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P-Stereogenic pincer iridium complexes: Synthesis, structural characterization and application in asymmetric hydrogenation
Journal of Organometallic Chemistry 791 (2015) 41e45
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P-Stereogenic pincer iridium complexes: Synthesis, structural characterization and application in asymmetric hydrogenation Zehua Yang a, Xuan Wei a, Delong Liu a, **, Yangang Liu a, Masashi Sugiya c, Tsuneo Imamoto c, d, ***, Wanbin Zhang a, b, * a
School of Pharmacy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China Organic R&D Department, Nippon Chemical Industrial Co., Ltd., Kameido, Koto-ku, Tokyo 136-8515, Japan d Department of Chemistry, Graduate School of Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 8 April 2015 Accepted 2 May 2015 Available online 21 May 2015
P-Stereogenic PNP type pincer iridium complexes PNPtBuMeIrH2Cl (3) and PNPtBuMeIrH3 (4) were synthesized in reasonable yields and characterized by 1H NMR, 13C NMR, 31P NMR, HRMS and/or single crystal X-ray diffraction. The ORTEP diagram shows that the coordination geometry around the iridium center of complex 3 is approximately octahedral. The chlorinated iridium complex (3) and/or the trihydride iridium complex (4) were used as catalysts in the asymmetric hydrogenation of ketones, olefins and quinoline to provide the desired products with up to 17% enantioselectivity. © 2015 Elsevier B.V. All rights reserved.
Keywords: P-Stereogenic phosphine ligand Pincer iridium complex Asymmetric hydrogenation
Introduction Pincer metal complexes, due to their rigid tridentate structural properties, tunable electronic density and steric hindrance, have received widespread attention over the last three decades [1]. These types of complexes can serve as efficient catalysts and show very high reaction activities in many kinds of catalytic synthesis [2e7]. Of these catalysts, pincer iridium complexes, such as the commonly used PCP [8e12] and PNP [13e16] type, have received little attention. Moreover, the majority of reported pincer iridium complexes are achiral; few examples of chiral pincer iridium complexes have been reported. In 2002, MoraleseMorales and coworkers developed a phenyl and tert-butyl substituted P-stereogenic PCP type pincer iridium complex and used it in the achiral
* Corresponding author. School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China. Tel./fax: þ86 21 5474 3265. ** Corresponding author. School of Pharmacy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China. *** Corresponding author. Organic R&D Department, Nippon Chemical Industrial Co., Ltd., Kameido, Koto-ku, Tokyo 136-8515, Japan. E-mail addresses: [email protected] (D. Liu), [email protected] (T. Imamoto), [email protected] (W. Zhang). http://dx.doi.org/10.1016/j.jorganchem.2015.05.002 0022-328X/© 2015 Elsevier B.V. All rights reserved.
transfer dehydrogenation of cyclooctane to cyclooctene with good reaction activity [17]. In 2009, Goldman et al. described the preparation of a series of methyl and tert-butyl substituted P-stereogenic PCP type pincer iridium complexes (racemic species) and applied them to alkane dehydrogenation reactions [18]. Recently, Corma, Iglesias and their co-workers developed a series of chiral NNN-pincer iridium complexes, however such species have not yet been utilized in asymmetric reactions [19]. Gamasa et al. synthesized a series of NNN type pincer iridium complexes with a Pybox skeleton, and applied them to asymmetric transfer hydrogenations with high conversions and excellent enantioselectivities [20e25]. These results indicate that chiral pincer iridium complexes can serve as efficient chiral catalysts in asymmetric synthesis, and hence the development of novel and more suitable chiral pincer iridium complexes is a worthwhile endeavor. To the best of our knowledge, asymmetric reactions catalyzed by PNP type chiral pincer iridium complexes have not yet been reported. Bidentate P-stereogenic phosphine ligands such as QuinoxP* and BenzP*, possessing small (methyl) and bulky (tert-butyl) alkyl groups have displayed excellent activity and enantioselectivity in asymmetric catalysis [26e29]. We therefore envisaged that the introduction of the P-stereogenic phosphine groups into the rigid pincer backbone of the complex, will produce a stable and effective P-stereogenic pincer type catalyst. Novel P-stereogenic PCP type
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Z. Yang et al. / Journal of Organometallic Chemistry 791 (2015) 41e45
pincer palladium complexes were synthesized and showed high reaction activity and good enantioselectivity in the asymmetric addition of diarylphosphines to nitroalkenes [30]. Recently, a Pstereogenic PNP type pincer-Pd complex was also prepared and applied to the asymmetric intramolecular hydroamination of amino-1,3-dienes, providing the desired products in good yields and with excellent regioselectivities and moderate enantioselectivities [31]. Herein we report the synthesis and structural characterization of two new P-stereogenic PNP type pincer iridium complexes and their application to asymmetric hydrogenation reactions.
Results and discussion Synthesis and characterization of pincer iridium complexes The P-stereogenic PNP type pincer iridium complex was synthesized in two steps using optically active (R,R)-2,6-bis[(boranato(tert-butyl)methylphosphino)methyl]pyridine (1) as the starting material (Scheme 1). Thus, the boranato groups in compound 1 were removed with 1,4-diazabicyclo[2,2,2]octane (DABCO) in degassed dry toluene at 80 C. The resulting chiral bisphosphine pyridine 2 was then reacted with [IrCl(coe)2]2 under hydrogen pressure to give the pincer iridium complex 3 as a white solid. The 1 H NMR spectrum shows one td peak at 19.39 ppm and one ddd peak at 23.15 ppm, both with an integral value equal to one proton. This indicates that two hydrides are coupled with each other and with the phosphorus atoms. The two hydrides are thought to occupy cis- and trans-positions relative to the nitrogen atom, forming an octahedral coordination structure. Two doublets are present at 37.75 and 21.82 ppm in the 31P NMR spectrum, indicating that in complex 3, two magnetically inequivalent phosphorus nuclei are present and coupled with each other. Addition of an excess amount of NaH to a tetrahydrofuran solution of 3 generated a trihydride pincer iridium complex 4. The 1H NMR spectrum shows two resonances in the hydride region; a td peak at 9.83 ppm and a multiplet at 19.38 ~ 19.45 ppm, with integral values equal to two protons and one proton, respectively. A singlet centered at 32.77 ppm in the 31P NMR spectrum of 4 indicates the presence of two phosphorus nuclei in an indistinguishable chemical environment. This information shows that the chlorine atom located at the iridium center is completely substituted by a hydrogen atom. Additionally, 4 is sensitive to air and moisture. Layering a concentrated dichloromethane solution of 3 with n-
Fig. 1. ORTEP drawing of pincer iridium complex 3. Ellipsoids are show at 30% probability levels. Hydrogen atoms except hydrides bond to iridium were omitted for clarity.
hexane provided a single crystal suitable for X-ray analysis. The ORTEP diagram of complex 3 is shown in Fig. 1. X-ray crystallographic analysis revealed the PNP pincer ligand adopts a meridional coordination mode, with two peaks in the Fourier map that could be assigned as hydrides to form the octahedral coordination geometry. Two of the phosphorus atoms are bound to the iridium with different bond lengths [2.2762(16), 2.2572(16) Å] (Table 1). The bond angle of the PeIreP is 164.19(6) , smaller than 180 due to the steric strain of the five-member chelate rings. This structural feature allows the iridium atom to readily coordinate with substrates during asymmetric catalysis. The large steric difference between the small methyl group and bulky tert-butyl group in complex 3 is expected to provide a satisfactory stereo control in asymmetric catalysis. Pincer iridium catalyzed asymmetric hydrogenation Since pincer iridium complexes are known to be efficient catalysts for the hydrogenation of C]O double bonds [15], we investigated the potential use of 3 and 4 in such reactions (Table 2). Thus, the asymmetric hydrogenation of acetophenone (5a) was carried out using both 3 and 4 as chiral catalysts in ethanol with KOH as a base. The reactions proceeded smoothly providing the target product in good conversions but with very low enantiomeric excesses (entries 1 and 2). Considering the convenience of the synthetic procedure, we selected complex 3 as a catalyst in subsequent investigations. When methanol was used as the solvent, the target product was obtained with a higher ee but a lower conversion (entry 3). The introduction of a hydroxyl group to the ortho-position of acetophenone resulted in full conversion to the desired product and a slightly improved ee (entry 4). Under similar reaction conditions, biomass-derived levulinic acid (5c) was hydrogenated to provide the corresponding cyclic product (6c) in moderate conversion and with 14% ee (Scheme 2).
Table 1 Selected bond lengths [Å] and angles [ ] for complex 3. Bond lengths [Å]
Scheme 1. Synthesis of pincer iridium complexes.
Ir1eN1 Ir1eP2 Bond angles [ ]
2.137(5) 2.2572(16)
Ir1eP1 Ir1eCl1
N1eIr1eP1 P2eIr1eP1 P2eIr1eCl1
82.39(13) 164.19(6) 100.42(6)
N1eIr1eP2 N1eIre Cl1 P1eIr1eCl1
2.2762(16) 2.4968(16) 83.18(13) 89.25(13) 85.83(6)
Z. Yang et al. / Journal of Organometallic Chemistry 791 (2015) 41e45 Table 2 Hydrogenation catalyzed by pincer iridium complexesa.
Entry
Catalyst
Solvent
R
Conversion (%)b
ee (%)c
1 2 3 4
3 4 3 3
EtOH EtOH MeOH MeOH
H H H OH
82 75 38 95
6 4 17 8
a
Reaction conditions: 0.5 mmol substrates, 0.005 mmol catalyst, 0.05 mmol KOH, 2 mL solvent, 50 atm H2, stirred at 25 C for 24 h. b Determined by 1H NMR. c Determined by chiral HPLC or GC.
Scheme 2. a Reaction conditions: 0.5 mmol substrates, 0.005 mmol catalyst, 0.05 mmol KOH, 2 mL solvent, 50 atm H2, stirred at 100 C for 24 h b Acidified to pH 3e4 and stirred for 1 h.
Substrates with a C]C double bond were also hydrogenated by complex 3 (Scheme 3). For example, olefins substituted with a cyano (5d) or nitro (5e) group, could be hydrogenated smoothly with almost quantitative conversions. Carboxyl substituted styrene (5f) could be hydrogenated by complex 3 in moderate conversion but very low enantioselectivity. Moreover, 2-methylquinoline (5g), possessing both C]C and C]N double bonds, could also be hydrogenated using this catalytic system but the conversion and ee values were low. Conclusion We have designed and synthesized novel P-stereogenic pincer iridium complexes 3 and 4 in reasonable yields using a short
43
synthetic route. The complexes were characterized by 1H NMR, 13C NMR, 31P NMR, HRMS and/or single crystal X-ray diffraction. The ORTEP diagram of 3 shows that the coordination geometry around the iridium atom is approximately octahedral. 3 and/or 4 were used as catalysts for the hydrogenation of ketones, olefins and quinoline derivatives to provide the desired products with moderate to excellent conversions (up to 99%) and up to 17% enantiomeric excess. Investigations to improve the reaction enantioselectivity are currently underway and will be reported in due course. Experimental section General procedures All air and moisture sensitive manipulations were carried out with standard Schlenk techniques or in a glove box under argon atmosphere. Column chromatography was performed using 100e200 mesh silica gels. All solvents were refined by the standard method of solvent manual. The commercial available reagents were purchased from Adamas-Beta Ltd., Energy Chemical Inc. or J&K Scientific Inc. and were used without further purification unless otherwise specified. The NMR spectra were recorded on a Varian MERCURY plus-400 (400 MHz, 1H; 101 MHz, 13C; 162 MHz, 31P) spectrometer with chemical shifts reported in ppm relative to the residual deuterated solvents or the internal standard tetramethylsilane or 85% phosphoric acid respectively. HRMS were performed on a solariX XR 7.0 T hybrid quadrupole-FTICR mass spectrometer (Bruker Daltonics, Bremen, Germany), which was equipped with an ESI/APCI/MALDI ion source. Melting points were measured with SGW X-4 micro melting point apparatus. Optical rotations were measured on a Rudolph Research Analytical Autopol VI automatic polarimeter using a 50 mm path-length cell at 589 nm. Enantiomeric excess values (ee) were measured on a Shimadzu LC-10Avp HPLC system and using Daicel Chiralcel OD-H, and OJ-H columns with n-hexane/isopropanol as eluant, or on a Shimadzu GC2010 plus gas chromatography system using Supelco Analytical b-dex 120 and Varian CP-Chirasil-Dex CB capillary columns with nitrogen as eluant and FID as detector. Synthesis of (R,R)-2,6-bis[(tert-butylmethylphosphino)methyl] pyridine (2) Under an argon atmosphere, in a 50 mL two neck flask, compound 1 (500 mg, 1.5 mmol) and 1,4-diazabicyclo[2.2.2]octane (DABCO, 496 mg, 4.4 mmol) were dissolved in distilled toluene (10 mL). The solution was stirred at 80 C for 8 h. After cooling to room temperature, degassed n-hexane 20 (mL) was added to the mixture and a white precipitate was generated. Removal of the precipitate followed by evaporation of the solvent under reduced pressure gave compound 2 as a yellow oil, which was used in the next step without further purification. Synthesis of PNPtBuMeIrH2Cl (3)
Scheme 3. a Reaction conditions: 0.5 mmol substrates, 0.005 mmol catalyst, 0.05 mmol KOH, 2 mL solvent, 50 atm H2, stirred at 25 C for 24 h b Esterification with MeI.
To a solution of [IrCl(coe)2]2 (660 mg, 0.7 mmol) in distilled toluene (40 mL) was added a solution of 2 (the entire product generated in the previous step, ca. 1.4 mmol) in distilled toluene (20 mL). The mixture was then transferred to a stainless autoclave in a glove box and was pressurized with hydrogen gas (2.5 MPa) and stirred at 100 C for 12 h. After cooling to room temperature, the pressure was released and the solvent was removed in vacuo. The residue was purified by column chromatography (ethyl acetate: petroleum ether ¼ 2: 1) to afford the pure product as a white solid (327 mg, 41%). m.p. 257e259 C. 1H NMR (400 MHz, CDCl3) d 7.52 (t, J ¼ 7.7 Hz, 1H), 7.23 (d, J ¼ 7.7 Hz, 2H), 3.99 (dd, J ¼ 16.7, 8.9 Hz, 1H),
44
Z. Yang et al. / Journal of Organometallic Chemistry 791 (2015) 41e45
3.78 (dd, J ¼ 16.8, 9.8 Hz, 1H), 3.60 (dd, J ¼ 15.9, 9.1 Hz, 1H), 3.22 (dd, J ¼ 16.7, 10.2 Hz, 1H), 1.80 (dd, J ¼ 9.0, 3.1 Hz, 3H), 1.70 (dd, J ¼ 8.4, 2.7 Hz, 3H), 1.31 (d, J ¼ 14.5 Hz, 9H), 1.08 (d, J ¼ 14.3 Hz, 9H), 19.39 (td, J ¼ 13.5, 7.6 Hz, 1H), 23.15 (ddd, J ¼ 17.9, 13.6, 7.6 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 162.8, 162.4, 135.7, 119.9 (dd, J ¼ 14.2, 9.0 Hz), 43.9 (d, J ¼ 24.7 Hz), 42.9 (d, J ¼ 25.3 Hz), 26.7 (d, J ¼ 4.1 Hz), 26.0 (d, J ¼ 4.5 Hz), 18.0, 17.7, 5.4 (d, J ¼ 3.1 Hz), 5.2 (d, J ¼ 2.8 Hz). 31P NMR (162 MHz, CDCl3) d 37.75 (d, J ¼ 359.0 Hz), 21.82 (d, J ¼ 360.8 Hz). HRMS (MALDI): calcd. for C17H32ClIrNP2 [MH]þ 540.13277, found 540.13348. IR (KBr disc) n/cm1: 2963, 2904, 2870, 2380, 1590, 1573, 1453, 1394, 1369, 1067, 1019, 891, 818, 746. [a]25D ¼ 21.9 (c 0.1, CHCl3). Synthesis of PNPtBuMeIrH3 (4) To a 50 mL two neck flask, complex 3 (100 mg, 0.2 mmol), NaH (739 mg, 18.5 mmol, washed by distilled n-hexane) and tetrahydrofuran (10 mL) were added under an argon atmosphere, and the mixture was stirred at room temperature for 12 h. The reaction mixture was filtered and the filtrate was concentrated in vacuo. The residue was washed with distilled n-hexane and recrystallized from tetrahydrofuran/n-hexane to give complex 4 as a yellow solid (58 mg, 62%). m.p. 225e227 C. 1H NMR (400 MHz, C6D6) d 6.79 (t, J ¼ 7.6 Hz, 1H), 6.47 (d, J ¼ 7.5 Hz, 2H), 3.30 (d, J ¼ 17.1 Hz, 2H), 2.80 (d, J ¼ 16.7 Hz, 2H), 1.85 (s, 6H), 1.14 (vt, J ¼ 6.9 Hz, 18H), 9.83 (td, J ¼ 17.0, 5.0 Hz, 2H), 19.38 ~ 19.45 (m, 1H). 31P NMR (162 MHz, C6D6) d 46.58 (s). HRMS (MALDI): calcd. for C17H33IrNP2 [MH]þ 506.17175, found 506.17158. Typical procedure for catalytic asymmetric hydrogenation A 10 mL tube equipped with a magnetic stirring bar, substrate (0.5 mmol), catalyst (0.005 mmol) and KOH (0.05 mmol), was vacuum-pumped and flushed with argon three times. A degassed solvent (2 mL) was then added and the tube was transferred to a stainless autoclave in a glove box. The autoclave was pressurized and evacuated with hydrogen three times, and finally charged with hydrogen at a pressure of 50 atm. The reaction mixture was magnetically stirred at the given temperature for 24 h. After cooling to room temperature, the pressure was released. The solvent was removed in vacuo and the crude product was examined by 1H NMR to determine the conversion. The pure product was obtained by flash column chromatography using ethyl acetate/petroleum ether as the eluant. 1-Phenylethanol (6a) Colorless oil [32]. 1H NMR (400 MHz, CDCl3) d 7.41e7.25 (m, 5H), 4.89 (qd, J ¼ 6.4, 3.0 Hz, 1H), 1.50 (d, J ¼ 6.5 Hz, 3H). HPLC Daicel ChiralPak OD-H, n-hexane/i-PrOH ¼ 95/5, 210 nm, 0.8 mL/min tR1 ¼ 11.1 min (major), tR2 ¼ 13.1 min (minor), ee ¼ 17%. 2-(1-Hydroxyethyl)phenol (6b) Colorless oil [33]. 1H NMR (400 MHz, CDCl3) d 8.00 (brs, 1H), 7.16 (t, J ¼ 7.7 Hz, 1H), 6.98 (d, J ¼ 6.3 Hz, 1H), 6.84 (dd, J ¼ 14.3, 7.0 Hz, 2H), 5.05 (q, J ¼ 6.6 Hz, 1H), 2.78 (brs, 1H), 1.57 (d, J ¼ 6.6 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 155.5, 129.1, 128.6, 126.6, 120.0, 117.2, 71.7, 23.6. GC Varian CP-Chirasil-Dex CB, 90 C isothermal, 1 mL/min tR1 ¼ 24.5 min (minor), tR2 ¼ 25.2 min (major), ee ¼ 8%.
g-Valerolactone (6c) Colorless oil [34]. 1H NMR (400 MHz, CDCl3) d 4.65e4.53 (m, 1H), 2.61e2.40 (m, 2H), 2.40e2.26 (m, 1H), 1.92e1.69 (m, 1H), 1.37 (dd, J ¼ 6.2, 2.4 Hz, 3H). GC Supelco Aalytical b-dex 120, 90 C isothermal, 1 mL/min tR1 ¼ 16.8 min (major), tR2 ¼ 17.6 min (minor), ee ¼ 14%.
2,3-Diphenylpropanenitrile (6d) Colorless oil [35]. 1H NMR (400 MHz, CDCl3) d 7.38e7.28 (m, 8H), 7.14 (d, J ¼ 6.4 Hz, 2H), 4.00 (t, J ¼ 6.7 Hz,1H), 3.23e3.11 (m, 2H). HPLC Daicel ChiralPak OD-H, n-hexane/i-PrOH ¼ 98/2, 210 nm, 0.6 mL/min tR1 ¼ 21.8 min (minor), tR2 ¼ 23.1 min (major), ee ¼ 7%. (2-Nitropropyl)benzene (6e) Colorless oil [36]. 1H NMR (400 MHz, CDCl3) d 7.36e7.24 (m, 3H), 7.17 (d, J ¼ 6.6 Hz, 2H), 4.78 (sextet, J ¼ 6.7 Hz, 1H), 3.33 (dd, J ¼ 14.0, 7.5 Hz, 1H), 3.01 (dd, J ¼ 14.0, 6.8 Hz, 1H), 1.55 (d, J ¼ 6.7 Hz, 3H). HPLC Daicel ChiralPak OJ-H, n-hexane/i-PrOH ¼ 90/10, 210 nm, 0.8 mL/min tR1 ¼ 12.5 min, tR2 ¼ 13.6 min, ee ¼ 0. 2-Phenylpropanoic acid (6f) Colorless oil [37]. 1H NMR (400 MHz, CDCl3) d 7.36e7.26 (m, 5H), 3.74 (q, J ¼ 7.2 Hz, 1H), 1.51 (d, J ¼ 7.2 Hz, 3H). HPLC Daicel ChiralPak OJ-H, n-hexane/i-PrOH ¼ 90/10, 210 nm, 1 mL/min tR1 ¼ 10.3 min (minor), tR2 ¼ 11.6 min (major), ee ¼ 5%. 2-Methyl-1,2,3,4-tetrahydroquinoline (6g) Colorless oil [38]. 1H NMR (400 MHz, CDCl3) d 7.00e6.96 (m, 2H), 6.63 (td, J ¼ 7.4, 1.1 Hz, 1H), 6.49 (dd, J ¼ 8.3, 1.1 Hz, 1H), 3.58 (brs, 1H), 3.47e3.36 (m, 1H), 2.86 (ddd, J ¼ 17.1, 11.5, 5.7 Hz, 1H), 2.75 (ddd, J ¼ 16.4, 5.2, 3.6 Hz, 1H), 2.01e1.86 (m, 1H), 1.66e1.56 (m, 1H), 1.23 (d, J ¼ 6.3 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 145.0, 129.5, 126.9, 121.4, 117.2, 114.3, 47.4, 30.4, 26.9, 22.9. HPLC Daicel ChiralPak OD-H, n-hexane/i-PrOH ¼ 99/1, 220 nm, 0.8 mL/min tR1 ¼ 14.1 min (minor), tR2 ¼ 16.1 min (major), ee ¼ 4%. Crystal structure determinations Single crystals of complex 3 were obtained from a concentrated solution of dichloromethane layered by n-hexane. The X-ray diffraction data were collected on an Oxford Diffraction Gemini A
Table 3 Crystallographic data for Complex 3. Formula Fw Cryst color Cryst size [mm] T (K) l (Å) Cryst syst Space group a (Å) b (Å) c (Å) a (deg) b (deg) g (deg) Z V (Å3) rcalcd (g cm3) m (cm1) q range (deg) Total no. of rflns No. of unique rflns R1 (I > 2s) wR2 (I > 2 s) R1 (all data) wR2 (all data) Abs corr Abs corr range Parameters/restraints Flack x param Rint S Res density (e Å3)
C17H33ClIrNP2 541.03 colorless 0.35 0.32 0.30 170(2) 0.71073 orthorhombic P 21 21 21 8.9472(5) 11.6097(7) 20.2296(15) 90.00 90.00 90.00 4 2101.3(2) 1.710 6.631 3.04e25.34 13,433 3827 0.0285 0.0531 0.0355 0.0558 multi-scan 0.2049e0.2410 3827/215 0.026(9) 0.0548 1.031 0.602/0.694
Z. Yang et al. / Journal of Organometallic Chemistry 791 (2015) 41e45
Ultra diffractometer with graphite monochromated Mo Ka radiation (l ¼ 0.71073 Å). The structures were solved by direct method using the SHELXS-97 [39] program, and all non-hydrogen atoms were refined anisotropically on F2 by the full-matrix least-squares technique, which used the SHELXS-97 [39] crystallographic software package. Details of the crystal structure determination of the complexes 3 are summarized in Table 3. Acknowledgments This work was partially supported by the National Natural Science Foundation of China (No. 21172143, 21172145, 21372152 and 21472123), Nippon Chemical Industrial Co., Ltd., and Shanghai Jiao Tong University (SJTU). We thank the Instrumental Analysis Center of SJTU for HRMS analysis. Appendix A. Supplementary material CCDC 1017083 for complex 3 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Appendix B. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2015.05.002. References , O. Wendt, Pincer and Pincer-type Complexes: Applications in [1] K. Szabo Organic Synthesis and Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Germany, 2014 (and references cited therein). [2] J.-J. Feng, X.-F. Chen, M. Shi, W.-L. Duan, J. Am. Chem. Soc. 132 (2010) 5562. [3] J.-I. Ito, K. Fujii, H. Nishiyama, Chem. e Eur. J. 19 (2013) 601. [4] X.-Q. Hao, J.-J. Huang, T. Wang, J. Lv, J.-F. Gong, M.-P. Song, J. Org. Chem. 79 (2014) 9512. [5] C. Haibach Michael, N. Lease, S. Goldman Alan, Angew. Chem. Int. Ed. 53 (2014) 10160. [6] P. Kang, S. Zhang, T.J. Meyer, M. Brookhart, Angew. Chem. Int. Ed. 53 (2014) 8709. [7] T. Zell, Y. Ben-David, D. Milstein, Angew. Chem. Int. Ed. 53 (2014) 4685.
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[8] D.W. Lee, C.M. Jensen, D. Morales-Morales, Organometallics 22 (2003) 4744. [9] I. Goettker-Schnetmann, M. Brookhart, J. Am. Chem. Soc. 126 (2004) 9330. [10] Z. Huang, J. Zhou, J.F. Hartwig, J. Am. Chem. Soc. 132 (2010) 11458. [11] S. Kundu, J. Choi, D.Y. Wang, Y. Choliy, T.J. Emge, K. Krogh-Jespersen, A.S. Goldman, J. Am. Chem. Soc. 135 (2013) 5127. [12] J. Choi, A.H.R. MacArthur, M. Brookhart, A.S. Goldman, Chem. Rev. 111 (2011) 1761. [13] M.T. Whited, R.H. Grubbs, Organometallics 27 (2008) 5737. [14] R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 131 (2009) 14168. [15] W. Li, J.-H. Xie, H. Lin, Q.-L. Zhou, Green. Chem. 14 (2012) 2388. [16] C. Cheng, B.G. Kim, D. Guironnet, M. Brookhart, C. Guan, D.Y. Wang, K. KroghJespersen, A.S. Goldman, J. Am. Chem. Soc. 136 (2014) 6672. [17] D. Morales-Morales, R.E. Cramer, C.M. Jensen, J. Organomet. Chem. 654 (2002) 44. [18] S. Kundu, Y. Choliy, G. Zhuo, R. Ahuja, T.J. Emge, R. Warmuth, M. Brookhart, K. Krogh-Jespersen, A.S. Goldman, Organometallics 28 (2009) 5432. [19] A.M. Rasero-Almansa, A. Corma, M. Iglesias, F. Sanchez, ChemCatChem 5 (2013) 3092. [20] D. Cuervo, J. Diez, M.P. Gamasa, J. Gimeno, P. Paredes, Eur. J. Inorg. Chem. (2006) 599. [21] P. Paredes, J. Diez, M.P. Gamasa, J. Organomet. Chem. 693 (2008) 3681. [22] P. Paredes, J. Diez, M.P. Gamasa, Organometallics 27 (2008) 2597. [23] P. Paredes, J. Diez, M.P. Gamasa, Polyhedron 28 (2009) 57. [24] P. Paredes, E. Vega, J. Diez, M.P. Gamasa, Organometallics 31 (2012) 3798. [25] E. de Julian, J. Diez, E. Lastra, M.P. Gamasa, J. Mol. Catal. A Chem. 394 (2014) 295. [26] H. Ito, S. Kunii, M. Sawamura, Nat. Chem. 2 (2010) 972. [27] T. Imamoto, K. Tamura, Z. Zhang, Y. Horiuchi, M. Sugiya, K. Yoshida, A. Yanagisawa, I.D. Gridnev, J. Am. Chem. Soc. 134 (2012) 1754. [28] I.D. Gridnev, Y. Liu, T. Imamoto, ACS Catal. 4 (2014) 203 (and references cited therein). [29] T. Imamoto, M. Sugiya, N. Oohara, K. Tamura, J. Synth. Org. Chem. Jpn. 72 (2014) 1084 (and references cited therein). [30] B. Ding, Z. Zhang, Y. Xu, Y. Liu, M. Sugiya, T. Imamoto, W. Zhang, Org. Lett. 15 (2013) 5476. [31] Z. Yang, C. Xia, D. Liu, Y. Liu, M. Sugiya, T. Imamoto, W. Zhang, Org. Biomol. Chem. 13 (2015) 2694. [32] L. Cao, J. Ding, M. Gao, Z. Wang, J. Li, A. Wu, Org. Lett. 11 (2009) 3810. [33] S.G. Davies, W.E. Hume, P.M. Roberts, J.E. Thomson, Tetrahedron 66 (2010) 8076. [34] A. Soelvhoej, R. Madsen, Organometallics 30 (2011) 6044. € fberg, R. Grigg, M.A. Whittaker, A. Keep, A. Derrick, J. Org. Chem. 71 [35] C. Lo (2006) 8023. [36] A. Fryszkowska, K. Fisher, J.M. Gardiner, G.M. Stephens, J. Org. Chem. 73 (2008) 4295. [37] Y. Zhang, Z. Han, F. Li, K. Ding, A. Zhang, Chem. Commun. 46 (2010) 156. [38] R.J. Pagliero, S. Lusvarghi, A.B. Pierini, R. Brun, M.R. Mazzieri, Bioorg. Med. Chem. 18 (2010) 142. [39] G.M. Sheldrick, Acta Crystallogr. Sect. A Found. Crystallogr. 64 (2008) 112.
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Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem
P-Stereogenic pincer iridium complexes: Synthesis, structural characterization and application in asymmetric hydrogenation Zehua Yang a, Xuan Wei a, Delong Liu a, **, Yangang Liu a, Masashi Sugiya c, Tsuneo Imamoto c, d, ***, Wanbin Zhang a, b, * a
School of Pharmacy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China Organic R&D Department, Nippon Chemical Industrial Co., Ltd., Kameido, Koto-ku, Tokyo 136-8515, Japan d Department of Chemistry, Graduate School of Science, Chiba University, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 January 2015 Received in revised form 8 April 2015 Accepted 2 May 2015 Available online 21 May 2015
P-Stereogenic PNP type pincer iridium complexes PNPtBuMeIrH2Cl (3) and PNPtBuMeIrH3 (4) were synthesized in reasonable yields and characterized by 1H NMR, 13C NMR, 31P NMR, HRMS and/or single crystal X-ray diffraction. The ORTEP diagram shows that the coordination geometry around the iridium center of complex 3 is approximately octahedral. The chlorinated iridium complex (3) and/or the trihydride iridium complex (4) were used as catalysts in the asymmetric hydrogenation of ketones, olefins and quinoline to provide the desired products with up to 17% enantioselectivity. © 2015 Elsevier B.V. All rights reserved.
Keywords: P-Stereogenic phosphine ligand Pincer iridium complex Asymmetric hydrogenation
Introduction Pincer metal complexes, due to their rigid tridentate structural properties, tunable electronic density and steric hindrance, have received widespread attention over the last three decades [1]. These types of complexes can serve as efficient catalysts and show very high reaction activities in many kinds of catalytic synthesis [2e7]. Of these catalysts, pincer iridium complexes, such as the commonly used PCP [8e12] and PNP [13e16] type, have received little attention. Moreover, the majority of reported pincer iridium complexes are achiral; few examples of chiral pincer iridium complexes have been reported. In 2002, MoraleseMorales and coworkers developed a phenyl and tert-butyl substituted P-stereogenic PCP type pincer iridium complex and used it in the achiral
* Corresponding author. School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China. Tel./fax: þ86 21 5474 3265. ** Corresponding author. School of Pharmacy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China. *** Corresponding author. Organic R&D Department, Nippon Chemical Industrial Co., Ltd., Kameido, Koto-ku, Tokyo 136-8515, Japan. E-mail addresses: [email protected] (D. Liu), [email protected] (T. Imamoto), [email protected] (W. Zhang). http://dx.doi.org/10.1016/j.jorganchem.2015.05.002 0022-328X/© 2015 Elsevier B.V. All rights reserved.
transfer dehydrogenation of cyclooctane to cyclooctene with good reaction activity [17]. In 2009, Goldman et al. described the preparation of a series of methyl and tert-butyl substituted P-stereogenic PCP type pincer iridium complexes (racemic species) and applied them to alkane dehydrogenation reactions [18]. Recently, Corma, Iglesias and their co-workers developed a series of chiral NNN-pincer iridium complexes, however such species have not yet been utilized in asymmetric reactions [19]. Gamasa et al. synthesized a series of NNN type pincer iridium complexes with a Pybox skeleton, and applied them to asymmetric transfer hydrogenations with high conversions and excellent enantioselectivities [20e25]. These results indicate that chiral pincer iridium complexes can serve as efficient chiral catalysts in asymmetric synthesis, and hence the development of novel and more suitable chiral pincer iridium complexes is a worthwhile endeavor. To the best of our knowledge, asymmetric reactions catalyzed by PNP type chiral pincer iridium complexes have not yet been reported. Bidentate P-stereogenic phosphine ligands such as QuinoxP* and BenzP*, possessing small (methyl) and bulky (tert-butyl) alkyl groups have displayed excellent activity and enantioselectivity in asymmetric catalysis [26e29]. We therefore envisaged that the introduction of the P-stereogenic phosphine groups into the rigid pincer backbone of the complex, will produce a stable and effective P-stereogenic pincer type catalyst. Novel P-stereogenic PCP type
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Z. Yang et al. / Journal of Organometallic Chemistry 791 (2015) 41e45
pincer palladium complexes were synthesized and showed high reaction activity and good enantioselectivity in the asymmetric addition of diarylphosphines to nitroalkenes [30]. Recently, a Pstereogenic PNP type pincer-Pd complex was also prepared and applied to the asymmetric intramolecular hydroamination of amino-1,3-dienes, providing the desired products in good yields and with excellent regioselectivities and moderate enantioselectivities [31]. Herein we report the synthesis and structural characterization of two new P-stereogenic PNP type pincer iridium complexes and their application to asymmetric hydrogenation reactions.
Results and discussion Synthesis and characterization of pincer iridium complexes The P-stereogenic PNP type pincer iridium complex was synthesized in two steps using optically active (R,R)-2,6-bis[(boranato(tert-butyl)methylphosphino)methyl]pyridine (1) as the starting material (Scheme 1). Thus, the boranato groups in compound 1 were removed with 1,4-diazabicyclo[2,2,2]octane (DABCO) in degassed dry toluene at 80 C. The resulting chiral bisphosphine pyridine 2 was then reacted with [IrCl(coe)2]2 under hydrogen pressure to give the pincer iridium complex 3 as a white solid. The 1 H NMR spectrum shows one td peak at 19.39 ppm and one ddd peak at 23.15 ppm, both with an integral value equal to one proton. This indicates that two hydrides are coupled with each other and with the phosphorus atoms. The two hydrides are thought to occupy cis- and trans-positions relative to the nitrogen atom, forming an octahedral coordination structure. Two doublets are present at 37.75 and 21.82 ppm in the 31P NMR spectrum, indicating that in complex 3, two magnetically inequivalent phosphorus nuclei are present and coupled with each other. Addition of an excess amount of NaH to a tetrahydrofuran solution of 3 generated a trihydride pincer iridium complex 4. The 1H NMR spectrum shows two resonances in the hydride region; a td peak at 9.83 ppm and a multiplet at 19.38 ~ 19.45 ppm, with integral values equal to two protons and one proton, respectively. A singlet centered at 32.77 ppm in the 31P NMR spectrum of 4 indicates the presence of two phosphorus nuclei in an indistinguishable chemical environment. This information shows that the chlorine atom located at the iridium center is completely substituted by a hydrogen atom. Additionally, 4 is sensitive to air and moisture. Layering a concentrated dichloromethane solution of 3 with n-
Fig. 1. ORTEP drawing of pincer iridium complex 3. Ellipsoids are show at 30% probability levels. Hydrogen atoms except hydrides bond to iridium were omitted for clarity.
hexane provided a single crystal suitable for X-ray analysis. The ORTEP diagram of complex 3 is shown in Fig. 1. X-ray crystallographic analysis revealed the PNP pincer ligand adopts a meridional coordination mode, with two peaks in the Fourier map that could be assigned as hydrides to form the octahedral coordination geometry. Two of the phosphorus atoms are bound to the iridium with different bond lengths [2.2762(16), 2.2572(16) Å] (Table 1). The bond angle of the PeIreP is 164.19(6) , smaller than 180 due to the steric strain of the five-member chelate rings. This structural feature allows the iridium atom to readily coordinate with substrates during asymmetric catalysis. The large steric difference between the small methyl group and bulky tert-butyl group in complex 3 is expected to provide a satisfactory stereo control in asymmetric catalysis. Pincer iridium catalyzed asymmetric hydrogenation Since pincer iridium complexes are known to be efficient catalysts for the hydrogenation of C]O double bonds [15], we investigated the potential use of 3 and 4 in such reactions (Table 2). Thus, the asymmetric hydrogenation of acetophenone (5a) was carried out using both 3 and 4 as chiral catalysts in ethanol with KOH as a base. The reactions proceeded smoothly providing the target product in good conversions but with very low enantiomeric excesses (entries 1 and 2). Considering the convenience of the synthetic procedure, we selected complex 3 as a catalyst in subsequent investigations. When methanol was used as the solvent, the target product was obtained with a higher ee but a lower conversion (entry 3). The introduction of a hydroxyl group to the ortho-position of acetophenone resulted in full conversion to the desired product and a slightly improved ee (entry 4). Under similar reaction conditions, biomass-derived levulinic acid (5c) was hydrogenated to provide the corresponding cyclic product (6c) in moderate conversion and with 14% ee (Scheme 2).
Table 1 Selected bond lengths [Å] and angles [ ] for complex 3. Bond lengths [Å]
Scheme 1. Synthesis of pincer iridium complexes.
Ir1eN1 Ir1eP2 Bond angles [ ]
2.137(5) 2.2572(16)
Ir1eP1 Ir1eCl1
N1eIr1eP1 P2eIr1eP1 P2eIr1eCl1
82.39(13) 164.19(6) 100.42(6)
N1eIr1eP2 N1eIre Cl1 P1eIr1eCl1
2.2762(16) 2.4968(16) 83.18(13) 89.25(13) 85.83(6)
Z. Yang et al. / Journal of Organometallic Chemistry 791 (2015) 41e45 Table 2 Hydrogenation catalyzed by pincer iridium complexesa.
Entry
Catalyst
Solvent
R
Conversion (%)b
ee (%)c
1 2 3 4
3 4 3 3
EtOH EtOH MeOH MeOH
H H H OH
82 75 38 95
6 4 17 8
a
Reaction conditions: 0.5 mmol substrates, 0.005 mmol catalyst, 0.05 mmol KOH, 2 mL solvent, 50 atm H2, stirred at 25 C for 24 h. b Determined by 1H NMR. c Determined by chiral HPLC or GC.
Scheme 2. a Reaction conditions: 0.5 mmol substrates, 0.005 mmol catalyst, 0.05 mmol KOH, 2 mL solvent, 50 atm H2, stirred at 100 C for 24 h b Acidified to pH 3e4 and stirred for 1 h.
Substrates with a C]C double bond were also hydrogenated by complex 3 (Scheme 3). For example, olefins substituted with a cyano (5d) or nitro (5e) group, could be hydrogenated smoothly with almost quantitative conversions. Carboxyl substituted styrene (5f) could be hydrogenated by complex 3 in moderate conversion but very low enantioselectivity. Moreover, 2-methylquinoline (5g), possessing both C]C and C]N double bonds, could also be hydrogenated using this catalytic system but the conversion and ee values were low. Conclusion We have designed and synthesized novel P-stereogenic pincer iridium complexes 3 and 4 in reasonable yields using a short
43
synthetic route. The complexes were characterized by 1H NMR, 13C NMR, 31P NMR, HRMS and/or single crystal X-ray diffraction. The ORTEP diagram of 3 shows that the coordination geometry around the iridium atom is approximately octahedral. 3 and/or 4 were used as catalysts for the hydrogenation of ketones, olefins and quinoline derivatives to provide the desired products with moderate to excellent conversions (up to 99%) and up to 17% enantiomeric excess. Investigations to improve the reaction enantioselectivity are currently underway and will be reported in due course. Experimental section General procedures All air and moisture sensitive manipulations were carried out with standard Schlenk techniques or in a glove box under argon atmosphere. Column chromatography was performed using 100e200 mesh silica gels. All solvents were refined by the standard method of solvent manual. The commercial available reagents were purchased from Adamas-Beta Ltd., Energy Chemical Inc. or J&K Scientific Inc. and were used without further purification unless otherwise specified. The NMR spectra were recorded on a Varian MERCURY plus-400 (400 MHz, 1H; 101 MHz, 13C; 162 MHz, 31P) spectrometer with chemical shifts reported in ppm relative to the residual deuterated solvents or the internal standard tetramethylsilane or 85% phosphoric acid respectively. HRMS were performed on a solariX XR 7.0 T hybrid quadrupole-FTICR mass spectrometer (Bruker Daltonics, Bremen, Germany), which was equipped with an ESI/APCI/MALDI ion source. Melting points were measured with SGW X-4 micro melting point apparatus. Optical rotations were measured on a Rudolph Research Analytical Autopol VI automatic polarimeter using a 50 mm path-length cell at 589 nm. Enantiomeric excess values (ee) were measured on a Shimadzu LC-10Avp HPLC system and using Daicel Chiralcel OD-H, and OJ-H columns with n-hexane/isopropanol as eluant, or on a Shimadzu GC2010 plus gas chromatography system using Supelco Analytical b-dex 120 and Varian CP-Chirasil-Dex CB capillary columns with nitrogen as eluant and FID as detector. Synthesis of (R,R)-2,6-bis[(tert-butylmethylphosphino)methyl] pyridine (2) Under an argon atmosphere, in a 50 mL two neck flask, compound 1 (500 mg, 1.5 mmol) and 1,4-diazabicyclo[2.2.2]octane (DABCO, 496 mg, 4.4 mmol) were dissolved in distilled toluene (10 mL). The solution was stirred at 80 C for 8 h. After cooling to room temperature, degassed n-hexane 20 (mL) was added to the mixture and a white precipitate was generated. Removal of the precipitate followed by evaporation of the solvent under reduced pressure gave compound 2 as a yellow oil, which was used in the next step without further purification. Synthesis of PNPtBuMeIrH2Cl (3)
Scheme 3. a Reaction conditions: 0.5 mmol substrates, 0.005 mmol catalyst, 0.05 mmol KOH, 2 mL solvent, 50 atm H2, stirred at 25 C for 24 h b Esterification with MeI.
To a solution of [IrCl(coe)2]2 (660 mg, 0.7 mmol) in distilled toluene (40 mL) was added a solution of 2 (the entire product generated in the previous step, ca. 1.4 mmol) in distilled toluene (20 mL). The mixture was then transferred to a stainless autoclave in a glove box and was pressurized with hydrogen gas (2.5 MPa) and stirred at 100 C for 12 h. After cooling to room temperature, the pressure was released and the solvent was removed in vacuo. The residue was purified by column chromatography (ethyl acetate: petroleum ether ¼ 2: 1) to afford the pure product as a white solid (327 mg, 41%). m.p. 257e259 C. 1H NMR (400 MHz, CDCl3) d 7.52 (t, J ¼ 7.7 Hz, 1H), 7.23 (d, J ¼ 7.7 Hz, 2H), 3.99 (dd, J ¼ 16.7, 8.9 Hz, 1H),
44
Z. Yang et al. / Journal of Organometallic Chemistry 791 (2015) 41e45
3.78 (dd, J ¼ 16.8, 9.8 Hz, 1H), 3.60 (dd, J ¼ 15.9, 9.1 Hz, 1H), 3.22 (dd, J ¼ 16.7, 10.2 Hz, 1H), 1.80 (dd, J ¼ 9.0, 3.1 Hz, 3H), 1.70 (dd, J ¼ 8.4, 2.7 Hz, 3H), 1.31 (d, J ¼ 14.5 Hz, 9H), 1.08 (d, J ¼ 14.3 Hz, 9H), 19.39 (td, J ¼ 13.5, 7.6 Hz, 1H), 23.15 (ddd, J ¼ 17.9, 13.6, 7.6 Hz, 1H). 13C NMR (101 MHz, CDCl3) d 162.8, 162.4, 135.7, 119.9 (dd, J ¼ 14.2, 9.0 Hz), 43.9 (d, J ¼ 24.7 Hz), 42.9 (d, J ¼ 25.3 Hz), 26.7 (d, J ¼ 4.1 Hz), 26.0 (d, J ¼ 4.5 Hz), 18.0, 17.7, 5.4 (d, J ¼ 3.1 Hz), 5.2 (d, J ¼ 2.8 Hz). 31P NMR (162 MHz, CDCl3) d 37.75 (d, J ¼ 359.0 Hz), 21.82 (d, J ¼ 360.8 Hz). HRMS (MALDI): calcd. for C17H32ClIrNP2 [MH]þ 540.13277, found 540.13348. IR (KBr disc) n/cm1: 2963, 2904, 2870, 2380, 1590, 1573, 1453, 1394, 1369, 1067, 1019, 891, 818, 746. [a]25D ¼ 21.9 (c 0.1, CHCl3). Synthesis of PNPtBuMeIrH3 (4) To a 50 mL two neck flask, complex 3 (100 mg, 0.2 mmol), NaH (739 mg, 18.5 mmol, washed by distilled n-hexane) and tetrahydrofuran (10 mL) were added under an argon atmosphere, and the mixture was stirred at room temperature for 12 h. The reaction mixture was filtered and the filtrate was concentrated in vacuo. The residue was washed with distilled n-hexane and recrystallized from tetrahydrofuran/n-hexane to give complex 4 as a yellow solid (58 mg, 62%). m.p. 225e227 C. 1H NMR (400 MHz, C6D6) d 6.79 (t, J ¼ 7.6 Hz, 1H), 6.47 (d, J ¼ 7.5 Hz, 2H), 3.30 (d, J ¼ 17.1 Hz, 2H), 2.80 (d, J ¼ 16.7 Hz, 2H), 1.85 (s, 6H), 1.14 (vt, J ¼ 6.9 Hz, 18H), 9.83 (td, J ¼ 17.0, 5.0 Hz, 2H), 19.38 ~ 19.45 (m, 1H). 31P NMR (162 MHz, C6D6) d 46.58 (s). HRMS (MALDI): calcd. for C17H33IrNP2 [MH]þ 506.17175, found 506.17158. Typical procedure for catalytic asymmetric hydrogenation A 10 mL tube equipped with a magnetic stirring bar, substrate (0.5 mmol), catalyst (0.005 mmol) and KOH (0.05 mmol), was vacuum-pumped and flushed with argon three times. A degassed solvent (2 mL) was then added and the tube was transferred to a stainless autoclave in a glove box. The autoclave was pressurized and evacuated with hydrogen three times, and finally charged with hydrogen at a pressure of 50 atm. The reaction mixture was magnetically stirred at the given temperature for 24 h. After cooling to room temperature, the pressure was released. The solvent was removed in vacuo and the crude product was examined by 1H NMR to determine the conversion. The pure product was obtained by flash column chromatography using ethyl acetate/petroleum ether as the eluant. 1-Phenylethanol (6a) Colorless oil [32]. 1H NMR (400 MHz, CDCl3) d 7.41e7.25 (m, 5H), 4.89 (qd, J ¼ 6.4, 3.0 Hz, 1H), 1.50 (d, J ¼ 6.5 Hz, 3H). HPLC Daicel ChiralPak OD-H, n-hexane/i-PrOH ¼ 95/5, 210 nm, 0.8 mL/min tR1 ¼ 11.1 min (major), tR2 ¼ 13.1 min (minor), ee ¼ 17%. 2-(1-Hydroxyethyl)phenol (6b) Colorless oil [33]. 1H NMR (400 MHz, CDCl3) d 8.00 (brs, 1H), 7.16 (t, J ¼ 7.7 Hz, 1H), 6.98 (d, J ¼ 6.3 Hz, 1H), 6.84 (dd, J ¼ 14.3, 7.0 Hz, 2H), 5.05 (q, J ¼ 6.6 Hz, 1H), 2.78 (brs, 1H), 1.57 (d, J ¼ 6.6 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 155.5, 129.1, 128.6, 126.6, 120.0, 117.2, 71.7, 23.6. GC Varian CP-Chirasil-Dex CB, 90 C isothermal, 1 mL/min tR1 ¼ 24.5 min (minor), tR2 ¼ 25.2 min (major), ee ¼ 8%.
g-Valerolactone (6c) Colorless oil [34]. 1H NMR (400 MHz, CDCl3) d 4.65e4.53 (m, 1H), 2.61e2.40 (m, 2H), 2.40e2.26 (m, 1H), 1.92e1.69 (m, 1H), 1.37 (dd, J ¼ 6.2, 2.4 Hz, 3H). GC Supelco Aalytical b-dex 120, 90 C isothermal, 1 mL/min tR1 ¼ 16.8 min (major), tR2 ¼ 17.6 min (minor), ee ¼ 14%.
2,3-Diphenylpropanenitrile (6d) Colorless oil [35]. 1H NMR (400 MHz, CDCl3) d 7.38e7.28 (m, 8H), 7.14 (d, J ¼ 6.4 Hz, 2H), 4.00 (t, J ¼ 6.7 Hz,1H), 3.23e3.11 (m, 2H). HPLC Daicel ChiralPak OD-H, n-hexane/i-PrOH ¼ 98/2, 210 nm, 0.6 mL/min tR1 ¼ 21.8 min (minor), tR2 ¼ 23.1 min (major), ee ¼ 7%. (2-Nitropropyl)benzene (6e) Colorless oil [36]. 1H NMR (400 MHz, CDCl3) d 7.36e7.24 (m, 3H), 7.17 (d, J ¼ 6.6 Hz, 2H), 4.78 (sextet, J ¼ 6.7 Hz, 1H), 3.33 (dd, J ¼ 14.0, 7.5 Hz, 1H), 3.01 (dd, J ¼ 14.0, 6.8 Hz, 1H), 1.55 (d, J ¼ 6.7 Hz, 3H). HPLC Daicel ChiralPak OJ-H, n-hexane/i-PrOH ¼ 90/10, 210 nm, 0.8 mL/min tR1 ¼ 12.5 min, tR2 ¼ 13.6 min, ee ¼ 0. 2-Phenylpropanoic acid (6f) Colorless oil [37]. 1H NMR (400 MHz, CDCl3) d 7.36e7.26 (m, 5H), 3.74 (q, J ¼ 7.2 Hz, 1H), 1.51 (d, J ¼ 7.2 Hz, 3H). HPLC Daicel ChiralPak OJ-H, n-hexane/i-PrOH ¼ 90/10, 210 nm, 1 mL/min tR1 ¼ 10.3 min (minor), tR2 ¼ 11.6 min (major), ee ¼ 5%. 2-Methyl-1,2,3,4-tetrahydroquinoline (6g) Colorless oil [38]. 1H NMR (400 MHz, CDCl3) d 7.00e6.96 (m, 2H), 6.63 (td, J ¼ 7.4, 1.1 Hz, 1H), 6.49 (dd, J ¼ 8.3, 1.1 Hz, 1H), 3.58 (brs, 1H), 3.47e3.36 (m, 1H), 2.86 (ddd, J ¼ 17.1, 11.5, 5.7 Hz, 1H), 2.75 (ddd, J ¼ 16.4, 5.2, 3.6 Hz, 1H), 2.01e1.86 (m, 1H), 1.66e1.56 (m, 1H), 1.23 (d, J ¼ 6.3 Hz, 3H). 13C NMR (101 MHz, CDCl3) d 145.0, 129.5, 126.9, 121.4, 117.2, 114.3, 47.4, 30.4, 26.9, 22.9. HPLC Daicel ChiralPak OD-H, n-hexane/i-PrOH ¼ 99/1, 220 nm, 0.8 mL/min tR1 ¼ 14.1 min (minor), tR2 ¼ 16.1 min (major), ee ¼ 4%. Crystal structure determinations Single crystals of complex 3 were obtained from a concentrated solution of dichloromethane layered by n-hexane. The X-ray diffraction data were collected on an Oxford Diffraction Gemini A
Table 3 Crystallographic data for Complex 3. Formula Fw Cryst color Cryst size [mm] T (K) l (Å) Cryst syst Space group a (Å) b (Å) c (Å) a (deg) b (deg) g (deg) Z V (Å3) rcalcd (g cm3) m (cm1) q range (deg) Total no. of rflns No. of unique rflns R1 (I > 2s) wR2 (I > 2 s) R1 (all data) wR2 (all data) Abs corr Abs corr range Parameters/restraints Flack x param Rint S Res density (e Å3)
C17H33ClIrNP2 541.03 colorless 0.35 0.32 0.30 170(2) 0.71073 orthorhombic P 21 21 21 8.9472(5) 11.6097(7) 20.2296(15) 90.00 90.00 90.00 4 2101.3(2) 1.710 6.631 3.04e25.34 13,433 3827 0.0285 0.0531 0.0355 0.0558 multi-scan 0.2049e0.2410 3827/215 0.026(9) 0.0548 1.031 0.602/0.694
Z. Yang et al. / Journal of Organometallic Chemistry 791 (2015) 41e45
Ultra diffractometer with graphite monochromated Mo Ka radiation (l ¼ 0.71073 Å). The structures were solved by direct method using the SHELXS-97 [39] program, and all non-hydrogen atoms were refined anisotropically on F2 by the full-matrix least-squares technique, which used the SHELXS-97 [39] crystallographic software package. Details of the crystal structure determination of the complexes 3 are summarized in Table 3. Acknowledgments This work was partially supported by the National Natural Science Foundation of China (No. 21172143, 21172145, 21372152 and 21472123), Nippon Chemical Industrial Co., Ltd., and Shanghai Jiao Tong University (SJTU). We thank the Instrumental Analysis Center of SJTU for HRMS analysis. Appendix A. Supplementary material CCDC 1017083 for complex 3 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Appendix B. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2015.05.002. References , O. Wendt, Pincer and Pincer-type Complexes: Applications in [1] K. Szabo Organic Synthesis and Catalysis, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Germany, 2014 (and references cited therein). [2] J.-J. Feng, X.-F. Chen, M. Shi, W.-L. Duan, J. Am. Chem. Soc. 132 (2010) 5562. [3] J.-I. Ito, K. Fujii, H. Nishiyama, Chem. e Eur. J. 19 (2013) 601. [4] X.-Q. Hao, J.-J. Huang, T. Wang, J. Lv, J.-F. Gong, M.-P. Song, J. Org. Chem. 79 (2014) 9512. [5] C. Haibach Michael, N. Lease, S. Goldman Alan, Angew. Chem. Int. Ed. 53 (2014) 10160. [6] P. Kang, S. Zhang, T.J. Meyer, M. Brookhart, Angew. Chem. Int. Ed. 53 (2014) 8709. [7] T. Zell, Y. Ben-David, D. Milstein, Angew. Chem. Int. Ed. 53 (2014) 4685.
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