chiral Ferrocenes in Asymmetric Catalysis; Dai, L.-X.; Hou, X.-L., Eds.; Wiley: Chichester, U.K., 2010. (2) (a) Tao, B.; Lo, M. M.-C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 353−354. (b) Fu, G. C. Acc. Chem. Res. 2004, 37, 542−547. (3) For a review, see: (a) Schaarschmidt, D.; Lang, H. Organometallics 2013, 32, 5668−5704. For selected recent examples, see: (b) Siegel, S.; Schmalz, H.-G. Angew. Chem., Int. Ed. Engl. 1997, 36, 2456−2458. (c) Günay, M. E.; Richards, C. J. Organometallics 2009, 28, 5833−5836. (d) Dendele, N.; Bisaro, F.; Gaumont, A.-C.; Perrio, 4624

DOI: 10.1021/acs.organomet.5b00730 Organometallics 2015, 34, 4618−4625

Article

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DOI: 10.1021/acs.organomet.5b00730 Organometallics 2015, 34, 4618−4625