NHTs Effect on the Enantioselectivity of Ru(II) Complex Catalysts

Article pubs.acs.org/Organometallics

NHTs Effect on the Enantioselectivity of Ru(II) Complex Catalysts Bearing a Chiral Bis(NHTs)-Substituted Imidazolyl-OxazolinylPyridine Ligand for Asymmetric Transfer Hydrogenation of Ketones Huining Chai,†,‡,∥ Tingting Liu,†,‡,∥ and Zhengkun Yu*,†,§ †

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, 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: Pincer-type ruthenium(II)-NNN complex catalysts bearing a chiral bis(NHTs)-substituted imidazolyl-oxazolinyl-pyridine ligand were synthesized and structurally characterized by NMR, IR, elemental analysis, and X-ray single-crystal crystallographic determinations. The two NHTs groups substituted on the imidazolyl moiety of the chiral NNN ligand exhibited a remarkable effect on the enantioselectivity of the Ru(II)-NNN complexes for the asymmetric transfer hydrogenation (ATH) of ketones. The Ru(II)-NNN complex bearing a chiral (NHTs)2substituted imidazolyl-(isopropyl)oxazolinyl-pyridine ligand exhibited excellent catalytic activity, reaching an enantioselectivity up to 99.9% ee for the target alcohol products.

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INTRODUCTION Enantioselective reduction of prochiral ketones to secondary alcohols by asymmetric catalysis is significant because it can provide a convenient and atom-efficient route to valuable alcoholic intermediates in pharmaceutical, agricultural, and synthetic chemistry.1 Noyori-type Ru(II) complexes containing a chiral monotosylated 1,2-diamine or aminoalcohol ligand2 have exhibited excellent catalytic activity for asymmetric transfer hydrogenation of ketones. Due to the ideal compromise between the steric and electronic properties of the arene and chiral diamine auxiliary, such Ru(II) complexes supported by a chiral N-tosylated 1,2-diamine can serve as efficient catalysts for the asymmetric reduction.3 Functionalization of the NH2 terminus with a tosyl group is crucially important to modify the steric and electronic properties of the ligand.4 Encouraged by this fundamental study, a great deal of relevant work has been devoted to this area. The Wills group combined a single p-toluenesulfonyl-functionalized chiral diamine ligand with an aromatic ring through an alkyl or sulfonyl linker to synthesize tethered ruthenium(II) complexes.5 Such a linker limits rotation of the aromatic ring, facilitates the retention of the catalytic central configuration, and thus increases the catalytic activity of the complex catalysts. Ikariya et al. prepared Ru(II) complexes of oxo-tethered chiral diamine-aryl ligands, which exhibited excellent catalytic activity in the asymmetric transfer hydrogenation and direct hydrogenation of ketones.6 Xiao et al. reported that Noyori−Ikariya catalysts also enabled efficient ATH reactions in neat water.7 Baratta et al. reported versatile © XXXX American Chemical Society

ruthenium(II) and osmium(II) complexes bearing a 2-aminomethylpyridine (ampy) ligand which have demonstrated efficient catalytic activity in the asymmetric transfer hydrogenation of ketones.8 Transition-metal complexes bearing a ligand with an NH functionality can be readily transformed and usually exhibit high reactivity; thus, such an “NH effect” strategy has been applied in the design of transition-metal complex catalysts for direct hydrogenation or transfer hydrogenation of ketones9,10 or for diverse organic transformations.11,12 Recently, we reported the catalytic behaviors of Ru(II) complexes bearing a pyridyl-based NNN ligand featuring a coordinative (NHTs)2-substituted imidazolyl moiety in the transfer hydrogenation reactions of ketones.13 Introduction of the NHTs functionality on the heteroaryl backbone of the coordinative moiety remarkably enhanced the catalytic activity of complex A (Chart 1). Intrigued by our ongoing investigation of pincertype transition-metal complex catalysts,14 we reasonably envisioned that introduction of the NHTs functionality to the ligand backbone of chiral Ru(II)-NNN complexes B and C15 might also lead to alteration of their catalytic properties. It should be noted that Ru(II) complexes B and C supported by a chiral imidazolyl-oxazolinyl-pyridine ligand can exhibit good catalytic activity in the ATH of ketones.15 Herein, we report the synthesis of Ru(II) complexes bearing a chiral Received: July 24, 2017

A

DOI: 10.1021/acs.organomet.7b00559 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Chart 1. Ruthenium(II)-NNN Complex Catalysts

Scheme 1. Synthesis of Ligands 5 and Complexes 6a

Legend: (i) NaHSO3, EtOH, 78 °C, 6 h, 40%; (ii) K4[Fe(CN)6], CuI, 1-methyl-1H-imidazole, 160 °C, 12 h, 77%; (iii) Zn(OTf)2, toluene, 110 °C, 2 days; (iv) Ru(PPh3)3Cl2, toluene, 110 °C, 2 h.

a

(NHTs)2-substituted imidazolyl-oxazolinyl-pyridine ligand and their catalytic behaviors in the asymmetric transfer hydrogenation of ketones.

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RESULTS AND DISCUSSION Synthesis of Ligands and Ru(II)-NNN Complexes. The reaction of 6-bromopicolinaldehyde (1) and N,N′-(4,5-diamino1,2-phenylene)bis(4-methylbenzenesulfonamide) (2) in the presence of NaHSO3 led to pyridyl-functionalized imidazole 3. Copper(I)-catalyzed cyanation of 3 with K4[Fe(CN)6] afforded intermediate compound 4. Subsequent condensation with (S)-valinol, (S)-tert-leucinol, or (S)-phenylglycinol in refluxing toluene gave chiral (NHTs)2-substituted imidazolyl-oxazolinylpyridine ligands 5a−c in 52−54% yields. Treatment of equimolar amounts of ligand 5 with RuCl2(PPh3)3 in refluxing toluene produced chiral pincer-type Ru(II)-NNN complexes 6a (85%), 6b (81%), and 6c (80%), respectively (Scheme 1). Neutral complex 6a could be transformed into the corresponding cationic complex 6d by recrystallization from acetonitrile/diethyl ether/ n-hexane at ambient temperature (eq 1). In a similar fashion, condensation of compound 4′15 with (S)-tert-leucinol afforded chiral ligand 5b′ (50%), which was reacted with RuCl2(PPh3)3 to form chiral Ru(II)-NNN complex 6b′ (88%) (Scheme 2).

Characterization of Ligands and Ru(II)-NNN Complexes. The NMR, HRMS, and/or elemental analyses of all the new compounds are consistent with their compositions. In the 1H NMR spectra determined in CDCl3, the benzimidazolyl NH functionality of ligands 5a−c appeared as singlets at 12.59, 13.01, and 11.61 ppm, respectively, while such NH resonance signals in ligand 5b′, complexes 6a−c, and 6b′ disappeared due to the H−D exchange in CD3OD or DMSO-d6 solutions. The 31 P NMR resonances of complexes 6a, 6b, and 6b′ were shown

Scheme 2. Synthesis of Complex 6b′a

a

Legend: (i) Zn(OTf)2, toluene, 110 °C, 2 days; (ii) Ru(PPh3)3Cl2, toluene, 110 °C, 2 h. B


Article

Organometallics

in 2-propanol. Thus, the catalytic behaviors of complexes 6a−d, 6b′, B, and C were comparatively investigated with 0.1 mol % Ru loading (Table 1). Due to the NHTs effect and effect of the alkyl or aryl functionality on the chiral oxazolinyl moiety of the ligand, these complexes exhibited various catalytic activities and enantioselectivities in the ATH reactions of acetophenone, 4′-chloroacetophenone, and 4′-methylacetophenone. The ketones were exclusively reduced to the corresponding (S)-alcohols in 93−97% yields with 94−98% ee by using complex 6a as the catalyst, whereas complex B only exhibited a poor enantioselectivity (20−59% ee).15 Unexpectedly, complexes 6b and 6b′ both showed very low catalytic activity to afford the (R)-alcohol products. Complex C15 demonstrated a catalytic activity superior to that of the NHTs-functionalized NNN ligand-Ru(II) complex 6c, reaching 90−98% yields and 70−86% ee for the target products. Cationic Ru(II)-NNN complex 6d exhibited a catalytic activity much lower than those for complexes 6a, B, 6c, and C but reached the second-best enantioselectivity among all the complexes. The coordinated acetonitrile molecule may prohibit the ketone substrate from interacting with the metal center of the complex catalyst, leading to a low catalytic activity for 6d. It is noteworthy that (NHTs)2functionalized complex 6a can exhibit a catalytic activity comparable to or better than that of its corresponding nonNHTs-functionalized counterpart B with a much better enantioselectivity, revealing a remarkable NHTs effect on the enantioselectivity. Except in the case of complex C bearing chiral (4′-phenyl)oxazolinyl-imidazolyl-pyridine as the ligand, the Ru(II) complexes supported by the (NHTs)2-functionalized NNN ligands exhibited much better enantioselectivities (Table 1). Asymmetric Transfer Hydrogenation of Ketones. Under the stated conditions, a variety of aromatic and heteroaromatic ketones were applied as substrates to explore the protocol generality by means of complex 6a as the catalyst (Table 2). As a comparison, the results using the non-NHTs functionalized NNN ligand-supported chiral Ru(II) complex catalyst, that is, complex C, are also presented. Complex 6a exhibited excellent catalytic activity for the ATH reactions of acetophenones bearing substituents such as Cl, Br, Me, and OMe, reaching 72−100% yields and 92 to >99.9% ee. The steric hindrance from the substituents in the substrates altered the reaction rates and product enantioselectivities. The best result was obtained in the case of 2′-bromoacetophenone, reaching 100% yield with >99.9% enantioselectivity (Table 2, entry 6). Only in the cases of 2′- and 4′-methoxyacetophenones could the reactions not reach a complete conversion for the ketone substrates (72−73% yields) with a relatively low ee value of 92% (Table 2, entries 12 and 14). In general, the ortho- and meta-substituted acetophenones could be reduced to the corresponding alcohols with a enantioselectivity higher than that of their para-substituted analogues. It is obvious that the chiral non-NHTs-functionalized NNN ligand-supported Ru(II) complex catalyst C exhibited a lower catalytic activity and a much lower enantioselectivity (70−97% ee) in comparison to complex 6a, and in the case of 4′-methoxyacetophenone only 72% yield and 58% ee were obtained (Table 2, entry 14). Unexpectedly, complexes 6a and C exhibited reversed catalytic activity and enantioselectivity in the case of 2′-CF3-acetophenone (Table 2, entry 15): that is, complex C demonstrated a higher catalytic activity than complex 6a and the corresponding alcohol product has the R configuration by using 6a as the catalyst, while an (S)-alcohol product was the product with complex

as singlets at 53.7, 53.9, and 53.2 ppm, respectively, in CD3OD, and that of complex 6c appeared at 32.9 ppm in DMSO-d6. The chemical shift difference of the 31P NMR signals between complex 6c and other complexes is attributed to the following two reasons: (a) the deuterated solvent effect of CD3OD and DMSO-d6 and (b) the coordination of DMSO-d6 to the ruthenium center of complex 6c as we previously reported.9e It should be noted that complex 6c was poorly soluble in CD3OD, so that its 31 P NMR in CD3OD was not successfully obtained for comparison. Due to the susceptibility of the coordinated acetonitrile molecule, complex 6d could not be dried under reduced pressure, and its NMR spectra were collected by using a sample directly picked up from the recrystallization mixture without dryness (see the Supporting Information for details). The CH3 proton resonance signal of the coordinated acetonitrile molecule appears at 2.70 ppm in CDCl3 as a singlet. In the 13C NMR spectrum, the coordinated CH3CN exhibits two resonance signals at 3.9 and 112.3 ppm, respectively. The 31P NMR resonance signal is shown at 51.7 ppm. The IR spectral analyses reveal the N−H absorption peaks of all the complexes at ∼3400 cm−1. Fortunately, the solid-state molecular structure of complex 6d was confirmed by an X-ray single-crystal structural analysis (Figure 1; see the Supporting Information for details). In the

Figure 1. Molecular structure of complex 6d. Hydrogen atoms are omitted for clarity.

solid state, the central ruthenium atom is situated in a distorted-octahedral environment surrounded by tridentate NNN ligand 5a, one PPh3, one chloride, and one acetonitrile molecule. The PPh3 ligand is positioned trans to the bonded chloride, and these two ligands are situated at the two sides of the NNN ligand plane with P−Ru−N angles of 88.5°−96.4° and P−Ru−Cl angle of 177.3°. The coordinated acetonitrile molecule is arranged trans to the pyridyl nitrogen atom, with a dissociated chloride anion around. The N(2)−Ru−N(7) angle is 174.7°. The Ru−P and Ru−Cl bond lengths are 2.29 and 2.43 Å, respectively, and those of the four Ru−N bonds are in the region of 1.98− 2.14 Å. These results suggest that the ruthenium atom is situated at the center of a distorted-octahedral environment. Remarkably, one of the NHTs functionalities is arranged syn to the isopropyl group of the oxazolyl moiety to reduce the steric hindrance. Comparison of the Catalytic Activities of Complexes 6. The asymmetric transfer hydrogenation reactions of ketones were conducted at 28 °C by using a 0.1 M ketone solution C


Article

Organometallics Table 1. Comparison of Catalytic Behaviors of Complexes 6a

a b

Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); Ru(II) catalyst, 0.1 mol %; ketone/iPrOK/Ru(II) cat., 1000/20/1; 0.1 MPa N2; 28 °C. Determined by GC analysis (column: β-DEX 225). cCited from ref 15.

catalyst C. Complex 6a also exhibited a catalytic activity inferior to that of complex C in the ATH reaction of 3′-CF3-acetophenone (Table 2, entry 16). Complex 6a facilitated the ATH reaction of 2-acetylnaphthalene (Table 2, entry 17). The ATH reaction of

heteroaromatic ketone 2-acetylfuran was carried out efficiently in the presence of complex 6 or C, but only 65−73% ee values were obtained (Table 2, entry 18). However, acetylpyridine did not react under the stated conditions, presumably due to its D


Article

Organometallics Table 2. Transfer Hydrogenation of Ketones Catalyzed by 6a and Ca

Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); catalyst, 0.1 mol %; ketone/iPrOK/Ru(II) cat., 1000/20/1; 0.1 MPa N2; 28 °C. Determined by GC analysis (column: β-DEX 225). cUsing 0.2 mol % of catalyst. dCited from ref 15. eDetermined by HPLC analysis on a Daicel Chiralpak IC column as reported in ref 16. fUsing 0.5 mol % of catalyst.

a b

(E)-4-Phenylbut-3-en-2-one did not react under the stated conditions due to its low reactivity. It should be noted that, except for 2′-fluoroacetophenone, complex 6a catalyzed the ATH reactions to produce (S)-alcohol products, while (R)-alcohols were obtained by using C as the catalyst. It has been confirmed that one of the NHTs functionalities in the (NHTs)2-benzimidazolyl of the ligand in the cationic counterpart of complex 6a, that is, complex 6d, is positioned syn to the isopropyl group attached to the oxazolyl

strong binding to the catalytic metal center. The ATH reactions of aliphatic and unsaturated ketones were also examined with complexes 6 and C as the catalysts under the standard conditions (28 °C, 0.1 mol % of catalyst), but the reactions seldomly occurred. When the catalyst loading was increased to 0.5 mol %, octan-2-one was reduced to octan-2-ol in 33% yield and 99% ee within 30 min in the case of using complex 6 as the catalyst, and complex C promoted the same reaction to give the target product in 15% yield with 99% ee (Table 2, entry 19). E


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Organometallics

(DMSO-d6, 100 MHz): δ 150.5 and 149.0 (Cq each), 143.7 (Cq), 141.3, 141.1 (Cq), 140.8, 135.9 (Cq), 133.1 (Cq), 129.8, 129.1, 128.0, 127.2, 125.5 (Cq), 120.9, 115.5, 107.0, 21.1. HRMS: calcd for C26H22BrN5O4S2 611.0297, found 611.0299. Synthesis of Compound 4.

moiety, which can remarkably enhance the enantioselectivity control for complex 6a.

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CONCLUSIONS In summary, Ru(II) complexes bearing a chiral (NHTs)2substituted imidazolyl-oxazolinyl-pyridine ligand have exhibited a remarkable NHTs effect on the enantioselectivity in the ATH reactions of ketones. The cooperativity of the imidazolyl NH and NHTs groups is of significance for such chiral pincer Ru(II)-NNN complex catalysts to exhibit excellent catalytic activity and enantioselectivity. The present method provides an alternative route to highly active chiral transition-metal complex catalysts.

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EXPERIMENTAL SECTION

A mixture of CuI (44 mg, 0.23 mmol), 3 (709 mg, 1.16 mmol), K4[Fe(CN)6] (426 mg, 1.16 mmol), and 1-methyl-1H-imidazole (7 mL) was heated with stirring at 160 °C for 12 h. After the mixture was cooled to ambient temperature, 20 mL of water was added and the mixture extracted with EtOAc (3 × 15 mL). The organic phase was dried over anhydrous MgSO4 and filtered. After all the volatiles were removed under reduced pressure, the resultant residue was purified by silica gel column chromatography (eluent petroleum ether (60−90 °C)/ ethyl acetate 1/1, v/v) to give 4 as a yellow solid (500 mg, 77% yield). Mp: 118−120 °C. 1H NMR (DMSO-d6, 400 MHz): δ 13.15 (s, 1 H, NH), 9.22 and 9.16 (s each, 2 H, 1″-NH and 1‴-NH), 8.47 and 8.11 (d each, J = 7.9 and 7.5 Hz, 1:1 H, 3-H and 5-H), 8.20 (t, J = 7.9 Hz, 1 H, 4-H), 7.60 (m, 4 H, 4″-H and 4‴-H), 7.35 (d, J = 7.1 Hz, 4 H, 5″-H and 5‴-H), 7.22 and 7.17 (s each, 1:1 H, 5′-H and 8′-H), 2.35 (s, 6 H, C6″-CH3 and C6‴-CH3). 13C{1H} NMR (DMSO-d6, 100 MHz): δ 150.3 and 149.4 (Cq each), 143.7 (Cq), 143.6, 140.9 (Cq), 139.5, 136.0 (Cq), 133.1 (Cq), 129.8, 132.4, 128.2 (Cq), 127.1, 125.6 (Cq), 125.2, 117.2 (Cq, −CN), 115.5, 107.0, 21.1. HRMS: calcd for C27H22N6O4S2 558.1144, found 558.1156. Synthesis of Ligand 5a.

General Considerations. All the manipulations of air- and/or moisture-sensitive compounds were carried out under a nitrogen atmosphere using the standard Schlenk techniques. The solvents were dried and distilled prior to use by the literature methods. 1H and 13 C{1H} NMR spectra were recorded on a 400 MHz spectrometer, and all chemical shift values refer to δTMS 0.00 ppm in CDCl3 (δ(1H), 7.26 ppm; δ(13C), 77.16 ppm), DMSO-d6 (δ(1H), 2.50 ppm; δ(13C), 39.52 ppm), and CD3OD (δ(1H), 3.30 ppm; δ(13C), 49.00 ppm). Elemental and HRMS analysis were achieved by the Analysis Center, Dalian University of Technology, and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. TLC analysis was performed by using glass-backed plates coated with 0.2 mm silica gel. Flash column chromatography was performed on silica gel (200−300 meshes). All chemical reagents were purchased from commercial sources and used as received unless otherwise indicated. X-ray Crystallographic Studies. X-ray diffraction studies for complex 6d were carried out on a SMART APEX diffractometer with graphite-monochromated Mo Kα radiation (λ= 0.71073 Å). Cell parameters were obtained by global refinement of the positions of all collected reflections. Intensities were corrected for Lorentz and polarization effects and empirical absorption. The structures were solved by direct methods and refined by full-matrix least squares on F2. All nonhydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions. Structure solution and refinement were performed by using the SHELXL-97 package. The X-ray crystallographic files, in CIF format, are available from the Cambridge Crystallographic Data Centre on quoting the deposition number CCDC 1560687 for 6d. General Synthetic Procedures. Synthesis of Compound 3.

An oven-dried 25 mL two-necked flask fitted with a reflux condenser was charged with 4 (250 mg, 0.45 mmol) and zinc triflate (179 mg, 0.49 mmol). The system was purged with nitrogen, and anhydrous toluene (5 mL) was added. The mixture was stirred for 5 min, and then a solution of (S)-valinol (51 mg, 0.49 mmol) in anhydrous toluene (5 mL) was added. The reaction was set to reflux for 24 h. After the mixture was cooled to ambient temperature, all of the volatiles were removed under reduced pressure. The resultant residue was subject to purification by silica gel column chromatography (eluent dichloromethane/methanol 50/1, v/v), affording 5a as a yellow solid (151 mg, 52% yield). Mp: 156−158 °C. [α]20D = +33.60 (c = 0.50, CH3OH). 1H NMR (CDCl3, 400 MHz): δ 13.01 (s, 1 H, NH), 8.24 and 7.90 (d each, J = 7.6 and 7.2 Hz, 1:1 H, 3-H and 5-H), 7.84 (t, J = 7.7 Hz, 1 H, 4-H), 7.51 and 7.47 (br each, 2:2 H, 4‴-H and 4″″-H), 7.35 (s, 1 H, 5″-H), 7.08 (br, 4:1 H, 5‴-H, 5″″-H and 8″-H), 4.52 and 4.23 (t each, 1:1 H, 3′-H), 4.15 (m, 1 H, 4′-H), 2.25 (s, 6 H, C6‴-CH3 and C6″″-CH3), 1.87 (m, 1 H, CH(CH3)2), 1.02 and 0.92 (d each, J = 6.7 and 6.7 Hz, 3:3 H, CH(CH3)2). 13C{1H} NMR (CDCl3, 100 MHz): δ 162.7 (Cq), 152.4 and 148.0 (Cq each), 146.0 (Cq), 144.2 and 144.0 (Cq), 142.1 (Cq), 138.1, 136.0 (Cq), 135.2 (Cq), 133.5, 129.7, 129.6, 127.6, 127.5, 125.2 (Cq), 124.7 (Cq), 123.5, 118.6, 108.4, 72.5, 71.1, 32.7, 21.6, 19.0, 18.3. HRMS: calcd for C32H32N6O5S2 644.1876, found 644.1885.

A mixture of 6-bromopicolinaldehyde (1; 72 mg, 2 mmol), N,N′-(4,5diamino-1,2-phenylene)bis(4-methylbenzenesulfonamide (2; 893 mg, 2 mmol), and NaHSO3 (218 mg, 2.1 mmol) in ethanol (20 mL) was refluxed with stirring for 6 h. The resultant mixture was cooled to ambient temperature. After removal of all the volatiles in vacuo, the residue was subject to purification by silica gel column chromatography (eluent dichloromethane/methanol 50/1, v/v), affording 3 as a yellow solid (490 mg, 40% yield). Mp: 168−170 °C. 1H NMR (DMSO-d6, 400 MHz): δ 12.95 (s, 1 H, NH), 9.20 (s, 2 H, 1″-NH and 1‴-NH), 8.19 and 7.70 (d each, J = 7.6 and 7.8 Hz, 1:1 H, 3-H and 5-H), 7.87 (t, J = 7.8 Hz, 1 H, 4-H), 7.61 and 7.34 (d each, J = 7.9 and 7.9 Hz, 4:4 H, 4″-H, 4‴-H and 5″-H, 5‴-H), 7.20 (s, 2 H, 5′-H and 8′-H), 2.33 (s, 6 H, C6″-CH3 and C6‴-CH3). 13C{1H} NMR F


Article

Organometallics Synthesis of Ligand 5b.

138.0 (Cq), 135.7 and 135.5 (Cq), 132.9 (Cq), 129.6, 129.5, 129.0, 128.8, 127.9 (Cq), 127.4, 126.7, 125.4 (Cq), 125.1 (Cq), 123.8, 117.6, 107.5, 75.6, 69.9, 21.5, 21.4. HRMS: calcd for C35H30N6O5S2 678.1719, found 678.1720. Synthesis of Complex 6a.

In a fashion similar to the synthesis of 5a, compound 4 (100 mg, 0.18 mmol) was reacted with (S)-tert-leucinol (23 mg, 0.20 mmol), and zinc triflate (73 mg, 0.20 mmol) to give 5b as a yellow solid (62 mg, 53% yield). Mp: 165−167 °C. [α]20D = +35.24 (c = 1.18, CH3OH). 1H NMR (CDCl3, 400 MHz): δ 12.59 (s, 1 H, NH), 8.17 (d, J = 7.2 Hz, 1 H, 3-H), 7.79 (m, 2 H, 5-H and 4-H), 7.41(br, 4 H, 4‴-H and 4″″-H), 7.31(s, 1 H, 5″-H), 7.00 (br, 4:1 H, 5‴-H, 5″″-H and 8″-H), 4.39 and 4.25 (t each, 1:1 H, 3′-H), 4.02 (t, 1 H, 4′-H), 2.18 (s, 6 H, C6‴-CH3 and C6″″-CH3), 0.87 (s, 9 H, C(CH3)3). 13 C{1H} NMR (CDCl3, 100 MHz): δ 162.9 (Cq), 152.8 and 148.2 (Cq each), 145.7 (Cq), 144.2 and 142.2 (Cq each), 138.2, 136.0 (Cq), 135.2 (Cq), 133.9, 129.7, 127.6, 125.2 (Cq), 124.5 (Cq), 123.6, 118.5, 109.0, 76.0, 70.0, 34.1, 26.1, 21.7. HRMS: calcd for C33H34N6O5S2 658.2032, found 658.2038. Synthesis of Ligand 5b′.

Under a nitrogen atmosphere, a mixture of RuCl2(PPh3)3 (96 mg, 0.10 mmol) and 5a (64 mg, 0.10 mmol) in toluene (5 mL) was refluxed for 2 h, forming a red-brown solid. After the mixture was cooled to ambient temperature, the precipitate was filtered off, washed with diethyl ether (3 × 10 mL), and dried under vacuum to afford 6a as a red-brown crystalline solid (92 mg, 85% yield). Recrystallization was conducted to further purify the complex in CH3CN/Et2O (1/3, v/v) at 25 °C. Mp: >320 °C dec. 1H NMR (CD3OD/CD2Cl2, 400 MHz): δ 8.27 (s, 1 H, 3-H), 7.87 (d each, J = 8.2 Hz, 1:1 H, 5-H and 4‴-H), 7.78 (m, 1:1 H, 4-H and 4‴-H), 7.63 (d, J = 8.2 Hz, 2 H, 5‴-H), 7.35, 7.24, and 7.04 (m each, 9:6:6 H, aromatic CH), 4.68 and 4.57 (t each, 1:1 H, 3′-H), 4.12 (t, 1 H, 4′-H), 2.96 (m, 1 H, CH(CH3)2), 2.43 and 2.26 (s each, 3:3 H, C6‴-CH3 and C6″″-CH3), 1.05 and 0.83 (d each, J = 7.1 and 6.7 Hz, 3:3 H, CH(CH3)2). 13C{1H} NMR (CD3OD, 100 MHz): δ 167.7 (Cq), 155.5 and 151.0 (Cq), 143.9 (Cq), 143.9(Cq), 141.2 (Cq), 136.0, 135.6 (Cq), 132.8, 132.7, 132.1 (Cq), 131.6 (Cq), 129.5, 129.2, 127.6, 128.0, 127.4, 123.8 (Cq), 123.2 (Cq), 121.5, 114.3, 111.4, 70.8, 70.3, 28.8, 20.2, 18.6, 13.2, 7.8. 31 1 P{ H} NMR (CD3OD, 162 MHz): δ 53.7 (PPh3). IR (KBr pellets, cm−1): ν 3435, 3064, 2926, 1954, 1631, 1578, 1481, 1436, 1410, 1335, 1284, 1161, 1091, 1032, 998, 945, 814, 746, 701, 662, 561, 548, 527, 501. Anal. Calcd for C50H47Cl2N6O5PRuS2: C, 55.66; H, 4.39; N, 7.79. Found: C, 55.94; H, 4.28; N, 7.60. Synthesis of Complex 6b.

In a fashion similar to the synthesis of 5a, compound 4′ (219 mg, 1.0 mmol) was reacted with (S)-tert-leucinol (129 mg, 1.1 mmol), and zinc triflate (400 mg, 1.1 mmol) to give 5b′ as a white solid (160 mg, 50% yield). Mp: 104−106 °C. [α]20D = +60.33 (c = 0.60, CH3OH). 1 H NMR (CD3OD, 400 MHz): δ 8.28 and 7.80 (d each, J = 7.8 and 7.6 Hz, 1:1 H, 3-H and 5-H), 7.97 (t, J = 7.8 Hz, 1 H, 4-H), 7.61 and 7.27 (m each, 2:2 H, phenyl H), 4.44 and 4.36 (t each, 1:1 H, 3′-H), 4.02 (t, 1 H, 4′-H), 0.94 (s, 9 H, C(CH3)3). 13C{1H} NMR (CD3OD, 100 MHz): δ 164.3 (Cq), 151.9 and 149.4 (Cq each), 146.6, 140.0 (Cq), 139.8, 125.1, 124.6, 124.0, 124.8 (Cq), 116.6, 77.1, 70.9, 34.8, 26.1. HRMS: calcd for C19H20N4O 320.1637, found 320.1636. Synthesis of Ligand 5c.

In a fashion similar to the synthesis of 6a, RuCl2(PPh3)3 (86 mg, 0.09 mmol) was reacted with 5b (60 mg, 0.09 mmol) to afford 6b as a red-brown solid (80 mg, 81% yield). Mp: >320 °C dec. 1H NMR (CD3OD, 400 MHz): δ 8.41 (s, 1 H, 3-H),7.95 (d, J = 8.0 Hz, 1 H, 5-H), 7.81 (m, 3 H, 4-H and 4‴-H), 7.66 (d, J = 8.0 Hz, 2 H, 5‴-H), 7.34, 7.22, and 7.02 (m each, 9:6:6 H, aromatic CH), 4.83 (dd, J1 = 3.8 Hz, J2 = 9.4 Hz 1 H, 3′-H), 4.30 (t, 1 H, 4′-H), 3.92 (dd, J1 = 3.6 Hz, J2 = 9.6 Hz 1 H, 3′-H), 2.42 and 2.27 (s each, 3:3 H, C6‴-CH3 and C6″″-CH3), 1.11 (s, 9 H, C(CH3)3).13C{1H} NMR (CD3OD, 100 MHz): δ 169.8 (Cq), 154.9 and 152.7(Cq each), 145.6 and 145.5(Cq), 142.2 (Cq), 137.5 (Cq), 136.5, 134.4 (Cq), 134.4, 134.3, 132.9 (Cq), 132.4(Cq), 131.4 (Cq), 131.1, 130.8, 128.9, 129.4, 128.7, 126.1, 124.0, 118.1, 110.0, 75.8, 73.8, 35.7, 26.8, 21.6. 31P{1H} NMR (CD3OD, 162 MHz): δ 53.9 (PPh3). IR (KBr pellets, cm−1): ν 3424, 3037, 2970, 1633, 1601, 1573, 1480, 1432, 1408, 1324, 1280, 1156, 1088, 940, 816, 752, 703, 663, 545, 529. Anal. Calcd for C51H49Cl2N6O5PRuS2: C, 56.04; H, 4.52; N, 7.69. Found: C, 55.97; H, 4.48; N, 7.74.

In a fashion similar to the synthesis of 5a, compound 4 (81 mg, 0.15 mmol) was reacted with (S)-phenylglycinol (22 mg, 0.16 mmol) and zinc triflate (58 mg, 0.16 mmol) to give 5c as a yellow solid (53 mg, 54% yield). Mp: 132−134 °C. [α]20D = +134.79 (c = 0.50, CH3OH). 1H NMR (CDCl3, 400 MHz): δ 11.61 (s, 1 H, NH), 8.27 and 7.99 (d each, J = 7.6 and 7.4 Hz, 1:1 H, 3-H and 5-H), 7.81 (t, J = 7.6 Hz, 1 H, 4-H), 7.44 (m, 4:1 H, 4‴-H, 4″″-H and 5″-H), 7.18 (m, 5:1 H, 2″‴-H, 3″‴-H, 4″‴-H and 8″-H), 7.00 and 6.94 (d each, J = 7.6 and 7.7 Hz, 2:2 H, 5‴-H and 5″″-H), 5.33 and 4.78 (t each, 1:1 H, 3′-H), 4.25 (t, 1 H, 4′-H), 2.22 and 2.12 (s each, 6 H, C6‴-CH3 and C6″″-CH3). 13C{1H} NMR (CDCl3, 100 MHz): δ 163.8 (Cq), 151.8 and 148.1 (Cq each), 145.8 (Cq), 143.9 (Cq), 141.8, 141.3 (Cq), G


Article

Organometallics

CH3CN/Et2O/n-hexane (1/3/1, v/v) at 25 °C. Mp: >320 °C dec. 1H NMR (CDCl3, 400 MHz): δ 7.82 (s, 1 H, 3-H), 7.56 and 7.40 (d each, J = 7.8 and 7.9 Hz, 3:2 H, 5-H, 4-H and phenyl CH), 7.17 (m, 4 H, phenyl CH), 7.03 and 6.93 (m each, 10:7 H, phenyl CH), 6.79 (s, 1 H, phenyl CH), 4.50 and 4.20 (t each, 1:1 H, 3′-H), 3.96 (t, 1 H, 4′-H), 2.70 (s, 3 H, CH3CN), 2.48 (m, 1 H, CH(CH3)2), 2.29 and 2.17 (s each, 3:3 H, phenyl-CH3), 1.09 and 1.05 (d each, J = 6.5 and 7.0 Hz, 3:3 H, CH(CH3)2). 13C{1H} NMR (CDCl3, 100 MHz): δ 164.9 (Cq), 146.8, 143.5, and 142.8 (Cq), 139.9 (Cq), 135.1 (Cq), 134.4 (Cq), 134.1 (Cq), 131.4, 130.0, 129.6, 129.1, 128.6, 127.2, 126.4 (Cq), 123.1 (Cq), 122.2 (Cq), 112.3 (Cq, CH3CN), 70.0, 70.0, 29.4, 20.7, 20.5, 19.0, 15.0, 3.9 (CH3CN). 31P{1H} NMR (CDCl3, 162 MHz): δ 51.7 (PPh3). IR (KBr pellets, cm−1): ν 3430, 3359, 3053, 2961, 2917, 2849, 1624, 1578, 1479, 1434, 1412, 1328, 1264, 1158, 1091, 1024, 944, 916, 808, 746, 699, 665, 560, 547, 529, 515, 499. Anal. Calcd for C52H50Cl2N7O5PRuS2: C, 55.76; H, 4.50; N, 8.75. Found: C, 56.70; H, 4.95; N, 8.60. Typical Procedure for the Catalytic Transfer Hydrogenation of Ketones. The catalyst solution was prepared by dissolving complex 6a (11 mg, 0.01 mmol) in isopropyl alcohol (50 mL). Under a nitrogen atmosphere, a mixture of ketone (2.0 mmol), 10 mL of the catalyst solution (0.002 mmol), and isopropyl alcohol (9.8 mL) was stirred at 28 °C for 10 min. Then, 0.2 mL of 0.1 M iPrOK (0.02 mmol) solution in isopropyl alcohol was introduced to initiate the reaction. At the stated time, 0.1 mL of the reaction mixture was sampled, immediately diluted with 0.5 mL of isopropyl alcohol precooled at 0 °C, and filtered through a short pad of Celite to remove the complex catalyst to quench the reaction. The resultant filtrate was used for GC or HPLC analysis. After the reaction was complete, the reaction mixture was quickly cooled to ambient temperature, filtered through Celite, condensed under reduced pressure, and then purified by silica gel column chromatography to afford the corresponding alcohol product, which was identified by comparison with the authentic sample through NMR and GC analysis.

Synthesis of Complex 6b′.

In a fashion similar to the synthesis of 6a, RuCl2(PPh3)3 (90 mg, 0.09 mmol) was reacted with 5b′ (30 mg, 0.09 mmol) to afford 6b′ as a red-brown solid (60 mg, 88% yield). Mp: >320 °C dec. 1H NMR (CD3OD, 400 MHz): δ 8.91 (m, 1 H, 3-H), 7.98 (d, J = 7.9 Hz, 1 H, 5-H), 7.83 (t, J = 7.9 Hz, 1 H, 4-H), 7.47, 7.24, and 7.10 (m each, 10:3:6 H, aromatic CH), 4.84 and 4.26 (t each, 1:1 H, 3′-H), 3.91 (dd, J1 = 3.8 Hz, J2 = 9.6 Hz, 1 H, 4′-H), 1.12 (s, 9 H, C(CH3)3). 13C{1H} NMR (CD3OD, 100 MHz): δ 169.8 (Cq), 155.2 and 153.0 (Cq each), 152.7 (Cq), 144.2 (Cq), 135.8 (Cq), 134.5, 134.4, 130.8, 129.0, 133.1 (Cq), 126.8, 126.0, 125.3, 124.1, 122.1, 113.9, 75.6, 73.9, 35.7, 26.8. 31 1 P{ H} NMR (CD3OD, 162 MHz): δ 53.2 (PPh3). IR (KBr pellets, cm−1): ν 3426, 3051, 2945, 1621, 1574, 1486, 1435, 1418, 1371, 1327, 1258, 1150, 1092, 948, 746, 700, 526. Anal. Calcd for C37H35Cl2N4OPRu: C, 58.89; H, 4.67; N, 7.42. Found: C, 59.01; H, 4.70; N, 7.36. Synthesis of Complex 6c.

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In a fashion similar to the synthesis of 6a, RuCl2(PPh3)3 (37 mg, 0.04 mmol) was reacted with 5c (26 mg, 0.04 mmol) to afford 6c as a red-brown solid (34 mg, 80% yield). Mp: >320 °C dec. 1H NMR (DMSO-d6, 400 MHz): δ 9.59 and 9.43 (s each, 2 H, 1‴-NH and 1″″-NH), 8.10 (d, J = 7.9 Hz, 1 H, 3-H), 7.89 (m, 1:1 H, 4-H and 5-H), 7.74 and 7.63 (d each, J = 7.5 and 7.6 Hz, 2:2 H, 4‴-H and 4″″-H), 7.50 (m, 5 H, 2″‴-H, 3″‴-H, 4″‴-H), 7.38 (m, 6:1 H, PPh3 and 5″-H), 7.22 (m, 6 H, PPh3), 7.11 (m, 3:4 H, PPh3 and 5‴-H and 5″″-H), 6.78 (s, 1 H, 8″-H), 5.08 and 4.99 (m each, 1:1 H, 3′-H), 4.92 (t, 1 H, 4′-H), 2.34 and 2.19 (s each, 6 H, C6‴-CH3 and C6″″-CH3). 13 C{1H} NMR (DMSO-d6, 100 MHz): δ 167.4 (Cq), 151.7 and 150.7 (Cq each), 148.8 (Cq), 143.8 (Cq), 143.6, 139.5 (Cq), 138.6 (Cq), 136.2(Cq), 135.5 (Cq), 135.0, 132.9, 129.8, 128.2, 130.3, 129.4 (Cq), 129.2 (Cq), 127.4, 127.0, 128.0, 125.1 (Cq), 123.7 (Cq), 123.6, 113.1, 108.0, 78.2, 68.4, 21.1, 21.0. 31P{1H} NMR (DMSO-d6, 162 MHz): δ 32.9 (PPh3). IR (KBr pellets, cm−1): ν 3436, 3316, 3050, 2924, 1631, 1596, 1572, 1479, 1434, 1410, 1327, 1274, 1160, 1090, 1032, 995, 938, 885, 813, 749, 701, 664, 616, 589, 561, 550, 531, 517, 496. Anal. Calcd for C53H45Cl2N6O5PRuS2: C, 57.19; H, 4.08; N, 7.55. Found: C, 57.01; H, 4.13; N, 7.45. Synthesis of Complex 6d.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00559. NMR spectra of the new compounds and X-ray crystallographic data for 6d (PDF) Cartesian coordinates of the calculated structure (XYZ) Accession Codes

CCDC 1560687 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.K.Y.: [email protected]. ORCID

Huining Chai: 0000-0001-8087-3458 Tingting Liu: 0000-0002-0156-3054 Zhengkun Yu: 0000-0002-9908-0017 Author Contributions ∥

H.N.C. and T.T.L. contributed equally to this work.

Notes

Single crystals of complex 6d suitable for X-ray crystallographic determination were grown from the recrystallization of complex 6a in

The authors declare no competing financial interest. H


Article

Organometallics

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Renaud, J.-L.; Cahard, D. Angew. Chem., Int. Ed. 2012, 51, 6467−6470. (d) Strotman, N. A.; Baxter, C. A.; Brands, K. M. J.; Cleator, E.; Krska, S. W.; Reamer, R. A.; Wallace, D. J.; Wright, T. J. J. Am. Chem. Soc. 2011, 133, 8362−8371. (e) Zeng, F. L.; Yu, Z. K. Organometallics 2009, 28, 1855−1862. (10) (a) Sandoval, C. A.; Ohkuma, T.; Muñiz, K.; Noyori, R. J. Am. Chem. Soc. 2003, 125, 13490−13503. (b) Ohkuma, T.; Koizumi, M.; Muñiz, K.; Hilt, G.; Kabuto, C.; Noyori, R. J. Am. Chem. Soc. 2002, 124, 6508−6509. (c) Doucet, H.; Ohkuma, T.; Murata, K.; Yokozawa, T.; Kozawa, M.; Katayama, E.; England, A. F.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. 1998, 37, 1703−1707. (11) (a) Cabrero-Antonino, J. R.; Alberico, E.; Drexler, H. J.; Baumann, W.; Junge, K.; Junge, H.; Beller, M. ACS Catal. 2016, 6, 47− 54. (b) Adam, R.; Alberico, E.; Baumann, W.; Drexler, H. J.; Jackstell, R.; Junge, H.; Beller, M. Chem. - Eur. J. 2016, 22, 4991−5002. (c) Elangovan, S.; Wendt, B.; Topf, C.; Bachmann, S.; Scalone, M.; Spannenberg, A.; Jiao, H. J.; Baumann, W.; Junge, K.; Beller, M. Adv. Synth. Catal. 2016, 358, 820−825. (12) (a) Kallmeier, F.; Dudziec, B.; Irrgang, T.; Kempe, R. Angew. Chem., Int. Ed. 2017, 56, 7261−7265. (b) Forberg, D.; Obenauf, J.; Friedrich, M.; Hühne, S. M.; Mader, W.; Motz, G.; Kempe, R. Catal. Sci. Technol. 2014, 4, 4188−4192. (c) Michlik, S.; Kempe, R. Nat. Chem. 2013, 5, 140−144. (13) Chai, H. N.; Liu, T. T.; Wang, Q. F.; Yu, Z. K. Organometallics 2015, 34, 5278−5284. (14) (a) Liu, T. T.; Chai, H. N.; Wang, L. D.; Yu, Z. K. Organometallics 2017, 36, 2914. (b) Chai, H. N.; Wang, Q. F.; Liu, T. T.; Yu, Z. K. Dalton Trans. 2016, 45, 17843−17849. (c) Du, W. M.; Wang, Q. F.; Wang, L. D.; Yu, Z. K. Organometallics 2014, 33, 974− 982. (d) Du, W. M.; Wang, Q. F.; Yu, Z. K. Chin. J. Catal. 2013, 34, 1373−1377. (e) Jin, W. W.; Wang, L. D.; Yu, Z. K. Organometallics 2012, 31, 5664−5667. (f) Ye, W. J.; Zhao, M.; Yu, Z. K. Chem. - Eur. J. 2012, 18, 10843−10846. (g) Du, W. M.; Wang, L. D.; Wu, P.; Yu, Z. K. Chem. - Eur. J. 2012, 18, 11550−11554. (h) Zhao, M.; Yu, Z. K.; Yan, S. G.; Li, Y. Tetrahedron Lett. 2009, 50, 4624−4628. (i) Zeng, F. L.; Yu, Z. K. Organometallics 2008, 27, 6025−6028. (j) Zeng, F. L.; Yu, Z. K. Organometallics 2008, 27, 2898−2901. (k) Deng, H. X.; Yu, Z. K.; Dong, J. H.; Wu, S. Z. Organometallics 2005, 24, 4110−4112. (l) Sun, X. J.; Yu, Z. K.; Wu, S. Z.; Xiao, W. J. Organometallics 2005, 24, 2959−2963. (15) Ye, W. J.; Zhao, M.; Du, W. M.; Jiang, Q. B.; Wu, K. K.; Wu, P.; Yu, Z. K. Chem. - Eur. J. 2011, 17, 4737−4741. (16) Zheng, L. S.; Llopis, Q.; Echeverria, P. G.; Férard, C.; Guillamot, G.; Phansavath, P.; Ratovelomanana-Vidal, V. J. Org. Chem. 2017, 82, 5607−5615.

ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (21672209) for support of this research.

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REFERENCES

(1) For selected reviews, see: (a) Echeverria, P.-G.; Ayad, T.; Phansavath, P.; Ratovelomanana-Vidal, V. Synthesis 2016, 48, 2523− 2539. (b) Noyori, R. Angew. Chem., Int. Ed. 2013, 52, 79−92. (c) Ikariya, T.; Blacker, A. J. Acc. Chem. Res. 2007, 40, 1300−1308. (d) Wu, X. F.; Xiao, J. L. Chem. Commun. 2007, 2449−2466. (e) Samec, J. S. M.; Bäckvall, J.-E.; Andersson, P. G.; Brandt, P. Chem. Soc. Rev. 2006, 35, 237−248. (f) Gladiali, S.; Alberico, E. Chem. Soc. Rev. 2006, 35, 226−236. (g) Tang, W.; Zhang, X. Chem. Rev. 2003, 103, 3029−3070. (2) (a) Zimbron, J. M.; Dauphinais, M.; Charette, A. B. Green Chem. 2015, 17, 3255−3259. (b) Pechácě k, J.; Václavík, J.; Přech, J.; Šot, P.; Janušcǎ ḱ , J.; Vilhanová, B.; Vavřík, J.; Kuzma, M.; Kačer, P. Tetrahedron: Asymmetry 2013, 24, 233−239. (c) Coll, M.; Ahlford, K.; Pàmies, O.; Adolfsson, H.; Diéguez, M. Adv. Synth. Catal. 2012, 354, 415−427. (d) Coll, M.; Pàmies, O.; Adolfsson, H.; Diéguez, M. Chem. Commun. 2011, 47, 12188−12190. (e) Ohkuma, T.; Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval, C.; Noyori, R. J. Am. Chem. Soc. 2006, 128, 8724−8725. (f) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008−2022. (g) Yamakawa, M.; Yamada, I.; Noyori, R. Angew. Chem., Int. Ed. 2001, 40, 2818−2821. (h) Haack, K.-J.; Hashiguchi, S.; Fujii, A.; Ikariya, T.; Noyori, R. Angew. Chem., Int. Ed. Engl. 1997, 36, 285−288. (i) Noyori, R.; Hashiguchi, S. Acc. Chem. Res. 1997, 30, 97− 102. (j) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 4916−4917. (k) Takehara, J.; Hashiguchi, S.; Fujii, A.; Inoue, S.-i.; Ikariya, T.; Noyori, R. Chem. Commun. 1996, 233−234. (3) (a) Vilhanova, B.; Vaclavik, J.; Sot, P.; Pechacek, J.; Zapal, J.; Pazout, R.; Maixner, J.; Kuzma, M.; Kacer, P. Chem. Commun. 2016, 52, 362−365. (b) Dub, P. A.; Gordon, J. C. Dalton Trans. 2016, 45, 6756−6781. (c) Eichenseer, C. M.; Kastl, B.; Pericas, M. A.; Hanson, P. R.; Reiser, O. ACS Sustainable Chem. Eng. 2016, 4, 2698−2705. (d) Vedejs, E.; Trapencieris, P.; Suna, E. J. Org. Chem. 1999, 64, 6724−6729. (e) Fujii, A.; Hashiguchi, S.; Uematsu, N.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 2521−2522. (4) (a) Zhang, B.; Wang, H.; Lin, G. Q.; Xu, M. H. Eur. J. Org. Chem. 2011, 2011, 4205−4211. (b) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 7562−7563. (5) (a) Hayes, A. M.; Morris, D. J.; Clarkson, G. J.; Wills, M. J. Am. Chem. Soc. 2005, 127, 7318−7319. (b) Hannedouche, J.; Clarkson, G. J.; Wills, M. J. Am. Chem. Soc. 2004, 126, 986−987. (6) (a) Touge, T.; Nara, H.; Fujiwhara, M.; Kayaki, Y.; Ikariya, T. J. Am. Chem. Soc. 2016, 138, 10084−10087. (b) Touge, T.; Hakamata, T.; Nara, H.; Kobayashi, T.; Sayo, N.; Saito, T.; Kayaki, Y.; Ikariya, T. J. Am. Chem. Soc. 2011, 133, 14960−14963. (7) (a) Wu, X. F.; Wang, C.; Xiao, J. L. Chem. Rec. 2016, 16, 2772− 2786. (b) Wang, C.; Wu, X. F.; Xiao, J. L. Chem. - Asian J. 2008, 3, 1750−1770. (c) Wu, X. F.; Xiao, J. L. Chem. Commun. 2007, 2449− 2466. (d) Wu, X. F.; Li, X. G.; Hems, W.; King, F.; Xiao, J. L. Org. Biomol. Chem. 2004, 2, 1818−1821. (e) Li, X. G.; Wu, X. F.; Chen, W. P.; Hancock, F. E.; King, F.; Xiao, J. L. Org. Lett. 2004, 6, 3321−3324. (8) For selected recent reports on ATH of ketones from the Baratta group, see: (a) Chelucci, G.; Baldino, S.; Baratta, W. Acc. Chem. Res. 2015, 48, 363−379. (b) Chelucci, G.; Baldino, S.; Baratta, W. Coord. Chem. Rev. 2015, 300, 29−85. (c) Putignano, E.; Bossi, G.; Rigo, P.; Baratta, W. Organometallics 2012, 31, 1133−1142. (d) Baratta, W.; Barbato, C.; Magnolia, S.; Siega, K.; Rigo, P. Chem. - Eur. J. 2010, 16, 3201−3206. (e) Baratta, W.; Ballico, M.; Chelucci, G.; Siega, K.; Rigo, P. Angew. Chem., Int. Ed. 2008, 47, 4362−4365. (f) Baratta, W.; Chelucci, G.; Herdtweck, E.; Magnolia, S.; Siega, K.; Rigo, P. Angew. Chem., Int. Ed. 2007, 46, 7651−7654. (9) (a) Zhao, B. G.; Han, Z. B.; Ding, K. L. Angew. Chem., Int. Ed. 2013, 52, 4744−4788. (b) Du, W. M.; Wu, P.; Wang, Q. F.; Yu, Z. K. Organometallics 2013, 32, 3083−3090. (c) Bizet, V.; Pannecoucke, X.; I


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