Asymmetric Rhodium(I)-Catalyzed C–C Activations with Zwitterionic

Article pubs.acs.org/Organometallics

Asymmetric Rhodium(I)-Catalyzed C−C Activations with Zwitterionic Bis-phospholane Ligands Evelyne Parker and Nicolai Cramer* Laboratory of Asymmetric Catalysis and Synthesis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), SB-ISIC, BCH4305, 1015 Lausanne, Switzerland S Supporting Information *

ABSTRACT: The development of unconventional ligand scaffolds is an important aspect to alter reaction pathways of transition-metal-catalyzed reactions. The nature of the counterion of cationic metal complexes plays an important role in the catalyst reactivity. We herein report a chiral anionic bidentate bis-phosphine ligand based on the popular phospholane scaffold. Subsequently, zwitterionic rhodium(I) complexes with no external counterion were synthesized, and their potential was evaluated in asymmetric carbon−carbon bond activation of cyclobutanones. This type of rhodium complex allows for a significantly lower reaction temperature than analogous cationic rhodium complexes and enables, for the first time, asymmetric transformations with up to 93.5:6.5 enantiomeric ratio.

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INTRODUCTION Advances in transition-metal catalysis are often closely linked to developments in ligand design. Chiral phosphines became of utmost importance for asymmetric catalysis.1 In addition to the organic ligand, the catalytic activity of transition-metal complexes can be largely modulated by the nature of the counterion.2 For instance, cationic metal complexes not only are more electrophilic at the metal center but also preserve free coordination sites for substrate binding. Such “cationic” species refer to Rh(I) complexes with weakly coordinating counterions such as BF4−, SbF6−, or BARF.3 For example, Dong et al. found that a careful adjustment of the coordinating strength of the counterion was key for intermolecular asymmetric ketone hydroacylations.4 The moderately coordinating nitrate anion had the right balance for reactivity and selectivity. On the other hand, strongly coordinating counterions such as alcoholates or carboxylates can also play an important role in catalysis. For instance, they enable rapid ligand exchange or transmetalation.5 A complementary and unique counterion approach was developed by Peters et al.6 They reported borate-bearing bisphosphine ligands 1, which form with monocationic transitionmetal organometallic zwitterions (Scheme 1). The location of the borate anion is spatially defined and fixed at a distance of around 4 Å from the coordinated metal center, impacting the electrophilicity of the bound metal. A consequence of the internal charge neutralization and the altered electrophilicity is differences in the mechanism and rate of ligand substitution. It was reported by Peters that zwitterionic Rh(I) complex 2 largely outperforms typical cationic rhodium complexes for the intramolecular hydroacylation of aldehyde 3, giving cyclopentanone 4 (Scheme 1).6c Moreover, the reaction proved to be more tolerant to the solvent, allowing for previously unsuitable coordinating and © 2014 American Chemical Society

Scheme 1. Anionic Bis-phosphine Ligands and Application of Their Rh(I) Complex in Intramolecular Hydroacylation

polar solvents. However, despite this potential, these zwitterionic complexes have not yet been systematically explored for catalytic reactions. Besides the single reported example on an intramolecular hydroacylation, the zwitterionic rhodium(I) complexes were so far tested only for the hydrogenation, hydrosilylation, and hydroboration of styrene.6c Surprisingly, this type of complex is so far elusive for asymmetric catalysis, as no chiral ligand featuring such a borate backbone has been yet reported. In our long-standing efforts to develop asymmetric C−H7 and C−C bond activations,8 we saw an opportunity to enhance the reactivity for C−C bond cleavages of cyclobutanones with tethered olefins with such zwitterionic complexes. Herein, we report the synthesis of a chiral zwitterionic Rh(I) complex based on the C2-symmetric Received: December 2, 2013 Published: January 23, 2014 780

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Organometallics

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phospholane architecture and demonstrate an initial application for challenging asymmetric carbon−carbon bond activations of cyclobutanones.

Scheme 3. Preparation of Metal Complex 8

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RESULTS AND DISCUSSION Synthesis of the Ligands and the Metal Complexes. The key precursor for the ligand synthesis, chlorodimethylphospholane 5, is accessible from the TMS-phospholane in a single step (Scheme 2).9 Subsequently, 5 was coupled in the Scheme 2. Synthesis of Ligand 1b

presence of catalytic amounts of CuBr·SMe2 and lithium bromide with methyl magnesium iodide to give methyl phospholane 6.10 Metalation of 6 with tert-butyllithium under neat conditions led to the lithiated species 7 in good yields. In turn, reaction of 7 with diphenyl boron chloride provided ligand 1b as its Li(THF)2 salt in moderate yield. X-ray quality crystals of 1b were obtained by layering a solution of the ligand in toluene with hexane (Figure 1).

Figure 2. X-ray structure of complex 8. The thermal ellipsoids are displayed at 40% probability, and hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): B−Rh, 4.144; P1−Rh, 2.295, P2−Rh, 2.293; C1/C2−Rh, 2.207/2.227, C4/C5−Rh, 2.197/2.199; P1−Rh−P2, 88.96.

range as observed by Peters.6c The important quadrant shielding properties of the C2-symmetric phospholane moiety, which is critical for efficient asymmetric induction, can be visualized in Figure 3. Properties of the Ligands. Upon treatment of complex 8 with two equivalents of hydrogen in a 2:1 mixture of acetonitrile and THF, clean hydrogenolysis of the norbornadiene moiety delivered presumed bis-acetonitrile species 9 with a JRh,P coupling of 166 Hz (Scheme 4). However, this species

Figure 1. X-ray structure of chiral anionic ligand 1b. The thermal ellipsoids are displayed at 40% probability. Two molecules of toluene and all hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): B−Li, 4.151, P1−Li, 2.553; P2−Li, 2.611, O1− Li, 1.916; O2−Li, 1.936; P1−Li−P2, 91.86; O1−Li−O2, 100.57; P1− Li−O1, 127.69; P1−Li−O2, 107.22; P2−Li−O1, 107.41; P2−Li−O2, 124.75.

Subsequent complexation of the rhodium(I) precursor [{Rh(nbd)Cl}2] with 1b gave rise to the desired zwitterionic Rh(I) complex 8 as a reddish powder in good yields (Scheme 3). Layering of a toluene solution of the complex 8 with pentane provided dark red crystals suitable for X-ray crystallographic analysis. Complex 8 shows the typical square planar geometry for rhodium(I) complexes (Figure 2). The distance between boron and rhodium is 4.144 Å, similar to the typical

Figure 3. X-ray structure of complex 8, front view, showing the quadrant shielding of the phospholane units. The thermal ellipsoids are displayed at 40% probability. The norbornadiene moiety and most hydrogen atoms are omitted for clarity. 781

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Scheme 4. Hydrogenolysis of the Norbornadiene Ligand of Complex 8 Leads to Presumed Complexes 9 and 10

proved stable only in solutions containing large amounts of acetonitrile, and no isolation was attempted. For instance, running the hydrogenolysis of 8 in an aromatic solvent or exposing 9 to noncoordinating solvents resulted in the generation of another species with a characteristically larger JRh,P coupling of 189 Hz. However, neither species could be isolated and further characterized. On the basis of the previous reports of Peters,6c a dimeric structure 10 is tentatively proposed. Due to the relatively low stability of the diene-free Rh(I) complexes, we performed all reactivity assessments for the catalysis by in situ hydrogenolysis of 8 immediately prior to use. Zwitterionic Rhodium Complexes in Catalytic Carbon−Carbon Bond Activations. We next aimed to evaluate the potential of the chiral complexes 8 for carbon−carbon bond cleavage processes.11 For instance, Ito and Murakami reported a rhodium(I)-catalyzed cleavage of cyclobutanones 11 (R = H, Scheme 5).12,13 The reaction is suggested to be initiated by an oxidative insertion of the rhodium(I) catalyst in the cyclobutanone acyl−carbon bond, providing rhoda(III)cyclopentanone 12. In turn, this species reacts with the appended vinyl group to deliver via rhoda(III)cycle 13 bicyclic ketone 14. The reported reaction conditions for this transformation are quite harsh, requiring 135 °C in order to enforce the C−C bond cleavage. As a result, BHT was required to prevent polymerization of the styrene substrate under those forcing conditions. An initial control experiment using conditions similar to those reported by Murakami showed that the quaternary center on cyclobutanone substrate 11a is well tolerated and gave good yields of corresponding ketone 14a (Table 1, entry 1). In a first evaluation of this transformation with Peters’ catalyst no conversion of 11 occurred (entry 2). We suspected that the

Scheme 5. Rhodium(I)-Catalyzed Carbon−Carbon Bond Activation of Cyclobutanones 11

norbornadiene unit would bind strongly to the rhodium center and therefore hamper exchange with substrate 11. By means of the aforementioned hydrogenative removal of the norbornadiene ligand to give 2, the resulting species proved to be remarkably active for the C−C bond activation, providing 14a in 87% isolated yield (entry 3). Notably, full conversion of substrate 11a is achieved already at a significantly reduced reaction temperature of 90 °C. We attributed this increased catalytic performance to the zwitterionic nature of the catalyst, as in comparison, structurally similar dppp- or dppe-derived cationic Rh(I) catalysts having tetrafluoroborate counterions were completely inactive under similar conditions (entries 4− 6). No significant change in reactivity was observed upon hydrogenolysis of [{Rh(cod)dppp}BF4]. Asymmetric Rhodium-Catalyzed Carbon−Carbon Bond Activation of Cyclobutanones. With the observed 782

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Table 1. C−C Bond Activations of Cyclobutanone 11aa

[Rh]a

entry c

1 2d 3d 4 5 6

solvent

[{Rh(cod)dppp}BF4] [Ph2B(CH2PPh2)2Rh(nbd)] 2 [{Rh(cod)dppp}BF4] [{Rh(cod)dppe}BF4] [{Rh(cod)dppe}BF4]

c

toluene DCE DCE DCE DCE dioxane

cationic and neutral Rh(I) complexes with a range of chiral phosphines based on the phospholane scaffold (entries 9−14). While most fail to give any product, sluggish reactions but rather good selectivities were seen with 1614 and 17 at much higher reaction temperatures in dioxane (entries 13 and 14).

temp [°C]

yieldb

135 90 90 90 90 130

79 0 87 0 0 0

With the optimized procedure, we then evaluated several cyclobutanone substrates 11 in the C−C cleavage process (Table 3). The enantioselectivity of the reaction was influenced by the nature of both the substituent R1 of the cyclobutanone and the substituent R2 on the aromatic ring. In comparison with the model substrate 11a, these changes might slightly disturb the geometry of initial binding of the complex to the olefin and carbonyl group and thus lead to reduced selectivities.

a

Reaction conditions: 0.05 mmol of 11a, 5 mol % [Rh], 25 mol % H2, 0.08 M in solvent for 12 h. bYield of isolated product 14a. c10 mol % of BHT was added. dNo H2.

increased reaction rates of the C−C bond activation with the achiral zwitterionic rhodium complex, we next aimed to evaluate the potential of a catalytic asymmetric version using our chiral complex 8. Again, the norbornadiene ligand was removed by in situ hydrogenolysis. Gratifyingly, the resulting complex 10 also led to smooth conversion of cyclobutanone 11a into ketone 14a (Table 2). Remarkably, the reaction proceeds only in DCE or CH2Cl2 (entries 1 and 2). In both solvents, the chiral complex led to a good enantiomeric ratio of 91:9 (DCE) and 93.5:6.5 (DCM). A wide range of other solvents are not compatible (entries 3−7). Lowering the reaction temperature to 60 °C slightly increased the selectivity of the reaction to 94.5:5.5 er, however at the expense of a poor conversion of 11a (entry 8). We compared the reactivity and the selectivity of the zwitterionic ligand against conventional

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CONCLUSIONS In summary, we have reported an anionic chiral bis-phosphine ligand that incorporates the chiral phospholane moiety. The olefin ligand can be removed in situ from the obtained zwitterionic rhodium(I) norbornadienyl complex to give a highly reactive complex. In a first application, we demonstrated the potential of these catalysts for challenging enantioselective C−C bond activations of cyclobutanones, providing access to chiral bicyclic ketones. An advantage of the zwitterionic nature of the catalyst is the enhanced reaction rate and hence reduced reaction temperatures for challenging carbon−carbon bond activations.

Table 2. Optimization of the Asymmetric C−C Activation of Cyclobutanone 11aa

entry

[Rh]a

solvent

temp [°C]

convb

1 2 3 4 5 6 7 8e 9

8 8 8 8 8 8 8 8 10 mol % 15 10 mol % [{Rh(cod)2}BF4] 10 mol % 16 10 mol % [{Rh(cod)2}BF4] 10 mol % 16 10 mol % [{Rh(cod)2}BF4] 10 mol % 16 5 mol % [{Rh(cod)2}2Cl2] 10 mol % 16 5 mol % [{Rh(cod)2}2Cl2] 10 mol % 17 5 mol % [{Rh(cod)2}2Cl2]

DCE DCM CH3CN toluene hexane acetone CCl4 DCM DCE

90 90 90 90 90 90 90 60 90

100 88 0 10 0 0 0 36 0

DCE

120

98% purity by 31P NMR, 95% estimated purity by EA). Optionally, this material was recrystallized from a mixture of benzene/toluene by layering with pentane. After being stored at −33 °C, X-ray quality crystals were collected. 1H NMR (400 MHz, C6D6, δ): 7.88 (br s, 4H, B(ortho-C6H5)), 7.37 (dd, 3JH−H = 7.5 Hz, 3JH−H = 7.5 Hz, 4H, B(meta-C6H5)), 7.18−7.13 (m, 2H, B(para-C6H5)), 3.36 (t, 3JH−H = 6.7 Hz, 8H, OCH2CH2 of THF), 2.15−2.01 (m, 2H,

EXPERIMENTAL SECTION

Chemicals and Reagents. All manipulations were carried out under an inert N2(g) atmosphere using standard Schlenk or glovebox techniques. Solvents were purified using a two-column solid-state purification system and degassed prior to use with three pump− freeze−thaw cycles. Unless otherwise noted, all other reagents and starting materials were purchased from commercial sources and used without further purification. (R,R)-1-Chloro-2,5-dimethylphospholane 5,10 ligand 16,14 and chlorodiphenylborane15 were prepared as previously reported. Physical Methods. The 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded at 293 K on a 400 MHz spectrometer at 298 K unless specified otherwise. Chiral HPLC analyses were conducted with a Chiralpak IC column. Elemental analyses were performed by the EPFL Elemental Analysis Facility. X-ray diffraction studies were carried out at the EPFL Crystallography Facility. The data collections for the crystal structures were performed at low temperature using Mo Kα radiation on a diffractometer equipped with a kappa geometry goniometer. The data were reduced by EvalCCD16 and then corrected for absorption.17 The solution and refinement were performed by SHELX.18 The structure was refined using full-matrix least-squares based on F2 with all non-hydrogen atoms anisotropically defined. Hydrogen atoms were placed in calculated positions by means of the “riding” model. Ligand 1b. A solution of tert-butyllithium (1.87 M in pentane, 7.3 mL) was introduced in a dry Schlenk flask equipped with a stir bar. The pentane was carefully removed in vacuo, providing tertbutyllithium as a white solid (0.87 g, 13.58 mmol). (2R,5R)-1,2,5Trimethylphospholane 6 (1.61 g, 12.4 mmol) was added at 0 °C. The 784

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(pentane/EtOAc, 9:1). HPLC separation (Chiralpak IC, 4.6 × 250 mm; 10% i-PrOH/hexane, 1 mL/min, 254 nm): tr(minor) = 10.5 min, tr(major) = 13.4 min; 91:8 er. Ketone 14b. Prepared from cyclobutanone 11b (8.7 mg). Yield: 8 mg (92%), colorless oil. 1H NMR (400 MHz, CDCl3, δ): 7.09 (d, 3 JH−H = 8.9 Hz, 1H), 6.76−6.58 (m, 2H), 3.78 (s, 3H), 3.40 (dt, 3JH−H = 5.4 Hz, 3JH−H = 3.3 Hz, 1H), 2.58 (dd, 2JH−H = 16.6 Hz, 4JH−H = 3.6 Hz, 1H), 2.52−2.32 (m, 3H), 2.29−2.20 (m, 1H), 2.02 (d, 2JH−H = 11.1 Hz, 1H), 1.45 (s, 3H) ppm. 13C{1H} NMR (100 MHz, CDCl3, δ): 210.0, 159.4, 149.9, 137.7, 123.8, 112.2, 107.6, 55.8, 55.4, 49.6, 47.6, 45.1, 38.5, 22.6 ppm. IR (ATR, cm−1): 2936, 2871, 2835, 1708, 1609, 1585, 1481, 1408, 1378, 1342, 1328, 1288, 1238, 1227, 1213, 1175, 1058, 1029, 867, 836, 812, 659, 504 cm−1. HRMS (ESI): calcd for [C14H17O2]+ 217.1223, found 217.1221. [α]20D = −67.7 (c 0.5, CHCl3). Rf = 0.28 (pentane/EtOAc, 9:1). HPLC separation (Chiralpak OZH, 4.6 × 250 mm; 5% i-PrOH/hexane, 1 mL/min, 254 nm): tr(minor) = 10.0 min, tr(major) = 11.8 min; 86:14 er. Ketone 14c. Prepared from cyclobutanone 11c (8 mg). Yield: 5 mg (62%), colorless oil. 1H NMR (400 MHz, CDCl3, δ): 7.23−7.10 (m, 3H), 7.11−7.04 (m, 1H), 3.47 (dt, 3JH−H = 5.1 Hz, 3JH−H = 3.4 Hz, 1H), 2.61 (dd, 2JH−H = 16.7 Hz, 3JH−H = 3.8 Hz, 1H), 2.60 (d, 2JH−H = 16.3 Hz, 1H), 2.47 (dq, 2JH−H = 16.6 Hz, 3JH−H = 2.0 Hz, 1H), 2.33− 2.20 (m, 2H), 1.93 (d, 2JH−H = 10.8 Hz, 1H), 1.87 (hept, 3JH−H = 7.5 Hz, 2H), 1.00 (t, 3JH−H = 7.5 Hz, 3H) ppm. 13C{1H} NMR (100 MHz, CDCl3, δ): 210.3, 147.3, 146.1, 127.4, 127.2, 123.2, 121.7, 54.0, 48.9, 47.9, 45.1, 39.1, 28.2, 8.7 ppm. IR (ATR, cm−1): 3067, 3019, 2938, 2879, 2857, 1710, 1472, 1459, 1408, 1381, 1334, 1311, 1214, 1180, 1019, 754, 621, 545, 486, 402 cm−1. HRMS (ESI): calcd for [C14H17O]+ 201.1274, found 201.1270. [α]20D = −19.0 (c 0.7, CHCl3). Rf = 0.41 (pentane/EtOAc, 9:1). HPLC separation (Chiralpak IC, 4.6 × 250 mm; 10% i-PrOH/hexane, 1 mL/min, 254 nm): tr(minor) = 10.8 min, tr(major) = 15.2 min; 78.5:21.5 er. Ketone 14d. Prepared from cyclobutanone 11d (11.7 mg). Yield: 6 mg (51%), colorless oil. 1H NMR (400 MHz, CDCl3, δ): 7.40−7.34 (m, 4H), 7.34−7.28 (m, 1H), 7.22−7.19 (m, 1H), 7.18−7.12 (m, 3H), 4.66 (d, 2JH−H = 12.3 Hz, 1H), 4.61 (d, 2JH−H = 12.3 Hz, 1H), 3.84 (d, 2 JH−H = 9.1 Hz, 1H), 3.72 (d, 2JH−H = 9.1 Hz, 1H), 3.56−3.41 (m, 1H), 2.78 (d, 2JH−H = 16.9, 1H), 2.64 (dd, 2JH−H = 17.1 Hz, 3JH−H = 3.9 Hz, 1H), 2.53−2.41 (m, 2H), 2.33 (ddt, 2JH−H = 11.0 Hz, 3JH−H = 5.4 Hz, 4JH−H = 2.8 Hz, 1H), 2.12 (d, 2JH−H = 11.2 Hz, 1H) ppm. 13 C{1H} NMR (100 MHz, CDCl3, δ): 209.8, 146.1, 145.7, 138.2, 128.4, 127.7, 127.6, 127.5, 127.4, 123.3, 122.0, 74.2, 73.5, 51.4, 49.3, 48.0, 45.2, 39.1 ppm. IR (ATR, cm−1): 3064, 3028, 2941, 2857, 1709, 1473, 1454, 1409, 1363, 1335, 1317, 1211, 1129, 1095, 1019, 754, 738, 698, 613, 497 cm−1. HRMS (ESI): calcd for [C20H21O2]+ 293.1536, found 293.1530. [α]20D = +3.11 (c 0.75, CHCl3). Rf = 0.25 (pentane/ EtOAc, 9:1). HPLC separation (Chiralpak IC, 4.6 × 250 mm; 10% iPrOH/hexane, 1 mL/min, 254 nm): tr(minor) = 12.8 min, tr(major) = 16.3 min; 71:29 er. Ketone 14e. Prepared from cyclobutanone 11e (13.3 mg). Yield: 11 mg (77%), colorless oil. 1H NMR (400 MHz, CDCl3, δ): 7.24−7.12 (m, 4H), 4.13 (d, 2JH−H = 9.7 Hz, 1H), 3.97 (d, 2JH−H = 9.7 Hz, 1H), 3.56−3.41 (m, 1H), 2.83 (d, 2JH−H = 16.9 Hz, 1H), 2.64 (dd, 2JH−H = 17.0 Hz, 3JH−H = 4.0 Hz, 1H), 2.48 (dtd, 2JH−H = 16.9 Hz, 3JH−H = 5.1 Hz, 4JH−H = 2.4 Hz, 2H), 2.25 (ddt, 2JH−H = 11.0 Hz, 3JH−H = 5.4 Hz, 4 JH−H = 2.8 Hz, 1H), 2.11 (d, 2JH−H = 11.0 Hz, 1H), 1.24−1.03 (m, 21H) ppm. 13C{1H} NMR (100 MHz, CDCl3, δ): 210.46, 146.43, 145.90, 127.56, 127.36, 123.23, 122.04, 68.00, 51.22, 50.73, 47.91, 45.25, 39.09, 18.04 (CH3, TIPS), 11.93 (CH, TIPS) ppm. IR (ATR, cm−1): 2941, 2890, 2865, 1714, 1461, 1130, 1098, 1070, 1015, 882, 804, 680, 494 cm−1. HRMS (ESI): calcd for [C22H35O2Si]+ 359.2401, found 359.2395. [α]20D = +5.5 (c 1, CHCl3). Rf = 0.59 (pentane/ EtOAc, 9:1). HPLC separation (Chiralpak IC, 4.6 × 250 mm; 2% iPrOH/hexane, 1 mL/min, 254 nm): tr(minor) = 9.6 min, tr(major) = 10.7 min; 83:17 er. Ketone 14f. Prepared from cyclobutanone 11f (10.5 mg). Yield: 9.7 mg (92%), colorless oil. 1H NMR (400 MHz, CDCl3, δ): 7.43−7.35 (m, 4H), 7.28−7.35 (m, 1H), 7.25−7.15 (m, 2H), 7.12 (td, 3JH−H = 7.2 Hz, 4JH−H = 1.6 Hz, 1H), 6.81 (d, 3JH−H = 7.5 Hz, 1H), 3.01 (d,

PCH), 1.97−1.77 (m, 6H, PCH, PCHCH2), 1.70−1.55 (m, 4H, BCH2P, PCHCH2), 1.40−1.18 (m, 16H, BCH2P, CH3, OCH2CH2 of THF), 1.09−0.98 (m, 6H, CH3), 0.98−0.92 (m, 2H, PCHCH2) ppm. 13 C{1H} NMR (100 MHz, C6D6, δ): 167.6−165.9 (m, B(ipso-C6H5)), 133.4 (B(ortho-C6H5)), 126.9 (B(meta-C6H5)), 123.0 (B(para-C6H5), 68.5 (OCH2CH2 of THF), 36.9 (PCHCH2), 36.4 (PCHCH2), 34.1 (PCHCH2), 33.8 (PCHCH2), 25.4 (OCH2CH2 of THF), 21.9 (br, CH3), 15.7−15.2 (m, BCH2P), 15.0 (CH3) ppm. 31P{1H} NMR (162 MHz, C6D6, δ): 2.80 ppm. HRMS (ESI): calcd for [C26H38BP2]− 423.2547, found 423.2558. [α]20D = +63.3 (c 1.0, THF). IR (ATR, cm−1): 3056, 3039, 2917, 2862, 2845, 1481, 1450, 1427, 1371, 1139, 1044, 919, 872, 769, 731, 701, 602, 573, 493, 407 cm−1. Mp = 134− 138 °C. Complex 8. A solution of [{Rh(nbd)Cl}2] (0.784 g, 1.70 mmol) in THF (17 mL) was added to a stirred solution of 1b (2.20 g, 17.0 mmol) in THF (17 mL) in a 100 mL Schlenk at 23 °C. After stirring for 3.5 h, the reaction mixture was concentrated under reduced pressure. Toluene (15 mL) was added to the residue, and the solution was then concentrated in vacuo (repeated twice). The residue was solubilized in a minimum of benzene, filtrated on a pad of Florisil, and eluted with benzene (50 mL) and toluene (20 mL). After concentration under reduced pressure, a deep red solid was obtained (1.52 g, 2.44 mmol, 72%, >99% purity by 31P NMR). Optionally, this material can be recrystallized by dissolution in benzene (20 mL) and toluene (10 mL), followed by layering with pentane. After being stored at −33 °C, X-ray quality crystals of 8 (735 mg, 1.13 mmol, 35%) were collected. 1H NMR (400 MHz, C6D6, δ): 7.81 (d, 3JH−H = 7.5 Hz, 4H, B(ortho-C6H5)), 7.38 (dd, 3JH−H = 7.5 Hz, 3JH−H = 7.5 Hz, 4H, B(meta-C6H5)), 7.15 (m, 2H, B(para-C6H5)), 4.44 (br s, 2H, CH CH), 4.25 (br s, 2H, CHCH), 3.41 (br s, 2H, CHCHCH2), 2.22−2.07 (m, 2H, PCH), 1.72−1.60 (m, 2H, BCH2P), 1.60−1.42 (m, 4H, PCHCH2), 1.33−1.23 (m, 10H, CHCH2CH, PCHCH2, CH3), 1.23−1.10 (m, 4H, BCH2P, PCH), 1.08−1.02 (m, 6H, CH3), 0.66− 0.53 (m, 2H, PCHCH2) ppm. 13C{1H} NMR (100 MHz, C6D6, δ): 165.7−164.2 (m, B(ipso-C6H5)), 132.3 (B(ortho-C6H5)), 127.3 (metaB(C6H5)), 123.4 (B(para-C6H5)), 83.8−83.6 (m, CHCH), 77.1− 76.9 (m, CHCH), 68.5 (br s, CHCH2CH), 54.0 (br s,  CHCHCH2), 37.2 (PCHCH2), 34.8 (d, 1JC−P = 13.0 Hz, PCH), 34.6 (d, 1JC−P = 13.1 Hz, PCH), 34.1 (PCHCH2), 32.6 (d, 1JC−P = 11.7 Hz, PCH), 32.5 (d, 1JC−P = 11.7 Hz, PCH), 20.0 (d, 2JC−P = 4.4 Hz, CH3), 19.9 (d, 2JC−P = 4.4 Hz, CH3), 13.4 (2 × CH3), 13.1−11.6 (m, BCH2P) ppm. 31P{1H} NMR (162 MHz, C6D6, δ): 45.1 (d, 1JP−Rh = 147.1 Hz) ppm. IR (ATR, cm−1): 3055, 2991, 2920, 2869, 2854, 1448, 1426, 1387, 1374, 1265, 1252, 1139, 1088, 1076, 935, 923, 862, 764, 731, 700, 636, 619, 577, 458 cm−1. HRMS (ESI): calcd for [C33H47BP2Rh]+ 619.2296, found 619.2295. [α]20D = +29.5 (c 1.0, THF). Mp = 240−244 °C (dec). Anal. Calcd for C33H46BP2Rh: C, 64.10; H, 7.50. Found: C, 64.14; H, 7.45. General Procedure for the Catalytic Asymmetric Reaction with Cyclobutanones 11. In a glovebox, the respective cyclobutanone (0.04 mmol) and complex 8 (2.47 mg, 4 μmol) were weighted into an oven-dried vial equipped with a magnetic stir bar. Dry and degassed DCE (0.5 mL) was added. The vial was sealed and removed from the glovebox. Then, hydrogen (0.45 mL, 0.5 equiv) was syringed into the homogeneous solution, promoting a color changed from orange to yellow. The reaction mixture was immersed in a preheated oil bath at 90 °C and stirred for 12 h. After cooling to ambient temperature, the mixture was filtered on a short pad of silica. The crude product was purified on a silica gel column eluting with pentane/EtOAc, giving ketones 14 as colorless oils. Ketone 14a. Prepared from cyclobutanone 11a (7.5 mg). Yield: 6.7 mg (90%), colorless oil. 1H NMR (400 MHz, CDCl3, δ): 7.23−7.08 (m, 4H), 3.51−3.39 (m, 1H), 2.67−2.56 (m, 1H), 2.55−2.33 (m, 3H), 2.25 (ddt, 2JH−H = 10.9 Hz, 3JH−H = 5.4 Hz, 4JH−H = 2.7 Hz, 1H), 2.03 (d, 2JH−H = 11.2 Hz, 1H), 1.47 (s, 3H) ppm. 13C{1H} NMR (100 MHz, CDCl3, δ): 209.9, 148.2, 145.7, 127.4, 127.2, 123.2, 121.3, 55.8, 49.3, 47.5, 44.8, 39.2, 22.8 ppm. IR (ATR, cm−1): 2932, 2871, 1708, 1473, 1458, 1407, 1378, 1325, 1260, 1213, 1182, 1060, 1020, 926, 754, 612, 536, 489, 460, 393 cm−1. HRMS (ESI): calcd for [C13H15O]+ 187.1117, found 187.1116. [α]20D = −11.7 (c 0.6, CHCl3). Rf = 0.35 785

dx.doi.org/10.1021/om4011627 | Organometallics 2014, 33, 780−787

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2

JH−H = 15.8 Hz, 1H), 2.85 (ddd, 2JH−H = 15.8 Hz, 4JH−H = 3.0 Hz, JH−H = 1.9 Hz, 1H), 2.68−2.53 (m, 2H), 2.43 (dt, 2JH−H = 16.2 Hz, 4 JH−H = 2.4 Hz, 1H), 2.27 (d, 2JH−H = 11.3 Hz, 1H), 1.57 (s, 3H) ppm. 13 C{1H} NMR (100 MHz, CDCl3, δ): 209.2, 148.2, 147.9, 143.7, 128.5, 127.7, 127.6, 126.9, 126.7, 123.4, 121.2, 58.2, 54.7, 51.6, 51.3, 44.1, 22.7 ppm. IR (ATR, cm−1): 3062, 3024, 2953, 2870, 1711, 1498, 1470, 1458, 1329, 757, 700, 648, 572 cm−1. HRMS (ESI): calcd for [C19H19O]+ 263.1430, found 263.1427. [α]20D = +1.2 (c 0.7, CHCl3). Rf = 0.39 (pentane/EtOAc, 9:1). HPLC separation (Chiralpak IC, 4.6 × 250 mm; 10% i-PrOH/hexane, 1 mL/min, 254 nm): tr(minor) = 9.4 min, tr(major) = 12.3 min; 83:17 er. Crystallographic Details for 1b. A total of 25 208 reflections (−22 < h < 21, −22 < k < 22, −10 < l < 14) were collected at T = 140(2) K in the range 4.65−73.43°, 7370 of which were unique (Rint = 0.0289), with Co Kα radiation (λ = 1.54178 Å). The structure was solved by direct methods. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated idealized positions. The residual peak and hole electron densities were 0.409 and −0.226 e Å−3, respectively. The absorption coefficient was 1.118 mm−1. The least-squares refinement converged normally with residuals of R(F) = 0.0473 and Rw(F2) = 0.1310 and GOF = 1.114 (I > 2σ(I)). Crystal data for C48H70BLiO2P2: Mw = 758.73, space group P31, trigonal, a = 18.2343(4) Å, b = 18.2343(4) Å, c = 11.9109(2) Å, α = 90°, β = 90°, γ = 120°, V = 3429.66(12) Å3, Z = 3, ρcalcd = 1.102 Mg/m3. Crystallographic Details for 8. A total of 22 577 reflections (−12 < h < 12, −19 < k < 13, −23 < l < 22) were collected at T = 140(2) K in the range 3.65−73.48°, 6145 of which were unique (Rint = 0.0233), with Co Kα radiation (λ = 1.54178 Å). The structure was solved by direct methods. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated idealized positions. The residual peak and hole electron densities were 0.197 and −0.227 e Å−3, respectively. The absorption coefficient was 5.529 mm−1. The leastsquares refinement converged normally with residuals of R(F) = 0.0206 and Rw(F2) = 0.0511 and GOF = 1.036 (I > 2σ(I)). Crystal data for C33H46BP2Rh: Mw = 618.36, space group P212121, orthorhombic, a = 10.37956(8) Å, b = 15.57637(13) Å, c = 19.28434(14) Å, α = 90°, β = 90°, γ = 90°, V = 3117.81(4) Å3, Z = 4, ρcalcd = 1.317 Mg/m3. 4

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REFERENCES

(1) Phosphorous Ligand in Asymmetric Catalysis; Börner, A., Ed.; Wiley-VCH: Weinheim, 2008. (2) For a review of ion pairing in transition metals, see: (a) Macchioni, A. Chem. Rev. 2005, 105, 2039−2073. (b) Fagnou, K.; Lautens, M. Angew. Chem., Int. Ed. 2002, 41, 26−47. For selected examples of counterion effects in asymmetric catalysis, see: (c) Ashimori, A.; Bachand, B.; Poon, D. J.; Overman, L. E. J. Am. Chem. Soc. 1998, 120, 6477−6487. (d) Nandi, M.; Jin, J.; Rajan Babu, T. V. J. Am. Chem. Soc. 1999, 121, 9899−9900. (e) Moreau, C.; Hague, C.; Weller, A. S.; Frost, C. G. Tetrahedron Lett. 2001, 42, 6957−6960. (3) Krossing, I.; Raabe, I. Angew. Chem., Int. Ed. 2004, 43, 2066− 2090. (4) Phan, D. H. T.; Kim, B.; Dong, V. M. J. Am. Chem. Soc. 2009, 131, 15608−15609. (5) (a) Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 2116−2119. (b) Amatore, C.; Le Duc, G.; Jutand, A. Chem.Eur. J. 2013, 19, 10082−10093. (6) (a) Thomas, J. C.; Peters, J. C. Inorg. Chem. 2003, 42, 5055− 5073. (b) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100−5101. (c) Betley, T. A.; Peters, J. C. Angew. Chem., Int. Ed. 2003, 42, 2385−2389. (d) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2003, 125, 8870−8888. (e) Lu, C. C.; Peters, J. C. J. Am. Chem. Soc. 2004, 126, 15818−15832. (7) (a) Albicker, M.; Cramer, N. Angew. Chem., Int. Ed. 2009, 48, 9139−9142. (b) Tran, D. N.; Cramer, N. Angew. Chem., Int. Ed. 2010, 49, 8181−8184. (c) Tran, D. N.; Cramer, N. Angew. Chem., Int. Ed. 2011, 50, 11098−11102. (d) Saget, T.; Lémouzy, S.; Cramer, N. Angew. Chem., Int. Ed. 2012, 51, 2238−2242. (e) Saget, T.; Cramer, N. Angew. Chem., Int. Ed. 2012, 51, 12842−12845. (f) Pham, M.; Ye, B.; Cramer, N. Angew. Chem., Int. Ed. 2012, 51, 10610−10614. (g) Ye, B.; Cramer, N. Science 2012, 338, 504−506. (h) Ye, B.; Cramer, N. J. Am. Chem. Soc. 2013, 135, 636−639. (i) Donets, P. A.; Cramer, N. J. Am. Chem. Soc. 2013, 135, 11772−11775. (j) Saget, T.; Cramer, N. Angew. Chem., Int. Ed. 2013, 52, 7865−7868. (k) Tran, D. N.; Cramer, N. Angew. Chem., Int. Ed. 2013, 52, 10630−10634. (8) (a) Seiser, T.; Cramer, N. Angew. Chem., Int. Ed. 2008, 47, 9294− 9297. (b) Seiser, T.; Roth, O. A.; Cramer, N. Angew. Chem., Int. Ed. 2009, 48, 6320−6323. (c) Waibel, M.; Cramer, N. Angew. Chem., Int. Ed. 2010, 49, 4455−4458. (d) Seiser, T.; Cramer, N. Chem.Eur. J. 2010, 16, 3383−3391. (e) Seiser, T.; Cramer, N. J. Am. Chem. Soc. 2010, 132, 5340−5342. (f) Seiser, T.; Cramer, N. Angew. Chem., Int. Ed. 2010, 49, 10163−10167. (g) Waibel, M.; Cramer, N. Chem. Commun. 2011, 345−348. (9) Holz, J.; Monsees, A.; Kadyrov, R.; Börner, A. Synlett 2007, 599− 602. (10) Donets, P. A.; Saget, T.; Cramer, N. Organometallics 2012, 31, 8040−8046. (11) For recent reviews of C−C bond activation, see: (a) Crabtree, R. H. Chem. Rev. 1985, 85, 245−269. (b) Jones, W. D. Nature 1993, 364, 676−677. (c) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870−883. (d) Murakami, M.; Ito, Y. Top. Organomet. Chem. 1999, 3, 97−129. (e) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759−1792. (f) Jun, C.-H. Chem. Soc. Rev. 2004, 33, 610− 618. (g) Satoh, T.; Miura, M. Top. Organomet. Chem. 2005, 14, 1−20. (h) Jun, C.-H.; Park, J.-W. Top. Organomet. Chem. 2007, 24, 117−143. (i) Necas, D.; Kotora, M. Curr. Org. Chem. 2007, 11, 1566−1591. (j) Seiser, T.; Cramer, N. Org. Biomol. Chem. 2009, 7, 2835−2840. (k) Aissa, C. Synthesis 2011, 3389−3407. (l) Cramer, N.; Seiser, T. Synlett 2011, 449−460. (m) Seiser, T.; Saget, T.; Tran, D. N.; Cramer, N. Angew. Chem., Int. Ed. 2011, 50, 7740−7752. (n) Murakami, M.; Matsuda, T. Chem. Commun. 2011, 47, 1100−1105. (o) Korotvicka, A.; Necas, D.; Kotora, M. Curr. Org. Chem. 2012, 16, 1170−1214. (12) Murakami, M.; Itahashi, T.; Ito, Y. J. Am. Chem. Soc. 2002, 124, 13976−13977. (13) For representative reports on metal-catalyzed carbon−acyl bond cleavage of cyclobutanones, see the following. Rh-catalyzed: (a) Murakami, M.; Amii, H.; Ito, Y. Nature 1994, 370, 540−541. (b) Murakami, M.; Takahashi, K.; Amii, H.; Ito, Y. J. Am. Chem. Soc.

ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization data for the cyclobutanones 11. Figures giving NMR spectra of all new compounds and HPLC traces for ketones 14. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC 966110 and 966111 contain the supplementary crystallographic data for 1b and 8. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac. uk/data_request/cif.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: nicolai.cramer@epfl.ch. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the EPFL and by the European Research Council under the European Community’s Seventh Framework Program (FP7 2007−2013)/ERC Grant agreement no. 257891. We thank Dr. R. Scopelliti for X-ray crystallographic analyses and Dr. R. Kadyrov (Evonik Industries AG) for generous donations of 2,5-dimethyl-1-TMS-phospholane. 786

dx.doi.org/10.1021/om4011627 | Organometallics 2014, 33, 780−787

Organometallics

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dx.doi.org/10.1021/om4011627 | Organometallics 2014, 33, 780−787